Hemimicelle concentration

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... 

Figure 3.11. Adsorption models for surfactants (a) model of Fuerstenau (b) model of Scamehorn and co-workers (c) model of Harwell and co-workers (12) (hmc, hemimicelle concentration)...

In applying these equations to surfactant adsorption research for surfactant concentrations greater than the hemimicelle concentration, Somasundaran et al. [26] put equation 2 into logarithmic form and differentiated to give ... 

Nonionic and ionic surfactant molecules adsorb on solid surface at water-insoluble solid-water interface such as silica gel-water and alumina-water interface through van der Waals, hydrophobic, hydrogen bonding, and polar/ionic interactions at low concentrations of surfactant. Then, at a relatively higher and specific concentration known as critical hemimicelle concentration (CHMC), the adsorption increases dramatically as hemimicelles form on the adsorbent involving forces that characterize hydrophobic, van der Waals, and polar/ionic forces. Hemimicelles are two-dimensional molecular aggregates whose structural and physicochemical behavior have not been studied as extensively as those of normal micelles and, consequently, they are not understood even at a very rudimentary level. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

Fig. 36 Schematic diagram of surfactant-adsorption on silica surface as monomer, hemimicelle, and at surfactant concentration above CMC where pyrene is bound to micelle...

Surface aggregates formed by ionic surfactant adsorption on oppositely charge surfaces have been shown to be bi layered structures (1.) and are called admicelles<2) in this paper, though they are sometimes referred to as hemimicelles. The concentration at which admicelles first form on the most energetic surface patch is called the Critical Admicellar Concentration (CAC) in analogy to the Critical Micelle Concentration (CMC), where micelles are first formed. Again, in much of the literature, the CAC is referred to as the Hemimicellar Concentration (HMC). 

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). 

Denoyel and coworkers [61] used a thermodynamic adsorption method to study the adsorption of nonionic and anionic surfactants onto silica, kaolin, and alumina particles. The surfactants formed various structures depending upon their concentrations in the solution. It was found that 2D hemimicelles with low coverage were formed at very low surfactant concentrations. With... 

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). 

Organic adsorbates that are more hydrophobic exhibit different adsorption behavior, particularly at higher concentrations. Long-chain fatty acids adsorb to oxide surfaces in part through surface complexation, as shown by electron spin resonance spectroscopy (32). At higher concentrations at the surface, however, favorable interactions between sorbed molecules (hemimicelle formation) appear to dominate and result in greater than monolayer adsorption (40, 41). Because humic substances (like the fatty acids) are amphiphilic, both surface complexation and hydrophobic interactions may be involved in the adsorption of humic substances on oxide surfaces. 

The floatability of quartz using alkylamines is shown in Fig. 2.20. The concentration corresponding to the sharp increase in floatability is taken as Chm (where Crm is concentration at which hemimicelles form on the solid surface), log Chm- is also linear with a slope equal to -0/2.303/ r. 4> was estimated to be —1.0/ r or 600 cal/mol(-CH2-) in agreement with that obtained from the zeta-potential data. 

Using the noncoulombic model, one can investigate the details of surfactant adsorption at the solid-water interface. The van der Waals interactions of the hydrocarbon tails oF the surfactant Ions lead to the abrupt condensation of tbe surfactant on the surface as the concentration of surfactant in solution increases. The formation of such a condensed film (called a hemimicelle) appeals to be neoessaty in tbe development... 

Information is obtained by modeling the competition between surfactant ions and other ions for adsorption sites on the floe surface. A statistical mechanical approach, as previously employed for couperative surface phasomena (e.g., hemimicelle formation), allows one to observe diet excessive concentrations of surfactant interfere with the flotation of paniculate material. This apparently results from the formation of a second bemimicelle of condensed surfactant on lop of the first, with the surfactant polar or ionic heads presented to the water this results in a hydrophilic surface (Fig. 17.2-4). The effects or ionic strength and surfactant hydrocarbon chain length on this behavior heve been modeled malhemeticaliy. 

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