340 JOURNAL OF COSMETIC SCIENCE CONCENTRATION DEPENDANCE OF CRITICAL STRESS MAY BE UTILIZED TO CHARACTERIZE GELS J. Mendoza•, 2 MS and M. Serpil Kislalioglu •, Ph.d. •Cosmetics and Personal Care Products Technology Program, College of Pharmacy, The University of Rhode Island, Kingston, R102881 and 2Chemex Chemical Manufacturing and Exporting Co., PO Box 1448, Tegucigapa, Honduras, C.A. INTRODUCTION In order to define concentration dependent rheological parameters, Bentonite RV ©, sodium carboxy methyl cellulose, methyl cellulose (Methocel A4C•), hydroxypropyl methyl cellulose (Methocel KM4•), pectin USP-100, Carbopol 971 NF ©, RLV•(lambda), VVllPF © (iota) and W711 • (kappa), carrageenans, guar gum U-NF and Water Lock (starch/acrylates/acryamide copolymers) A-100 ©, A-180 ©, DD-223 © and G-400 © were studied with oscillatory viscometry at nondestructive and destructive modes. The elastic (G'), viscous (G") and complex moduli (G*), complex viscosity (q*), strain (•,) and the phase angles (o0 were measured at the linear region where G' is independent of the shear stress. Critical stress ((•c) at (G') and (G") were also determined from the instruments output. EXPERIMENTAL Preparation of the gels: The gels, with exception of Carbopo1971 ©, were prepared by adding sterilized distilled water to the polymer powder to obtain the pre-selected (w/w) concentration. The mixtures were left for swelling for 24 hours. They were homogenized using a Fisher Scientific (Pittsburgh, PA) Dyna- mix stirrer at 1000 rpm for 1 hr. Carbopol 971 © was prepared with 1N NaOH neutralization. The concentrations used varied from 0.3% to 7.0% with no less than a 3 fold increment for each range. Five different concentrations per gel were uscck Rheological measurements: The measurements were taken within 2448 hours of preparation. The theological behavior of the gels was characterized at each concentration using a Boldin Instruments Rheometer Model CVO. A stainless steel, plate and plate spindle (number 4) with 1 nun gap was used at a strain range of 0.00075 - 15 nun and frequency of 0.05 Hz. Measurements were taken at 25 øC. RESULTS AND DISCUSSION Description and comparison of flow properties of the gels: The gel strengths varied from 0.001Pa to 7,600 Pa. The flow types were both shear thinning and shear thickening. With increasing stress, the G" of sodium carboxy methylcellulose, methylcellulose, pectin and guar gum decreased, whereas G" of Carbopol 971 © increased. The elastic nature of Carbopol 971 © was also obvious from low o[ (10 ø) at all concentrations studied (0.5 - 5.0%). Pectin demonstrated a gradual decline in G' that is characteristic of a low molecular weight polymer with a broad molecular weight distribution. The degree of sulfate esterification influences the viscoelastic properties of carmgeenans. At the same concentration, the o• of iota carmgeenan was almost 90 ø. It has a lesser degree of esterification than lambda carrageenan. Kappa earmgeenan, having the least degree of esterffication, demonstrated purely viscoelastic behavior at 1% concentration. Its o• is around 20 ø. Water Locks form solid gels with high degree of G' which was in the order of 500 - 1000 Pa at 1% concentrations. With exception of Water l_x)ck A-180 ©, they showed an increment in G" with increased stress, denoting dilatant behavior. Identification of concentration dependent theological parameters: The linear, exponential, logarithmic and power equations were tested as models to seek a relationship between the (G'), (G"), 01'), (G*), (,/) and (o0 at the linear region and ((•c) at (G') and (G") respectively, and concentration. The only parameter that provided a relationship with concentration in the gels studied was the (•, at (G') which can be written as ((•,=o• + s. C), where s is called the sensitivity index, Table I. The sensitivity indices given in Table I can be further categorized into three groups: In gels where • 10, the internal elasticity increases greatly with small increments in concentration. In gels with s=l-10, the concentration moderately influences internal elasticity. The gels with • 0.01-0.9 poorly respond to concentration changes. Sensitivity index may be useful for quick evaluation of the gels based on their concentration. The model proposed was applicable to gels of different chemical structures, molecular weights, molecular weight distributions, chain structures and viscosity types therefore, it may be considered as a tmiversal model to describe the gel strength with reference to concentration.

2000 ANNUAL SCIENTIFIC SEMINAR 341