Chem. Res. Chin. Univ., 2016, 32(4), 682―688

doi: 10.1007/s40242-016-5428-8

Differences in Adsorption of Anionic Surfactant AOT by Calcium Oxalate: Effect of Crystal Size and Crystalline Phase SUN Xinyuan, DING Yiming, WEN Xiaoling and OUYANG Jianming* Institute of Biomineralization and Lithiasis Research, Jinan University, Guangzhou 510632, P. R. China Abstract The adsorption of anionic surfactant sodium diisooctyl sulfosuccinate(AOT) onto calcium oxalate monohydrate(COM) and dihydrate(COD) with sizes of 50, 100 nm, 1, 3 and 10 μm was comparatively studied to simulate the interaction between urinary crystallites and urine components. The adsorption quantity of different concentrations of AOT onto COD and COM with different sizes was detected using a UV-Vis spectrophotometer. The crystalline phase transition of COM and COD before and after adsorption was analyzed by X-ray powder diffraction and Fourier transform infrared spectrometry. The zeta potential of the crystal surface after adsorption of different concentrations of AOT was measured using a zeta potential analyzer. The adsorption quantity of AOT on COM and COD with different sizes was ranked in the following order: 50 nm>100 nm>1 μm>3 μm>10 μm. The adsorption quantity of COM was greater than that of COD with the same size because the density of the positive charges on the COM surface was higher than that on COD surface. With the increase of AOT concentration, the adsorption curves of the large-sized COM and COD(3 and 10 μm) were S-type, whereas the adsorption curves of the small-sized COM and COD(50 nm, 100 nm and 1 μm) were linear. The adsorption capacities of small-sized COM and COD were much greater than those of the 3 and 10 μm crystals. On the basis of the above results, we proposed a molecular model to summarize the absorption of AOT onto COM and COD crystals. Small crystals exhibit a large specific surface area and high surface energy. Thus, the adsorption capacities of them are stronger than those of large crystals. Overall, this study implies that small crystals can easily absorb anionic molecules in urine and may easily adhere to a negatively charged cell surface, thereby increasing the severity of cell injury. Keywords Crystal size; Calcium oxalate; Anionic surfactant; Surface adsorption; Adsorption model

1

Introduction

The formation of calcium oxalate stones is closely related to the compositions of both urine and urine microcrystalline[1,2]. The main factors leading to the formation of urinary stones include the supersaturation of calcium oxalate in urine; the nucleation, growth, and aggregation of urine microcrystalline; the decrease of urine inhibitors or the increase of promoters; and the injury of renal tubular epithelial cells. Numerous organic substances exist in human urine; many of them are surface active substances, such as amino acids[3,4], bile salt[5], albumin[1] and phospholipids[6,7]. These surface active substances can significantly affect the nucleation, growth, aggregation, and crystal phase transformation of calcium oxalate in urine. However, the urine system is complex, and more than 700 species exist in urine. Thus, a representative substance is commonly used to study the effects of a certain kind of substance on urinary stone formation. The common anionic surfactants used in the simulation of calcium oxalate biomineralization are sodium diisooctyl sulfosuccinate(AOT, Fig.1) and sodium

dodecyl sulfate(SDS)[8―10]. AOT is a double-chain substance, and its structure is similar to that of phospholipids in humans [such as dipalmitoylphosphatidylcholine(DPPC)]; hence, AOT is an ideal model substance to systematically study the effects of urine surface active molecules on the formation and transformation of mineral crystals, such as calcium oxalate[11]. For example, the effects of AOT on the crystallization of

Fig.1

Molecular structure of AOT

——————————— *Corresponding author. E-mail: [email protected] Received November 6, 2015; accepted April 29, 2016. Supported by the National Natural Science Foundation of China(No.21371077). © Jilin University, The Editorial Department of Chemical Research in Chinese Universities and Springer-Verlag GmbH

No.4

SUN Xinyuan et al.

calcium oxalate and the adsorption of AOT on calcium oxalate monohydrate(COM) and dihydrate(COD) have been reported in many previous studies[8,9]. AOT can promote the formation of COD and increase the negative charges on the crystal surface after AOT adsorption. Morphology and crystalline phases were evidently changed with the increase in the anionic surfactant concentration of SDS[10]; the adsorption of SDS on the ( 1 01) plane of COM can strongly inhibit the growth of COM, thereby promoting the nucleation and growth of COD. However, the size effect of calcium oxalate crystals on the adsorption was not explored in these studies. Surfactants generally exhibit unique properties, such as self-assembly and easy adsorption on a crystal interface. The type and concentration of surfactants can affect the morphology and aggregation(suppression or promotion) of calcium oxalate. These factors can also influence the growth kinetics and phase transformation of crystals. Many researchers have reported the effects of other acidic organic substances and their analogues on the crystal habit of calcium oxalate. Fischer et al.[4] studied the effects of polyglutamic acid with different chain lengths on the formation of calcium oxalate. Polyglutamic acid with a chain length of 10(Glu10) induced the generated COD with tetragonal bipyramid morphology, whereas a chain length of 20(Glu20) induced the formation of elongated COD crystals, showing dominant (100) faces. The elongation of the COD crystals can be explained through the specific adsorption of the acidic peptides to the (100) faces with a high density of positive charge, promoting growth in the [001] direction and resulting in large (100) faces. Thus, the differences of crystal plane properties in COM and COD crystals, particularly the difference in charge density, play an important role in the specific adsorption of the additives on a crystal plane, thereby affecting the growth habit of calcium oxalate. Stone formation is closely related to the deposition of urine microcrystalline in the urinary system. Research has shown that the transient time of urine microcrystalline across the kidney is 5―10 min. The retention time is very short for crystals to grow large enough to block the urinary tract; therefore, crystal aggregation is the most important step to rapidly increase the size of urine microcrystalline[12,13]. When the surface of urine microcrystalline adsorbs anionic surfactant molecules, the negative charge density on the surface of the microcrystalline will increase. As a result, the electrostatic repulsion between the crystals will also increase. The growth and aggregation of crystals will be prevented, which is beneficial to inhibit the formation of urinary stones. Given that various surface active molecules and urinary crystals with different sizes, morphologies, and crystalline phases(such as COM and/or COD) exist in urine[14,15], we explored the adsorption of a representative anionic surfactant, AOT, onto COM and COD crystals of different sizes(50, 100 nm, 1, 3 and 10 μm). We aimed to reveal the interaction between urinary crystals and anionic molecules in urine, as well as the negatively charged cell surfaces, thereby exploring the influence of crystal sizes, crystalline phases, and surface

683

adsorption on stone formation.

2 2.1

Experimental Reagents and Instruments

Methylene blue, sodium dihydrogen phosphate(NaH2PO4), sodium chloride(NaCl), chloroform(CHCl3) were all purchased from Guangzhou Chemical Reagent Factory of China (Guangzhou, China). AOT was purchased from Shanghai Kai Yang Biotechnology(Shanghai, China). All the chemicals used in this study were of analytical grade. The water used in all the experiments was double distilled water. The instruments used include a UV-Vis spectrophotometer (Cary 500, Varian, USA); a Fourier-transform infrared spectrometer(FTIR, Nicolet, American); a Zetasizer Nano-ZS nano particle sizer(Malvem, UK); a D/max2400 X-ray powder diffractometer(XRD, Rigaku, Japan), a KQ3200 DE ultrasonic instrument(Kunshan, China); a centrifugal sedimentation machine(TGL-16C, Shanghai, China).

2.2 Preparation of COM and COD with Different Sizes COM and COD crystals with different sizes(about 50, 100 nm, 1, 3 and 10 μm) were prepared by changing the concentration of reactants, reaction temperature, solvent, mixing manner, and stirring speed. The XRD and FTIR results showed that all the prepared COM and COD crystals were pure target products. The detailed preparation and characterization processes can be referred to our recent paper[15].

2.3

Adsorption Experiments of AOT

The amount of adsorbed AOT was determined via the depletion method[8]; 50 mg of COM or COD crystals were added to 12 mL of AOT solutions at different concentrations (c0). The suspension was dispersed by ultrasonic processing for 10 min and then placed into a thermostat water bath at 37 °C. The suspension was centrifuged after adsorption for 24 h, and the residual concentration of AOT was measured(ceq). The adsorbed amount(Qads) of AOT on COM or COD was calculated using the formula Qads=V(c0–ceq)/m, where V is the volume of the solution, and m is the mass of the crystals. The zeta potential of the suspension was detected. The maximum experimental concentration of AOT was 450 mg/L, and the solution became turbid over this concentration. The equilibrium concentration of AOT in the solution after adsorption was determined using a modified methylene blue method. In the range of working concentrations of AOT, the calibration line followed the equation y=0.1137x−0.0149, R2=0.9985, where x is the concentration of AOT and y is the absorbance at 280 nm wavelength.

2.4 XRD and FTIR Characterization of Crystals After Adsorption The crystal and AOT suspension was centrifuged to remove the supernatant after standing for 24 h, then the crystals

684

  Chem. Res. Chin. Univ.

were collected and dried at 55 °C overnight, and submitted for XRD and FTIR characterization.

3

Results and Discussion

3.1 XRD and FTIR Spectra of COM and COD Crystals After Adsorption Fig.2 shows the XRD patterns of COM and COD with different sizes after adsorption of AOT for 24 h. All the COM crystals were detected with the diffraction peaks appearing at 2θ=15.06°, 24.45°, 30.25° and 38.36°, which can be assigned

Vol.32 –1

and 1319 cm , respectively, while those of COD are at ca. 1648 and 1327 cm–1. In the fingerprint region, the absorption peaks of COD and COM do not change significantly. The absorption peaks of COM appear at about 947, 885 and 669 cm–1, while those of COD appear at about 916 and 609 cm–1. COM and COD shared the absorption peak at about 781 cm–1, which is assigned to the stretching vibration of C―C, but the peak of COM is very sharp and high, while that of COD is relatively broad.

to ( 1 01), (020), ( 2 02) and (130) planes of COM crystals(PDF No. 20-0231), respectively[16]. All the COD crystals were detected with the diffraction peaks appearing at 2θ=14.24°, 20.03°, 32.19° and 40.21°, which can be assigned to (200), (211), (411) and (213) planes(PDF No. 17-541), respectively[16]. Compared with the XRD results before adsorption[15], the crystalline phases of COM and COD with different sizes after adsorption of AOT were unchanged.

Fig.3

FTIR spectra of COM(A) and COD(B) with different sizes after AOT adsorption Crystal size: a. 50 nm; b. 100 nm; c. 1 μm; d. 3 μm; e. 10 μm.

Fig.2 XRD patterns of COM(A) and COD(B) with different sizes after AOT adsorption Crystal size: a. 50 nm; b. 100 nm; c. 1 μm; d. 3 μm; e. 10 μm.

Fig.3 shows the FTIR spectra of COM and COD with different sizes after adsorption of AOT. The COM crystals after adsorption have a broad absorption peak at 3485―3050 cm–1, which is assigned to the absorption peak of O―H bond in crystal water and is split into five small absorption peaks. While COD crystals with different sizes have a single broad absorption peak at about 3450 cm–1[17,18], which is one of the characteristic peaks that distinguish COD from COM. The asymmetric stretching vibration(νas) and symmetric stretching vibration(νs) of carboxyl group(COO–) in COM are at ca. 1621

That is, COM and COD crystals of different sizes after absorption for 24 h in the AOT solution still showed the typical infrared absorption peaks of COM and COD, indicating that the crystalline phases had not changed, which is consistent with the results of XRD analysis. A total of 40% COD was transformed into thermodynamically stable COM after placement in 0.3 mol/L NaCl aqueous solutions for 1 d, then completely transformed into COM after 7 d[11]. COD can stably exist in an AOT solution for at least 24 h.

3.2

Adsorption Curve of Different-sized COM

The adsorption curves of different-sized COM to AOT are shown in Fig.4(A). For the three small-sized COM crystals (COM-50 nm, COM-100 nm, and COM-1 μm), the adsorption quantity of AOT almost increases in a straight line with the increase of c(AOT)(Fig.4 curves a―c), whereas the two largesized COM crystals(COM-3 μm and COM-10 μm) present the common S-type adsorption curve(Fig.4 curves d and e)[8,11]. The adsorption quantities of the large-sized COM crystals are much smaller than those of the small-sized ones.

No.4

SUN Xinyuan et al.

685

repulsion forces between the negatively charged groups of AOT in the bulk solution and crystal surfaces. Therefore, the increase of adsorption quantity decelerated again with the increase in AOT concentration. When c(AOT) reached 220 mg/L, the adsorption gradually saturated[8,19,20]. That is, the difference of the interaction mode between the solution AOT and the COM surface was the essential cause of the S-type adsorption curve.

Fig.4

Adsorption curves of COM(A) and COD(B) crystals with various sizes to AOT Crystal size: a. 50 nm; b. 100 nm; c. 1 μm; d. 3 μm; e. 10 μm. The insets are the enlarged figures of adsorption curves of COM and COD crystals with sizes of 3 and 10 μm.

3.2.1 Adsorption COM-10 μm

Curves

of

COM-3 μm

and

The S-type adsorption curves of COM-3 μm and COM-10 μm(Fig.4 curves d and e) can be divided into three phases. Phase I: when c(AOT)<100 mg/L, the adsorption quantity of COM slowly increased with the increase of c(AOT). AOT were mainly adsorbed on the crystal surface of COM with the negative charged “head”(sulfonate) through Coulomb force, and the two hydrophobic tails pointed “out” to the solution[Fig.5(A) and (B)]. AOT can form monolayer, micelle, or vesicle in aqueous solution. At low concentration, the adsorption of AOT on the solid surface mainly occurred through monolayer adsorption, and no interaction existed among the AOT molecules in this phase[19]. Phase II: when 100 mg/L 200 mg/L], the slope of the isotherm decreased again. With the further increase of c(AOT), a close bilayer or “micelles” formed on the surface[Fig.5(E) and (F)], increasing the negative charges on the crystal surface, and thus increasing the

Fig.5

Structural diagram of AOT molecules adsorbed on COM surface

(A), (B) Phase I: c(AOT)<100 mg/L, early stage (A), adsorbing some molecules, later stage (B), forming monolayer adsorption; (C), (D) phase II: 100 mg/L200 mg/L, forming close bilayer or “micelles”.

The critical micelle concentration(cmc) of AOT in aqueous solution is 6.8×10–4 mol/L[9], that is, 302.3 mg/L. The AOT concentration used in this study was 30―450 mg/L. Accordingly, AOT can form close bilayer or micelles in solid surface at high experimental concentrations[21]. Stocker et al.[19] studied the thickness of the AOT adsorption layer on a calcite surface by using atomic force microscopy and neutron reflection. When the AOT concentration is lower than cmc, AOT forms a monolayer adsorption. When the AOT concentration is near cmc, the AOT hydrophobic portion will attract each other to form micelles. Specifically, AOT can form a bilayer or micelle adsorbed to the hydrophilic crystal surface. The adsorption curve(Fig.4) is consistent with that reported in the literature[19], and the surface properties of calcium oxalate are similar to those of calcium carbonate. Thus, the adsorption of AOT on calcium oxalate crystal is similar to that on calcite. Considering that the crystal surface is not perfectly flat, AOT can easily transform from bilayer into micelles. To further analyze the change in adsorption quantity, the adsorption model of AOT on calcium oxalate crystal is shown in Fig.5.

3.2.2 Adsorption COM-100 nm

Curves

of

COM-50 nm

and

As shown in Fig.4(A) curves a and b, the adsorption curves of the two nano-sized crystals(COM-50 nm and COM-100 nm) were linear. For nano-COM crystals, the

686

  Chem. Res. Chin. Univ.

Vol.32

[19,22,23]

, thus significantly increasing the adsorption quansites specific nature of the surface and interface effects on nanopartity of AOT. In this study, the adsorption of AOT on the two ticles significantly affects adsorption. Nanoparticles exhibit a nano-sized crystals did not reach saturation adsorption under large specific surface area(Table 1) and numerous exposed the maximum concentration[c(AOT)=450 mg/L]. Some atoms on the particle surface. These exposed atoms contain adsorption sites on the crystal surface were not adsorbed by many unsaturated bonds leading to several defects on the AOT. Hence, the adsorption amount increased linearly with the crystal surface. These regions with defects demonstrate a strong increase of c(AOT). adsorption capacity, reaction activity, and many adsorption Table 1 Crystal size, specific surface area, pore volume, pore diameter and maximum adsorption quantity of COM and COD crystals Crystal

Crystal size using SEM

Hydrodynamic size/nm

Specific surface area, SBET/(m2·g–1)

Pore volume/ (mm3·g–1)

Pore diameter/nm

Maximum adsorption quantity, Qmax/(mg·g–1)

COM-50 nm COM-100 nm

(47.7± 6.2) nm (92.1±10.4) nm

1342 1400

26.3 14.7

49.2 37.3

7.49 10.10

93.5 89.6

COM-1 μm

(0.91±0.22) μm

1205

13.6

37.7

11.10

83.6

COM-3 μm

(2.65±0.43) μm

2792

1.51

3.3

6.71

24.9

COM-10 μm

(9.67±1.76) μm

—

0.83

1.3

6.37

19.6

COD-50 nm

(44.1±8.7) nm

953

40.8

95.7

9.37

88.7

COD-100 nm

(98.3±8.1) nm

1353

21.4

40.9

7.63

83.1

COD-1 μm

(0.92±0.31) μm

1053

9.12

31.5

13.83

71.4

COD-3 μm

(3.41±0.57) μm

2494

1.36

1.2

4.61

19.1

COD-10 μm

(9.58±0.97) μm

—

0.80

1.1

5.71

16.1

3.2.3

Adsorption Curve of COM-1 μm

The adsorption curve of COM-1 μm was linear, similar to that of COM-50 nm and COM-100 nm, and was mainly associated with its specific surface area. As shown in Table 1, the specific surface area of COM-1 μm(13.6 m2/g) was significantly greater than those of the two other micron-sized COM crystals and even close to that of COM-100 nm(14.7 m2/g). COM-1 μm exhibited a larger pore volume(37.7 mm3/g), which was more than 10 times larger than those of the two other micron-sized COM crystals(1.3―3.3 mm3/g), resulting in the large specific surface area of COM-1 μm. SEM images [Fig.6(A)] show that the surface of COM-1 μm is rough with many micropores, which resulted in a large specific surface area. This finding may be ascribed to the preparation process of the rapid direct mixing of CaCl2 and K2Ox solution. In the preparation of the 10 crystals, four nanocrystals and COM-1 μm were prepared through the rapid direct mixing of two reactants. The adsorption quantity was positively correlated to the specific surface area of crystals, but the values were usually indirectly proportional. For instance, the specific surface area of nano TiO2 is about 44 times larger than that of bulk TiO2, but the adsorption quantity of nano TiO2 is only about 20 times of its bulky counterpart[24]. The specific surface areas of the crystals were detected under nitrogen atmosphere, and the adsorption experiments were performed in water environment. COM-50 nm and COM-100 nm crystals easily aggregated in aqueous solution because of their low absolute zeta potential; their hydrodynamic sizes were similar to those of COM-1 μm, which ranged between 1.2 μm and 1.4 μm(Table 1). Such serious aggregation of COM-50 nm and COM-100 nm crystals decreased the number of active sites exposed on the crystal surface, thereby reducing their adsorption ability. Therefore, the adsorption quantity ranked in the following order: COM-50 nm>COM-100 nm>COM-1 μm. However, the difference of adsorption quantity was much less than the dif-

ference of the initial sizes of crystals(SEM size).

3.3 Adsorption Curves of COD with Different Sizes As shown in Fig.4(B), the adsorption curves of COD with different sizes are similar to those of COM. With the increase of c(AOT) for the three small-sized COD crystals(50 nm, 100 nm and 1 μm), the adsorption quantity of AOT increased almost linearly[c(AOT)<450 mg/L]. However, none of the three groups reached saturated adsorption. COD-50 nm and COD-100 nm crystals also easily aggregated in aqueous solution; therefore, their hydrodynamic sizes were similar to those of COD-1 μm(Table 1). The adsorption quantity of the three small-sized COD crystals was ranked as follows: COD-50 nm> COD-100 nm>COD-1 μm. However, the difference of adsorption quantity in the three small-sized crystals was less than the difference of their specific surface areas. For the two large-sized COD crystals(3 and 10 μm), their adsorption quantities were much less than those of the three small-sized COD crystals, and their adsorption curves were all S-type. At low AOT concentrations[c(AOT)=30―80 mg/L], the adsorption quantity increased slowly(phase 1); when the concentrations increased from 80 mg/L to 180 mg/L, the adsorption quantity increased rapidly(phase 2); and when the concentration exceeded 180 mg/L, the adsorption quantity reached a stable saturation stage(phase 3). The adsorption curve of COD-1 μm was also linear. Although the specific surface area of COD-1 μm was not obviously higher than those of COD-3 μm and COD-10 μm (Table 1), the morphology of COD-1 μm differed from those of COD-3 μm and COD-10 μm(Fig.6). COD-1 μm presented a thicker (100) plane, and the proportion of the (100) plane in all the planes was significantly higher than those for COD-3 μm and COD-10 μm[15]. The density of Ca2+ ion on the (100) plane was significantly higher than that on the (101) plane[25]. The specific surface area of COD-1 μm was higher than those of

No.4

SUN Xinyuan et al.

COD-3 μm and COD-10 μm. Therefore, more adsorption sites existed in COD-1 μm than in COD-3 μm and COD-10 μm, and the adsorption quantity of COD-1 μm was significantly greater than those of COD-3 μm and COD-10 μm.

Fig.6

SEM images of COM-1 μm(A) and COD-1 μm (B) and structural diagrams of COM-1 μm(C) and COD-1 μm(D)showing their crystal planes The arrows show the microporous surfaces of COM-1 μm and COD-1 μm crystals.

3.4 Zeta Potential Changes of COM and COD After AOT Adsorption Zeta potential reflects the changes in crystal surface charge[26]. To detect the changes in the surface charge of the COM and COD crystals after adsorbing AOT, the zeta potential was determined. On the basis of the adsorption curve type of the different-sized crystals(Fig.4), we selected the 50 nm and 3 μm COM and COD crystals to study the change mechanisms of the zeta potential after adsorbing AOT(Fig.7). The changing trend of the zeta potentials of COM-50 nm and COD-50 nm(Fig.7 curves a and b) was almost consistent with the changing trend of the adsorption quantity(Fig.4), that is, the absolute value of zeta potential increased linearly with the increase of c(AOT). The adsorbed AOT on the crystal surface increased linearly with the increase of AOT concentration; thus, the negative charge on the crystal surface also increased linearly. The changing trend of the zeta potentials of COM-3 μm and COD-3 μm was incompletely consistent with its S-type adsorption curve. Although the adsorption quantity of large-sized crystals slowly increased in the beginning with the increase of c(AOT)(Fig.4), the zeta potential rapidly became negative. When c(AOT) reached 200 mg/L, the zeta potential slowly became negative then reached a steady state. This result may be associated with the adsorption mode of AOT on the

687 [11]

crystal surface, as reported in the current study . Given a low concentration of AOT, the “head” groups of the AOT monolayer were adsorbed on the crystal surface[8,11]. With each additional AOT molecule adsorbed on the crystal surface, a more negative charge will be generated. These adsorbed sulfonic acid ions can directly contact the high-energy sites of the COM and COD crystal surfaces, particularly the crystal surfaces with a positive charge, thus offsetting the mass of positive charges on the crystal surface and rapidly increasing the zeta potential despite a low AOT concentration[11]. When c(AOT) further increased, AOT formed a bilayer or half micelles on the crystal surface. A large proportion of the negative charges of AOT molecules was shielded by outer AOT molecules. The apparent charge number of crystal surface was less than the number of absorbed AOT molecules. Therefore, the increase of zeta potential slowed down.

Fig.7

Zeta potential of COM and COD with different sizes changed with AOT concentration a. COM-50 nm; b. COD-50 nm; c. COM-3 μm; d. COD-3 μm.

3.5 Comparison of Maximum Adsorption Quantities of COM and COD The maximum absorption quantities(Qmax) of COM and COD in the experimental range of AOT concentration(30―450 mg/L) are shown in Fig.8. With the increase in COM or COD size, the Qmax to AOT decreased consequently. The Qmax of COM and COD with different sizes was ranked as follows: 50 nm>100 nm>1 μm>3 μm>10 μm. The main factors affecting Qmax are the specific surface area and surface energy of the crystals. Small-sized crystals do not only exhibit large specific surface area and more adsorption sites but also yield high surface energy, thus increasing their adsorption capacities and quantities[22]. In addition to the specific surface area of crystals, the crystal phase also affects the Qmax[27]. The Qmax of COM was consistently slightly higher than that of the same-sized COD. Although the specific surface area of COD was consistently greater than that of COM with the same size, the Qmax of COD was slightly less than that of COM. Aside from the influences of the specific surface area and surface energy of crystals on the adsorption quantity, the structural differences between COM and COD are also important. The main crystal plane of COM is a positively charged ( 1 01) plane, whereas COD is mainly a tetragonal bipyramid; the charge density of COD is low because its main crystal plane arranges a large number of water molecules. The only region with high charge density of

688

  Chem. Res. Chin. Univ.

COD is that of the two vertices of a double cone, but this region accounts for a very small portion of the crystal surface. Thus, the quantity of positively charged COM was obviously higher than that of COD with the same size, resulting in significantly greater capability of COM than COD to absorb anionic AOT.

Vol.32

Gohar M. N., Saudi J. Kidney Dis. Transpl., 2013, 24, 630 [3] Illsinger S., Schmidt K. H., Lu T., Vaske B., Bohnhorst B., Das A. M., Amino Acids, 2010, 38, 959 [4] Fischer V., Landfester K., Munoz-Espi R., Cryst. Growth Des., 2011, 11, 1880 [5] Saso L., Grippa E., Gatto M. T., Silvestrini B., Int. J. Urol., 2001, 8, 124 [6] Wiessner J. H., Hasegawa A. T., Hung L. Y., Mandel G. S., Mandel N. S., Kidney Int., 2001, 59, 637 [7] van Meer G., Voelker D. R., Feigenson G. W., Nat. Rev. Mol. Cell. Bio., 2008, 9, 112 [8] Tunik L., Furedi-Milhofer H., Garti N., Langmuir, 1998, 14, 3351 [9] Tunik L., Addadi L., Garti N., Füredi-Milhofer H., J. Cryst. Growth, 1996, 167, 748 [10] Wei X. X., Yang J., Li Z. Y., Su Y. L., Wang D. J., Coll. Surf. A, 2012, 401, 107 [11] Sikiric M., Filipovic-Vincekovic N., Babic-Ivancic V., Vdovic N.,

Fig.8

4

Maximum adsorption quantity of AOT onto COM and COD with different sizes

Conclusions

The adsorption behavior of AOT onto small-sized COM and COD(50 nm, 100 nm and 1 μm) was significantly different from that onto large-sized crystals(3 and 10 μm). The former showed linear adsorption, whereas the latter displayed S-type adsorption. Small-sized crystals exhibit larger specific surface area, more adsorption sites, and higher surface energy; thus, they demonstrate stronger adsorption capacity than large-sized crystals, and their adsorption quantities were much higher than those of large-sized crystals. The adsorption quantities of AOT onto COM and COD with different sizes were ranked as follows: 50 nm>100 nm>1 μm>3 μm>10 μm. The crystal size, specific surface area, aggregation, and even crystal structural differences between COM and COD all affect the adsorption quantity. The adsorption quantity of COM was greater than that of the same-sized COD, probably because the charge density of COM was higher than that of the same-sized COD. Our study simulated the adsorption behavior between calcium oxalate and anionic molecules in urine by using a chemical approach, which will provide some theoretical bases to reveal the interaction between urinary crystalline and urinary components.

Furedi-Milhofer H., J. Coll. Interf. Sci., 1999, 212, 384 [12] Kok D. J., Khan S. R., Kidney Int., 1994, 46, 847 [13] Finlayson B., Reid S., Invest. Urol., 1978, 15, 442 [14] Verdesca S., Fogazzi G. B., Garigali G., Messa P., Daudon M., Clin. Chem. Lab. Med., 2011, 49, 515 [15] Sun X. Y., Ouyang J. M., Liu A. J., Ding Y. M., Gan Q. Z., Mater. Sci. Eng. C, 2015, 57, 147 [16] King M., Mcclure W. F., Andrews L. C., Powder Diffraction File Alphabetic Index, Inorganic Phases-organic Phases, International Center for Diffraction Data, Pennsylvania, 1992 [17] Ouyang J. M., Duan L., Tieke B., Langmuir, 2003, 19, 8980 [18] Selvaraju R., Thiruppathi G., Raja A., Spectrochim. Acta A, 2012, 93, 260 [19] Stocker I. N., Miller K. L., Welbourn R. J., Clarke S. M., Collins I. R., Kinane C., Gutfreund P., J. Colloid Interface Sci., 2014, 418, 140 [20] Walsh R. B., Wu B., Howard S. C., Craig V. S., Langmuir, 2014, 30, 2789 [21] Paria S., Khilar K. C., Adv. Colloid Interf., 2004, 110, 75 [22] Wang X., Jiang Z. Y., Xie Z. X., Zheng L. S., Acounts Chem. Res., 2014, 42, 308 [23] Rnsari R., Shahabodini A., Faghih Shojaei M., Mohammadi V., Gholami R., Physica E, 2014, 57, 126 [24] Song L., Yang K., Jiang W., Dua P., Xing B., Coll. Surf. B, 2012, 94, 341 [25] Jung T., Kim W. S., Choi C. K., J. Cryst. Growth, 2005, 279, 154 [26] Chung T. H., Wu S. H., Yao M., Lu C. W., Lin Y. S., Hung Y., Mou C.

References [1] Farmanesh S., Ramamoorthy S., Chung J., Asplin J. R., Karande P., Rimer J. D., J. Am. Chem. Soc., 2014, 136, 367 [2] Hussein N. S., Sadiq S. M., Kamaliah M. D., NorAkmal A. W.,

Y., Chen Y. C., Huang D. M., Biomaterials, 2007, 28, 2959 [27] Du P., Zhao J., Mashayekhi H., Xing B., J. Phys. Chem. C, 2014, 118, 22249