Polymeric surfactants for enhanced oil recovery. Part III—interfacial tension features of ethoxylated alkylphenol-formaldehyde nonionic surfactants

The interfacial tensions (IFT) of four low molecular weight groups of ethoxylated octylphenol-, dodecylphenol-, tetradecylphenol- and hexadecyl-phenol—formaldehyde polymeric surfactants were determined using the spinning drop method. Some noteworthy features of the interfacial behaviour of dilute aqueous solutions of 16 of these compounds and homologous hydrocarbons are discussed. An important feature is that these surfactants behave similarly to monomeric ones in their hydrocarbon scan, that is they have a minimum IFT value against a particular member of a homologous hydrocarbon series. The magnitudes of the tension at minimum (γ_min) values obtained in this study are of the order of ‘ultralow’ (10^?2-10^?3 mNm^?1). The n_min values of these polymeric nonionic surfactants decrease with increasing hydrophilicity, that is decrease with the increase of ethylene oxide units condensed per mole of alkylphenol unit in the polymeric surfactants studied. In this case, the downward shift in n_min is smaller and apparently not linearly related to the number of EO units. Increasing the hydrophobicity of these polymeric nonionics, that is increasing the length of the alkyl chain from C₈ to C₁₆, resulted in an increase in the n_min values obtained. For each of the investigated groups, the lowest γ_min values are obtained with polymeric surfactants having the highest EO content. The optimum low tension performance occurs at the low end of the equivalent alkane carbon number scale (at EACNs below 6). Under the influence of added electrolytes these EACNs were shifted to higher values.