Investigating the effect of transcutol on the physical properties of an O/W cream

Topical cream product development typically requires screening of excipients and emulsifiers to kinetically stabilize otherwise thermodynamically metastable systems. The conventional concepts of hydrophilic-lipophilic balance for selecting emulsifiers often work with mixtures of oil and water phases. However, in the presence of polar organic solvents, other methodologies must be employed for formulation selection during the development process. This work demonstrates how tensiometry can be used along with other characterization techniques to elucidate failure mechanisms in oil in water (O/W) creams. Emulsion systems were prepared with varying amounts of the co-solvent, Transcutol® P (TP). Physical characterization by optical microscopy, LUMiSizer® and DSC identified a critical concentration of Transcutol® P (>25 wt. %) at which the microstructure, melting temperature, and relative stability of the cream changed significantly. Interfacial tension (IFT) values were also found to decrease with increasing amount of Transcutol® P in the absence of emulsifiers, indicating an increase in miscibility and the formation of a one phase system. At higher levels of Transcutol® P (40% w/w), decreasing the emulsifier content did not reduce droplet size. These experiments demonstrated that a minimum inherent immiscibility between the emulsion phases in the absence of emulsifiers is required for a stable two-phase emulsion system. We show here how the stability of a complex, polar oil – polar solvent emulsion can be determined by a combination of factors including droplet size/distribution, melting temperature, apparent viscosity, and IFT.