Bulk solution region, adsorption

Reactant concentrations Cyj in the bulk solution, as well as the Galvani potential between the electrode and the bulk solution (which is a constituent term in electrode potential E), appear in kinetic equations such as (6.8). However, the reacting particles are not those in the bulk solution but those close to the electrode surface, near the outer Helmholtz plane when there is no specific adsorption, and near the inner Helmholtz plane when there is specific adsorption. Both the particle concentrations and the potential differ between these regions and the bulk solution. It was first pointed out by Afexander N. Frumkin in 1933 that for this reason, the kinetics of electrochemical reactions should strongly depend on EDL structure at the electrode surface. 

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

In general, the adsorption of a surfactant on particles with previously adsorbed polymer can be influenced by (i) a reduction of surface area available for adsorption as a result of the presence of adsorbed polymer, (ii) possible interactions between polymer and surfactant in the bulk solution or in the interfacial region (that is, surfactant with loops, tails or trains of adsorbed polymer molecules), (iii) the steric effect of adsorbed polymer, preventing approach of surfactant molecules for adsorption at the surface, or (iv) possible electrostatic effects if polymer and/or surfactant are charged species. 

Adsorption is the net accumulation of matter on the sohd phase at the interface with an aqueous solution or gaseous phase. In this process, the solid surface is the adsorbent and the matter that accumulates is the adsorbate. Adsorption also may be defined as the excess concentration of a chemical at the subsurface solid interface compared to that in the bulk solution, or the gaseous phase, regardless of the nature of the interface region or the interaction between the adsorbate and the sohd surface that causes the excess. Surface adsorption is due to interactions between electrical charges, or nonionized functional groups, on mineral and organic constituents. 

Region 2 is characterized by a marked change in the slope of the adsorption isotherms. This results from the onset of association of the hydrocarbon chains of the surfactant ions adsorbed in the Stem plane. The mean separation distance of adsorbed ions under these conditions is about 70 A., which approximates the mean separation distance in bulk at the c.m.c. In such adsorption phenomena, there is a relationship between this asociation and the formation of micelles in bulk solution. For example, electrokinetic studies (1) on quartz at neutral pH showed that alkylammonium ions associate in the Stem plane when their bulk concentration is approximately one hundredth of the c.m.c. This association which has been called hemimicelle formation (3), gives rise to a specific adsorption potential which causes the adsorption to increase markedly and brings about a reversal in the sign of the potential at the Stem plane. The hemimicelle concentration, that is the bulk concentration necessary... 

The thickness of the adsorbed region B which consists of a number of layers, each with Ms sites, is assumed to be <5 Pf where 6 and P are constants. Silberberg47) assumes that <5 = 1/2 and P = 1. Beyond this region is there a homogeneous bulk solution of polymer volume fraction < > from which adsorption has taken place. Except for solvent, all that is found in the region B are loop segments of the adsorbed polymer chains. Thus, the fraction of adsorbed polymer segments is expressed by... 

The transport of the growth unit(s) from the bulk solution through the hydrodynamic boundary layer to a region adjacent to the adsorption layer of the crystal. This is often referred to as bulk transport-controlled, volume diffusion-controlled, or simply transport-controlled. 

The adsorption isoterms of SDS on polyvinyl chloride (PVC) latex particles in the presence of various concentrations of sodium ions in the bulk solution, have been determined by Bibeau and Matijevic (2 ). In the latices which were examined by us, the surface concentration of emulsifier was confined to the region well below the saturation level. Under these circumstances only a negligible fraction of the total amount of emulsifier will be dissolved in the aqueous phase. 

In a liquid binary solution, this accumulation is accompanied by the corresponding displacement of another component (solvent) from the surface region into the bulk solution. At equilibrium a certain amount of the solute will be accumulated on the surface in excess of its equilibrium concentration in the bulk solution, as shown in Figure 2-6. Excess adsorption E of a component in binary mixture is defined from a comparison of two static systems with the same liquid volume Vo and adsorbent surface area S. In the first system the adsorbent surface considered to be inert (does not exert any surface forces in the solution) and the total amount of analyte (component 2) will be no = VoCo. In the second system the adsorbent surface is active and component 2 is preferentially adsorbed thus its amount in the bulk solution is decreased. The analyte equilibrium concentration Ce can only be measured in the bulk solution, so the amount VoCe is thereby smaller than the original quantity no due to its accumulation on the surface, but it also includes the portion of the analyte in the close proximity of the surface (the portion U Ce, as shown in Figure 2-6 note that we did not define V yet and we do not need to define... 

The immobilization of dsDNA and ssDNA to surfaces can be attained very easily by adsorption. No reagents or DNA modifications occur since the immobilization does not involve formation of covalent bonds with the surface. Surface immobilization of ssDNA by covalent binding, described by Thompson and co-workers [12], is convenient in DNA-hybridization sensing because it enables probe structure flexibility with respect to changes in its conformation to occur, such that hybridization can take place without the probe being removed from the sensor surface. However, non-selective adsorption of non-complementary ssDNA, added to bulk solution, also occurs at multiple sites in the interstitial regions on the sensor surface between immobilized ssDNA strands. The effects of selective and non-selective binding influence the detection of hybridization of the immobilized strands, as found by ICrull and co-workers [13]. 

Equation (a), with set equal to ( >, is surprisingly successful in describing the effect of varying the double-layer structure upon the kinetics of electrochemical reactions at Hg electrodes, at least in the absence of specific adsorption of the supporting electrolyte (i.e., when the inner-layer region adjacent to the electrode contains only solvent molecules). However, this does not necessarily imply that average electrostatic interactions provide the sole contribution to the work terms, because contributions may arise from other sources that remain constant under these conditions. In particular, inner-sphere pathways commonly involve reaction sites within the outer Helmholtz plane. Consequently, the overall work terms consist of separate contributions from transporting the reactant from the bulk solution to this outer plane and from this plane to the reaction site within the inner layer. The latter will then be independent of and, therefore, influence only k j.. in Eq. (a). 

Chemical formulas of crystalline salts whose ions tend to be specifically adsorbed do not reflect complex solution chemistry of these salts. Apparently the adsorption systems are simple, but in fact the solutions are multicomponent systems, and the coexisting species can substantially differ in their affinities to the surface. The speciation in the interfacial region can be completely different from that in bulk solution. Many analytical methods do not distinguish between particular species, and the results representing overall sorption behavior of all species involving the element of interest are obtained. Fortunately, some results obtained by means of spectroscopic methods can be resolved into pieces of information regarding... 

Equation 3.108 predicts a higher local concentration of cations near a negatively charged clay surface than in bulk solution, and a lower concentration of anions near the surface than in solution. Figure 3.24 shows this predicted distribution of monovalent cations and anions near the clay surface for two different concentrations of electrolyte in solution. More modem statistical mechanical models of this clay interfacial region have predicted that ion-ion correlation (electrostatic) effects should cause deviations from this classical picture, such as the positive adsorption of anions at intermediate distances from the surface when the cation is divalent or multivalent. 

The solute and solvent molecules present in any solution have different intensities of attractive force fields, and also have different molecular volumes and shapes. A concentration difference between the surface region and the bulk solution occurs because the molecules that have the greater fields of force tend to pass into the interior, and those with the smaller force fields remain at the surface. The Gibbs surface layer of a solution is more concentrated in the constituents that have smaller attractive force fields, and thus whose intrinsic surface free energy is smaller than the interior. As we stated in Section 3.3, this concentration difference of one constituent of a solution at the surface is termed adsorption. In qualitative terms, if the solution has a smaller surface tension than its pure solvent, the solute is concentrated in the surface layer indicating a positive adsorption according to... 

In mineral-reagent systems, surface precipitation has been proposed as another mechanism for chemisorption. The solubility product for precipitation and the activities of the species at the solid-liquid interface determine the surface precipitation process. Under appropriate electrochemical conditions, the activity of certain species can be higher in the interfacial region than that in the bulk solution and such a redistribution can lead to many reactions. For example, the sharp increase in adsorption of the calcium species on silica around pH 11 has been shown to be due to surface precipitation (Somasundaran and Anan-thapadmanabhan, 1985 Xiao, 1990). Similar correlations have been obtained for cobalt-silica, alumina-dodecylsulfonate, calcite/apatite/dolomite-fatty acid, francolite-oleate and tenorite-salicylaldoxime systems. The chemical state of the surfactant in the solution can also affect adsorption (Somasundaran and Ananthapadmanabhan, 1985). 

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