Supercritical fluid swelling of liquid crystal films

Processing of surfactant templated nano-structured silica films using compressed carbon dioxide as interpreted from in situ fluorescence spectroscopy

The local environment and dynamics of compressed carbon dioxide (CO(2)) penetration in surfactant templated silica film synthesis is interpreted from the in situ fluorescence emission spectra of pyrene (Py) and a modified pyrene probe. Pyrene emission in cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) templated silica films is monitored immediately after casting and during processing with gaseous and supercritical (sc) CO(2) (17-172 bar, 45 °C). The solvatochromic emission spectra of pyrene in CTAB templated films suggest CO(2) penetration in both the micelle interface and its interior. An anchored derivative of pyrene, 1-pyrenehexadecanoic acid (C(16)-pyr), is established for probing CPB films, where the pyrene moiety is preferentially oriented toward the micelle interior, thus limiting quenching by the pyridinium headgroup of CPB. CO(2) processing of CPB templated silica films results in an increase in the time scale for probe mobility, suggesting an increased time scale of silica condensation through CO(2) processing. The mobility of C(16)-pyr increases with pressure from gaseous to sc CO(2) processing and persists for over 5 h for sc CO(2) processing at 172 bar and 45 °C compared to about 25 min for the unprocessed film. The delivery of CO(2) soluble solutes to specific regions of surfactant templated mesoporous materials is examined via the nonradiative energy transfer (NRET) between pyrene and CO(2)-solubilized naphthalene.

Swelling of ionic and nonionic surfactant micelles by high pressure gases

The influence of different solvent environments on the size, shape, and characteristics of surfactant micelles of Pluronic F127 and CTAB was investigated by small-angle neutron scattering (SANS). SANS experiments were undertaken on dilute micellar surfactant solutions of F127 and CTAB that between them were exposed to liquid and supercritical carbon dioxide, liquid propane, ethane, and heptane under various pressures and temperatures. Swelling of the surfactant micelles could be directly related to the solubility of the solvents within the micelles, especially within their cores. Carbon dioxide produced the largest swelling of the Pluronic F127 micelles, compared to propane and ethane, which mirrors the solubility of the gases in the PPO core of the micelles. Conversely, the extent of swelling of the cores of CTAB micelles was greater with propane compared to carbon dioxide, which again relates to the solubility of the solvents in the alkane core of the CTAB micelles.

Enhanced stabilization of vesicles by compressed CO2

In this work, we studied the effect of compressed CO2 on the stability of vesicles formed in a dodecyltrimethylammonium bromide (DTAB)/sodium dodecyl sulfate (SDS) mixed surfactant system by combination of phase behavior and turbidity study, and UV-vis and fluorescence techniques. It was discovered that compressed CO2 could enhance the stability of vesicles significantly. This new and effective method to stabilize vesicles has some unique advantages over conventional methods. For example, the size and stability of the vesicles can be easily controlled by CO2 pressure; the method is greener because CO2 is a green reagent and it can be released completely after depressurization, which simplifies postseparation processes in applications. The main reason for CO2 to stabilize the vesicles is that CO2 molecules can insert into the hydrophobic bilayer region to enhance the rigidity of the vesicle film and reduce the size of the vesicles, which is different from that of conventional cosolvents (e.g., alcohols) used to stabilize vesicles. On the basis of this discovery, we developed a method to prepare hollow silica spheres using tetraethoxysilane as the precursor and CO2-stabilized vesicles as the template, in which CO2 acts as both the stabilizer of the vesicular template and the catalyst for the hydrolysis reaction of the precursor, and other cosolvents and catalysts are not required. Besides, the size of the silica hollow spheres prepared can be controlled by the pressure of CO2.