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.
Highlights
Surfactants are substances that find applications almost everywhere in our life
The experimental data for concentrations smaller than the respective critical micelles concentration (CMC) were fitted by the reorientation model, and are shown as solid lines
The surface tension isotherms can be fitted with good accuracy by the reorientation model, which accounts for the possibility for the surfactant molecules to adsorb in two states characterised by different surface areas
Summary
A huge number of technologies are based on the action of surface-active molecules and would not properly work without them. Prominent examples of their application include the production of detergents, personal care and the many emulsifiers and foaming agents used for various industrial and consumer products. About 50% of the total surfactant production [1] are non-ionic surfactants, and among these, poly(ethoxylated) molecules are the most frequent, such as Tweens or Tritons. These technical surfactants are not chemically pure but consist of many components with different size distributions of the alkyl chain [2] and ethylene oxide chain [3,4]
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