114 research outputs found
Međudjelovanje nabijenih makroiona putem štapićastih nabijenih molekula
A short review of recent theoretical advances in studies of the interaction between highly charged systems embedded in a solution of rod-like molecules is presented. The system is theoretically described by the functional density theory, where the correlations within the rod-like molecules are accounted for. We show that for sufficiently long molecules and large surface charge densities, an attractive force between like-charged surfaces arises due to the spatially distributed charges within the molecules. The added salt has an influence on the condition for the attractive force between like-charged surfaces. The theoretical results are compared with Monte Carlo simulations. Many phenomena motivate the study of the interaction between like-charged surfaces (DNA condensation, virus aggregation, yeast flocculation, cohesion of cement paste).Predstavljen je kratak pregled nedavnih teorijskih dostignuća u istraživanju međudjelovanja nabijenih sustava u otopini štapićastih molekula. Sustav je opisan teorijom funkcionala gustoće uzevši u obzir odnose među štapićastim molekulama. Pokazano je da se u slučaju dovoljno dugačkih molekula i velike površinske gustoće naboja javlja privlačna sila između nabijenih ploha zbog prostornog rasporeda naboja u molekulama. Dodatak soli utječe na privlačnu silu među nabijenim plohama. Teorijski rezultati uspoređeni su sa simulacijama Monte-Carlo. Mnoge pojave potiču proučavanje međudjelovanja nabijenih površina (kondenzacija DNA, agregacija virusa, flokulacija kvasca, povezivanje cementne paste
Electrostatic correlations on the ionic selectivity of cylindrical membrane nanopores
We characterize the role of electrostatic fluctuations on the charge selectivity of cylindrical nanopores confining electrolyte mixtures. To this end, we develop an extended one-loop theory that can account for correlation effects induced by the surface charge, nanoconfinement of the electrolyte, and interfacial polarization charges associated with the low permittivity membrane. We validate the quantitative accuracy of the theory by comparisons with previously obtained Monte-Carlo simulation data from the literature, and scrutinize in detail the underlying forces driving the ionic selectivity of the nanopore. In the biologically relevant case of electrolytes with divalent cations such as CaCl2 in negatively charged nanopores, electrostatic correlations associated with the dense counterion layer in the channel result in an increase of the pore coion density with the surface charge. This peculiarity analogous to the charge inversion phenomenon remains intact for dielectrically inhomogeneous pores, which indicates that the effect should be observable in nanofiltration membranes or DNA-blocked nanopores characterized by a low membrane permittivity. Our results show that a quantitatively accurate consideration of correlation effects is necessary to determine the ionic selectivity of nanopores in the presence of electrolytes with multivalent counterions.Peer reviewe
Role of Multipoles in Counterion-Mediated Interactions between Charged Surfaces: Strong and Weak Coupling
We present general arguments for the importance, or lack thereof, of the
structure in the charge distribution of counterions for counterion-mediated
interactions between bounding symmetrically charged surfaces. We show that on
the mean field or weak coupling level, the charge quadrupole contributes the
lowest order modification to the contact value theorem and thus to the
intersurface electrostatic interactions. The image effects are non-existent on
the mean-field level even with multipoles. On the strong coupling level the
quadrupoles and higher order multipoles contribute additional terms to the
interaction free energy only in the presence of dielectric inhomogeneities.
Without them, the monopole is the only multipole that contributes to the strong
coupling electrostatics. We explore the consequences of these statements in all
their generality.Comment: 12 pages, 3 figure
Interaction between macroions mediated by divalent rod-like ions
Attractive interactions between identical like-charged macroions
in aqueous multivalent salt solution arise due to ion-ion
correlations. The mean-field level Poisson-Boltzmann (PB) theory
does not predict such behavior for point-like structureless ions.
Various multivalent ions, such as certain DNA condensing agents
or short stiff polyelectrolytes, do have an internal, often
rod-like, structure. Applying PB theory to the
generic case of divalent rod-like salt ions, we find attraction
between like-charged macroions above a critical distance between
the two individual charges of the rod-like ions. We calculate
this distance analytically within linearized PB theory.
Numerical results for the non-linear PB theory indicate strong
enhancement of the tendency to mediate attractive interactions
Separated cross sections in \pi^0 electroproduction at threshold at Q^2 = 0.05 GeV^2/c^2
The differential cross sections \sigma_0=\sigma_T+\epsilon \sigma_L,
\sigma_{LT}, and \sigma_{TT} of \pi^0 electroproduction from the proton were
measured from threshold up to an additional center of mass energy of 40 MeV, at
a value of the photon four-momentum transfer of Q^2= 0.05 GeV^2/c^2 and a
center of mass angle of \theta=90^\circ. By an additional out-of-plane
measurement with polarized electrons \sigma_{LT'} was determined. This showed
for the first time the cusp effect above the \pi^+ threshold in the imaginary
part of the s-wave. The predictions of Heavy Baryon Chiral Perturbation Theory
are in disagreement with these data. On the other hand, the data are somewhat
better predicted by the MAID phenomenological model and are in good agreement
with the dynamical model DMT.Comment: 6 pages, 4 figure
A measurement of the axial form factor of the nucleon by the p(e,e'pi+)n reaction at W=1125 MeV
The reaction p(e,e'pi+)n was measured at the Mainz Microtron MAMI at an
invariant mass of W=1125 MeV and four-momentum transfers of Q^2=0.117, 0.195
and 0.273 (GeV/c)^2. For each value of Q^2, a Rosenbluth separation of the
transverse and longitudinal cross sections was performed. An effective
Lagrangian model was used to extract the `axial mass' from experimental data.
We find a value of M_A=(1.077+-0.039) GeV which is (0.051+-0.044) GeV larger
than the axial mass known from neutrino scattering experiments. This is
consistent with recent calculations in chiral perturbation theory.Comment: 14 pages, 5 figures, uses elsart.cl
High-precision Studies of the He(e,ep) Reaction at the Quasielastic Peak
Precision studies of the reaction He(e,ep) using the
three-spectrometer facility at the Mainz microtron MAMI are presented. All data
are for quasielastic kinematics at MeV/c. Absolute cross
sections were measured at three electron kinematics. For the measured missing
momenta range from 10 to 165 MeV/c, no strength is observed for missing
energies higher than 20 MeV. Distorted momentum distributions were extracted
for the two-body breakup and the continuum. The longitudinal and transverse
behavior was studied by measuring the cross section for three photon
polarizations. The longitudinal and transverse nature of the cross sections is
well described by a currently accepted and widely used prescription of the
off-shell electron-nucleon cross-section. The results are compared to modern
three-body calculations and to previous data.Comment: 4 pages, 3 figures. Submitted for publication in Phys. Rev. Let
Coherent \pi^0 threshold production from the deuteron at Q^2 = 0.1 GeV^2/c^2
First data on coherent threshold \pi^0 electroproduction from the deuteron
taken by the A1 Collaboration at the Mainz Microtron MAMI are presented. At a
four-momentum transfer of q^2=-0.1 GeV^2/c^2 the full solid angle was covered
up to a center-of-mass energy of 4 MeV above threshold. By means of a
Rosenbluth separation the longitudinal threshold s wave multipole and an upper
limit for the transverse threshold s wave multipole could be extracted and
compared to predictions of Heavy Baryon Chiral Perturbation Theory.Comment: 7 pages, 7 figures, latex2
Measurement of Rlt and Atl in the 4He(e,e'p)3H Reaction at pmiss of 130-300 MeV/c
We have measured the 4He(e,e'p)3H reaction at missing momenta of 130-300
MeV/c using the three-spectrometer facility at the Mainz microtron MAMI. Data
were taken in perpendicular kinematics to allow us to determine the response
function Rlt and the asymmetry term Atl. The data are compared to both
relativistic and non-relativistic calculations.Comment: To be published in the European Physical Journal
Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the \Delta(1232) resonance
The recoil proton polarization has been measured in the p (\vec e,e'\vec p)
pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2
and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz
Microtron. Due to the spin precession in a magnetic spectrometer, all three
proton polarization components P_x/P_e = (-11.4 \pm 1.3 \pm 1.4) %, P_y =
(-43.1 \pm 1.3 \pm 2.2) %, and P_z/P_e = (56.2 \pm 1.5 \pm 2.6) % could be
measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR =
(-6.4\pm 0.7_{stat}\pm 0.8_{syst}) % was determined from P_x in the framework
of the Mainz Unitary Isobar Model. The consistency among the reduced
polarizations and the extraction of the ratio of longitudinal to transverse
response is discussed.Comment: 5 pages LaTeX, 1 table, 2 eps figure
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