IKHIBITIOS O F FOAMSKG
429
IKHIBITION OF FOAMISG. 111*2
X MECHAKISM FOR
THE
R ~ P T U ROFE LIQUIDFILMS BY ANTIFOAMISG AGEXTS SYDKEY ROSS
Department o j Chemistry, Rensselaer Polytechnic Institute, T r o y , .Vew Y ork Received March 2, 1949
The similarity, pointed out by Edser (4),between the spreading of a rupture in a soap bubble and the spreading of a film of oil over pure water suggested to some workers with antifoaming agents that the presence in a thin film of a substance that spreads could be considered equivalent to the existence of a rupture in the film, with the obvious exception that in the case of the spreading substance a thin film of the substance mould remain, whose stability would however be so slight that it would soon vanish. Leviton and Leighton (8) thus connected in a qualitative way the spreading power of an agent with its effectiveness as an antifoam. In a previous publication on this subject (11) the results of bulk foaming tests were shown to have a large degree of correspondence with the action of an additive on a single film. At that time a mechanical model for the rupture of single films was discussed, based on the observation that effective antifoaming agents spread on the surface of the liquid film before they cause it to rupture. This discussion was limited to insoluble additives but the insolubility of the additive is a widespread condition, found in more than 90 per cent of successful antifoaming agents. A subsequent study by Robinson and Woods (10) described in some detail a mechanism of bubble coalescence. According t o this picture two bubbles will coalesce “as a result of a triple coincidence, namely the practically simultaneous collision of two air bubbles with the same droplet of the dispersed foam inhibitor,” It is probable, however, that this relatively rare three-body coincidence need not be invoked. The consideration of the force vectors of the various surface tensions involved shows that under the proper conditions droplets of the antifoaming agent are actually drawn by surface tension forces into the liquid film existing between bubbles. The proper conditions are that YF
+
YDfF!
> YD
where the subscript F refers to the original (foaming) liquid, D refers to the antifoaming agent, and the same letters primed refer to each liquid saturated with 1 The paper now included as the first of this series (reference 11) was written in collaboration with Professor J . W. McBain in connection with an investigation sponsored and financed by the Sational Advisory Committee for Aeronautics. The present and succeeding studies are a continuation of this work, but have been conducted without dependence on the previous sponsorship. 2 Presented before the Division of Colloid Chemistry a t the 116th Sational Meeting of the American Chemical Society, which was heid in Atlantic City S e w Jersey, September 18-23, 1949; see Abstracts of Papers, p , 1BG.
430
SYDNEY ROSS
the other. Robinson and Woods have defined an “entering coefficient”, E , by the expression
E = YI
+
YDF
- YD
For values of E > 0 the antifoaming agent is drawn into the liquid film and bridges the two air bubbles. For values of E < 0 the antifoaming agent will be ejected from the liquid film should it happen by chance to arrive near it. It is reasonable to follow Harkins’ treatment of “spreading coefficients” (5) and define three different “entering coefficients”, EDp, E D ~ Fand , EDtF,-named, respectively, the initial, the semi-initial, and the final entering coefficients.
+ + +
EDF =
YF
ED‘F =
YR
ED!,#=
Y F ~
- YD - YD,
YD~F#
(1)
YD,F,
(2)
-
Y D ~ F ~ YD,
(3)
The initial entering coefficient exists when the anlifoaming agent has been newly added to the foaming system. The semi-initial entering coefficient exists when the droplets of the insoluble antifoaming agent have been saturated by the foaming liquid; it is probable that the numerical value of the semi-initial entering coefficient does not differ much from that of the initial entering coefficient. The final entering coefficient exists when both foamer and antifoamer are mutually saturated. The condition for the liquid antifoam to spread on the surface of the foaming liquid is that YF
+
> YD’F‘
YD
Harkins (5) has defined the initial, semi-initial, and final spreading coefficients as follows:
SDF
= YF
SD,F = SD’Ft
YF
- YD,F* - YD - YD‘F, - Y D ,
= YF, -
YD’F,
-
YD,
(4) (5)
(6)
The same remarks that were made to distinguish the three entering coefficients apply to the three spreading coefficients. Harkins (5) has shown by calculation of the Gibbs free energy that a positive value of the initial spreading coefficient, S D F , means that the antifoam liquid D will spread as a duplex film on the foaming liquid F. He has also demonstrated that such duplex films are unstable and after a time will gather into a liquid lens in equilibrium with a monolayer film. The time required for this transformation is that taken for the initial spreading coefficient to change to the final spreading Coefficient,i.e., the time required for mutual saturation of the two liqcids. The value of the final spreading coefficient is always negative.
INHIBITION OF FOAMING
43 1
The relation between entering and spreading coefficients is obtained by combining the previous equations:
EDCF
+ = 2YD'F' +
EDfFt
= 2yD,r,
EDF
E
2yDrFF
SDF
(7)
SD'F
(8)
+ SD'F,
(9)
It can be seen from these equations that it is possible for the entering coefficient to be positive and the spreading coefficient to be negative. Such cases would be associated with pairs of liquids having relatively high interfacial tensions. A different situation exists for pairs of liquids having relatively low interfacial tensions. There the entering coefficient is approximately equal to the spreading coefficient. The mechanism proposed for the rupture of liquid films is: (1) Entering: The entering coefficient must be positive so that a drop of the antifoaming agent is drawn into the liquid film between bubbles. (8) Spreading: The spreading coefficient must be positive so that after the drop has made itself a part of the liquid film it will spread out as a duplex film on each side of the original film between the bubbles. ( 3 ) Rupture: As the antifoaming agent spreads as two duplex films the original droplet of the agent thins out, drawing the two bubbles closer together and squeezing out the original liquid from the film between them. As this process continues the liquid film develops a thin spot at the place where the droplet of antifoaming agent was originally situated. At the same time the presence of the duplex film lowers the surface tension inside the bubble, as spreading could not take place at all unless Y F > YD. This produces an increase in the radii of curvature of the bubbles, which serves further to decrease the thickness of the film between them. The total effect is the replacement in the film of a liquid capable of sustaining stable films by a liquid which does not possess that property. The film is thinnest precisely at the place where it is composed entirely of the antifoaming agent and it is therefore at this spot that rupture of the film can be expected to take place. This mechanism differs from that suggested by Robinson and Woods in requiring entering and spreading, rather than entering alone, as a necessary preliminary to film rupture. A positive spreading coefficient, rather than a positive entering coefficient, is taken as the criterion for an effective antifoaming agent. The following combinations of effects are possible:
Initial coejin'ents less than zero: By equation 7 if the entering coefficient is negative, the spreading coefficient is also negative. In such a case the antifoaming droplet does not even enter the liquid film and the substance would not be a defoamer by this mechanism. ( 2 ) E greater than zero; S less than zero. This could happen when the entering coefficient is not very large, or if the interfacial tension is high, though the num(1) E
432
SYDNEY ROSS
ber of agents in this category is probably not large. The material enters the liquid film but does not spread over it as a duplex film. I t will remain as a drop or liquid lens in equilibrium with a monolayer of D spread on the surface of F . It does not seem that this situation would necessarily lead to a rupture of the film. Single bubbles have been examined that were formed underneath the surface of water in a trough on which an insoluble monomolecular layer was present. The bubbles carried the monomolecular film upwards as they reached the surface, so that at rest the outermost surface of the liquid film that enclosed the bubble was covered with the monomolecular layer. The stability of the single bubbles thus produced was measured as a function of the degree of compression of the monomolecular film (12, 13). The stability of the bubble is found to be low at low compressions, where the monomolecular film is gaseous and so lacks coherence. As the degree of compression of the film increases, interactions between the molecules on the surface takes place and a greater degree of cohesion of the film is reached; the stability of the single bubble correspondingly increases. At higher compressions, however, the monomolecular layer may become too coherent and show a tendency in the solid surface phase to pile up and reconstruct the three-dimensional crystal. The lack of fluidity in the surface film is accompanied by a decrease in the stability of the single bubbles. The bubble stability, therefore, has a maximum value at a degree of film compression of the appropriate fluidity, neither too vaporous to lack coherence nor too solid to lack elasticity. A similar maximum has been observed as a function of bulk concentration in solutions of ethyl alcohol (2). Calculations made by the author’s students (3) have shown that, in this case also, the maximum of bubble stability occurs at astate of the adsorbed surface film of the appropriate fluidity. These considerations apply also to a drop or liquid lens of defoamer in equilibrium with a monomolecular layer of defoamer on the surface of a liquid capable of foaming. It should be considered that a mixed monolayer is now present on the surface of any liquid film, and that this mixed monolayer may be capable of either impairing or improving the cohesion of the original film. Impairment of the stability will follow when the agent is composed of small molecules, such as relatively short chain alcohols. The molecules interact with those of the foamproducing liquid and lessen the cohesion that is required to maintain the stability of the film, or they may even entirely substitute themselves for the molecules of the foam-forming substance at the surface of a liquid film. The monolayer now lacks the coherence necessary to maintain a film and the solution is no longer able t o sustain a foam (7, 14). If the agent in the mixed monolayer has a chemical constitution such that it either does not affect or even improves the coherence of the original film, then there will be no antifoaming effect of the agent. The gist of these considerations is that if the liquid D enters the film and does not spread, it may still act as an antifoaming agent, but it does so by a different mechanism than the one suggested as operative when the agent both enters the film and then spreads as its own duplex film. (5)E greater than zero; S greater than zero: This is the situation where the liquid
Ih-HIBITIOS O F FOBYISG
433
D enters the film of F and then spreads as a duplex film on each side of it. By the proposed mechanism this leads to the rupture of the film. Fznal coeficients According to Harkins ( 5 ) the final spreading coefficient is always negative. Consequently only tn-o cases are possible here, corresponding to the first two combinations of the initial coefficients. After the two liquids have become mutually saturated, the liquid D either does not enter the liquid film of F at all or, having entered it, does not spread as a duple film. This leads to the interesting practical conclusion that a good antifoaming agent must eventually lose much of its antifoaming effectiveness. It has a positive initial spreading coefficient but after a period of contact with the foaming system its spreading coefficient becomes negative. When the interfacial tension is low, the final entering coefficient is also negative and the antifoaming agent will not even enter the film between the bubbles (case 1). If the interfacial tension is large, the agent may enter but will not spread as a duplex film (case 2). Direct experimental testing of this prediction is complicated by the changes that have taken place in the foaming system itself by the time the final coefficients are established. The liquid F is noTv saturated with the liquid D and, quite apart from considerations of surface tension, may now be so altered in the structure of its surface phase that it no longer supports a foam to the extent that it did formerly. Nevertheless the observation of a loss of effectiveness of the antifoam agent after a period of use is well known to those xho have experience with the practical part of the subject. This loss of effectiveness is usually accelerated by a rise in temperature, so much so that it is common to find antifoaming agents good at room temperature that are permanently impaired on raising the temperature. The widespread occurrence of these t n o related observations indicates a general cause rather than a series of specific causes. S o t enough studies have been made of the effect of the passage of relatively long periods of time on systems containing antifoaming agents. Another observation that is readily described by the present mechanism is the presence of a residue of fine stable bubbles on the surface of a solution to which an effective antifoaming agent has been added. The major quantity of foam is successfully broken and the small bubbles remaining in the surface are usually of no practical disadvantage. Their presence may be ascribed to some of the droplets of liquid D that have already attained their final spreading coefficient and so provided a stabilizing monolayer of D on the surface of F . The series of foaming solutions and their effective antifoaming agents that 11-erefirst devised and investigated by Ross and McBain (11) provides a large number of systems suitable for further theoretical studies. h selection of some of these systems was taken by Robinson and Woods (IO), to compare entering and spreading coefficients with the results of foam inhibition. They measured both surface tensions and interfacial tensions, using a Cenco-du Nouy tensiometer with a platinumiridium ring 4.00cm. in diameter. The surface tensions were measured on the phases separately, not mutually, saturated. The ring corrections were made by
434
SYDNEY ROSS
the method of Harkins and Jordan (6), which requires the measurement of the density of each liquid. Table 1 is a comparison of initial spreading coefficients and the results of a test that measured the foam-inhibiting action of the additive on three known foaming systems. The data are compiled from the publications of Robinson and Woods (10). In general, table 1 reveals a high degree of correspondence between a positive initial spreading coefficient and effective foam inhibition by an additive. The few instances where the initial spreading coefficient is negative and the initial entering coefficient is positive (case 2) are indicated by an asterisk. Here the additive enters the film but does not spread as a duplex film. Only four examples were discovered, and in each case the additive showed definite antifoaming action. Another observation of interest is the behavior of the well-known silicone oil defoaming agent. This material displayed the highest positive values for the initial spreading coefficients and so should have shown the most pronounced action as a foam-inhibiting agent. It is therefore surprising that in systems A and C only moderate antifoaming action is reported, while for system B no antifoaming action at all is reported. Experience with this agent has shown that foaming tests must be carried out with particular regard to the degree of dispersion of the agent in the foaming liquid. Silicone oil has an exceptionally high interfacial tension, which provides a high energy barrier to its dispersal as an emulsion. Once adequately dispersed, silicone oil has proved itself to be an exceptional defoaming agent. The lack of correlation of the data of Robinson and Woods is caused by poor dispersion of the silicone oil. Experiments by the author have demonstrated excellent defoaming action for silicone oil on systems A, B, and C when the agent is sufficiently dispersed. This outstanding exception in the results of Robinson and Woods therefore disappears. Even with the exception noted the results display a definite agreement with the requirements of the hypothetical concept. Of thirty-five additives having positive spreading coefficients on a certain foaming system only two were found to fail as antifoaming agents ; of nineteen additives having negative spreading coefficients only four were found to have antifoaming properties. Perfect correspondence between theory and experiment is prevented by uncontrolled variables, such as the rate of departure from the initial spreading coefficient to the final spreading coefficient after the additive has been added to the foaming system. Antonoff (1) has emphasized that in some cases it is a matter of some practical difficulty, requiring the passage of a surprisingly long period of time, for mutual saturation of two liquids to be completed. Such systems would retain a defoamed property for a relatively long time. They are usually characterised by a high interfacial tension. Systems with a low interfacial tension are likely t o approach the condition of the final spreading coefficient more rapidly and so lose their defoamed property sooner. Another difficulty is the impossibility (at present) of predicting the behavior of an agent of the type given by case 2, which enters the film and produces a mixed monolayer, but does not spread as a duplex film. The few data available
435
INHIBITION OF FOAMING
show antifoaming action for this situation, but our discussion revealed the possibility of foam stabilizing by some agents of this type. I t is not to be supposed that the mechanism of film rupture by the formation of a duplex film of the agent is more than one among a number of different ways of reducing film stability. McBain (9) has stated that the lack of general correlations between defoaming ability and other physical properties has led to an undue insistence on the insolubility of the antifoaming agent as a sine qua non of its effectiveness. Although considerably less common, many examples are known of TABLE 1 Comparison of spreading coeflcients and foam-inhibiting action Initial spreading coefficients (8) and foam-inhibiting action ( F ) of various agents on three different foaming systems: A = 7.75 per cent Aerosol OT in triethanolamine; B = 1.5per cent Sacconol NR in water: C = 5.5 per cent Aerosol OT in diethylene glycol
I
I-
N A S OF ADDITIVE
NUMBER
1II
1 Ethyl oleyl glycol orthophosphate
1... . . 2 . ,. . . . 3.,, , . 1 4. , . . . , 5.. . . . . . . 7... . . , , . 8... . . . . . 9., , , , . , 10.., . . . . 11,. . . . . . ' 1 2 . .. . , . . ' 13.,. , , , 15... . , , , , 16... . . . . 1 17... , . , , . ' 18... . . , . . 1 19 , . . . , , , 22... . . . . . ,
I
, ,
~
,
,
~
* Agent
Trioctyl tripolyglycol tetrapolyphosphate Glyceryl monoricinoleate 2-.4mino-2-methyl-l-propanol Tetraoctyl pyrophosphate Carbitol maleate hlonoijleyl dipolyglycol orthophosphate Diethylene glycol monooleate Diglycol dinaphthenate 2-Amino-2-ethyl-l,3-propanediol Sapamine MS Polymerized dimethyl silicone Penetrol 60 Diethylene glycol Ethyl phosphate Polyoxyalkylene sorbitan monooleate n-Sonyl alcohol n-Butyl phthalate
SYSTEM
A
SYSTEY
B
1
SYSTEY
C -
F
s
-
4.2 10.8 -0.2 3.4 3.8 -0.1 5.7 4.3 3.3 -4.5 6.0
E M
-\
8.8 2.7 -7.8 6.1 1.5 8.1
E M
- 4.3
N 1-1.4 E 1.0 M 1.4 M E 2.0 M 3.4 E N E E E 1.9 ,I' 4.0 E N - 9.0 N 1-5.2 N E 2.0 N 5.6 E M
!-
~
1-
~
'
M N M
E M E
2.6
with negative initial spreading coefficient and pos
ve entering coefficient.
defoaming by soluble agents. They are usually described as acting by adsorption displacement of the foaming material at the surface of the film by the antifoaming agent, and so correspond to case 2 described above, where a monolayer of the additive is formed on the film. This clearly is not enough by itself to produce a defoamed system, since, if that were true, every material that lowers surface tension, and so is positively adsorbed at the surface, would be expected to act as a defoaming agent; unfortunately, defoaming agents are not come by thus readily. It is probable that in many cases of defoaming by soluble agents, specific chemical reactions are taking place at the surface, causing the chemical destruction of the foam stabilizer.
436
NEW BOOKS SUMMARY
A combination of the concepts of the entering coefficient of Robinson and Woods and the spreading coefficients of Harkins is used to furnish a mechanical description of some observed phenomena associated with the destruction of liquid films by antifoaming agents. REFERENCES (1) ANTOKOFF, G . : Arch. Biochem. 6 , 199 (1945). (2) CLAYTON, W . : The Theory of E m u l s i o n s and their Technical Treatment, 3rd edition, p. 44, London (1935). (3) DAUBER,L . : “The Surface Structure of Alcohol-Water Solutions,” B. Chem. Eng. Thesis, Rensselaer Polytechnic Institute, 1949. (4) EDSER,E . : Trans. Faraday SOC.17, 664 (1922). (5) HARKINS, W. D . : J. Chem. Phys. 9 , 5 5 2 (1941). (6) HARKINS, W. D . , ASD JORDAN, H . F.: J. Am. Chem. SOC.62, 1751 (1930). (7) LACHAMPTE, F., A N D DERYICHIAS, D . : Bull. soc. chim. France 1946,495. (8) LEVITOK, A . , AND LEIGHTON, A.: J. Dairy Sci. 18, 105 (1935). (9) MCBAIN,J. W., AND COWORKERS: Xatl. Advisory Comm. Aeronautics, Wartime Rept. ARR KO.4105 (1944). (10) ROBISSON,J. W., A N D WOODS,W. W.: Katl. Advisory Comm. Aeronautics, Tech. Note No. 1025 (1946): J. So?. Chem. Ind. 67, 361 (1948). (11) Ross, S., AND MCBAIN,J. W.: Ind. Eng. Chem. 36, 570 (1944). (12) TALMUD, D., AND S C C H O ~ O L S K AS.: J AZ,. physik. Chem. 164, 277 (1931). A , : Acta Physicochim. U.R.S.S. 13, 265 (1940). (13) TRAPEZNIKOV, (14) TRAPEZNIKOV, A , , ASD REBIKDER, P. A , : Compt. rend. acad. sci. U.R.S.S. 18, 427 (1938).
KEW BOOKS Physical Methods of Organic Chemistry. Vol. I, Part I. Second edition. ARNOLD WEISSBERGER, Editor. 1084 pp. New York: Interscience Publishers, Inc., 1949. Price: $12.50. Volumes I and I1 of the first edition of this work have been used extensively as references in this laboratory in studying the physical properties of fluorocarbons. An examination of P a r t I of the second edition indicates that this edition is a much more useful compilation of methods for determining physical properties. Recent techniques and up-to-date data have been included in some of the old chapters as well as the new ones. The four new chapters, “Temperature Measurement,” “Temperature Control,” “Determination of Vapor Pressure,” and “Determination with the Ultracentrifuge,” broaden the scope of the book. Each author outlines the fundamentals o[ the respective subjects and also presents a description of pertinent apparatus and a list of references. For example, in the chapter on calorimetry sufficient description of each type of calorimeter is given so t h a t the reader may make a selection of a desired method, and then pursue further knowledge on the subject in the many up-to-date literature references given. In many instances, an evaluation of the limitations and usefulness of a method is given along with its description. The chapter on the determination of vapor pressure is a definite asset to the book. This new section presents general and special methods of determining vapor pressures, as well as mathematical treatment of experimental data, including the estimation of critical constants. Chapters on temperature measurement and control, density, boiling and condensation temperatures, solubility, and viscosity should be particularly helpful t o the organic chemist in the identification of new compounds. The chapter on density contains a new section on isotope analysis. The chapter on microscopy is very complete, including the presentation of the fundamentals, techniques, and applications of the electron microscope. This book is highly recommended as a reference work for any research or analytical laboratory. GEORGED . OLIVER.
