A hydrophilic group. In the sulfate ester, an oxygen link is provided, forming the acid group of many commercial surface-active agents in which a little sulfonic acid and much long-chain sulfate ester are linked to fatty acids and oils. Examples of reagents used in mineral processing include Lissapol LS (sodium salt of oleic acid chloride and panisidine sulfonic acid); and Aerosol (sulfosuccinic acid diester).