Thermodynamic study on the surface formation of the mixture of water and ethanol

The formation of the water + ethanol mixture/air surface was studied by measuring the surface tension as a function of temperature and the mole fraction of ethanol in the mixture. The surface composition, the entropy Δs, and energy Δu of surface formation were evaluated by adopting the thermodynamic equations. It was shown that the surface was enriched in ethanol molecules compared with the bulk solution. The values of Δs and Δu were examined closely by estimating the contribution from the nonideal mixing in the bulk solution and that at the surface to Δs and Δu. It was concluded that the surface formation is driven by entropy at lower x₂ and by energy at higher x₂ respectively, and that the surface formation becomes more energetically favorable with rising temperature.