Isentropic compressibility behaviour of dilute solutions of alcohols in water

Abstract

The apparent molal adiabatic compressibility (ϕ_K) at 5, 15, 20, 25, 30 and 45°C have been obtained for dilute aqueous solutions of the following monofunctional aliphatic alcohols, from accurate sound velocity measurements carried out by the phase comparison interferometer and density measurements by the differential float balance: MeOH, EtOH, n-PrOH, n-BuOH, i-PrOH, i-BuOH, s-BuOH and t-BuOH. The limiting apparent molal compressibility (ϕ^°_K) values decrease with the increase in the chain length of solute molecules whereas the temperature coefficient ∂ϕ^°_K/∂_T are positive and increase with chain length. At 5°C the ϕ^°_K values for all the alcohols except MeOH are negative. The ϕ_K(x₂) curves have negative slopes for all alcohols except n-PrOH and n-BuOH. The negative and positive slopes of ϕ_K(x₂) curves suggest occupation of cavities and solute–solute interactions, respectively. The temperature dependence of ϕ^°_K further enables a distinction to be made in the way the alcohols dissolve in water: substitutional dissolution for lower alcohols and interstitial dissolution for the higher members, as proposed by Franks and Ives.