Pressure induced phase transitions of TTAB-micellar solutions studied by SANS and Raman spectroscopy

Abstract

Using small angle neutron scattering in combination with optical Raman spectroscopy, phase transitions in the micellar system tetradecyl-trimethylammonium bromide (TTAB) have been investigated in a wide range of temperatures, pressures and surfactant concentrations. Allowing for the sluggish kinetics of crystallisation, the phase diagram in the p-T-c space could be determined that is dominated by a pronounced two-phase region of coexisting liquid and solid phases at elevated pressures.

Self-assembly of metal–ligand coordinated charged vesicles

“…One has noted that even binary surfactant/water systems have complex phase behavior. These self-assembled structures of surfactants in solutions can be controlled by tuning the related parameters such as temperature [2,3], pressure [4,5], ionic strength (the addition of salts) [6,7], nature of the counterions [8], the composites [9] and structures of surfactants [10]. Among the self-assembled aggregates formed by surfactants or their mixtures in solutions, vesicles which are more turbid than the L 1 -phase (micelles), optically isotropic and no long range order have attracted great attention because of their applications or potential applications in selective membranes, drug delivery, microreactors, and cosmetic applications [11][12][13][14][15].…”

Pressure-induced phase transitions in micellar solutions

“…4 shows a sequence of optical spectra obtained in the frequency regime of CH 3 /CH 2 -bending vibrations. The crystallization is associated with a pronounced narrowing of the vibrational lines along with an increase of the trans-conformers as shown by Haramagatti et al [3]. During the transition that occurs on a time scale of hours, a broad and unstructured spectrum is observed that might correspond to a gel-like intermediate state.…”

“…It is shown that the pressure-induced transition appears at the same values of pressure and temperature as the crystallization on cooling. Moreover, a thermal hysteresis of about 8 K or 400 bar is observed just as in the case of salt-free solutions [3].…”

“…[3], the slope dp/dT is about 53 bar/K for salt-free solutions at small surfactant concentrations. The present results show that there is a shift of the liquid-solid phase boundary towards higher temperatures by 5 K or to smaller pressures (by 400 bar) when 0.2 M NaBr is added.…”

“…Therefore, we have undertaken a systematic study of the phase behaviour of ionic surfactants in a wide range of thermodynamic parameters such as surfactant and salt concentration, temperature and pressure. Having already described the effect of pressure on the phase transition of pure aqueous TTAB-(C 14 H 29 N(CH 3 ) 3 Br) solutions in [3], we want to focus here on the influence of NaBr-additives.…”