Surfactants shear behaviour

Wtistneck, R., Kragel, J., Miller, R., Wilde, P.J., Clark, D.C. (1996). Adsorption of surface-active complexes between p-casein, p-lactoglobulin and ionic surfactants and their shear rheological behaviour. Colloids and Surfaces A Physicochemical and Engineering Aspects, 114, 255-265. 

In other cases, several discrete relaxation times or distributions of relaxation times can be found [39]. This is typically the case if the stress relaxation is dominated by reptation processes [42 ]. The stress relaxation model can explain why surfactant solutions with wormlike micelles never show a yield stress Even the smallest applied stress can relax either by reptation or by breakage of micelles. For higher shear rates those solutions typically show shear thinning behaviour and this can be understood by the disentanglement and the orientation of the rod-like micelles in the shear field. 

Efficient degreasing was found to be closely connected to the three-phase state and hence to the ultra-low interfacial tension between water and oil [170]. The so far unidentified mechanism of degreasing of animal skins could be understood and explained. Correlation of results obtained from phase behaviour measurements and degreasing experiments revealed that Eusapon OD shows the best degreasing performance and lead to the clarification of the four-step process of degreasing as shown in Fig. 10.13 [171]. The first step is the penetration of the surfactant into the skin. In a second step the natural fat is solubilised. A microemulsion phase coexists with a fat- and a water-excess phase and the interfacial tension between water and oil is ultra-low. On the surface of the skin dilution of the microemulsion with pure water, i.e. reduction of the salt concentration in the float, leads to the formation of a stable emulsion via shearing. The stable emulsion prevents the deposition of the fat on the skin and enables the transport of the natural fat away from the skin. 

This approximate theoretical model which attempts to explain the anomalous behaviour of protein/surfactant mixtures was recently confirmed for the HSA/CioDMPO mixture as an example [137]. The equilibrium surface tension isotherms for mixed and pure CioDMPO solutions at 22°C are shown in Fig. 2.25. It is seen that for c > 10" mol/1, the two isotherms are almost identical. For these CioDMPO concentrations the adsorption of HSA is negligible. The conclusion concerning the sharp change in the composition of the surface layer within a narrow CioDMPO concentration range is supported by the analysis of the surface shear viscosity t s of mixed monolayers [137]. 

A frequently encountered behaviour for longer chain surfactants, say Cu or above, is that at low or intermediate concentrations the viscosity starts to increase rapidly with concentration. This is exemplified in a plot of the (zero-shear) viscosity versus concentration in Figure 19.20. Here, the micelles grow with increasing concentration, at first to short prolates or cylinders and then to long cylindrical or thread-like micelles (Figure 19.21). 

Figures 10.13-10.18 present the rheological behaviour of viscoelastic vesicle phases. Both the shear moduli Go and the yield stress values Gy increase with increasing total surfactant concentration that is seen in Figure 10.13. Below concentration of 1 wt%, both quantities drop to zero, what means that the vesicles are no longer densely packed under these conditions and thus are not restricted to a fixed position. Above 1 wt%, both quantities increase almost linearly with the concentration and Go is about one order of magnitude higher than Gy. This confirms that the vesicles must be deformed by about 10% until they can pass each other and the solutions start to flow like liquids.

Hoffmann, H. and Ulbricht, W., Vesicle phases of surfactants and their behaviour in shear flow, Tenside Surf. Det., 35, 421-438 (1998). 

---