Experimental and Thermodynamic Study on the Temperature-Dependent Surface Activity of Some Polyether Siloxane Surfactants at the Water–Air Interface

Measurements of the surface tension of aqueous solutions of some trisiloxane surfactants containing various polyether groups (HOL7, HOL9, and HOL12) at 293 K, 303 K, and 313 K were performed. The studied surfactants were synthesized by hydrosilylation reaction and their structural analysis was carried out by the ¹H NMR, ¹³C NMR, ²⁹Si NMR, as well as FT-IR techniques. The thermal stability of HOL7, HOL9, and HOL12, as well as their molecular weight distributions, were also studied. On the basis of the obtained experimental results of the surface tension of aqueous solutions of HOL7, HOL9, and HOL12, the activity of the studied surfactants at the water-air interface was determined and discussed in the light of intermolecular interactions. Using the measured values of the surface tension, the Gibbs surface excess concentration, the area occupied by the surfactant molecule in the adsorption layer, and the standard Gibbs free energy of adsorption of the studied surfactants at the water-air interface were also calculated. Based on the obtained thermodynamic parameters of adsorption of the studied surfactants at the water-air interface, temperature, as well as a number of polyether groups in the hydrophilic part of surfactant, impact on particular surfactant adsorption was deduced. In general, the changes in the standard Gibbs free energy of adsorption of the studied surfactants at the water-air interface indicate that their adsorption tendency decreases with decreasing temperature. In addition, that tendency also diminishes as the number of the polyether groups in the hydrophilic part of the surfactant increases.