On the importance of the purity of surfactant solutions in determining their adsorption kinetics

Abstract

By means of a theoretical experiment it is demonstrated, that a barrier in the adsorption kinetics of a surfactant can be simulated by the presence of surface-active impurities. Hence, the application of models for the diffusion-kinetic-controlled and/or kinetic controlled adsorption presumes the proof of the surface chemical purity of the surfactant solution under investigation.Key words: Adsorption kinetics, model of diffusion-kinetic-controlled adsorption, effect of impurities on adsorption kinetics.Investigating~ the properties of surfactant adsorption layers the purity of the surfactant solutions used plays an important role. In particular the dynamic properties are strongly influenced by surface-active impurities [1][2][3]. In the evaluation of dynamic surface tension measurements of surfactant solutions in addition to the model of the diffusion-controlled adsorption [4,5] there are also applied models which take into consideration a special mechanism for describing the process of transferring the molecules from the state of solution to that of adsorption [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. This paper is to demonstrate by means of a "theoretical" experiment "how surface-active impurities may effect the adsorption kinetic behaviour of surfactants and what kind of misinterpretation may be induced by their presence. The "theoretical" experiment shall be carried out in analogy with a practical one. The surface activity of the impurity is assumed to be stronger than that of the surfactant. The adsorption of the mixture of surface active compounds shall be described by a generalized Langmuir-isotherm. roo-rl (t)-G (t)] (2)(c~-surface tension, cro-surface tension of pure water, R -gas constant, T-absolute temperature, t-time). Now we assume that the experimental findings are represented by such a dependence o (t), which -on its turn -is evaluated from the standpoint of adsorption kinetics. Furthermore as usually done in practice we assume that there will be no impurity in the surfactant solution under investigation. Then the model of the diffusion-kinetic-controlled adsorption [21,23] is used for the evaluation. Figure 1 represents the "experimental" dependence (e-e) as well as a family of curves cx (t) which relate to different kinetic constants of the model of diffusion-kinetic-controlled adsorption. These dependencies are analogous to the results which Borwankar and Wasan [21] obtained for the dynamic surface tension of n-decanoic acid solutions when applying a corresponding kinetic model to the experimental data given by Defay and Hommelen [24,25]. The experimental findings of [24,25] may lead to the conclusion of a hindered adsorption [21]. Just the same situation is met in the case of the above mentioned "theoretical" experiment, i. e. by the installation of an impurity in the surfactant solution system a hindered adsorption mechanism would be simulated for the true surfactant (main component).W527

Dynamic Surface Tension Measurement with a Dynamic Wilhelmy Plate Technique

“…This result could be caused by the surface-active impurity in the surfactant samples, which mimic a slow continuation of the adsorption process, and simulate an adsorption barrier, although the main surfactant as well as the contaminant adsorb via diffusion controlled mechanism. Similar effect of contamination in the surfactant sample on the adsorption kinetics has been observed and discussed by other researchers (28,29).…”

“…For long-time approximation, the surface adsorption is closed to saturation and surfactant molecules may have to cross an energy barrier to be adsorbed onto the aqueous/air interface, [28] where R is the gas constant, T is the absolute temperature, and Q is the energy barrier term.…”

A two-step model for surfactant adsorption at solid surfaces

“…In toluene, head group-solvent interactions would be expected to be much weaker as toluene shows far less polar character, and hence this energy barrier would be lower. Alternative models which have been employed to account for adsorption dynamics at the airwater interface suggested that surface-active impurities can give rise to an apparent barrier [16]. However, in the work described here any impurities co-adsorbed at the surface would be detected owing to the high sensitivity of ATR: impurities are not observed in the signals, so that this may be discounted as the origin of the barrier.…”

Novel Results on the Adsorption Properties of n-Alkyldimethylphosphine Oxides at the Air/Water Interface

“…After having discovered that reliable investigations on the adsorption of surfactants require a particular grade of the surfactant solutions' purity − some basic new adsorption properties were detected. One of these concerns the finding that, opposite to common sense, the cross-sectional area A min (limiting surface area demand per molecule adsorbed) is not constant within a series of soluble homologues but decreases with increasing chain length until the A min value of the corresponding insoluble homologues is reached.…”