Effect of Urea on Biomimetic Systems:  Neither Water 3-D Structure Rupture nor Direct Mechanism, Simply a More “Polar Water”

Abstract

Analysis of a variety of properties of supramolecular aggregates in aqueous urea supports an explanation for the urea effect that differs from traditional explanations based on the direct mechanism of urea−water solvation or the indirect mechanism via rupture of the three-dimensional (3-D) structure of water. The urea-induced effects investigated are increases in amphiphile critical micelle concentrations, ionization degrees (α), and aggregation numbers; decreases in percolation thresholds of reversed micelles; expansion of minimum areas of monolayers; increases in the radii of gyration of polyelectrolytes; changes in morphologies of sodium bis-2-ethylhexylsulfosuccinate thin films on glass substrates; and direct evidence for urea-induced reduction in ion pairing. All of these effects are attributed to an urea-induced enhancement of the hydrophilic properties of water that results in more strongly solvated polar groups and ions and a reduction in ion pair formation. The implications of this analysis for urea effects on protein 3-D structure are briefly highlighted.

Urea decreases specific ion effects on the LCST of PMMA-block-PDMAEMA aggregates

“…Although there are many plausible explanations for the effects presented here, taking into account previous studies, it is likely that urea is affecting the way the anions bind (and exchange) to the positively charged interface of the polymer micelles, as it does to other similar systems [2,19] . It has been proposed that urea gets into the solvation layers impairing close contacts of the ion with charged surface to make specific interactions.…”

“…It has been proposed that urea gets into the solvation layers impairing close contacts of the ion with charged surface to make specific interactions. In that fashion, all the ions at high urea concentration behave as solvated species and the interaction with an opposite charged surface is roughly the same for all of them [2,3,5] . In opposition to higher concentrations where other effects can be claimed [16] , the explanation based on ion binding is preferred because salt concentrations studied here are relatively low (up to ~0.1mol L -1 ) where the electrostatic interactions are dominant.…”

“…Urea is a very interesting and valuable additive, widely used as a protein denaturant but having also effects on systems like micelles, vesicles and reverse micelles [1,2] . Despite these very known actions of this molecule, the exact mechanism of urea-driven denaturation is still under debate [3,4] .…”

Interesting Viscosity Changes in the Aqueous Urea–Ionic Liquid System: Effect of Alkyl Chain Length Attached to the Cationic Ring of an Ionic Liquid

“…species due to the hydrophobic hydration [16,18]. The urea molecules can form hydrogen bonds with these icebergs forming the urea-water complexes [47][48][49][50][51].…”

“…Weakening of the hydrophobic bonding results in loosening of the ionic liquid structure and hence the viscosity is reduced. Also, the presence of urea in water increases the relative permittivity of water [47,[49][50][51], which helps in further weakening of the coulomb interactions prevailing between [C 8 mim] ? and Br -.…”