Temperature-sensitive properties of occluded hydration centers in direct hexagonal (H_I) mesophases

Temperature-induced structural transformations of a direct hexagonal mesophase were investigated by SAXS, rheology, DSC, and NMR. The investigated mesophase region consists of two distinct direct hexagonal structures. The primary structure, detected at lower water content (33.0–39.0 wt%, 25 °C), is oriented as a bidiscontinuous assembly; it is composed of “occluded hydration centers” (OHCs) and direct surfactant-oil aggregates. The secondary structure, detected at higher water content (39.5–42.5 wt% water, 25 °C), is composed of an unbound water continuous phase and direct surfactant-oil aggregates. The OHCs within the bidiscontinuous mesophase were found to enhance the mesophase's order (correlation length (ξ)) and storage modulus (G′). The content of the unbound water occluded within the OHCs is temperature dependent, due to the ethoxylates’ water binding properties. At temperatures below 18 °C, the water in the bidiscontinuous mesophase (<39.0 wt% water) remains bound, resulting in a decreased mesophase order (correlation length (ξ)) and storage modulus (G′). Elevated temperatures increase the unbound water content, causing the unbound water domains to swell. Above the critical swelling point, the unbound water domains interlink to form a continuous phase, reducing the systems’ range order (ξ) and the storage modulus (G′). Better understanding the temperature dependence of the direct hexagonal mesophase enables mapping of the critical factors affecting the controlled delivery vehicles for non-bioavailable drugs.