Multiple solutes, adsorption

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Adsorption and ion exchange share so many common features in regard to apphcation in batch and fixed-bed processes that they can be grouped together as sorption for a unified treatment. These processes involve the transfer and resulting equilibrium distribution of one or more solutes between a fluid phase and particles. The partitioning of a single solute between fluid and sorbed phases or the selectivity of a sorbent towards multiple solutes makes it possible to separate solutes from a bulk fluid phase or from one another. 

Leckie [14] emphasized the advantage of chemical speciation over overall distribution coefficients in adsorption modeling. On the other hand, in many theoreticar studies of adsorption even the speciation in solution is neglected and only the total concentration of dissolved species is taken into account. One probable reason of paying no attention to well-known experimental facts is that some authors use adsorption equations borrowed from gas adsorption, and obviously these equations are not suitable to deal with multiple solution species involving the adsorbate. 

A number of recent studies involving the adsorption of solutes from solution by mineral surfaces have resulted in data suggesting multiple-site adsorption. That is, several different arrays of sites are postulated, each of which fulfills the requirements of the Langmuir model. For example, Fig. 9.10 shows data for phosphate adsorption on... 

Another characteristic of a partitioning process in contrast to a true surface adsorption is that for multiple solutes there is no competitive interaction. This is illustrated in Figure 3.9, where the sorption of parathion, an organophosphate, on soil is not affected by the presence of lindane and vice versa. ... 

For adsorption of multiple solutes from solution, there has been a beginning on ikt work of establishing a dieorettcal background [72,83]. In this case, the adsorption is a function not only of the relative, but also of the total, solute concentration, just as for vapor mixtures, where the relative and total pressure are both important. 

The SCLF model has several important features (1) adsorption of polyfimc-tional, multisegmented solute molecules, such as surfactants, can be modeled (2) the effects of flexibility of multisegmented solute and solvent molecules can be accoimted for (2) the volume fraction profiles of the solute molecular segments in the adsorbed layer (lattice layers near the solid surface) can be determined (3) effects of counter ions and multiple solute molecules can be modeled (4) electrostatic interactions are considered (5) lateral interactions can be modeled (6) since the model is based on the probability distribution of conformations of the entire solute molecule, the probability of conformations such as trains, loops, tails, and bridges can be determined speeifically and (7) the mean field approximation applied to solute distribution within eaeh lattic layer results in computation time being linearly proportional to the number of segments in each solute or solvent molecule. 

In addition to these three simple forms of adsorption, two other types of bonds are important but harder to explain. In some cases, adsorption depends on solute shape. One good example is adsorption of antibiotics on custom-synthesized ion-exchange resins. There, successful separation can involve solute adsorption on several adjacent, non-ionic sites. Such multiple site adsorption tends to reflect the solute s molecular shape and, hence, is of special value for expensive solutes. 

Micellar flooding is a promising tertiary oil-recovery method, perhaps the only method that has been shown to be successful in the field for depleted light oil reservoirs. As a tertiary recovery method, the micellar flooding process has desirable features of several chemical methods (e.g., miscible-type displacement) and is less susceptible to some of the drawbacks of chemical methods, such as adsorption. It has been shown that a suitable preflush can considerably curtail the surfactant loss to the rock matrix. In addition, the use of multiple micellar solutions, selected on the basis of phase behavior, can increase oil recovery with respect to the amount of surfactant, in comparison with a single solution. Laboratory tests showed that oil recovery-to-slug volume ratios as high as 15 can be achieved [439]. 

One can further elaborate a model to have a concrete form of /(ft), depending on which aspect of the adsorption one wants to describe more precisely, e.g., a more rigorous treatment of intermolecular interactions between adsorbed species, the activity instead of the concentration of adsorbates, the competitive adsorption of multiple species, or the difference in the size of the molecule between the solvent and the adsorbate. An extension that may be particularly pertinent to liquid interfaces has been made by Markin and Volkov, who allowed for the replacement of solvent molecules and adsorbate molecules based on the surface solution model [33,34]. Their isotherm, the amphiphilic isotherm takes the form... 

For the two methods, the resulting grafting of functional monomers, e.g. acrylic acid and acrylamide, has been measured by multiple reflection IR spectra, ESCA spectra, and dye adsorption from an aqueous solution of crystal violet. The measurements indicate that the inert surfaces of the polymer substrates are modified by a complete surface layer of the grafted monomers. 

For an initial concentration of 2.5 pg/ml of GFP solution, the maximum amount of GFP adsorbed on Aerosil was 120.8 jLtg/g, whilst using calcined SBA-15 was 166.5 pg/g, corresponding to 72.5% and 99.9% of the initial concentration, respectively. This result is an indication that the inner surface of the mesoporous material plays a key role in the immobilisation of guest molecules. The relatively small difference in adsorption between the two materials suggests also that the GFP molecules might form multiple layers on the Aerosil nanoparticle surface. 

Attention was paid early on to solution pH, and in particular, to a surface — bulk proton balance. Various models of hydroxyl chemistry have been developed in colloid science literature [21], Perhaps the simplest and most common model assumes a single type of OH group and amphoteric behavior (i.e., one set of Kx and K2 from Figure 6.1). More complicated models invoke multiple OH groups and proton affinity distributions [22]. It will be demonstrated below that the simpler type has worked well for the revised physical adsorption (RPA) model. 

Metal Ion Adsorption in Mixtures of Multiple Solid Phases. One of the arguments put forth for extending the concepts of solution coordination chemistry to heterogeneous systems is the hypothesis that the mineral components of soils or sediments can be considered as ligands which compete for complexation of adsorbates. To this end, it is important to know the relative ability of different mineral surfaces to complex solutes. 

Moreover, the unique adsorption properties of GEC allowed the very sensitive electrochemical detection of DNA based on its intrinsic oxidation signal that was shown to be strongly dependent of the multi-site attachment of DNA and the proximity of G residues to GEC [100]. The thick layer of DNA adsorbed on GEC was more accessible for hybridization than those in nylon membranes obtained with genosensors based on nylon/GEC with a changeable membrane [99,101,102]. Allhough GEC has a rough surface, it is impermeable, while nylon is more porous and permeable. DNA assays made on an impermeable support are less complex from a theoretical standpoint [7] the kinetics of the interactions are not compUcated by the diffusion of solvent and solutes into and out of pores or by multiple interactions that can occur once the DNA has entered a pore. This explained the lower hybridization time, the low nonspecific adsorplion and the low quantity of DNA adsorbed onto GEC compared to nylon membranes. 

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