Study of Electrostatic Interaction between Charged Surfactant Vesicles and Ionic Molecules by Bulk and Fluorescence Correlation Spectroscopy Measurements

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Phospholipid vesicles (Liposomes) for controlled release applications such as drug and gene delivery have attracted great interest. However, lack of long term stability and low solute encapsulation efficiency limit the usage of liposomes in many areas. In this thesis, charged surfactant vesicles that are formed spontaneously in mixtures of single-tailed surfactants are investigated as an alternative for liposomes in applications where improved long-term capture of charged organic molecules is desirable. The system of interest is dilute solutions of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS).

It is shown that charged surfactant vesicles have a high efficiency for encapsulating oppositely charged probe molecules with extremely slow release rates. Several probe molecules, both anionic and cationic, were studied including the cancer chemotherapeutic drug doxorubicin (Dox). All probe molecules were captured at high efficiency (ca. 20-70%) when the vesicle bilayer was of opposite charge from the probe molecule; when the charge of vesicle and probe molecule was the same, encapsulation was diminished (ca. 0-8%). Strong electrostatic interaction between surfactant vesicles and charged molecules are responsible for the extremely high encapsulation efficiency. The vesicle/probe formulations are stable for weeks to months due to the inherent stability of these vesicles which form spontaneously and are believed to be equilibrium structures. These properties allow surfactant vesicles to be used to selectively separate oppositely charged dye molecules, and this is demonstrated.

Fluorescence correlation spectroscopy (FCS) was used to gain deeper understanding into the role of electrostatics in the capture of charged probe molecules by charged surfactant vesicles. FCS measures the diffusion of fluorescent probe molecules in aqueous solutions at very low concentrations (10-9-10-8 M) and distinguishes between rapidly diffusing single molecules and slowly diffusing molecules that are adsorbed on a vesicle bilayer. This method is sensitive enough to rapidly determine the fraction of probe molecules bound to the bilayer interface in a given sample. Binding isotherms were constructed from FCS measurements in which a series of solutions were measured by holding the dye concentration constant while increasing the vesicle concentrations. The resulting isotherm yields a measure of binding energy. Comparisons of binding energies show that probe/bilayer interactions are mainly governed by charge-charge interactions but may also depend on the size and structure of the surfactant counter ions.

Our findings provide useful guidelines for implementing surfactant vesicles in biotechnological applications and also serve as an intriguing example of charge-mediated bilayer interactions.