Effect of composition on empirical stability trends for oil-in-water emulsions
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University of Cape Town
Relative stabilities of paraffin oil-in-water emulsions were determined empirically as a function of increasing ionic strength, surfactant type or concentration, droplet size, pH and calcium(II) concentration. Stabilities were measured by droplet size changes, planar interface oil droplet coalescence times, creaming rates and photography. Stability trends between methods were compared. Conclusions were derived in terms of creaming and coagulation. The trends below were discussed in terms of theories of emulsion stability. 1. Increased surfactant concentrations stabilised the emulsions against creaming. Coalescence trends were complex: an optimal cetylpyridinium chloride concentration stabilised the emulsions. Beyond that concentration, stability was reduced. Because of its low HLB, increased sorbitan sesquioleate concentrations destabilised emulsions towards coalescence. 2. Smaller droplets stabilised all the emulsions despite the increased polydispersity. 3. Increased ionic strengths accelerated creaming. Coalescence was faster for cetylpyridinium chloride because of reduced droplet repulsion. Sodium dodecyl sulphate resisted coalescence at all ionic strengths due to the restabilisation predicted by the Stochastic model. 4. pH did not affect emulsions containing a pH-stable surfactant (sodium dodecyl sulphate). Emulsion stability was reduced with acid- or base-labile labile surfactants (sorbitan sesquioleate, cetylpyridinium chloride) due to reduced ability to lower the surface tension during emulsification, or hydrolysis. 5. Ca⁺² destabilised emulsions containing anionic surfadants (sodium dodecyl sulphate and laurate) by co-ordination, but had little effect on a cationic emulsifier (cetylpyridinium chloride) to which it did not co-ordinate. The destabilisation of anionic-based emulsions was due to the formation of oil-wettable solid salts and the removal of the o/w surfactant. 6. Low stabilities of sorbitan sesquioleate-based emulsions were attributed to Bancroft's rule and the low hydrophile-lipophile balance of sorbitan sesquioleate.
Wilson, K. 1996. Effect of composition on empirical stability trends for oil-in-water emulsions. University of Cape Town.