Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

In this work, the binding between sodium oleate (SO), sodium laurate (SL), sodium dodecyl sulfate (SDS), and sodium dodecylphosphonate (SDP) and iron oxide nanoparticles was systematically investigated using isothermal titration calorimetry (ITC). Comparing the heat exchanged during the isothermal titration with the corresponding surfactant adsorption isotherm, in the cases of SO and SDP, a strong binding takes place at low surfactant concentrations. The binding enthalpy at this low surfactant concentrations depends on the type of surfactant anionic head group. For C12 surfactants, the phosphonate group produced the strongest endothermic binding, followed by the exothermic binding with the carboxylate group, followed by weak exothermic interaction with the sulfate group. For carboxylate surfactants, longer surfactant tails result in larger exothermic binding. Surfactants that exhibited large binding enthalpies also produced more stable suspensions. The Langmuir (L), Freundlich (F), and Langmuir Freundlich (L F) adsorption models were used to interpret the adsorption isotherms during the titration with sodium oleate. The L F adsorption isotherm model was selected to calculate the heat of the formation of the SO monolayer and bilayer on the iron oxide nanoparticles. The L F model reflects the finite or limited adsorption of the Langmuir model, but accounts for non-homogeneous adsorption of the Freundlich model that help account for surfactant self-assembly before and after adsorption. Coupling the adsorption model with the titration data is possible to calculate the real heat of adsorption of the surfactants on the metal oxide.