Theoretical Insight into Preferential Interaction Issues and Solution Structure, and Contentious Apparent Hydrated Molar Volume of Cosolute

Background: There seems to be a mathematical or a conceptual error in an equation whose substitution into other equations for the determination of an apparent hydrated molar volume (V1) of a cosolute leads to an incorrect answer.

Objectives: The objectives are 1) To show theoretically that the preferential interaction parameter (PIP) is an extensive thermodynamic quantity, 2) rederive new equations and reexamine various equations related to solution structure, 3) apply derived equation for the determination of V1, and 4) determine m-values and cognate preferential interaction parameter (PIP).

Methods: The research is mainly theoretical and partly experimental. Bernfeld method of enzyme assay was adopted for the generation of data.

Results and Discussion: The investigation showed that equation linking chemical potential of osmolyte to solution structure is dimensionally invalid; PIP was seen as a thermodynamically extensive quantity. Equations for the graphical determination of V1 of the osmolyte were derived. With ethanol alone, there were - m-value and + PIP; with aspirin alone, there were + m-value and - PIP. There was a change in sign in m-value with sucrose and ethanol/aspirin mixture, and a change in sign in PIP when the latter is taken as function of [ethanol]/[aspirin] and [sucrose](c3).

Conclusion: A solution structure is as usual determined by either a relative excess or a deficit of the solution component either in the bulk or around the macromolecular surface domain; the PIP remains thermodynamically an extensive quantity. To be valid there is a need to introduce a reference standard molar concentration or activity to some equations in literature. The slope Capture3.PNG from one of the equations seems to give a valid value for V1 (V1 is «1; is activity coefficient). A known destabiliser may behave as a stabiliser being excluded. Like ethanol, aspirin as cosolute is destabilising and opposed by sucrose.