A Thermotropic Phase Transition in Polyelectrolyte−Surfactant Complexes As Characterized by Deuterium NMR

Abstract

Deuterium nuclear magnetic resonance (2H NMR) spectroscopy of mixed cationic plus zwitterionic surfactant micelles exposed to the anionic polyelectrolyte poly(acrylic acid) (PACA) has been used to detect a thermotropic transition undergone in the phase-separated stoichiometric cation:anion complexes so formed. The deuteron labels were located on the quaternary methyls of either the cationic surfactant cetyltrimethylammonium bromide (CTAB-γ-d 9) or the zwitterionic surfactant hexadecylphosphocholine (HDPC-γ-d 6). The 2H NMR spectra of CTAB-γ-d 9 consisted of a superposition of two Pake doublet spectral components, indicating the presence of two surfactant populations undergoing different types of anisotropic motional averaging. With decreasing temperature, and over a narrow temperature range, the Pake doublet with the larger quadrupolar splitting increased in intensity at the expense of that with the smaller quadrupolar splitting, demonstrating the occurrence of a highly cooperative thermotropic transition between two states represented by the two Pake doublets. This thermotropic transition was manifested by CTAB-γ-d 9 but not by HDPC-γ-d 6, attesting to its origin in the electrostatic interactions between the cationic and anionic species. The midpoint temperature of this transition decreased with increasing CTAB:HDPC ratio of the surfactant micelles. The 2H NMR T1 relaxation time of both Pake doublet spectral components increased with increasing temperature but was a factor of 2 shorter in the Pake doublet with the larger quadrupolar splitting, indicating that the molecular motions contributing to relaxation were fast on the 2H NMR time scale but hindered in the low-temperature versus high-temperature states. The overall results are consistent with a thermotropic transition within the PACA/CTAB/HDPC complexes from a high-temperature normal hexagonal (HI) liquid crystalline phase to a low-temperature lamellar (Lα) liquid crystalline phase.

Intermolecular Interactions in Polyelectrolyte and Surfactant Complexes in Solution

“…When enough surfactant is present to reach the charge-neutralization point (at or near a 1:1 molar charge ratio), all the free charges on the polyelectrolyte are neutralized by the charged surfactant heads and this is the charge neutralization concentration (CNC) [77]. This leads to the formation of precipitates, ordered structures, and gels that phase separate [9,39,59,101,102,103]. Increasing the surfactant concentration further allows the hydrophobic surfactant tails to interface with the now neutral precipitate and the charged heads to interact with water.…”

Nonstoichiometric polymer-surfactant complexes obtained in a lamellar lyotropic medium

“…The type of structures formed depends on factors such as the global composition [12], the length of the surfactant alkyl chain [13], the length of the polymer chain [14], the charge density of the polyelectrolyte [15], or the presence of cosurfactants [16]. The composition of these complexes is usually assumed to be stoichiometric with equal amounts of the two species of opposite charge [3,6,[17][18][19], but in most of the systems studied, their detailed stoichiometry has not been characterized, and in a few cases, nonstoichiometric complexes have also been formed [20,21].…”

CO2-Induced Microstructure Transition of Surfactant in Aqueous Solution: Insight from Molecular Dynamics Simulation

“…19 Macdonald et al reported a thermotropic phase transition in polyelectrolyte-surfactant system of PACA/CTAB/HDPC, in which the hexagonal liquid crystal phase (H I ) was found to change into lamellar bilayer (LR) by decreasing temperature. 20 Wang studied the influence of pH on the microstructures of surfactant bis(amidoethyl-carbamoylethyl) octadecylamine in aqueous solution. The surfactant was observed to form micelles, vesicle, and lamellar structure at pH ) 2.0, 6.8, and 12.0, respectively.…”