Excess partial molar enthalpies of 2-butoxyethanol and water in 2-butoxyethanol–water mixtures

Abstract

Excess partial molar enthalpies of 2-butoxyethanol (BE) and water, (I = BE or H₂O), were measured from 25 to 35 °C in the entire concentration range. The results indicated that there are three concentration regions bounded at about x_B = 0.02 and x_B = 0.5, x_B being the mole fraction of BE. In each region, the concentration and temperature dependence of (I = BE or H₂O), is distinctively different from those in the other regions, and appears to support the following views: In the water-rich region, x_B < 0.02, BE molecules cause an enhancement in the structure of water, and this effect spans a long range via a structurally enhanced network of water. The solute–solute (BE–BE) interaction is repulsive and of a long range character in terms of enthalpy. As x_B increases, the repulsive solute–solute interaction becomes stronger sharply to the threshold value, x_B ≈ 0.017, whereupon the mode of such mixing no longer becomes possible. In the intermediate range, 0.02 < χ_B < 0.5, a new scheme whereby BE molecules tend to associate is predominant. Two clathrate-like aggregates of the types h[BE(H₂O)_k] with k = 55 and 4 may exist together with BE clusters (BE)_n In the third region, 0.5 < x_B, BE molecules are exactly in the same environment as in pure liquid, while water molecules are almost in the same environment as in pure water. The solution consists of BE and water clusters. Keywords: excess partial molar enthalpy, 2-butoxyethanol–water.