Multiple W/O/W emulsions—Using the required HLB for emulsifier evaluation

Stable emulsions are best formulated with emulsifiers or combinations of emulsifiers, which possess HLB values close to the required HLB of the oil phase. In this work, we have investigated the application of this established method to the development of multiple emulsions. This is of particular interest, since multiple emulsions are highly sensitive in terms of variations of the individual components as a result of the presence of two thermodynamically unstable interfaces. However, multiple W/O/W emulsions are potential skin delivery systems for water-soluble active pharmaceutical ingredients as a result of their pronounced encapsulation properties. Firstly, a suitable primary emulsion was developed based on required HLB determinations of the investigated oils. Secondly and based on the required HLB, multiple W/O/W emulsions were developed using the most appropriate primary emulsion and 1% of hydrophilic emulsifier blends in order to stabilise the second interface. In order to find the appropriate mixtures of hydrophilic emulsifiers, the required HLB for the primary W/O emulsion was determined using two different chemical classes of emulsifier blends, i.e. polyethoxylated ethers and polyethoxylated esters. The physicochemical parameters of the formulations were characterised by means of rheological measurements, droplet size and creaming volume observations as well as by means of conductivity analysis. As discovered, all methods are appropriate for determining the required HLB determination with the exception of the rheological data. Referring to the primary emulsions tested, required HLB values of 4.3–4.7 using paraffin as the oil phase resulted in stable emulsions. Irrespective of the emulsifiers used, the finest droplets, lowest conductivity and minimal creaming volume were obtained for the multiple emulsions at required HLB values between 15 and 15.5 using paraffin as the oil phase. What is more, using a polyethoxylated ether instead of a polyethoxylated fatty acid ester resulted in more stable multiple emulsions.