Sept., 1956
EFFECTS OF ANTIFOAMING AGENTSON SURFACE-PLASTIC SOLUTIONS
1255
THE INHIBITION OF FOAMING. VII. EFFECTS OF ANTIFOAMING AGENTS ON SURFACE-PLASTIC SOLUTIONS BY SYDNEY Ross AND J. N. BUTLER Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N . Y . Received February 9>,1966
Solutions of soa s, commercial synthetic detergents and proteins produce foams of outstanding stability b virtue of plastic surface films. Soyutions of USP grade sodium lauryl sulfate are shown to form surface-plastic films. l g i n g the sodium lauryl sulfate solution, or changing the pH with sulfuric acid or with sodium h droxide, or the addition of small amounts of antifoaming agents to the solution-each of these actions tends to retard or iestroy the formation of the surface-plastic film. r h e addition of commercial antifoams to protein solutions is also shown to reduce or inhibit the normal surface plasticity of such solutions. The mechanism of antifoaming action in surface-plastic solutions is, therefore, related to the destruction of the surface-plastic film by the antifoam.
The surface of certain types of solution shows an enhanced viscosity compared to that of the underlying bulk ‘liquid. The solutions themselves may be dilute and have normal, L e . , Newtonian, flow behavior, yet the existence of a plastic film at the surface can be demonstrated readily.. Focusing an ultramicroscope on the surface of a solution of sodium stearate that contained some finely-divided barium sulfate, Wilson and Riesl report: “At first the colloidal particles of barium sulfate were observed to be in violent Brownian motion, but in a few minutes after preparing the fresh surface this (motion) was replaced by a slower vibrational movement in which all the particles in the field moved in unison, as if suspended in a jelly.” Gelation of the surface film was observed to build up for an hour after a fresh surface had been formed; the final thicknesses attained in diff went solutions varied from 10 to 40 p . The importance of surface plasticity in stabilizing foams has been amply demonstrated. Solutes that impart surface plasticity to aqueous solutions include proteins,2 saponin13soaps4 and detergents6; precisely those solutes best known to produce stable and plastic foams, such as meringue, whipped cream, fire-fighting foams and shaving lather. The sequent paper of the present series6 reported the experiments of J. N. Butler on the inhibitory effect of tributyl phosphate on the surface plasticity of aqueous sodium lauryl sulfate solution. To the same effect Criddle and Meader’ have recently reported the destruction of surface plasticity of an oil solution by the addition of dimethylsilicone. Tributyl phosphate is a well-known commercial antifoam for aqueous systems, and dimethylsilicone is an antifoam for hydrocarbons. .An important part of the mechanism of their antifoaming action is, therefore, suspected to reside in their inhibition or destruction of the surface plasticity of the solution. The present paper reports the effects of a number of agents on the surface plasticity of sodium (1) R. E. Wilson and E. D. Ries. “Colloid Symposium Monograph,” Vol. 1, Williams and Wilkins, Baltimore, M d . , 1923, p. 145. (2) J. Plateau, MLm. acad. roy. sci. bela., (81 97, 49, (1869). (3) S. A. Shorter, P h i l . Mag., 71, 560 (1909). (4) E . J. Burcik, J. R . Sears and A . Tiltotson, J . Colloid Sci., 9, 281 (1954). (5) A. G. Brown, W. C. Thuman and J. W. MaBain, i b i d . , 8, 491 (1953). (6) 5. Ross, Symposium on Foams and Their Applications, A. C. 9. Meeting at Minneapolis, Minn., Sept. 16, 1955. (7) D. W . Criddle and A. L. Meader, Jr., J. A , p p l . P h y s . , 16, 840 (1955).
lauryl sulfate solution and egg albumin solution. It is shown that antifoaming agents for these solutions all behave in a similar way in inhibiting the formation of surface plasticity. Apparatus and Materials.-Surface viscosity was studied by means of a torsion-pendulum surface viscometer similar to that of Wilson and Ries.1 A sharp-edge cylinder was used instead of a disc to reduce end-effects and disturbance of the liquid surface; a diagram of the apparatus was provided in reference 6. The amplitude of the oscillation was plotted on a logarithmic vertical scale us. the swing number on a linear horizontal scale. When the movement of the pendulum is damped in a Newtonian fluid, the logarithmic decrement ( d ) can be calculated by d = ( l / n ) log A J A z
(1)
where A1 and A Zare the amplitudes a t the beginning and end, respectively, of a set of n swings. This e uation does not give a constant value of d for any but gewtonian fluids. For a plastic surface the decrease of apparent viscosity a t greater amplitudes is reflected in the smaller slope of the curve of A us. n. For purposes of comparison the relative steepness of the curve is measured as the slope of the chord between amplitudes 5 and 10. The “apparent surface viscosity” is defined as the value of d thus obtained. Because of the small surface area of the cylinder edge, the shearing stress in these measurements is small, which accounts for the high value of d for non-Newtonian surfaces compared with that for the surface of pure water. The value of d for 0.10% sodium lauryl sulfate is 0.202; whereas the value of d for the water surface is 0.0071-a ratio of 28 to 1. The values obtained by this method are reproducible within a precision of 2% for any single determination and 6% for a set of determinations on different solutions. Errors caused by temperature variation were small, as the temperature was controlled to within 0.1 ’. The materials used are listed below in Table I. The solutions were prepared by dissolving a weighed amount of the surface active agent in a known volume of water in which the antifoam had previously been dissolved, or suspended in fine droplek by mixing in a Waring Blendor. The solutions were usually used for measurements of surface viscosity immediately after preparation.
Results For most of the present investigation an aqueous solution of USP grade sodium lauryl sulfate was used, as this solute has already been the subject of an extensive study of surface viscosity by Brown, Thuman and M c B a k 6 These authors have shown that carefully purified sodium lauryl sulfate has a Newtonian surface viscosity little different from that of pure water; the addition of small amounts of lauryl alcohol or sodium octadecyl sulfate (less than 1% of the dry powder) causes the formation of plastic viscosity a t the surface. Small quantities of these substances are almost certainly present in the USP grade of sodium lauryl sulfate.
SYDNEY Ross AND J. N. BUTLER
1256
Vol. 60
TABLE I MATERIALS USED,TRADE NAMES,COMPOSITION, COMMERC I A L SOURCE Rlaterial
Trade Name
Sodium lauryl ...... sulfate, USP Sodium alkyl Nacconol aryl sulfonate NRSF Dioctyl sodium Aerosol OT sulfosuccinate Egg albumin, ...... dry 2-Ethylhexanol Octyl alcohol
Source
Fisher Sci. Go.
Concn.. wt./vol.
0.1%
Natnl. Aniline 0 . 1yo Divn. Am. Cyanamid 0 . 1 %
co.
Merc,k and Co.
0.1%
Carbide and Carbon Chemicals Corp. Methylisohtyl- Hexyl alcohol Carbide and Carbon carbinol Chemicals Corp. Trioctyl phos- TOF Plasticizer Carbide and Carbon phate Chemicals Corp. Tributyl phosphate Sorbitan mono- Span 20 laurate Polyoxyethylene Tween 80 sorbitan monooleate
Commercial Solvents Corp. Atlas Powder co. Atlas Powder co.
The type of measurement used in the present investigation permits the observation of the change of apparent surface viscosity as the surface ages. In Fig. 1 are reported the observations for the change of apparent surface viscosity with the age of the surface. Comparisons are made for the same solution immediately after its preparation and again after remaining in bottle for one day, two days and ten days. The older solutions take increasingly longer to build up plasticity at a newlyformed surface.
Time in minutes. Fig. 2.-The effect of sulfuric acid on the apparent Burface viscosity of 0.10% sodium lauryl sulfate solutions.
p 250 X -e
200
.-s 0" ."cp 150 Y
g
$
100
2
*
$
50
4 I
2
3
4
6
8
0
20
33 40
60 BO
Time in minutes. Fig. 3.-The effect of Sodium hydroxide on the aPPment surface viscosity of 0.10% sodium lauryl sulfate.
tards the formation of surface plasticity. The effect of addition of base is much less pronounced a t first; there is hardly any change in the rate of surface aging from pH 7.0 to 11.85, but small additions of base beyond that point soon destroy completely all surface plasticity in the solution. The effects of several commercial antifoams on the surface plasticity of freshly prepared solutions of sodium lauryl sulfate are twofold: first, the freshly formed surface of the solution containing the antifoam has a Newtonian viscosity, close to or identical with that of water itself; second, after an induction period, whose extent depends on the concentration of antifoam present, there is a sudden increase of the apparent surface viscosity, accomI 2 4 6 S I 0 20 40 60 ea panied with increasingly greater deviation from Newtonian toward plastic behavior of the surface. Time in minutes. Fig. 1.-Aging of solutions of sodium lauryl sulfate: Typical of the results obtained are Figs. 4 and 5 , apparent surface viscosity versus age of the surface, for soh- which report, respectively, the effects of 2-ethyltions of 0.10% Sodium lauryl sulfate kept UP to 10 days. hexanol and methylisobutylcarbinol. A compafiRather similar effectsto those obtained with older son of the inhibitory action Of antifoaming agents solutions were observed on fresh solutions by chang- can be made by measuring the number of minutes, required for the apparent surface ing the pH, either with HzS04or NaOH. I n Fig. 2 designated TI/*, the effect of decreasing p H is reported, and in Fig. 3 viscosity to reach half of its ultimate maXiInUm the effect of increasing pH. From pH 7.0 to pH value. Table 11reports Ti/, for a number of agents 2.45, adding acid lowers the initial value of the ap- that have been found to inhibit the foaming of parent surface viscosity, but raises the final value, 0.10% sodium lauryl sulfate solution. The range of Below pH 2.45, adding more acid increasingly re- concentrations found effective for foam inhibition
Sept., 1956
EFPECTS OF ANTIFOAMING AGENTSON SURFACE-PLASTIC SOLUTIONS
is reported in column 2 of Table 11; the information is derived from a report by S. Ross* and from unpublished results obtained in this Laboratory by P. &$pel. There is a significant parallelism between the concentration ranges for effective foam inhibition and those in which the agents show a inhibitory or destructive action on the form a plastic surface film.
g
1257
250
TABLE I1 THEINHIBITION OF SURFACE PLASTICITY IN 0.10 LAURYLSULFATE SOLUTIONS BY ANTIFOAMIN Antifoaming agent
2-Ethylhexanol
Concn. for foam Concn. inhibition (7%) used (%I
0.005to 1.0
0.01 .05
Ti/
< 1
Methylisobutyfcarbinol
0.10 to 1.0
0.10
5.0 18 '100 < 1
1.00
>loo
Tributyl phosphate
0 , O l to 0.1
0.01
50% Methylisobutylcarbinol plus 50% tributyl phosphate
approx. 0.01
0.005
.07 .10
.lo
IO0
The apparent surface viscosity albumin increased linearly with t leveling off was observed. The tion ( O . l O ~ o ) formed a plastic surf 4 6 810 20 40 60 80 idly than the lower concentration (0.01%). AddiTime in minutes. tion of tributyl phosphate to the 0.10% solution de- Fig. 5.-The effect of methylisohutylcarbinol, (t comcreased the rate of formation of the plastic surface; mercial nntifoam, on the apparent surface viscosity of tributyl phosphate (0.05%) added to the more di- O.lo% sodium lauryl lute (0.01%) albumin solution caused an induction slightly different from water, The addition of anperiod of about ten minutes before the plastic sur- tifoams to these solutions suppressed the foam, and face commenced to form, which it then did slightly was usually accompallied wit11 a small decrease in more rapidly than before. The m ~ h a n i s mfor the the surface viscosity. .411 the surfaces were Newformation of a plastic Surface film in proteins is tonian, and none of the systems showed any change probably a surface polymerization; denaturation of the surface viscosity with the age of the surface, of the protein a t the solution-air interface is also as had been ohserved in the sodium lauryl sulfate known to take place. solutions. The action of antifoams cannot therefore Span 20 (Atlas €'owder cos) is the agent used be ascribed i n every system to the destruction of commercially to prevent foaming of egg white dur- the plastic surface films of the foam. A discusing dehydration-' The addition of as little as sion of other mechanisms of antifoaniing action will 0.001% of Span 20 to a 0.01% egg albumin Solution be foulld it1 reference 6, completely inhibited the formation of a plastic surDiscussion face film. The surface viscosity of the system remained Newtonian and indistinguishable from that The production of plastic surfaces in solutions of of water. synthetic detergents and soaps is usually ascribed The general XW.hWkm of antifoaming fiction is hydrolysis products, such as fatty a ~ c o h o ~ s ~ only partly elucidated by these filldillgs* It has to or acid soaps.4 In the present study of sodium been demons"rated that not foaming lauryl sulfate the dependence of the surface plasticsolutes develop surfac*plastic In the on p H suggests the formation of lauryl alcohol present investigation Nacconol NRSF, Triton X- ity by the reaction 100, hexadecyldimethylbenzylammoniiim chloride Ci2HuSOdHz0 CizHuOH + HSOdand Aerosol OT, all solutes that produce stable foams in aqueous solution, were tested, and all were It is now well recognized that reactions that occur found t o have Newtonian surface viscosities only slowly in the bulk solution may take place rapidly s. Ross, "The Inhibitionof F o a m i n g , " Rens. Polytech, Inst. a t the solution-air interface. The relatively slow Bull., E n g . Sci. Series, 63 (1950). production of a plastic surface film in solutions
+
ENZOFERRONI AND GABBIELLA GABRIELLI
1258
where bulk hydrolysis is negligible can be well accounted for by the formation of hydrolysis products taking place only a t the surface. The apparent depletion of 10-day old solutions of sodium lauryl sulfate (see Fig. 1) can be traced to another cause. Solutions kept even for one day developed a slight haze; on standing for ten days, a definite precipitate formed; the aging process was accelerated by allowing a larger surface to be ex-
Vol. 60
posed t o the air. These facts suggest that atmospheric oxidation may be taking place, changing the lauryl alcohol a t the surface to insoluble lauric acid. The bearing of the present findings on the mechanism of antifoaming action, a t least for solutions in which the stability of the foam can be directly traced to the presence of plastic surface films, needs no further comment.
DETERMINATION OF THE EQUILIBRIUM CONSTANT BY MEANS OF SURFACE TENSION MEASUREMENTS BY ENZO FERRONI AND GABRIELLA GABRIELLI Institu,to d i Chimica Fisica, Dell Universita d i Firenze, Firenze, Italy Received June 24. 1966
+
+
For the system IZ CZH5OH CC14, the surface tension shows a minimum corresponding to a molar ratio 1:l i.e., corresponding to the complex I+&HbOH. The correlation between concentration of the complex and decrease of surface tension (evaluated as a difference between the mean of the surface tensions of the single components and the measured surface tension) is studied. This correlation appears to be linear in the range 0.0120-0.011 M . A general method of calculation for the determination of the apparent equilibrium constant of the complex is proposed. The average value of the constant is found to be 1.236, practically coincident with those obtained by other workers using different methods.
The correlation between surface tension minima and complex formation in solution has been mentioned by several workers. We have previously studied the formation of molecular compounds (antipyrine-chloral hydrate) and of complex ions (halides of cadmium and halides of potassium).3 I n this work it has been emphasized that any correlation between surface tension minima and molarity ratio of the components of the complex could only be deduced after a study of the surface tension behavior of the single components. It is known t,hat many salt solutions display surface tension minima a t low concentrations. These minima have been described by Jones and Rays4 Such minima can be present in ternary systems too, and therefore it would be wrong t o relate these minima to a possible complex (or molecular association). Surface-active substances, moreover, can have surface tension minima a t very low concentrations. I t is known, for example, that sodium oleate shows three minima a t concentrations 12.2 X 13.9 X respecof 7.5 X tively, corresponding to the formation of unimolecular films.5 Minima of surface tension can also be caused by the part,it,ionof a surface-active solute between the int.erface liquid-gas and another interlayer (which may already exist or may be formed in the liquid a t a certain concentration), as for instance for a micelle “solution.” I t is known, moreover, that the surface tension shows a niinimum value corresponding to the criti( 1 ) M. R. Nayar, L. N. Srivastavn and K. \’. Nayar, J. Indian Chem. S o c . , 29, 241 (1952). (2) E. Ferroni, C. Cahrielli and hf. Giarfuglia, Ric. Sci., 26, 539 (1955). (3) l? Ferroni and G. Gabrielli, A n n . Chim., in press. (4) G. Jones and W.A. R a y , J. A m . Chem. Soc., 69, 187 (1937); 63, 288 (1941); I. Langrnuir, Science, 88, 430 (1937); J . Chem. Phys., 6 , 873 (1938). ( 5 ) P. Lecornte Dii Nouy. “Equilibres Suporficiels des Solution Colloidalea,” Ed. Masson, 1929, p. 98.
I
.. .
. ..~.
.
.
cal micellar concentration.6 Tt is obvious that all these minima are completely independent of the formation of a hypothetical complex. Except where minima are found in the surface tension-activity curves of the single components or are due to the formation of unimolecular films or correspond to the critical micellar concentration, it may be inferred that minima in surface tension, as measured by the ring method, will only be found where ionic or molecular complexes are formed. To study the correlation between surface tension activity and complex concentration, the system of two components forming a single complex in inert solvent has been chosen as particularly suitable. The system CzH50H Iz in CCh seems to satisfy this condition. Experimental
+
The surface tensions have been measured with the tensiometer used in our institute.’ The given values have been read directly on the tensiometer, without the Harkins and Jordan corrections. It follows that greater values are obtained with respect to those corrected or obtained by other methods. The temperature during the experiments was kept constant a t 25 i 0.1’ using a jacketed container around which the liquid from a thermostat was circulated. Ethanol, iodine and carbon tetrachloride were of highest grade Merck quality and were further purified by the usual methods. System CzHsOH-CC14.-The capillary activity of C~HBOH in CC1, has been studied in the concentration range 0.0025-0.05 &I. The values of the surface tension for the different concentrations are given in Table I, each being the average of five measurements. Except in the range 0.0025-0.0078 M , the surface tension deceases as the molarity of the solution increases until, after a certain point, it remains constant. This is the normal behavior. System T2-CClr.-The variations in the value of surface tension with concentration, for the range 0.0025-0.05 M , are reported in Table 11. (6) J. L. Moilliett and B. Collie, “Surface Activity,” Spon Ltd., London, 1951, p. 14. (7) G. Piccardi, Brevetto Italian0 No. 462316, February 195&March
1951.
