Surfactant adsorption wetting process

Alcohols and surfactant molecules compete for adsorption on the stationary phase. Alcohol addition reduces the amount of adsorbed surfactant [19,20, 23]. Table 6.5 lists the amounts of adsorbed surfactant when micellar mobile phases with or without alcohol was used [7]. The surfactant desorption is dependent on the alcohol chain length [7, 19]. Surfactant desorption is linked to a Of reduction that should decrease the HETP (eq. 6.6). The kinetics of the surfactant adsorption-desorption process is enhanced by alcohol addition [5, 7,21]. Spectroscopic studies have shown that short chain alcohols (C1-C3) wet a monomeric C18 silica bonded layer without changing its organization. Oppositely, long chain alcohols (C7-C10)... 

To estimate the influence of the surfactant adsorption on the capillary forces, the wetting tension yiv cos was calculated from the values given in Fig. 10a. The results drawn in Fig. 10b show for both measurement series a minimum of the capillary forces exactly at the concentration ceff. The capillary forces are reduced by about 20% compared to water. This confirms the hypothesis that the reduction of the pattern collapse is caused by a hydropho-bizing of photoresist processed with the threshold dose by cationic surfactant adsorption. Unfortunately the inverse ADS A method could not be applied at relative surfactant concentrations >0.2 since the bubbles became unstable due to the lower surface tension. Thus it cannot be estimated how the wetting tension evolves at higher concentrations. 

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. 

A relatively new field is the use of flotation in wet textile processes [73]. The ( -potential of cotton fibres in aqueous solutions is negative, therefore they are effectively floated by cationics like quaternary ammonium salts, e.g. dodecyl trimethyl ammonium chloride. Sysilia et al. [74] have established, by measuring the electrokinetic potential, a clear rule between the positive surface charge of chromite and flotation efficiency. At low pH, chromite was effectively floated by fatty acid soaps, the ions of which are negatively charged under these conditions. The surfactant adsorption is reversible which is indicative of its physical nature. 

In recent time, main attention was focused on the treatment of fiber surfaces, wetting control, and the penetration rate of fabrics by corresponding process solutions. According to [169], the imbibition of fabrics takes place as result of two processes a) a bulk imbibition, since the strands of yarn behave as an assembly of capillaries b) a surface rise due to the formation of surfactant adsorption layers. 

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. 

While surfactant adsorption on weakly polar surfaces such as polyesters and polymethylmethacrylate is often sufficiently nonspecific to allow the use of models based on nonpolar sohds, interactions with more polar ionic surfaces tend to be more complicated. Even those cases, however, can be successively analyzed in terms of the concepts described above, so that the modification of wetting characteristics by surfactant adsorption can be predicted with reasonable confidence, possibly saving a great deal of time (= money) in various processes. 

The kinetics of surfactant adsorption is a fundamental problem of interfacial science playing a key role in various processes and phenomena, such as wetting, foaming and stabilization of liquid films. Since the pioneering theoretical work of Ward and Tordai in the 1940s [1], it has been the object of thorough experimental and theoretical research [2]. 

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). 

Rosen and Dahanakake [3] explore the properties of a large number of industrial surfactants with respect to their performance in industrial processes and formulations and lead the readers to a conclusion that surfactant performance depends on measurable and tangible properties such as surface and interfacial tensions, effectiveness of adsorption, molecular surface area, efficiency of adsorption, wetting, spreading coefficients, CMC values, Krafft and cloud points, and solubilization capacity, among others. Such an approach seems to be beneficial and relevant for rather simple diluted manufactured emulsions, foams, or dispersions in which the number of components is limited and the measured parameters reflect the product s characteristics. However, when dealing with more concentrated and complex systems such as natural and processed foods in which several different types of components are present, it seems that simplification is... 

The adsorption of surface-active materials onto a solid surface from solution is an important process in many situations, including those in which we may want to remove unwanted materials from a system (detergency), change the wetting characteristics of a surface (waterproofing), control the triboelectric properties of a surface (static control), or stabilize a finely divided solid system in a liquid where stability may otherwise be absent (dispersion stabilization). In these and many other related applications of surfactants or amphiphilic materials, the ability of the surface-active molecule to situate itself at the solid-liquid interface and produce the desired effect is controlled by the chemical natures of the components of the system the solid, the surfactant, and the solvent. The following discussions summarize some of the factors related to chemical structures that significantly affect the mechanisms of surfactant adsorption and the orientation with which adsorption occurs. 

The foregoing is an equilibrium analysis, yet some transient effects are probably important to film resilience. Rayleigh [182] noted that surface freshly formed by some insult to the film would have a greater than equilibrium surface tension (note Fig. 11-15). A recent analysis [222] of the effect of surface elasticity on foam stability relates the nonequilibrium surfactant surface coverage to the foam retention time or time for a bubble to pass through a wet foam. The adsorption process is important in a new means of obtaining a foam by supplying vapor phase surfactants [223]. 

---