Effect of methylcellulose on the stability of oil-in-water emulsions

The objective of this study was to investigate how polymers used as auxiliary emulsifiers improve the stability of oil-in-water emulsions. One stable emulsion and three unstable emulsions were formulated with 30% mineral oil and an emulsifier blend of Tween 40 and Span(R) 20. The stable emulsion (SE) contained 2% emulsifier blend optimized for maximum stability. One unstable emulsion, UE1, was formulated to contain 0.5% of the same emulsifier blend as the SE formulation. Two unstable emulsions were formulated to contain an unbalanced emulsifier blend, one with excessive hydrophilic emulsifier (UE2) and one with excessive lipophilic emulsifier (UE3). A series of emulsions was prepared containing increasing amounts of methylcellulose for each base emulsion. Creaming and change in particle size were measured to evaluate stability. The addition of the polymer to the stable emulsion caused instability leading to creaming and eventual oil separation. This effect of the polymer was more pronounced in UE1 emulsions. However, the addition of the polymer improved the stability of the UE2 and UE3 series of emulsions. The polymer also caused a reduction in the particle size of UE3 emulsions and a proportionally larger increase in the viscosity of UE2 emulsions. These results suggest that (i) methylcellulose could act as a hydrophilic emulsifier only in the absence of Tween(R) 40, (ii) methylcellulose and Tween(R) 40 associate to form a complex and (iii) the concentration of Tween(R) 40 is the determining factor for the stability of emulsions. A model of the methylcellulose-Tween(R) 40 association and its effect at the mineral oil-water interface is proposed.