4
THE
pH
OF MIXTURES OF AQUEOUS SOAP SO and MONOMERS
-k 4-
CHARLES F. FRYLING AND EDWIN W. HARRINGTON The B. F. Goodrich Company, Akron, O h i o The addition of monomers to aqueous soap solutions is accompanied b y t i o changes in p H ; an initial decrease which is attributed to solubilization, with the formation of micelles in the aqueous phase, is followed b k a n increase which is the result of solution of fatty acids in the monomer phase. This phenomenon lends support to the theory that the emulsion polymerization of synthetic rubber is initiated in the aqueous phase, and not at the interface or in the monomer phase. Other experimental facts in support of this theory are presented. It is suggested that the formation of micelles in aqueous solution is an important factor in the initiation of the reaction.
+ *
The sodium myristate solutions were prepared by neutralization with sodium hydroxide of Armour’s Neofat-13, for which the following composition is claimed: myristic acid 90%, lauric acid 4%, palmitic acid 4%, and unsaturated acids 2 7 . Merck’s U.S.P. oleic acid was used in making sodium oyeate. The monomers were taken from lots which had been subjected t o polymerization tests and found satisfactory. With the exception of acrylonitrile, all monomers contained a small concentration of ant ioxidant-type stabilizer.
OME three years ago the addition of acrylonitrile to an aqueous soap solution was found to lower the p H from 9.5 to 8.5. An increase in pH, due to transfer of fatty acid or acid soap (i.e., sodium hydrogen myristate) to the organic phase was expected. Since the sample of acrylonitrile was free of acidic impurities, this anomaly was investigated further. The addition of acrylonitrile to a soap solution was then found to exhibit two distinct effects. The first is a lowering of pII. A sharp break in the curve is observed when sufficient acrylonitrile has been added to form a second phase, and further addition results in the expected increase of pH. This phenomenon is of interest for several reasons. It provides a convenient method for dekrmining the approximate solubility of organic liquids in aqueous solutions of fatty acid soaps. At the same time, solubilization appears to be intimately connected with the process of soap hydrolysis. Finally, i t may become possible to relate the catalytic effect of the soaps employed in emulsion copolymerizations to their ability to bring about solubilization. That the phenomena observed are of a general nature was indicated by investigations conducted on acrylonitrile, styrene, methyl methacrylate, isoprene, benzene, and methyl ethyl ketone.
S
Table I and Figure 1 give the data obtained. On addition of acrylonitrile to an 85% neutralized solution of myristic acid, the pH decreased uniformly from an initial value of 9.28 to 8.10. The addition of the first milliliter of acrylonitrile was accompanied by a distinct change in the appearance of the system. The opaque soap solution became clear and transparent. When the minimum was reached, the system suddenly became cloudy owing to the formation of a second, dispersed phase. This so-called cloud point served as an independent end point in determining the solubility of the organic phase in the soap solution, and was useful at higher pH’s where the characteristic V-curve was not obtained. As long as the system was homogeneous, ionic equilibrium was established quickly, and the experimental points fell uniformly on a smooth curve. On the upslope of the curve, however, some time was required for the attainment of equilibrium, probably because of the time required for the transfer of myristic acid or sodium hydrogen myristate, as the case may be, to the organic phase. The behavior of 100% neutralized myristic acid was similar to that exhibited by the 85% neutralized acid. A slight increase in p H was observed at the start, the measured value rising from
VARIATION OF p H
A Beckman pH meter with external glass and calomel electrodes was used. These were immersed in the soap solution contained in a beaker provided with a glass stirring propeller and a thermometer. Precipitation of the emulsion at the salt bridge interface introduced discrepancies in the recorded values. This was partially corrected in later experiments by immersing the calomel electrode in a separate beaker of saturated potassium chloride solution with a slightly higher liquid level than the soap solution. The two beakers were connected through a U-tube filled with potassium chloride solution and provided with a stopcock, the plug of which was kept moistened. The end of the U-tube in contact with the soap was drawn down to a capillary and bent upward. Before each reading the stopcock WELS given a half turn t o allow a fresh surface of potassium chloride solution to be formed a t the capillary tip. The organic liquid under investigation was added in small amounts to the soap solution from a buret. After the pII reading reached constancy, it was recorded and organic liquid was again added. The actual manipulation was similar to that of an electrometric titration. Soaps were prepared from fatty acids by heating with aqueous sodium hydroxide solutions. Those prepared by addition of an equivalent amount of alkali were termed “100% neutralizcd”. Most of the curves under consideration were obtained a t lower degrees of neutralization. This nomenclature leads to rather cumbersome expressions, such as “a 2Oj, solution of 115% neutralized soap” (or more accurately “fatty acid”); but it expresses exactly the meaning intended.
~~
~~
~
TABLEI. VARIATIONOF pH ON ADDITIONOF ACRYLONITRILE TO A N AQUEOUS SOLUTION OF 85% NEUTRALIZED MYRISTIC ACID (Sodium myristate soap, 8 5 7 neutralized, 2% concentration, 200 ml. volume; acrylonitrile, fressly steam-distilled; room temperature) ACNQ ACN ACN ACN M1. pH MI. pH MI. pH M1. pH 9.28 13 8.42 39 8.30 0 26 8.12 14 1 9.18 8.40 8.31 27 8.13 40 15 8.37 28 2 45 9.08 8.37 8.16 8.98 8.33 8.41 29 8.18 3 16 50 4 8.90 8.29 8.46 8.19 17 30 55 8.84 8.25 8.48 8.20 18 60 5 31 8.54 32 8.78 8.21 8.21 19 65 6 8.19 8.56 8.21 20 33 8.71 70 7 8.63 8.59 8.22 21 75 34 8 8.16 22 35 8.62 8.25 8.13 80 9 8.58 8.67 8.26 8.10 23 36 10 8.53 90 8.27 24 11 8.72 8.10 37 8.49 100 8.28 25 12 8.10 38 a.45 ACN = acrylonitrile.
114
0
February,
1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Variation of pH upon Addition of Acrylonitrile to a 2% Solution of 85% Neutralized Sodium Myristste Soap at Room Temperature. (Initial Volume 200 MI.)
..
ML Or ACRYLONlrRlLE
1---~ ' A . 2% SOAP, 100% NEUT
I
le.
0.
11s
If hydrolysis of the soap is involved, it should be possible t o suppress the effect by the presence of excess sodium hydroxide. When a 2% sodium myristate solution was neutralized to l85%, a solid soap gel was obtained. Therefore a 1 per cent solution of 185% neutralized myristic acid was used. Under these conditions, the only effect on p H from addition of acrylonitrile was a very slight increase. However, the cloud point was observed at 22 ml. Similar results were also obtained at 125% neutralization. These results are included in Figure 2. The investigation of styrene-sodium oleate systems was somewhat m9re difficult. U.S.P. oleic acid, a mixture of several fatty acids, was employed. The addition of small amounts of styrene did not result in the formation of a clear homogeneous solution, possibly because of the presence of insoluble sodium stearate. This prevented observation of the cloud point. I n addition, the solubility of styrene in sodium oleate solution is low. Nevertheless the shape of the curves was similar to those obtained with acrylonitrile and sodium myristate. Table I11 presents data obtained by adding various organic liquids to aqueous soap solutions. The results indicate that a decrease in pH, followed by an increase, is a general phenomenon. However, methyl ethyl ketone, a very soluble substance, does not show the sharp minimum exhibited by the other substances listed in Table 111. Its behavior is illustrated in Figure 3.
I V o S O A R 125"hNEUT.
9.0 1
IO 20 30 M L . OF A C R Y L O N I T R I L E
6 .E
Figure 2. Variation of p H upon Addition of Acrylonitrile to Sodium Myristate Soap Solution at 28" C. (Initial Volume 200 MI.)
10.12 to 10.20 for a 0.25-ml. addition of acrylonitrile. At this point the liquid soap solution was converted to a soft gel. Further addition (0.75 ml.) brought about reliquefaction of the soap solution with the formation of a clear homogeneous solution similar to that previously observed. The minimum was observed a t 23.2 ml. per 200 ml. of soap solution. Allowing for experimental error, this is identical with the 23.5 ml. per 200 ml. obtained with 85Oj, neutralized soap. The data are presented in Table I1 and plotted in Figure 2.
I
6 .C
* 7.5
20
40 60 M L OF M E T H Y L ETHYL KETONE
Figure 3. Variation of p H upon Addition of M e t h y l Ethyl Ketone to a 2% Solution of 85% Neutralized Sodium Myristate Soap at Room Temperature (Initial Volume 200 MI.)
By this method it should be possible to investigate the variation of solubility as a function of soap concentration. The reTABLE 11. VARIATIONOF pH ON ADDITIONOF ACRYLONITRILE sults obtained with acrylonitrile using 180 ml. of 85% neutralized TO AN AQUEOUS SOLUTIONOF 1 0 0 ~ oNEUTRALIZED MYRISTIC myristic acid at 30" C. are presented in Figure 4. Since aqueous ACID solutions at the higher soap concentrations at room temperature (Sodium myristate soap, 100% neutralized, 2% concentration, 200 ml. are solid gels, i t was necessary to add sufficient acrylonitrile from volume; acrylonitrile, freshly steam-distilled: 28' C.) the buret to liquefy the systems before p H measurements could ACN, ACN, M1. pH Solution M1. pH Solution be made. However, that portion of the curve which exhibited the 15.30 9.43 Very trans3)&rent 0 10.12 minimum was easily established. At the higher soap concentra9.39 Same 16.70 0.1 10.18 9.37 Same 17.86 10.20 0.25 G'ei. tions the range of p H was less than at lower concentrations. For 9.32 Same 18.68 10.18 0.45 Gel 0.45 3.90 5.00 6.00 7.00 8.00 9.10 12.32 14.00
10.18 9.90 9.87 9.81 9.76 9.71 9.68 9.57 9.51
Gel Liquid Liquid Very liquid Same Same Very transpai.ent Same Same
19.65 20.38 21.70 23.58 24.88 27.00 29.40 33.30 39.20
9.30 9.27 9.22 9.20 9.20 9.22 9.26 9.30 9.35
Same Same Cloudy Cloudy Cloudy Cloudy Cloudy Cloudy Cloudy
The addition of acrylonitrile to 200 ml. of a dilute sodium carbonate solution gave a p H which was constant within 0.1 unit up to 50 ml. addition. This was interpreted as showing that the change in p H in the case of the soap solution was attributable to interreaction of soap and acrylonitrile.
BY TABLE 111. SOLUTION OB VARIOUS SUBSTANCES MEASURED pH MINIMUMI N 850/, NEUTRALIZED SOAPSoLumox
Organic Phase Acrylonitrile Acrylonitrile Methyl methacrylate Styrene Stvrene Isoprene Benzene Methyl ethyl ketone
2 % Sodium Soap S o h . Myristate Oleate Myristate Myristate Oleate Oleate Myristate .Myristate
Approx. ,-- PHSoly., M1./ Initial At min. 100 M1. Soap 9.28 8.10 11.8 9.42 8.22 12 3 9.48 8.39 3.0 9.49 9.31 2.0 9.45 9 32 1 5 9 . 7 5 (?) 9 . 6 0 (1) 2.2 9.32 9.10 0.6 9.50 7.50 40 (1)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
116
Vol. 36, No. 2
example, the spread from the initial to the minimum pH for a 0.5% solution was 1.32 units; for the 4y0 solution it was only 1.17 units. Similar results for styrene and isoprene in 85% neutralized U.S.P. oleic acid are given in Figure 5. Because of the errors involved in making determinations of the minimum, too great reliance cannot be placed upon their accuracy. The experimental points a t the minimum tend to round off, as shown in Figure 2. I n addition, since the measurements were made in open beakers, evaporation of both water and added organic liquid introduces uncertainties in the actual amounts of two of the components present. Presumably these errors could be decreased to some exFigure 4. Solubility of Acrylonitrile in tent by conducting 85% Neutralized Sodium Myrirtate at the titration in an 30” C. (Initial Volume 1 8 0 MI.) e n c l o s e d system. 26 Considering these limitations, about the only interpretation of the data in 24 $ 5 Figures 4 and 5 is that the solubility increases with in2 2 crease in the soap SODUM MYRISTATE IN THE A W E W S PHASE 12 concentration. Since d e t e r m i n a ISOPRENE e tions of the soluSTYRENE 0 bility of monomers 8 in soap solutions show promise in understanding the 4 p h e n o m e n a of emulsion polymerization, they should be obtained by a 2 4 6 e % S O D I U M OLEATE IN THE AQUEWS PHASE d i f f e r e n t method with a high degree Figure 5. Solubility of Styrene and Of accu’acy* It is Isoprene in 85% Neutralized Sodium suggested that such Oleate at 30”C. (Initial Volume 1 8 0 MI.) measurements should be made in the presence of varying amounts of the organic phase and then extrapolated back to the saturated solution-Le., to the cloud point. Such a procedure would avoid errors due to solution of the fatty acids in the organic phase.
is my opinion that solubilization takes a long time to reach equilibrium and that, while Fryling has waited long enough to reach apparently constant pH values after additions of hydrocarbons, the solubilization is not a t equilibrium. However, this ‘instantaneous’ measurement that he makes may be a good indication of the total solubilization that occurs. It may be especially important and useful in the synthetic rubber problem because it is this instantaneous solubilization that would cause the reaction to begin initially in the soap sol. Before the period required for complete solubilization to reach equilibrium is reached, the reaction will have already begun and may be well on its way to completion.” I n other words, reaction 1 is a highly simplified representation of the hydrolysis equilibrium. It is not unreasonable t o suppose that the incorporation of organic molecules in soap micelles is accompanied by a decrease in micellar water content. The decrease in viscosity of soap solutions on addition of monomers suggests that such may well be the case. Consequently, while it, is proposed that the p H is lowered by the formation of organic. micelles, according to the following equilibrium,
DECREASE IN pH AFTER ORGANIC ADDITION
INCREASE IN pH O N FURTHER ORGANIC ADDITION
Although it has proved comparatively easy to explain the rise in p H following the appearance of the second phase, the initial decrease in p H is puzzling. If the hydrolysis equilibrium of soap is represented by
If the fatty acid in equilibrium 1 is removed from the aqueous phase by solution in the organic phase, the shift of equilibrium will be toward the right, resulting in an increase in OH- concentration and a rise in pH. It should be immaterial whether the fatty acid goes into the organic phase in the form of free fatty acid RCOOH, dimerized fatty acid (RCOOH),, or the acid soap RCOOH.RCOONa. However, the shift of equilibrium 1 to the right should affect all the equilibria involved and eventually result in a decrease in the concentration of organic micelles.
a;
~~
RCOO-
__
+ Ea+ + HzO
RCOOH
+ Na+ + OH-
(1)
it is necessary to decide whether the primary effect of the monomer is on the right- or left-hand side of the equilibrium. The most reasonable explanation seems to be that ionized soap is removed from the left of the equilibrium by the formation of micelles with the monomer. The possibility that organic micelle formation would be accompanied by a decrease in hydrolysis was suggested by J. W. McBain. I n considering the formation of organic micelles, it should be kept in mind that soap and water form micelles, and that fatty acids are presumably solubilized by soaps. According to a communication from McBain, it would not make much difference whether the soap were in solution or had crystallized out as curd or gel. The “crystalline curd fibers can also incorporate and solubilize hydrocarbons-just like micelles”. A suggestion was made by one of McBain’s collaborators, K. Johnson: “It
RCOOwhere ACiY
+ Na+ + n(ACX) e[(ACN),.RCOONa] =
(2)
acrylonitrile
the possibility should be kept in mind that the process is considerably more complicated than indicated by this formulation. I. M. Kolthoff became sufficiently interested in the phenomenon under discussion to conduct some independent investigations. According to his unpublished results, addition of acrylonitrile to a 2 4 % solution of sodium myristate causes a marked increase of electrolytic conductance up to the point of saturation. Addition of organic liquids to dilute (0.5% or less) solutions causes a decrease of conductance. The decrease in p H was also observed in dilute sodium myristate solutions in which, according to Kolthoff, there are hardly any micelles. His investigation of p H substantiates the results presented in this paper. However he attributes the pH effect simply to a decrease of hydrolysis due to an increase of the solubility of the fatty acids in the aqueous phase. Accordingly, Kolthoff is unwilling to accept the explanation for the decrease of pH based on the formation of organic micelles according to equilibrium 2. However, before i t can be claimed that an increase in conductance disproves the theory of organic micelle formation, it would seem necessary to show that the conductance cannot be ascribed to the greater number of conducting organic micelles formed or to their higher mobility.
DECREASE IN pH DURING EMULSION POLYMERIZATION
A decrease in pH during the course of emulsion polymerization is usually observed. Previously this has been attributed to oxidation products generated by the peroxide employed as a polymerization initiator, hydrolysis of one of the monomers, or formation of potassium hydrogen sulfate when potassium persulfate is the polymerization initiator. On occasion, however, inexplicable examples of an increase in pH toward the end of a polymerization have been observed. It follows from what has been presented that, as the monomer phase is removed from an emulsion system by polymerization, a decrease
February, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
in pH is to be expected due to retransfer of fatty acid to the aqueous phase. It might even be expected that complete utilization of the monomers would be accompanied by an increase in p H toward the end of the reaction. I n other words, during polymerization it is necessary t o think of passing from right t o left along a curve such as that presented in Figure 1. However, the actual changes in p H observed during a polymerization are not simple because oxidation and hydrolytic effects do appear, and the observed pH change is further complicated by absorption of soap on the latex particles. It must be concluded that the change of pH during emulsion polymerization is a complex process, and that one of the factors involved is retransfer of fatty acid to the aqueous phase.
It is not intended to imply that a numerical correlation exists between solubilization and emulsion polymerization velocity; rather, the more useful polymerization emulsifiers are found among those that exhibit considerable solubilization.
’
S O A P MICELLES A S P O L Y M E R I Z A T I O N CATALYSTS
Whether the initiation of emulsion polymerization takes place in the interface, in the organic phase, or in the aqueous phase is difficult t o decide. The following experimentally determined facts indicate that the polymerization process may start in the aqueous phase. This agrees with the conclusions’ of Fikentscher (1). 1. With an undis ersed acrylonitrile-water-potassium persulfate system, it can %e visually demonstrated that polymerization takes place exclusively in the aqueous phase. Acrylonitrile, purified for polymerization, is added to a 6% aqueous solution of potassium persulfate a t room temperature. A portion of the acrylonitrile dissolves in the aqueous phase while the greater portion forms a well-deiined, clear upper phase. After standing quietly for a short time, the aqueous phase becomes cloudy and eventually opaque, due to formation of polyacrylonitrile. No evidence of polymer is detectable in the upper phase; as the result of an interesting phenomenon, it can be observed that no polymer is formed a t the interface. The aqueous phase remains clear and transparent for about 5 mm. below the interface, as though acrylonitrile, diffusing into the aqueous phase to replenish that removed by polymerization, has had insufficient time to undergo polymerization during its diffusion through the upper 5 mm. of the aqueous phase. However, if the partially polymerized system is stirred, the polyacrylonitrile is preferentially wetted by acrylonitrile and passes immediately into the upper phase. Consequently if such a system were submitted to continuous agitation, its appearance might lead to the erroneous conclusion that polymerization had taken place either in the acrylonitrile phase or a t the interface. 2. “Emulsion” polymerization of synthetic rubber will take place even though the mixture of monomers is separated from the other ingredients of the recipe by the vapor phase. The monomers evaporate, pass into the aqueous phase, and copolymerize without any discernible evidence of a monomer phase in contact with the aqueous solution. Under these conditions the initiation of polymerization is a homogeneous process, and no part of it (i.e., the initiation) can be attributed to interfacial reactions. 3. Water-soluble initiators, such as potassium persulfate, hydrogen peroxide, and sodium perborate, are effective initiators i n emulsion polymerization of synthetic rubber. 4. I n many recipes stable emulsification of the organic phase is not obtained until after polymerization starts. 5. The emulsion polymerization velocity of many recipes at low soap concentrations is almost linearly proportional to the concentration of the soap. This catalytic effect appears to favor the theory of initiation of polymerization in the aqueous phase. 6 . The utility of various substances as polymerization emulsifiers follows approximately the same order as their solubilizing action, as measured for propylene by McBain and Soldate (3). The idea that emulsion polymerization of synthetic rubber is initiated i n the aqueous phase was tentatively formulated by one of us before publication by Fikentscher. Subsequent experience has furnished considerable oonfirmation of this viewpoint. Fikentscher’s report, however, had little influence on this because he did not present the experimental basis of his theory. According to a recent communication from H. Mark, Fikentscher’s conclusion was largely based on the observation that the emulsified particles decreased in size as polymerization prooeeded. With emulsion-polymerized synthetic rubber latices, this observation cannot be confirmed because it is necessary in this caee to consider the variation in size of two types of particles-namely, those consisting of monomers and those consisting predominantly of copolymer. 1
117
These facts indicate that the micelle formation responsible for solubilization of the organic phase in an aqueous solution may be an important factor in the initiation of so-called emulsion polymerization. The prevalent idea that emulsion polymerization occurs a t the interface seems to be founded solely on the observation that polymerization is faster in emulsion systems than in homogeneous systems, and that orientation ‘11 the interface provides a mechanism to explain this increased rate. It appears evident that a micellar structure, such as that described by McBain ( 2 ) and consisting of alternate layers of soap, water, soap, and organic liquid, is capable of furnishing as plausible an explanation as can be advanced from the interface theory. It may go even further and explain differences existing between copolymers obtained a t low yields and those made a t higher conversions, if it is assumed that polymerization is initiated in the aqueous phase and that the subsequent polymer growth occurs in the organic phase. Consequently those growing polymer molecules, which in the growth process fail to penetrate a phase interface, are readily terminated, possibly by free radicals which would otherwise act as initiators and give low molecular weight products. Despite the evidence indicating that initiation of polymerization generally occurs in the aqueous phase in the process of emulsion polymerization of rubberlike copolymers, it cannot be assumed that under different conditions initiation in the organic phase, or even in the interface, is impossible. The well known processes employing benzoyl peroxide for initiation are probably examples of initiation in the organic phase. Rather it is suggested that recognition of the locus of initiation of polymerization may provide a useful method of classifying various types of emulsion polymerization. Such a classification was foreshadowed by TrommsdoriT (4) in his differentiation between “pearl” and “emulsion” polymerization. SUMMARY
1. The pH of an aqueous soap solution decreases on solution o monomers and increases after the formation of a second phase. .2. The method developed is convenient for determining the approximate solubility of organic liquids in soap solutions. 3. These effects contribute to the pH changes observed during emulsion polymerization. Other contributing factors are oxidstion, hydrolysis, and adsorption of soap by latex particles. 4. The decrease of pH is attributed to solubilization resultin from the formation of micelles, while the increase is attribute! to solution of fatty acids in the organic phase. 5. Solubilization of monomers is believed to play an important role in the initiation of polymerization. The evidence indicates that initiation may occur in the aqueous phase. 6. Determination of the locus of initiation may provide a useful method for classifying so-called emulsion polymerizations. ACKNOWLEDGMENT
The writers wish to thank J. W. McBain and I. M. Kolthoff for their interest and helpful suggestions, and R. J. Houston for assistance in conducting pH determinations. LITERATURE CITED
(1) Fikentscher, H.,Angew. Chem., 51, 433 (1938). (2) McBain, J. W., “Solubilization and Other Factors in Detergent Action”, in “Advances in Colloid Science”, Vol. 1,p. 124 (1942). (3) McBain, J. W., and Soldate, A. M., J . Am. Chem. SOC.,64, 1556 (1942). (4) Trommsdorff, E.,“Kunststoffe aus Polymerisaten von sthylenderivaten”, in Houwink’s “Chemie und Technologie der Eunststoffe”, pp. 304-60 (1939). Paersrn~r~~n before the Division of Rubber Chemistry at the 106th Meeting of the A M ~ R I C A CABMICAL N SOCIXITY, Detroit, Mich.
.
