Self-Assembly at High Pressures:  SANS Study of the Effect of Pressure on Microstructure of C₈E₅ Micelles in Water

Abstract

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.

Pressure Reentrant Assembly: Direct Simulation of Volumes of Micellization

“…In particular, the nonionic surfactant headgroup volume is positive and comparable to the total micellization volume at ambient pressure and subsequently decreases with increasing pressure to near zero at 2500 bar, consistent with small-angle neutron-scattering experiments indicating ethoxy headgroup dehydration with pressurization. 29 The C 10 E 5 tailgroup contribution is shifted downward relative to those of the ionic surfactants, crossing zero at pressures in the range of 500 to 1000 bar. Adding the nonionic headgroup and tailgroup association volumes results in a crossover in the total volume at pressures near 1500 bar.…”

Lower consolute boundaries of the nonionic surfactant C8E5 in aqueous alkali halide solutions: An approach to reproduce the effects of alkali halides on the cloud-point temperature

“…This study builds on the material of an earlier report, on how alkali halides change the properties (position in temperature and shape) of the lcb of C 8 E 5 [17]. There are definite advantages in using aqueous solutions of C 8 E 5 in that quite extensive results from spectroscopic [9,[22][23][24][25][26][27], thermodynamic [28][29][30][31], and modelling [32,33] studies exist. The effects of the presence of alkali halides in C 8 E 5 micellar solutions have been studied by time-resolved fluorescence quenching (aggregation behaviour and dynamics) [9], dynamic light scattering (particle and collective properties) [26], mixing calorimetry (apparent enthalpy of the surfactant) [31], and by a statistical-thermodynamical model (consolute boundaries) [33].…”

High Pressure Phase Diagram of an Aqueous PEO-PPO-PEO Triblock Copolymer System via Probe Diffusion Measurements

“…Here, we extend this work by demonstrating that DWS measurements are well suited for rapidly characterizing morphological phase transitions in complex fluids at high pressure. Specifically, the microrheology of an aqueous triblock copolymer solution is measured over a full range of temperatures and hydrostatic pressures exceeding 200 MPa, utilizing a high pressure transmission scattering cell originally designed for SANS experiments. , Aqueous Pluronic solutions are known to exhibit a rich phase behavior as a function of both temperature and pressure. − As the material undergoes a morphological phase transition, there is a distinct change in tracer particle mean square displacement temporal behavior, thereby providing a facile method for rapidly constructing high pressure phase diagrams for complex fluids.…”