Effect of Urea on Ion Pair Formation. the Hydrophilic Effect of Urea

Denaturant effect of urea has been interpreted by two mechanisms, the direct interaction of the additive with biomolecules or lyophilic colloidal aggregates or the indirect mechanism via rupture of the water 3D structure. In contrast we focused that urea produces a higher polarity of aqueous mixtures that causes the denaturation. In the realm of this effect is how charges, dipoles and so in solutions and in suspensions are stabilized by urea addition. For example, ion association is in the center of the interactions that leads to ionic micelle formation. Here, using (79) Br NMR line broadening and solubility data of bis (tri-methyl)-alpha,omega-alkanediammonium, (1-n-1)Br-2 (n = 2-4) (bolaform salts) and of tetramethylammonium bromide and the effect of adding perchlorate, we demonstrated the effect of urea in weakening ion pair association and stabilizing solvent separated ions. The high association constant of perchlorate with ammonium salts was used to titrate the ammonium-bromide interactions. Bromide counterions in bolaform salts derivative having two, three, and four methylene spacers or in tetramethyl ammonium salts were replaced by perchlorate. Although addition of urea to water is well known to lead to an increase of the dielectric permittivity, urea leads to a decrease of anion selectivity and the new pairs bind unspecific both anions. Since ion pairing between ionic surfactants head groups and its counterions is directly correlated to the critical micelle concentration (CMC) values, "loss of selectivity" explains the increase in the ionic surfactants CMC's with increase in urea concentration. (C) 2017 Elsevier B.V. All rights reserved.