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Surface second harmonic generation (SSHG) has been used to study the two successive acid/base equilibria of eosin B at the air/water interface. Surface tension measurements have shown that the neutral and anionic forms of eosin B are not surface-active. Nevertheless, SSHG was able to follow the appearance and the disappearance, as a function of the aqueous bulk pH, of the three different species at the interface despite the low number of molecules per unit surface present there. The two surface values were measured at = 4.0 and = 4.2, and both were found to be shifted to larger values as compared to the corresponding ones measured by UV−vis spectroscopy in aqueous bulk solution, at pK a1 = 2.2 and pK a2 = 3.7, respectively. These values indicate that the neutral and the monoanionic forms of eosin B are favored at the interface compared to the monoanionic and the dianionic forms, respectively. A wavelength analysis of the SH response of the dianionic form of eosin B at this interface suggests a lower hydration of the molecule at the air/water interface compared to the bulk solution, in agreement with the observed smaller stabilization of the charged species at the interface. The interfacial orientation has been measured for the doubly-charged and neutral forms of eosin B and found at 33° and 31°, respectively.

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