on the Viscosity of Emulsions

J. Soc. Cosmetic Chemists, 19, 683-697 (Sept. 16, 1968) Effect of Initial Surfactant Locations on the Viscosity of Emulsions TONG JOE LIN, Ph.D.* Presented December 6, 1967, New York City Synopsis--Viscosities of emulsions immediately following homogenization were studied as a function of HLB and the initial surfactant locations. Keeping the total surfactant concentration constant, the ratios of the initial concentration of the hydrophilic surfactant to that of the lipophilic surfactant in each phase were varied. The experimental results indicate that the initial locations of the surfactants not only affect the initial viscosity of the emulsions but also the emulsion stability, particle size distribution, and emulsion type as well. INTRODUCTION There are literally unlimited ways by which a given emulsion can be prepared (1, 2). Unquestionably, the method of preparation has a great influence on the physical properties of the finished emulsion. Stanko et al. (3) conducted a series of expehments with a mineral oil emulsion stabilized with nonionic surfactants. They discovered that the method of addition, the rate of addition, and the temperature of each phase at the time of emulsification all had some effects on the droplet size distribution, viscosity, and the emulsion stability. Using a two-level fractional factohal design, Benson et al. (4) conducted an extensive investigation of six preparative variables including: order of addition, emulsifier location, emulsifier concentration, proportion of water, emulsification temperature, and type of agitation. Four oils were used and thirteen different surfactants were employed in their investigation. They discovered that the chemical and physical nature * Max Factor & Co., 1655 N. McCadden Place, Hollywood, Calif. 90028. 68.3

684 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the emulsifiers and oils had most significant effect on the stability of the emulsions, and less than half of the systems studied showed statistically significant dependence on the preparative variables. Most of the emulsions in cosmetic preparations are stabilized with a mixture of hydrophilic and lipophilic surfactants. These surfactants may be incorporated into the emulsion in many different ways. Becher (1) describes four different ways of preparing an emulsion depending on the methods of incorporating emulsifying agents: 1. Agent-in-water method. The emulsifying agent is dissolved in the water. 2. Agent-in-oil method. The emulsifying agent is dissolved in the oil phase. The mixture is then added to the water or water is added to the mixture. 3. Nascent soap (in situ) method. The fatty acid is predissolved in the oil and the alkaline part in the water so that the soap is spontaneously formed during emulsification. 4. Alternate addition method. The water and oil are added alter- nately, in small portions, to the emulsifying agent. Since most emulsifying agents have some solubility in both oil and water, if a mixture of a hydrophilic and a lipophilic surfactant is dissolved in the oil phase prior to emulsification, a great part of the hydrophilic surfactant and a small part of the lipophilic surfactant will undoubtedly migrate to the aqueous phase after emulsification until an equilibrium is established. A similar situation will occur when the surfactant mixture is dissolved or dispersed in the water prior to emulsifica- hon. It can be reasoned that if the rate of surfactant migration from one phase to another is finite, the initial location of the surfactant may have significant effect on the physical properties of the freshly prepared emulsion. Since many surfactants have marked effects on the viscosity of the phase in which they are dissolved or dispersed, the initial surfactant location can, therefore, influence the viscosity of a fresh emulsion before it reaches equilibrium. Undoubtedly, the rate of surfactant migration in an emulsified system would be a complex function of the viscosities of both phases, surfactant concentration, surfactant solubility, surfactant diffusivity, interfacial area, the property of the interfacial film, etc. It is expected that the migration rate of a surfactant from the internal phase to external phase would be relatively low if the viscosity of the internal phase were sufficiently high or if the resistance of the interfacial film to mass

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 685 transfer were high. Conceivably, slow surfactant migration may be one of the reasons why some emulsions exhibit time-dependency of the rheological properties. In this work, systems initially containing varying proportions of surfactants in each phase were emulsified under carefully controlled conditions and the viscosities of the freshly formed emulsions were determined immediately after homogenization. In addition to the viscosity, effects of the initial surfactant location on the particle size distribution, emulsion type, and emulsion stability were also investigated. The main purpose of this work was not to examine the causes of time-dependency, but rather, an attempt was made to explore the ways by which initial surfactant locations affected the properties of the freshly prepared emulsions. EXPERIMENTAL Since the viscosity of an emulsion is strongly dependent on the preparative variables, efforts were made to control all variables to insure the reproducibility of the experiments. In most instances, coarse emulsions were prepared in a 2-1. rectangular clear plastic vessel shown in Fig. 1. The mixing equipment used was Model ELB Experimental Agitator Kit* designed for bench-scalexperimental purposes. The kit consists of 3 hp motor, variable speed drive, and various types of calibrated impellers. The impeller chosen for most of the experiments was a 2 -in. flat 6-blade turbine. The location of the impeller was 3 in. from the bottom of the vessel and, unless stated otherwise, the mixer speed was set at 400 rpm. In more viscous systems two 2 -in. impellers set apart by 3 in. were used. The mixing time used was mostly 3 minutes but in some viscous systems a longer mixing time was used. To keep the emulsion reasonably stable, the coarse emulsion prepared in the vessel was immediately passed through an ultrasonic homogenizer once. The homogenizer used was a laboratory size Minisonic IV Ultrasonic Emulsifiert which operates on the mechanical cavitation principle. The valve setting used was such that the rate of discharge of water at 24øC was 1960 cc/min. Prior to emulsification, emulsifiers were dissolved or dispersed in each phase with a laboratory propeller mixer at 600 rpm. These liquids were * Manufactured by Chetnineer Inc., Dayton, Ohio. p Manufactured by Sonic Engineering Corporation, Norwalk, Conn.

686 JOURNAL OF THE SOCIETY OF COSMETIC CItEMISTS then placed in a constant temperature bath until the temperature reached 24 = 0.1 øc. The water phase was first placed in the emulsification vessel and the oil phase was then slowly placed on the top of it. This operation was done very carefully to avoid emulsification prior to turning on the mixer. All viscosity measurements were done with Brookfield Synchrolectric Viscometer Model LVT,* and particle size distributions were determined from the enlarged photographs taken through a microscope. Emulsion stability was judged by placing the emulsion in a graduated cylinder and determining the degree of separation with time. The oil used for the experiments is a light mineral oilt and deionized water was used for the water phase. Surfactants used include: Tween 80, Arlacel 80,t, Amerchol L-101,õ and Solulan 98.õ These surfactants are commercial grades and they were used without further purification. The following HLB values suggested by the manufacturers were used for computing the HLB of the surfactant mixtures: Tween 80 15 Ariaeel 80 4.3 Amerchol L-101 8 Solulan 98 13 RESULTS AND DISCUSSION Effect on Emulsion Viscosity The effect of initial locations of the hydrophilic surfactant on the immediate viscosities of emulsions stabilized with Tween 80-Ariaeel 80 is shown in Fig. 2. The total surfactant concentration was kept at 5% by weight in all cases but the HLB values for each system varied from 6 to 14. The amount of the hydrophilic surfactant (Tween 80) originally present in the aqeuous phase was varied from 0 to 100% of the total amount employed. All the lipophilic surfactant (Arlacel 80) was placed in the oil phase prior to emulsification in this series of experiments. The emulsions contained 30% mineral oil and the coarse emulsion prepared in the emulsification vessel was passed through the homogenizer once. * Manufactured by Brookfield Engineering Lab., Stoughton, Mass. t Carnation P-I oil, Witco Chemical Co., Sonneborn Division, New York, N. ¾. $ Tween 80 (polyoxyethylene sorbitan monooleate) and Arlacel 80 (sorbitan monooleate), Atlas Chemical Industries, Wihnington, Del. õ Amerchol L-101 (multisterol) and Solulan 98 (ethoxylated lanolin), American Cholesterol Products, Edison, N. J.

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 687 I THERMOMETER TWEEN 80- ARLACEL 80 (5% TOTAL) LEVEL - RBINE o o IMPELLER Figure 1. 5" Emulsification vessel I I I I 0 20 40 60 80 moo % TWEEN 80 IN AQUEOUS PHASE Figure 2. Effect of initial hydrophilic surfactant location on the immediate viscosity of emulsions The emulsions obtained showed pseudoplastic behavior. Since the main interest was to determine the relative effects of the initial surfactant locations on emulsion viscosity, Brookfield viscometer readings on 100 scales were used rather than the absolute viscosity units. The viscometer readings were obtained with the No. 1 spindle at 30 rpm. As indicated in Fig. 2, for the emulsions having relatively low HLB values, the immediate viscosity following homogenization increases with the amount of Tween 80 initially present in the aqueous phase. As the HLB value increases beyond 8, this effect appears to become smaller and finally at HLB 14, the initial location of Tween 80 has no appreciable effect on the immediate viscosity of the emulsion. In Fig. 3, the systems studied were identical to the corresponding systems given in Fig. 2. However, instead of varying Tween 80, the ratio of the lipophilic surfactant, Arlacel 80, in the oil phase and the aqueous phase was varied. All the hydrophilic surfactant was placed in the aqueous phase before emulsification. The data show that the effect of varying Arlacel 80 in the oil phase was not as pronounced as the effect of varying Tween 80 in the aqueous phase. And again, virtually no effect was observed in the high HLB systems. As pointed out by a number of authors ($-7), emulsion viscosity is a very complex function of the viscosity of the external phase, volume fraction of the dispersed phase, viscosity of the internal phase, particle size distribution, nature of the interfacial film, emulsifier concentration, the extent of flocculation, etc. If the rate of surfactant migration is rela-

688 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS w/o,/ o/w TWEEN 80 - ARLACEL 80 ( õ% TOTAL) 30 % OIL HLB 12 - LB 14. 0 I I I I I 0 20 40 60 80 I00 % ARLACEL 80 IN OIL PHASE ( 5% TOTAL) TWEEN 80- ARLACEL80 HLB 6 7'0% OIL o 2o,,o ;,o eo,oo % TWEEN 80 IN AQUEOUS PHASE Figure 3. Effect of initial lipophilic surfactant location on the immediate viscosity of emulsions Figure 4. Effect of initial hydrophilic surfactant location on viscosity and type of emulsions tively slow, conceivably, the location of initial surfactant in the aqueous phase can affect many of the above mentioned factors. In this work, no attempt was made to isolate each factor and determine the manner by which the emulsion viscosity is influenced. However, as it will be shown later, the initial surfactant location can have a significant effect on the particle size distribution of the emulsion formed and, consequently, on the emulsion viscosity. Effect on the Type of Emulsion In the systems where the HLB of the surfactants and the volume of the internal phase are such that it can form either an O/W or W/O type emulsion, the initial surfactant location appears to play an important role in determining the type of the emulsion. In the systems investigated, placing of the entire surfactants in the oil phase appeared to promote formation of a W/O emulsion or a multiple emulsion. On the other hand, placing of the surfactants in their respective phases (i.e., the hydrophilic surfactant in water and lipophilic surfactant in oil) seemed to encourage formulation of an O/W emulsion. For example, in a 70% mineral oil system stabilized with Tween 80- Arlacel 80 combination at HLB 0, the viscosity increased sharply when the initial concentration of Tween 80 in the aqueous phase was increased from 0 to 10% (Fig. 4). Conductivity measurements indicated that the

-- EFFECT OF SURFACTANT LOCATION ON EMULSIONS 689 --W/O [ O/W = w/o o/w ; = ( 5 % TOTAL) TWEEN 80 - ARLACEL 80 / HLB I0 / 8 SOLULAN 98 - AMERCHOL L-IOI ( 5% TOTAL) ß HLB I0 60% OIL 70ø/001L./ 20 40 60 80 00 % TWEEN 80 IN AQUEOUS PHASE Figure 5. Effect of initial hydrophilic surfactant location on viscosity and type of emulsions 0 20 40 60 BO I00 % SOLULAN 98 IN AQUEOUS PHASE Figure 6. Effect of initial hydrophilie surfactant location on viscosity and type of emulsions sharp increase in the emulsion viscosity was due to phase inversion from W/O type to O/W type. Figure 5 shows the viscosity change for a similar system at HLB 10. In this system, the inversion took place when the amount of Tween 80 initially in the aqueous phase corresponded to approximately 70% of the total Tween 80 used. Figure 6 shows a similar curve for a Solulan 98- Amerchol L-101 system. In these three systems the viscosity readings were taken with the No. 3 spindle at 30 rpm. It has been known that the type of surfactants as well as their concentration have marked effect on the type of emulsion formed (8, 9). The present work indicates that not only the surfactant type and concentration are important, but the initial distribution of the hydrophilic surfactant prior to emulsification can also have a significant effect on the type of the emulsion formed. Davies reasoned that the type of emulsion formed as the result of shaking of a mixture of oil and water with an emulsifying agent is determined by the relative coalescence rates (10). He suggested that: and, Rate 2 O/W emulsion preferentially stable if Rate >> 1 Rate 2 W/O emulsion preferentially stable if --- << 1 Rate 1

090 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS where: Rate 1 = coalescence rate of an O/W emulsion Rate 2 = coalescence rate of a W/O emulsion From thermodynamic considerations, Davies further suggested that the ratio of coalescence rates could be related to the HLB value of the surfaetant and also to the partition coefficient of the surfaetant by the following equation: where: C = collision factor for Rate 1 C2 = collision factor for Rate 2 c = surfactant concentration in water Co = surfactant concentration in oil 0 = fraction of interface covered CiRate C2 Rate 21 - (c,,' \ / ø'7 ø (1) This equation qualitatively agrees with Bancroft's rule that the phase in which the emulsifying agent is more soluble will be the continuous phase (11). If Co and c can be considered as the initial surfactant concentrations in each phase, it can be shown that the above equation also qualitatively explains the results of the present work. For example, the data presented in Fig. 5 indicate that when the c is small (or Co is large) the system will form a W/O emulsion. Examination of the above equation would also indicate that reduction of c,,,/co ratio would favor a W/O emulsion. In some systems studied which had relatively low HLB values, multiple emulsions were observed under the microscope. An example of such a system at HLB 6 is shown in Fig. 7. A conductivity measurement indicated that the continuous phase of this emulsion was water. As can be seen from the photograph, most of the oil droplets contain some very small water particles and the result is a (W/O)/W type emulsion. Interestingly, the initial surfactant locations were also found to influence the formation of such an emulsion. The emulsion shown in Fig. 7 was prepared by initially placing all the surfactants in the oil phase. Figure 8 shows a microphotograph of an identical system stabilized with Tween 80-Arlacel 80 at HLB 6. The only difference between this emulsion and the previous one is the fact that all the surfactants in this sys-

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 691 Figure 7. Microphotograph of a 30% mineral oil system stabilized with 5% Tween 80-Arlacel 80 at HLB 6. All surfactants initially in the oil phase tern were dispersed in the aqueous phase prior to emulsification. It is clearly seen that, although there are some multipl emulsion droplets in the photograph, most of the oil droplets do not contain another phase. If the formation of a multiple emulsion can be regarded as the tendency of the system to form an inverted emulsion (W/O), the equation of Davies can be again used to explain the observedifference. Effect on Droplet Size Distribution Droplet size distributions of the freshly prepared emulsions were determined from enlarged microphotographs. Due to the limited resolution of the optical microscope used, it was not possible to obtain accurate measurements of emulsions containing many submicron range droplets.

692 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 8. Microphotograph of a 30% mineral oil system stabilized with 5% Tween 80-Arlacel 80 at HLB 6. All surfactants initially in the aqueous phase For this reason, measurements were limited to the systems homogenized at a relatively low homogenizing pressure corresponding to 786 cc/min throughput of water at 24 øc. Between 500 to 2000 droplets were measured for each system and the result was expressed in terms of mean volume diameter, din, defined by the following equation: dm= (2) Zn where n is the number of droplets and d is the droplet diameter. The results for the systems containing 30% mineral oil stabilized with Tween 80-Arlacel 80 mixture at HLB 10 are given in Table I.

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 098 Table Effect of Initial Surfactant Locations on Mean Droplet Diameter (Tween 80-Arlacel 80 at HLB 10, All Arlacel 80 Dissolved in the Oil Phase Prior to Emulsification) I Combined Surfactant Concentration in the System (% by weight) Tween 80 Initially in the Aqueous Phase (% of total) Mean Volume Diameter, d (cm) 1 100 2.8 X 10-3 1 40 2.8 X 10-3 1 0 4.8 X 10-3 8 100 2.8 X 10-3 3 20 2.5 X 10-3 3 0 3.2 X 10-3 Although the amount of data presented here does not allow generalization, for the systems investigated, however, there is a tendency to produce a coarser emulsion as less hydrophilic surfactant is present in the aqueous phase prior to emulsification. A possible explanation for the observed difference in the droplet size distribution may be given from the surfactant migration viewpoint. In order that the freshly formed droplets remain stable, the surfactant molecules must be adsorbed at the oil-water interface. From the dynamic surface tension measurements made by a number of authors using the oscillating jet method, it is evident that a finite time is required for the system containing surfactants to reach its surface equilibrium (12-14). Conceivably, the presence of a surfactant in the oil phase or aqueous phase prior to emulsification can affect the accessibility of the surfactant at the interface and hence the droplet size distribution. Effect of Emulsion Stability The effect of the initial surfactant location on emulsion stability was studied both by a shaking method as well as by using the emulsification vessd described in Fig. 1. In the shaking experiment, the oil phase and the aqueous phase containing various amount of surfactants were shaken together in an enclosed jar using a mechanical shaker for a predetermined length of time. The emulsion thus formed was then poured into a graduated cylinder and the amount of separation was observed as a function of time. However, in many systems, shaking produced a considerable amount of foam which made it difficult to interpret the data obtained. For example, in a series of experiments using Tween 80 and Arlacd 80, the emulsion prepared by

694 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (5 % TOTAL} 40- TWEEN 80-ARLACEL 80 HLB I0 / - 20- co 0 20 40 60 80 I00 % TWEEN 80 IN AQUEOUS PHASE Figure 9. Effect of initial hydrophilic surfactant location on emulsion stability predissolving all Tween 80 in the aqueous phase was less stable than an equivalent emulsion prepared by initially placing Tween 80 in the oil phase. However, the former emulsion produced a considerably greater amount of foam than the latter emulsion during the shaking. The presence of foam during and after emulsification could have affected the emulsions in many ways: (a) Foam could damp the shaking action to reduce the intensity of mixing operation, (b) adsorption of the surfactants at the liquid-air interface could deplete the surfactants needed at the oil-water interface to stabilize the emulsion, (c) retaining of the liquid in the foam laminas could affect true reading of the degree of separation. For this reason, shaking experiments were abandoned and a limited study was made using the turbine mixer in the emulsification vessel. The emulsions prepared in the emulsification vessel were virtually free of air bubbles and the data were fairly consistent. The example shown in Fig. 9 represents a series of emulsion stabilized with Tween 80 and Arlacel 80 at the required HLB value of the mineral oil. The emulsion stability was determined by measuring the per cent of the volume of water separated in a graduated cylinder. In this system, placing of the surfactants in their respective phases produced a less stable emulsion than the one prepared by placing all surfactants in the oil phase. However, this observation did not apply to others as an exactly opposite effect was noted in many other systems. Since emulsion stability is an extremely complex function of droplet size distribution, rheological properties of each phase, surfactant type, concentration, etc., the limited experimental data obtained here do not

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 69.5 permit generalization. However, it is evident that the initial surfactant distribution can have a significant effect on the stability of some systems. Davies discovered that HLB values of many surfactants could be calculated from the empirically determined structural group numbers (10, 15). He further suggested the following relationship between the coalescence rates and HLB number: (C Rate ' ) = 2.20(HLB-7) (3) In Rate Combining equations 1 and 3, an equation relating the HLB number to the partition coefficient of the surfactant can be obtained: (HLB-7) = 0.36 In (c ) (4) If the rate oœsurfactant migration is relatively slow, and if equation 4 is applicable, the surfactant mixture in an emulsified system can have different HLB values depending on the distribution of the constituent in the oil and water phases. Assuming that an emulsion having a maxi4 mum stability is obtained when the HLB value of the surfactant system equals that of the oil phase, it is clear that the initial surfactant location may be a significant factor affecting the emulsion stability. The HLB system developed by Griffin has been a very useful tool for emulsion chemists in spite of its limitations (16-18). The present work suggests that for the systems where the rate of surfactant migration is slow, it may be necessary to take this factor into account when determining the HLB value by an empirical method. For example, a common practice of preparing an emulsion is to mix all emulsifiers into the oil phase before emulsification (19). Conceivably, for a certain system, determination of the required HLB value of the oil by such method may give a different value from that obtained by mixing all emulsifiers into the aqueous phase prior to emulsification. CONCLUSIONS Experimental studies using various nonionic surfactants indicated that the initial surfactant location could have significant effect on the physical properties of the emulsion produced. The emulsion viscosity immediately after homogenization was affected by the initial surfactant location although the effect appeared to diminish with the increase of the HLB number of the systems studied. Placing of the surfactants in the oil phase appeared to encourage the formation of a W/O emulsion or a multiple emulsion. The experimental results were consistent with the

090 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS theory advanced by Davies (10). The emulsion droplet size distribution was also influenced by the initial surfactant location. Placing of the hydrophilic surfactant in the aqueous phase favored formation of emulsions having smaller droplets than the emulsions formed by placing all surfactants in the oil phase. The effect of initial surfactant locations on emulsion stability was also significant. Due to the complexity of the nature of emulsions, it is difficult to generalize the results beyond the scope of the experiments. It is probable, however, the initial surfactant location plays an important role in determining the characteristics of many practical emulsions. Particularly, if the systems are such that the rate of surfactant migration is slow, the initial surfactant location may be an important consideration from the manufacturing viewpoint. The optimum initial surfactant location for a given system would be dependent on the end results desired. For example, if the final emulsion should be a W/O emulsion but the phase volume is such that an Of W emulsion can be formed, it would probably be best to place all the surfactants in the oil phase to avoid phase inversion. On the other hand, if it is desired to produce an O/W emulsion having finest droplet size, it may be better to place all the surfactants in the aqueous phase. If the surfactant migration from one phase to another can produce viscosity change and if such a change is undesirable from the product viewpoint, it may be best to distribute the surfactants according to their solubility in each phase prior to emulsification. ACKNOWLEDGMENT The author gratefully acknowledges the assistance of John C. Lambrechts in collecting the experimental data and the assistance of William E. Gardner of Sloan Research Industries in preparing microphotographs. (Received December 8, 1967) REFERENCES (1) Beeher, P., Emulsions: Theory and Practice, 2nd ed., Reinhold Publishing Corp., New York, 1965, pp. 267-325. (2) Sumner, C. G., Clayton's Theory of Emulsion and Their Technical Treatment, 5th ed., Chemical Publishing Co., New York, 1954, pp. 480-528. (3) Stanko, G. L., Fiedler, W. C., and Sperandio, G. J., The effect of physical factors on the formation of cosmetic emulsions, J. Soc. Cosmetic Chemists, 5, 29-50 (1954).

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 697 (4) Benson, F. R., Griffin, W. C., and Truax, H. M., Statistical approach to common variables in emulsion preparation, Ibid., 13, 437-48 (1962). (5) Osipow, L. E., Sinface Chemistry, Theory and Industrial Applications, Reinhold Publishing Corp., New York, 1962, pp. 305-9. (6) Sherman, P., The Influence of Emulsifier Concentration on the Rheological Properties of Emulsions, in Sherman, P., Rheology of Emulsions, The MacMillan Co., New York, 1963, pp. 73-90. (7) Becher, P., Emulsions: Theory and Practice, 2nd ed., Reinhold Publishing Corp., New York, 1965, pp. 59-85. (8) Sherman, P., Factors influencing emulsion viscosity and stability, Research (London), 8, 396-401 (1955). (9) Becher, P., The effect of the nature of the emulsifying agent on emulsion inversion, _f. Soc. Cosmetic Chemists, 9, 141-8 (1958). (10) Davies, J. T., and Rideal, E. K., Interfacia! Phenomena, Academic Press, New York, 1963, pp. 366-83. (11) Bancroft, W. D., The theory of emulsification, J. Phys. Chem., 17, 501-19 (1913). (12) Addison, C. C., The measurement of surface and interfacial tension at fresh surface by the vibrating jet method, Phil. Mag., 36, 73-100 (1945). (13) Rideal, E. K., and Southerland, K. L., The variation of the surface tension of solution with time, Trans. Faraday Sot., 48, 1109-23 (1952). (14) Schwartz, A.M., and Perry, J. W., Surface Active Agents: Their Chemistry and Technology, Vol. 1, Interscience, New York, 1943, pp. 286-8. (15) Davies, J. T., A quantitative kinetic theory of emulsion type, I. Physical chemistry of the emulsifying agent. Proc. Intern. Congr. Surface Activity, 2nd, London, 1957, 1, 426-38. (16) Griffin, W. C., Classification of surface-active agents by "HLB," _f. Soc. Cosmetic Chemists, 1, 311-26 (1949). (17) Griffin, W. C., Calculation of HLB values of nonionic surfactants, Ibid., 5, 249-56 (1954). (18) Griffin, W. C., Ranauto, H. J., and Adams, A.D., Further studies on emulsion systems, Am. Perfumer Cos etics, 81, 31-42, (Sept., 1966). (19) Guide to the Use of Atlas Surfactants and Sorbito! in Cosmetic and Pharmaceutica! Products, Atlas Chemical Industries, Wilmington, Del., 1965, p. 40.