164 JOURNAL OF COSMETIC SCIENCE The increase in interlayer spacing with water content is determined by equation 1: d = do(1 + R)/(1 + otR) (1) in which d is the interlayer spacing, d o is interlayer spacing extrapolated to zero water content, R is the volume ratio of water to surfactant, and ot is the volume fraction of added water penetrating from layer A to layer B (Figure 8). In the same manner, the hydrocarbon is partitioned between zones C and B. R, d, and d o are all known, and may be calculated for each R value. However, the d o value remains constant, and ot is, hence, changing for different values of R. The limiting value for R -- 0 is a more useful value for characterizing the water penetration. Algebraic manipulations and neglecting second order terms in R give a simple expression for (x = 1 - (0d/0R)R_•o/d o (2) This equation leads to d = d o for ot = 1 (complete penetration) and d = do(1 + R) for ot = 0 (water localized to the space between the polar group A). The values (Tables I and II) are used to find the location and activity of the addition of acid and white oil. Addition of acid caused a reduction of the d o value, the thickness of the lipid layer, A + B, which means that acid is located in layer B of LLC white oil, which is insoluble in water, increased d o and hence is localized in layer C (the space between the terminal methyl group of the surfactant). In addition, adding the acid reduced ot to negative values. At first, these values may seem surprising a negative ot value means that the interlayer spacing increases more than that which corresponds to the added water. The explanation for such a response lies with the interaction with the other compounds present. The acid is located in zone B and moved to zone C by addition of water such a modification of the behavior will formally give a negative ot value for water. For the white oil, no negative value of ot shows that water is located in A and B and that added white oil is located in zone C. REFERENCES (1) I. Effendy, C. Kwangsukstith, J. Y. Lee, and H. I. Maibach, Functional changes in human starturn comeurn induced by topical glycolic acid: Comparison with all trans retinoic acid, Acta Derm. Venereol. (Stockh.), 75,455-458 (1995). (2) J. R/Sding and C. Artmann, The salts of hydroxycarboxylic acids--Non irritant, potent active sub- stances,J. SOFW, 121, 1018-1021 (1995). (3) B. Langlois and S. E. Friberg, Evaporation from a complex emulsion system,.]. Soc. Cosmet. Chem., 44, 23-34 (1993). (4) R.Y. Lochhead and C.J. Rulison, An investigation of the mechanism by which hydrophobically- modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions, Colloids Surf, 88, 27-30 (1994). (5) T. Moaddel and S. E. Friberg, Phase equilibria and evaporation rates in a four components emulsion, J. Disp. Sci. Techn., 16, 69-97 (1995). (6) S.E. Friberg, T. Huang, and P. A. Aikens, Phase changes during evaporation from a vegetable oil emulsion stabilized by a polyoxyethylene{20} sorbitan oleate, Tween © 80, Colloids Surg. (in press). (7) S. E. Friberg, T. Young, R. Mackay, J. Oliver, and M. Breton, Evaporation from a microemulsion in the water aerosol OT-cyclohexanone system, Colloids Surf, 100, 83-92 (1995). (8) S. E. Friberg, T. Moaddel, and A. J. Brin, Interfacial transfer of vitamin E acetate during evaporation of its emulsion, J. Soc. Cosmet. Chem., 46, 255-260 (1995).

j. Cosmet. sci., 53, 165-173 (May/June 2002) Stability estimation of emulsions of isopropyl myristate in mixtures of water and glycerol C. A. AYANNIDES and G. KTISTIS, Department of Pharmaceutical Technology, School of Pharmacy, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece. Accepted for publication January 31, 2002. Synopsis Phase studies were carried out on systems consisting of isopropyl myristate, polysorbate 80, glycerol, and water. The stable oil-in-water emulsion regions were identified. An influence of the glycerol-to-water ratio on the area of existence of stable emulsions was obtained. The Coulter counter technique was used to determine the droplet size in oil-in-water emulsions. A decrease in average particle size with an increase in glycerol and polysorbate concentration was observed 24 hours after the preparation. Rheologically, the emulsions displayed NewtonJan behavior. Their viscosities increased with increasing glycerol and polysor- bate concentrations. The influence of glycerol and polysorbate concentrations on the cream separation of one-month-old emulsions indicated an increase in emulsion stability with the increase in glycerol and polysorbate concentrations. The use of a polysorbate 80 concentration of 5% by weight can be proposed for stable oil-in-water emulsions of isopropyl myristate in glycerol-and-water mixtures. INTRODUCTION There has been renewed interest in emulsions as a vehicle for delivering pharmaceutical and cosmetic substances, since they have been found to have several advantageous characteristics, including the enhancement of the bioavailability of the drug substance. Oil-in-water emulsions are a convenient form for the administration of lipophilic sub- stances (1,2). This form has several advantages but also the disadvantage of physical instability, which is of great importance for pharmaceutical and cosmetic technology (3,4). The formulation of the most possibly stable emulsions is, therefore, a field of study in pharmaceutical and cosmetic technology (5). In the cosmetic field, emulsification is one of the most useful tools. It is the process of dispersing one material throughout another in separate droplets and, for industry's purposes, effecting a dispersion that will retain its physical characteristics for a reason- able time. Non-ionic surfactants are widely used in cosmetic emulsions because of their lack of toxicity and low sensitivity to additives. These non-ionic surfactants adsorb onto Address all correspondence to G. Ktistis. 165