On the Utility of Energy-Volume Coefficient (Internal Pressure) to Study Excess Partial Molar Volumes of Alcohols in Aqueous Solutions at 298.15 K

Abstract

Using fine and precise data of density (r) at different temperatures, speed of sound (u) and specific heat (C_p) at 298.15 K, available in literature, the calculations of isothermal compressibility (k_T) in the limiting concentration range (0-0.25 M) for aqueous alcohol solutions (methyl to n-pentyl and t-butyl alcohols) have been made. The coefficient of thermal expansion (a_P) and isothermal compressibility (k_T) of solutions were used to obtain energy-volume coefficient p_int = (¶U/¶V)_T property as a function of alcohol concentration. It was found that p_int increases with the increase in concentration, the extent becoming more as the chain length of alcohol molecules increase. Applying the Gibson-Trait equation of state, the calculations of excess partial molar volume (`V₂^E) of alcohols were made, which are negative and again show dependence on chain length. Further calculations of excess internal pressure (p_int^E) were made using volume fraction statistics for aqueous alcoholic solutions except for n-pentanol-H₂O system. It is observed that solutions of methanol, ethanol and tert-butanol exhibit negative p_int^E but of n-propanol and n-butanol show positive p_int^E, increasing in magnitude with increase in concentration of alcohol. These results are explained on the basis of hydrophobic hydration of alcohols and persistence of hydrophobic interaction between the non-polar parts of alcohol down to lowest concentration studied in case of higher alcohols. It has been shown that pure liquid as a standard state is inadequate to obtain excess partial molar volumes of solute in water. The p_int^E profiles subtly differentiate the mode of interactions of alcohols, either by substitutional dissolution or by interstitial dissolution indicating the importance of structural changes of solvent water.