Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy

Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference

“…However, as discussed previously, 6,10,[19][20][21] this description does not suffice in presence of charged interfaces since surface charges may induce an extra DC electric field, E DC , which, in addition to the IR-and VIS-fields, E IR and E VIS , also acts on nonlinear active molecules in the proximity of the surface. This gives rise to an additional third order (w (3) ) contribution to the V-SFG response which depends on the penetration depth of the DC field and may even exceed the w (2) -response.…”

“…15 For particularly large penetration depths, the third order contribution vanishes completely and only the field-independent V-SFG response of the bonded interfacial layer (w (2) ) survives. 15 The V-SFG intensity may thus not necessarily increase with number of molecules affected by the DC field as has been generally assumed, 6,7,10,20,24,25 but decreases if the DC field reaches the magnitude of the coherence length of the probed non-linear process.…”

“…These values for the w (3) /w (2) ratio seem reasonable compared with previous studies of water at charged surfaces. 20,23 Please note that for simplicity we assume that the charge density is constant for all salt concentrations. Small variation with concentration cannot be excluded as the surface charge density of silica (À0.06 C m À2 o s 0 o À0.02 C m À2 ) 2,4,14,26,27 is typically around the critical surface charge density (s 0,crit ) above which counterion condensation occurs.…”

“…where E DC is the surface DC field, and Dk z represents the phase mismatch 20 accumulated for different points along the coordinate perpendicular to the silica/water interface z. By performing the integration with E DC ðzÞ ¼ À dfðzÞ dz and by considering an exponentially decaying potential f(z) = f 0 (c) Â exp(Àk(c)z) along the z-direction (perpendicular to the silica/water interface), the V-SFG intensity dependence on ionic strength can be formulated as follows: Besides the intensity of the involved optical beams and the non-linear susceptibilities of water, the resulting V-SFG intensity is determined by the ionic strength-dependent Debye length, k(c), and surface potential, f 0 (c), (as described by the Grahame equation) as well as the phase mismatch along the probing depth, Dk z , whose explicit definitions are given in the ESI † (eqn (S1)-(S3)).…”

Surface pH and Ion Affinity at the Alcohol-Monolayer/Water Interface Studied by Sum-Frequency Spectroscopy

“…, the surface charge density σ may already be large enough to yield a significant field-induced 24 , we can find Ψ(σ) using Eq. (1) and then deduce σ from Ψ(σ) with the help of Levin's theory that relates σ and E 0 (z).…”