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Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study
1.J. O. M. Bockris and A. K. N. Reddy, Modern Electrochemistry 2A: Fundamentals of Electronics, 2nd ed. (Plenum, New York, 2001).
2.Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984).
13. in front of is needed to compensate the phase difference between surface and bulk SFG. See Ref. 12 for detail.
14.In a strict sense, or is not constant but dependent on in the OH stretching region because of the vibrational resonance. According to the calculation of the complex Fresnel factor and reflectivity, the phase shifts due to and are , which is about the size of experimental uncertainty of the phase. Thus, we limit our discussion to a large phase shift . The present spectra were not normalized to the dependence of and .
29.The bulk concentration of the “free” counterion is for 10 mM CTAB due to the micellization. [T. Asakawa, H. Kitano, A. Ohta, and S. Miyagishi, J. Colloid Interface Sci. 242, 284 (2001).] The corresponding thickness of Gouy-Chapman layer is 6.9 nm.
32.M. Oh-e, A. I. Lvovsky, X. Wei, and Y. R. Shen, J. Chem. Phys. 113, 8827 (2000);
32.note that the sign of is opposite to ours in their paper because of the different sign of the denominator in Eq. (4).
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Complex spectra of air/water interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the first time. In contrast to the neat watersurface, the signs of for two broad OH bands are the same in the presence of the charged surfactants. The obtained spectra clearly showed flip-flop of the interfacial water molecules which is induced by the opposite charge of the head group of the surfactants. With the sign of theoretically obtained, the absolute orientation, i.e., up/down orientation, of water molecules at the charged aqueous surfaces was uniquely determined by the relation between the sign of and the molecular orientation angle. Water molecules orient with their hydrogen up at the negatively charged aqueous interface whereas their oxygen up at the positively charged aqueous interface.
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