Interaction of water and oxyethylene groups in lyotropic liquid-crystalline phases of poly(oxyethylene) n-dodecyl ether surfactants studied by 2H nuclear magnetic resonance spectroscopy

Abstract

N.m.r. quadrupole splittings (Δ-²H) of heavy water have been measured as a function of composition and temperature (268–338 K) in hexagonal (H₁) and lamellar (L_α) liquid crystals formed from a range of poly(oxyethylene) surfactants.

Measurements have been made on the L_α phase of C₁₂EO₃, C₁₂EO₄, C₁₂EO₅, C₁₂EO₆ and C₁₂EO₄OMe, and the H₁ phase of C₁₂EO₅, C₁₂EO₆ and C₁₂EO₈. A few data are presented for the metastable gel phases of C₁₂EO₅ and C₁₂EO₆(at 268 K). Increasing the surfactant concentration causes an initial sharp increase in Δ, followed by a levelling off or a shallow maximum. The values of Δ generally decrease with increasing temperature at high surfactant concentrations for all the surfactants studied. This is also true at low surfactant concentrations, unless increasing temperature causes a transition from micellar solution (L₁) to L_α where Δ may increase with temperature close to this phase boundary.

There is a general decrease of Δ with increasing EO length. The molecular processes contributing to the observed Δ values are discussed, and this is followed by a description of water binding equilibria. The experimental data can be fitted using a model where water binds collectively to the head groups. For each data set three parameters are required, these being the number of bound water molecules (r), the equilibrium constant of binding (K_B) and the order parameter of the bound water.