Salt Effect on the Formation of Dye Aggregates at Liquid/Liquid Interfaces Studied by Time-Resolved Surface Second Harmonic Generation

Abstract

The present study reports on the effect caused by sodium salts added to a solution of malachite green in a liquid/liquid interfacial system probed by the time-resolved surface second harmonic generation (TRSSHG) technique. This effect is known as "salting-out effect" and is shown to reveal two main issues: salts added to the bulk, first, cause a reduction of the dye solubility and, second, stimulate the adsorption of malachite green cations at the interface, changing the equilibrium constant between the dye molecules adsorbed at the interface and those being dissolved in the bulk. The increased adsorption at the interface is observed in the TRSSHG experiment as a relative increase of the aggregates' contribution to the measured time profile. However, depending on the nature and properties of salt anions, the mechanisms responsible for enhancing the population of interfacial aggregates can differ. This study explains such mechanisms for NaCl and NaSCN: addition of NaCl leads to an increase of the malachite green adsorption at the interface followed by the formation of aggregates, whereas the addition of NaSCN leads rather to the formation of aggregates already in the bulk with their further migration toward the interface. A simple quantitative description of the salting-out effect based on a modified Frumkin-Fowler-Guggenheim model also has been proposed. It has been shown to give a good agreement with the experiment with NaCl, i.e., when the formation of dye aggregates in the bulk solution can be neglected

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

“…This behavior is at odds with most results obtained so far with organic molecules adsorbed at liquid/water or air/water interfaces where aggregation was found to be higher at the interface than in the aqueous phase. 19,[28][29][30][31][32][33] This result also reveals that the adsorption of YOYO1 at the interface has the same effect as intercalation into DNA, i.e. a disruption of the intramolecular H-dimer.…”

Effects of Some Operating Parameters on Sorption and Desorption Kinetics Related to Spotted Golden Thistle Stalks and Methylene Blue; Isotherm Study at Optimal pH and Sorbent Regeneration

“…Unless for moderate amounts, the salt presence was beneficial to MB removal [ Figure 2(a)]; this result bodes well for using SML as an unconventional sorbent in view of the high salt levels in real effluents. The antagonistic behaviour observed when various amounts of NaCl were added may be due to the preponderance of phenomena such as changes in the dye solubility [20] and in its degree of ionisation [21], compression or expansion of the double electric layer around the sorbent [22,23], competition involving cations from the salt [24] or the screen effect [3].…”

Artificial Photosynthesis at Soft Interfaces

“…[15] In order to follow these processes at the interface, we require a time-resolved surface-sensitive technique. As described in this issue by the group of Prof. E. Vauthey in Geneva, Time Resolved-Surface Second harmonic Generation (TR-SSHG) [16] should be suitable for carrying out these measurements at free-standing polarised water|1,2-dichloroethane interfaces. The goal of this study is to measure the potential dependence of the charge separated state, in the other words to investigate how the local electric field created upon polarisation of the interface influences the charge separation.…”