Rheological study and thermodynamic analysis of the binary system (water/ethanol): Influence of concentration

Water is the most widely used solvent in the chemical and pharmaceutical industry, since it is the most physiological and best tolerated excipient. However, in some cases water cannot be used as a solvent because the active substance or solute is insoluble or only slightly soluble in water. For this and other reasons, nonwater solvents may be used possessing the common characteristic of being soluble or mixable in water; as a result, such solvents can be used to prepare binary or tertiary mixtures, etc., with different purposes such as increasing water solubility, or modifying the viscosity or absorption of the dissolved substance, for example. Ethanol, along with other alcohols either alone or in water-alcohol solutions involving different proportions, are widely used in the pharmaceutical industry as excipients in different formulations, or as solvents. Ethanol-water systems are characterized by the so-called volume contraction phenomenon, which is in turn accompanied by a considerable increase in the viscosity of the system. This is attributed to the increase in size of the molecular package secondary to solvation or the formation of hydrogen bonds between the alcohol and water - a phenomenon referred to as viscous synergy. The formation of hydrogen bonds between alcohol and water modifies with temperature, thus leading to variations in the viscosity of the system. The present study investigates the viscous synergy of systems comprising pure alcohol in water at different concentrations, determining the proportion of alcohol-water at which maximum viscous synergy occurs, along with the correlation between the viscosity increments and density. The ratio between the maximum viscosity reached by the mixture and the viscosity of pure alcohol is expressed by the enhancement index defined as: E-n = n(max)/n(0). Likewise, and since the viscosity of these systems varies with temperature, a thermodynamic study has been made to determine the activation energy of the ethanol-water mixture as a function of concentration.