Charge Transfer Interactions in Micelles and Vesicles. Inter‐ and Intramolecular Probes of Solubilization Site Polarity

Abstract

Abstract. A study of intra-and intermolecular charge transfer interactions with organic reagents which can be solubilized in micelles and vesicles is reported. The first part of the paper details the study on the formation of intermolecular charge transfer complexes between aromatic hydrocarbons ("enes") and the methyl viologen dication; these complexes show a weak charge transfer transition which is dependent on solvent polarity. A study of the charge transfer transition in homogeneous solutions and microheterogeneous media shows that the complexes occur for micelles at a reasonably polar "interfacial" site. A quantitative study of complex formation for several different substrates and different surfactant media reveals that the precise micropolarity for various combinations varies and hence indicates that the solubilization site provided for a given set of reagents in a given detergent is variable and cannot be too precisely defined. Studies of two polyenes containing electron donor and electron acceptor substituents within the same molecule have also been carried out. In the case of 4-nitro-4'-methoxystilbene (NMS) it has been found that the triplet-triplet absorption spectrum observed following laser flash excitation is sensitive to both solvent polarity and viscosity; in micelles, NMS exhibits a single moderately polar solubilization site in good agreement with that reported for the aforementioned charge transfer complexes. In contrast, it has been found that vesicles solubilize NMS in two distinctively different solubilization sites. One of the solubilization sites is attributed to an "interfacial" location while the other appears less polar but more viscous. Studies of the analogous diene, NND, have focused on its fluorescence and indicate the presence once again of a site which is sensitive to the hydrocarbon chains in bilayer vesicles. The hydrocarbon-sensitive site in vesicles for both NMS and NND could be associated with a "wedge"-shaped or cleft site in the vesicles which exists most prominently below the phase transition temperature of the vesicle.Micelles and vesicles have many properties in common; both are microheterogeneous media formed by selfassembly of surfactant or amphiphilic molecules in aqueous solution. The driving force for forming both types of assemblies is the so-called "hydrophobic effect." Additionally, both micelles and vesicles have the ability to solubilize a diverse array of reagents varying from inorganic ions to quite lipophilic organic molecules. Although there have been many investigations of the structures of micelles and vesicles by a variety of methods, both direct and indirect, considerable controversies exist due, at least in part, to frequent conflicts between the structural information inferred from different techniques.':" Some major persistent questions concerning micelles include the extent of chain order, the degree of water penetration of "wetness", and a whole range of uncertainties as to the nature of "solvent-solute" interactions when various substances are so...

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