Excited-State Dynamics of Charged Dyes at Alkane/Water Interfaces in the Presence of Salts and Ionic Surfactants

Abstract

The excited-state dynamics of the cationic dye malachite green (MG) and of the dianionic dye eosin B at the dodecane/water interface has been investigated using femtosecond time-resolved surface second harmonic generation (TR-SSHG). By using different probe wavelengths, the contributions of monomeric and aggregated MG to the signal could be spectroscopically distinguished. The effect of the addition of a small amount of surfactants was found to strongly depend on the relative charges of surfactant and dye. For surfactant/dye pairs with opposite charges, the TR-SSHG signal is dominated by the contribution from aggregates, whereas for pairs with the same charges, the signal intensity becomes vanishingly small. These effects are explained in terms of electrostatic interactions between surfactants and dyes that favor either attraction of the dye toward the interface or its repulsion toward the bulk. As a very similar behavior is observed with MG upon addition of NaSCN, we conclude that, in this case, this effect reflects the affinity of SCN¯ for the interface. On the other hand, the guanidinium cation was found to have a different effect than that of a positively charged surfactant on the SSHG signal of MG, indicating this cation does not accumulate in the interfacial region.

Exploring Fluorescent Dyes at Biomimetic Interfaces with Second Harmonic Generation and Molecular Dynamics

“…Compared to DPPG, DPPC has a zwitterionic head with an ammonium end group that should lead to substantial Coulombic repulsion of the cationic dye from the interface. A qualitatively similar effect of the charge of the surfactant head on the SSHG signal from ionic dyes has recently been observed …”

“…This is due to the excess concentration of these anions in the interfacial region relative to the bulk − and thus to the Coulombic attraction of the oppositely charged dye. A similar effect was reported with anionic surfactant sodium dodecyl sulfate (SDS) but only at concentrations lower than that required to form a monolayer . Indeed, at higher SDS concentrations, this enhancement effect totally vanished and the population of absorbed dye was apparently similar to that without any surfactant.…”

“…A qualitatively similar effect of the charge of the surfactant head on the SSHG signal from ionic dyes has recently been observed. 28 The free energy of adsorption of 1 at the dodecane/water and dodecane/DPPG/water interfaces obtained from the PMF profiles shown in Figure 3 amounts to −43.8 ± 6.6 and −48.6 ± 4.4 kJ/mol, respectively. This excellent agreement between the SSHG results and the MD simulations supports the validity of both the model and the applied force field.…”

“…A similar effect was reported with anionic surfactant sodium dodecyl sulfate (SDS) but only at concentrations lower than that required to form a monolayer. 28 Indeed, at higher SDS concentrations, this enhancement effect totally vanished and the population of absorbed dye was apparently similar to that without any surfactant. This was explained in terms of the competition for adsorption between the dye and SDS, which is strongly in favor of the latter given its strong amphiphilic nature.…”

Ultrafast excited-state dynamics at interfaces: fluorescent DNA probes at the dodecane/water interface

“…[18][19] The probe pulses (~100 fs, ~2 µJ at the interface) were generated with a collinear optical parametric amplifier (TOPAS-C, Light Conversion), whereas the pump pulses (~80 fs, ~1 µJ at the interface) used for the TR-SSHG measurements were produced by a non-collinear optical parametric amplifier and were circularly polarized (TOPAS White, Light Conversion). The SSHG spectra were recorded by scanning the TOPAS-C with 2 nm steps.…”