20 - ·en 10 ' a. a. 0 DEAD SEA SALTS IN COSMETIC EMULSION Period of storage = one week Storage condition : room temperature 0.00 0.05 0.10 0.15 Salt concentration (w/w % ) 0.20 5 20.65(1/s) 0.25 Figure 1. The effect of DS salt concentration on the apparent viscosity of cream samples stored for one week at room temperature and measured at three shear rates: ■ 7 .387 (11sec), • 20.65 (11sec), and -'. 40.32 (11sec). viscosity with increasing DS salt concentration from 0.0 to 0.05 wt% was observed, reaching the first minimum point at 0.05%. In the second region, the apparent viscosity increased with DS salt concentration to reach the first maximum in viscosity, which occurred at 0.07 wt% DS salt content. A sharp decrease in the apparent viscosity was then observed, reaching a minimum point at 0.08% DS salt concentration. A higher maximum point than that at 0.0 and 0.07 wt% in the apparent viscosity was reached at 0.15 wt% DS salt concentration. A decrease in the apparent viscosity was noticed in the fourth region when the DS salt concentration was increased from 0.15 to 0.25 wt%. Similar behavior was found for the samples subjected to different shear rates over the entire concentration range, as can be clearly seen in Figure 1. In order to be sure that the pervious variation of viscosity with salt concentration is real and not a reflection of experimental error, part of the samples was prepared in duplicate. As can be seen in Figure 2, the reproducibility of a sample preparation is very high where the maximum error in the apparent viscosity at a constant salt concentration is less than 4.0%. In addition, Figure 2 shows that experimental error did not affect the general trend of the apparent viscosity of cream samples with the salt concentration. Based on the function of sodium cetearyl sulfate (anionic emulsifier) used in this sys­ tem, the oil droplet surface will be surrounded by a negatively charged layer. This layer works as a bridge between the oil and water phases consequently, the electrostatic repulsion between oil droplets will prevent them from coalescence. Ionization to cations and anions occurs when the DS salt is added. The negatively charged ions (e.g., Cr)

6 10 - 8 "' �L II - 'in 6 "' · - 4 C: - � a. 2 - 0 I 0.00 JOURNAL OF COSMETIC SCIENCE I I 0.05 I :::::c:: I I =-== :::::c:: - I I I 0.10 0.15 I Storage time = one weak Storage conditions: room temp. Shear rate= 20.65 (1/s) I I I 0.20 Salt concentration (w/wo/o) J[ 0.25 Figure 2. Error bars of the apparent viscosity of the prepared creams measured at shear rate = 20.65 (11sec). move toward the negatively charged layer. This results in an increase in the ionic strength of the negatively charged layer and subsequently capturing more water mol­ ecules in the void space between the aggregated oil droplets. This may explain the increase in the viscosity of the cream upon the addition of salt. The appearance of maximum viscosity may be explained by the concentration at a certain salt level where a maximum capacity of the layer for the anionic charge is reached. As more DS salt is added, positively charged ions (e.g., Na+, Mg+2 ... ) start neutralizing some of the negative charge. This action results in a weaker bridge, resulting in water molecules with lower attraction. Thus, the oil droplets will have a greater tendency to coalesce, resulting in a reduction in the apparent oil-phase volume. The viscosity reduction of the body cream samples is the result of a decrease in the apparent phase volume of the dispersed droplets. The explanation for the appearance of two maxima in the apparent viscosity at 0.07 and 0.15 wt% DS salt concentration (see Figure 1) might be attributed to the presence of different ions (e.g., Mg +2 , Ca +2 , K+, Na+, ... ) with different ionic strength in the added DS salt. Silvander et al. (9) investigated the effects of electrolyte addition to a cosmetic emulsion. Monovalent sodium chloride had practically no influence on viscosity. Cal­ cium chloride, on the other hand, had a large impact on viscosity, even at low concen­ trations. The resulting increase in viscosity was due to flocculation that led to an increase in apparent phase volume. A similar behavior was obtained with magnesium chloride, with the difference that the maximum in viscosity was shifted to higher electrolyte concentrations. This was interpreted as such because magnesium bound more strongly to the hydration water than did calcium.