Adsorption Behavior of Surfactant–Polyacrylamide Mixtures with Kaolin N.V. Sastry * and P.N. Dave Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India

ABSTRACT: The adsorption behavior of three surfactants— hexadecylpyridinium bromide (HPyBr), sodium dodecylbenzene sulfonate (SDBS), and Triton X-100 (TX-100)—on kaolin from aqueous solution is monitored as a function of pH. The nature and shape of the adsorption isotherms are typical and highly dependent on the surfactant structure. All three surfactants adsorb on kaolin beyond the limits of monolayer coverage. Isotherms for HPyBr fit the Langmuir equation well. SDBS isotherms are typical of a two-stage process, and TX-100 adsorption isotherms are Sshaped. The pH-sensitive adsorption of each surfactant and the models used to represent the isotherms are considered together in explaining the mechanism of adsorption in each case. The complex mixture adsorption behavior from polyacrylamide–surfactant mixtures is also determined in acidic and basic pH media. Three polyacrylamides, catam1, anam1 and nonam1, are chosen to represent cationic, anionic, and nonionic polymers, respectively. Both competitive and synergistic effects are noted in the mixture adsorption depending upon the polymer–surfactant pair and the mode of introduction of a second component in the presence of another. The results are explained by considering various factors such as changes in the nature of solvent power of the media, interaction between the polymer and surfactant in the bulk solution as well as at the kaolin surface, blocking of surface sites by a preadsorbed component, and the change in the conformation state of polymer chains Paper no. S1126 in JSD 2, 459–472 (October 1999). KEY WORDS: Adsorption mechanism, competitive and synergistic effects, individual and mixture adsorption, kaolin, polyacrylamides, surfactants.

Adsorption of surfactants at the solid–liquid interface is of broad interest in different fields of application, such as ore flotation, tertiary oil recovery, detergents, cosmetics, and pharmaceuticals (1–5). The efficiency of a surfactant in any of these application areas is due to the high surface activity of the surfactant at the solid–liquid interface. Also, loss of surfactants has been reported when they are used in tertiary oil- recovery processes, in which surfactant micellar solutions are injected into the rock formation to release trapped oil. Surfactant loss is traced mainly to its adsorption on to rock formations or other porous solid minerals like clay. Moreover, the surfactants are well-characterized *To whom correspondence should be addressed. E-mail: [email protected] Copyright © 1999 by AOCS Press

model substances in establishing and understanding their adsorption mechanism on oppositely charged hydrophilic and hydrophobic substrates, oleophobic and uncharged hydrophobic surfaces, etc. Hence, the adsorption studies of surfactants not only are of an academic interest but also have great techno-economic value. Kaolin is a reactive clay mineral and is an important and unique substrate with a capacity to adsorb positively charged, negatively charged, and even uncharged surfactant species. The surface chemistry of kaolin is unique and it is now well known that its surface is heterogeneous in nature and has both cation and anion exchange sites (6). The presence of an electrical double layer, a definite cation exchange capacity (CEC), anion exchange capacity (AEC), relatively large surface area, and plate-like structure offer a wide variety of opportunities for physicochemical studies, namely, adsorption, flocculation and dispersion, electro optical, and rheological measurements. The presence of a dual charge and pH sensitivity allows interesting studies to be made under variable solution conditions such as pH and ionic strength. The key factors governing the adsorption behavior of surfactants on charged surfaces are (i) interaction of the ionic head group of the surfactant with oppositely charged sites on the solid surface; (ii) interaction of hydrophobic chains with the surface; and (iii) lateral interactions within adsorbed layers. The mechanism of adsorption of ionic and nonionic surfactants on charged surfaces is well understood. However, a general acceptance of the isotherm shape for each case and the development of a universal predictive model that fits the adsorption data have not emerged so far. A bilayer model has generally been proposed for the adsorption of alkyltrimethylammonium or pyridinium halides on kaolinite, but the structure of the adsorption layer is treated differently by different authors. It is now well established that cationic surfactants initially form a monolayer on the negative sites of kaolinite by electrostatic forces. However, views differ on the formation of the second layer. Malik et al. (7) and Sjoblom and Soderlund (8) assumed that the second layer was formed due to weaker van der Waals forces between the alkyl chains of surfactant molecules that stand erect at the surface. Wierer and Dobias (9), however, have concluded that cationic surfactant molecules start adsorbing on less reactive sites near the first occupied ones, and that the alkyl chains start to as-

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sociate due to van der Waals forces at a still higher surface coverage. Coadsorption of counterions increases and a second layer is thus built up. Hanna and Somasundaran (10) reported a complex shape in adsorption isotherms of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) on Nakaolinite with a maximal peak near the critical micelle concentration (CMC). Adsorption tends to be zero, or even negative, in concentrated surfactant solutions. The Somasundaran group (11,12) made further investigations to explain the appearance of maxima in the isotherms by constructing hystersis patterns for the adsorption–desorption process under carefully controlled solution conditions, and they concluded that adsorption involves a fast ion exchange and/or electrostatic adsorption step and a slow step in which the dissolution of aluminum species from kaolinite takes place resulting in the formation of anionic surfactant–Al+3 complexes at low surfactant concentration, retardation/redissolution of precipitates above the CMC, and bulk precipitation upon dissolution. Scamehorn et al. (13,14), however, have observed no maxima in the isotherms for pure alkylbenzene sulfonates on kaolinite, but the isotherm was divided into four regions. The authors accounted for the different regions by pointing to formation of a first monolayer, hemimicelle formation, a two-dimensional phase transition in hemimicelles, and finally a limiting region. Barakat et al. (15) reported that adsorption isotherms for alkylbenzene sulfonates on kaolinite were of the high-affinity type with an initial steep rise followed by an abrupt plateau region. Sastry et al. (16) noted that SDBS adsorption on Na-kaolinite involves two stages in which the formation of surface aggregates (hemimicelles) preceding the first monolayer was considered. Microcalorimetrically measured adsorption enthalpies for these systems further supported the formation of bilayer structures (17). More data exist on the adsorption of nonionic surfactants on hydrophobic and hydrophilic surfaces compared to ionic surfactants. Most of the studies dealt with adsorption of the Triton series of surfactants on highly hydrophilic surfaces such as silica. The data reveal that adsorption of nonionic surfactants also occurs in two stages similar to the ionic surfactant adsorption. Several models involving the formation of bidimensional interfacial aggregates (18) and hemimicelles (19,20) have been suggested. Polymers and surfactants are used together in many industrial processes such as flotation, enhanced oil recovery, and detergency. Phosphate-free detergents contain polycarboxylates, as well as a mixture of anionic and nonionic surfactants. After the washing process, the effluent is passed through water treatment plants, where it comes in contact with high-molecular-weight charged polyacrylamides which are used as flocculents. The presence of polymers and surfactants together can affect the selectivity of the process through interactions between opposite or similarly charged polymer–surfactant pairs in the bulk solution as well as at the surface. These interactions are expected to affect not only

the amount of the adsorption but also the manner of polymer–surfactant orientation. Both effects have a direct bearing on the efficiency of a given process. Interactions of charged or uncharged polymers and surfactants in the bulk solution have been studied extensively (21). A literature survey on the adsorption from polymer–surfactant mixtures at the solid–liquid interface revealed that only a few studies had been made, mostly involving a cationic polymer with anionic/cationic surfactants (22), anionic polymers with anionic/nonionic surfactants (23–25), and nonionic polymers with an anionic surfactant (26,27). The competitive effects and formation of polymer– surfactant complexes in the bulk state, as well as at the surface, were mainly considered in explaining the mixture adsorption. Most of the above individual and mixture adsorption studies were done on different samples of kaolinite, and the results were treated differently. If such studies are made using different surfactants and the same substrate surface under identical solution conditions, they may yield useful insights into the process at the solid–liquid interface. Hence, with an aim of understanding the adsorption mechanism of individual surfactants and their mixtures with charged and uncharged polyacrylamides on kaolin, this paper presents results on the adsorption of a cationic surfactant, hexadecylpyridinium bromide (HPyBr); an anionic surfactant, SDBS; and a nonionic surfactant Triton (TX-100) on to same sample of kaolin under identical solution conditions. The adsorption isotherms for each class of surfactant were explained by considering appropriate models. Complex adsorption from polyacrylamide–surfactant mixtures on kaolin was also followed under both acidic and basic pH conditions. The combination of either oppositely charged or similarly charged, and even uncharged polyacrylamide–surfactant pairs were studied. Mixture adsorption in the simultaneous mode, as well as adsorption of one component on kaolin that was saturated with the second component, were monitored. Several factors, such as competitive and synergistic effects, were considered to explain the adsorption behavior of each component in the presence of another.

EXPERIMENTAL PROCEDURES The cationic surfactant was practical-grade HPyBr from Fluka (Buchs, Switzerland). It was further purified several times from an acetone/ethanol solvent mixture. The anionic surfactant was SDBS from Aldrich (Weinheim, Germany). SDBS was further recrystallized from ethanol. The nonionic surfactant was TX-100 (C8H17–C6H4 –(OCH2–CH2)10OH) (TX-100) of scintillation grade from Ubichem (Staines, England). It was used as received. As per the manufacturer data, TX-100 was polydispersed with a Poisson distribution in the number of oxyethylene groups. The CMC of all surfactants were determined by using conductometric and dye solubilization methods. Our experimental values are given in Table 1 along with comparable literature data.

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SURFACTANT–POLYACRYLAMIDE ADSORPTION TABLE 1 Critical Micelle Concentrations (CMC) of Surfactants in Watera Surfactant HpyBr SDBS TX-100

Experimental (mol·dm−3) DS Conductance 8.5 × 10−4 1.35 × 10−3 3.75 × 10−4

8.7 × 10−3 1.35 × 10−3 —

Literature 9.00 × 10−3 (25°C)b 1.30 × 10−3 (25°C)c 3.35 × 10−3 (25°C)d

a Temperature was 28°C; DS, dye solubilization; HPyBr, hexadecylpyridinium bromide; SDBS, sodium docecylbenzenesulfonate; TX-100, C8H17–C6H4–(OCH2–CH2)10OH. b Reference 28. c Reference 29. d Reference 30.

Polyacrylamide samples were donated by Allied Colloids Ltd. (Hamburg, Germany). They were used as received. The cationic and anionic polyacrylamides are random copolymers. These random copolymers have a copolymer composition of 30 mol%, adjusted according to the comonomer feed ratio. The cationic polyacrylamide (catam1) is a random copolymer of acrylamide and trimethylamine ethylacrylate chloride. The anionic copolymer (anam1) consists of acrylamide and acrylic acid moieties. Nonionic polyacylamide (nonam1) is a high-molecular-weight homopolymer. Viscosity average molecular weights of catam1, anam1, and nonam1 polymers are 1.54 × 106, 1.51 × 106, and 1.98 × 106 g·mol−1, respectively. The charge density of catam1 and anam1 polymers are 423 and 328 C·g−1, respectively. The details of the characterization of these polymers have been described elsewhere (31). Kaolin was obtained from Sigma Chemical Company (St. Louis, MO). Particles less than 2 µm in size were separated from the kaolin suspensions in water by repeated centrifugation, and the final residue was dialyzed every 2 h against triple-distilled water until the supernatant conductivity became less than 10 µS/cm. The solid residue obtained after final centrifugation was dried at 80°C. The CEC of the sample was estimated by ammonium ion adsorption using an ion-selective electrode and was found to be 3.8 meq/100 g. The BET surface area from nitrogen adsorption was estimated to be 15 m2/g. Triple-distilled water from an all-Pyrex glass still was used in the preparation of solutions and aqueous suspensions. The pH of kaolin suspensions was adjusted by Fischer-certified 0.1 N HCl or 1 N NaOH solutions.

METHODS CMC estimation. The conductivities of anionic and cationic surfactant solutions for CMC determinations were measured with a Systronics (Ahemdabad, India) conductometer having a dip-type conductometer cell and a cell constant of about 1.0 cm−1. Dye solubilization experiments were done by shaking an excess of orange OT (a water-insoluble azo dye) in surfactant solutions for 48 h at room temperature. Once equilibrium was reached, the excess insoluble dye was sepa-

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rated by centrifugation and filtration. The supernatant solution containing solubilized dye was diluted with ethyl alcohol and water, with the final ratio of ethyl alcohol and water adjusted to 2:1. Absorbance of the solution containing solubilized orange OT was measured at its λmax (470 nm) using a digital Systronics spectrophotometer. Adsorption isotherms. Adsorption isotherms of surfactants on kaolin (10 g/L) from aqueous solutions were constructed based on the depletion method. The method involves premixing the adsorbent (pre pH-conditioned aqueous suspension of particles) with a solution of known surfactant concentration and shaking the mixture for 48 h on a reciprocal shaker. The initial concentrations for all surfactants in each set were selected in such a way that the concentrations were evenly spread below, near, and above the CMC values. The suspensions were then centrifuged at a constant temperature close to 30 ± 2°C for 1 h at a speed of 10,000 rpm on a C-30 Remi Cooling research centrifuge (Mumbai, India). Surfactant concentration was measured by ultraviolet absorption on a Shimadzu spectrometer model 160 (Kyoto, Japan), using the second derivative method. New calibration curves of peak height vs. surfactant concentration were constructed each time. The amount of surfactant adsorbed, expressed in µmol/g (of kaolin), was obtained from the difference in concentration of the surfactant solution before and after adsorption equilibrium. The concentrations by this method have a standard error of ± 1%. Adsorption from polymer–surfactant mixtures was monitored by adopting two experimental approaches, namely, simultaneous adsorption and adsorption of one component onto pretreated kaolin (pretreated with another component by giving a fixed initial dose corresponding to the plateau levels). Preparation of polymer–surfactant solutions, especially when the components are oppositely charged, was carefully done as per the procedure described below to avoid either any visible turbidity or macrophase separation. After a few trials, the following procedure was adopted for preparing polymer–surfactant mixture solutions for each type of experiment. Simultaneous adsorption. For constructing the surfactant adsorption isotherms in the presence of a fixed amount or concentration of each type of polymer, stock surfactant solutions (0.01 mol·dm−3) were prepared directly by weighing the required amount of surfactant and dissolving it in 0.08% of each polymer solution (freshly prepared in tripledistilled water). Then the concentrations needed for constructing the adsorption isotherms of surfactant were obtained by diluting the stock surfactant solution with 0.08% polymer solution. However, this procedure was unsuitable for preparing SDBS in catam1 because a fibrous, white, thin gel precipitate was formed upon mixing. In this case, we prepared 50 mL of a SDBS stock solution (0.1 mol·dm−3) in triple-distilled water. Stock SDBS solution (5 mL) was diluted by adding 45 mL of 0.08% catam1 solution (in triple-distilled water) to yield a 0.01 mol·dm−3 SDBS solu-

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tion. We noticed no precipitation or visible turbidity when the mixture was prepared in this manner. For constructing polymer adsorption isotherms from polymer–surfactant mixtures, the same procedure was adopted as outlined in the preceding paragraph. A 0.1% stock polymer solution in 100 mL of surfactant solution (2 × 10−3 mol·dm−3 of HPyBr, 1.5 × 10−3 mol·dm−3 of TX-100, 4 × 10−3 mol·dm−3 of SDBS) was prepared by weighing the required amount of solid polymer directly into surfactant solutions. Further dilutions were made using the surfactant solution as solvent. In the case of the catam1–SDBS mixture, a 100-mL sample of 0.2% of catam1 solution was first prepared in triple-distilled water. Then 50 mL of catam1 solution was mixed with 50 mL of stock SDBS solution to get a 0.1 % catam1 solution in SDBS. The concentration of SDBS stock solution in the latter case was selected such that it was either equal to 4 × 10−3 mol·dm−3 or exceeded it in every case. No visible turbidities or precipitation was observed in either case. Adsorption onto pretreated kaolin. Kaolin suspensions (10 g/L) were prepared as usual at a desired pH with fixed concentrations of surfactants or polymer (corresponding to the same concentration as in the simultaneous adsorption), and the adsorption was allowed to take place during an equilibration period of 48 h. Suspensions were centrifuged at a speed of 10,000 rpm. Supernatants were discarded and the settled residue yielded kaolin particles saturated with either a surfactant or a polymer. Individual adsorption isotherms were run as usual. The concentrations of surfactants and polymer were determined per the procedure described earlier. New calibration curves for each species were, however, constructed from the mixture solutions and no effect was observed on the linearity of the curves drawn for individual species in the presence of a second component. Viscosity measurements were made on polymer–surfactant solutions using a modified Ostwald-type capillary viscometer.

FIG.1. Adsorption isotherms for the cationic surfactant hexadecylpyridinium bromide (HPyBr) on kaolin.

the equilibrium concentration, the rate of rise in adsorption decreases until the equilibrium concentration is equal to half the CMC value. At higher equilibrium concentrations, a plateau is observed for all three curves. The beginning of the plateau corresponds to the CMC. Furthermore, an increase in pH has little effect in the initial stage of adsorption. The pH effect increases adsorption levels in the intermediate and plateau regions. The surface-charge characteristics of kaolin have been determined from acid–base titrations (Sastry, N.V. and Dave, P.N., unpublished data). The edges and basal planar surfaces of kaolin were found to behave differently. In acidic pH media, most of the edge aluminol groups are protonated and generate a positive charge, while the basal planes carry a permanent negative charge. With a rise in pH, positive sites on the edge faces disappear at a point of zero charge (pH ≈ 6.2 in this case) and the surface is fully negatively charged. Adsorption of a cationic surfactant on

RESULTS AND DISCUSSION Individual adsorption of surfactants. Adsorption isotherms of HPyBr, SDBS, and TX-100 on kaolin are represented by plotting the adsorbed surfactant concentration against the equilibrium, or free concentration, as shown in Figures 1–3. The effect of pH on adsorption is also shown for each surfactant. Adsorption measurements were done on the same adsorbate (kaolin). It is interesting to note from Figures 1–3 that the shape of the isotherm and magnitude of plateau adsorption and its pH dependence are typical and distinct for each type of surfactant. Adsorption of cationic surfactant on kaolin. Adsorption isotherms for HPyBr on kaolin at pH values of 4.0, 7.0, and 9.0 are shown in Figure 1. The isotherms are of the highaffinity type with a steep increase in adsorption level within the initial concentration range. With an increase in

FIG.2. Adsorption isotherms for anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on kaolin.

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TABLE 2 Summary of Langmuir Fittings of the Adsorption Isotherms of Cationic Surfactant on Kaolin in Watera pH

Γmax (µmol/g) Experimental Calculated

4.0 7.0 9.0 a

FIG. 3. Adsorption isotherms for nonionic surfactant C8H17–C6H4– (OCH2–CH2)10OH (TX-100) on kaolin.

kaolin may thus be due mostly to ionic interactions (electrostatic attractions and van der Waals forces up to its CEC and van der Waals forces only beyond it. Formation of a second layer in which weak hydrophobic forces are operative occurs only when the electrostatic adsorption is complete. The CEC of kaolin was 42 µeq/g of kaolin. Thus the plateau values (Fig. 1) of 195, 209, and 216 µmol/g are in excess of the CEC of kaolin. The excess adsorbed surfactant can only be explained by considering the formation of a bilayer in which the hydrophobic alkyl chains of surfactant molecules interact with the alkyl chains of molecules that are already adsorbed on kaolin through electrostatic attractions. Harwell (32) proposed an admicelle model for such a bilayered adsorption. However, his hypothesis maintained that the surfactant aggregation that produces the bilayered structure occurs on a given patch of heterogeneous surface at a certain critical admicelle concentration, which appears as a break in the curve. No such breaks are seen in Figure 1, and it did not fit his model. Adsorption isotherms for dodecylpyridinium chloride on Na kaolinite from salt solution were found to be S type (17), which also is not the case with our results. However, Malik et al. (7) and Sjoblom and Soderlund (8) reported high-affinity Langmuir isotherms for cetylpyridinium bromide/kaolinite, cetylpyridinium chloride/quartz, and kaolin and dodecylpyridinium bromide/silica gel systems. The curves of Figure 1 are fitted to the Langmuir model, and the calculated values of plateau adsorption value, Γmax, and K, the binding constant, are summarized in Table 2. The agreement between experimental and fitted adsorption values as shown in the figure indicates that our explanation for HPyBr adsorption on kaolin is reasonable. Adsorption of anionic surfactant on kaolin. Adsorption isotherms for SDBS are shown in Figure 2. The curves are characteristic of a two-stage process with apparent multilayer formation. The isotherms showed an initial steep rise

180 191 210

184 199 209

T (K) 3070 4831 5200

HpyBr was used as the surfactant. See Table 1 for abbreviation.

in adsorption with a narrow first plateau, and then a sudden increase followed by a second plateau. The second plateau appears just after the equilibrium concentration exceeds the CMC. The nature of the curve can be explained in the following manner: Adsorption in the first stage is initially due to strong interactions between the negatively charged head groups and positively charged adsorption sites on the kaolin surface in acidic and neutral pH media. The adsorption is not restricted to this electrostatic attraction. As the bulk concentration of surfactant is increased, additional surfactants are adsorbed by the lateral alkyl–alkyl hydrophobic interactions, and small surface aggregates are formed around the primarily adsorbed species. These surface aggregates are called hemimicelles. At concentrations greater than CMC, the adsorption level reaches a nearly constant value (second plateau region). The arrangement of molecules in the adsorbed layer at the second stage of adsorption is in the form of closely packed bi- and multilayers. Zhu and Gu (33) have developed a general isotherm based on mass action treatment. The relation used to represent the isotherm is given as Γ =

Γα k1 C (1/ n + k 2 C n − 1 ) 1 + k1 C (1 + k 2 C n − 1 )

[1]

where Γ is the adsorbed amount in moles/g; Γα is the saturation or upper limiting adsorption value, i.e., Γ2nd (at C >> CMC); C is the equilibrium concentration in mol/L, n is the aggregation number, i.e., number of surfactant molecules in the hemimicelles, and k1 and k2 are equilibrium constants for the first and second stages, respectively. This equation is successfully applied to describe the adsorption of ionic and nonionic surfactants on solid surfaces (34,35). We have applied Equation 1 to our results taking the experimental values of Γ and C (shown as points in Fig. 2). The best-fitting curves in Figure 2 are obtained from nonlinear regression analysis by varying the value of aggregation number n. The analysis allows an evaluation of k1 and k2, the equilibrium constants for adsorption of monomers in the monolayer (or first layer), and for formation of surface aggregates. Results of the analysis are summarized in Table 3. It can be seen that the aggregation number for hemimicelles is about 3. It can be further seen from Figure 2 that adsorption levels in general decrease with an increase in pH, and about a threefold reduction was noted in the plateau value when pH was raised from 4.0 to 9.0.

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TABLE 3 Parameters of the Adsorption Isotherm Equation for the Adsorption of Anionic Surfactant on Kaolin in Watera pH

Γα (µmol/g)

n

k1

k2

4.0 7.0 9.0

8.2 6.1 4.2

3.0 3.0 3.5

4.8 ± 1.5 × 104 4.1 ± 1.3 × 104 4.8 ± 3.1 × 104

1.62 ± 0.21 × 106 1.49 ± 0.21 × 106 2.80 ± 0.70 × 107

a

SDBS was used as the surfactant; Γα, saturation adsorption value; n, aggregation number; k1 and k2 are equilibrium constants for the first and second stages, respectively. See Equation 1 for more details. See Table 1 for abbreviation.

that the mass-action model should work for nonionic surfactant adsorption systems. A suitable adsorption isotherm can then be derived in the following manner. Consider the aggregation of n monomers of nonionic surfactant on a surface site (S) to form hemimicelles (surface aggregates, abbreviated hm), where n is the aggregation number of the hemimicelle. At equilibrium, S + n-monomer = hemimicelle

[2]

K = ahm/as an

[3]

and The molecular area per adsorbed molecule can be calculated from the plateau adsorption or surface saturation values. Calculations using Γα values from Table 3 reveal that molecular areas of 33.2, 44.9, and 63.9 Å/SDBS molecule are needed to account for the saturation values at pH 4.0, 7.0, and 9.0, respectively. The molecular area value of 33.2 Å/SDBS molecule at pH 4.0 is in agreement with reported values for closely packed layers of SDBS on kaolin (10) and dodecylbenzenesulfonate on CaCO3 and Ca3(PO4)2 (36). The increase in area per adsorbed SDBS molecule with increase in pH indicates that packing in the first layer at basic pH conditions is loose. Thus, two-step adsorption isotherms and pH-sensitive adsorption values on kaolin clearly suggest that the positive-edge faces of kaolin play a major role in the initial anchoring stage (monolayer formation) in great part by electrostatic interactions (anion-exchange process). The second stage of adsorption involves hemimicelle formation through lateral interactions. However, the relatively large amounts adsorbed at basic pH, where electrostatic repulsion between adsorbate–adsorbent is expected, indicate that not only must electrostatic interactions be considered but also other types, such as ligand exchange, hydrophobic interactions and entropic effects. Microcalorimetrically measured adsorption enthalpies for SDBS adsorption on kaolin were indeed found to be exothermic in the initial stage followed by endothermic values at surface saturation concentrations (9,16,19). Exothermic enthalpies are characteristic of strong electrostatic-attractive interactions while endothermic values indicate weaker attractive forces (hydrophobic or entropy effects). Adsorption of a nonionic surfactant on kaolin. Adsorption isotherms of TX-100 on kaolin exhibited an S-type curve at all pH values (Fig. 3). After an initial minimal adsorption, the values increase sharply. As the concentration increases further, adsorption reaches a plateau above the CMC. Thus, only one adsorption plateau above the CMC is observed. Such S-type adsorption isotherms for nonionic surfactants on silica gel have previously been reported (37). The absence of an initial first plateau in the isotherms can be qualitatively explained because neither electrostatic nor strong interactions between the adsorbate and adsorbent are present in these systems. Thus, as a first approximation, the adsorbed amounts should be considered the result of surface micellization. It is reasonable to postulate

where a is the activity of nonionic surfactant monomers in solution, and for dilute solutions a = C, which is the concentration of nonionic surfactant, if C < CMC. The activities of hemimicelles and surface sites are ahm and as, respectively, and K is the equilibrium constant. Approximately, ahm and as can be given by ahm = Γ/n

[4]

as = (Γα − Γ)/n

[5]

and

respectively. Γ is the amount of surfactant adsorbed at C and Γα is the amount adsorbed in the limiting adsorption at high concentrations. Thus, Equation 3 becomes K = Γ/(Γα − Γ) Cn

[6]

Γ = (Γα K Cn)/(1 + K Cn)

[7]

and

Equation 7 has a form which can be considered a combination of the Langmuir and Freundlich adsorption isotherms. At low concentrations Equation 7 reduces to a Freundlich isotherm. If we take the logarithm of Equation 6, we obtain, after rearrangement, log [(Γ/(Γα − Γ)] = log K + nlog C

[8]

Equation 8 has a linear form, and to test the validity of this model, a plot of log [Γ/(Γα − Γ)] against log C gives a straight line. The slope of the straight line should give the value of n, and the intercept should be log K. If the value of n > 1, hemimicelles occur. If multisite adsorption occurs (i.e., each adsorbed molecule occupies more than one site), then n should be < 1, but > 0. When n = 1, Equation 6 reduces to the Langmuir adsorption equation, indicating monolayer formation. The value of n can never be less than zero. Experimental adsorption values for TX-100 were fitted to Equation 8. The parameters evaluated by linear regres-

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TABLE 4 Parameters for the Representation of Adsorption Isotherms of Nonionic Surfactant Adsorption on Kaolin in Watera pH

n

Log K

K

hmc (mmol/dm3)

−∆hmG° (kJ/mol)

∆mG° (kJ/mol)

4.0 7.0 9.0

5.2 3.8 5.3

19.2 14.0 19.0

1.51 × 1019 1.09 × 1014 1.02 × 1019

0.13 0.12 0.17

21.09 20.97 20.43

20.19 20.19 20.19

a TX-100 was used as the surfactant; n, slope; log K, intercept; K, equilibrium constant; hmc, hemimicelle concentration; ∆hmG°, standard free energy of hemimicellization; ∆mG°, free energy of bulk micellization.

sion analysis are summarized in Table 4. The model fits the experimental data well, as the solid lines representing the fitted values pass through the experimental points in Figure 3. The hemimicelle concentration (hmc) was taken from the initial inflections in the isotherms. The hmc value roughly equals one-fourth to one-half the CMC value. From the value of K, the standard free energy of hemimicellization (∆hm Go) for one mole of surfactant can be calculated by using the equation − ∆hmGo = (1/n) RT ln K

[9]

and similarly the free energy of bulk micellization is calculated by − ∆mGo = RT ln CMC

[10]

where R = gas constant in Joules and T = absolute temperature. It is interesting to see from the data of Table 4 that the values of − ∆hmGo and −∆mGo are close in magnitude and match in sign. Thus as expected, the hemimicellization and bulk micellization have analogous natures. The following mechanism is proposed for adsorption of TX-100 on kaolin. When the surfactant concentration is small, a weak adsorption of the molecule in the form of monomers is observed and the adsorption varies linearly with the concentration in this initial stage. The oxyethylenic part of the molecule is attached to surface hydroxyl groups via hydrogen bonding, and the lateral hydrophobic interactions between hydrophobic groups are negligible. The molecules tend to lie flat on the surface, however, at hmc, the cooperative interactions (adsorbate–adsorbate) are dominant, and a large change in the adsorbed amount is observed (rising part of the isotherms). This increased amount causes not only a change in the reorientation of adsorbed molecules but also the appearance of surface aggregates due to lateral alkyl–alkyl interactions. This mechanism is consistent with the above proposed model. There are few adsorption studies of TX-100 on kaolin. Denoyel and Rouquerol (19) measured the adsorption of TX-100 on kaolin. The isotherm at pH 3.8 consisted of two parts. The first part is L shaped, but with an ill-defined plateau and

the second part is a step before the CMC and the final plateau. However, Kronberg et al. (38) think that adsorption takes place only on one of the basal faces of kaolin, and they did not see the first step. The adsorption isotherm was fitted to the Langmuir model. Both of the reported plateau adsorption values of 20 µmol/g (19) and 22.6 µmol/g (38) for TX-100 on kaolinite are in agreement with our value of 24.5 µmol/g at pH 4.0. It can be further seen from Figure 3 that an increase in pH produces a marginal decrease in adsorption levels. This sensitivity of adsorption to pH indicates that both the silanol and aluminol groups of the lateral and basal faces of kaolin act as adsorption sites through hydrogen bonding. An increase in pH deprotonates not only the lateral aluminol but also the silanol and aluminol groups on the basal faces, thereby reducing the free hydroxyl sites needed for hydrogen bonding. Effect of polymer on surfactant adsorption. The adsorption of HPyBr, SDBS, and TX-100 in the presence of a catam1, anam1, and nonam1 was measured in acidic and basic media. The respective adsorption isotherms are shown in Figure 4 A–C. It can be seen from Figure 4A that HPyBr adsorption is depressed by all polymers both in acidic and basic media. Adsorption suppression was in the order, catam1 > anam1 > nonam1. An increase in pH led to higher suppression in HPyBr adsorption in the presence of the polymers. A closer examination of Figure 4B shows that catam1 and nonam1 have a marginal effect on SDBS adsorption both in acidic and basic pH media because only a slight decrease in adsorption levels was noted. Anam1 decreases the SDBS adsorption by almost half in acidic media and at the same time enhances the adsorption by almost 1.5–2 times in basic pH media. An initial increase in adsorption values followed by a sharp decrease was noted for TX-100 adsorption at both pH levels (Fig. 4C). The order of decrease in adsorption value at concentrations around and above CMC showed that nonam1 depresses maximal adsorption of TX-100 almost 4.5 to 5 times, followed by anam1 and catam1. Adsorption of individual surfactants on polymer pretreated kaolin particles was also studied, and the respective adsorption isotherms are shown in parts of Figure 5 A–C. Examination of Figure 5A and B reveals a general pattern. The adsorption of either HPyBr or SDBS in acidic

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FIG. 4. Simultaneous adsorption isotherms of (A) HPyBr; (B) SDBS; and (C) TX-100 on kaolin in the presence of a fixed concentration (0.08 g/dL) of polyacrylamides; (a) pH 4.0 and (b) pH 9.0. Catam1, cationic polyacrylamide; random copolymer of acrylomide and trimethylamine ethylacrylate chloride; anam1, anionic copolymer of acrylamide + acrylic acid; nonam1, high-molecular-weight homopolymer of nonionic polyacrylamide.

as well as basic pH media is lower by almost 1.5 to 2 times on kaolin particles presaturated (with a polymer concentration corresponding to the plateau region of individual isotherms) with similarly charged polymers, namely catam1 and anam1. At the same time, the coating of kaolin particles with nonam1 affects HPyBr and SDBS adsorption slightly. The saturation of kaolin particles with the polymer containing chains of opposite charge with respect to the surfactant increases the adsorption of HPyBr and SDBS at both pH values, with the exception of HPyBr adsorption in basic pH media where a decrease of adsorption on anam1-coated particles was noted. Adsorption isotherms for TX-100 as shown in Figure 5C reveal that adsorption is decreased almost by half an order of magnitude at acidic pH on nonam1 presaturated kaolin particles and by four times at basic pH, while the TX-100 adsorption isotherms on catam1- and anam1-coated particles lie between those of pure and nonam1-coated particles. Effect of surfactant on polymer adsorption. The adsorption isotherms of catam1, anam1, and nonam1 in the presence of each surfactant are shown at both pH values in Figure 6 A–C. The curves presented in the figure do not show any general trend, and variations in adsorption values are highly sensitive not only to the pH of the media but also to the type of surfactant and polymer. Increased adsorption of catam1 in low pH media means that the presence of surfactant increases adsorption levels irrespective of their

charge in acidic media. The increase was higher in the presence of SDBS followed by HPyBr and TX-100. In basic pH media, SDBS increases the catam1 adsorption while a decreasing trend was noted in the presence of HPyBr and TX100. The enhanced levels of adsorption of a cationic polymer in the presence of sodium dodecyl sulfonate were also observed by Moudgil and Somasundaran (22) and Arnold and Bruer (39). As seen in Figure 6B, the effect of the simultaneous presence of surfactant on anam1 adsorption is highly dependent on pH. A decrease in adsorption was noted by the presence of HPyBr, SDBS, and TX-100 in acidic media, while HPyBr and SDBS enhanced the adsorption levels and TX-100 had a decreasing effect in the basic media. A similar trend in the effects due to simultaneous presence of surfactants was noted on the adsorption of nonam1 in acidic and basic pH media. Blocking adsorption sites on kaolin by preadsorbed surfactant molecules can affect adsorption of the polymers. This is shown in Figure 7 A–C. Examination of the curves revealed interesting but complex trends. A general decrease in polymer adsorption was observed when kaolin was presaturated with a similarly charged surfactant with the following exception: The saturation of the kaolin surface by HPyBr in acidic media does not show a decreasing effect on catam1 adsorption, and in fact, a slight increase was noted. Another feature of these curves is that presatu-

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FIG. 5. Adsorption isotherms of (A) HPyBr; (B) SDBS, and (C) TX-100 on polyacrylamide saturated kaolin suspensions; (a) pH 4.0 and (b) pH 9.0. See Figures 1–3 for abbreviations.

ration of kaolin with surfactants results in increased adsorption levels when the surfactant and polymer have opposite charges. The presence of HPyBr increases nonam1 adsorption while SDBS lowers the adsorption of nonam1 in acidic and basic media. Adsorption results from the polymer–surfactant mixtures present a complex picture. As discussed elsewhere (32), adsorption of polymers on the kaolin surface is highly dependent upon pH and is thought to involve electrostatic interactions (ion exchange reactions), hydrogen bonding, and ligand exchange in addition to van der Waals forces and entropic forces. Similarly, cationic and anionic surfactants initially anchor through electrostatic interactions followed by lateral hydrophobic interactions resulting in the formation of small surface aggregates (hemimicelles, in the case of anionic and nonionic surfactants) and multilayered admicelles (in case of cationic surfactant). Same forces are thus expected to play a decisive role in adsorption from the polymer–surfactant mixtures. In addition to these forces, the bulk interactions between a given pair of polymer and surfactant species arise from the change in solvent power of the medium and the associated interactions in the bulk solution or at interfaces as a result of electrostatic, hydrogen bonding, and hydrophobic forces between the polymer and surfactant species. Changes in polymer conformation toward coiling or expansion, because of either charge neutralization (when the polymer and surfactant have op-

posite charge) or screening of charged groups by counterions of surfactant species, influence polymer adsorption. An attempt was made to characterize the bulk polymer–sur-factant interactions using dye solubilization and viscosity methods. The CMC of all three surfactants were determined by dye solubilization method in the presence of a fixed concentration of each type of polymer. The values are summarized in Table 5. It can be seen that CMC values for all three surfactants used in this study decrease in the presence of all three types of polymers in acidic and basic media. The decrease in CMC values of charged surfactants is greater in the presence of polymer of opposite charge, i.e., HPyBr/anam 1 and SDBS/catam1 systems. The general decrease in CMC values can be explained by the fact that the charged polymer partially neutralizes the charge on the surfactant micelles, and few polymer–surfactant neutral complexes are formed. The presence of such neutral polymer–surfactant complexes makes the water a poorer solvent and thus enhances the surface activity of free surfactant molecules. Similarly, the partial neutralization of charge on surfactants by oppositely charged polymers promotes micellization and therefore both the above factors contribute to the decrease in CMC values. We have not noted any visible turbidity or precipitation in polymer–surfactant solutions in the concentration range of our study. The addition of surfactants to polymer solutions also af-

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FIG. 6. Simultaneous adsorption isotherms of (A) cationic polyacrylamide (catam1); (B) anionic polyacrylamide (anam1); and (C) nonionic polyacrylamide (nonam1) in the presence of a fixed concentration of surfactants; (a) pH 4.0 and (b) pH 9.0. For abbreviations see Figures 1–3.

fects conformation due to changes at the electrical double layer around polyions as observed in the presence of simple salts. Sodium ions from SDBS and bromide ions from HPyBr decrease the repulsion between negative and positive charges carried by anam1 and catam1 polymers and thus reduce coil expansion. The reduced viscosity of catam1, anam1, and nonam1 polymer solutions in the presence of surfactant micelles is plotted as a function of polymer concentration (Figs. 8 and 9). A general decrease in the reduced viscosity of polymer solutions is observed in the presence of all surfactants, irrespective of charge. The de-

crease in reduced viscosity is large in the presence of a surfactant of opposite charge because of charge neutralization. A large change in the magnitude of reduced viscosity was noted in catam1/surfactants at pH 4.0 and anam1/surfactants at pH 9.0. Similarly, narrow changes in reduced viscosities were noted in anam1/surfactants at pH 4.0 and catam1/surfactants at pH 9.0. The decrease in reduced viscosity of nonam1 polymer/surfactant mixtures appears to be less sensitive with the exception of nonam1/TX-100, where the decrease was maximal in both media. Large changes in reduced viscosities in catam1 and anam1 poly-

FIG. 7. Adsorption isotherms of (A) catam1; (B) anam1; and (C) nonam1 onto surfactant-saturated kaolin suspensions; (a) pH 4.0 and (b) pH 9.0. See Figure 1–3 and 6 for abbreviations.

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TABLE 5 CMC (mol/dm3) of Surfactants in the Presence of Polymers from DS Measurementsa Mixture HPyBr/water HPyBr/catam1 HPyBr/anam HPyBr/nonam SDBS/water SDBS/catam SDBS/anam1 SDBS/nonam1 TX-100/water TX-100 / catam1 TX-100 / anam1 TX-100 / nonam1

pH 4.0

pH 9.0

0.85 × 10−3 0.71 × 10−3 10.70 × 10−3 10.81 × 10−3 1.35 × 10−3 10.90 × 10−3 1.10 × 10−3 1.10 × 10−3 0.375 × 10−3 0.345 × 10−3 0.355 × 10−3 0.345 × 10−3

0.85 × 10−3 0.78 × 10−3 0.71 × 10−3 0.81 × 10−3 1.35 × 10−3 1.10 × 10−3 1.20 × 10−3 1.30 × 10−3 0.375 × 10−3 0.350 × 10−3 0.350 × 10−3 0.350 × 10−3

a Catam1, cationic polyacrylamide, random copolymer of acrylamide and trimethylamine ethacrylate chloride; anam1, anionic copolymer of acrylamide and acrylic acid; nonam1, high-molecular weight homopolymer of nonionic polyacrylamide; see Table 1 for other abbreviations.

acryamides in acidic and basic pH media can be related to the fact that both these polymers in the respective pH media acquire a fully extended conformation in water. Thus it may be that the fully extended polymer chains are more susceptible in the presence of surfactants. Schwartz and François (40), Siffert and Bocquenet (23), and Bocquenet and Siffert (24) have also observed the similar conformational changes of charged polymers in the presence of ionic surfactants. Methemitis et al. (41) also reported a sharp fall in reduced viscosity of hydrolyzed polyacrylamide solutions in the presence of anionic surfactants. The authors have accounted for the conformational changes by taking into account the site binding of counterions either on the polyions or the micelles. In considering the various factors that control the adsorption of individual polymer or surfactant species on kaolin as well as the bulk interactions between polymer surfactant mixtures, the following qualitative treatment is used in explaining the adsorption of polymer–surfactant mixtures. Simultaneous adsorption of surfactants in the presence of polymers. Competition between similarly charged polymers and surfactants for sites on kaolin is a reasonable explanation for the decrease in adsorption of HPyBr, SDBS, and TX-100 in the presence of catam1, anam1, and nonam1 in acidic and basic media. The increasing effect of SDBS adsorption by anam1 in basic conditions, where both the polymer and kaolin are highly negative, cannot be explained by taking only competitive effects into consideration. Electrostatic repulsion between anam1 and SDBS may drive the surfactant onto the surface, as noted by Sastry et al. (16). The initial increase in TX-100 adsorption in the presence of all three types of polymers in acidic and basic media may be attributed to the enhanced surface activity of TX-100, as indicated by smaller CMC values in the presence of each of the polymers (Table 5). However, when the concentration of free TX-100 reaches a value close to, or

FIG. 8. Reduced viscosity vs. polyacrylamide concentration profiles in the presence of a fixed concentration of surfactants at pH 4.0; HpyBr (2 × 10−4 mol·dm−3), SDBS (4.0 × 10−3 mol·dm−3), and TX-100 (1.5 × 10−3 mol·dm−3). See Figures 1–4 for abbreviations.

greater than, CMC, the competing effects become significant. This is clear from the observation that the plateau value of TX-100 is diminished by 4.5 to 5 times in the presence of nonam1, as the acrylamide and ether groups compete for free hydroxyl groups on the kaolin surface via hydrogen bonding. Surfactant adsorption onto prepolymer-treated kaolin. Sharing of the same surface sites on kaolin by similarly charged polymers and surfactants and nonam1 and TX-100 can be demonstrated further from the absorption results of surfactants onto prepolymer-treated kaolin particles. A decreased level of adsorption (by 1.5 to 2 times) of HPyBr and SDBS at both pH values on kaolin particles presaturated with the catam1 and anam1 polymers, respectively, was noted. Similarly, a large decrease in TX-100 adsorption values was noted when nonam1 polymer was preadsorbed, whereas adsorption of HPyBr and SDBS was affected only

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FIG. 9. Reduced viscosity vs. polyacrylamide concentration profiles in the presence of a fixed concentration of surfactants at pH 9.0; HPyBr (2 × 10−4 mol·dm−3), SDBS (4.0 × 10−3 mol·dm−3), and TX-100 (1.5 × 10−3 mol·dm−3). See Figures 1–3 for abbreviations.

slightly under these conditions. Preadsorption of catam1 and anam1 had little effect on TX-100 adsorption, as expected. Enhanced adsorption was measured for HPyBr and SDBS on kaolin particles presaturated with oppositely charged anam1 and catam1. Increased levels of adsorption of HPyBr and SDBS can be explained by considering the electrostatic attraction between the loops of the polymer chains and surfactant molecules. Simultaneous adsorption of polymers in presence of surfactants. Changes in the conformation of polymer chains toward chain coiling or expansion in the presence of surfactants produce significant changes in polymer adsorption. Changes in reduced viscosities of polymers are reflected in the adsorption of catam1 in acidic media. The reduction of reduced viscosities of catam1 followed the order SDBS > HPyBr > TX-100, and similarly increased levels of polymer

adsorption were noted in the same order. A decrease in reduced viscosity indicated a reduction in the size and surface of molecules because of chain coiling due to partial charge neutralization or interactions of surfactant counterions with the electrical double layer of the charged polymer, producing a screening effect. Thus, the small surface requirement of coils enhances adsorption levels. However, in the basic media, the polymer chain of catam1 is coiled because of interunit contacts between the positive charge and negative carboxylate charges (generated by hydrolysis of amide in basic pH conditions) and thus the addition of a surfactant produced only small variations in reduced viscosity values. Decreased adsorption of catam1 in the presence of HPyBr and TX-100 can be attributed to competitive effects among all species for the same sites on kaolin. However, SDBS presence increases catam1 adsorption, probably because of formation of an interfacial layer due to the lateral interactions between the alkyl chains of surfactant sandwiched between the polymer chains. Competition for the negative sites on kaolin between catam1 and HPyBr may be responsible for decreased adsorption of catam1 in the presence of HPyBr. Anam1 has both acrylamide and acrylic acid moieties randomly distributed along the chain. The dissociation of the carboxylic group is minimal at pH 4.0, and hence, the chain is in a coiled state. As already noted, the presence of surfactant affects the conformation because of a change in solvent quality. The increase in anam1 adsorption in the presence of HPyBr and SDBS in basic pH media can be explained by considering both the partial neutralization of negative charge on the polymer chain by HPyBr micelles and the screening effects exerted by sodium ions from SDBS on the fully dissociated carboxylic polyanions. Both of these factors favor coiling of the polymer chain and therefore decrease the surface of macromolecules and enhance adsorption. Competitive effects were further confirmed by adsorbing polymers by presaturating the kaolin surface by HPyBr, SDBS, and TX-100. Adsorption levels of catam1 and anam1 polymers decrease as the availability of vacant sites decreases on HPyBr- and SDBS-treated kaolin particles. Similar effects were also reported during the adsorption of anionic polymers onto SDS-treated hematite particles (43,44) and SDBS-treated kaolin (16). The authors have concluded that the chronology of component addition affects the adsorption of each components. Enhanced adsorption values for catama1 and anam1 onto SDBS- and HPyBr-saturated particles may be explained by taking into account the presence of the dual charges (positive edges and negative basal planes) of kaolin in acidic media and the deflocculated state of kaolin in basic pH media due to the destruction of card-house structures or edge-face associations. Adsorption of an oppositely charged surfactant reduces the number of neighboring unfavorable sites and hence, enhances adsorption of polymer. For example, adsorption of HPyBr on negative

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basal planes would facilitate strong contacts between negative anam1 and positive edge sites and vice versa. Deflocculation of kaolin particles creates additional surface area that otherwise would not have been available and hence adsorption of polymer is increased.

ACKNOWLEDGMENT The authors acknowledge financial support from the University Grants Commission, New Delhi under a major research project scheme.

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40.

41.

42.

43.

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JR–Sodium Dodecyl Sulfate Complex on the Surface of Alumina, Ibid 13:103 (1985). Schwartz, T., and J. François, Limites de solubilité des polyacrylamides partiellement hydrolyses en présence d’ions divalents, Makromol. Chem. 182:2775 (1981). Methemitis, C., M. Morcellet, J. Sabbadin, and J. François, Interaction Between Partially Hydrolyzed Polyacrylamide and Ionic Surfactants, Eur. Polym. J. 22:619 (1986). Gebhardt, J.E., and D.W. Fuerstenau, The Effect of Preadsorbed Polymers on Adsorption of Sodium Dodecyl Sulfonate on Hematite, Miner. Metall. Process., August:164 (1986). Gebhardt, J.E., and D.W. Fuerstenau, Flotation Behavior of Hematite Fines Flocculated with Polyacrylic Acid, ACS Symp. Ser. 253:291 (1984).

N.V. Sastry is currently a reader in chemistry at the Department of Chemistry, Sardar Patel University. He has been teaching chemistry at the post-graduate level for the last 13 years. He was a DAAD fellow at the Institut für Angewandte Physikalische Chemie, Forschungszentrum Juelich, Germany, from 1991–1993. His main research interest is solution chemistry, especially polymer/surfactant and nonelectrolyte solutions. Sastry has published more than 35 research papers and acted as principal investigator on major research projects sponsored by national funding agencies. P.N. Dave is presently working as lecturer in chemistry at Nirma Institut for Science and Technology, Ahmedabad. India.

[Received January 28, 1999; accepted August 13, 1999]

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