Inhibition of Foaming. VIII. Changes in Electrical Conductivity of

THE JOURNAL OF

PHYSICAL CHEMISTRY (Regiitered in U. 5. Patent Office)

VOLUME 61

(0Copyright, 1957, by the American Chemioal Society)

OCTOBER 18, 1957

NUMBER 10

t

INHIBITION OF FOAMING. VIII. CHANGES IN ELECTRICAL CONDUCTIVITY OF COLLOIDAL ELECTROLYTE SOLUTIONS ON ADDITION OF NON-IONIC FOAM STABILIZERS AND FOAM INHIBITORS BYSYDNEYRoss AND T. H. BRAMFITT Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N , Y . Received February $6, 1067

The effects of non-ionic foam stabilizers and foam inhibitors on the structure of the micelles produced spontaneously in solutions of colloidal electrolytes has been investigated by means of electrical conductivity measurements. At concentrations above the c.m.c. foam stabilizers reduce specific conductance and foam inhibitors increase specific conductance; which facts are interpreted as resulting from increased micellization caused by foam stabilizers, and reduced micellization caused by foam inhibitors. The solubilization of foam inhibitors and the antifoaming action occur simultaneously, and both phenomena reach their optimum a t the same concentration of additive. An extended “surface semi-micelle” is postulated as co-existing with the micelles in bulk; like them, capable of solubilizing additives; unlike them, surface-active; and responsible by the cohesion of its structure for the relative stabilities of foams.

.I

The formation of micelles within, and the surface activity of, a solution of a colloidal electrolyte both arise from the same cause: the spontaneous reduction of the free energy of solute molecules; which can be effected either by association within the bulk solvent (micelle formation), or by an increase of solute concentration a t surfaces and interfaces. As both effects are thus related, any additives, such as foam stabilizers or foam inhibitors, known to affect the structure of the surface layer, could well be expected to show parallel behavior in affecting the structure of micelles. The measurement of electrical conductivity is a readily available method to elicit information about micelles in solutions of colloidal electrolytes, and t o reveal changes in micelles brought about by the addition of specific non-ionic agents. Cert,ain well-known foam inhibitors and foam stabilizing agents were selected as additives to solutions of colloidal electrolytes; the colloidal electrolytes themselves were selected t o cover the widest possible range of types and behavior: Hyamine 1622, a cationic quaternary ammonium salt that has a non-plastic surface’ in aqueous solution; sodium oleate, an anionic soap that has a non-plastic surface in aqueous solution’; and sodium lauryl sulfate, an anionic detergent that has a plastic surface in aqueous solution.‘ (1) 8. Rosa and J. N. Butler, THIS JOURNAL,80, 1266 (1956).

The interaction of foam-stabilizing additives and detergents in mixed micelles has been reported previously by Schick and Fowkes,2 who determined changes in the critical micelle concentration at 55” by the dye titration method. Schick and Fowkes discovered that “additives that lower critical micelle concentration are those capable of enhancing foam stability.” Their finding is confirmed by those results of the present report that bear on foam-stabilizing agents; we would incorporate this conclusion in the more general statement that any additives tending to promote the spontaneous formation of micelles act as foam-stabilizing agents, and those that hinder formation of ,micelles act as foam-inhibiting agents. Methods and Materials Resistance measurements were made with an A. C. Impedance Bridge (General Radio Co., Type 650-A),and a dip-type conductivity cell with bright platinum electrodes (Fisher Catalog 111, No. 9-382). The null point of the Bridge was detected by a Heathkit Oscilloscope (Model OM-1). To increase the sensitivity of the detection of null point, a variable capacitance was connected in parallel with the Bridge. The solutions were contained in a 1-liter beaker, .which was suspended in a constant temperature bath maintained at 24.7 =!c 0.02’. Two types of experiment (2) M. J. Schick and F. M . Fowkes, Paper presented to the Division of Colloid Chemistry, at the 130th National Meeting of the Arnerioan Chemioal Sooiety, Atlantic City, N. J., September 19, 1956: Abstraots I, No. 24.

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SYDNEY Ross AND T.H. BRAMFITT

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Vol. 61

TABLEI IDENTIFICATION OF MATERIALS USED Trade name

Hyamine 1622

D-C Antifoam A

Agent

Chemioal name, or type

p-Diisobutylphenoxyethoxyethyldimethylbenzylammonium chloride Sodium oleate Sodium lauryl sulfate Tributyl phosphate Methylisobutylcarbinol Polymethylsiloxane n-Decyl alcohol Lauryl alcohol

were conducted: the original solution of the colloidal electrolyte plus additive was diluted volumetrically with water plus additive, from a mixture of the two containing the additive in the same concentration as in the original solution; hence the concentration of additive was kept constant and the concentration of the colloidal electrolyte was the only variable (Fig. 1). I n the second series of experiments the concentration of colloidal electrolyte was constant, and the additive concentration varied (Figs. 2, 3, 4). All the readings were corrected for the conductivity of the diluting mixture. The solutions were agitated continuously with an electric stirrer throughout the course of each experiment. Observations of solubilization, by turbidity, and foam stability, by the method of Ross and Miles,* were made on some of the solutions here reported. The materials used, their purity and source, are reported in Table I.

W 0

Purity

Source

98.8%

Rohm and Haas Co.

Technical

Fisher Scientific Go. Fisher Scientific Co. Commercial Solvents Corp. Carbide and Carbon Chemicals Corp. Dow-Corning Corp. Eaatman Kodak Co. Eastman Kodak Co.

u. s. P.

Technical Technical Technical C.P. Technical

angle. The experimental data in the transition range of concentrations do not conform to an abrupt discontinuity, but give continuous curves linking together the two linear portions; the range of transition concentrations is occasionally wide, though not sufEciently so to render uncertain the ascription of the straight lines. In order to report a quantity of data most succinctly, each experimental run is characterized by the value of the concentration at the vertex of the angle (the critical micelle concentration, c.m.c.), and by a function M that denotes the relative change of slope of the second (higher concentration) line with respect to the first. If no colloidal micelles were formed in these solutions, a single straight line would suffice t o describe the variation of L, with concentration, at least in the range of dilute solutions; the function M is chosen to equal zero for this situation; it is defined as M = tan 81

z

a 0

3 0

z

u 0 0

Y L)

L u?

0 CONCENTRATION OF SURFACTANT.

Fig. 1.-Generalized description of the effects of foam stabilizers and foam-inhibitors on the relation between specific conductance and concentration of a colloidal electrolyte (“surfactant”): OCD represents conductivities of the surfactant alone; OAB represents the surfactant in the presence of a constant concentration of a foam-stabilizing agent; OEF represents the surfactant in the presence of a constant concentration of a foam-inhibiting agent.

Experimental Results The experimental results of the first series of conductivity measurements are described qualitatively in Fig. 1, in which the specific conductance, La,is plotted against the concentration of colloidal electrolyte. I n every series of measurement the specific conductance at low concentrations is described by a straight line through the origin; and at higher concentrations by another straight line such that the two lines together form an obtuse (3) J. Ross and G. D. Miles, Oil and S o a p , I S , 99 (1941),

J,

- tan

tan

I-

J

81

where tan 81 is the slope of the first line, and tan ez that of the second. M can be thought of as a measure of the degree of micellization of a colloidal electrolyte in solution. A general outline and summary of the results for these experiments is given by Fig. 1. The lines OC, CD describe the specific conductance of a colloidal electrolyte; the slope of OC is tan el, the slope of CD is tan 02, and the concentration at C is the c.m.c. Addition of small amounts of nonionic additives, either foam stabilizers or foam inhibitors, does not affect the specific conductance in the range OA, where no micelles are present; a t higher concentrations, however, the specific conductances may be described by AB or EF, instead of by CD. The results of the present work indicate that foam stabilizers change the description of the conductances from CD to that of a line of lesser slope, AB; foam inhibitors change it from CD to that of a line of greater slope, EF. The above description is qualitative and general; in particular, the points A, C and E do not always occur in that order; sometimes C and E coincide, and occasionally E may be a little below C. The truth of the general description given by Fig. 1 can be verified by the data in Table 11, which reports the observed results for c.m.c. and function M of a number of colloidal electrolytes and additives.

I

b

1263

INHIBITION OF FOAMING

Oct., 1957

TABLE I1 CHANGES IN ELECTRICAL CONDUCTIVITY OF COLLOIDAL ELECTROLYTE SOLUTIONS AT 24.7" ON ADDITION OF FOAM STABILIZERS OB FOAM INHIBITORS Colloidal eleotrolyte

Additive

Hyamine-1622 Hyamine-1622 Hyamine-1622 Hyamine-1622 Hyamine-1622 Hyamine-1622 Hyamine-1622 Sodium lauryl sulfate Sodium lauryl sulfate Sodium lauryl sulfate Sodium lauryl sulfate Sodium lauryl sulfate Sodium lauryl sulfate Sodium oleate Sodium oleate Sodium oleate Sodium oleate Sodium oleate

None D-C Antifoam A Tributyl phosphate Tributyl phosphate 2-Ethylhexanol n-Decyl alcohol Lauryl alcohol None Tributyl phosphate Tributyl phosphate Tributyl phosphate n-Decyl alcohol Lauryl alcohol None Tributyl phosphate Tributyl phosphate n-Decyl alcohol Lauryl alcohol

The results of the second type of experiment are reported in Figs. 2, 3 and 4: Fig. 2 reports the effects of added tributyl phosphate, a foam inhibitor, on the specific conductances of three different solutions of colloidal electrolytes, each at a fixed concentration greater than its c.m.c.; Fig. 3 reports similar effects with methylisobutylcarbinol, another foam inhibitor; Fig. 4 reports the effects of ndecyl alcohol and lauryl alcohol, both of them foam stabilizers, on the specific conductances of 0.10% sodium oleate. These data bear out the findings of the experiment first reported: foam stabilizers decrease, and foam inhibitors increase, the specific conductivity of a solution of a colloidal electrolyte a t any concentration above its c.m.c. Discussion of Results Observation of the foam stabilities of a series of solutions of the same composition as some of those reported in Figs. 2 and 3, showed that the effect of the foam inhibitor augmented with increased concentration, accompanied by a concomitant increase in the specific conductance of the solution. The foam inhibiting agent achieved its maximum effect a t the concentration a t which the specific conductance first reached a constant value. Continued addition of agent beyond that amount caused a decrease in foam-inhibiting action, and in some cases greater quantities of agent actually stabilized foam. Throughout these additions, however, there was no further increase in specific conductance; it was also observed that the additional amounts added produced first a haze and ultimately an emulsion of the agent in the solution. By an application of Maxwell's equation for the conductivity of an emu1sionI4there ought to be a small but definite decrease of specific conductance as the concentration of the emulsified phase increases; the calculated values are too small, however, at the concentrations used, t o be apparent in the present measurements . (4) 8. Ross, J . Soo. Cosmetic Chem., 7 , 184 (1955).

Foam stabilizer (S) or foam inhibitor (I)

Additive concn.

...

I I I I

..

...

.20 .10 .05 .07

0

S S I

I

.13 .13 .18 .16 .07 * 09 .07 .05 .04 .03 .04

I

.05 .20 .40 .07 .07

14

.09 .ll

..

...

I

02

M

0.53 .52 .43 .32 .42 .42 .54 .47 .47 .40 .31 .61 .62 .48 .48 .47 .58 .48

0.16 .17 .15 .14 .13

a .

Trace 0.05 .50 .05 .05 .05

C.m.c. (wt. %)

I

I

I

04

06

I

I

06

(25

I

1.0

Yo TRIBUTYL PHOSPHATE.

Fig. 2.-The effects of added tributyl phosghate (foaminhibitor) on the specific conductance at 24.7 of colloidal electrolytes at fixed concentration: 0 = 0.4% sodium lauryl sulfate; 0 = 0.40% H amine 1622; 0 = 0.20% sodium oleate. The foam s t a b i h e s refer to 0.40% Hyamine 1622 plus tributyl hosphate a t 25", as measured by the Ross and Miles metho%( height of foam in cm. after five minutea).

Solubilization of the foam-stabilizing or the foaminhibiting agent was observed in the first series of experiments; it took place as the mixture of water and agent was used to dilute the solution of colloidal electrolyte. The agents were almost completely insoluble in water and, immediately after their addition t o the solution, were still visible in the form of large droplets, which soon disappeared as a result of the continuous agitation of the solution. As the dilution proceeded beyond the c.m.c., a turbidity caused by droplets in a fine degree of dispersion made its appearance, evidently due to the release of the additive from the mixed micelles as the latter became disaggregated. I n the second series of experiments (Figs. 2 , 3 and 4) the limits of solubilization were reached, not by diluting the colloidal electrolyte below its c.m.c., but by adding agent beyond the amount that could be solubilized by a given Concentration of colloidal

SYDNEY Ross AND T. H. BRAMFITT

1264

I

0

I

1.0

2.0 3.0 % METHYLISOBUTYLCARBINOL

.

4.0

Fig. 3.-The effects of added methylisobutylcarbinol (foam inhibitor) on the specific conductance a t 24.7” of colloidal electrolytes a t fixed concentration: 0 = 0.40ojb sodium lauryl sulfate; 0 = 0.40% sodium oleate. 21

I

0

0.1

I

0.2

0.3

% FOAM STABILIZER.

Fig. 4.-The effects of added foam stabilizers on the specific conductance at 24.7” of 0.10% sodium oleate: 0 = addition of lauryl alcohol; 0 = addition of n-decyl alcohol. The foam stabilities refer to 0.10% sodium oleate with additions of n-decyl alcohol, as measured by the Ross and Miles method (height of foam in cm. after five minutes).

electrolyte. The limits of solubilization were marked in every case both by the appearance of haze and by a cessation of changes in the specific conductance. This point also corresponded to the limit of action of the foam-inhibiting agents. The significant correlation of these experiments is, therefore, that specific electrical conductance, solubilization and foam inhibiting action all reach their greatest effect at the same concentration of agent. The above finding illuminates some of the obscurities of the mechanism of foam inhibiting action. It has long been known, and even enunciated as a prin~iple,~ that foam inhibitors are usually insoluble in the solvent in which they are effective; but, that the insoluble additive is, in fact, solubilized by the micelles of the foaming agent, has not previously been reported, though it could reasonably have been inferred. The surface layers of foam lamellae can be considered as a two-dimensional, or at least an extended, semi-micelle, in which addi(5) C. H. Fiske, J . B i d . Chsm., 86, 411 (1918).

Vol. 61

tives can be incorporated as readily as in the micelles of the bulk solution. At equilibrium, the free energy change directing an additive toward the surface semi-micelle or the bulk micelles would be one and the same; and when the limit of solubilizing accommodation by the micelles had been reached, it would be reached simultaneously at the surface. On this basis, the finding that foam inhibiting action, solubilization of the additives and specific conductance have their maximum effects concomitantly is readily understood : the surface layer can now incorporate no more additive molecules, each one of which has had a share in reducing the cohesion of the solute molecules originally present; the micelles in the bulk solution simultaneously reject incorporation of more additive molecules, or cannot reach any stable rearrangement of micellar sizes and structure at greater ratios of additive to colloidal electrolyte; and, as no further alteration now occurs in the structure of the mixed micelles, more of the non-ionic additive has little effect on the electrical conductivity of the system. The results for the foam-stabilizing agents supplement those of Schick and Fowkes,2 who found in every case a lowering of the c.m.c. by such agents. The present investigation was directed originally only to a conductimetric determination of the c.m.c., with the expectation that the added antifoam would cause an increase in the c.m.c. as a reflection of a reduced tendency to micelle formation; and, conversely, that a foam stabilizer would show a decrease in the c.m.c., as an indication of a greater tendency to produce spontaneously a mixed micelle. The results indicated, however, that variation in the c.m.c. is less significant than the general trend of the conductivity data, which indicates the presence of a foam inhibitor by an increase in specific conductance and the presence of a foam stabilizer by a decrease in specific conductance. If micelles did not form at all in solutions of colloidal electrolytes, the specific conductances would be greater than is actually found to be the case. Any general decrease or increase in specific conductance above the c.m.c. is therefore interpreted as reflecting a greater or less tendency to produce micelles. It is thereby concluded that foam inhibitors caused a reduced tendency of the solute to form micelles and that foam stabilizers cause an enhancement of the tendency to form micelles. Schick and Fswkes2 suggest that “the more effective foam-stabilizing additives are solubilized into the palisade layers of detergent micelles or into surface films; whereas the less effective additives are solubilized into the interior of the micelles.” This hypothesis is in line with the increased cohesion of the surface films that would result from a more effective alignment of van der Waals forces in the mixed palisade layer, The foam inhibitors, we suggest, are solubilized, not so much by being incorporated in existing micelles of the surfactant, as by causing a breakdown in size and an increase in number of mixed micelles, in which it may no longer be possible to distinguish an external palisade layer and a micelle interior.

a

e

C

c

Oct., 1957

INHIBITION OF FOAMING

DISCUSSION TRVIN A. LICI~TMAN.-RedLiCtiOn of the free energy of solute molecules by sorption within a micelle or by increase in concentration at foam surfaces are not strictly comparable thermodynamic processes. Micelles represent a stable configuration of matter, whereas foam has a transient existence, a t least under the experimental conditions employed. It might be worthwhile to consider a rheological method of determining foam strength under conditions where the foam is relatively stable. SYDNEY Ross.-Foam surfaces as such are not considered in our discussion. We suppose that the composition and stability of a foam is dependent on the character of the liquid, particular1 the surface of the liquid, from which the foam is produced: The thermodynamic com arison that we drew was between the micelles in the bulg liquid and that a t the liquid surface; both types of micelle “represent a stable configuration of matter.” M. B. EPSTEIN.-It is stated that sodium lauryl sulfate WRS selected as “an anionic detergent that has a plastic surface in aqueous solution.” However, it is now well established that solutions of pure sodium lauryl sulfate have %on-plastic” surfaces, on the basis of the independent work of A. G. Brown, A. C. S. Lawrence, J. Ross and their respective co-workers, among others. The sodium lauryl sulfate used in this work was of the USP grade, which is described in the pharmacopoeia as a mixture of sodium alkyl sulfates consisting chiefly of sodium lauryl sulfate. It may, accordin to the pharmacopoeia, contain as much as 4% of unsubated alcohols and 10% (combined) of sodium chloride and sodium sulfate. The alkyl groups in much USP grade material vary from Clo t o Cia, often the proportion of Cle being 61% and of C l r , 23%. The C.M.C. of pure sodium lauryl sulfate has been established as 0.234% by K. J. Mysels and confirmed by others. This is quite different from that given here, namely, 0.13%. The data reported in this paper pertain to an admittedly complex system and therefore the conclusions may re uire substantial modification before being applied to pure so%um lauryl sulfate. SYDNEYRoss.-In order to obtain a solution that will exhibit surface plasticity it is necessary to resort to systems of more than two components. A comparison of the behavior of surface-plastic solutions and those of surface fluidity is of great interest; in our paper we have shown them to behave in the same way toward foam inhibitors, which might not have been suspected in advance because of the different mechanisms by which these two types of surface film stabilize foam. Our interest lies in determining the physical mechanism of foam inhibition, and for that purpose the physical characteristics of the foaming solution are more pertinent than its chemical purity. MAX BENDER.-Question was raised as to the Maxwell equation being used by the authors to calculate the conductivity of an emulsion, whether allowance was in the equation for conducting electric current by the actual cataphoresis of the emulsion globules. SYDNEY Ross.-The Maxwell equation and a description of some experiments by which it has been tested are reported in reference 4 of our paper. The equation does not contain any. terms that express, directly or indirectly, the electrical charge a t the globule interface, and so makes no allowance for cataphoresis of emulsion globules. We conclude, therefore, that where t,he equation is found to hold true, cataphoresis is not a significant factor in the total conductivity. LEO SHEmovsKY.-Only two examples of foam properties are given, and the stated relation to conductance is not shown, I n Fig. 2, for example, there appears to be little change in conductance with change in foam properties. Furthermore, it is stated that the variation in C.M.C. is of relatively small significance, and this is also indicated by the data given in Table 11. Even if the extent of adsorption were indicated from conductance, this would not give either the composition or properties of the foam.

1265

The materials used were limited to a few mixtures and do not represent a wide range of types. The sodium oleate used is a mixture of soaps and U.S.P. sodium lauryl sulfate contains homologs of sodium dodecyl sulfate as well as lauryl alcohol, I n this case, the effect of addition of more lauryl alcohol on the mixture is reported. The properties of both solution and foam are a function of both detergent and additive, and not of either one alone. I n view of these considerations, the general statements regarding the effect of additives are not tenable. SYDNEY Ross.-In both Figs. 2 and 4 examples are given in which the stated relation between foam stability and conductance are shown. In Fig. 2 the magnitude of the change in conductance is slight, but the effect is real and reproducible. Some of the changes of C.M.C. listed in Table I1 are small, and because of the difficulty in determining the value with precision we have warned against leaning too heavily on such small changes. Our conclusion “that at concentrations above the C.M.C. foam stabilizers reduce specific conductance and foam inhibitors increase specific conductance” is based, however, on the totality of the accumulated evidence. which is tmticularlv immessive in Figs. 2, 3 and 4. We have already met the objection that some of our solutions do not contain pure solute. Before offering this statement as criticism it behooves critics to show why in anv parliculur case urity of the solute is necessary before valid inductions can {e drawn from experimental results. I

.

J. J. BIKERMAN.-Figure 1 of the paper seems to be an example of a quite general rule that additions which increase the solubility of the foaming agent depress the foaminess of the solution while foaminess is raised by additions which lower the solubilit (see, e.g., J. J. Bikerman, “Foams,” Reinhold Publ. Zorp., New York, N. Y., 1953, p. 84). SYDNEYRoss.-The chorus of critics having exhausted the theme that the results are not true, Dr. Bikerman raises the antiphonal chant: (‘The results are not newl”

ROBERT S. HANSEN (Communicated).-Debye and Anacker have suggested a very reasonable mechanism for the reduction of the critical micelle concentration by organic non-electrolytes. By what mechanism do you believe the antifoaming agents increase the C.M.C. or reduce micellization? Unleaa the ordinary and “semi” micelles are identical in structure, which apparently is not proposed, i t seems surprising that solubility of agents in them should be precisely the same, Le:, that they should have the same “solubilizing accommodation.” Once the “solubilization accommodation” of the interior micelles is exceeded the activity of agent becomes fixed a t unity (since the agents considered are insoluble). Isn’t this the simplest explanation for the failure of further addition to influence conductivity or foam inhibition appreciably? SYDNEY Ross (Communicated).-( 1) We report only one example of a significant increase of C.M.C., namely, the addition of tributyl phosphate to a solution of U.S.P. sodium lauryl sulfate. This solution originally contained free lauryl alcohol, and its C.M.C. of 0.13% was less than the value of 0.234% published for pure sodium lauryl sulfate. The increase of C.M.C. on addition of tributyl phosphate did not bring the value up to that of the pure solute, showing that lauryl alcohol in the original micelle is replaced by tributyl hos hate. This replacement is a clue to the mechanism wiereEy tributyl phosphate acts as a foam inhibitor in the surface-plastic solution of U.S.P. sodium lauryl sulfate. We have no evidence to offer however, that the C.M.C. of a pure solute is raised on addition of an antifoaming agent. (2) We did not mean to imply that the solubility of agents in the ordinary micelles is precisely the same as in the surface semi-micelles. Our statement is merely that the limit of solubilizing accommodation is reached simultaneously in both types of micelle,