Spreading of surfactant

INSOLUBLE MONOLAYERS SPREADING OF SURFACTANTS ON AQUEOUS SURFACES... 

The presence of gradients of I (and thus of a) on the interface drives a flow in fhe thin film that is mainly responsible for the spread of surfactant on fhe surface of the film. 

If there is only one surfactant, the wave velocity can be interpreted as the rate of spreading of surfactant over the interface. Actually, the word spreading is to some extent misleading, since it is the interface that moves, taking the surfactant with it. Putting v = dx/dt, integration of Eq. (10.18)... 

FIGURE 10.31 Vessel with a solution of surfactant (denoted by short lines), of which the surface is immediately enlarged (frame 2). Spreading of surfactant (3) and adsorption (4). y — surface tension. See text. Highly schematic. 

Assume that the experiment illustrated in Figure 10.31 is done with a solution of Na-stearate and with one of Na-myristatc. Each solution has a surface tension of 50 mN m The barrier is moved over 5 cm, thereby increasing the AW surface area by a factor of two. For which solution is the spreading of surfactant fastest What is the time needed to restore the initial value of y for each solution ... 

Adsorption isotherms can be applied to any surface. In the following we focus our attention on surfaces covered with adsorption layers under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from soluble adsorption layers for example. Another phenomenon is the spread of surfactant molecules tangential to the surface that effect takes place if the adsorption layer is inhomogeneous (cf. Fig. 1.1). 

A method to obtain nano-particles at air/water interfaces has been described in [287]. Spreading of surfactant-coated metallic, semi-conducting, magnetic and ferroelectric nanoparticles on water surfaces results in the formation of monoparticulate thick films which then can be transferred, layer by layer, to solid substrates. These films can find potential applications in advanced electronic and electro-optical devices. Here and further, we give only typical examples of using surfactants in novel technologies. A more detailed description can be found in a new edition of Surfactants Science Series [288]. 

The spreading of surfactants forced the water to flow away from the initial location of th drop thus creating a depression where only a thin film of liquid subsisted. The periphery of thi depression, i.e. the liquid front had a sharp increase in thickness. The water layer wa horizontal and the drop was carefully placed, hence, there was no preferred direction, and th edge was circular but some modulation could appear in the experiment after a few seconds Note that the surfactant occupied more surface area of the film than the depressed zone sine the talc powder is pushed ahead of it. The dependence on time of the radius of the surfactan patch is given in Fig. 3 using log-log plot. 

Spreading of surfactant soiutions over hydrophobic substrates... 

Sudden extension of an interface or sudden local application of a surfactant to an interface, however, can produce a fairly large -gradient, and in such a case spreading of surfactant can occur. This proceeds as a longitudinal wave, according to Lucassen. He derived for the linear velocity of such a wave on an air/water surface... 

Starov, V.M., Kosvintsev, S.R., Velarde, M.G., 2000. Spreading of surfactant solutions over hydrophobic substrates. J. CoUoid Interface Sci. 227,185. 

In this chapter we consider the kinetics of spreading of surfactant solutions over hydrophobic and porons substrates. In spite of the wide use of these processes, currently we are not in the position to answer even basic questions in this area, such as how surfactant molecules are transferred in a vicinity of the three-phase contact line. 

It is obvious that adsorption of surfactant molecules behind the moving meniscus results in a decrease of the bulk surfactant concentration from the capillary inlet in the direction of the moving meniscus. However, as we show in this section, the major process, which determines penetration of surfactant solutions into hydro-phobic capillaries or spreading of surfactant solutions over hydrophobic substrates, is the adsorption of surfactant molecules onto a bare hydrophobic substrate in front of the moving three-phase contact line. This process results in a partial hydrophilization of the hydrophobic surface in front of the meniscus or drop, which, in its turn, determines spontaneous imbibition or spreading. 

SPREADING OF SURFACTANT SOLUTIONS OVER HYDROPHOBIC SUBSTRATES... 

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