Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.

Graphical abstractComparison of the TX-100 and CTAB activity in the mixed monolayer (a12S) at the water–air interface calculated from the Sprow and Prausnitz equation to the standard Gibbs free energy of adsorption of the TX-100 and CTAB mixture (ΔGadso) at this interface calculated from the Langmuir equation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► At low concentration of the TX-100 and CTAB mixture they adsorb independently. ► The synergetic effect of the TX-100 and CTAB mixtures is confirmed by their activity. ► CTAB high concentration affects to a smaller extent TX-100 adsorption than the reverse. ► TX-100 and CTAB mixture ΔGadso minimum is in agreement with the maximum synergetic effect.