DEAD SEA SALTS IN COSMETIC EMULSION 9 Aranberri et al. (10) found that in emulsion systems with added salt, water evaporation virtually ceased before all the water present was lost, probably as a result of oil-drop coalescence resulting in the formation of a water-impermeable oil film at the emulsion surface. A control experiment was conducted on a sample placed in a closed container where no change in weight was observed. In addition, the viscosity of this sample did not show significant change. This result supports our previous hypothesis that the increase in cream viscosity with storage time is caused by the evaporation of water from the emulsion. The samples containing 0.07 and 0.15% DS salt showed a maximum viscosity over the entire range of storage time tested. With the increase in storage time, the two maxima showed less increase in apparent viscosity than in the salt-free sample. EFFECT OF STORAGE TEMPERATURE In this part of our investigation the effect of storage conditions was studied. Body cream samples were stored up to four months at (i) room temperature, (ii) 45°C, and (iii) 8 ° C. The concentration of the Dead Sea salt varied between 0.0 and 0.15 wt%. Figure 6 shows the effect of storage temperature on the apparent viscosity of the cream samples stored for one week. An increase in the apparent viscosity at constant shear rate with increasing storage temperature can be clearly observed. The increase in the viscosity of -en 'ui en · � a. a. 20 18 16 14 12 10 ., ...... .. .. .. .. .. ' , , ______..- •,_EEi- .......---········ Storage time = one weak Shear rate= 7.387 (1/second) * EE ■ 8 °C room temperature 45 °C 8 ----l----�--�--.l,-------,--------,----'---r----,----------'j 0.00 0.04 0.08 0.12 Salt concentration (w/w%) 0.16 Figure 6. The effect of DS salt concentration on the apparent viscosity of cream samples stored for one week at different temperatures.

10 JOURNAL OF COSMETIC SCIENCE creams with increasing storage temperature may be due to the increase in the rate of water evaporation. With increasing storage temperature, the evaporation of water in­ creases, resulting in a viscosity increase. In a similar study, Korhonen et al. (3) found that the consistency of a cream containing a non-ionic surfactant, PG 25 soya sterol and sorbitan trioleate, increased with increasing storage temperature. At longer storage periods a clear effect of the storage temperature on the viscosity of the salt-free sample can be seen, where a significant increase in the apparent viscosity is observed (see Figure 7). The salt-free sample stored for two months at 45 ° C showed the highest viscosity beyond the viscosity range that can be measured by the viscometer. The effect of increasing the storage temperature on the viscosity of the salt-free sample was more pronounced than that observed for the salted sample. However, the salt-free samples showed a higher rate of increase in apparent viscosity with storage time than that observed for the salted samples. CONDUCTIVITY MEASUREMENTS The conductivity test is a rapid method for determining the stability of creams based on the change in the physical characteristics of the cream (11). Conductivity measurements of cream samples were performed at room temperature and over various storage periods up to four months. Conductivity values of the creams increased as the amount of added DS salt was in­ creased. This may reflect the higher level of free ions within the cream samples as the 40 35 u, 30 - ·en 25 u, · 20 a. 15 10 Storage time = 2 months Shear rate = 7.387 (1/second) * EB 0.00 ..... -·····•··.,, __ _ _ _ _ ___ __ :_:_� ■ II room temperature __ .. ----------m ... ...... .... ·rn.. __________________ 0.04 0.08 0.12 Salt concentration (w/w%) room temperature 45 ° c 0.16 Figure 7. The effect of DS salt concentration on the apparent viscosity of cream samples stored for two months at different temperatures.