Langmuir, Vol.29, No.15, 4789-4795, 2013
Thermodynamic Properties of Bridging Clusters in Thin Films of Water between Hydrophobic Surfaces Assessed from Surface Force Isotherms
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 degrees 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 (Delta G(c), Delta H-c, and T Delta S-c) 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 Delta H-c and T Delta S-c are both negative quantities, with the latter being larger than the former, which implies a positive excess Gibbs energy of a cluster, Delta G(c) = Delta H-c - T Delta S-c. Typically, for temperatures between 10 and 40 degrees C, these cluster properties are of the order of some k(B)T units, corresponding to 10(-4)-10(-3)k(B)T 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.