527 research outputs found
Glasslike Behavior in Aqueous Electrolyte Solutions
When salts are added to water, the viscosity generally increases suggesting
the ions increase the strength of the water's hydrogen-bond network. However,
infrared pump-probe measurements on electrolyte solutions have found that ions
have no influence on the rotational dynamics of water molecules implying no
enhance-ment or breakdown of the hydrogen-bond network. Here we report optical
Kerr-effect and dielectric relaxa-tion spectroscopic measurements, which have
enabled us to separate the effects of rotational and transitional motions of
the water molecules. These data show that electrolyte solutions behave like a
supercooled liquid approaching a glass transition in which rotational and
translational molecular motions are decoupled. It is now possible to understand
previously conflicting viscosity data, nuclear magnetic resonance relaxation,
and ultrafast infrared spectroscopy in a single unified picture
Microglobule formation and a microscopic order parameter monitoring the phase transition of aqueous poly(N-isopropylacrylamide) solution
The coil-to-globule transition of poly(N-isopropylacrylamide) (pNIPAm) in water is generally believed to be driven by hydrophobic interaction between the isopropyl groups of its side chains. However, it is still unclear how dehydration and critical fluctuations of the polymer chains are correlated. Here, we use small-and wide-angle x-ray scattering and dielectric relaxation spectroscopy to cover a wide range of the relevant length and time scales, enabling us to grasp an overall picture of this phase transition. We find that the hydration number of pNIPAm decreases only moderately with temperature up to about 6 K below its spinodal temperature T-S, but then drops steeply on approaching T-S. This rapid dehydration is coupled to a mean-field-like power-law divergence of the correlation length xi, representing fluctuations of the density order parameter. Real-space decoding of an observed interference peak reveals partial-globule formation even far below T-S and demonstrates that the polymer-rich phase above T-S can be understood as a high-density assembly of the microglobules. Strikingly, condensation of the microglobules and the divergence of xi do not run parallel. Instead, the condensation occurs only above T-S and is completed about 6 K above T-S. The local number density of the microglobules, exhibiting a steplike increase just above T-S, should be identified as an additional microscopic order parameter governing the phase transition of pNIPAm
Hydration and self-aggregation of a neutral cosolute from dielectric relaxation spectroscopy and MD simulations: the case of 1,3-dimethylurea
The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielec. relaxation spectroscopy. The expt. provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a sep. slow-water relaxation in addn. to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration nos. were calcd., showing that each 1,3-DMU mol. effectively freezes the reorientation of 1-2 water mols. Addnl., a significant amt. of solvent mols., decreasing from ~39 at infinite diln. to ~3 close to the soly. limit, is retarded by a factor of ~1.4 to 2.3, depending on concn. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound H2O mols. Mol. dynamics (MD) simulations of selected solns. revealed a notable slowdown of water rotation for those solvent mols. surrounding the Me groups of 1,3-DMU and strong binding of ~2H2O by the hydrophilic carbonyl group, corroborating thus the exptl. results. Addnl., the simulations revealed 1,3-DMU self-aggregates of substantial lifetime
Evidence for cooperative Na+ and Cl- binding by strongly hydrated L-proline
In Nature, L-proline (Pro) is a ubiquitous and highly effective osmolyte protecting cells against osmotic stress. To understand this effect, knowledge of the hydration of Pro and its interactions with dissolved salts is essential. We studied these properties by combining statistical mechanics and broadband dielec. spectroscopy and found that Pro remains strongly hydrated up to high amino acid concns. This is also the case upon NaCl addn. to a 0.6M Pro soln. Here, addnl. a Pro·NaCl aggregate is formed with a stability const. of K° ≈ 0.95...1.25 M-1, where Na+ and Cl- cooperatively bind to adjacent carboxylate-oxygen and ammonium-hydrogen atoms, resp
Cation-Binding of Glutamate in Aqueous Solution
Interactions of the cations Li+, Na+, Mg2+, and Ca2+ with L-glutamate (Glu–) in aqueous solution were studied at room temperature with dielectric relaxation spectroscopy in the gigahertz region. Spectra of ∼0.4 M NaGlu with added LiCl, NaCl, MgCl2, or CaCl2 (c(MCln) ≤ 1.5 M) were evaluated and experiments supplemented by density functional theory and 3D reference interaction site model (3D-RISM) calculations. In addition to the modes found for aqueous NaGlu, namely, the reorientation of free Glu– ions (peaking at ∼1.6 GHz), of moderately retarded H2O molecules hydrating the carboxylate moieties of Glu– (∼8.4 GHz), of the cooperative resettling of the H-bond network of bulk water (∼20 GHz), and its preceding fast H-bond flip (∼400 GHz), an additional low-frequency relaxation at ∼0.4 GHz was detected upon the addition of the four salts. In the case of NaGlu + MgCl2(aq) and NaGlu + CaCl2(aq), this mode could be unequivocally assigned to an ion pair formed by the cation and the side-chain carboxylate moiety of Glu–. For NaGlu + LiCl(aq), either this species or a backbone-[Li+–H2O–Cl––Glu–] triple ion is formed. Binding constants increase in the order Li+<Mg2+<Ca2+. For NaGlu + NaCl(aq), an assignment of the ∼0.4 GHz mode to ion pairs or triples was not plausible. Accordingly, its origin remains speculative here
Insight into the Hydration of Cationic Surfactants: A Thermodynamic and Dielectric Study of Functionalized Quaternary Ammonium Chlorides
Hydrophobic interactions are one of the main thermodynamic driving forces in self-assembly, folding, and association processes. To understand the dehydration-driven solvent exposure of hydrophobic surfaces, the micellization of functionalized decyldimethylammonium chlorides, XC10Me2N+Cl-, with a polar functional group, X = C2OH, C2OMe, C2OC2OMe, C2OOEt, together with the "reference" compound decyltrimethylammonium chloride, C10Me3N+Cl-, was investigated in aqueous solution by density measurements, isothermal titration calorimetry (ITC), and dielectric relaxation spectroscopy (DRS). From the density data, the apparent molar volumes of monomers and micelles were estimated, whereas the ITC data were analyzed with the help of a model equation, yielding the thermodynamic parameters and aggregation number. From the DRS spectra, effective hydration numbers of the free monomers and micelles were deduced. The comprehensive analysis of the obtained results shows that the thermodynamics of micellization are strongly affected by the nature of the functional group. Surprisingly, the hydration of micelles formed by surfactant cations with a single alkyl chain on quaternary ammonium is approximately the same, regardless of the alkyl chain length or functionalization of the headgroup. However, notable differences were found for the free monomers where increasing polarity lowers the effective hydration number
Structure and dynamics in protic ionic liquids: a combined optical Kerr-effect and dielectric relaxation spectroscopy study
The structure and dynamics of ionic liquids (ILs) are unusual due to the strong interactions between the ions and counter ions. These microscopic properties determine the bulk transport properties critical to applications of ILs such as advanced fuel cells. The terahertz dynamics and slower relaxations of simple alkylammonium nitrate protic ionic liquids (PILs) are here studied using femtosecond optical Kerr-effect spectroscopy, dielectric relaxation spectroscopy, and terahertz time-domain spectroscopy. The observed dynamics give insight into more general liquid behaviour while comparison with glass-forming liquids reveals an underlying power-law decay and relaxation rates suggest supramolecular structure and nanoscale segregation
Small anisotropy of the lower critical field and -wave two-gap feature in single crystal LiFeAs
The in- and out-of-plane lower critical fields and magnetic penetration
depths for LiFeAs were examined. The anisotropy ratio is
smaller than the expected theoretical value, and increased slightly with
increasing temperature from 0.6 to . This small degree of anisotropy
was numerically confirmed by considering electron correlation effect. The
temperature dependence of the penetration depths followed a power
law() below 0.3, with 3.5 for both and
. Based on theoretical studies of iron-based superconductors, these
results suggest that the superconductivity of LiFeAs can be represented by an
extended -wave due to weak impurity scattering effect. And the
magnitudes of the two gaps were also evaluted by fitting the superfluid density
for both the in- and out-of-plane to the two-gap model. The estimated values
for the two gaps are consistent with the results of angle resolved
photoemission spectroscopy and specific heat experiments.Comment: 10 pages, 5 figure
Sulphur limitation and early sulphur deficiency responses in poplar: significance of gene expression, metabolites, and plant hormones
The influence of sulphur (S) depletion on the expression of genes related to S metabolism, and on metabolite and plant hormone contents was analysed in young and mature leaves, fine roots, xylem sap, and phloem exudates of poplar (Populus tremula×Populus alba) with special focus on early consequences. S depletion was applied by a gradual decrease of sulphate availability. The observed changes were correlated with sulphate contents. Based on the decrease in sulphate contents, two phases of S depletion could be distinguished that were denominated as ‘S limitation’ and ‘early S deficiency’. S limitation was characterized by improved sulphate uptake (enhanced root-specific sulphate transporter PtaSULTR1;2 expression) and reduction capacities (enhanced adenosine 5′-phosphosulphate (APS) reductase expression) and by enhanced remobilization of sulphate from the vacuole (enhanced putative vacuolar sulphate transporter PtaSULTR4;2 expression). During early S deficiency, whole plant distribution of S was impacted, as indicated by increasing expression of the phloem-localized sulphate transporter PtaSULTR1;1 and by decreasing glutathione contents in fine roots, young leaves, mature leaves, and phloem exudates. Furthermore, at ‘early S deficiency’, expression of microRNA395 (miR395), which targets transcripts of PtaATPS3/4 (ATP sulphurylase) for cleavage, increased. Changes in plant hormone contents were observed at ‘early S deficiency’ only. Thus, S depletion affects S and plant hormone metabolism of poplar during ‘S limitation’ and ‘early S deficiency’ in a time series of events. Despite these consequences, the impact of S depletion on growth of poplar plants appears to be less severe than in Brassicaceae such as Arabidopsis thaliana or Brassica sp
- …