Dynamic Processes of Adsorption and Wetting

Most technological processes (spraying, coating, etc.) work under dynamic conditions and improvement of their efficiency requires the use of surfactants that lower the liquid surface tension under these dynamic conditions. The interfaces involved (e.g. droplets formed in a spray or impacting on a surface) are freshly formed and have only a small effective age of some seconds or even less than a millisecond. 

The most frequently used parameter to characterise the dynamic properties of liquid adsorption layers is the dynamic surface tension (which is time dependent quantity). Techniques should be available to measure yLV function of time (ranging from a fraction of a millisecond to minutes and hours or even days). 

To optimise the use of surfactants, polymers, and mixtures of them, specific knowledge of their dynamic adsorption behaviour rather than equilibrium properties is of great interest [20]. It is, therefore, necessary to describe the dynamics of surfactant adsorption at a fundamental level. 

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The formation of emulsions or microemulsions is conneeted with several dynamic processes the time dependence of surface tensions due to the kinetics of adsorption, the dynamic contact angle, the elasticity of adsorption layers as a mechanic surface property influencing the thiiming of the liquid films between oil droplets, the mass transfer across interfaces and so on. Kahlweit et al. (1990) have recently extended Widom s (1987) concept of wetting or nonwetting of an oil-water interface of the middle phase of weakly-structured mixtures and microemulsions. They pointed out that the phase behaviour of microemulsions does not differ from that of other ternary mixtures, in particular of mixtures of short-chain amphiphiles (cf for example Bourrell Schechter (1988). 

The presence of surfactants or wetting agents in textile treatment solutions can also introduce other complications in the understanding of the dynamics of the wetting process. Because surfactants adsorb at the SL interface as well as the LV interface, as the liquid front moves across fresh solid surface, adsorption processes will tend to deplete the concentration of available surfactant and may cause localized changes in both ctlv and 6. In many cases, however, adsorption rates at the SL interface is much slower than that at LV interfaces, so that such effects can be taken into consideration without too much difficulty. 

Important factors are the physical nature of the powder surface (particle size, pore size, porosity, environment, roughness, pretreatment). The dynamic wetting process is therefore influenced by the rates of ingredient dissolution and surfactant adsorption and desorption kinetics (25). 

The value of the critical micelle concentration (CMC) is an important parameter in a wide variety of industrial applications involving adsorption of surfactant molecules at interfaces, such as foams, froths, emulsions, suspensions, and surface coatings. It is probably the simplest means of characterizing the colloid and surface behaviour of a surfactant solute, which in turn determines its industrial usefulness. Many industrial processes are also dynamic processes in that they involve a rapid increase in interfacial area, such as foaming, wetting, emulsification and solubilization. First, the available monomers adsorb on to the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration (i.e. CMC) is low, the micellar breakup time or diffusion of monomers to the newly created interface can be rate-limiting steps in the supply of monomers, which is the case for many nonionic surfactant solutions (3). 

It is important to remember that many dynamic wetting processes act in the presence of wetting agents as a special type of surfactants. In contrast to very low displacements of the contact line where no shear stresses exist and the adsorption equilibrium is established, at higher speeds the modelling of the process (overlap of surface energetic and hydrodynamic forces) becomes very difficult and a lot of boundary conditions must be simplified. 

Instead of UHV conditions, a wet process was applied by Kunitake and coworkers in order to realize an equilibrium polymerization on the surface [136]. In this case, iodine-modified Au(111) substrates (I/Au(lll)) were dipped into aqueous solutions of tetraamine 64 and dialdehyde monomers 65-67 (Figure 28.28b). A dynamic adsorption-desorption equilibrium and a high lateral mobility of the adsorbed monomers on I/Au(lll) allowed for their surface polycondensation based on Schiff-base formation. The polycondensation was found to be sensitive to the solution conditions (such as pH), as well as to the choice of substrates. When the products were observed directly using STM, fragments with a periodic order were seen to have formed on the surface, although the lateral sizes still required some improvement (Figure 28.29). 

Equilibrium and dynamic wetting and spreading processes, adhesion, physical adsorption, chemisorption and heterogeneous catalysis, spectroscopic and optical studies of surfaces, flow through porous media... 

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