Gemini surfactant–water mixtures: some physical–chemical properties

The phase diagram of the water-Gemini 16-4-16 system has been investigated and the phase boundaries were determined. DSC and optical microscopy were used to define the region of existence of the different phases. No liquid crystalline phases have been observed, however, a two-phase region and a wide gel phase follow the solution region. The solution region can be highly viscous, depending on composition and temperature. Surface tension and electrical conductance experiments have been performed, to define micelle formation and counter-ion binding to micelles. Interactions and motions over short distances were studied by 1H-NMR relaxation experiments. The drastic decrease of spin–spin relaxation time, T2, with Gemini composition ( ≈2 wt.%) was explained in terms of particle growth. Pulsed field gradient spin-echo (PGSE) NMR experiments were used to determine water and surfactant self-diffusion. Some modifications in the micellar structure were inferred on increasing the Gemini content in the mixture. Dynamic rheological experiments were performed for probing the solution microstructure. The observed high solution viscosity and the shear relaxation processes were rationalized in terms of the presence of entangled threadlike aggregates at a moderate concentration (≈ 4 wt.%). According to the Bohlin theory of flow as a cooperative phenomenon, the number of the micellar aggregates correlated to each other, and the interaction strength between the micellar units was obtained as a function of Gemini concentration.