NMR and SANS Studies of Aggregation and Microemulsion Formation by Phosphorus Fluorosurfactants in Liquid and Supercritical Carbon Dioxide

Abstract

(1)H NMR relaxation and diffusion studies were performed on water-in-CO(2) (W/C) microemulsion systems formed with phosphorus fluorosurfactants of bis[2-(F-hexyl)ethyl] phosphate salts (DiF(8)), having different counterions (Na(+), NH(4)(+), N(CH(3))(4)(+)) by means of high-pressure in situ NMR. Water has a low solubility in CO(2) and is mainly solubilized by the microemulsion droplets formed with surfactants added to CO(2) and water mixtures. There is rapid exchange of water between the bulk CO(2) and the microemulsion droplets; however, NMR relaxation measurements show that the entrapped water has restricted motion, and there is little "free" water in the core. Counterions entrapped by the droplets are mostly associated with the surfactant headgroups: diffusion measurements show that counterions and the surfactant molecules move together with a diffusion coefficient that is associated with the droplet. The outer shell of the microemulsion droplets consists of the surfactant tails with some associated CO(2). For W/C microemulsions formed with the phosphate-based surfactant having the ammonia counterion (A-DiF(8)), the (1)H NMR signal for NH(4)(+) shows a much larger diffusion coefficient than that of the surfactant tails. This apparent paradox is explained on the basis of proton exchange between water and the ammonium ion. The observed dependence of the relaxation time (T(2)) on W(0) (mole ratio of water to surfactant in the droplets) for water and NH(4)(+) can also be explained by this exchange model. The average hydrodynamic radius of A-DiF(8) microemulsion droplets estimated from NMR diffusion measurements (25 degrees C, 206 bar, W(0) = 5) was R(h) = 2.0 nm. Assuming the theoretical ratio of R(g)/R(h) = 0.775 for a solid sphere, where R(g) is the radius of gyration, the equivalent hydrodynamic radius from SANS is R(h) = 1.87 nm. The radii measured by the two techniques are in reasonable agreement, as the two techniques are weighted to measure somewhat different parts of the micelle structure.

PFG NMR diffusion experiments for complex systems

“…The appropriate choice of the gradient values provides CTP selection in DQDiff, but by itself does not provide convection compensation. The latter is achieved by setting the value of x to [21] where it is assumed, following Eq.…”

Anisotropic reversed micelles with fluorocarbon-hydrocarbon hybrid surfactants in supercritical CO2

“…This suggests exchanging monovalent counterions for divalent cations can result in viscosity-enhancing rod-like reversed micelles. In addition, double-FC-tail phosphate surfactants were also found to be efficient µE stabilisers, the most favourable case stabilizing W0 up to 45 [23][24].…”

Continuous Tuning of Silver Nanoparticle Size in a Water‐in‐Supercritical Carbon Dioxide Microemulsion

“…Another factor to consider is the dependence of the size of the micellar water core on pressure and temperature. Small‐angle neutron scattering (SANS) experiments carried out by Xu et al 8. showed that an increment in pressure or temperature had an impact on the size of the water droplets.…”