Hydrophobic solid surface

The wetting of a hydrophobic solid surface by an aqueous medium is considerably helped by the addition of surface-active agents. Wa is increased and yLG is decreased (Figure 6.4b), so that, from the Young-Dupre equation, 0 is reduced on two counts. 

Surfactants adsorb on solid surfaces due to hydrophobic bonding, electrostatic interaction, acid-base interaction, polarisation of rr electrons and dispersion forces. Hydrophobic bonding occurs between the hydrophobic surfactant tail and the hydrophobic solid surface (tail down adsorption with monolayer structure) or between the hydrophobic tails of the surfactant adsorbed on the hydrophilic solid surface and the hydrophobic tails of the surfactant from the liquid phase (head down adsorption with bilayer structure) [54, 55]. 

Electrostatic interactions occur between the ionic head groups of the surfactant and the oppositely charged solid surface (head down adsorption with monolayer structure) [56]. Acid-base interactions occur due to hydrogen bonding or Lewis acid-Lewis base reactions between solid surface and surfactant molecules (head down with monolayer structure) [57]. Polarisation of jt electrons occurs between the surfactant head group which has electron-rich aromatic nuclei and the positively charged solid surface (head down with monolayer structure) [58]. Dispersion forces occur due to London-van der Waals forces between the surfactant molecules and the solid surface (hydrophobic tail lies flat on the hydrophobic solid surface while hydrophilic head orients towards polar liquid) [59]. 

FIGURE 4.27 Wetting a hydrophobic solid surface with the aid of a wetting agent. 

Water-soluble polymers coat hydrophobic solid surfaces with multilayers and thus render the solid hydrophilic (i.e., wetting). The number of adsorbed chains (or the amount of polymer adsorbed) per surface site (or unit weight of adsorbent) is related to the volume fraction of segments in each layer. As the length of the chains increases,... 

The nature of the adsorption of surfactants at both the hydrophilic and hydrophobic solid surfaces has been subject to extensive studies, and a number of excellent recent reviews exist [27-29]. The structure of the adsorbed layer, at both the hydrophilic and hydrophobic surfaces, has been the subject of much conjecture. From the form of the adsorption isotherm at the hydrophilic surface, the cooperative nature of the adsorption was established, and the evolution of the structure with concentration was inferred [30] (see Fig 3). 

The last mentioned method is particularly connected with the wetting behaviour of a solid surface by a solution. If a hydrophobic solid surface is not wetted in such a way that a... 

It has been shown recently that this unique property is part of the much broader concept of a surfactant-enhanced spreading. Aqueous solutions of ethoxylated alcohols (C,E,) rapidly wet moderately hydrophobic solid surfaces [4,5]. The same effect was observed for certain double-chained anionic and cationic hydrocarbon surfactants [6]. A detailed analysis of spreading data obtained at a Si-surfactant solution/liquid hydrocarbon interface led to the conclusion that this rapid spreading can be understood as a Marangoni flow driven process [7]. Although the role of... 

Using the noncoulombic ttrodel, one can investigate the details of surfactant adsorption at the solid-water interface. The van der Waals interactions of the hydrocarbon tails of the surfactam ions lead to the iforupt condensation of the surfactant on the surface as the concentration of surfactant in solution increases. The fonrration of such a condensed film (called a hemimicelle) appears to be necessary in the development of a hydrophobic solid surface (Fig. 17.2-3). 

So far the interaction between surfactant molecules and hydrophobic solid surfaces with extreme water repellence properties has not been studied extensively so that only limited reference data are available, despite the large potential interest. 

Individual, single surfactant systems have been so far deeply investigated at hydrophobic solid surfaces to study their role as wetting regulators while the interest has increased in the study of mixtures of ionic and non-ionic surfactants with sufficiently long poly(ethylene oxide) chain [5,6]. 

Horinek D, Netz RR (2007) Specific ion adsorption at hydrophobic solid surfaces. Phys Rev Lett 99 226104... 

Dupre s scheme may be complicated by the possibility of the adsorption of liquid phase molecules on a bare solid substrate. Such adsorption can be neglected for hydrophobic solid surfaces. 

Interactions between Hydrophobized Solid Surfaces in Nonpolar Liquids... 

Li, D. and Neumann, A. W., Contact angles on hydrophobic solid surfaces and their interpretation, J. Colloid Interface ScL, 148, 190-200 (1992). 

This chapter has discussed the issue of boundary conditions at smooth hydrophobic and rough hydrophilic surfaces and has then given emphasis to effective boundary conditions for flow past hydrophobic solid surfaces with special textures that can exhibit greatly enhanced ("super ) properties, compared to analogous flat... 

Y. Takata, J.-H. Cho, B. Law, M. Aratono, Ellipsometric search for vapor layers at liquid-hydrophobic solid surfaces, Langmuir 22(2006)1715-1721. 

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