Isothermal titration calorimetric studies of alkyl phenol ethoxylate surfactants in aqueous solutions

Two commercially available alkyl phenol ethoxylates (APEs), i.e. octyl phenol ethoxylate (Triton X-series) and nonyl phenol ethoxylate (Tergital NP-series), were studied at different temperatures using the isothermal titration calorimetric (ITC) technique. Both the critical micelle concentration (CMC) and the thermodynamic parameters associated with the surfactant micellization were determined. Increasing the length of hydrophilic PEG segments produces a higher CMC and a larger micellization enthalpy. Increasing the size of the hydrophobic alkyl segments leads to a sharp decrease in the CMC. With increasing temperatures, both the CMC and the micellization enthalpy decrease. The thermodynamic parameters revealed that the micellization of APE surfactant is an entropy driven process. Scaling relationships between the lengths of the PEG segments and CMC or micellization enthalpy were developed, i.e. CMC=0.0335N−0.0171; ΔHm=6.5 ln(N)−6.2 for Triton X-series, and CMC=0.0082N−0.0068; ΔHm=5.3 ln(N)−3.8 for Tergital NP-series, where N represents the number of the oxyethylene repeat units in the surfactant molecules. The thermodynamic parameters obtained from ITC were compared with those determined using the van’t Hoff analysis.