An equation of state describing hydrophobic interactions

Abstract

Several thermodynamic properties for the process of dissolution of pure hydrocarbons into water are found to be linearly related to the number of hydrogens on the hydrocarbon molecule. From the correlations found for the Gibbs energy change, enthalpy change, and heat capacity change, along with the use of an average minimum solubility temperature, an equation of state for the hydrophobic effect is derived. The entropy change upon dissolution per hydrocarbon hydrogen atom is close to -R In 2. A model based upon a "tetrahedrally" localized water molecule with one corner defined by a carbon-hydrogen group and the other three corners defined by water molecules is used to estimate the observed entropy and heat capacity changes.The availability of enthalpies of solution into water as a function of temperature for a set of hydrocarbons (1) has made possible the evaluation of corresponding entropies of solution from solubility measurement (2). In the course of correlating these results with structural features of the hydrocarbons studied, a significant descriptive parameter was found to be the number of hydrogens on the hydrocarbon species. A similar feature has also been implied in calculations for partial molar heat capacities of organic solutes in water. Linear correlations of the hydrophobic Gibbs energy and the molar volume (2) or surface area (3-5) of a given family of hydrocarbon groups have been obtained, but they do not provide detailed molecular insight into the nature of aqueous solutions of hydrocarbons.The enthalpies of solution of seven hydrocarbon liquids (benzene, toluene, ethyl benzene, propyl benzene, pentane, hexane, and cyclohexane) (1) are characterized by a positive linear dependence upon temperature and a temperature near room temperature where the enthalpy of solution is zero. It is convenient to describe the temperature dependence of the Gibbs energy change from this temperature of minimum solubility. This procedure enables the development of an empirical equation of state for hydrophobic interactions. At room temperature the enthalpy changes upon solution are small, and the dominant factor in the Gibbs energy of dissolution is the entropy change. The relation of these thermodynamic parameters to the number of hydrogen atoms in a hydrocarbon molecule leads to a particularly simple molecular picture of the nature of water molecules localized about each hydrocarbon hydrogen atom. It is hoped that this description will be useful in estimating thermodynamic properties of hydrophobic interactions in biological materials. Hydrophobic Gibbs energy change We define the hydrophobic Gibbs energy change AGO for the transfer process of taking hydrocarbon molecules from the pure hydrocarbon liquid phase to the standard state of the hydrocarbon in a dilute aqueous phase. In terms of chemical poten- [l]A correlationof the hydrophobic Gibbs energy change has been demonstrated with the carbon chain length for hydrocarbon families, such as normal saturated alkanes, and alkenes, as well as with aliphatic a...

Energetics of hydrogen bonding in proteins: A model compound study

“…The negative contribution (-7.0 J K" mol") for the apolar hydrogen is similar to that published previously (Gill & Wadso, 1976). The negative contribution of the hydroxyl, on the other hand, is considerably less in magnitude than that reported by Privalov and Makhatadze (1990) based on gas dissolution studies.…”

“…Additionally, the results of such studies provide important information on the interaction between solute functional groups and water through the changes in entropy and heat capacity. Such information has been invaluable in understanding the hydrophobic effect (Gill & Wadso, 1976;Baldwin, 1986; and in modeling the temperature dependence of biological processes in solution (Privalov & Makhatadze, 1992;Gomez et al, 1995).…”

Entropy and enthalpy convergence of hydrophobic solvation beyond the hard-sphere limit

“…(3)- (6) obviously rests on the assumption of additivity of the contributions of individual chemical groups to the total solvation entropy and enthalpy, an assumption which has been independently verified. 13,47,48 The other, more serious, assumption is that it is sufficient to sort the solvation contributions of all chemical groups into just two groups, hydrophilic and hydrophobic, characterized by the variable X , an assumption which we show not to be entirely true in this paper. In fact, the concept of entropy convergence treated theoretically by the information theory approach and revised scaled particle theory in some sense goes beyond the additivity concept outlined above, 19,22,23 since here it is shown that the entropy of solvation of hard spheres converges to a universal value at one temperature for a whole range of different sphere radii.…”

Thermodynamic Investigation on Micellization of Cationic Gemini Surfactants with Nitrophenoxy Groups in Hydrophobic Chains

“…由于表面活性剂的亲水部分在 胶束形成前后始终与水接触, 而疏水部分在胶束形成前 暴露于水相, 在胶束形成后才会迁移到聚集体的非极性 内核中, 因此, 表面活性剂分子的疏水部分所处环境的 变化更大. ΔC p,mic 的数值正是反映表面活性剂在胶束化 前后所处环境变化的热力学参数, 该数值的大小能够近 似地反映表面活性剂分子中暴露于水相的疏水部分面 积在胶束化过程中的变化情况 [12,13] . 因此, 通过 ΔC p,mic 的数值可以推测表面活性剂的聚集体结构.…”