DEAD SEA SALTS IN COSMETIC EMULSION 7 An additional point of interest is the decrease in viscosity when the salt concentration increased from 0.0 to 0.05 wt%. This small effect is due to the decrease in electrostatic screening length and is often referred to as a secondary electroviscous effect (9). One the other hand, a decrease in the apparent viscosity of body cream samples with different DS salt concentrations was observed as the shear rate was increased (see Figure 1). This implies that the cream behaves like a shear thinning material. Figures 3 and 4 show the effect of DS salt on apparent viscosity for samples stored at room temperature at different storage periods. It can be clearly observed that the apparent viscosity of cream samples increased with storage time and that this effect was more pronounced at high storage time (two and three months). In a similar study, Korhonen et al. (3) studied the effect of storage time on the rheological behavior of creams with four different surfactant combinations. They found that the viscosity (con­ sistency) of the cream decreased during storage, suggesting that other factors have affected our creams in the increase of viscosity with storage time. This phenomenon can be attributed to the evaporation of water from the emulsion. Evaporation of the water­ continuous phase causes compression of the emulsion, with progressive distortion of the oil drops and thinning of the water films separating them (10). To verify this result, the weight of two cream samples, one with DS salt and one salt-free, was observed. As can be seen in Figure 5, the weight of the cream samples decreases with storage time because of water evaporation. It is interesting to see in Figure 5 that the rate of water evaporation decreases in the presence of DS salt. This can explain the results presented in Figure 4, where the salt-free sample exhibits maximum viscosity after three months of storage. 60 --,-----------------.----------- Storage condition: room temperature Shear rate= 4.45 (1/s) storage time: - "' cti E!::., 40 � 'cii "' · � 20 C. C. EJ One week Two months 0 -1--- -r---.----.----,------,,------,,------,,------,,------,------, 0.00 0.05 0.10 0.15 Salt concentration (w/w % ) 0.20 0.25 Figure 3. The effect of DS salt concentration on the apparent viscosity of cream samples stored for different periods at room temperature.

8 JOURNAL OF COSMETIC SCIENCE Storage condition: room temperature 30 5 0.3 Shear rate= 12.37 (1/s) 100 Salt concentration (w/w % ) 0 0 Storage time (day) Figure 4. The effects of DS salt concentration and storage time on the apparent viscosity of cream samples stored at room temperature. rn rn 2.00 -.--------------,----.--------r------.-------, 1.50 Storage conditions: room temperature Salt content= 0.0% Salt content= 0.15 % _9 1.00 ---""----------1--------1----�::.......J....-----+------+-------I 0.00 -1£....------t-----+------+-----+------+-------I 0 40 80 120 Storage Time (day) Figure 5. Weight losses in cream samples as a function of storage time.