94 JOURNAL OF COSMETIC SCIENCE 50- 40- 3O 20 t0 -10 70- [• ABIL WE 09 %• ABIL + 25% gtycerine F77• ABIL + 40% glyce•ine ,•V•-• ABIL + 50% glycerine 15øC 40øC • DC Q2-5200 - •'• DC + 25% G • DC + 40% G ',,• DC + 50% G 15øC 40oc 60ø6 TEMPERATURE øC Figure 4. A. Percent evaporation of Abil WE 90 with different concentrations of glycerine (25 %, 40%, and 50%) during ten days at 15 ø, 40 ø, and 60øC. B. Percent evaporation of DC Q2-5200 with different concentrations of glycerine (25%, 40%, and 50%) during ten days at 15 ø, 40 ø, and 60øC. glycerine was 4% for Abil WE 09 and about 10% for DC Q2-5200. At 60øC, evapo- ration was 25% for Abil WE 09 and slightly higher for DC Q2-5200. FREEZING ASSAYS As described above for evaporation tests, the silicones and glycols were tested separately and as mixtures with different concentrations of glycol. The temperatures tested were 2 o, -10 ø, and -28øC.

BEHAVIOR OF SILICONE SURFACTANTS 95 After 24 hours at 2 ø and -10øC, none of the samples had frozen except PEG 400 alone, which froze after two hours at 2øC and after 30 minutes at - 10øC. At -28øC, all samples froze within five minutes. These results show that the type of glycol used in the mixture and the concentration of glycol had little effect on the freezing point of the two silicones tested here. RHEOLOGICAL STUDY Influence of temperature on viscosity. In a given fluid, shear stress (force per unit surface area required to produce sharing action, F = F/S) is related to shear rate • = dy/dt, a measure of the speed at which the intermediate layers move with respect to each other). New- tonian fluids are those in which shear rate (7) is directly proportional to shear stress (F), and its proportionality constant is defined as viscosity (qq). The reciprocal term (1/qq) is designated fluidity. The resulting rheogram is a straight line (10-13). •1 = F/• The viscosity of simple (i.e., pure) liquids (molecular solutions) is influenced by com- position, temperature, and pressure, increasing slowly with pressure and decreasing rapidly with temperature. The changes in viscosity with temperature are described by an equation similar to the Arrhenius formula: qq = Ae E/RT where A -- molecular weight-dependent constant, T = absolute temperature, E -- acti- vation energy for flow between molecules, and R = gas constant. At high temperature, hydrogen bonds break down, and E and qq decrease significantly (14). Our analyses showed that both surfactants apparently display newtonian behavior: vis- cosity remained constant across a relatively large range of values of shear stress (I') up to a point at which flow changed from laminar to turbulent. In both cases viscosity decreased as temperature increased (Figure 5). This was particularly evident in DC Q2-5200, in which "dynamic" (i.e., apparent) viscosity ranged from 4000 cP at 15øC to 850 cP at 50øC. In Abil WE 09 the effect of temperature was weaker: viscosity was 315 cP at 15øC and 125 cP at 50øC. At low shear rates viscosity was greater at higher temperatures (Figure 6A not so in Figure 6B). The difference in chemical composition in the two surfactants probably accounted for the difference in rheological behavior. At very low shear rates the behavior of both silicones, however, was not newtonian, as can be seen in Figure 6. This effect has been reported for a number of other compounds for example, the behavior of blood (15) at low shear rates is non-newtonian, whereas at high shear rates behavior becomes newtonian. The dependence of apparent viscosity on shear rate is shown in the absence (Figures 6A, 6B) and in the presence (Figures 7A, 7B) of added glycols. As observed, a newtonian behavior (constant viscosity) is found for shear rates above 0.85 s -1. However, for lower shear rates values a non-newtonian behavior is observed, and viscosity decreases when the shear rate is increased, reflecting the resistance of chemical groups to flow under shear stresses these groups are larger in DC Q2-5200 than in Abil WE 09, hence the larger viscosities found in the former case. When the shear rate is sufficiently high, the polymer