Electrolyte screening effect on the photoprotolytic cycle of excited photoacid in ice

Time-resolved emission was used to measure the photoprotolytic cycle of an excited photoacid as a function of temperature, both in liquid water and in ice, in the presence of an inert salt. The inert salt affects the geminate recombination between the transferred proton with the conjugate base of the photoacid. We used the Debye-HUckel theory to express the screening of the Coulomb electrical potential by the inert salt. We find that in the liquid phase the measured screening effect is small and the Debye-HUckel expression slightly overestimates the experimental effect. In ice, the screening effect is rather large and the Debye-HUckel expression under estimates the measured effect. We explain the large screening in ice by the "salting-out" effect in ice that tends to concentrate the impurities to confined volumes to minimize the ice crystal energy.