Surfactants adsorption characteristics

Since concentrations xj, X2, X3 represent equilibrium values (i.e. concentrations in the bulk phase after adsorption takes place) it is impossible to prepare the original samples of ternary solutions in such a way that the X2/X3 ratio stays constant without prior knowledge of the adsorption isotherm. This is the reason that adsorption isotherms seem to depend on the solid/liquid ratio in the system. An increase in the amount of the solid phase increases the total amount of surfactants adsorbed, which results in a change of X2/X3 ratio and a shift of the experimental point on the adsorption isotherm surface. Obviously, this effect is more pronounced in systems with large differences in individual surfactant adsorption characteristics. 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

In addition to the mobility control characteristics of the surfactants, critical issues in gas mobility control processes are surfactant salinity tolerance, hydrolytic stability under reservoir conditions, and surfactant propagation. Lignosulfonate has been reported to increase foam stability and function as a sacrificial adsorption agent (392). The addition of sodium carbonate or sodium bicarbonate to the surfactant solution reduces surfactant adsorption by increasing the aqueous phase pH (393). 

In both cases, overall adsorption and especially that of sulfonate (or "primary surfactant in the composition of most micellar systems used for EOR) are considerably reduced by simply adding a second product having low adsorption characteristics (NP 30 EO in the above example). This is why we have called this strongly hydrophilic surfactant a desorbent. 

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

Kawai, T Tsutsumi, K. Adsorption characteristic of surfactants and phenol on modified zeolites from their aqueous solutions. Colloid Polymeric Science, 1995 273, 787-792. 

It has been found that the 1,3-dioxolane ring corresponds to approximately two oxyethylene units with regard to effect on the critical micelle concentration and adsorption characteristics [42]. Thus, surfactant type I in Fig. 14 should resemble ether sulfates of the general formula R-(0CH2CH2)20S03Na. This is interesting since the commercial alkyl ether sulfates contain two to three oxyethylene units. 

Miller, R., Fainerman, V.B., Makievski, A.V., Kragel, J., Wiistneck, R. (2000b). Adsorption characteristics of mixed monolayers of a globular protein and a non-ionic surfactant. Colloids and Surfaces A Physicochemical and Engineering Aspects, 161, 151-157. 

The characteristics of surfactant adsorption isotherm on solid surface are generally analysed by the plot of log Ns versus log Ce based on eqn 2.24 or the plot of log T versus log Ce based on eqn 2.25. These plots show four region isotherms as shown in Figure 2.4. 

A characteristic of the early neutron reflectivity studies of nonionic surfactant adsorption was some variability in the pattern of adsorption. This was investigated in more detail and more systematically by McDermott et al. [55], who compared the adsorption of Ci2E6 onto a range of different substrates, amorphous silica, crystalline quartz, and the oxide layer on a silicon single crystal. The adsorbed surfactant was found to form a bilayer with an overall thickness 49 4 A, with a structure similar to that determined in the previous studies (see Fig. 4). 

The added surfactant molecules intended for CMP slurry stabilization can adsorb not only onto the abrasive particle but also onto the surface of the wafer to be polished. Depending on the extent of such adsorption, the added surfactant may influence the CMP process in several ways such as change in friction behavior of the slurry, modification of removal rate and selectivity, alteration of defectivity level, and shift in post-CMP profile. In this section the impact of surfactant adsorption on the removal rate, selectivity, and post-CMP cleaning characteristics will be discussed. 

Ethoxylated surfactants were chosen for study based on predicted foaming properties, thermal and chemical stability, and adsorption characteristics. Only foaming properties are discussed herein. 

Correlation equations relating surfactant chemical structure to performance characteristics and physical properties have been established. One atmosphere foaming properties of alcohol ethoxyl-ates and alcohol ethoxylate derivatives have been related to surfactant hydrophobe carbon chain length, ethylene oxide content, aqueous phase salinity, and temperature. Similar correlations have been established for critical micelle concentration, surfactant cloud point, and surfactant adsorption. 

Alcohol ethoxylates and alcohol ethoxylate derivatives were chosen for study based on their predicted foaming properties, thermal and chemical stability, salinity tolerance, and adsorption characteristics. Table 1 illustrates the classes of surfactants used and the shorthand surfactant naming system employed. Except when noted, surfactants were developmental ENORDET surfactants from Shell Chemical Company or were research samples synthesized in our laboratory or at Koninlijke/Shell Laboratorium, Amsterdam. AES 810-2.6A was obtained from GAF Corporation. 

The surfactant systems used for mobility control in miscible flooding do not form a surfactant rich third phase, and lack its buffering action against surfactant adsorption. Furthermore, for obvious economic reasons, it is desirable to keep the surfactant concentration as low as possible, which increases the sensitivity of the dispersion stability to surfactant loss. Hence, surfactant adsorption is necessarily an even greater concern in the use of foams, emulsions, and dispersions for mobility control in miscible-flood EOR. The importance of surfactant adsorption in surfactant-based mobility control is widely recognized by researchers. A decision tree has even been published for selection of a mobility-control surfactant based on adsorption characteristics (12). 

In dishwashing, one must consider soil and surfactant adsorption to both polar and nonpolar surfaces. Metals (aluminum, stainless steel, carbon steel, cast iron, silver, and tin), siliceous surfaces (china, glass, and pottery), and organics (polyethylene, polypropylene, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and wood) present a wide variety of surface characteristics. They span the range of high interfacial free energy (metals and many ceramics) to low interfacial free energy (hydrocarbon polymers) surfaces [27,28],... 

We can summarise that the problem of diffusion-controlled adsorption kinetics of reorientable surfactant molecules is formulated and solved even for the case when the reorientation process within the surface layer requires some time. It was shown that this non-instantaneous reorientation can result in either acceleration or deceleration of the surface tension decrease, depending on the adsorption characteristics of the different molecular states, and on the actual surface lifetime [227] faster (for medium n values) or slower (large n values) decrease of y is caused by an oversaturation of the surface layer by the state possessing maximum molar area. 

The measurements of the characteristics of transverse surface waves are possible when the ratio of oscillation amplitude to wavelength is less than 0.1 % and all perturbations are really small [105]. The potential of relaxation methods was appreciated already by Lucassen (1975) who used the oscillating barrier method [94, 95]. However, for most surfactants the characteristic adsorption times correspond to frequencies, which are inaccessible for this method. The application of surface wave techniques to micellar solutions relates to later time [96 - 105]. 

The theoretical models proposed in Chapters 2-4 for the description of equilibrium and dynamics of individual and mixed solutions are by part rather complicated. The application of these models to experimental data, with the final aim to reveal the molecular mechanism of the adsorption process, to determine the adsorption characteristics of the individual surfactant or non-additive contributions in case of mixtures, requires the development of a problem-oriented software. In Chapter 7 four programs are presented, which deal with the equilibrium adsorption from individual solutions, mixtures of non-ionic surfactants, mixtures of ionic surfactants and adsorption kinetics. Here the mathematics used in solving the problems is presented for particular models, along with the principles of the optimisation of model parameters, and input/output data conventions. For each program, examples are given based on experimental data for systems considered in the previous chapters. This Chapter ean be regarded as an introduction into the problem software which is supplied with the book an a CD. 

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