Abstract:

Micelles, a kind of surfactant aggregate formed in water, may be swollen to a generally limited extent upon addition of a liquid hydrophobic compound. Swollen micelles have attracted considerable research attention because they can enhance the solubility of the said hydrophobic compound. The development of swollen micelles is of significant interest in terms of both scientific and industrial applications, such as drug delivery, oil recovery, and soil remediation. While there have been many studies focusing on micellar solubilization, several questions remain unanswered: the capacity to quantitatively solubilize the drug in drug delivery, the interaction between micelles and non-polar oil when microemulsions are not formed, and the differences and similarities between swollen micelles and microemulsions. Comprehensive understanding of and insight into swollen micelles will be helpful to tailor surfactants for industrial applications. Herein, we reviewed the recent progress in the field of swollen micelles in terms of solubilization capacity, solubilization site, micellar morphology, etc. First, the UV spectrophotometry results demonstrate that the solubilization capacity of micelles is related to their molecular structures and surfactant properties. The solubilization is also dependent on the composition and nature of the hydrophobic compounds, the presence of electrolytes, temperature, etc. Second, the solubilization site may be located in the micellar core, the palisade layer of the micelle, the micelle surface, or the hydrophilic shell of the micelle, depending on the property of the solubilized compounds and the morphology of the micelles. In general, the micellar aggregation number increases with increasing oil concentration; high concentration of oil causes the formation of spherical micelles, while high concentration of oil results in ellipsoidal micelles. Furthermore, the micellar size increased gradually with increasing oil concentrations. Finally, the differences and similarities between swollen micelles and microemulsions were clarified. It is believed that microemulsions can be considered as swollen micelles, but there has been some strong evidence that differentiates swollen micelles and microemulsions. Based on our results, we believe that microemulsions can be considered as swollen micelles, but all micellar solutions cannot be swollen to the extent of microemulsions, unless the specific structural requirements and conditions are satisfied. Overall, understanding the properties of swollen micelles and how they transform to microemulsions not only provide theoretical support for practical applications of surfactants, but can also be used to design new surfactants.

Key words: Surfactant, Swollen micelle, Microemulsion, Solubilization