The adsorption tendency of cetylpyridinium bromide at water–air interface and micelles formation in the presence of propanol

The effect of propanol concentration on the adsorption behaviour at water–air interface and micelle formation of cetylpyridinium bromide (CPBr) was investigated by surface tension measurements of aqueous solutions of CPBr and propanol mixtures. The measured values of surface tension as a function of propanol concentration in the range from 0 to 6.67 M were compared with those calculated from Miller et al. equation. From comparison it results that the mixtures of the CPBr and propanol in the surface monolayer behaved as ideal ones if the monolayer was formed by adsorption of CPBr and propanol from the solution in the concentration range of CPBr corresponding to unsaturated layer of CPBr in the absence of propanol. However, at concentration of CPBr in aqueous solution corresponding to saturated monolayer of CPBr without propanol the deviation from ideal behaviour was observed, and the pressure of the mixed monolayer cannot be predicted by Miller et al. equation derived for the mixtures of the homological series of surfactants or surfactant and alcohols having comparable hydrocarbon chain. On the basis of the obtained experimental and calculated results it was suggested that the aggregation process influences not only CPBr adsorption at solution–air interface but also the micelle formation in the bulk phase. The obtained relationship between CMC and propanol concentration indicates that CMC values considerably increase at the propanol concentration close to that at which probably aggregation of propanol molecules takes place. Above this concentration no formation of CPBr micelles was observed. The measured values of the surface tension were used in Gibbs equation for calculations of surface excess concentration of CPBr and propanol.Next, taking into account the calculated values from this equation and assuming a certain thickness of the interface region, the values of CPBr and propanol concentration in this region were determined. The values for propanol are going through a maximum corresponding to propanol concentration in the bulk phase equal to 1.07 M. On the basis of the values of CPBr and propanol concentration in the surface layer and the bulk phase, the values of the standard free energy of adsorption were calculated. The values of the standard free energy of adsorption obtained in this way were compared with those calculated from the modified Langmuir equation.