Intramolecular electron density redistribution upon hydrogen bond formation in the anion methyl orange at the water/12-dichloroethane interface probed by phase interference second harmonic generation

Surface second harmonic generation (SSHG) studies of the azobenzene derivat ive p-dimethylaminoazobenzene sulfonate, often referred as Methyl Orange (M O), at the neat water/1,2-dichloroethane (DCE) interface is reported. The t wo forms of the anionic MO dye, which are usually observed in bulk solution , with one form being hydrogen bonded to a water molecule through the azo n itrogens (MO/H2O) and the other form not being hydrogen bonded (MO) have al so been observed at the water/DCE interface. Their equilibrium constant has been compared with the corresponding bulk solution and found to be identic al. The adsorption equilibrium of the two forms has been determined and the Gibbs energy of adsorption measured to be -30 kJ mol(-1) for both forms. F rom a light polarisation analysis of the SH signal, the angle of orientatio n of the MO transition dipole moment was found to be 34 +/- 2 degrees for M O and 43 +/- 2 degrees for MO/H2O under the assumption of a Dirac delta fun ction for the angle distribution, a difference explained by the different s olvation properties of the two forms. Furthermore, the wavelength dependenc e analysis of these data revealed an interference pattern resulting from th e electronic density redistribution within the hydrated anionic form occurr ing upon the formation of the hydrogen bond with a water molecule. This int erference pattern was clearly evidenced with the use of another dye at the interface in order to define a phase reference to both forms of Methyl Oran ge.