Colloid & Polymer Sci. 253, 754-759 (1975)

j. t-Ieyrovs@ fnstitute of Phydcal Chemislry and Electrochemistry, Czechoslovak Academy of Sciences, Prague ( ~SSR)

Dynamic surface tension of foaming solutions and dispersions J. K]oubek With 8 figures and 1 table (Received May 16, 1974)

Introduction In the preceding papers the maximum bubble pressure method for the measurement of the dynamic surface tension (yt) of surfactaut solutions was studied (1-3). The principal advantage of this method resides in the fact that it makes it possible to measure 7t practically from the zero age of the surface to the equilibrium surface tension, y~, also in solutions equilibrating very slowly, viz. by extrapolation. The disadvantage consists in a considerable scattering of the results. During measurements in some solutions serious difficulties may arise from the creeping of a liquid into the capillary at lower rates of bubbling which ultimately may cause a drop formation inside of the capillary and its clogging. By the creeping of the liquid the effective radius of the capillary decreases and, consequently, higher values of yt are erroneously determined. The narrowing of the capillary may bring it about that after a certain prolongation of the interval between two consecutive bubbles the measured pressure does not drop respectively any more and, consequently, unreal values of 7e are obtained. Garner and Garfias (4) also mentioned the difficulties with the creeping of liquid into the capillary in the determination of surface tension from the dimensions of the bubble profile. Furthermore, the measuremegt may be affected or even made impossible by the hydrophobization of the capillary surface which may prevent an immediate restoring of the meniscus on the inside capillary edge after the bubble detachment, and thereby a continuous stream of bubbles arises (1). Last but not least, there are difficulties W 976

with measuring of foaming solutions when the foam fills the measuring cell. For surface ages exceeding a few seconds the drop weighing method may be advantageously used. It is a lengthy method but yields very accurate results (5) even for solutions creating difficulties for the application of the maximum bubble pressure method. In the two above methods the relationship between the measured interval of the bubble or drop formation and t h e effective surface age (i.e. period in which the static surface would reach ya) was studied (2, 5). The respective corrections enabling to measure 7~ in dependency on the age of the static surface of the solution were suggested. For the thus evaluated results a good agreement of yt of dodecyl sulfate solutions with the two above methods and with the static Wilhdmy plate method was found (5). This paper deals with the results obtained in technical surfactant solutions and dispersions by the maximum bubble pressure and the drop weighing methods.

Experimental Surface tension Measurements by the maximum bubble pressure method were carried out on the apparatus described in a previous paper (1) and evaluated according to the empirical relationship, y = 3 . 7 5 8 Ap ° (dyn/em) (cf. ref. (5)). Measurements by means of the stalagmometer and the way of their evaluations were described in the preceding paper (5). The results represent values of the dynamic surface tension for a static surface of the age t (sec) referred to.

K/oubek, Dynamic surface lendon of foaming solutions and dispersions

755

Table 1. Characteristics of measured solutions and dispersions Text tag

Features*)

A 27 C D E

s s d d d

f n n f f

Viscosity

(cp)

Extrapolated Ye (dyn/cm) maximum pressure stalagmometer

1.216 2.114 2.234 1.009 1.015

60.4-63.5 45.0 42.5-56.7 53.6--60.9 51.0--55.6

52.0-52.8 52.8-55.6 42.5 43.7 42.5

*) * = solution d = dispersion, f = foaming, n = non-foaming

Solutions and dispersions

the results obtained by the two methods, one Characteristics of solutions and dispersions are should realize that in the maximum bubble given in table 1. Distilled water and the following pressure m e t h o d all points lying on one curve technical agents were used to their preparation: resulted from one experiment during which the A, dark red brown foaming solution of 0.2% interval between two consecutive bubbles was sodium dinaphthylmethane disul{onate, 0.1% congradually prolonged. E v e n if the points condensed phenol sulfonate, 0.2°{) polyvinylalcohol; B, non-foaming solution after breaking of the dis- stitute a continuous curve, this does not attest persion C; to the reliability of results, T h e actual scatterC, non-foaming unstable dispersion of 0.5% con- ing of results is shown in fig. 1 by the curves of centrated paraffin dispersion (paste), 0.2o/0 carboxy- three experiments demonstrating differences methyl cellulose ; D, red brown foaming stable dispersion of 0.5O/o up to 5 dyn/cm. O n the contrary, each point acrylate vinylacetate copoIymer, 0.1% condensed on the curve determined by drop weighing on product of formaldehyde, cresol and naphtholsulfonic the stalagmometer represents an independent acid; measurement and the continuity of the curve E, light brown foaming stable dispersion of 0.5% acrylate vinylacetate copolymer, 0.25% dried sulfite plotted according to the points clearly attests waste liquor, 0.05% sodium dinaphthylmethane di- to an excellent reproducibility of the results. sulfonate. T h e curves obtained by the maximum bubble pressure method show a relatively quick reaching of ye. If we consider the inferior Results and discussion reproducibility of experiments and relatively T h e determined yt values in dependency high ye values, the presented results have small on the surface age in a strongly foaming solu- credibility, particularly because the solution tion A are presented in fig. ~. When comparing contains a mixture of macromolecular compounds having diverse molecular weights. In such case a slow equilibration may be expected. Hence, it is obvious that the stalagmometer yields accurate results whereas in the maximum bubble pressure m e t h o d the sources of errors mentioned in the introduction of this paper a', come to the fore. In fig. 2 there are the results obtained with solution 2? prepared by the separation of the 60 - ~ . - 4- -solid part after the b r e a k d o w n of the little stable paraffin dispersion. T h e dispersion medium B was slightly turbid and nonfoaming. T h e dependency of y~ u p o n the age 55 of the surface gives similar results by the both so oo 9o 15o methods; the maximum difference in the range t measured amounts only to about 1 dyn/cm, Fig. 1. Dynamic surface tension, yt (dyn/cm) of solu- the dependency course, however, is steeper tion A at 20 °C in dependency on the surface age, t (sec). Empty circles, drop weighing method. Full in the maximum bubble pressure method, Fig. and half-full circles, maximum bubble pressure method 3 displays the dependency 1/@o--yt) on 1it i

i

756

Colloid and Polymer Science, Vol. 253 • No. 9

7O \•

60 t

~"-. r

n

30

I

60

n

90

120

t

Fig. 2. Dynamic surface tension, ?t (dyn/cm), of solution B at 20 °C in dependency on the surface age, t (sec). Empty circles, drop weighing method. Full circles, maximumbubble pressure method /. / /

~,//

O.?5

iiiiiI~ o

1

0.10

._._..o,_.,a~--4>'--

0.05 i

0,05

i

1/t

0,10

Fig. 3. Dependencyof reciprocal values of the spreading pressure (y0 --y~) on the surface age for solutions A and B by the drop weighing method. Full circles and dashed line, solution B by the maximum bubble pressure method for both solutions A and B. The extrapolation of curves to 1 / t = 0 results in the equilibrium surface tension ge included in table 1. According to the maximum bubble pressure method ye values of the two solutions differ. However, we consider both of them inaccurate in view of the above mentioned improbable course of yt and the scattering of the values measured (too high y~ and ye resulting from the narrowing of clogging of the capillary) in solution A and in view of the comparison with below mentioned suspension C (too low ye value of B owing to the hydrophobization of the capillary) in solution B. The orienting measure-

ment of the contact angle of water on glass washed in advance by the above solutions showed that in the latter case (B) a partial hydrophobization of glass took place. According to the stalagmometric method the extrapolated ye of solutions A and B are practically identical. The viscosity of solution B is considerably higher and, therefore, a higher stability of its foam might have been expected. However, since B, contrary to A, is non foaming, neither ye nor viscosity can have major influence upon the foaming power of the solution. According to B u r c i k (5) the foaming power is determined by the rate of the y t decrease of the solution in a range of age measured by the jet method, i.e. approximately up to 0.03 sec. Fig. 3 displays considerable differences between curves for foaming solution A and non-foaming solution B signifying a steeper decrease of ya at a very low age and showing a slower decrease at a higher surface age of A compared with B. The foaming power seems to be related to the y~ course not only at an age in the order of hundredths of seconds but also at an age in the order of seconds. Fig. 4 shows the y~ course for the little stable paraffin dispersion C which while standing separated itself spontaneously and within one day of quiet standing created clearly delimited layers, the upper one being a concentrated paraffin dispersion, the lower one

70 \.

6O

50

30

60

90

120

t

Fig..4. Dynamic surface tension, y~ (dyn/cm), of dispersion C in dependency on the surface age, * (sec) at 20 °C. Empty circles, drop weighing method. Full circles, maximum bubble pressure method. Crosses indicate the dependencyfor solution B according to the maximumbubble pressure

757

Kloubek, Dynamic surface tension of foaming solution, and disdoerdons

containing turbid dispersion medium B. The scattering of y~ values in the stalagmometric measurement of C should be attributed to the changeability of the contents of the dispersed component caused by the instability of the dispersion. Consequently, the dispersed component strongly influences the values measured. On the contrary, there is a striking identity of values measured by the maximum bubble pressure method for dispersion C and the dispersion medium B itself whose values are plotted once again for comparison in fig. 4. As evident from fig. 5 and table

areas o f the heterogeneous surface. As a statistical mean was y already calculated in molecular dispersions (2). Figs. 6 and 7 display examples o f / t in stable dispersions D and 27. In both instances the

65

'// Z/ 0.15

I ¢7

45

// 1

/i

~.-~ 0.10

Ii

//

//'//II

30

60

90

t

/ / •

120

Fig. 6. Dynamic surface tension, y~ (dyn/cm), of dispersion D in dependency on the surface age, t (sec), at 20 °Co Empty circles, drop weighing method. Full and half-full circles, maximum bubble pressure method

_

o . o 5

i I

O.'05

1/t

0110

55 D~'O--

Fig. 5. Dependency of reciprocal values of the spreading pressure (y0 -- 7~) on the surface age for dispersion C. Empty circles drop weighing method. Full circles, maximum bubble pressure method

ye determined from the course on the stalagmometer is in dispersion C approximately by 10 dyn/cm lower than that in dispersion medium B. On the contrary, the ye value determined by the maximum bubble pressure method is very inaccurate and may take a very wide range whose upper value is close to that determined stalagmometrically for dispersion medium B and whose lower value is identical with that determined f o r B according to the maximum bubble pressure. Hence it seems that on the stalagmometer y~ values corresponding to the dispersion as a whole are determined, while the maximum bubble pressure is more influenced by the dispersion medium. Under the condition that y of the suspended particles amounts to approximately 30 dyn/cm and that the particles take 50 per cent of the surface, then y~ determined on the stalagmometer presents a statistical mean y of the individual

~--

--

-

-

5O

g, 45

I

ao

do

10

9

I

120

Fig. 7. Dynamic surface tension, ~, (dyn/cm), of dispersion /~ in dependency on the surface age, t (sec), at 20 °C. Empty circles, drop weighing method. Full and half-full circles, maximum bubble pressure method

reproducibility of stalagmometrical measurements, contrary to the instable dispersion C, was excellent. Contrariwise, the reproducibility of the results obtained by the maximum bubble pressure method was very bad. In dispersion D differences exceed 7 dyn/cm, in dispersion E 4 dyn/cm. As evident from extrapolated

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Colloid and Polymer So#nee, Vo/. 253 • No. 9

'

r)

'

o

o

E

o

0040 1

~o-~ 0035

i

0.05

i

1/t

0.10

Fig. 8. Dependency of reciprocal vaiues o~ the spreading pressure ( y 0 - Yt) on the surface age for dispersions D and E by the drop weighing method

values in fig. 8 and table 1, the 7, values are lower according to the stalagmometer than according to the maximum bubble pressure. The last two specimens had the same content of the dispersed component. In accordance with the considerably higher concentration of the components dissolved in the dispersion medium, one might expect, on one hand, a steeper initial decrease of 7~ and, on the other hand, a lower 7e value in 17 which was really displayed by the both methods (cf. figs. 6, 7 and table 1). According to the stalagmometer ye is lower for 17 only by 1.2 dyn/cm than for D while according to the maximum bubble pressure 7e is lower for 17 at least by 2.6 dyn/cm (as shown by the measured minimum values). However, the actual difference for the maximum bubble pressure may be much higher since according to the course of curves (fig. 7) in suspension 17 a clogging of the capillary occurred and thereby an unreal higher value of ye is considered. The difference in 7t is higher, e.g. for the surface age of 10 sec. a value for 17 was determined by at least 10 dyn/sec lower than for D. This shows that the maximum bubble pressure determines surface tension (if correct values are obtained) corresponding predominantly to a dispersion medium. Although dispersion C has the same 7e as dispersion E, and approximately even as D, the comparison of figs. 4, 6 and 7 reflects the fact that in C there is a substantially slower initial decrease but a steeper decrease of y, for a higher surface age. T h a t is also evident

from the comparison of curves in figs. 5 and 8. As already noted before in connection with solutions referred to in fig. 3, this course of dependency of 7e upon the surface age signifying slower adsorption, or slower equilibrating, is connected with a lower foaming power of the solution. Concurrently, also dispersions D and t7 foamed considerably while dispersion C was non-foaming, even though the viscosity of C is higher than that of D and 17 (see table I). Ultimately, when comparing foaming solution A and non-foaming dispersion C having the initial speed of the decrease of ?,, approximately the same, we see a quicker equilibrating (higher ye) in A. Similarly as in solutions also in dispersions it is evident that neither 7e nor viscosity play the main role in the foaming power but that the course of the dynamic surface tension is a factor of importance. The comparison of values arrived at by the maximum bubble pressure method and by the stalagmometer yields interesting results, even though in the former case the scattering of measurements is considerable. In spite of the fact that the work with the stalagmometer is a laborious and tedious one, it makes it possible to determine very well reproducible values and, consequently, may present important contribution both for applied and theoretical research of various solutions, emulsions and suspensions. Summary The way of evaluating the dynamic surface tension, yt, by the method of drop weighing described in one of the preceding papers (5) was used for technical surfactant solutions and for water dispersions. The above procedure was compared with the maximum bubble pressure method. The results of drop weighing are much better reproducible and, moreover, the maximum bubble pressure yietds incorrect values in many instances. It is displayed that neither the equilibrium surface tension nor the viscosity of studied solutions and dispersions have major influence upon their foaming power which, however, is related to the course of dependency of yt on the surface age. Furthermore, the results indicate that in dispersions the resulting yt determined on the stalagmometer represent a statistical mean of the surface tension of dispersed particles and the dispersion medium while in the maximum bubble pressure it is rather closer to the value of the dispersion medium. Zusammenfassung Der in einer vorangehenden Arbeit (5) beschriebene Vorgang zur Bewertung der dynamischen Ober-

Kloubek, Dynamic surface tension of foaming solutions and dis~Oersions fl~chenspannung 7~ nach der Methode des Tropfenwiegens wurde f/it L6sungen technischer Dispersionen bentitzt. Die Resultate wurden mit Werten nach der Methode des maximalen Blasendrucks verglichen. Die Methode des Tropfenwiegens kennzeichnet sich dutch eine bessere Reproduzierbarkeit, und aul3erdem hietet der maximale Blasendruck in manchen F~illen unrichtige Werte. Es wurde gezeigt, dab weder Gleichgewichtsoberflfichenspannmag noch ¥iskosit~t einen bestimmenden EinfluB auf die Schaumkraft bewirken, die jedo'ch mit der Zeitabh~ngigkeit yon ye zusammenhfingt. Die Ergebnisse deuten welter an, dab yt der Dispersionen nach den Stalagmometermessungen als ein statistisches Mittel der Oberfl~ichenspannung der dispergierten Teilchen und des Dispersionsmediums erscheint, wfihrend ye nach der Methode des maximaten Blasendrucks mehr zum Wert des Dispersionsmediums angen~hert ist.

759

~erences 1) Kloubek, J., Tenside 5,. 317 (1968). 2) Kloubek, J., J. Colloid Interface Sci. 41, 1, 7, 17 (1972). 3) Kloubek, J., Tenside Detergents (in print). 4) Garner, F. H. and F..1. Garfias, J. Colloid Inter~ace Sci. 26, 253 (1968). 5) Kloubek, J., Colloid & Polymere Sci. (in press). 6) Burcik, E. J., J. Colloid Sci. 5, 421 (1950).

Author's address : Dr. J-. Kloubek J. Heyrovsk3~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague (CSSR)