Purpose: To determine the influence of the solubilization of various polar oils used in topical formulations on the apparent molal volumes of a non-ionic surfactant derived from castor oil.
Methods: The polar oils studied included isopropyl myristate (IPM), neopentyl glycol diethylhexanoate (NGDO), diisopropyl sebacate (DIS) and diisopropyl adipate (DIA). Polyoxyl 40 hydrogenated castor oil (Kolliphor® RH 40) was the model surfactant with diethylene monoethyl ether (DEG), ethanol and 2-propanol being the co-surfactants. The densities of the microemulsions with and without the oils in various co-surfactants were measured using the pycnometric method at 26 ˚C from which the apparent molal volumes were calculated. The locations of the oils within the microemulsions were determined by UV spectroscopy.
Results: The apparent molal volumes of Kolliphor® were independent of the surfactant concentration and the co-surfactant/water proportion with ethanol and 2-propanol at the range in which microemulsions formed. At infinite dilution, the apparent molal volumes of the micellar surfactants were statistically the same (p<0.05) with all three co-surfactants (2300 ± 50 ml/mol). The addition of all oils to the system resulted in an increase in the micellar surfactant volumes depending on the amount solubilized. The effect of the co-surfactant on the magnitude of the change in the apparent molal surfactant volumes were as follows: DEG>2-propanol>ethanol. UV studies showed that in systems with DEG, the oils were solubilized in regions closest to the hydrophobic core while in systems with ethanol and 2-propanol, the oils were located in the palisade layer.
Conclusion: The combination of the density and UV data show that the solubilization of polar oils by the Kolliphor® resulted in an increase in the size of the microemulsion which was related to the location of the solubilized oil. In systems where the oils were located closest to the hydrophobic core, the changes in the apparent molal volumes were the greatest due to pooling of the oil in the core. The differences in the results with ethanol and 2-propanol is most likely due to 2-propanol, as a branched-chain alcohol, further increasing the spacing between surfactant molecules compared to ethanol.