Adsorption kinetics of surfactant mixtures

The adsorption kinetics of surfactant mixtures is a field rarely investigated. Systematic studies do not exist at all and therefore, we can give here only few information. As it was shown by the generalised model of Sutherland of Eq. (4.19) the adsorptions of the components of a mixture are independent at sufficiently low concentrations so that the dynamic surface tension y(t) can be calculated easily. The concentrations of the three compounds calculated for Fig. 4.34 differ 

The graphs shown in Fig. 4.35 are the dynamic surface tensions of three mixtures of CioDMPO and CmDMPO measured with the maximum bubble pressure method MPT2 (O) and ring tensiometer TE2 (O). Although there is a general theoretical model to describe the adsorption kinetics of a surfactant mixture, model calculations are not trivial and a suitable software does not exists. 

The graphs contain calculations for the main component of the mixture only, i.e. the shorter chain surfactant CioDMPO at a concentration 300, 100 and 30 times higher as that of the second component CmDMPO, respectively. Up to an adsorption time of about ten seconds the curve calculated for D = 210 cmVs agrees quantitatively with the experimental points. Only at this time the second component starts to adsorb and deviations in y(t) are observed. With increasing CMDMPO concentration the deviation increases and the total surface tension decrease becomes larger as expected for the adsorption of a mixture. The intermediate plateau is visible at the lowest C14DMPO concentration but fades with further increase as the adsorption of this component sets in earlier. 

The described results demonstrate very well how the adsorption kinetics allows to distinguish between the contributions of different compounds of a mixture to the measured surface tension change. Using the correct adsorption model (adsorption isotherm and equation of state) a quantitative description of the adsorption kinetics of mixtures would be possible, however, no programmes are available at present and analytical solutions do not exist. 

Adsorption kinetics, mainly studied by dynamic surface tension measurements, shows many features very much different from that of typical surfactants (Miller et al. 2000). The interfacial tension isotherms for standard proteins such as BSA, HSA, (3-casein and (3-lactoglobulin were measured at the solution/air interface by many authors using various techniques. The state of the art of the thermodynamics of adsorption was discussed in Chapter 2 while isotherm data for selected proteins were given in the preceding Chapter 3. Here we want to give few examples of the dynamic surface pressure characteristics of protein adsorption layers. 

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