Abstract
The artificial sweetener, aspartame, is a dipeptide, consisting of a hydrophobic phenylalanine methyl ester entity and a hydrophilic aspartyl residue. In this work, its solubilization in five different types of water in oil (w/o) microemulsions (MEs) was investigated. The stabilizing surfactants and cosurfactants of the MEs were ME 1, sodium di-2-ethylhexyl sulfosuccinate (AOT) and no cosurfactant; ME 2, maleic anhydride α-olefin copolymer and dimethyl amino ethanol; ME 3, sucrose ester monostearate and n-butanol; ME 4, l-α-phosphatidylcholine and n-butanol; and ME 5, mono- and diglycerides of fatty acids and l-α-phosphatidylcholine. The maximum amount that could be dissolved (boundary concentration) was determined by adding powdered aspartame to a heated ME and cooling to a specified temperature under controlled conditions. The solutions that remained clear for at least 4 days were regarded as stable, and those of the stable solutions with the highest aspartame concentrations were taken as having the boundary concentrations. From the solubility data, the distribution of the aspartame molecules between the w/o interface and the water pools was calculated, and the results were correlated with the molecular structure and ionic state of the surfactant. The results show that aspartame can be solubilized in all the investigated MEs to an extent, exceeding by far its solubility in pure water, and that overall solubilization is most efficient in water/isooctane MEs stabilized with AOT (ME 1). While the aspartame solubility in the water pools was comparable in all the investigated MEs, the aspartame/surfactant molar ratio at the w/o interface was found to decrease with decreasing polarity of the stabilizing surfactant at the interface. In addition to the solubilization studies, the effect of aspartame on some properties of ME 1 was investigated. It was found that aspartame lowers the interfacial tension at the water/isooctane/AOT interface and, under certain conditions, stabilizes unstable mixtures of water, isooctane, and AOT. The results of small-angle X-ray scattering measurements show that the ME droplet sizes increase in the presence of aspartame molecules and their shapes change from near spherical to ellipsoidal. It was concluded that aspartame acts as a cosurfactant in water/isooctane MEs stabilized with AOT.
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