The inapplicability of the DLVO theory to multilayered anionic bilayers is found in terms of the co-ion valence dependence of the lamellar repeat distance. Most of the added salt is expelled from the inter-lamellar space to the bulk due to the Gibbs-Donnan effect on multiple bilayers with the bulk. The electrostatic double-layer interaction is well expressed by the formula recently proposed by Trefalt. The osmotic pressure due to the expelled ions, rather than the van der Waals interaction, is the main origin of the attractive force between the bilayers.
(Phys. Rev. E, 96, 040601(R) (2017))
Effects of monovalent and monatomic salts on lamellar repeat distances d of nonionic surfactants (monomyristolein and C12E2) are investigated using small-angle X-ray diffraction. The lamellar repeat distances d (sum of thicknesses of a bilayer and a sandwiched water layer) increase as the increase in the salt concentration with strong anion dependence (Br- > Cl-). The increase of the thickness of the water layer is found to dominate the increase in d. Since the anion dependence of the increase is inconsistent with the ion dependence of the amount of the hydration water we reported previously (Hishida et al., J. Chem. Phys., 2015), the hydration repulsion classically considered is contradicted as the origin of the increase in d. As a result, the increase in d cannot be explained by the existing model of the interactions between neutral charged bilayers. Temperature dependence of d also conrms that a new
mechanism of the effect of ions on d should be considered. The new mechanism seems to relate to the water structure beyond the hydration water, which depends on the ion species.
(J. Sol. Chem., 45, 1612-1619 (2016))
The behavior of water molecules at the surface of nonionic surfactant (monomyristolein) and effects of monovalent ions on the behavior are investigated using the heterodynedetected vibrational sum frequency generation (HD-VSFG) spectroscopy. It is found that water molecules at the surface are oriented with their hydrogen atoms pointing to the bulk, and that the degree of orientation depends on the anion strongly but weakly on the cation. With measured surface potentials in those saline solutions, it is concluded that the heterogeneous distribution of anions and cations in combination with the nonionic surfactant causes the water orientation. This heterogeneous distribution well explains the contrasting order of anions and cations with respect to the ion size in the Hofmeister series.
(J. Chem. Phys., 142, 171101 (2015))
Change in lamellar repeat distances of neutrally charged lipids upon addition of monovalent salts was measured with small-angle X-ray scattering for combinations of two lipids (PC and PE lipids) and six salts. Large dependence on lipid head group is observed in addition to those on added cation and anion. The ion and lipid dependences have little correlation with measured surface potentials of lipid membranes. These results indicate that the lamellar swelling by salt is not explained through balance among interactions considered previously (van derWaals interaction, electrostatic repulsion emerged by ion binding, etc.). It is suggested that effect of water structure, which is affected by not only ions but also lipid itself, should be taken into account for understanding membrane-membrane interactions, as in Hofmeister effect.
(Langmuir, 30, 10583-10589 (2014))
It has been unclear whether the role of water in the self-assembly of soft materials and biomolecules is influential or water is just a background medium. Here we investigate the correlation between hydration state of lipid membrane and structural phase transition of the membrane between lamellar and inverted-hexagonal phases, as an intermediate process of membrane fusion, by using the complementary techniques of X-ray scattering and terahertz (THz) spectroscopy. By comparing two lipid species, our results indicate that the structural changes of the lipid membrane depend on the behavior of the surrounding water, especially in the second hydration layer, in addition to the molecular shape of lipids. The water behaves differently at each membrane surface owing to the different hydrophilicities of the lipid head groups.
(J. Phys. Soc. Jpn., 83, 044801 (2014))
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