D. Fennell Evans

Micelle formation in ethylammonium nitrate, a low-melting fused salt

Abstract Critical micelle concentrations (CMC) are determined from surface tension measurements for alkyltrimethylammonium bromides and alkylpyridinium bromides at 50°C and for Triton X-100 at 20 and 50°C in ethylammonium nitrate, a low-melting anhydrous fused salt. The CMCs are approximately 5 to 10 times larger than those observed in water. From the change of CMC with surfactant chain length, the free energy of transfer of a methylene group from the fused salt to the micelle interior is calculated to be −370 cal/mole compared to −680 cal/mole for a similar transfer from water to the micelle. It is concluded that in respect to solvophobic behavior ethylammonium nitrate and water show a number of similarities.

Critical micelle concentrations for alkyltrimethylammonium bromides in water from 25 to 160°C

Critical micelle concentrations were determined by conductance measurements for decyl-, dodecyl-, tetradecyl- and hexadecyltrimethylammonium bromide in water at 25, 60, 95, 130, and 160°C. The results are discussed in terms of the equilibrium model and the nonlinear Poisson-Boltzmann model for micelle formation. The free energies of transferring a methylene group from water to the oil-like interior of the micelle are found to be −781 at 25°C, −796 at 60°C, −819 at 95°C, −815 at 130°C, and −787 at 160°C cal-mol−1.

Diffusion in surfactant solutions

Abstract The diffusion coefficients in water of Triton X-100 and sodium dodecyl sulfate were measured as a function of concentration using the Taylor dispersion technique. For Triton X-100, a nonionic surfactant, the diffusion coefficient drops from 7.4 × 10 -7 cm 2 /sec at 0.45 g/liter to 6.45 x 10 -7 cm 2 /sec at 5 g/liter. The diffusion coefficient of methyl yellow solubilized in Triton X-100 is close to that of the surfactant. This behavior is quantitatively consistent with a chemical equilibrium between monomer and micelle. For sodium dodecyl sulfate, an anionic surfactant, the diffusion coefficient increases from 1.76 x 10 -6 cm 2 /sec at 0.01 M to 4.53 x 10 -6 cm 2 /sec at 0.125 M . The increase is less when 0.1 M NaCl is added. The diffusion coefficient of the methyl yellow solubilized by the SDS is significantly less than that of the surfactant, particularly at low ionic strength. This behavior can be quantitatively explained by including electrostatic coupling between monomer, micelle, and counterion.

Surfactant diffusion: New results and interpretations

Abstract Data for surfactant diffusion are reported for sodium dodecyl sulfate at 25°C and tetradecyltrimethylammonium bromide at 25, 90, and 135°C, as measured by Taylor tube dispersion. These data are analyzed in terms of two limiting forms of theory, one appropriate to “slow” reaction rates, the other to “fast” rates. It is shown that the usual extrapolation to the critical micelle concentration to infer intrinsic diffusion constants is not permissible. The data are explicable if transport occurs by a process wherein ionic micelles disassociate, diffuse as monomers, and reassemble into micelles. This is directly contrary to current ideas on diffusion of surfactants.

Micelle formation above 100°C

Critical micelle concentrations (CMC) were determined for tetradecyltrimethylammonium bromide in water from 25 to 166°C using a flow conductance cell. Over this temperature range the CMC increases by a factor of 10 and the degree of micelle dissociation increases from 0.25 to 0.65. The values of ΔH and ΔS for micellization are determined using the equilibrium model and are discussed in terms of the Benzinger compensation hypothesis for aqueous solutions.

The activity and stability of alkaline phosphatase in solutions of water and the fused salt ethylammonium nitrate

The fused salt ethylamonium nitrate has several properties which resemble those of the biologically important solvent water. In order to shed light on the role of solvent in determining protein structure we have examined the influence of ethylammonium nitrate on the activity and stability of the enzyme alkaline phosphatase. Significantly, although reduced enzymatic activity was observed in ethylammonium nitrate solutions up to 60% (v/v) in water, the enzyme was stable to brief exposure to solutions as high as 80% (v/v) in the fused salt.

Simultaneous diffusion of ions and ion pairs across liquid membranes

Abstract The flux of tetrabutylammonium nitrate (Bu 4 NNO 3 ) across a liquid membrane of n-heptyl cyanide varies with the salt concentrations in dilute solution, but with the concentration squared in more concentrated solution. The results are consistent with a mechanism which includes parallel diffusion of ions and ion pairs. This type of behavior, which will be common in homogeneous membranes of low dielectric constant, is a form of facilitated diffusion in which the salt acts as its own carrier.

Thermodynamic Properties of the Ethylammonium Nitrate + Water System: Partial Molar Volumes, Heat Capacities, and Expansivities

Partial molar volumes at 15, 25, and 45°C and partial molar heat capacities and expansivities at 25°C for ethylammonium nitrate + water mixtures are reported. The results are compared with those for other aqueous cosolvents, particularly hydrazine and ammonium nitrate.

Theoretical and experimental basis for a specific countertransport system in membranes.

A sodium ion carrier transport system contains the antibiotic monensin in the membrane and uses a pH gradient as an energy source. The experimental results are in accord with theoretical predictions based on a mechanism which is understood on a molecular level.

The Rideal Lecture. Vesicles and molecular forces

Properties of some double-chained surfactants which form spontaneous thermodynamically stable unilamellar vesicles in water are reviewed and analysed in terms of current ideas on self-assembly and recent direct force measurements. The reasons for the apparent success of older theories are discussed. The conclusion is that the older description in terms of primitive model double-layer theory, van der Waals, and additional hydration forces has to be abandoned for a rigorous theory of self-assembly. That conclusion does not abrogate the usefulness of the present theoretical framework in providing a predictive rationale for many systems and in biological problems. This is illustrated by the use of cationic surfactants as immunosuppressants, bacteriocides and other applications.