Head-group ion

The surface charge density arising from the unpaired head-group ions is... 

Ion distribution around a spherical micelle can also be described with models that consider that polarizable and not very hydrophilic species (such as Br ) interact both coulombically and by a specific, noncoulombic, interaction [26]. This latter interaction allows the ion to intercalate at the micellar surface and to neutrahze an equivalent number of head groups. Ion distribution around a micelle is then calculated by solving the Poisson-Boltzmann equation (PBE) in the spherical symmetry with allowance for specific interactions via a Langmuir or Volmer isotherm [31]. The original kinetic treatment for a micelle of radius a, aggregation number iV in a cell of radius R yields [31] ... 

The situation changes radically when the ratio of DDP to cation is increased. It was found that when the ratio reaches 2 1, a self-assembly occurs where the two pyrrolidone head groups ion-dipole bond to the cation and then the alkyl chains self-assemble into a rigid post (Figure 2.7). These posts are randomly distributed on the surface so that when plates come together face to face, the posts do not interdigitate. 

The key requirements for using Si-Cl functional initiators to produce polymers carrying Si Cl termini by carbenium ion polymerization are i) Si-Cl should be inert toward aUcylaluminum coinitiators, ii) Si-Cl should not react with propagating carbenium ions, in) chain transfer to monomer should be negligible so as to end up with one Si-Cl head-group per polymer chain. 

Surfactants are widely used in industrial chemistry to modify the behavior of aqueous solutions. Common surfactant head groups include carboxylate (— CO2), sulfonate (-SO3), sulfate (-OSO3 ), and ammonium (-NH3 ). The negative charge of anionic head groups usually is neutralized by Na , and the positive charge of ammonium usually is neutralized by Cl . These ions are used because they are nontoxic and their salts are highly soluble. 

Cevc, G. Watts, A. Marsh, D., Titration of the phase transition of phosphatidylserine bilayer membranes. Effects of pH, surface electrostatics, ion binding, and head group hydration. Biochemistry 20, 4955 -965 (1981). 

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

Scheme 1 gives a representation of an approximately spherical micelle in water, with ionic head groups at the surface and counterions clustered around the micelle partially neutralizing the charges. Counterions which are closely associated with the micelle can be assumed to be located in a shell, the so-called Stern layer, the thickness of which should be similar to the size of the micellar head groups. Monomeric co-ions will be repelled by the ionic head groups. The hydrophobic alkyl groups pack randomly and parts of the chains are exposed to water at the surface (Section 2). 

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