highboiling

High-boiling, or high-boiling-point, refers to liquids or solvents that have relatively high boiling points at standard atmospheric pressure. The designation is relative and depends on context; in many chemical applications, substances with boiling points above roughly 150–200°C at 1 atm are regarded as high-boiling. These materials are contrasted with low- or mid-boiling solvents used for rapid distillation.

Boiling point is determined by intermolecular forces and molecular structure. Heavier molecules, strong hydrogen bonding, and strong dipole interactions raise boiling points, while branching and flexible chains may lower it by reducing surface area and cohesive interactions. At standard pressure, high-boiling substances may require higher energy input to reach boiling and can exhibit high viscosities and low vapor pressures.

Handling and purification: High-boiling solvents are commonly purified by vacuum distillation to minimize exposure to air and decomposition. They may form azeotropes with water or other solvents, requiring azeotropic distillation, entrainers, or alternative separation methods. Their low volatility makes complete removal challenging, affecting product purity and process design.

Examples and notes: Common high-boiling solvents include dimethyl sulfoxide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, and certain glycols and glycol ethers. Some classes, such as ionic liquids, are effectively non-volatile and do not boil under typical conditions. The term is widely used in chemistry, coatings, and materials science to describe processing steps that involve heating to high temperatures or extended residence times.