waterinsolubility

Water insolubility refers to the condition in which a substance dissolves very little or not at all in water. It is typically used to describe solids and, less often, gases and polymers. Solubility in water depends on molecular structure, polarity, hydrogen-bonding potential, and the balance between lattice energy or cohesive forces within the solid and the energy gained from interactions with water.

Several factors influence water insolubility. Nonpolar and bulky molecules tend to have weak interactions with water, leading to high free-energy costs for dissolution. Ionic compounds vary widely: some release ions that strongly hydrate, while others form lattices with high lattice energy that resist dissolution. Temperature often alters solubility; for many solids, solubility increases with temperature, though some substances show little change or opposite trends. pH, presence of salts, and solvent co-solvents can also modify apparent solubility.

Measurement and classification rely on solubility data at a given temperature. Substances are often described as insoluble or sparingly soluble when their maximum dissolution in water is very small, typically expressed in mass per unit volume or molarity. For sparingly soluble salts, the solubility product (Ksp) provides a quantitative framework to predict dissolution and precipitation under different conditions.

Examples of water-insoluble materials include many hydrocarbons (oils and waxes), certain polymers (polyethylene), and inorganic solids such as barium sulfate and silica. In pharmaceuticals, water insolubility can limit bioavailability and motivate formulation strategies like salt formation, co-crystals, particle size reduction, or surfactants. Environmentally, water-insoluble contaminants may persist and require specialized remediation approaches.