Presence Of Monovalent Salt Research Articles

Salt dependence of compression normal forces of quenched polyelectrolyte brushes

We obtained mean-field expressions for the compression normal forces between two identical opposing quenched polyelectrolyte brushes in the presence of monovalent salt. The brush elasticity is modeled using the entropy of ideal Gaussian chains, while the entropy of the microions and the electrostatic contribution to the grand potential is obtained by solving the nonlinear Poisson–Boltzmann equation for the system in contact with a salt reservoir. The interplay between these distinct contributions upon variations of the ionic strength determines the brush thickness. For the polyelectrolyte brush we considered both an isotropic charged slab as well as a longitudinally heterogeneous charge profile obtained using a self-consistent field theory. Using the Derjaguin approximation, we related the planar-geometry results to the realistic two-crossed cylinders experimental setup. Theoretical predictions are compared to experimental measurements of the salt dependence of the compression normal forces between two quenched polyelectrolyte brushes formed by the adsorption of diblock copolymers poly(tert-butyl styrene)-sodium poly(styrene sulfonate) onto an octadecyltriethoxysilane hydrophobically modified mica surface.

Rodlike Polyelectrolytes in the 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 $\xi$ is elucidated. A good agreement with the experimental results is obtained.

Donnan equilibrium and the osmotic pressure of charged colloidal lattices

We consider a system composed of a monodisperse charge-stabilized colloidal suspension in the presence of monovalent salt, separated from the pure electrolyte by a semipermeable membrane, which allows the crossing of solvent, counterions, and salt particles, but prevents the passage of polyions. The colloidal suspension, that is in a crystalline phase, is considered using a spherical Wigner-Seitz cell. After the Donnan equilibrium is achieved, there will be a difference in pressure between the two sides of the membrane. Using the functional density theory, we obtained the expression for the osmotic pressure as a function of the concentration of added salt, the colloidal volume fraction, and the size and charge of the colloidal particles. The results are compared with the experimental measurements for ordered polystyrene lattices of two different particle sizes over a range of ionic strengths and colloidal volume fractions.

Counterion Effects on Colloid Stability of Cationic Vesicles and Bilayer-Covered Polystyrene Microspheres

The major factor responsible for the very low colloid stability (W) of dioctadecyldimethylammonium (DODA) chloride, bromide, or acetate vesicles in the presence of monovalent salt is identified. Large DODAAc vesicles prepared in 0.6 M d-glucose remain stable over the entire range of NaAc concentrations tested (0−150 mM NaAc). The pH measured for this highly concentrated d-glucose solution is 5.1−5.3. Because acetic acid is a neutral and small molecule that can freely permeate the DODAAc vesicle, upon addition of an acidic NaAc plus d-glucose solution to the vesicle outside, acetic acid forms and reaches the vesicle interior acting as an acetate carrier; acetate binds to the inner vesicle surface and eliminates any charge asymmetry between the inner and outer surface. Counterions such as chloride or bromide do not cross the membrane to reach the inside. Thereby asymmetry of the charge distribution generated upon external addition of NaCl or NaBr, respectively, to DODACl or DODABr vesicles cannot be relaxed...

Pentachlorophenol-induced change of ζ-potential and gel-to-fluid transition temperature in model lecithin membranes

We have determined ζ-potentials for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) membranes by measuring the electrophoretic mobility of multilayered vesicles and the temperatures of the gel-to-ripple-to-fluid phase transitions of sonicated vesicles by a photometric method. Some conclusions are: (1) The ζ-potentials of DMPC and DPPC vesicles become negative due to adsorption of ionized pentachlorophenol (PCP), (2) their magnitude changes, step-like, on gel-to-fluid transition and (3) the temperature of the step-like change in ζ-potential decreases with an increase in PCP concentration. (4) PCP exhibits a large effect on membrane structure: It induces an isothermal phase change from the ordered to disordered state, which is enhanced by monovalent salt in the aqueous phase. (5) Both ionized and unionized PCP decrease the melting phase transition temperature and abolish the pretransition, (6) the unionized species increases the melting transition width and (7) the ionized species is more potent in abolishing the pretransition. (8) The shorter chain lipid (DMPC) is more sensitive to the presence of PCP; the maximum decrease in ΔT t is 13 K (DMPC) and 7 K (DPPC) in the presence of ionized PCP. We have shown experimentally, by comparing the ΔT t from photometric studies with the density of adsorbed PCP derived from ζ-potential isotherms, that (9) the shift of the melting phase transition temperature increases linearly with the density of adsorbed PCP. (10) In contrast to membranes made of negatively charged lipids, the transition temperature of DMPC and DPPC membranes in the presence of PCP further decreases in the presence of monovalent salt. The salt effect is due to screening of the membrane surface leading to enhanced adsorption of ionized PCP and a depression in transition temperature. (11) It is shown that both the adsorption and the changes of gel-to-fluid phase transition temperature can be described in terms of the Langmuir-Stern-Grahame model and (12) proposed that future studies of membrane toxicity of PCP should be focused on its pH dependence.

Characterization and production of a new extracellular polymer from Pseudomonas sp. GSP-910

A new strain, Pseudomonas sp. GSP-910 has been isolated from soil and has been found to produce large quantities of an extracellular, highly viscous polysaccharide in a simple salt medium. Good polymer production (6.16 g·l-1) occurs on a sucrose-containing medium (2%) at high phosphate concentration (80 mM·l-1) and 0.5 g·l-1 of nitrogen source NH4Cl. The relative proportions of sugars in the polymer are: glucuronic acid 8.8%, glucose 28.07%, galactose 56.8%, and it is partially acetylated (6.32%). The isolated polymer exhibits higher viscosity at dilute concentrations than xanthan gum and it is stable at different temperatures, over a wide range of pH and in the presence of monovalent salt. In the presence of divalent cation (CaCl2 0.5%), 910-gum in aqueous solution (1%) solidifies to a resilient gel.

Salt dependence of the kinetics of the lac repressor-operator interaction: role of nonoperator deoxyribonucleic acid in the association reaction.

The kinetics of binding of lac repressor protein and operator deoxyribonucleic acid (DNA) have been studied as a function of monovalent and divalent cation concentration. The salt dependence of the association and dissociation rate constants has been interpreted in light of recent theoretical analyses based on Manning's counterion condensation model. The bell-shaped dependence of the association rate constant on salt concentration evidences a role for nonoperator DNA binding in the repressor's search for the operator site on a large DNA molecule. At intermediate mono- or divalent cation concentrations, the association rate goes through a maximum. At lower cation concentrations, it decreases and becomes dependent on DNA concentration; the high affinity of repressor for nonoperator DNA confines the protein to the DNA. At higher cation concentrations, the association rate decreases and becomes dependent on the weak affinity of repressor for nonoperator DNA. The kinetic data are fit to the theory of Berg and Blomberg [Berg, O. G., & Blomberg, C. (1978) Biophys. Chem. 8, 271] for the salt dependence of association kinetics with coupled diffusion, using published values of the affinity for nonoperator DNA. From this fit, one-dimensional diffusion of repressor along the DNA chain is estimated to be about 4 times faster on MgDNA than on NaDNA. At higher cation concentrations, the salt dependence of the association and dissociation rate constants is consistent with a preequilibrium mechanism for the association reaction [Lohman, T. M., deHaseth, P. L., & Record, M. T., Jr. (1978) Biophys. Chem. 8, 281]. Agreement between literature values (corrected for the presence of Mg2+) and experimental values of the rate constants in the presence of monovalent salt is quite good.