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Phase behavior and small-angle neutron scattering (SANS) measurements are reported for poly(ethylene-co-1-butene) (PEB) dissolved in pentane, pentane plus ethane, and supercritical ethane to pressures of 2000 bar and temperatures of 130 and 150 °C. The solution microstructure and solvent quality are probed using the high-concentration isotopic labeling technique at the mixture critical concentration to determine the variation of intra-and intermolecular correlations on approach to the phase boundary with isothermal changes in pressure. The SANS results for the three solutions show that the intramolecular radius of gyration remains close to the unperturbed value over the entire pressure range investigated. However, the intermolecular correlation length changes dramatically as the phase boundary is approached and is approximately 3 times larger in ethane as compared to pentane even at 1000 bar from the phase boundary. The intermolecular correlation length data suggest that ethane is a poorer quality solvent than pentane at conditions close to the phase boundary even though both solvents have similar mass densities.

We present the results of a high-pressure small-angle neutron scattering study of the effect of pressure on surfactant microstructure. The study was carried out on a solution of 1 wt % C 8 E 5 in D 2 O at 29.4 °C and pressures up to 310 MPa. The C 8 E 5 micelles that form under these conditions are noninteracting. We find that applying pressure leads to a pronounced decrease in the micelle radius of gyration and the forward scattering intensity over the pressure range from ambient to 150 MPa. The partial molecular volume of the surfactant and the extent of hydration of the surfactant head groups in the micelle were also determined using the method of solvent contrast variation. Both quantities decrease with the application of pressure up to 150 MPa. Core-shell model fits to the scattering spectra over the entire q-range indicate that the shell radius decreases, while the hydrophobic core radius increases slightly with pressure. The pressure dependence of the shell radius is notably similar to that observed for the radius of gyration. Collectively, these observations lead to the conclusion that the effect of pressure on C 8 E 5 micellization is to induce the dehydration of surfactant head groups and the collapse of the hydrophilic micelle shell at pressures between ambient and 150 MPa.