Statistical Partitioning of Oil Phase Composition and Oil-to-Water Ratio Effects on Cream Emulsion Rheology

Creams are formulated by adjusting the oil-phase composition and the oil-to-water ratio, but the relative importance of these coupled variables is rarely quantified. We varied the liquid paraffin:white petrolatum ratio (P:V, 0:4 to 4:0) and the oil:water mass ratio (O:W, 1:8 to 8:1) in a 5 × 5 factorial design with a fixed cetostearyl alcohol/cetomacrogol emulsifier system. Twenty-one compositions formed creams. The four combining high paraffin and high oil contents did not, and this boundary was treated as a separate outcome. Microstructure and rheology indicated O/W creams at O:W ≤ 2:7, a transitional region at 4.5:4.5 and W/O creams at O:W ≥ 7:2. Viscosity was compared at 1 s–1, a shear rate typical of dispensing and gravity-driven flow that lies on the stable flow branch of every cream. Two-way ANOVA on all 21 creams attributed 64% of the variance in viscosity and 50% of that in log G′ uniquely to the oil:water ratio, compared with 8% and 14% to oil-phase composition, whose effect depended on oil content. This dominance reflects control of the continuous phase rather than droplet crowding. In O/W creams, G′ (0.7–1.4 kPa) was nearly independent of both factors, consistent with an aqueous lamellar gel network. In W/O creams, the petrolatum-structured oil phase governed the response, and replacing 25% of the petrolatum with liquid paraffin lowered G′ about 10-fold. Oil-phase composition is therefore a regime-specific modifier that also determines whether a W/O cream can form, which provides a practical basis for cream formulation.