The Hofmeister series arises from specific ion-water interactions and water structure polarization
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Peer-reviewed literature establishes that the Hofmeister series arises from specific ion-water interactions and ion-specific ordering or polarization of surrounding water molecules.
The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH2-groups, which follows the Hofmeister series: SO42- > Cl- > NO3- > Br- > ClO4- > SCN-. The minimal orientation angle of CH2-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO4-, and SCN-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH2-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.
Our ongoing interest in the puzzling physical properties of liquid water arises from water!s presence in daily life, and its importance in technical, chemical, and biological processes. As water is already interesting alone, the addition of solutes considerably broadens the spectrum of observed phenomena. For this reason the structure and dynamics of water in the vicinity of solutes have been studied for decades. One of the most challenging phenomena in this respect is the so-called Hofmeister effect, first reported by Franz Hofmeister in 1888. He made the observation that different salts have different efficiencies in salting-out proteins, while some salts have no effect. Most importantly, the effectiveness of the anions and cations seems to assume a particular specific order. Moreover, these specific ion effects are ubiquitous in chemistry and biology, and similar ordering of the ions is observed for numerous macroscopic properties including surface tension, chromatographic selectivity, colloid stability, and protein-denaturation temperatures. The best approach to understanding these ion effects is to focus on the simple solvation of the ions. Consequently, the Hofmeister series has been speculated to reflect different ordering powers of ions, usually anions, on the surrounding water molecules. Hence the ionic sequence has been thought as ranging from stabilizing “kosmotropes” to disruptive “chaotropes”. The structure-making (kosmotrope) and structure-breaking (chaotrope) i
Beyond electrostatic, the interaction of ions with their close environment depends also on their hydration shell and the water dynamic in this environment. Non-negligible water mediated effects can indeed influence many physical chemistry processes such as cloud point, protein salting-in or out, bubble coalescence, the topology of bilayers...and polymer self-assemblies. This type of interaction is qualified of "ion specific". During the last 10 years, our team have shown that these effects can be exalted with nanometer-sized ions. These latter can indeed bind to electrically neutral matter in solution although these ions are highly charged but always characterized by low charge density. This nano-ion specific effect, called also superchaotropic effect in referring to an extension of the “Hofmeister series”, arises from the partial dehydration of both the nano-ion and the solute or surface in interaction that lead to a significant gain in enthalpy of the combined system.[1] The characterization of this effect is strongly supported by scattering experiments, neutron and x-ray, always in a very complementary manner (to build phase diagram) and by other spectral techniques that allow to precisely defined the predominant chemical functions in interaction with the nano-ions that can vary depending on their supramolecular environment. In this presentation we will describe two examples of Keggin-type polyoxometalate nano-ions in interaction with either Poly(N-isopropylacrylamide) PNI
Molecular properties at air–liquid and liquid–liquid interface hold the key to many processes involving molecular transport across phase boundaries from aerosol formation to carbon cycling and material separation using solvent extraction techniques. Using dibutyl phosphate (DBP) as a representative for partially aqueous soluble surfactants, the specific ion effect (SIE) of the Hofmeister series cations Cs + , Na + , Li + , and Mg 2+ on the partition and interaction between surfactant molecules and water molecules in the air–aqueous interface are investigated using vibrational sum frequency generation spectroscopy and surface tension measurements. In the presence of 1 mM and 1M bulk aqueous phase ionic strength salt concentrations, fundamental qualitative relationships are observed for the salting out of DBP relative to bulk aqueous phase nitrate salt concentrations and the specific cations species. At 1 mM ionic strength, the interfacial charge and hence the interfacial potential modulates the electrostatic interactions; in particular, the counter cations partially screen the negatively charged interface induced by the DBP in a direct Hofmeister order. At 1M ionic strength, the electric field at the interface or interfacial potential is effectively neutralized, and the counter cations promote the partitioning of DBP to the interface depending on their specific interaction with the DBP head group and metal ion hydration properties. The present results lay a foundation to study
From the ion point-of-view specific ion effects (SIE) arise as an interplay of ionic size and shape and charge distribution. However in aqueous systems SIE invariably involve water, and at surfaces they involve both interacting surface groups and local fields emanating from the surface. In this review we highlight the fundamental importance of ionic size and hydration on SIE, properties which encompass all types of interacting forces and ion-pairing phenomena and make the Hofmeister or lyotropic series of ions pertinent to a broad range of systems and phenomena. On the other hand ionic hydrophobicity and complexation capacity also determine ionic behavior in a variety of contexts. Over the years we have carried out carefully designed experiments on a few selected soft matter model systems, most involving zwitterionic phospholipids, to assess the importance of fundamental ionic and interfacial properties on ion specific effects. By tuning down direct Coulomb interactions, working with different interfacial geometries, and carefully tuning ion-lipid headgroup interactions it is possible to assess the importance of different parameters contributing to ion specific behavior. We argue that the majority of specific ion effects involving relatively simple soft matter systems can be at least qualitatively understood and demystified.
Chaotropes adsorb to the gel according to their position in the Hofmeister series, with the most chaotropic species adsorbing most strongly. ++Chaotropes adsorb to the gel less strongly in the presence of chaotropes (a salting in effect) and more strongly in the presence of polar kosmotropes (a salting out effect). Polar kosmotropes do not adsorb to the gel, and are sieved through the gel according to their position in the Hofmeister series, with the most kosmotropic species having the largest relative hydrodynamic radii. The hydrodynamic radii of polar kosmotropes is increased by chaotropes and decreased by polar kosmotropes. These results suggest that a chaotrope interacts with the first layer of immediately adjacent water molecules somewhat less strongly than would bulk water in its place; a polar kosmotrope, more strongly. Published in The Journal of biological chemistry (1986)
Abstract
Interactions between ions and water at hydrophobic interfaces within ion channels and nanopores are suggested to play a key role in the movement of ions across biological membranes. Previous molecular dynamics (MD) simulations have shown that the affinity of polarizable anions to aqueous/hydrophobic interfaces can be markedly influenced by including polarization effects through an electronic continuum correction (ECC). Here, we designed a model biomimetic nanopore to imitate the polar pore openings and hydrophobic gating regions found in pentameric ligand-gated ion channels. MD simulations were then performed using both a non-polarizable force field and the ECC method to investigate the behavior of water, Na
+
and Cl
−
ions confined within the hydrophobic region of the nanopore. Number density distributions revealed preferential Cl
−
adsorption to the hydrophobic pore walls, with this interfacial layer largely devoid of Na
+
. Free energy profiles for Na
+
and Cl
−
permeating the pore also display an energy barrier reduction associated with the localization of Cl
−
to this hydrophobic interface, and the hydration number profiles reflect a corresponding reduction in the first hydration shell of Cl
−
. Crucially, these ion effects were only observed through inclusion of effective polarization which therefore suggests that polarizability may be essential for an accurate description for the behavior of ions and water within hydrophobic nanoscale pores, especially those that conduct Cl
−
.
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