Microemulsions Based on Soybean Phosphatidylcholine and IsopropylmyristateEffect of Addition of Hydrophilic Surfactants

Abstract

This work reports the effect of the addition of n-alkyl β-D-maltosides (C n G 2 ), sucrose monododecanoate, and sodium taurocholate on the phase behavior and microstructure of a microemulsion based on water, 1-propanol, soybean phosphatidylcholine (SbPC), and isopropylmyristate (IPM). The self-diffusion coefficients of the components were determined with the pulsed field gradient NMR technique. The spontaneous curvature (H 0 ) of the surfactant monolayer separating the aqueous and oil domains was found to increase with an increasing proportion of hydrophilic surfactant for all the hydrophilic surfactants studied. The microstructure of the microemulsion, deduced from the self-diffusion coefficients and the conductivity measurements, changed from an oil-continuous structure to oil-swollen micelles through a bicontinuous structure. The general phase behavior could be described with a simple model where H 0 was assumed to vary linearly with the changing weight fraction of hydrophilic surfactant. The molar ratio of C n G 2 /SbPC needed to obtain a balanced microemulsion ∈bal was found to increase linearly with n for n ≥ 10. The ∈bal for C 12 G 2 was 0.71. There was no observed difference in ∈bal between sucrose monododecanoate and C 12 G 2 , but sucrose monododecanoate was found to increase the flexibility of the surfactant film to a greater extent than C 12 G 2 . Sodium taurocholate increased H 0 more effectively than C 12 G 2 (∈ bal = 0.22) but did not destabilize the surfactant film to the same degree.