Adsorption of hydroxypropyl methylcellulose at the liquid/liquid interface and the effect on emulsion stability

This paper investigates the physico-chemical properties of the aqueous solutions of three different types of hydroxypropyl methylcellulose (HPMC 2208, HPMC 2906 and HPMC 2910) and those of HPMC stabilized oil-in-water emulsions containing medium-chain triglycerides (MCT) as dispersed phase. The mean molecular weight and molecular dimensions of HPMC in aqueous solutions were calculated from the intrinsic viscosity, which was obtained by fitting viscosity data to the Huggins equation. The dynamic adsorption behavior of HPMC at the water/MCT interface has been studied using the axisymmetric drop shape analysis. Measurements of the dynamic interfacial tension give information on the structure of the adsorption layer. The equilibrium interfacial tension at the critical aggregation concentration depends on the HPMC substitution type, whereby HPMC 2910 shows the lowest values. The adsorption process can be described by a diffusion-controlled model. The emulsion viscosity versus phase volume relationship fits well to predicted values from the Krieger-Dougherty equation, indicating that HPMC-stabilized emulsions can be described with the hard-sphere model. The thickness of the adsorbed laver. calculated from the maximum phase volume fraction, decreases with increasing phase volume. The emulsion stability depends on HPMC substitution type. Emulsions stabilized by HPMC 2208 and HPMC 2906 as compared with HPMC 2910 show higher stability of the droplet size when stored in a temperature cycle test for 6 months. (C) 2000 Elsevier Science B.V. All rights reserved.