Spreading of Surfactant Solutions over Hydrophobic Substrates

4 SPREADING OF SURFACTANT SOLUTIONS OVER HYDROPHOBIC SUBSTRATES 

Let a small water drop be placed on a hydrophobic surface. If the drop is small enough, then the effect of gravity can be ignored. Accordingly, the radius of the drop base, R(t), has to be smaller than the capillary length, a, and hence. 

First of all, let us consider expression (1.2) from Chapter 1, Section 1.1 for the excess free energy, 1 , of the droplet on a solid substrate  

The foregoing expression shows that the excess free energy decreases if (1) the liquid-vapor interfacial tension, y, decreases, (2) if the liquid-solid interfacial tension, y, deaeases, and (3) the solid-vapor interfacial tension, y, increases. 

Let us assume that, in the absence of surfactant, the drop forms an equilibrium contact angle above nil. If the water contains surfactants, then three transfer processes take place from the liquid onto all three interfaces surfactant adsorption at both (1) the inner liquid-solid interface, which results in a decrease of the solid-liquid interfacial tension, y,, (2) the liquid-vapor interface, which results 

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

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. 

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. 

Svitova TF, Hill RM, Radke CJ (2001) Spreading of aqueous trisiloxane surfactant solutions over liquid hydrophobic substrates. Langmuir 17 335-348... 

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