Oppositely charged polymers and surfactants show a complex phase behavior with large regions of solubility and insolubility dependent on the concentrations of the species present. Here, a series of quaternized hydroxyethyl cellulose (cationic) polymers have been characterized by pulsed-gradient spin−echo NMR (PGSE-NMR) and electrophoretic NMR (eNMR) in simple aqueous (D 2 O) solutions and in combination with the oppositely charged (anionic) surfactant, sodium dodecyl sulfate (SDS). Analysis of the effective charge on the polymer derived from both the eNMR and PGSE-NMR results yields a readily interpretable insight into the polymer behavior; the effective charge on the polymer at infinite dilution shows a linear relationship with the degree of modification. On addition of low concentrations of SDS, typically C surf < 5 mM, the surfactant interacts with the charged polymers, leading to substantial changes in the dynamics of the system (polymer diffusion, viscosity). At these levels of surfactant addition, there is no macroscopic phase separation. Further, with the absence of an interaction with the parent, the uncharged polymer strongly suggests that the SDS only interacts with the charged moieties present on the functionalized side groups and not the polymer backbone. Ultimately, the charge on the soluble polymer/ surfactant complex was found to depend linearly on the level of surfactant binding across a series of polymers with differing levels of modification with the charge becoming effectively zero at the macroscopic phase separation boundary.
Coacervation is widely used in formulations to induce a beneficial character to the formulation but non-equilibrium effects are often manifest. Electrophoretic (eNMR), pulsedgradient spin-echo NMR (PGSE-NMR) and small-angle neutron scattering (SANS) have been used to quantify the interaction between low molecular cationic poly(diallyldimethylammonium chloride) (PDADMAC) and the anionic surfactant sodium dodecylsulphate (SDS) in aqueous solution as a model for the precursor state to such nonequilibrium processes. The NMR data show that within the low surfactant concentration onephase region, an increasing surfactant concentration leads to a reduction in the charge on the polymer and a collapse of its solution conformation, attaining minimum values coincident with the macroscopic phase separation boundary. Interpretation of the scattering data reveals how the rod-like polymer changes over the same surfactant concentration window, with no discernible fingerprint of micellar type aggregates, rather the emergence of disc-like and lamellar structures. At the highest surfactant concentration, the emergence of a weak Bragg peak in both the polymer and surfactant scattering suggest these pre-cursor disc and lamellar structures evolve into paracrystalline stacks which ultimately phase separate. Addition of the non-ionic surfactant hexa(ethylene oxide) dodecyl ether (C12E6) to the system seems to have little effect on the PDADMAC/SDS interaction as determined by NMR, merely displacing the
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