28,017 research outputs found
Polyelectrolyte Solutions with Multivalent Salts
We investigate the thermodynamic properties of a polyelectrolyte solution in
a presence of {\it multivalent} salts. The polyions are modeled as rigid
cylinders with the charge distributed uniformly along the major axis. The
solution, besides the polyions, contain monovalent and divalent counterions as
well as monovalent coions. The strong electrostatic attraction existing between
the polyions and the counterions results in formation of clusters consisting of
one polyion and a number of associated monovalent and divalent counterions. The
theory presented in the paper allows us to explicitly construct the Helmholtz
free energy of a polyelectrolyte solution. The characteristic cluster size, as
well as any other thermodynamic property can then be determined by an
appropriate operation on the free energy
Variation in a host-epiphyte relationship along a wave exposure gradient
The red alga Polysiphonia lanosa (L ) Tandy is an obligate epiphyte that primarily occurs on the fucoid brown algal basiphyte Ascophyllum nodosum (L) Le Jolis In the present study we examine how epiphytic interactions between P lanosa and A nodosum vary along a wave exposure gradient within the southern Gulf of Maine, USA P lanosa was most dense on protected shores, however because the stature of P lanosa was greater on exposed than on sheltered shores, greater biomass occurred In exposed habitats Epiphytlc P lanosa pnmanly attached to inlured vegetative bssue at exposed sites, while ~ t osc currence was primarily receptacular at sheltered sites A significantly stronger correlation was found between host receptacle abundance and epiphyte abundance at a protected low than an exposed site As a result, the distribution of epiphytes along the host S stlpe vanes at different sites We suggest that changes in the distribution and abundance of P lanosa across this wave exposure gradient are highly influenced by vanations in the distribution and persistence of suitable attachment sites on the host plant Because both the quantity and quality of attachment sites vanes w t h exposure, we hypothesize that d~fferenpt rocesses limit or de t e rm~neP lanosa populations in different locations In protected sites P lanosa may be limited by the presence of adequate substrata (inlured bssue and lateral pits) where successful recruitment may occur By contrast at exposed sites the supply of P lanosa sporelings, rather than quantity of appropnate substrata, may limlt population size
Rod-like Polyelectrolytes in Presence of Monovalent Salt
We investigate the properties of rigid polyelectrolyte solutions in presence
of monovalent salt. The free energy within the Debye-H\"uckel-Bjerrum (DHBj)
theory [M. E.
Fisher and Y. Levin, {\it Phys. Rev. Lett.} 71, 3826 (1993)] is constructed.
It is found that at thermodynamic equilibrium the polyelectrolyte solution
consists of clusters composed of one polyion and various counterions. The
distribution of the cluster densities is determined by finding the minimum of
the Helmholtz free energy. The osmotic pressure and the average charge of the
cluster are found and their dependence on Manning parameter is
elucidated. A good agreement with the experimental results is obtained.Comment: 11 pages, Revtex (using twocolumn style), 15 figures and postscript
file. Submitted to Macromomelecule
Complex Formation Between Polyelectrolytes and Ionic Surfactants
The interaction between polyelectrolyte and ionic surfactant is of great
importance in different areas of chemistry and biology. In this paper we
present a theory of polyelectrolyte ionic-surfactant solutions. The new theory
successfully explains the cooperative transition observed experimentally, in
which the condensed counterions are replaced by ionic-surfactants. The
transition is found to occur at surfactant densities much lower than those for
a similar transition in non-ionic polymer-surfactant solutions. Possible
application of DNA surfactant complex formation to polynucleotide delivery
systems is also mentioned.Comment: 5 pages, latex, 3 figure
Amphiphile Adsorption on Rigid Polyelectrolytes
A theory is presented which quantitatively accounts for the cooperative
adsorption of cationic surfactants to anionic polyelectrolytes. For high salt
concentration we find that the critical adsorption concentration (CAC) is a
bilinear function of the polyion monomer and salt concentrations, with the
coefficients dependent only on the type of surfactant used. The results
presented in the paper might be useful for designing more efficient gene
delivery systems
High performance dash on warning air mobile, missile system
An aircraft-missile system which performs a high acceleration takeoff followed by a supersonic dash to a 'safe' distance from the launch site is presented. Topics considered are: (1) technological feasibility to the dash on warning concept; (2) aircraft and boost trajectory requirements; and (3) partial cost estimates for a fleet of aircraft which provide 200 missiles on airborne alert. Various aircraft boost propulsion systems were studied such as an unstaged cryogenic rocket, an unstaged storable liquid, and a solid rocket staged system. Various wing planforms were also studied. Vehicle gross weights are given. The results indicate that the dash on warning concept will meet expected performance criteria, and can be implemented using existing technology, such as all-aluminum aircraft and existing high-bypass-ratio turbofan engines
Charge inversion in DNA-amphiphile complexes: Possible application to gene therapy
We study a complex formation between the DNA and cationic amphiphilic
molecules. As the amphiphile is added to the solution containing DNA, a
cooperative binding of surfactants to the DNA molecules is found. This binding
transition occurs at specific density of amphiphile, which is strongly
dependent on the concentration of the salt and on the hydrophobicity of the
surfactant molecules. We find that for amphiphiles which are sufficiently
hydrophobic, a charge neutralization, or even charge inversion of the complex
is possible. This is of particular importance in applications to gene therapy,
for which the functional delivery of specific base sequence into living cells
remains an outstanding problem. The charge inversion could, in principle, allow
the DNA-surfactant complexes to approach negatively charged cell membranes
permitting the transfection to take place.Comment: Latex, 5 figure
Flexible Polyelectrolytes with Monovalent Salt
We present a model for describing flexible polyelectrolytes in a good solvent
a nd in the presence of monovalent salt . The molecule composed by monomers
is characterized by the end to end distanc e and the
number of associated counterions . At high tem peratures the polyelectrolyte
behaves as a neutral polymer (). Dec reasing the temperature, the
macromolecule changes from this extended configurat ion() to a
stretched form (). At even lower temp eratures, above the
Manning condensation threshold, the polyelectrolyte collapse s (). Our results show good agreement with simulations
- …