Counterion

Abstract: Polyelectrolytes are polymers carrying either positively or negatively charged ionizable groups. The properties of these polymers in solutions and at charged surfaces depend on the fraction of dissociated ionic groups, solvent quality for polymer backbone, solution dielectric constant, salt concentration, and polymer‐substrate interactions. In this review, we summarize the current development of theoretical models describing properties of polyelectrolyte solutions and adsorption of charged polymers at surfaces and interfaces. We discuss in detail the conformational properties of polyelectrolyte chains in dilute and semidilute solutions, the phenomenon of counterion condensation, the necklace structure of polyelectrolytes in poor solvent conditions for polymer backbone, the dynamics of polyelectrolyte solutions, the surface overcharging by adsorbed polyelectrolytes and its implication for assembled polyelectrolyte multilayers. q 2005 Elsevier Ltd. All rights reserved.

Abstract: We have developed a general thermodynamic analysis of monovalent ion effects on the observed association constants K obs of ligand-nucleic acid interactions. Our approach is based on the binding theory of Wyman (1964) and the polyelectrolyte theory of Manning (1969) . In the case of model ligands such as Mg 2+ or short oligolysines, where there is no anion binding by the ligand, the dependence of K obs on monovalent ion (M + ) concentration results from the release of M + counterions from the nucleic acid in the association reaction. We find that, for these systems, log K obs is a linear function of log [M + ]. The slope of such a graph yields the number of charge interactions, or ion pairs, formed between ligand and nucleic acid; the intercept of a linear extrapolation to a 1 m -M + standard state yields the non-electrostatic component of the binding free energy. From an analysis of the data of Latt & Sober (1967) on the interactions of oligolysines with polyribonucleotides, we have concluded that the dominant factor driving complex formation between these charged ligands and the nucleic acid is the entropic contribution from the release of counterions. Counterion release also appears to drive the non-specific interactions of proteins with nucleic acids.

Abstract: The dependence of polyelectrolyte multilayer thickness on salt concentration, salt type, solvent quality, deposition time, and polymer concentration is evaluated. Polymers are deposited on spinning silicon wafers. For the strong polycation/polyanion pair studied, film thickness is approximately proportional to the number of layers and the salt concentration. The irreversibility of overall molecule adsorption is indicated by the lack of exchange of surface (radiolabeled) for solution polymer. The hydrophobic nature of the driving force for polymer sorption is illustrated by the choice of salt counterion or solvent. Analyzed within the framework of ion exchange, the net energy of ion pair formation is not high, at most a few kT. Salt, competing with polymer segments for the surface, permits localized rearrangements. In the mechanism proposed, excess polymer is accommodated within several layers, rather than in one layer of loops and tails. Steric barriers coupled with slow conformational changes are respons...

Abstract: This paper summarizes theory, experimental techniques, and the reported data pertaining to the zeta potential of silica and silicon with attention to use as microfluidic substrate materials, particularly for microchip chemical separations. Dependence on cation concentration, buffer and cation type, pH, cation valency, and temperature are discussed. The Debye-Huckel limit, which is often correctly treated as a good approximation for describing the ion concentration in the double layer, can lead to serious errors if it is extended to predict the dependence of zeta potential on the counterion concentration. For indifferent univalent electrolytes (e.g., sodium and potassium), two simple scalings for the dependence of zeta potential on counterion concentration can be derived in high- and low-z limits of the nonlinear Poisson-Boltzman equation solution in the double layer. It is shown that for most situations relevant to microchip separations, the high-z limit is most applicable, leading to the conclusion that the zeta potential on silica substrates is approximately proportional to the logarithm of the molar counterion concentration. The z vs. pH dependence measurements from several experiments are compared by normalizing the z based on concentration.

Abstract: Gradients of ammonium sulfate in liposomes [(NH4)2SO4]lip. > [(NH4)2SO4]med. were used to obtain 'active' loading of amphipathic weak bases into the aqueous compartment of liposomes. The loading is a result of the base exchange with the ammonium ions. This approach was applied to encapsulate anthracyclines and acridine orange inside the liposomes at very high efficiency (> 90%). Doxorubicin was accumulated in the aqueous phase of the liposomes where it reached a level as high as 100-fold the doxorubicin concentration in the remote loading medium. Most of the intraliposomal doxorubicin was present in an aggregated state. The active entrapment and loading stability were dependent on liposome lipid composition, lipid quality, medium composition and temperature, as well as on the pKa and hydrophobicity of the base. The ammonium sulfate gradient approach differs from most other chemical approaches used for remote loading of liposomes, since it neither requires preparation of the liposomes in acidic pH, nor to alkalinize the extraliposomal aqueous phase. The stability of the ammonium ion gradient is related to the low permeability of its counterion, the sulfate, which also stabilizes anthracycline accumulation for prolonged storage periods (> 6 months) due to the aggregation and gelation of anthracycline sulfate salt.