![]() ![]() This makes the hydrophobic effect the main driving force that causes metal complexes with organic ligands to be transferred from aqueous solutions to inert organic solvents. Both approaches predict a negative free energy change for removing hydrophobic solutes from water, which means their spontaneous transfer to organic solvents. Another approach to the problem, based on the work required to form cavities able to accommodate the solute molecule of a given size and shape in both liquid phases, assumes that the free energy of cavity formation is more positive in water than in common organic solvents. This value almost entirely corresponds to a negative entropy change that results from the ordering of the water structure, while the negative enthalpy of hydrogen bonds formation is compensated to a great extent by the positive enthalpy of removing the hydrocarbon molecule from its own liquid phase. The positive contribution from one methylene group to the standard free energy of dissolution of liquid hydrocarbons in water, Δ G S o(CH 2) ≈ 3.6 kJ mol − 1 at 25☌, significantly decreases with increasing temperature. Poor solubility of liquid hydrocarbons in water, decreasing with increasing chain length, is the result of local structuring (by means of hydrogen bonds) of liquid water around the hydrophobic surface of the solute molecule. The hydrophobic effect increases the thermodynamic activity of large hydrophobic molecules of metal complexes formed in the aqueous phase of solvent extraction systems, which promotes their transfer from the aqueous to the organic phase. Jerzy Narbutt, in Liquid-Phase Extraction, 2020 4.4.1 Hydrophobic effect
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