Hydrocarbon chain disorder in C-phase potassium caprate and its associated diffuse scattering

Structural studies of the crystalline monoclinic C phase of the potassium soap K⁺.C₁₀H₁₉O^-₂ (space group P2₁/a, a = 8.145 ± 0.044, b = 5.726 ± 0.010, c = 28.309 ± 0.061 Å, β = 94° 35' ± 46', Z = 4) stable above 349 K are reported. Analysis of the Bragg diffraction data demonstrates that the heavy end group and the first four carbon atoms in the hydrocarbon chain, C(1) to C(4), retain the ordered configuration of the room-temperature A phase. The remainder of the chain, C(5) to C(10), is disordered. The disordered segments of chain adopt an average parallel packing on a pseudo-hexagonal lattice. The chain axes are approximately parallel to the inter-layer long spacing d(001). It is proposed that the disorder is due to rotational isomerism about the bonds between atoms C(3) and C(4), C(5) and C(6), C(7) and C(8). This model is consistent with available thermodynamic data. Strong X-ray diffuse scattering associated with, and extending between, the reciprocal-lattice points 20l and 11l has been observed from the C-phase soap. This diffuse scattering is interpreted in terms of the proposed disorder. The localization of the diffuse scattering about the specified lattice points is accounted for by the presence of intermolecular correlations.