Refractive index matching and clear emulsions.

Refractive index (RI) matching is a unique way of making clear emulsions to meet market trends. However, RI matching has not been sufficiently investigated in terms of physical principles and methodologies. Snell's law (n2 sin r2= n1 sin r1) is applicable to cosmetic emulsions. When oil phase and water phase have equal RI (n2 = n1) values, light will not bend as it strikes obliquely at the emulsion interface. Instead, light is transmitted through the emulsion without refraction, which produces clarity. Theoretical RI values in solution can be calculated with summation of the product of the weight percentage and refractive index of each ingredient (RI(mix) = [W1 x n1 + W2 x n2 + W3 x n3 + + Wn x nn]Wtau). Oil-phase RI values are normally at 1.4 or higher. Glycols are used to adjust the water phase RI, since they typically have larger RI values than water. Noticeable deviations from calculated RI values are seen in experimentally prepared solutions. Three basic deviation types are observed: negative, positive, and slightly negative or positive, which can occur in glycol aqueous solutions at different concentrations. The deviations are attributed to changes in molecular interaction between molecules in solution, which can lead to changes in specific gravity. Negative RI deviation corresponds to a decrease in specific gravity, and positive RI deviation corresponds to an increase in specific gravity. RI values will deviate from calculated values since an increase or decrease in specific gravity leads to a change in optical density.

Solubilization of sodium cocoyl isethionate.

Sodium cocoyl isethionate (SCI) has been a predominant ingredient in syndet bar formulation for more than thirty years. Although cost effective and well recognized for good skin compatibility, SCI is not regularly found in liquid detergent systems due to its limited solubility in water. This study focuses on the understanding of enthalpy of solubilization, equilibrium of solubilization, and the structures and properties of sodium cocoyl isethionate and various surfactants. The purpose of this exercise is to help the formulator to find appropriate surfactant systems to keep sodium cocoyl isethionate in aqueous solution. The solubility of SCI in water is unfavorable in terms of enthalpy of solvation. When setting up equilibrium of solubilization, there are three possible phases, and three methods have been developed to prevent SCI from recrystallizing in aqueous solutions. The first focuses on tying CI ions within micelles made of secondary surfactants. The second focuses on the exchange of sodium ions with ammonium ions (and/or triethanolammonium). The third centers on emulsification of SCI and the subsequent change of micelles into emulsified oil drops. A combination of two or three of these methods will enable the formulator to use SCI as the primary surfactant in liquid detersive systems.