A linear relation between the cloud point and the number of oxyethylene units of water-soluble nonionic surfactants valid for the entire range of ethoxylation

Abstract

The following linear equation correlates the cloud point (CP) of water-soluble polyoxyethylated nonionic surfactants (NSs) with the average number p of oxyethylene units per molecule: (p−p 0)/ CP=a+b(p−p 0). Here p 0 is the smallest value of p that confers solubility in cold water: In a homologous series of NSs, it belongs to the surfactant with CP=0 °C. Plots of CP versus p for five representative homologous series of NSs consist of three segments: A steeply ascending, nearly straight line, a transition region that ranges from p=15–22 to p=20–28, and a nearly horizontal plateau that approaches asymptotically the CPs of polyethylene glycols with molecular weights between 30,000 and 4400. These CPs range from 113 to 130 °C. Most CPs for NSs were taken from the literature or measured on commercially available samples; eight CPs above 100 °C were measured on newly synthesized surfactants. Previously published linear equations correlating CP with p cover only NSs with p<16 and CPs<100 °C: They apply only to the ascending segment of the CP versus p plots. Our equation covers the entire plots and applies to the full range of NSs, including extensively polyoxyethylated NSs with p⩾100. It can be used for selecting specific NSs for high-temperature applications. The hydrophile–lipophile balance of the surfactant with p= p 0 oxyethylene units, namely, HLB 0, is a novel quantitative measure of the hydrophobicity of the hydrocarbon moiety of the relevant homologous NS series. Its value reflects the size, composition, and structure of the hydrocarbon moiety.