Pressure effects on the Raman spectra of micelles: Pressure induced phase transition and structural changes in aqueous sodium oleatea)

Is Micellar Catalysis Green Chemistry?

Many years ago, twelve principles were defined for carrying out chemical reactions and processes from a green chemistry perspective. It is everyone's endeavor to take these points into account as far as possible when developing new processes or improving existing ones. Especially in the field of organic synthesis, a new area of research has thus been established: micellar catalysis. This review article addresses the question of whether micellar catalysis is green chemistry by applying the twelve principles to micellar reaction media. The review shows that many reactions can be transferred from an organic solvent to a micellar medium, but that the surfactant also has a crucial role as a solubilizer. Thus, the reactions can be carried out in a much more environmentally friendly manner and with less risk. Moreover, surfactants are being reformulated in their design, synthesis, and degradation to add extra advantages to micellar catalysis to match all the twelve principles of green chemistry.

Role of water structure on the high pressure micellization and phase transformations of sodium dodecanoate aqueous solutions

The aim of this work is to study the micelle formation and possible subsequent transformations of sodium dodecanoate aggregates in aqueous solutions at pressures up to 700 MPa. This pressure range is much larger than in most available studies on surfactant solutions and allows for evaluating the possible effect of the low-density to high-density water transformation on the aggregative behavior of the surfactant. The speed-of-sound and attenuation coefficient were determined at 298.15 K as a function of pressure at concentrations up to 0.13 mol kg(-1) in water. The speed-of-sound behavior with concentration is maintained up to pressures around 350 MPa. The attenuation coefficient, initially insensitive to pressure, exhibits a sudden increase around 250 MPa, reaching a maximum around 350 MPa and a plateau above 500 MPa in the case of the highest studied surfactant concentrations. From the analysis of the changes observed in these properties, it was possible to extend the concentration-pressure phase diagram of sodium dodecanoate at constant temperature. Some peculiarities found were: (1) the critical micellar concentration reaches a maximum around 170 MPa, (2) the micellar phase disappears above 400 MPa, (3) a phase transformation starts around 250 MPa, setting the solubility limit of the surfactant at concentrations around 0.06 mol kg(-1) in this pressure region, and (4) further transformations occur between 350 and 500 MPa. We discuss in length the possibility that such transformations might be driven by structural changes linked to the so-called low-density-water to high-density-water transition.

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

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.