Microemulsions: A commentary on their preparation

Transcription

1 j. Soc. Cosmet. Chem., 39, (May/June 1988) Microemulsions: A commentary on their preparation HENRI L. ROSANO, JOHN L. CAVALLO, DAVID L. CHANG, and JAMES H. WHITTAM, Department of Chemistry, City University of New York, New York, NY (H. L. R. ), Clairol Research Labs, Stamford, CT (D. L. C. ), and Shaklee U.S., Inc. San Francisco, CA (J.H.W.), General Foods USA, Tarrytown, NY (J.L.C.). Received February 24, Synopsis A detailed description on the concept of microemulsions, how they can be formed, factors which affect stability, and recent applications are the focus of this commentary. A simple method for determining the amount of primary surfactant for a given microemulsion is shown, as well as data which emphasize the importance of the order of mixing based on thermodynamicalculations for six different microemulsion systems. We conclude that the formation reactions of these microemulsionstudies are entropy-driven reactions and thus quite different from the formation of coars emulsions in their thermodynamic proper- ties. INTRODUCTION Emulsions play a key role in many of the cosmetics we use today. Through the years much has been written on the formation and stability of these oil-dispersed-in-water (o/w) or water-dispersed-in-oil (w/o) systems. Nevertheless, the cosmetic formulator still seeks to understand and create the most favorable cosmetically eloquent and functional products possible. Aesthetically appealing products can be formulated as transparent o/w or w/o dispersions called microemulsions. The possible application for these systems range from products with an extended shelf life to delivery systems for active ingredients. The pioneering work on microemulsions started in 1943 (1). It was only in 1959 that Schulman and coworkers (2) coined the word microemulsion for these clear transparent dispersions. These systems offer a great deal of uniqueness, not only because of their novel transparency, but also because of the small dispersed phase, usually having a droplet diameter between fk. Today there are numerous theories on the nature of their formation and stability, yet the practical aspects of their preparation are still vague. Nevertheless, as can be seen by some recent patents (Table I), specific applications for microemulsions are being recognized. This review will elucidate one practical 201

2 202 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS approach for the scientist or cosmetic chemist to consider when formulating these unique transparent systems. The general ingredients required to make such transparent dispersions consist of an oil phase, an aqueous phase, a primary surfactant (which will determine whether the emulsion is of the o/w or w/o type), and a second surfactant, known as a cosurfactant. Typically the cosurfactant is a medium chain length (C 5 - C o) alcohol. The primary surfactant is mainly absorbed at the oil/water interface and determines the initial curvature of the dispersed phase. The cosurfactant also interacts at the interface to form a mixed duplex film. The cosurfactant appears to act in a dynamic state. Initially it causes a transitory lowering of the interfacial tension necessary during the formation of the dispersion. Secondly, as the cosurfactant distribution reaches equilibrium, it further distributes at the interface to become part of the interfacial film (oil/aqueous/surfactant molecules) with the primary surfactant. At this point, the system can be a thermodynamically stable swollen mixed micelie (o/w) or inverse mixed micelie (w/o) system. SPECIFIC COMBINATIONS FORM MICROEMULSIONS What will become clear to the formulator is that unlike coarse emulsions, the formula- Table I Microemulsion Technologies Documented in Recent Patents Patent (Date) 1. Tertiary oil recovery US (860701), US (851203). 2. Diesel fuel composition, lubricant, antifreeze solution US (860708), US (860812), (860909) 3. Pharmaceutical JP (860304) 4. Ink US (831011) 5. Liquid detergent, household products JP (850612), GB (850313) 6. Reaction medium US (860909), US (850313) 7. Drug delivery system Jp (860304) 8. Fiber finishes for soil and water repelling US (860128) 9. Herbicide, pesticide JP (851118) 10. Personal care products US (840717)

3 PREPARATION OF MICROEMULSIONS 203 tion of a stable microemulsion is more specific and thus greatly limited in variety. Coars emulsions can be stabilized by a wide variety of surfactants, application of highenergy apparatus (turbine mixers, sonicators, etc.), and excipient ingredients to alter viscosity and separation (gums, gelling agents). In general, stable microemulsions are very specific in the ratio of surfactant/cosurfactant used to disperse phase A in a continuous phase B. Six systems are described in Table II. An example of this specificity is the emulsion system consisting of 2 X 10-3 mole stearic acid, 2 ml oil, and 16 ml N KOH (System 5, Table II). The authors (3) reported that out of fifty-two alcohols, only five were suitable cosurfactants capable of producing transparent o/w systems. More recently, Rosano and coworkers (4) have made a systematic study on the formation of microemulsions prepared with alkyl sodium sulfate (with the alkyl chain containing 12 to 16 carbon atoms) as the surfactant, and alkanes (octane through hexadecane) as the oil, an aqueousolution of alkyldimethylamine oxide (dodecyl through hexadecyl) as the cosurfactant, and aqueous 5% NaC1, ph 12 brine solution as the continuous phase. Table III summarizes the systems studied. It is readily seen that out of the forty-five combinations, only fifteen resulted in the formation of transparent microemulsions, demonstrating the highly specific nature of the process. THE METHOD A recognized and classical approach to microemulsion formulation is to utilize phase diagrams. Friberg et al. have been great supporters of this technique and have elucidated the methodology in numerous papers (5-8). A major drawback to this approach is the time it takes to develop the phase diagram, especially when one has a variety of surfactants and cosurfactants and oils at his disposal for use and evaluation. Recently, automated systems have been commercialized to offset the laborious titrations to determine the various phases. The chief drawback at the present time is the cost of such instrumentation, as well as accuracy. An alternative simpler approach is discussed below. Rosano (9) suggested a way to determine the minimum amount of primary surfactant needed for a particular system. In this theoretical calculation, the minimum amount of primary surfactant required for the microemulsion is calculated by determining the Table Microemulsion Systems Continuous Dispersed Systems phase phase Surfactant Cosurfactant 1 X ml water 2 ml n-octane 0.5 ml C9Phenol-1.5-EO Csphenol-9-EO ml C9phenol-4-EO 2 X ml water 2 ml n-decane 0.5 ml C9Phenol- 1.5-EO C9Phenol- 10-EO ml C9phenol-4-EO 3 X ml water 2 ml n-hexadecane 0.5 ml C9Phenol-1.5-EO C9Phenol-9-EO ml C9Phenol-4-EO 4 X ml toluene 2 ml water 1.98 X 10-3M SDS 1-pentanol 5 X ml N KOH 2.3 ml n-hexadecane 2.3 X 10-3M stearic acid 1-pentanol 6 x ml 5% NaC1 saline 1 ml n-decane 1 gm SDS DDAO II

4 204 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Microemulsion Formation From a Given Oil/Aqueous System Cosurfactant (ADAO) Na cetyl sulfate (surfactant) Paraffin oil phase LDAO C8 C 10 C 12 C 14 C 16 C16 C14 C (90%)* 3.0 (88%) Cosurfactant (ADAO) Na myristyl sulfate (surfactant) Paraffin oil phase LDAO C8 ClO C12 C14 C16 C16 C14 C (100%) 3.6 (96%) 6.9 (95%) 5.4 (97%) 7.8 (92%) 7.3 (96%) Cosurfactant (ADAO) Na lauryl sulfate (surfactant) Paraffin oil phase LDAO C8 C 10 C 12 C 14 C 16 C (100%) C (90%)* 2.1 (98%) C (99%) 3.3 (100%) 3.O (9O%) 4.O (88%) Primary surfactants: Sodium alkyl sulfates. Cosurfactants: Alkyl dimethylamine oxide (ADAO). Oil phase: C8, C o, C 2, C 4, C 6 n-paraffins. Temperature: 30øC. *Viscous gel. ( )% transmittance (clarity) of system at 520 nm. = non-clear system. surface area or, more readily, the mono-molecular interfacial film area necessary to form between the dispersed phase and the continuous phase. If a given volume V (ml) of a phase is dispersed into spherical droplets of radius r ( ), the total volume of the dispersed phase can be expressed as V = a(4/3)'rrr 3 (1) where a is the total number of droplets formed by the dispersed phase. The total surface area A of the droplets is A = a4'rrr 2 (2) A = c n (3) where c and n are the area per surfactant molecule and the number of surfactant molecules, respectively. Combining Eqs. 1, 2 and 3 and solving for n, we find n = (3V)/c r (4) The value of c can be obtained from the surface tension vs. log concentration plot of the surfactant solution or from monolayer measurements. For microemulsions, the

5 PREPARATION OF MICROEMULSIONS 205 upper limit of r is 40 'nm(1/4 the wavelength of visible light), while the lower limit is set by the surfactant chain length. This simple calculation provides the minimum amount of surfactant necessary to cover the interface. It does not take into accounthe amount of surfactanthat is dispersed in both the aqueous and oil phases and other aggregates that may form in solution. Using the estimated amounts of surfactants, a coars emulsion of dispersed phase in continuous ß - ' formed by simple mixing o,-,,, rnonr since viscosity level. are always low. To determine if a transparent dispersion possible, a cosurfactant is gradually added to the coarsemulsion. In general, most cosurfactants are liquids and can be easily titrated into the emulsion while gentle mixing is appliedß If the system does not turn clear after adding the cosurfactant in an amount equivalento that of the primary surfactant, the system can be considered unacceptable. The formulator then has an option to alter the components of the system. In general, the cosurfactant is usually the nonspecificomponent, and so it is first altered. For example, if octanol was the cosurfactant used, one would consider selecting an alcohol with a different hydrocarbon chain length such as decanol or hexanol. New solutions of the base coarsemulsion are prepared and titrated with the alcohol of choice. Cosurfactantshould be chosen such that they do not prefer either the continuous or dispersed phase. The proper cosurfactant will be one that will migrate to the oil-water interface and form a mixed surfactant/cosurfactant film. The next option available if the cosurfactant selection fails is to consider a new primary surfactant. Recalculation of the amount of surfactant is made and repeat titrations with the cosurfactant are conducted. If the microemulsion does not form by variation of the cosurfactant and primary surfactant, the dispersed phase should be altered, in a logical manner. For example, if the dispersed phase is an alkane, either increase or decrease the chain length of the alkane depending on the requirements of the formulation. An example of this routine is shown in Table III. While at first appearing complicated, this progression can be conducted rapidly by redetermining a variety of cosurfactant/ surfactant and dispersed-phase optionsß ORDER OF MIXING The question on the order or method of preparation of microemulsions has long been debated. One school of thought is that these systems are thermodynamically stable and thus the order of mixing is inconsequential to product stability. Another approach (10) is to consider these transparent dispersions as true emulsions, and thus the stability is a function of preparation. We will attempt to shed some further light on this question by the following experiments. Vapor pressurexperiments were undertaken for o/w microemulsions to gain some insight into the mechanism of particle interaction in these systems (11). The microemulsions were prepared with 20 ml 5% NaC1, ph 11.2 (NaOH) solution, I gm sodium

6 206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tetradecyl sulfate (STS), varying amounts of octane, and titrated to clarity with dodecyldimethylamine oxide (DDAO, 30% active). Using the Clausius-Clapeyron equation ln(p/po) = -(AHvR)(1/T - 1/To) (7) where p and Po are the vapor pressure at T and To, H v the heat of vaporization, and R the gas constant, the vapor pressure and heats of vaporization can be determined. Figure ] 2 VOL. OCTAN[ ml Figure 1. Vapor pressure diagram for the microemulsions system: 5% NaCI (ph 12)/sodium hexadecyl sulfate/n-octane/dodecyldimethylamine oxide between 15-40øC. Plotted as a function of amount of n-oc- tane.

7 PREPARATION OF MICROEMULSIONS depicts the vapor pressure as a function of n-octane volume at various temperatures. The presence of an oil significantly reduces the vapor pressure of the solution, similar to regular colloidal solutions or high-molecular-weight polymer solutions, whereby the vapor pressure of the system is less than that of the continuous phase. Also revealed in the plots is the existence of two microemulsion regions distinguished by the sharp transition. In the low-pressure region and hence lower oil content, the microemulsion particles are isolated non-interacting droplets encapsulated by a surfactant film and are dispersed uniformly in the continuous medium. At high oil contenr, the rise in the vapor pressure is a reflection of the dynamic equilibrium between particle merging and reforming. Using the data in Figure 1, it is possible to estimate the thermodynamic properties of this system with increasing adsorption of cosurfactant. For a given microemulsion system at a particular temperature, the minimum amount of cosurfactant required to form the microemulsion varies with the amount of continuous phase in the system while the quantities of surfactant and dispersed phase are being held constant. Plotting the amount of cosurfactant vs. the volume of continuous phase reveals a linear relationship (4,9,12,13). The mole fraction of the cosurfactant at the interface, x, and in the aqueous phase, x b, can be determined from the intercept and the slope, respectively. From the equation AG - -RT In Xi/Xb (5) the change in free energy which corresponds to the adsorption of cosurfactant at the oil-water interface in the presence of surfactant film during microemulsification can be calculated. If the the same procedure is repeated at various temperatures and the change in free energy is plotted vs. temperature, the entropy change accompanying cosurfactant adsorption can be calculated via -- AS = ( /AG/ /T)p Table II lists the six systems investigated and Table IV summarizes the calculated free energy, enthalpy, and entropy values for each of the systems. The values of AG are all negative, signifying that microemulsification is a spontaneous process. However, the driving forces are small; therefore, other factors must be considered to explain the formation of microemulsions satisfactorily. Also consider systems 1 and 3: the difference in their AG values resulted from a difference in carbon chain length of the oils and AG changes with a value of J/mol per CH 2 unit. It is interesting to note that in the case of w/o microemulsions, the AG changes with a value of J/mol per CH 2 unit (13). Based on these calculations and experiments, the formation of this set of microemulsions is an entropy-driven process (4,12). The large increase interfacial area and the formation of a flexible surfactant/cosurfactant mixed film which produces a transitory low interfacial tension between oil and water oppose the entropy of the system. In microemulsification the free energy decrease caused by the entropy of dispersion outweighs the increase interfacial free energy from dispersion. For the systemstudied, the entropy values are all positive with one exception. Ruckenstein (14) showed that the adsorption of surfactant at the surface of a drop is another factor which favors dispersion; an increase interfacial area is accompanied by a decrease in free energy of an adsorbed

8 208 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS surfactant molecule compared to that of a molecularly dispersed state in the bulk phase. More recently, Rosano (12) has shown that the adsorption of cosurfactant molecules at the drop surface also reduces the system free energy. Miller and Neogi (15) developed a thermodynamic model for dilute microemulsion systems which includes the combined effects of surfactant film bending and dispersion. The duplex film model was used for the interface. Their model yields interesting predictions of microemulsion behaviors, but some of the parameters used to characterize bending effects are not readily related to known properties of the surfactant (16). Thus the free energy associated with formation of microemulsions is negative, but quite Table Thermodynamic Properties of O/W Microemulsions in Table II AG AH AS System (kj/mole) (kj/mole) (kj/k mole) X X X X X X 10-2 IV small. Therefore, the order of mixing plays an important role in accelerating the formation of these systems. SUMMARY Microemulsions are uniquely stable systems for the cosmetic formulator to consider. They, in general, are more difficult to formulate compared to regular emulsions due to the specificity of formulation and, in many cases, the order of mixing. An approach is summarized to help develop such systems in an orderly and rapid time period. REFEREES (1) T. P. Hoar and J. H. Schulman, Transparent water-in-oil dispersions: The oleopathic hydromicelle, Nature, 152, 102 (1943). (2) J. H. Schulman, W. Stoeckenius, and L. M. Prince, Mechanism of formation and structure of microemulsions by electron microscopy, J. Phys. Chem., 63, 1677 (1959). (3) H. L. Rosano, T. Lan, A. Weiss, W. E. F. Gerbacia, and J. H. Whittam, Transparent dispersions: An investigation of some of the variables affecting their formation, J. Colloid Interface Sci., 72, 233 (1979). (4) H. L. Rosano, J. L. Cavallo, and G. B. Lyons, Microemulsion Systems (Marcel Dekker, New York, 1987), p (5) S. Friberg, Microemulsions and their potentials, Chem. Technology, 6, (1976). (6) R. Zana and J. Lans, Dynamics of microemulsion, in Microemulsions.' Structure and Dynamics (CRC, Boca Raton, FL, 1987), pp (7) A. Belloca and D. Roux, Phase diagram and critical behavior of a quanternary microemulsion system, Microemulsions: Structure and Dynamics (CRC, Boca Raton, FL, 1987), pp (8) S. Friberg and P. Bothorel, in Microemulsions.' Structure and Dynamics, (CRC, Boca Raton, FL, 1987), p. 219.

9 PREPARATION OF MICROEMULSIONS 209 (9) H. L. Rosano, Microemulsions, J Colloid Interface Sci., 44, 242 (1973), and Method for Preparing Microemulsions U.S. Patent 4, 146, 499 March 27, (10) W. Gerbaciand H. L. Rosano, Microemulsions: Formation and stabilization, J. Colloid Interface Sci., 44, 242 (1973). (11) J. L. Cavallo, and H. L. Rosano, Vapor pressure measurements of an o/w microemulsion system, J. Phys. Chem., 90, 6817 (1986). (12) H. L. Rosano, and G. B. Lyons, Free energy, enthalpy and entropy changes during the formation of a n-hexadecane/potassium stearate/water/1-pentanol microemulsion system, J. Phys. Chem., 89, 363 (1985). (13) K. S. Birdi, Microemulsions: Effect of alkyl chain length of alcohol and alkane, Colloid Polymer Sci., 260, 628 (1982). (14) E. Ruckenstein, The origin of thermodynamic stability of microemulsions, Chemo Phys. Lett., 57, 517 (1978) (15) C. A. Miller and P. Neogi, Thermodynamics of microemulsions: Combined effects of dispersion entropy of drops and bending energy of surfactant films, Aiche, J., 26, 212 (1980). (16) S. Mukherjee, C. A. Miller, and T. Fort, Theory of drop size and phase continuity of microemulsions, J. Colloid lnterfacesci., 91, 223 (1983).