Thermodynamics, Kinetics and Dilational Visco-Elasticity of Adsorbed CnEOm Layers at the Aqueous Solution/Air Interface

Abstract

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

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

“…It is seen from Tables 1 and 2 that only the surface activity b 1 and the minimum molar area ω 10 vary with the number of states. Note, some of the parameter values summarised in Tables 1 and 2 for s = 2 deviate from those given in [19], where these values have been optimised for the reorientation model s = 2 with regard to the dynamic surface tensions. Quite expectedly, fitting of C 14 EO 4 experimental isotherm using the multistate Model R0 with s = 5 (red solid line) also leads to a good agreement with experimental dependencies of surface tension on the surfactant concentration.…”

“…Leaping ahead, however, it should be noted that the optimisation of the Model R0 parameters with respect to the very sensitive dilational visco-elasticity isotherm at a fixed oscillation frequency leads to less good agreement with the experimental rheologic data; therefore, in what follows, we concentrate on the results calculated using the multistate adsorption Model R. In this connection, it is worth mentioning that other values of s are also physically reasonable, but owing to the large time required for these simulations, we report here only about results for natural numbers. Thus, the best agreement of the model with the experimental data has not been performed such that we searched for an optimum value for s. [19]; filled symbols, this work. The respective theoretical isotherms are calculated for s = 2 (dotted curves), s = 5 (dashed curve), and s = 10 (solid curve) with the parameter sets presented in Tables 1 and 2.…”

“…Experimental surface tension isotherms of C 14 EO 4 (blue symbols and curves) and C 14 EO 8 (yellow symbols and curves) surfactant solutions. Open symbols, [19]; filled symbols, this work. The respective theoretical isotherms are calculated for s = 2 (dotted curves), s = 5 (dashed curve), and s = 10 (solid curve) with the parameter sets presented in Tables 1 and 2.…”

“…The experiments were performed with the profile analysis tensiometer PAT2 in the buoyant bubble mode at a fixed temperature of 25 • C. All details for measuring the dynamic surface tensions and dilational viscoelasticity were identical to those reported in [19]. In this publication, intending mostly to test the applicability of the proposed model, only one surface oscillation frequency (0.1 Hz, which is at the upper limit of the applied experimental tool) was used.…”

“…Although the two-state reorientation model describes the adsorption layers of C n EO m best, the most recent systematic work on C 10 EO 5 , C 10 EO 8 , C 12 EO 5 , C 12 EO 8 , C 14 EO 4 , and C 14 EO 8 adsorbed at the water/air interface demonstrated that there are still some open questions [19]. While the main goal was to describe all measured dependencies (dynamic and equilibrium surface tensions and dilational surface visco-elasticity as functions of the surfactant bulk concentration) with the same set of values for the model parameters, it turned out that the adsorption kinetics required the assumption of a diffusion coefficient that decreases with time.…”