ABSTRACT
Extensive deuterium NMR relaxation data are presented for two specifically deuterium labeled surfactants forming bicontinuous cubic phases with water. 2H spin-lattice (R1) and spin-spin (R2) relaxation rates were measured over an extended frequency range from 2 to 60 MHz. The data are interpreted with an existing theoretical framework for spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. We show that the theory succeeds in accounting for the data and that the defining parameters of the theory, correlation times and order parameters, are in agreement with related data in other surfactant phase situations. Specifically, we obtain the surfactant self-diffusion coefficient over the minimal surface in one unit cell and show that it is in agreement with the corresponding macroscopic NMR diffusion data. By measuring two additional NMR relaxation parameters for each carbon on the surfactant hydrocarbon tail, we demonstrate how order parameter and correlation time profiles can be obtained. Finally, we analyze published molecular dynamics trajectories for a bicontinuous cubic phase. The analysis provides further support for the theoretical framework used to interpret relaxation data.
ABSTRACT
In the course of a long-term effort to cope with surface force data for thin films of water between hydrophobic surfaces, we have applied the bridging-cluster model (Eriksson, J. C.; Henriksson, U. Bridging-cluster model for hydrophobic attraction . Langmuir 2007, 23, 10026 - 10033) to the recently published surface force isotherms for water films between hexadecylthiolated gold surfaces in the thickness range of 20-100 nm and temperature range of 10-40 °C (Wang, J.; Yoon, R.-H.; Eriksson, J. C. Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces. J. Colloid Interface Sci. 2011, 364, 257 - 263). We show that these isotherms can be faithfully reproduced on the basis of the bridging-cluster model. The thermodynamic excess properties (ΔGc , ΔHc , and TΔSc) of linear clusters that are assumed to bridge the core of the films were calculated from the experimental surface force isotherms. A crucial step taken was to infer two-dimensional ideal mixing of the clusters with the surrounding film water. We find that ΔHc and TΔSc are both negative quantities, with the latter being larger than the former, which implies a positive excess Gibbs energy of a cluster, ΔGc = ΔHc - TΔSc. Typically, for temperatures between 10 and 40 °C, these cluster properties are of the order of some kBT units, corresponding to 10(-4)-10(-3)kBT per water molecule entailed. Our analysis yields support of the notion that elongated aggregates can arise in thin films of water between hydrophobic surfaces driven by entropy of mixing.
Subject(s)
Organogold Compounds/chemistry , Thermodynamics , Water/chemistry , Gold/chemistry , Hydrophobic and Hydrophilic Interactions , Sulfhydryl Compounds/chemistry , Surface PropertiesABSTRACT
A new model is proposed to account for the long-range hydrophobic attraction repeatedly observed for thin water films between two stable (solid) hydrophobic surfaces. The model is based on the notion of structurally organized, elongated water clusters that span the gap between the hydrophobic surfaces. Two features are noted: (i) Mixing entropy due to the mixing of the clusters and the remainder of the water in the thin film is explicitly taken into account. (ii) A term is invoked that depends inversely on the film thickness, which accounts for the free-energy change associated with reorganizing the film as the film thickness varies. Fitting to experimental surface force data resulted in parameter values of reasonable magnitudes. The model developed covers film thicknesses from about 2 nm and above. On this basis, the amazingly long range of the hydrophobic attraction can be attributed to the formation of bridging, quasi-cylindrical clusters having a radius on the order of 1 to 2 nm.