Divalent head groups, surfactants

The technique of vertical dipping for construction of multilayer LB films containing metal ions is depicted in Figure 3.5.2. Centrosymmetric bilayers, with metal ions sandwiched between the polar head groups in the film, are deposited onto vertically mounted substrates passed down and then up through the metal/surfactant monolayer at the air/water interface. The structure of the film is represented by the bis chelate coordination of the carboxylate function of the fatty acid around a divalent metal ion. This type of structure has been determined for a number of metal ions including Cd, Co, Mn, Pb and Zn (24 and ref. within). The structure of CoSt (25), inferred from infrared (IR) and XRD studies, is depicted in Figure 3.5.3a (25). Centrosymmetric films, where the area of the film transferred to the substrate... 

As a rule, the liquid subphase from which the monolayer is transferred is water or, more often, an aqueous electrolyte solution. Indeed, when ionic surfactant molecules are involved low-molecular-weight ions play an important role in stabilizing the interaction between the charged head groups in the F-type film. In particular, divalent (cat)ions serve to stabilize films of monovalent (an)ionic surfactants. 

Further, just as in micelle formation, the addition of counterions can reduce the repulsion between the head groups of the anionic surfactants by compressing the electrical double layer between them. This compression acts to increase the adsorption. This increase in adsorption was not observed for addition of divalent cations to the cationic surfactants systems, however. 

We note that for nonionic surfactants, the addition of salt is about 100 times less efficient compared with ionic surfactants. In the latter case, there is a remarkable effect even if the salt concentration is on the order of 10 mM, while for nonionic surfactants there is an effect for salt concentrations even on the order of 1 M. In the latter case, the mechanism is that salt dehydrates the EO chain, while in the former case the mechanism is that the salt increases the counterion binding and hence decreases the ionic repulsion between the surfactant head groups. Thus, by knowing the mechanisms, we conclude that, for the ionic systems, adding a divalent counterion would be much more efficient compared with a monovalent counterion. For the nonionic surfactant systems, the anion is the deciding partner, while changing the cation hardly affects the CPP of the system. Here di- and trivalent anions are much more efficient compared with monovalent anions. 

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