Concentration Fluctuations and Surface Adsorption in Hydrogen-Bonded Mixtures

Abstract

Concentration fluctuations of the 1,4-butanediol + 1-dodecanol mixture have been characterized by laser light-scattering experiments at two temperatures. The results show that both components have a clear tendency toward heterocoordination that increases with temperature. The combination of the concentration fluctuation results with the surface tension measurements have provided precise values of the excess surface adsorption Γ 21 as a function of the composition. Γ 21 shows a maximum for a mole fraction of 1-dodecanol of x 2 ≈ 0.02; the maximum of Γ 21 decreases with increasing temperature. The position of this maximum contrasts with that at which the local compositions show extrema in the bulk. The surface entropy versus the composition curve shows maxima and minima throughout the composition range. This behavior resembles that found for some water-alcohol mixtures. Using the experimental surface tension data, we predicted the excess surface adsorption using a lattice-fluid model that takes into account the existence of hydrogen bonds between the two components. In spite of the fact that the model accounts reasonably well for the volumetric properties of the system, poor predictions are obtained for Γ 21 .

Thermodynamics of Ionized Monolayers: Surface Manometry on Very Low Density Spread Monolayers of Sodium Octadecyl Sulfate at the Air/Water Interface and Analysis of Ionic Double Layer Contributions to the Isotherms

“…This is misleading since it will include an electrostatic contribution (in addition to Π el ) to allow for the fact that the charge distribution is discrete, not averaged, in both the plane of the surface charge and in the counterion region of the double layer. It will also include the contribution from any changes in van der Waals interaction between the monolayer chains induced by charging the head groups and from association of counterions with the ionic head groups of the monolayer substance. , There is no thermodynamic reason why the second term in eq should be zero, as has been widely assumed. Furthermore, the two terms in that equation are not independent since both are functions of α.…”

“…It will also include the contribution from any changes in van der Waals interaction between the monolayer chains induced by charging the head groups and from association of counterions with the ionic head groups of the monolayer substance. 8,41 There is no thermodynamic reason why the second term in eq 6 should be zero, as has been widely assumed. Furthermore, the two terms in that equation are not independent since both are functions of R. Correspondingly, as discussed below for the present data, the contribution to the surface pressure from the van der Waals energy of interaction of the monolayer chains of the hypothetical uncharged monolayer may not be independent of R at a given Γ od and hence will be coupled to Π el for the charged monolayer in eq 4.…”

“…A valuable and extensive review of the literature on the adsorption of water-soluble ionic surfactants at the A/W interface is given by Prosser and Franses . Most of this literature takes surfactant solution concentrations as activities, an approximation which is often significantly in error, and experiments of thermodynamic quality are not always attained. − Discussions on the merits of competing forms of the Gibbs Adsorption Isotherm adapted by inclusion of electrostatic terms relating to the charge densities at and near the surface remain contentious. Prosser and Franses give a comprehensive analysis of the use of model adsorption isotherms for water-soluble ionized surfactants, using ionic double layer models and ion distributions which employ the Poisson−Boltzmann (PB) equation.…”

Water Activity from Equilibrium Molecular Dynamics Simulations and Kirkwood-Buff Theory

“…Numerical integration is largely insensitive to the number of concentrations used for the analysis (Section S7.2)water activities calculated from experimental KBIs of the ethanol/water systems using 20 and 5 concentrations differ at maximum of 2%. Estimates of osmotic compressibility derived from solute–solute interactions can be used to obtain osmotic pressure when T and chemical potential of water is effectively held constant. ,,, From a series of NPT simulations as used in this work, water activity can be estimated through the chemical potential derivative. ,, The chemical potential of a substance in the condensed phase is given by μ = μ° + RT ln a where μ° is the standard state chemical potential and the activity is the product of the activity coefficient and mole fraction a = γx . While these properties can more formally be expressed in molecular concentrations, , we follow the molar unit conventions to make the connection to macroscopic systems.…”

“…34,45,59,60 From a series of NPT simulations as used in this work, water activity can be estimated through the chemical potential derivative. 46,61,62 The chemical potential of a substance in the condensed phase is given by μ = μ°+ RT ln a where μ°is the standard state chemical potential and the activity is the product of the activity coefficient and mole fraction a = γx. While these properties can more formally be expressed in molecular concentrations, 63,64 we follow the molar unit conventions to make the connection to macro-scopic systems.…”