ABSTRACT
Processes at interfaces are ubiquitous in nature. They occur in respiration or photosynthesis reactions, for example, and one of the elementary steps in these mechanisms often involves the transfer of charged species across the interface. As a result, any experimental studies attached to the investigation of these transfer reactions entail the development of a surface sensitive technique. For charged species transfer reactions, the methodology of choice has long been derived from standard electrochemistry, since the transfer of a charged particle across an interface gives rise to a current which is amenable to detection by conventional electrochemical techniques [1]. The field of liquid-liquid electrochemistry has greatly benefited from these general ideas [2-4]. A major drawback of this approach, though, is that the distinction between the different transferring species is not an easy matter. This is an important problem in many aspects relevant to biological processes, as the transfer of an electron across the interface is often coupled to the simultaneous transfer of an ion. More selective techniques have been devised, and the most successful rely on spectroscopy principles. Indeed, the signature of a transferring species may be obtained from its absorption or emission spectrum and therefore UV-visible absorption and fluorescence spectroscopy have been employed extensively [5-10]. The use of other techniques like resonance Raman spectroscopy has also been reported [11, 12]. Nevertheless, linear optical techniques have no intrinsic surface specificity and therefore require an optimized optical configuration to gain surface specificity. The geometry of choice in this case is the total internal reflection (TIR) geometry whereby the light impinges onto the interface from the medium of highest refractive index n1, usually the organic phase, with an angle of incidence larger than the critical angle given by arcsin(n2/n1). The electromagnetic wave present in the low-index phase (index n2) is thus an evanescent wave for which the penetration depth is only of the order of 100 nm. This depth is only a fraction of the diffusion layer in mass transport limited processes but is still far greater than the Debye screening length and
therefore precludes any studies addressing the problem of adsorption or double layer effects at interfaces. This is clearly a major limitation, circumvented only on rare occasions [13, 14] This problem has led to the development of nonlinear optical techniques that do have a much more reduced probing depth at the interface owing to symmetry rules. The simplest nonlinear techniques are the three wave mixing techniques, namely sum frequency and difference frequency generation (respectively SFG and DFG) whereby two fundamental photons of frequency ω1 and ω2 are converted into one photon at the frequency ω1+ω2 in SFG or ω1−ω2 in DFG [15]. In the simple case where a single fundamental frequency is used, two photons at the frequency ω are converted into one photon at the frequency 2ω. The technique is called second harmonic generation (SHG) and is probably the one that is the most widely used because of the simplicity of the experimental arrangement. Although originally applied in surface science to study molecular adsorption on clean surfaces under high-vacuum conditions, the field has rapidly expanded to other domains [16, 17]. The first study of liquid-liquid interfaces has been reported by S.G. Grubb et al. in 1988 for the orientation of compounds at the watercarbon tetrachloride interface [18]. Subsequently, the structure and the dynamics of liquid interfaces have been the focus of interest and the problem of charge transfer reactions across the polarized liquid-liquid interfaces has only been addressed recently [19, 20].
