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

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.