Behavior of cetyltrimethylammonium bromide and Triton X-100 mixture at solution–air interface in presence of short-chain alcohols

Surface tension measurements of the aqueous solutions of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixture with methanol or propanol in the whole range of alcohol concentration and in the range of surfactant mixture concentration from 1 × 10−6 to 1 × 10−3 M were carried out. Based on the obtained results, the surface excess concentration values of this mixture and alcohols were calculated from the equation of Gibbs adsorption isotherm as well as from the Guggenheim-Adam one. Applying these surface excess concentration values, the composition of the mixed surface layer was established and compared with the hypothetic monolayer determined from the surface excess concentration values of the surfactant mixture and alcohols on the assumption that their adsorption was independent. The composition of the TX-100 and CTAB mixture was also calculated from the Rosen et al. concept on the assumption that water+alcohol was a mixed solvent and compared to the composition of the mixed monolayer formed from the independent adsorption of TX-100 and CTAB. From this comparison it results that there is a synergism in the reduction of the water surface tension at a certain composition of the surfactant mixture in the presence of alcohols. This synergism was confirmed by the standard free energy of the surfactant mixture adsorption calculated from both the Langmuir equation and the excess Gibbs free energy of TX-100 and CTAB mixing in the monolayer in the presence of methanol or propanol.