N,N'-Hexadecanoyl l-2-diaminomethyl-18-crown-6 surfactant: synthesis and aggregation features in aqueous solution.

Bolas surfactants can be inserted into bi-layers and may operate as permanent holes in such membranes. Significant synthetic work and an exhaustive characterisation of their properties in the bulk was performed. On this purpose, the phase diagram of the system composed by water and 1,16-hexadecanoyl-bis-(2-aminomethyl)-18-crown-6 (termed Bola A16) was investigated in a wide temperature and concentration range. No liquid crystalline phases were observed and a large micellar solution was present, up to about 50 surfactant wt%. Surface tension experiments defined adsorption and micelle formation. The low observed cmc value is important for pharmacological applications, in fact, considering intravenous administration, only micelles with low cmc value can exist in blood. Nuclear magnetic resonance experiments determined both water and surfactant self-diffusion. According to the aforementioned experiments, slight, if any, modifications in the structure of micelles were inferred on increasing Bola A16 content. Dynamic rheological experiments probed the solution micro-structure. The observed rheological behaviour is newtonian. The solution viscosity and the shear relaxation processes were rationalized assuming the presence of spherical aggregates, occurring up to high surfactant content. The viscometric behaviour was rationalised in terms of a former theory of flow as a cooperative phenomenon. The number of micelles coordinated each other during the viscous flow and the interaction strength between them was obtained as a function of Bola A16 concentration. Such value is close to unity and practically independent of surfactant content in the whole concentration range we investigated. This behaviour points out that little, or none, interactions among micellar aggregates occur. The absence of shear induced changes in the aggregate shape implies no change in drug delivery properties under flow, this is useful in the pharmaco-dynamics field, since drug delivery usually operates in mechanically stressed conditions. Thanks to the above properties, the material results particularly suitable for application in pharmaceutical field, may solubilize lipid membranes and selectively transport ions across them. Ancillary effects, such as the uptake of counter-ions in the crown ether, are to be considered.