Nitrobenzene, water uptake

Specific roles of the so-called co-surfactants (commonly, but not necessarily alcohols) have been examined by various workers [122, 126, 136] some points are discussed here. For example, a critical thermodynamic analysis in conjunction with experimentations led Eicke [ 136] to the conclusion that a co-surfactant should decrease the interfacial free energy under isothermal conditions, while causing an uptake of water into the microemulsion and extension of its domain. The anionic surfactant AOT assists the formation of large reverse microemulsion domains (high water uptake) in different ternary systems without help from a co-surfactant (Section 2.2), but cationic surfactants do generally need this fourth component. In spite of this, enhanced solubilization by the addition of (small quantities of) a co-surfactant has been observed by various workers in AOT systems. Eicke [136] used cyclohexane, benzene, carbon tetrachloride and nitrobenzene in the system AOT/ isooctane/water and found considerable water uptake (the fraction of the oil phase, i.e. isooctane was 0.8 or more). With increasing polarizability or polarity of the CO-surfactant, the water uptake decreased. 

Results obtained with nitrobenzene as the adsorbate indicate that solution pH has little effect on equilibrium uptake because nitrobenzene molecules are the dominant species across the entire range of solution pH. However, as commented above, the H-bonding of water to some surface functionalities also plays a role. A maximum uptake is obtained at pH = pHpzc because it is here that dispersion interactions are maximized. These results... 

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