Sulphonates are the salts/esters of the sulphonic acid with the general structure R-SO2-O-X, where X can be an organic residue or a cation. In contrast to the esters of sulphuric acid, sulphonates have R-S and not R-O-S bonds as sulphates do. Sulfonates differ from the sulfones (R-SO2-R'), which do not have a hydroxyl group. Sulphonic acids are strong acids. Acids and sulphonates are usually water-soluble or easily miscible with water.
Sulfonic acids in which the residual group R is aliphatic are referred to as alkanesulfonic acids and their salts and esters as alkylsulfonates or also as alkanesulfonates. For example, magnesium lauryl hydroxypropyl sulfonates (INCI) are used as surfactants in cosmetic formulations.