260 JOURNAL OF COSMETIC SCIENCE readings of oil fluids result from the similarity in both the molecular structure and the interaction between these molecules. However, there is a noticeable RI deviation in water-phase preparation. In matching RI values of water and oil phases, one uses water (low RI value) and glycols (high RI values). In addition, the molecular interaction between glycol and water is different from the interaction between those molecules by themselves. Therefore, the formula needs further refinement to assure proper index matching. Various glycols are used to raise the refractive index of the aqueous phase to match that of silicone oil (RI - 1.4) or isoparaffin (RI - 1.43 ). The refractive index values for several glycols as a function of concentration are shown in Figure 2 for a two-component system. The glycol aqueous concentrations vary from 10% to 80%. Different deviations are seen in the plot. Glycerin shows negative deviation and three glycols show positive deviation. Hexylene glycol shows the largest positive deviation. Glycereth-7 shows very little deviation. All deviations are concentration-dependent. Deviations are compared with the calculated values shown in Figures 3-5. Negative deviation is observed for glycerin in aqueous solution (Figure 3). The largest negative deviation is seen for glycerin at the concentration of 30%, where there is a -0.51 % deviation (1.367-1.37395 == -0.00695) from the calculated value. 1.48 1.44 � Cl) 1.4 -----+-- Propyl G Cl) -a-Butyl G 1.36 �Hexyl G -e-Glycerin �?l(-G lycereth-7 1.32 0 20 40 60 80 100 Percentage of Glycol Figure 2. Concentration-dependent RI values of several glycol aqueous solutions.

REFRACTIVE INDEX MATCHING 261 1.48 1.44 -� 1.4 i:: � � 1.36 • RI Glycr Ext RI Glycr Cal 1.32 0 20 40 60 80 100 Percentage of Glycerin Figure 3. Theoretical and experimental concentration-dependent RI values of glycerin aqueous solution. Solid line is experimental and dashed line is theoretical. Positive deviation is seen for hexylene glycol aqueous solution in Figure 4. It is the largest positive deviation among all the studied glycols (Figure 2). The experimental RI value of hexylene glycol is even larger than the value of glycerin solution at concentra­ tions in the range of 10-35%. The largest deviation of the refractive index for hexylene glycol is at the concentration of 60%, where it deviates 0.97% from the calculated value. Figure 5 demonstrates slightly negative deviation at lower concentration and slightly positive deviation at higher concentration for glycereth-7 aqueous solution. Glycereth-7 is a unique case since deviations run from slightly negative at low concentration (below 40%) to slightly positive at high concentration (above 50%). Negative deviation at lower concentration is comparable to what is observed in aqueous glycerin, and positive deviation at higher concentration is comparable to that of hexylene glycol. As a way of explaining deviation, the specific gravities of aqueous solutions of glycerin and three glycols (propylene glycol, butylene glycol, and hexylene glycol) at 50% by weight were measured and are listed in Table IV. The specific gravity of glycerin solution is lower than the calculated value (average of two components), which indicates a slight increase in volume after glycerin and water are mixed. The volume increase might be related to the difference in intermolecular interactions between water mol-