Krafft Points and Cloud Points of Polyoxyethylated Nonionic Surfactants

Abstract

Abstract Polyoxyethylated nonionic surfactants (POSs) are more soluble in cold than in hot water. On heating, they precipitate reversibly from aqueous solution at and above characteristic temperatures, which are called cloud points (CPs) because the solutions turn cloudy/opaque. The CPs of normally distributed POSs are correlated with the average number p of oxyethylene units per molecule by the linear equation (p – p0)/CP=A+B(p – p0). In a homologous series of POSs, the smallest value of p conducive to solubility in ice-cold water, p0, belongs to the surfactant with CP=0°. Its hydrophile-lipophile balance, HLB0, is a quantitative measure of the hydrophobicity of the parent fatty alcohols, CmH2m+1OH, fatty acids and alkylphenols. New CPs, especially of POSs with p approaching p0, are combined with published CPs in this study. The order of increasing HLB0 and hydrophobicity of the parent compounds, accompanied by decreasing CPs of their ethoxylates, is: saturated linear primary alcohols [m=18 (lowest HLB0=8.2), 16, 14.6, 13.3, 12.7, 12.0]<oleyl alcohol<saturated primary alcohols with multiple methyl branches (m=10, 13)<Guerbet alcohols (2-hexyldecanol, 2-butyloctanol)<p-nonyl (i.e., triisopropyl)phenol<p-octyl (i.e., diisobutyl)phenol<esterified saturated linear acids [m=18, 12 (highest HLB0=11.4)]. Interpolation by means of the equation above with the appropriate values of p0, A and B supplies the CP for any homologous POS belonging to one of the 14 groups investigated, based on its degree of ethoxylation p.