Surface tension measurements show that chaotropic salting-in denaturants are not just water-structure breakers

Since the salting-in agents guanidinium chloride, urea, and lithium perchlorate increase the surface tension of water, the salting-in phenomenon does not reflect easier cavity formation in water. Therefore, these salting-in agents must be directly contributing to the solvation of a solute such as benzene in water, probably by a direct solvation interaction. The increased surface-tension effects do not overbalance these solvation effects since they are smaller than the large surface-tension increases with lithium chloride, a typical salting-out agent. The salting-in agent tetra-n-butylammonium chloride differs in that it lowers the surface tension of water. Thus, it probably contributes both to easier cavity formation and to direct solvation of the substrate. The previous findings that most salting-in agents switch to become salting-out agents in other polar solvents such as ethylene glycol and formamide but that tetra-n-butylammonium chloride does not switch in these solvents can be understood in terms of relative polarities.