Adsorbed layer hemimicelles

All of this suggests that at the c.i.p. the first layer is completed. The surfactants are not adsorbed as hemimicelles or admicelles [7—9]. 

Pyrene and dinaphthylpropane fluorescent probes have been used to investigate the structural details of adsorbed layer of SDS at the alumina-water interface. This study supports the basic concepts of hemimicellization. Adsorption isotherm turns out to be S-type. The values of microviscosity of adsorbed layer and normal micelle are 90 to 120 and 8 cP, respectively. The mobility of the probe... 

Note these data also indicated a preference for DS over the competing chloride concentration of between 30 and 60. This implies the dodecylsulfonate was accumulated in the diffuse double layer surrounding the alumina relative to its bulk solution concentration more than an order of magnitude more preferentially than the inorganic chloride adsorbate. As a consequence, it is perhaps not surprising that Fuerstenau and Wakamatsu (1975) observed the accumulation of hemimicelles on the alumina at only about 400 /jM (pH 7.2) and about 7 fjM (pH 5.2) bulk DS-concentrations (see discussion of hemimicelles). 

The reversal of the direction of the electro-osmotic flow by the adsorption onto the capillary wall of alky-lammonium surfactants and polymeric ion-pair agents incorporated into the electrolyte solution is widely employed in capillary zone electrophoresis (CZE) of organic acids, amino acids, and metal ions. The dependence of the electro-osmotic mobility on the concentration of these additives has been interpreted on the basis of the model proposed by Fuerstenau [6] to explain the adsorption of alkylammonium salts on quartz. According to this model, the adsorption in the Stern layer as individual ions of surfactant molecules in dilute solution results from the electrostatic attraction between the head groups of the surfactant and the ionized silanol groups at the surface of the capillary wall. As the concentration of the surfactant in the solution is increased, the concentration of the adsorbed alkylammonium ions increases too and reaches a critical concentration at which the van der Waals attraction forces between the hydrocarbon chains of adsorbed and free-surfactant molecules in solution cause their association into hemimicelles (i.e., pairs of surfactant molecules with one cationic group directed toward the capillary wall and the other directed out into the solution). 

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. 

An alternative route is to use diblock copolymers in a selective solvent. In this case, one of the blocks (say, the A block) is not soluble. This has two consequences. In solution, free chains are not stable they self-assemble into structures such as micelles or lamellae where the insoluble A chains form the core they are screened from the solvent by a corona (which is, of course, some kind of spherical brush) of B chains. On the surface, several situations may arise. In case the A blocks make contact with the surface, either they can form a homogeneous (complete) wetting film from which a brush of B chains protrudes or a heterogeneous layer of packed hemimicelles can arise. It is also possible that the B chains adsorb, without any contact between A and the surface. In that case the micelles remain almost intact. The polymer-solvent interaction of the A chain has a large influence the worse the solvent, the less solvent is taken up in A domains, and the denser the B chains are packed at the A-solvent interface. Since A segments can adsorb in the form of an insoluble film, where many of them have no direct contact with the substrate, their crowding limits the brush density much less severely than when they would be soluble. 

The adsorption isotherm for the quartz adsorbent shows a bilayer character. The fitted value of the first plateau of the isotherm corresponds to the close-packed monolayer coverage, with an area of 0.3 nm occupied by one HDP. It may be concluded from the amount adsorbed at the second plateau that further adsorption of the surfactant occurs in at least two additional layers, or rather via surface hemimicelle formation. 

The adsorption of DDA on Silicagel R and precipitated silica powder (Fig. 3) was investigated at concentrations lower than 1 mmol/dm, i.e., well below its CMC (14.8 mmol/dm [38]). The adsorption isotherms show constant adsorption above the equilibrium concentration of 0.6 mmol/dm. The amount of DDA adsorbed is lower than that of HDP by one order of magnitude for both substrates. DDA is dissolved at low pH, and therefore the acidity of the systems was adjusted to pH 2 with HCl solution. In such an acidic medium, the surface of the adsorbents is weakly positively charged [15]. In spite of this, the adsorption of cationic surfactants occurs to a small degree. The occurrence of the second step of the isotherm for the precipitated silica can be explained in terms of the second layer or surface hemimicelle formation. 

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