Head group ordering in the lyomesophases of the dipotassium hexadecanedioate-water system studied by ¹³C and ²H NMR

Abstract

Carbon-13 NMR measurements are reported for the hexagonal (hex) and lamellar (lam) mesophases formed by water and dipotassium hexadecanedioate (isotopically enriched in the carboxyl group). The average anisotropic ¹³C chemical shift tensor in both mesophases has uniaxial symmetry and is almost independent of temperature with average values (relative to TMS) of (σ_∥)^hex = 171·3 ppm, (σ₁)^hex = 192·4 ppm, <σ_∥>^lam = 200·2 ppm, <σ_⊥>^lam = 177·1 ppm and σ_iso = 184·8 ppm. These results together with earlier quadrupole interaction data of the α-methylene deuterons, and a simple model for the structure of the −CαH₂COOK moiety are used to estimate the effective order parameters, n, of the disoap head groups along the direction normal to the water interfaces. The results are n^hex ≈ 0·44 and n ^lam ≈ 0·27.