Multilamellar Structures Induced by Hydrophilic and Hydrophobic Ions Added to a Binary Mixture ofD2Oand 3-Methylpyridine

Abstract

Phase transition is observed between one-phase disordered phase and an ordered phase with multilamellar (onion) structures in an off-critical mixture of D2O and 3-methylpyridine (3MP) containing a salt at 85mM. The salt consists of hydrophilic cations and hydrophobic anions, which interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by D2O. The onion phase is realized only for small volume fractions of 3MP (in D2O-rich solvent).PACS numbers: 82.45. Gj, 61.20.Qg, 64.75.Cd, 81.16.Dn Binary mixtures of water and organic solvent have been used extensively to study universal aspects of critical behavior and phase separation dynamics. However, not enough attention has yet been paid to unique ion effects in such mixtures, where preferential hydration around each ion should affect the composition fluctuations [1,2]. Salts composed of small hydrophilic ions can drastically change the phase behavior even at small concentrations [3,4,5,6,7]. More strikingly, many groups have observed long-lived heterogeneities (sometimes extending over a few micrometers) in one-phase state [8,9] and a third phase visible as a thin plate formed at a liquid-liquid interface in two-phase state [10]. These observations were reproduced in different experiments using solvents and salts of high purity, so they should be regarded as ioninduced supramolecular aggregates. We also comment on a mixture of water+isobutylic acid (IA), where IA partly dissociates into H + and Butyrate − ions. There, the mobility of H + ions much decreases at large IA contents in one-phase state [11] and the third phase also appears around an interface in two-phase state [10]. Similar phenomena have often been observed in various soft matters such as polymers, gels, colloids, and mixtures containing surfactants. Though not well understood, the solvation (ion-dipole) interaction among ions and polar molecules should play a major role in these phenomena together with the Coulombic interaction among charges. [1,2] Recently, the solvation effect on phase behavior was * Electronic address: hideki.seto@kek.jp theoretically studied in polar mixtures including the case of antagonistic ion pairs composed of hydrophilic and hydrophobic ions [12]. Such cations and anions interact differently with the composition fluctuations, leading to a charge-density-wave phase for sufficiently large solvation asymmetry even at small salt concentrations. In a smallangle neutron scattering (SANS) experiment, Sadakane et al. [13] found a peak at an intermediate wave number Q m (∼ 0.1Å −1 ) in a near-critical mixture of D 2 O and 3-methylpyridine (3MP) containing sodium tetraphenylborate NaBPh 4 at 100 mM. The volume fraction of 3MP, denoted by φ 3MP , was chosen to be 0.3...

The Effect of Tetraphenylphosphonium Chloride on Phase Behavior and Nanoscale Structures in a Mixture of D2O and 3-Methylpyridine

“…As described in a previous paper, the lamellar structure is stabilized by the balance between the entropic effect of the ions and the electrostatic double-layer repulsion. 7 This balance is easily disturbed by the size difference between the ions and the solubilities of the ions in the solvents. Thus, it is possible that the delicate balance required to stabilize the lamellar structure is disrupted in the present mixture.…”

“…This was the first observation of an ordered lamellar structure induced by the preferential solvation of an antagonistic salt. 7 In this study, we tried to confirm the effect of preferential solvation on a D 2 O/3MP mixture in the presence of a different kind of antagonistic salt, tetraphenylphosphonium chloride (PPh 4 Cl) (referred to as "inversely antagonistic" salt), composed of hydrophobic cations and hydrophilic anions. The phase behavior observed by visual inspection indicates that the effect of adding the "inversely antagonistic" salt is the same as that of adding the "normally antagonistic" salt, NaBPh 4 .…”

The effect of antagonistic salt on a confined near-critical mixture

“…Our purpose is a calculation of the effective potential between the confining surfaces for the samples studied experimentally in Refs. [2,3,5]. In order to model the particular samples, we assume that a = 0.4nm,Φ = 0 for the samples with near-critical composition, λ * D is given in Eq.…”

“…[2,3,5]. In the experiments the antagonistic salt sodium tetraphenylborate (NaBPh 4 ) was added to the 3−methylpyridine (3MP) and heavy water mixture near its lower critical point (LCP).…”

Selective Solvation in Aqueous Mixtures: Interface Deformations and Instability

“…Jpn. 81 (2012) SA004 SA004-2 coexistence curve [11,12], and (iii) mesophase formation in aqueous mixtures for an antagonistic salt [2][3][4][13][14][15].…”