Abstract
The compounds containing nitroxyl radical (NO˙) are actively used as spin probes to examine colloid systems, including lipid membranes and micelles. Their electron paramagnetic resonance spectrum provides information about the composition of the medium, in particular, the content of water there. Yet, the proper treatment of the measurement results demands understanding the microscopic characteristics of the molecular probe. In the present paper, we extend our previous studies on the microscopic state of acid-base and solvatochromic probes in surfactant micelles to the field of spin probes. We report the results of molecular dynamics simulation of a common spin probe, methyl-5-doxylstearate, in micelles of anionic (sodium n-dodecyl sulfate, SDS) and cationic (n-dodecyltrimethylammonium bromide, DTAB) surfactants. The localization of the molecule within the micelles, its shape, composition of the local environment, hydration were quantified and compared with the available relevant experimental data. No significant dissimilarity was found in the characteristics of the probe molecule in both kinds of micelles. However, the characteristics of the O˙ atom carrying the unpaired electron are pronouncedly different, namely, in DTAB micelles it is less hydrated and forms less hydrogen bonds with water. Similar situation was observed for the COO group. The main reason was found to be the interactions with cationic surfactant headgroups, which screen the O˙ atom and COO group from water. These findings allowed revisit the point of view that the surface layer of DTAB micelles as a whole is less hydrated in comparison to that of the SDS ones.
Highlights
The structure of micelles of ionic surfactants and their surface layers is an area of active research by modern experimental methods
The distribution function of the distance (DFD)'s between micelle center of mass (COM) and S or N atoms of the surfactant headgroups were plotted to outline the location of the micelle surface
The localization of the molecule was examined and the hydration of the molecule and its moieties was calculated by counting water atoms in their microenvironments and counting hydrogen bonds made with water molecules
Summary
The structure of micelles of ionic surfactants and their surface layers is an area of active research by modern experimental methods. The properties include fluorescence spectrum (for fluorescence probes), apparent acidity constant (for acid-base probes), UV/vis absorption spectrum (for solvatochromic probes) and electron paramagnetic resonance spectrum (for spin probes). This change may be related to such micelle properties as microviscosity, electrostatic potential, polarity, hydration, respectively [1,2,3,4,5,6]. To establish this relation, the knowledge of the state of the molecular probe in the micelle is necessary.
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