Electrolytes Change the Interfacial Water Structure but Not the Vibrational Dynamics

Abstract

Heterogeneous ozone chemistry occurring on aerosols is driven by interfacial chemistry and thus affected by the surface state of aerosol particles. Therefore, the effect of electrolytes on the structure of interfacial water has been under intensive investigation. However, consequences for energy dissipation rates and mechanisms at the interface are largely unknown. Using time-resolved sum frequency generation spectroscopy, we reveal that the relaxation pathway is the same for neat water–air as for aqueous solutions of Na2SO4 and Na2CO3. We further show that similar lifetimes are extracted from all investigated systems and that these lifetimes show an excitation frequency dependent relaxation time from 0.2 ps up to 1 ps. Hence, despite static SFG on the same systems revealing that the interfacial aqueous structure changes upon adding electrolytes, the vibrational dynamics are indistinguishable for both pure water and different electrolyte solutions.

Orientation independent vibrational dynamics of lipid-bound interfacial water

“…Here the Gaussian function accounts for the instrument response function and has a width of s = 0.16 ps. 32 The exponential decay describes the time scale of the spectral diffusion and shows comparable decay times of 0.47 ps AE 0.07 ps for DOPC and 0.5 ps AE 0.1 ps for DOCPe.…”

“…A similar trend has already been reported for the neat water-air, as well as the water-DPTAP interface. 8,32,38 At the lowest pump frequency, there is neither a significant difference between the cross and diagonal peaks, nor between the opposing water orientations. At 2400 cm À1 pump frequency, there is a 1s difference between the two water orientations, and at the 2500 cm À1 pump frequency, there is no significant difference between the lipids for the diagonal peak, but a 1s difference in t 1 at the cross-peak lifetime for the two water orientations.…”

“…Heating effects are accounted for within the fitting model and thus do not need to be subtracted beforehand. 8,9,[32][33][34] A trace consists of three sections. For negative times, e.g.…”

“…For further details see a previous publication, where the analytical solution to this model was presented. 32 The lifetime of the vibrationally excited state amounts to t 1 , with the system decaying into an intermediate state. The decay from the intermediate state into a heated ground state is characterized by a lifetime of t 2 for which 0.7 ps has been used.…”

“…The decay from the intermediate state into a heated ground state is characterized by a lifetime of t 2 for which 0.7 ps has been used. 32,37 Using this model, we obtain the lifetime of the excited vibrational states for different interfacial water orientation. Fig.…”