Variation in nitroxide probe chain flexibility within sodium dodecyl sulfate hemimicelles

Abstract

Three nitroxide spin probes (5-, 12-, and 16-doxylstearic acid) were used to investigate the structure of sodium dodecyl sulfate (SDS) micelles and the interfacial layer between water and alumina formed by SDS (hemimicelles of SDS). It was found that whereas the rotational correlation times of the three probes differ in SDS micelles, the effective microviscosities of the probes are the same. In contrast, the microviscosity within the SDS hemimicelles varies according to the distance from the alumina surface: the nitroxide sees a more ordered environment as it is placed closer to the alumina. This represents the first report of variations in flexibility (microviscosity) within a hemimicelle. The science of the water-solid interfacial layer formed by adsorbed long-chain surfactants on solids has been studied extensively for its value in the fields of enhanced oil recovery,2 fl~tation,~ detergen~y,~ l ~brication,~ and microelectronics.6 Considerable experimental evidence is consistent with the postulate that such surfactants form localized aggregates (termed hemimicelles) on the solid surface.’ The evidence for hemimicelles has generally been indirect, and hemimicelle structure and dynamics have been inferred from measurements of bulk properties using such techniques as adsorption isotherms, {potentials, particle wettability, and heats of adsorption.8 Recently we reported the use of two types of molecular probes for use in spectroscopic investigations of the hemimicelle microstructure formed by sodium dodecyl sulfate (SDS) adsorbed on alumina. The first involved studies of pyrene and dinaphthylpropane fl~orescence,~ while the second involved the spin probe 16-doxylstearic acid (l), a stable nitroxide radical, for use with ESR spectroscopy.i0 The carboxylate functionality of this latter probe adsorbs on the positively charged alumina surface. Our results indicated that at pH 6.5 this probe aggregates mostly by itself on the surface at low SDS concentrations and forms cohemimicelles with the SDS at higher SDS concentrations. The relative anisotropy observed in an ESR spectrum is directly related to the rotational mobility of the probe, a term that can be correlated with the probe’s microviscosity.” Using this correlation, we were able to deduce that the environment within the SDS hemimicelle is relatively viscous. In both these studies of the microenvironment of the SDS hemimicelle, the data reported represent a single probe position within the hemimicelle (or a time average of many depths from the surface). Spin probes have been used extensively to study the microviscosity and micropolarity of membranes, multilayers, and micelles.12 In particular, doxylstearic acid spin probes have been useful in determining these parameters for different probe depths within phospholipid bilayers and micelles. Such studies have provided information on the orientation of the probes within the systems as well as information on the microenvironmental differences within the bilayer or micelle. In this communication, we wish to report the use of three doxylstearic acid derivatives, 16-, 12-, and 5-doxylstearic acids (1, 2, and 3, respectively) in