Colloids and Surfaces A: Physicochemical and Engineering Aspects
Adsorption of hydroxypropyl methylcellulose at the liquid/liquid interface and the effect on emulsion stability
Introduction
Emulsions are thermodynamically unstable systems and tend to separate in order to minimize their interfacial area between the aqueous and the oil phase. When we refer to stable emulsions, we mean systems that show no significant changes during a certain time of observation, i.e. the rate of coalescence is sufficiently low. For the preparation of such stable emulsions, the use of emulsifiers is a necessary prerequisite. According to the IUPAC definition [1], emulsifiers are surfactants that stabilize emulsions by absorbing at the interface and thereby lowering the interfacial tension.
Hydroxypropyl methylcellulose (HPMC), a so-called polymeric emulsifier, is able to absorb at liquid interfaces and to lower the interfacial tension. The use of HPMC as an emulsifier for oil-in-water emulsions was first described by Daniels and Barta [2]. The formation of stabilizing interfacial layers requires, first, the diffusion of the polymer from the solution bulk to the interface and, second, its adsorption at the interface. Conformation of polymer adsorption layers may be described according to the train–loop–tail model [3]: Lipophilic segments along the polymer chain attach to the interface and form trains, which are separated by hydrophilic loops and tails, that extend into the aqueous phase. The properties of the interfacial layers, especially the ratio of train/loop and tail segments, and the elasticity of the layer, determine the emulsion stability, particularly the coalescence stability of the oil droplets [4].
The principal aims of this paper are: (1) characterization of the molecular dimensions of HPMC in aqueous solution using viscometry; (2) quantitative and qualitative evaluation of the adsorption process by dynamic interfacial tension measurements using axisymmetric drop shape analysis (ADSA); (3) determination of the thickness of the HPMC interfacial layers from emulsion viscosity; and (4) examination of the storage stability of emulsion samples using mainly the droplet size as a stability indicating parameter.
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Materials
HPMC 2208, HPMC 2906 and HPMC 2910 [5], [6] (Metolose 90 SH 100, 65 SH 50 and 60 SH 50, Shin Etsu; J-Tokyo) were used without further purification. Table 1 lists some main characteristics of the polymers: viscosity of 2 % w/w aqueous solutions (shear rate, D=10 s−1), percent of methyl- and hydroxypropyl content, and mean degree of methyl-substitution (DS), respectively, and mean molar degree of hydroxypropyl-substitution (MS) [7].
Medium-chain triglycerides (MCT) [5] (Miglyol 812, batch 960614;
Molecular weights and molecular dimensions
Mean molecular weights and dimensions of HPMC conformation in aqueous solution were calculated from the intrinsic viscosity, [η], of the polymer. The viscosity, η, of HPMC solutions and, therefore, [η] of HPMC depends primarily on the molecular weight and degree of polymerization; it is not influenced by the type of substitution (DS and MS) [11]. To obtain [η], the viscosity data were fitted to the Huggins equation [12], which was derived from the Einstein viscosity law [13], [14]. The Huggins
Conclusions
Measurements of the dynamic interfacial tension between aqueous solutions of HPMC and MCT indicate that the adsorption behavior of HPMC can be described by a diffusion-controlled model. The critical aggregation concentration for all HPMCs appears at approximately 2×10−7 mol l−1. Obviously, molecular weight and/or type of substitution have no substantial impact on the CAC. Compared with surfactants of lower molecular weight, the CAC is reached at much lower concentrations. The equilibrium
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