Peter Stilbs

How to study microemulsions.

The application of various experimental techniques for studying the properties of ternary mixtures H 2 O-oil-nonionic amphiphile is demonstrated. Based on detailed knowledge of the phase behavior, in particular, the positions and extensions of the three-phase bodies, the origin of the ultralow interfacial tensions and strong opalescence of the solutions is elucidated. Within the three-phase body one finds a maximum of the velocity of phase separation of the three phases that is correlated to a minimum of the viscosity and a jump of the electric conductivity. The properties of the homogeneous microemulsions are measured along a path through the phase prism at constant amphiphile concentration but varying oil/water ratio. The dynamics of the microstructure is studied by viscosity, dynamic light scattering, and electric field and pressure jump relaxation, as well as by ultrasonic absorption. The characteristic dimensions of the structure are determined by SANS and SAXS, NMR self-diffusion, electric conductivity, and freeze fracture electron microscopy.

Fourier transform NMR pulsed-gradient spin—echo (FT-PGSE) self-diffusion measurements of solubilization equilibria in SDS solutions

Abstract Solubilization equilibria for homologous series of alcohols, methyl ketones, aromatics, and crown ethers with SDS micelles in D 2 O solution at 25°C were determined. The measurements are based on multicomponent self-diffusion studies through an improved version of a Fourier transform NMR pulsed-gradient spin-echo technique. As expected, the degree of solubilization increases strongly with the number of carbon atoms in the solubilizate alkyl chains, corresponding to an increment in the standard free energy for transfer of solubilizate molecules from the aqueous to the micellar phase (ΔG p o ) of ≈ −2.6 kJ mole −1 for each −CH 2− group. Hydrocarbon chain branching in the solubilizate molecules decreases the degree of solubilization, as compared to conditions for n -alkyl compounds. Crown ether solubilization is found to increase strongly with ring size. Comparative studies with lauryltrimethylammonium bromide micelles do not show this trend, suggesting that the effect can be ascribed to the sodium binding capability of the larger crown ethers, and that crown ethers are “solubilized” at the SDS micellar surface.

Fourier transform NMR self-diffusion and microemulsion structure

A novel Fourier transform pulsed-gradient spin-echo 1H and 13C NMR method was employed to obtain multicomponent self-diffusion data for seven microemulsion systems. These included ionic surfactant-cosurfactant (short- or long-chain alcohol)-hydrocarbon-water, nonionic surfactant-hydrocarbon-water, and ionic surfactant (of the swelling type)-hydrocarbon-water systems. For the short-chain alcohol (butanol or pentanol) systems both water, hydrocarbon, and alcohol self-diffusion are very rapid over wide concentration regions. In contrast to micellar solutions and certain liquid crystalline phases there is apparently no marked separation into hydrophobic and hydrophilic domains; this is considered to be due to the alcohol having a compatibility with both aqueous and hydrocarbon environments as well as internal interfaces. There appear to be no extended well-defined structures in these systems. Instead the microemulsions are argued to have very flexible and low-order internal interfaces which open up and reform at a short time scale. It seems reasonable to assume a polydispersity in aggregate size and shape and it appears to be a clear possibility that there are mainly quite small aggregates. The nonionic surfactant system shows strong resemblances to the short-chain alcohol system while the remaining systems (long-chain alcohol or absence of alcohol) show a more pronounced separation into hydrophobic and hydrophilic regions.

Effect of inorganic salts on the micellar behaviour of ethylene oxide-propylene oxide block copolymers in aqueous solution

The aggregation behaviour of two ethylene oxide-propylene oxide block copolymers (PEO-PPO-PEO) in aqueous solution has been investigated in the presence of added salts (KCNS, KI, KBr, KCl and KF) by viscosity, cloud point, light scattering, pulse gradient spin echo NMR, and solubilization measurements. The salts have a strong effect on the cloud points of the pluronics. Both P-85 and L-64 form micelles which increase in size and change into elongated shapes when the cloud point is approached. The changes of size and shape of the micelles, revealed by the intrinsic viscosity and rheological properties, seem to occur at the same temperature relative to the cloud point, independent of the nature of the salt. The onset of micelle formation is also shifted in the same direction as the cloud point by the salts, but by a much smaller amount.

Effect of alcohol cosurfactant length on microemulsion structure

Results pertaining to the effect of varying the chain length of the alcohol cosurfactant from methanol to n-octanol are reported. The studies were performed for different combinations of surfactant and hydrocarbon. The influence of the alcohol chain length on the self-diffusion behavior seemed to be invariant with respect to replacing an anionic surfactant by a cationic one, to hydrocarbon type, and to changing surfactant alkyl chain length. The results demonstrate that the structure of cosurfactant-type microemulsion varies strongly with the cosurfactant. 19 references.

A comparative study of micellar solubilization for combinations of surfactants and solubilizates using the fourier transform pulsed-gradient spin—echo NMR multicomponent self—diffusion technique

Abstract The degree of solubilization for all combinations of 6 surfactants (sodium dodecanoate, sodium dodecyl sulfate, sodium octylbenzene sulfonate, sodium phenylundecanoate, dodecyltrimethylammonium bromide, and dodecylhexaethyleneglycolmonoether) and 12 solubilizates (1-pentanol, 2-pentanol, 3-pentanol, tert-pentanol, methyl-n-propyl ketone, diethyl ketone, benzene, benzyl alcohol, crown 18-6, crown 12-4, 1,4-dioxane, and ethyl glycol) have been determined at a surfactant concentration of 50 mg/ml D 2 O solution and low total solubilizate/surfactant ratios. The most striking observations concern the solubilization behavior of the nonionic surfactant; with the aliphatic type it solubilizates weakly, with aromatics as strongly as the other surfactants, and with crown ethers hardly at all. With the exception that dodecyltrimethylammonium bromide micelles solubilize ketones more weakly in a relative sense, the remaining five surfactants exhibit very similar solubilization characteristics. NMR-based evidence with regard to the site of solubilization of aromatics is discussed.

Micellar breakdown by short-chain alcohols. A multicomponent FT-PGSE-NMR self-diffusion study

Abstract By multicomponent self-diffusion studies it is demonstrated that solubilized short-chain n -alcohols cause breakdown of SDS micelles, as manifest in rapidly increasing self-diffusion for SDS as well as for trace amounts of cosolubilized (CH 3 ) 4 Si (TMS) above a certain total alcohol/SDS ratio. These effects are considerably enhanced at higher SDS/D 2 O ratios and for alcohol chain lengths above C 2 . The results are consistent only with highly disordered structures (as compared to micelles or inverted micelles) in solution at higher alcohol content and have general relevance for a general understanding of microemulsion systems. On the basis of observations of 1 H NMR line broadening effects and initially decreasing SDS and TMS diffusion coefficients, indications of initial micellar growth are also found at moderate total alcohol/SDS ratios upon solubilization of propanol, butanol, and pentanol into SDS micelles.

Substrate binding to cyclodextrins in aqueous solution: A multicomponent self-diffusion study

It is demonstrated that substrate binding to α- and β-cyclodextrins (CD) in solution can conveniently and directly be monitored from multicomponent self-diffusion data on these solutions, using the Fourier Transform NMR pulsed-gradient spin-echo technique. Included are aromatics and a series of alcohols ranging from methanol to octanol. Experimentally it was found thatn-alcohols associate more strongly with α-CD than with β-CD. As the bulkiness of the alcohol increased, binding to β-CD was enhanced while the reverse effect was observed in the case of α-CD. For both cyclodextrins it was found thatn-alcohol complexation in the homologous series was attributable to an increment in standard free energy of complexation of ∼ −3.0 kJ/mol for each −CH2− group, suggesting that the binding mechanism is of a hydrophobic nature.

Chemical nuclear spin-lattice relaxation in the rotating frame. Application to hindered internal rotation in ureas

Fourier transform T1ϱ measurements are used for the determination of rates and activation parameters for hindered internal rotation in ureas. The quantity ΔS≠ hin hindered internal rotation in urea in a hydrogen-bonding solvent is found to be strongly positive ( ≈20 cal mole−1 K−1) and ΔG≠ for the previously unknown barrier in tetramethylurea is estimated to be 6.1 ± 0.1 kcal mole−1, corresponding to a coalescense temperature in the region of −150°C. The direct methyl 13C14N coupling in tetramethylurea (7 Hz) has also been determined through T1ϱ measurements.

Molecular dynamics and NMR study of methane-water systems

Molecular dynamics of methane-water systems have been studied through computer simulations (MD), nuclear spin relaxation and self diffusion methods (NMR). MD simulation results give support to the concept of hydrophobic hydration by enhancing the water structure around the methane to create a clathrate. Spontaneous contacts between methanes is observed only at high densities, while so-called solvent-separated configurations are present in all simulations. The dynamics of the solvent water molecules is slowed down. In particular, the reorientational motion is sensitive to the presence of methane in solution. The simulated translational diffusion of methanes as solutes is also slowed down, but is higher than observed experimentally. The rotational motion of methanes is very sensitive to variation of the density while, the translational motion of methane is more constant and follows that of water. Methane acts nearly as a free rotor. The simulated reorientational correlation times for methane in water soluti...