Effect of ionic strength on aggregation of nile red and coumarin 30 in aqueous medium: primary kinetic salt effect or salting-out effect?

Abstract

The effect of ionic strength on the aggregation of planar dyes like nile red (NR) and coumarin-30 (C30) in aqueous medium has been explored. The dyes are known to undergo dimerization, resulting in fading of their respective colors in the visible range. The present study demonstrates that the fading process is accelerated appreciably upon increasing ionic strength of the solution through addition of soluble salts. Experiments consist of variation of cations (Na⁺, Mg²⁺ and Al³⁺) with different valencies in a series of salts keeping the anion same and a similar set with a variation of anions (NO₃⁻, SO₄²⁻ and PO₄³⁻), keeping the cation same. The question of involvement of ‘primary kinetic salt effect’ or ‘salting-out effect’ for accelerating the aggregation process has also been resolved. Using Na⁺, K⁺ and NH₄⁺ ions with the same counterpart NO₃⁻, our experimental results do not show any differential effect, in terms of making the aggregation process faster, and hence rule out any effect of Hofmeister series on the self-aggregation process. The detailed study explicitly establishes that it is the ‘primary kinetic salt effect’ and not the ‘salting-out effect’ that is involved in the present case.