(520c) NMR Investigation of Surfactant Structure Effects On Mixing in Cationic/Carbohydrate Surfactants

Mixtures of cationic and non-ionic surfactants have recently been the subject of a number of investigations due to the synergistic effects that arise in their mixing and properties.  In these mixed micelles, the molecular interactions between the surfactants plays a determining role in micellization thermodynamics and therefore characteristics such as critical micelle concentration (cmc), micelle size, aggregation number, etc.  Although several investigations have reported global thermodynamics of mixed surfactants with the help of a regular solution theory-based interaction parameter (β) (see for instance the review of Ruiz in Sugar-Based Surfactants: Fundamentals and Applications, Surfactant Science Series vol. 143, CRC Press: 2009, p. 413), this parameter is only an indicator of net enthalpy changes associated with mixing in micelles.  The relative positions of the surfactants in the mixed micelle and the interaction sites responsible for observed enthalpy changes in cationic-non-ionic surfactants are not as thoroughly understood.

Here, a variety of NMR (nuclear magnetic resonance) techniques are applied to gain insight into the relative positions and the interaction sites between alkyltrimethylammonium-based cationic and carbohydrate-based (non-ionic) alkyl ether surfactants in mixed micelles assembled in D₂O (deuterated water).  Our main objective is to determine combinations of surfactants in which the sugar head group is positioned slightly outside of the micelle so that there is a good chance that this surfactant combination can be used in future experiments to provide good interactions with a metal oxide matrix for molecular imprinting.  We use spin-spin relaxation measurements (T₂ analysis) and nuclear Overhauser effect spectroscopy (1D NOE spectra and 2D NOESY) for this purpose. Effects of carbohydrate head group hydrophilicity (gluopyranoside vs. xylopyranoside) and alkyl tail length are investigated. The NOESY evidence suggests that in mixed micelles of n-octyl-b-D-glucopyranoside (C8G1) and hexadecyltrimethylammonium bromide (C₁₆TAB), the glucose headgroup is located just inside of the corona region of the micelle, and therefore that it is not exposed to the external solution.  Trends in the T₂ values and NOE spectra indicate that changing the headgroup to the less hydrophilic xylopyranoside exacerbates the problem by driving the headgroup more deeply into the core of the mixed micelle.  We also study the effects of the relative lengths of the alkyl tails in the cationic and carbohydrate surfactants.  As the chain length of the cationic surfactant decreases, the favorable interaction with the sugar headgroup found for the C8G1 / C_nTAB mixture is known to decrease [Ruiz]. Our NMR results are consistent with the carbohydrate being “pushed” outwards towards the corona of the micelle as this occurs, although in extreme cases (such as when the length of the cationic surfactant tail is less than that of the carbohydrate surfactant) this may have the undesirable consequence of reducing the level of mixing between the two surfactants in micelles.