Structuring of Organic Solvents around Solutes and Crystal Surfaces



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An important aspect of crystalline material synthesis from solution crystallization involves solvent structuring at the crystal-solvent interface. The presence of lyophilic/lyophobic groups at the crystal-solution interface affects the solvent structuring around the crystal surface and incorporation of solute molecules into the crystal lattice. Solvent structuring around the crystal lattice may be ordered or disordered depending on the groups exposed on the crystal surface. In general, the effect of solvent structuring on the molecular level interactions between solid sorbate on solid sorbent is poorly understood, to this end we have studied the thermodynamic parameters of crystallization for a group of supramolecules known as “Porphyrins” in organic solvents. Thermodynamic parameters of crystallization act as molecular probes for the presence of solvent structuring. We studied non-polar and charged polar porphyrins containing two and four carboxylic groups. The hydrophobic porphyrin, Etioporphyrin I was crystallize from the solvents chosen for the study (octanol, butanol, DMSO and Octanoic acid). We find that hydrophobic interactions between the porphyrin molecule and aliphatic chain of the alcohols contributes to the enthalpy and entropy of crystallization thus influencing the solubility. The presence of carboxylic group in Octanoic acid results in higher interaction between the acidic carboxylic group and basic NH groups of Etioporphyrin I, resulting in a positive entropy of crystallization. The different crystal habit of Etioporphyrin I crystallized from different solvents like alcohols, polar aprotic solvent, DMSO and Octanoic acid indicates the importance of solute-solvent interaction during crystallization. Studies on polar porphyrins indicate that in case of solvents having long aliphatic chain the solvent structuring around the solute molecules play a crucial role in crystallization.



Solvent structuring, Porphyrins, Crystallization