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We examine the ordering of the Lower Consolute Solution Temperatures (LCSTs) for a set of dimethylpyridines. Density functional theory (DFT) is used. The equilibrium geometries and binding energies of dimers, each comprised of a pair of dimethylpyridines in a sandwich conformation and one H2O molecule at a pivotal site between the nitrogens (the 2:1 dimer), are calculated. It was shown previously that dimer formation in the water-rich zone of the phase diagram has a crucial role in dimethylpyridine demixing. In the resulting dimer diffusion, large hydrophobic clusters of mostly organic content, which expel water and promote phase separation, are assembled. In this description, phase separation requires the formation of 2:1 dimers, but it is the cleavage of hydrogen bonds of the neighboring H2O molecules, which stimulates the diffusion and the subsequent separation dynamics at the LCST. In the present study, we investigate this model and calculate the interaction strength of the external hydrogen bonds. This is obtained as the difference in electronic energy between the 2:1 dimer and the dimer augmented by one or two H2O molecules. The results are compared to the known LCST hierarchy in five dimethylpyridines (DMP): 2,6-DMP > 2,4-DMP > 2,5-DMP > 3,4-DMP > 3,5-DMP. The complexes are derived using high level Kohn–Sham DFT including dispersion terms. The hydrophobic–hydrophilic properties are accounted for by the solvation model, employed for the mixed medium of 60%-water and 40%-organic content. This is simulated by combination of model descriptors of water and DMP in the parameterization scheme of the polarizable continuum model. The calculation results agree with the experimental evidence. [ABSTRACT FROM AUTHOR]

Phase separation in substituted pyridines in water is usually described as an interplay between temperature-driven breakage of hydrogen bonds and the associating interaction of the van der Waals force. In previous quantum-chemical studies, the strength of hydrogen bonding between one water and one pyridine molecules (the 1:1 complex) was assigned a pivotal role. It was accepted that the disassembly of the 1:1 complex at a critical temperature leads to phase separation and formation of the miscibility gap. Yet, for over two decades, notable empirical data and theoretical arguments were presented against that view, thus revealing the need in a revised quantum-mechanical description. In the present study, pyridine-water and 2,6-dimethylpyridine-water systems at different complexation stages are calculated using high level Kohn-Sham theory. The hydrophobic-hydrophilic properties are accounted for by the polarizable continuum solvation model. Inclusion of solvation in free energy of formation calculations reveals that 1:1 complexes are abundant in the organically rich solvents but higher level oligomers (i.e., 2:1 dimers with two pyridines and one water molecule) are the only feasible stable products in the more polar media. At the critical temperature, the dissolution of the external hydrogen bonds between the 2:1 dimer and the surrounding water molecules induces the demixing process. The 1:1 complex acts as a precursor in the formation of the dimers but is not directly involved in the demixing mechanism. The existence of the miscibility gap in one pyridine-water system and the lack of it in another is explained by the ability of the former to maintain stable dimerization. Free energy of formation of several reaction paths producing the 2:1 dimers is calculated and critically analyzed. [ABSTRACT FROM AUTHOR]