Cosolvency chemical potential

Smith PE (2004) Local chemical potential equalization model for cosolvent effects on biomolecular equilibria. J Phys Chem B 108(41) 16271-16278... 

Use of consistent thermodynamic conditions for enhancer formulations Permeation enhancement efficacy of a CPE is a function of its chemical potential, temperature, pressure, and cosolvent amongst other thermodynamic parameters. These thermodynamic conditions need to be standardized for all the enhancers that are being tested to create direct comparison of their efficacies in increasing skin permeation. 

The present paper is devoted to the local composition of liquid mixtures calculated in the framework of the Kirkwood—Buff theory of solutions. A new method is suggested to calculate the excess (or deficit) number of various molecules around a selected (central) molecule in binary and multicomponent liquid mixtures in terms of measurable macroscopic thermodynamic quantities, such as the derivatives of the chemical potentials with respect to concentrations, the isothermal compressibility, and the partial molar volumes. This method accounts for an inaccessible volume due to the presence of a central molecule and is applied to binary and ternary mixtures. For the ideal binary mixture it is shown that because of the difference in the volumes of the pure components there is an excess (or deficit) number of different molecules around a central molecule. The excess (or deficit) becomes zero when the components of the ideal binary mixture have the same volume. The new method is also applied to methanol + water and 2-propanol -I- water mixtures. In the case of the 2-propanol + water mixture, the new method, in contrast to the other ones, indicates that clusters dominated by 2-propanol disappear at high alcohol mole fractions, in agreement with experimental observations. Finally, it is shown that the application of the new procedure to the ternary mixture water/protein/cosolvent at infinite dilution of the protein led to almost the same results as the methods involving a reference state. 

The Fluctuation Theory of Solutions—also known as Fluctuation Solution Theory, Kirkwood-Buff Theory, or simply Fluctuation Theory— provides an elegant approach relating solution thermodynamics to the underlying molecular distributions or particle number fluctuations. Here, we provide the background material required to develop the basic theory. More details can be found in standard texts on thermodynamics and statistical mechanics (Hill 1956 Munster 1970). Indeed, the experienced reader may skip this chapter completely, or jump to Section 1.2. A list of standard symbols is also provided in the Prolegomenon to aid the reader, and we have attempted to use the same set of symbols and notations in all subsequent chapters. Throughout this work we refer to a collection of species (1, 2, 3,...) in a systan of interest. We consider this to represent a primary solvent (1), a solute of interest (2), and a series of additional cosolutes or cosolvents (3,4,...) which may also be present in the solution. However, other notations such as A/B or u/v is also used in the various chapters. All summations appearing here refer to the set of thermodynamically independent components (n in the mixture unless stated otherwise. Derivatives of the chemical potentials with respect to composition form a central component of the theory. The primary derivative of interest here is defined as... 

A third illustration of the use of FST for open systems involves the effects of cosolvents or additives on the solubility of a solute in a solvent. If one follows the solute solubility curve, at a fixed temperature and pressure, then the chemical potential of the solute at saturation remains constant as it is in equilibrium with the solid solute. Hence, the effect of an additive on the molar solute solubility can be expressed in terms of derivatives of this curve taken at constant T, p, and P2. Using these constraints in Equation 1.43 and taking the appropriate derivatives, one immediately finds (Smith and Mazo 2008)... 

Both expressions are valid for systems containing any nnmber of components at any composition. The chemical potential derivatives are provided by the expressions for an component system. For low solnte concentrations, the former equation also provides an expression for the standard volume change for the process. A particularly common sitnation involves the effect of a single cosolvent on the conformational eqnilibrinm (n= 1, D N) of an infinitely dilute solute. In this case, Equation 1.98 then rednces to... 

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

The solubility of contaminants in subsurface water is controlled by (1) the molecular properties of the contaminant, (2) the porous media solid phase composition, and (3) the chemistry of the aqueous solution. The presence of potential cosolvents or other chemicals in water also affects contaminant solubility. A number of relevant examples selected from the literature are presented here to illustrate various solubility and dissolution processes. 

One of the primary components in the cost of cosolvent flushing technology is the cost of the cosolvent solution. Reuse of the flushing solution has shown the potential to greatly reduce the cost of treatment by reducing both chemical costs and the treatment and disposal costs of the extracted contaminants (D21314Z, p. 5). 

Many factors potentially can affect the distribution of an organic chemical between an aqueous and solid phase. These include environmental variables, such as temperature, ionic strength, dissolved organic matter concentration, and the presence of colloidal material, and surfactants and cosolvents. In addition, factors related specifically to the experimental determination of sorption coefficients, such as sorbent and solid concentrations, equilibration time, and phase separation technique, can also be important. A brief discussion of several of the more important factors affecting sorption coefficients follows. 

Penetration. In whole wood, accessibility of the treating reagent to the reactive chemical sites is a major consideration. To increase accessibility to the reaction site, the chemical must penetrate the wood structure. Penetration can be achieved by causing the wood structure to swell. If a reagent potentially capable of modifying wood does not cause the wood substance to swell, then catalyst may be necessary. If both the reagent and catalyst are unable to cause the wood to swell, a workable cosolvent could be added to the reaction system. 

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