Ionic headgroup repulsions

Electrolyte screening In electrolyte double-layer theory of ionic micellar surfaces, counterions added as salt to the bulk solution increase the counterion concentration between the headgroups at the micellar interface and reduce the headgroup repulsions. 

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

This calculation is for spherical micelles, but a similar calculation could be used to obtain estimates of salt concentrations for ionic wormlike micelles. Such salt concentrations for wormlike micelles are expected to be increased in comparison to spherical micelles. In fact, the addition of counterions or a sufficient increase in surfactant concentration often leads to a transition from spherical micelles to wormlike micelles. As the free counterion concentration in solution increases, so does the counterion binding. As a result, electrostatic repulsion between the charged head-groups is increasingly shielded and the mean cross-sectional (effective) headgroup... 

Comparing CMCs of a nonionic and an ionic surfactant with approximately equal head-group area (C12E08, C12Pyr and C12E02S0y) makes it apparent that the CMC of the nonionic surfactant is the lowest. It also demonstrates the effect of electrostatic repulsion. While hydrophobic interactions drive micellization, they are counteracted by steric and electrostatic interactions of the headgroups, both of which limit the coverage of the interface with surfactant molecules. 

The interpretation of the above results is made more difficult by a supplementary phenomenon due to the aqueous phase ionic strength. Increasing the density of charges in the vicinity of the surfactant headgroups causes a screening effect not only between surfactant and protein but also in the repulsive interaction between surfactant polar heads they come closer and the micellar size decreases (43). As a consequence, Goklen and Hatton (48) noted a decrease in the amount of water in AOT system when increasing KCl concentration. However, it is difficult from these results to state precisely whether the decrease in protein extraction is due to the decrease of electrostatic interactions or to a size exclusion effect. 

Hydrotropes have also the ability to insert in the interfacial film of systems based on ionic surfactants, thereby decreasing the electrostatic repulsion of the headgroups, which results in a decrease of their Krafft... 

An excellent example of counterion influence is the quite different thermal behavior of double-chain l-methyl-3,5-bis(n-hexadecyloxycarbonyl)pyridinium ion in crystals with iodide or chloride as counterion [4]. The iodide salt revealed three phase transitions solid crystalline-solid crystalline at —326 K, solid crystalline-liquid crystalline at —358 K, and liquid crystalline-isotropic liquid at —378 K. The X-ray diffraction pattern of the liquid crystalline phase could be best rationalized in terms of a smectic-H phase. The chloride anion could be unfavorable for liquid crystalline behavior because of its smaller ionic radius relative to the iodide anion. Less shielding of the positive charges of the pyridinium rings by the chloride counterion leads to increased electrostatic repulsion between headgroups. 

A detailed physicochemical model of the micelle-monomer equilibria was proposed [136], which is based on a full system of equations that express (1) chemical equilibria between micelles and monomers, (2) mass balances with respect to each component, and (3) the mechanical balance equation by Mitchell and Ninham [137], which states that the electrostatic repulsion between the headgroups of the ionic surfactant is counterbalanced by attractive forces between the surfactant molecules in the micelle. Because of this balance between repulsion and attraction, the equilibrium micelles are in tension free state (relative to the surface of charges), like the phospholipid bilayers [136,138]. The model is applicable to ionic and nonionic surfactants and to their mixtures and agrees very well with the experiment. It predicts various properties of single-component and mixed micellar solutions, such as the compositions of the monomers and the micelles, concentration of counterions, micelle aggregation number, surface electric charge and potential, effect of added salt on the CMC of ionic surfactant solutions, electrolytic conductivity of micellar solutions, etc. [136,139]. 

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