Solubility parameter and hydrophilic-lipophilic balance of nonionic surfactants

The solubility parameters of various polyoxyethylated nonionic surfactants were compared with their hydrophilic-lipophilic balance (HLB) numbers. The compounds included three homologous surfactant series based on dodecanol, octylphenol, and fatty acid esters of sorbitan, respectively, a polyoxyethylated sorbitol ester, and a polyethylene glycol. Solubility parameters were calculated from measured heats of vaporization for the polyoxyethylated dodecanol series and from molar attraction constants for all three surfactant series. The values remained nearly constant and independent of the degree of polyoxyethylation, increasing at most by 1 (cal/cm3) 1/2 while the HLB increased from 0 to 10. This discrepancy arose because HLB values are based on emulsification experiments, in which the polyoxyethylene or polyol moiety of the surfactants is hydrated, while the solubility parameter was calculated for anhydrous conditions. When the solubility parameter was corrected for hydration by including a hydrogen-bonding component, plots of HLB versus this new solubility parameter were nearly linear and parallel for the three series of surfactants, with slopes of 5.0 +/- 0.2. The three lines were spaced apart only approximately 1.2 (cal/cm3) 1/2 despite structural differences among the surfactants, indicating that the chemical nature of the hydrocarbon moiety exerts only a limited effect on the solubility parameter. The HLB, which considers only the weight percent of the hydrocarbon moieties of nonionic surfactants and completely disregards differences in structural features, is, therefore, not as bad an approximation as had previously been thought.