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SYDNEY Ross, A. F. HUGHES, M. L. KENNEDY AND A. R. MARDOIAN
Vol. 57
THE INHIBITION OF FOAMING. V. SYNERGISTIC EFFECTS OF ANTIFOAMING AGENTS BY SYDNEY Ross, A. F. HUGHES, M. L. KENNEDY AND A. R. MARDOIAN Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N . Y . Received March d , 1065
Synergistic effects of antifoaming agentasare investigated for foams of two synthetic detergents, Aerosol OT and Nacconol NRSF. The defoaming systems are (a) mixtures of Ucon 50-HB-3520 and 2-ethylhexanol and (b) mixtures of tributyl phosphate and methylisobutylcarbinol. The first of these shows a correspondence between antifoaming action and the free energy of spreading of the antifoam composition on the detergent solution. The second system shows no such correspondence, but a more complex effect related to the separate defoaming action of each constituent of the composition.
In the industrial use of antifoaming agents it has frequently been found that a mixture of two agents is more effective than either agent separately. This cooperative effect is known as synergism. In his report of industrial practices Rossi mentions a mixture of 2-ethylhexanol and diisobutylcarbinol, and again a mixture of Ucon lubricant 50-HB-3520 and heptanol-3, where this synergistic effect has been found. Despite its frequent use, there has not yet appeared any examination of this effect directed toward an understanding of its cause and a better basis for its further exploitation. Two synergistic systems that have some general interest are reported in this paper. The first pair of agents is Ucon 50-HB-3520 and 2-ethylhexanol, which is a combination similar to one mentioned in reference (1). The second is a mixture of tributyl phosphate and methylisobutylcarbinol ; these agents have already been investigated separately by Ross and Young2 and shown to have widely differing effects. It was felt that mixtures of the two might, by a combination of their separate effects, display synergistic action. The first pair of agents was tried on a foaming solution containing 0.10% Aerosol OT. The second pair of agents was tried on a foaming solution containing 0.50% Nacconol NRSF and 0.75% sodium silicate (cf. reference 2). Materials and Methods.-The sources are listed in Table I.
materials and their
two factors are not measured in complete isolation even by this expedient. The free energy of spreading of each solution of the two agents on the surface of the foaming system was calculated from surface and interfacial tensions by the equation
AFs = (YD 4- YDF) - YF where YD is the surface tension of the solution of the two agents, YF is the surface tension of the foamin solution and YDF’is the interfacial tension between the two fiquids. The surface and interfacial tensions were measured using the Cenco-du Nouy precision form tensiometer and the Cepcodu Noiiy interfacial tensiometer, using the ring corrections of Harkins and Jordan.8 Results.-The results obtained for the foam stabilities in the presence of different concentrations of the synergistic mixtures are depicted in Figs. 1 and 2. On each figure for comparison is given the corresponding free energy of spreading in ergs/cm.*. The antifoams of Fig. 1, 2-ethylhexanol and Ucon 50HB-3520 on a foaming solution of 0.10% Aerosol OT, are used at a total concentration of 0.50% for each combination. The antifoams of Fig. 2, methylisobutylcarbinol and tributyl phosphate on a foaming solution of 0.50% Nacconol NRSF and 0.75% sodium silicate, are used a t a total concentration of 2.0% for each combination.
Discussion.-An examination of the two figures shows that apparently a correspondence exists in one system between the foam stability, Lr, and the free energy of spreading of the antifoam on the foaming solution (Fig. l), and that no such correspondence is observed in the other system (Fig. 2). The mechanisms by which the agents act are therefore deduced to differ.
TABLE I DESCRIPTIONS AND SOURCES OF MATERIALS USED Description
source
Nacconol NRSF
Trade name
Sodium alkyl aryl sulfonate, salt free
Aerosol OT Ucon 50-HB-3520 Methylisobutylcarbinol 2-E thvlhexanol Tributyl phosphate
Dioctyl sodium sulfosuccinate Polyalkylene glycol, water miscible (CHa)&HCH&H(OH)CH8 CHICH(C2HdCH~(CH~)2-CHOH (C,H90j*P:0
National Aniline Division, Allied Chemical and Dye Corporation American Cyanamid Co. Carbide and Carbon Chemicals Corp. Carbide and Carbon Chemicals Corp. Carbide and Carbon Chemicals Corp. Commercial Solvents Corp.
The foam stabilities were measured as described in reference (2). Three indexes, designated LI, L, and Lt, are obtained from the data. Of the three, only Lr can properly be called the unit of foam stability, as it is actually a measure of the average time that 1 ml. of f o a m i . e . , the mixture of the phases, not either one separately-has a n existence of its own. The remaining two indexes, LI and L,! give an approximate account of the relative parts played in the total instability of the foam by drainage of liquid from the foam films and by rupture of the flms to release gas, res ectively. They give an approximate account only as tiese (1) 8. Ross, Rsnrselaer PaEytech. Inst. Ball.; Eng. Sci. Ser., No. 53. (2) 8, Rosa and Q. J. Young, Ind, Png. Chrm., 48, 2629 (1961),
(a) Correspondence between Foam Stability and Free Energy of Spreading.-A qualitative correspondence between defoaming action and the spreading coefficient (numerically equal but opposite in sign to the free energy of spreading) had already been pointed out for a majority of systems investigated by Robinson and Woods4and again by (3) W. D. Harkins and H. F. Jordan, J . Am. Chem. Sac., 14, 1761 (1930). (4)
J. V. Robinson and W. W,Woods, J . SPC.Chem, Tnd. (London),
97, 381 (1948),
00t., 1953
SYNERGISTIC EFFECTS OF ANTIFOAMING AGENTS
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-0-
FREE ENEROY OF SPREADING
I
I
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a
100
Fig..l.-The effects of 0.50% of various mixtures of Ucon 50-HB-3520 and 2-ethylhexanol on a foam produced by 0.10% Aerosol OT, and the free energies of spreading of these mixtures on the detergent solution.
Ross.6 The spreading coefficient is one of several methods of estimating the degree of polarity of an organic liquid, or the hydrophilic-lipophilic balance of a mixture. This balance determines the surface activity of an agent, and has been reported to have an optimum value for antifoaming action on aqueous systemse6 A similar effect was discovered by Morse and Moss’ in an evaluation of defoamers for use during yeast manufacture. Tall oil (a source of fatty acids) condensed with different amounts of ethylene oxide was used as a series of defoamers, with a variable hydrophilic-lipophilic balance. The most effective defoaming occurred in the range of molar ratios around 1 to 1. The free energies of spreading were not measured for the varying composition of this system. It might be expected, however, that they would show a similar correspondence with defoaming action to that shown in Fig. 1. It is significant for comparison with the next system to be discussed, that there is no clear separation of the functions of each of the two constituents. The correspondence between good defoaming action and a large negative free energy of spreading could be interpreted as due to a displacement of the original foam-stabilizing surface layer by a more
IO0
%METHYL ISOBUTYL CARBINOL
X TRIBUTYL PHOSPHATE
Fig. 2.-The effects of 2.0% of various mixturee of tributyl phosphate and methylisobutylcarbinol on a foam produced by a solution of 0.50% Nacconol NRSF.and 0.75% sodium silicate, and the free energies of spreading of these mixtures on that solution.
surface-active but less cohesive or resilient surface layer of antifoam. (b) Lack of Correspondence between Foam Stability and Free Energy of Spreading.-Figure 2 presents an interesting example of a real synergistic, effect that shows no correspondence with the free energy of spreading. The two agents concerned, methylisobutylcarbinol and tributyl phosphate, have already been studied separately (reference 2), and shown to affect foams in quite different ways. Tributyl phosphate greatly increases the rate of liquid drainage but has little direct effect in reducing the strength of the thinned liquid film. Methylisobutylcarbinol both increases the rate of liquid drainage, though not to the same extent as tributyl phosphate, and has also a pronounced weakening effect on the liquid films, which rupture while still TABLE I1 FOAM STABILITIES (SECONDS) (Solution containing 0.50% Nacconol NRSF and 0.75% sodium silicate) Agent
(5) 6. Ross. TRISJOURNAL, 64, 429 (1950). ( 6 ) “Atlas Surface Active Agents,” Atlas Powder Co., Wilmington, Del. (7) R. E. Morse and H. V. Moss, Znd. Enu. Chsm., 44, 346 (1952).
No agent Methylisobutylcarbinol Tributyl phosphate
Concn., %
6.00 4.00
LI
LP
Lf
350 135
2080 180 1060
1720 170 845
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SYDNEY ROSS, A. F. HUGHES, M. L. KENNEDY AND A. R. MARDOIAN
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10 20 30 40 50 60 70 Volume of liquid in foam (ml.). Fig. 4.-The effect of 2.0% of each of two antifoaming agents and of a mixture of the two on the variation of foam density with volume of liquid in the foam. 90 MIC-10 T B P represents a composition of 90% methylisobutylcarbinol and 10% tributyl phosphate.
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Volume of liquid in foam (ml.) Fig. 3.-The effect of antifoaming agents on the variation of foam density with volume of liquid in the foam. The foaming solution contains 0.50% Naoconol NRSF and 0.75% sodium silicate. MIC = methylisobutylcarbinol; T B P = tributyl phosphate (data of G. J. Young).
relatively thick. The following results, from reference 2, reflect these modes of behavior. Figure 3, based on data of G. J. Young, shows the variation of the density of the foam with the volume of liquid in the foam. Very low densities indicate a tenuous system of liquid films enclosing the gas phase. A rapidly increasing foam density (reading from right to left as the foam ages) indicates rupture of liquid films and loss of the gas phase. With tributyl phosphate present in the foam, the films drain to the same ultimate thickness before rupture, arriving a t the condition of critical film thickness, however, in about half the time required in the absence of tributyl phosphate. With methylisobutylcarbinol in the foam, the foam
density never reaches as low a value as in the absence of the agent. The minimum in this curve is produced by the rapid escape of gas from the foam, which proceeds a t a greater rate than the escape of liquid. Figure 4 shows the two effects in combination a t total concentrations of 2%. The action of methylisobutylcarbinol is probably related to its large negative free energy of spreading, by virtue of which it may a t 6% concentration largely displace the original stabilizing film from the surfaces of the foam. The action of tributyl phosphate is probably related to its reduction of the surface viscosity of the original stabilizing film, reaching a maximum at 4% concentration, which then permits more rapid flow of interfilm liquid though without destroying the intrinsic strength of the film. The two agents are here combined a t concentrations below their optimum effects (total of 2% concentration), so that one effect does not dwarf the other. The combination of these two effects is clearly a more complex phenomenon than is represented by the previous system, and can no longer be found to correspond with the measured free energies of spreading.
