Solvation Effects of Ions and Ionic Surfactants in Polar Fluids (original) (raw)
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Nonionic and ionic surfactants at an interface
EPL (Europhysics Letters), 2008
A Ginzburg-Landau theory is presented on surfactants in polar binary mixtures, which aggregate at an interface due to the amphiphilic interaction. They can be ionic surfactants coexisting with counterions. Including the solvation and image interactions and accounting for a finite-volume fraction of the surfactant, we obtain their distributions and the electric potential around an interface in equilibrium. The surface tension is also calculated. The distribution of the adsorbed ionic surfactant is narrower than that of the counterions. The adsorption is marked for hydrophilic and hydrophobic pairs of ionic surfactant and counterions
Ginzburg-Landau theory of solvation in polar fluids: Ion distribution around an interface
Physical Review E, 2006
We present a Ginzburg-Landau theory of solvation of ions in polar binary mixtures. The solvation free energy arising from the ion-dipole interaction can strongly depend on the composition and the ion species. Most crucial in phase separation is then the difference in the solvation free energy between the two phases, which is the origin of the Galvani potential difference known in electrochemistry. We also take into account an image potential acting on each ion, which arises from inhomogeneity in the dielectric constant and is important close to an interface at very small ion densities. Including these solvation and image interactions, we calculate the ion distributions and the electric potential around an interface with finite thickness. In particular, on approaching the critical point, the ion density difference between the two phases becomes milder. The critical temperature itself is much shifted even by a small amount of ions. We examine the surface tension in the presence of ions in various cases.
Effect of ionic solute on a near-critical binary aqueous mixture confined between charged walls with different adsorption preferences is considered within a simple density functional theory. For the near-critical system containing small amount of ions a Landau-type functional is derived based on the assumption that the correlation, ξ, and the Debye screening length, κ −1 , are both much larger than the molecular size. The corresponding approximate Euler-Lagrange equations are solved analytically for ions insoluble in the organic solvent. Nontrivial concentration profile of the solvent is found near the charged hydrophobic wall as a result of the competition between the short-range attraction of the organic solvent and the electrostatic attraction of the hydrated ions. Excess of water may be present near the hydrophobic surface for some range of the surface charge and ξκ. As a result, the effective potential between the hydrophilic and the hydrophobic surface can be repulsive far fro...
Ion Solvation in Liquid Mixtures: Effects of Solvent Reorganization
Physical Review Letters, 2012
Using field-theoretic techniques, we study the solvation of salt ions in liquid mixtures, accounting for the permanent and induced dipole moments, as well as the molecular volume of the species. With no adjustable parameters, we predict solvation energies in both single-component liquids and binary liquid mixtures that are in excellent agreement with experimental data. Our study shows that the solvation energy of an ion is largely determined by the local response of the permanent and induced dipoles, as well as the local solvent composition in the case of mixtures, and does not simply correlate with the bulk dielectric constant. In particular, we show that, in a binary mixture, it is possible for the component with the lower bulk dielectric constant but larger molecular polarizability to be enriched near the ion.
Inorganica Chimica Acta, 2000
We report molecular dynamics studies on the interfacial distribution of ionic species of different size, shape and topology at a water/chloroform interface: hydrophilic K + Cl − , K + SCN − and K + Pic − ions, amphiphilic ammonium NTMA + cations and farnesylphosphate FPH − anions, tetrahedral hydrophobic AsPh 4 + and BPh 4 − ions, with different counterions. Contrasted distributions are observed. The K + Cl − and K + SCN − ions sit almost exclusively in the water phase, but SCN − is less 'repelled' than Cl − by the interface. The Pic − anions are partly adsorbed at the interface and dissolved in the water phase where they display remarkable p-stacking interactions. Amphiphilic NTMA + cations or FPH − anions adsorb and dilute at the interface. Less expected is the high surface activity of symmetrical AsPh 4 + and BPh 4 − ions, with marked counterion effects. The two ions fully adsorb at the interface in the AsPh 4 + BPh 4 − salt, while in the Na + BPh 4 − or AsPh 4 + Cl − salts, they display an equilibrium between the organic phase and the interface. Crossed comparisons between the different solutions reveal the important role of counterions on the distribution of a given ionic species. These results are discussed in relation to experimental data.
Solvation structure and dynamics of solutes in mixed polar solvents.
Molecular dynamics simulations of Na+ Cl− ion pairs in acetonitrile - dimethyl sulfoxide mixtures have been performed as a function of salt concentration and the composition of mixtures. Temperature dependence of the potential of mean force (PMF) is studied to assess the enthalpy and entropy contributions to the PMFs. Stability of contact ion pair increases with increase in temperature and decreases with the salt concentration. In higher salt concentrations, free energies of contact ion pair formation seem to approach a limiting value. Formation of the ion pairs is governed by the entropy irrespective of concentration of salt and mixture compositions.
Solubilization of uncharged molecules in ionic surfactant aggregates. 2. Phase equilibria
The Journal of Physical Chemistry, 1992
This is the second of two papers dealing with the solubilization of nonionic solubilizates into ionic surfactant aggregates. In the first paper the solubilization into micellar aggregates was treated. Now we shall look at equilibria between the micellar phase and other phases, the lamellar phase and the reversed micellar phase. The model used is an extension of a model presented earlier dealing with three-component systems: ionic surfactant-water-long-chain4 alcohol (JBnsson, B.; WennerstrBm, H.
Interaction of Sodium Ions with Cationic Surfactant Interfaces
Chemistry - A European Journal, 2006
Background: The stability of colloidal systems is determined by a delicate interplay of various intermolecular forces. Supramolecular microstructures, long-range ordered liquid crystals, and also some simple biological systems are often used as models that reflect the role of different intermolecular forces in stabilization processes and in the evolution of microstructure. For more than half a century the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has underpinned the intuition of colloid scientists on these forces. The apparent successes of DLVO theory derive from its simplicity, which allows the extraction of the essential physics of the problem of lyophobic colloid stability. Its starting point is the ansatz that colloidal particle interactions (stability or coagulation of dispersions) is determined by the balance of two separate forces: double-layer repulsion and van der Waals attraction. However, the two forces are intimately coupled, and the separation of forces is an invalid approximation. This limits the applicability of the theory to low electrolyte concentrations (1 10 À3 to 5 10 À2 m). In particular, it loses its predictive ability for numerous colloidal phenomena that are dominated by the problem of ion specifici-
Journal of Physical Chemistry B, 2004
We report a molecular dynamics study on the distribution of spherical hydrophobic ions S + and S-(radius ≈ 5.5 Å) and hydrophilic counterions (halide X-; alkali M +) at a water-"oil" interface, where "oil" is modeled by chloroform. The results reveal the surface activity of S + and S-, with marked counterion effects. The S + Ssalt fully adsorbs at the interface, which is electrically neutral, while in the S + Xseries, the anion concentration near the interface decreases in the Hofmeister order I-> Br-> Cl-> F-, thus increasing the change in interfacial electrostatic potential ∆φ. A similar effect is observed with the S-M + salts, when Cs + is compared to Na +. We also investigate the effect of ion charge sign reversal, and find a larger ∆φ for S + Nathan S-Na + salts, in relation with the higher hydration of the fictitious Naanion compared to the isosteric Na + cation. The effect of the magnitude of the ion charge is studied with the divalent S 2+ vs S 2ions and Navs Na + counterions. Despite their mutual repulsion, the S 2+ or S 2like-charged species tend to self-aggregate at the interface and in water as a result of hydrophobic association and, again, differences in distributions are observed upon sign reversal. With regard to the treatment of electrostatics, the Ewald and Reaction Field methods qualitatively yield similar trends, but the latter underestimates the repulsion between like ions at the interface and thus exaggerates the calculated difference in interfacial potential ∆φ. When compared to standard calculations, our results point to the importance of the treatment of cutoff boundaries on the distribution of hydrophilic counterions near the interface. Implications of these results concerning the mechanism of assisted ion transfer are discussed.