Monosolvates

A monosolvate is a chemical compound crystallizing with exactly one molecule of solvent incorporated into each formula unit. This 1:1 solvation stoichiometry distinguishes monosolvates from polysolvates, which contain more than one solvent molecule per formula unit, and from solvates where solvent is only loosely associated and may be lost readily upon storage or heating.

Monosolvates form during crystallization from a solvent or by controlled solvent exchange, when the lattice energy and hydrogen-bonding network stabilize a fixed amount of solvent within the crystal. Water is a common solvent for hydrates, but monosolvates can involve many organic solvents as well. A well-known example is theophylline monohydrate, where one water molecule is integrated per theophylline molecule.

Properties of monosolvates can differ markedly from the anhydrous form, affecting solubility, dissolution rate, hygroscopicity, thermal behavior, and stability. Desolvation, the process of solvent loss upon heating or drying, can produce the anhydrous form or a different solid form, sometimes with partial or complete loss of the crystalline structure. Analytical techniques such as single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry are commonly used to characterize solvent content, occupancy, and thermal stability.

In pharmaceutical and materials chemistry, the existence of monosolvates can influence processing, storage, and performance. Distinguishing monosolvates from hydrates, solvates with higher solvent content, and co-crystals is important for understanding their formation, stability, and practical handling. Notation typically denotes the solvate with a dot, e.g., compound·solvent, and a water-based monosolvate may be referred to as a monohydrate.