Adsorption of cetyltrimethylammonium bromide and propanol mixtures with regard to wettability of polytetrafluoroethylene. I. Adsorption at aqueous solution–air interface

Measurements of surface tension of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and propanol mixtures (γL) for 1×10−51×10−5, 1×10−41×10−4, 6×10−46×10−4, and 1×10−3 M1×10−3 M concentrations of CTAB as a function of propanol concentration in the range from 0 to 6.67 M at 293 K were carried out. The obtained results indicate that there is first-order exponential relationship between the surface tension and propanol concentration in the solution at constant CTAB concentration. These results were compared with those calculated from the equations derived by von Szyszkowski, Joos, Miller et al. From the comparison it resulted that the values of γL determined by the Szyszkowski equation are correlated with those measured only in a limited propanol concentration range because of changes of the constant related to the specific capillary activity in this equation as a function of propanol concentration, particularly in the range of its high concentration. In the case of the modified Joos equation there is a correlation between the calculated and measured values of γL only at a very low concentration of propanol. The values of the surface tension of aqueous solutions of CTAB and propanol mixtures determined by the relationships of Miller et al. at CTAB concentration, corresponding to unsaturated surface layer in the absence of propanol, are close to those measured, but there are bigger differences between the calculated and measured values of the surface tension for solutions at a constant value of CTAB concentration close to CMC. However, the values of the surface tension of aqueous solution of CTAB and propanol mixtures calculated from the modified Miller et al. equation, in which the aggregation process of alcohol molecules at water–air interface was taken into account, are in excellent agreement with those measured. The measured values of the surface tension and the Gibbs equations were used for determination of the surface excess of CTAB and propanol concentration at solution–air interface. The obtained results indicate that at the constant concentration of CTAB equal to 1×10−51×10−5 and 1×10−4 M1×10−4 M there is a maximum of excess concentration of propanol in the surface region at its bulk concentration close to 1 M. Using the calculated values of the surface excess concentration of propanol and CTAB at solution–air interface and assuming the proper thickness of the interface region, the total values of their concentration in this region were evaluated. Next, the standard surface free energy of CTAB and propanol mixtures adsorption was calculated. The calculated values of this energy indicate that the tendency to adsorb molecules of CTAB and propanol decreases with increasing propanol concentration probably because of entropy of adsorption decrease resulting from water structure destruction by propanol molecules.

Changes of water surface tension under influence of CTAB and propanol mixtures are presented. Curves 1, 2, 3, and 4 represent solutions at constant CTAB concentration equal to 1×10−51×10−5, 1×10−41×10−4, 6×10−46×10−4, and 1×10−31×10−3, respectively.Figure optionsDownload as PowerPoint slide