Gels from small molecules in organic solvents: structural features of a family of steroid and anthryl-based organogelators

Abstract

Luminescent organogels formed by the small molecules cholesteryl 4-(2-anthryloxy) butanoate (CAB) and 2,3-bis-n-decyloxyanthracene (DDOA) in decane and alcohol solvents have been studied by scattering techniques. The results are compared with previous studies of hydrocarbon-based gels of an androstanol derivative (STNH) analogue. Rheology experiments confirm that the materials, obtained through a sharp transition from a solution to a soft solid, have all of the characteristics of gels. for instance, the thermoreversible DDOA–octan-1-ol gel exhibits a yield stress (σ*≈ 550 Pa at C= 1 wt.%), and its elasticity (G′≈ 10⁴ Pa) suggest the existence of numerous and/or strong junction zones. The neutron and X-ray scattering curves of the gels formed by CAB, DDOA and STNH consist of a mix of the form-factor of the aggregates and the structure factor of their organised interacting domains. The diameter (d) and conformation of the aggregates, long and rigid fibres, are sensitive to the type of solvent. For CAB, d= 160 Å in decane and 192 Å in butan-1-ol (assuming homogeneous circular cross-sections); for DDOA, interfaces of large bundles of fibres are revealed by the scattering technique [ca. 100 < (d/Å) < ca. 500]; for STNH, d≈ 100 Å with fibres being made by at least two protofilaments. The fibres are interconnected by ordered ‘junction zones’ which are part of a random heterogeneous three-dimensional network. For CAB in decane, the ‘nodes’ are lyotropic organisations (nematic-like) obtained through a swelling of the hexagonal packing of the crystalline solid-state. The intercolumnar spacings of the CAB fibrils varies from 75.6 Å(solid state) to ca. 102 Å(lyotropic microdomains). In alcohols, the structural organisation is different, with a packing close to that of the solid-state. For STNH in hydrocarbons, ordered microdomains are also responsible for the mechanical cohesion of the gel. The DDOA gel crystallinity is attributed to ‘dry’ microcrystallites analogous to the hexagonal packing of the related crystalline powder. These structural results have been confirmed by electron microscopy measurements. Structural correlations derived from studies of this family of gelators may bear upon the gelation process.