 The shape of the aggregates depends on the chemical structure of the surfactants, namely the balance in size between the hydrophilic head and hydrophobic tail.A measure of this is the hydrophilic-lipophilic balance (HLB).Surfactants reduce the surface tension of water by adsorbing at the liquid-air interface.The relation that links the surface tension and the surface excess is known as the Gibbs isotherm.