A Multistate Adsorption Model for the Adsorption of C14EO4 and C14EO8 at the Solution/Air Interface

Abstract

The dynamic and equilibrium properties of adsorption layers of poly (oxyethylene) alkyl ether (CnEOm) can be well described by the reorientation model. In its classical version, it assumes two adsorption states; however, there are obviously surfactants that can adsorb in more than two possible conformations. The experimental data for C14EO4 and C14EO8 (dynamic and equilibrium surface tensions and surface dilational visco-elasticity as measured by bubble profile analysis tensiometry) are used to verify if a reorientation model with more than two possible adsorption states can better describe the complete set data of CnEOm adsorption layers at the water/air interface. The proposed refined theoretical model allows s different states of the adsorbing molecules at the interface. The comparison between the model and experiment demonstrates that, for C14EO4, the assumption of s = 5 adsorption states provides a much better agreement than for s = 2, while for C14EO8, a number of s = 10 adsorption states allows an optimum data description.

A Multistate Adsorption Model for the Characterization of C13DMPO Adsorption Layers at the Aqueous Solution/Air Interface

“…For example, the nonionic surfactants with flexible ethylene oxide chains (C n EO m ) can allow a large variety of conformations in the adsorption layer, as discussed previously. 15 More compact molecules like the nonionic surfactant C 13 DMPO should have a smaller number of conformations. Nevertheless, also for such molecules, more than two adsorption states are imaginable, depending on the conformation and/or orientation of the molecules at the interface and the available free interfacial area.…”

“…Similar results were obtained by Rusanov who discussed the impact of molecular orientation in terms of changes in the adsorption layer thickness. 27 To keep a minimum number of model parameters, several simplifications were proposed previously, 15 and we use all of them here as well. Namely, we assume that the molar area increment between two neighboring adsorption states is constant:…”

“…14 After originally two coexisting states of adsorbed molecules were assumed, a generalization of the reorientation model to more than two adsorption states allows us to discuss adsorption layers in a multistate adsorption situation, as was demonstrated for the two poly (oxyethylene) alkyl ethers C 14 EO 4 and C 14 EO 8 in a previous study. 15 In the literature, various adsorption models are proposed, in some cases to improve the quality of agreement between the theory and experiment and in other cases to find appropriate methods to describe the surface behavior of new types of surfactants adequately. Wegrzynśka et al proposed a surface quasi two-dimensional electrolyte model to describe the adsorption layer properties of newly synthesized ionic surfactants.…”

Prediction of Aqueous Solution Surface Tension of Some Surfactant Mixtures and Composition of Their Monolayers at the Solution—Air Interface

“…So far, the ω has been expressed as equal to 1 Γ max (Γ max is the maximal Gibbs surface excess concentration) but as mentioned above, π is related to kΓ max . Therefore, it seems that should be equal to 1 kΓ max . It appears that the values of the surface tension of aqueous solution of CTAB, CPyB, SDDS, SDS, TX100, TX114, TX165, FC1 and FC2 calculated from Equation (3) are close to the measured ones if the abovementioned conditions are satisfied (Figures 1-4).…”

“…Application of surfactants is very often due to their adsorption at the water-air interface. As a result, a surface monolayer is formed, which reduces the water surface tension [1][2][3][4][5][6]. The reduction magnitude depends on the kind of surfactants and/or surfactant mixtures and their concentration in the aqueous solution.…”