]. Cosmet. Sci., 59, 1-14 Oanuary/February 2008) Determination of optimal dead sea salt content in a cosmetic emulsion using rheology and stability measurements BASIM ABU-JDAYIL, Chemical & Petroleum Engineering Department, United Arab Emirates University, Al Ain, United Arab Emirates and HAZIM A. MOHAMEED and ABEER BSOUL, Department of Chemical Engineering, Jordan University of Science and Technology, 2 2110 Irbid, Jordan. Accepted for publication August 15, 2007. Synopsis Dead Sea mud and salts are known for their therapeutic and cosmetic properties. The presence of Dead Sea (DS) salts in different types of cosmetics has affected the stability and the flow properties of the finished products. In this study, an attempt was made to find the optimum Dead Sea salt content in a cosmetic emulsion (model of body cream) using both rheology and stability measurements. The rheological properties were tested during a four-month storage period at three different storage temperatures: 8 ° C, room tem­ perature, and 45 ° C. In addition to rheological measurements and centrifuge tests, the conductivities of the emulsion samples were also determined. The centrifuge tests showed that the cream samples containing more than 0.25 wt% of DS salt showed phase separation. The addition of DS salt to the cosmetic emulsion led to two maxima in the emulsion viscosity at salt contents of 0.07 wt% and 0.15 wt%. However, the emulsion samples containing 0.15 % of DS salt was considered the optimum sample since it contained the maximum amount of salt and exhibited the maximum viscosity at all tested conditions. It was found that the viscosity of the emulsion is increased with storage time and storage temperature. This behavior was accompanied by a decrease in conductivity. This behavior was explained by water evaporation from the emulsion. However, it has been shown that the presence of DS salt in the cosmetic emulsion significantly reduces the rate of water evaporation. The conductivity measurements reflect the rate of water evaporation, and the presence of DS salt reduces the rate of conductivity. Conductivity is observed to decrease with storage time and temperature. INTRODUCTION In Jordan, a growing industry was established to produce different types of cosmetics. Cosmetics that have Dead Sea salts and/or mud in their formulas are the main products of this industry. The Dead Sea is known worldwide to be one of the world's richest sources of salts and minerals. But the presence of Dead Sea salts in various cosmetics has resulted in many problems during the production and in the stability of the end products. A typical problem is the maximum amount of Dead Sea salt that can be added 1

2 JOURNAL OF COSMETIC SCIENCE to the product without affecting stability and consistency. Phase separation is another problem observed in many samples of locally produced Dead Sea creams and mud masks. Many investigators have reported on the close relationship of emulsion rheology and stability relative to several structural parameters (1-5). Rheological techniques are pow­ erful tools to study the behavior of cosmetic emulsions. Changes in the rheological properties of cosmetic emulsions represent an important early warning of the impending failure of the product (3 ). Rheological measurements are now required in various pharmaceutical and cosmetic industries including, but not limited to (a) quality control, (b) storage stability under various weather and transportation conditions, (c) correlation with sensory assessment and consumer evaluation, (d) the effect of formulation on consistency, and (e) prediction of flow behavior under manufacturing or production environment conditions (e.g., pumping, mixing, milling, and packaging). Various cosmetic products have different rheological behaviors. For example, body creams require a high viscosity at rest in order to stay in the hands of the consumer while being taken out of the bottle, but on the other hand, a subsequent shear thinning behavior is required for the ease of spreading and applying the creams onto the skin (6). Modern Dead Sea cosmetics have been developed to meet the demands of new regula­ tions, technical opportunities, and today's consumer expectation for higher quality standards and proven performance. As an example of the application of this approach, Maor et al. (7) describe the development of new cosmetics formulations, based on "osmoter," a special Dead Sea mineral composition, and the evaluation of this formu­ lation effect on the depth of skin wrinkles by a controlled assay. In this study, body cream (a cosmetic emulsion model) containing Dead Sea salt was compounded using the conventional techniques in emulsion preparation. The addition of salt(s) to the emulsion resulted in a significant change in rheological behavior and stability. For this reason, rheological and stability measurements were used to evaluate the prepared cosmetic emulsion in order to find the optimum content of Dead Sea salt that can produce a stable cream with maximum viscosity. MATERIALS AND METHODS MATERIALS The components used in the cream formulation were divided into three groups: water phase, oil phase, and preservative. Oil-phase components were lanolin (pastel) consisting mainly of sterol (C27H45OH), petrolatum (white petroleum), paraffin oil, cetearyl al­ cohol (stied), sodium cetearyl sulphate (lan N), and glycerin stearate. The oil-phase components contribute to 24.1 % (wt%) of the total components used in the formulation of the body cream. The water-phase components were propylene glycol (PG), glycerin, Dead Sea salt, and RO water. The preservatives used were 5-chloro-2-methyl 4-iso thiazolin-3-one, 2-methyl 4-iso thiazolin-3-one, propyl paraben, and methyl paraben. Table I shows the percentages of the components used in the formulation of the body cream. The materials used for the body cream formulation were supplied by Ammon Dead Sea Co. (Amman, Jordan), which in turn were exported from Henkle Co. (Ger­ many). The Dead Sea salt consists mainly ofK+ (114500 mg/kg), Mg + 2 (81200 mg/kg),