Surfactant interactions with oppositely charged species

6 Surfactant interactions with oppositely charged species 

The possible interactions that may occur between a surfactant ion and an oppositely charged organic ion are outlined in Fig. 6.39. 

Tomlinson et al, [271] gave the following derivation of for an interaction between an anion and a cation thus 

Complexation has frequently been correlated with the hydrophobic character of one (or both) of the interacting ions [273-279]. Details of the interaction between a series of dyes and alkyltrimethylammonium bromides have been published [277], The structure of the dyes used, tartrazine (XXVI) amaranth (XXVII) carmoisine (XXVIII) and erythrosine (XXIX) are shown below. These are all important colours used in the food, drug and cosmetic industries. Phase separation diagrams were constructed to indicate the relationship between surfactant concentration and the anisotropic solution-coacervate boundary. Differences between the interactions of a hydrophilic dye, tartrazine and amaranth, carmoisine and erythrosine which have both hydrophobic and hydrophilic moieties were exhibited. Tartrazine appears to behave like a simple electrolyte interacting simply with the charged groups at the micellar surfaces while the other dyes complexed and were solubilized as a complex in addition to interacting with the micelle surface [277]. These dyes also induced the formation 

The viscosity of these mixed systems can vary considerably due to the complex interactions and the formation of colloidal particles. The effect of surfactant concentration and alkyl chain length on the specific viscosity of amaranth solutions is illustrated in Fig. 6.43. Viscosity increases sharply as the ratio of dye to surfactant increases up to the point where the system coacervates. Ratios of surfactant to dye at the maximum agree with the ratios for compatability in these systems. Dye solutions which contain short-chain homologues have a smaller viscosity maximum than those which contain long-chain homologues. 

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In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

The second situation appears when a solute is chromatographed with an oppositely charged surfactant, where electrostatic attraction occurs between both species. Electrostatic attraction between solute and micelle will complement any hydrophobic interaction, and thus, it can be expected that the solute will remain in the mobile phase for a longer period of time, decreasing the retention. However, electrostatic and hydrophobic interactions with the stationary phase are often sufficiently large to offset micellar attraction and thus thee retention will increase. That is why dissociated phenol and 2-naphthol are retained to a greater extent with DTAB than with SDS on a C18 column [2],... 

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