Direct measurement of sub-Debye-length attraction between oppositely charged surfaces.

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
Physical Review Letters (Impact Factor: 7.94). 09/2009; 103(11):118304. DOI:10.1103/PhysRevLett.103.118304
Source: PubMed

ABSTRACT Using a surface force balance with fast video analysis, we have measured directly the attractive forces between oppositely charged solid surfaces (charge densities sigma(+), sigma(-)) across water over the entire range of interaction, in particular, at surface separations D below the Debye screening length lambda(S). At very low salt concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at D<lambda(S) agrees well with predictions based on solving the Poisson-Boltzmann theory, when due account is taken of the independently-determined surface charge asymmetry (sigma(+) not equal to |sigma(-)|).

0 0
  • [show abstract] [hide abstract]
    ABSTRACT: A common method for creating hydrophobic monolayers on charged surfaces is by self-assembly of ionic surfactants from solution. Several factors are important in controlling the structure and properties of such layers: the hydrophobic interactions between adjacent chains, the electrostatic interactions between adjacent headgroups, and electrostatic interactions between the headgroups and the surface charges. We have discovered that the surfactant counterions can have a remarkable effect on the hydrophobicity and hydrophobic interactions of a self-assembled layer. The experimental system was stearoyl(C18)trimethylammonium surfactant with iodide, bromide or chloride counterion (STAI, STABr, and STACl respectively) self-assembled onto mica substrates. Changing the surfactant counterions alters the wetting properties of hydrophobic monolayers on mica. Using a surface force balance we have carried out direct measurements of the interaction force between two surfactant-coated surfaces across water, revealing a strong effect of counterion on the normal interactions. Paradoxically, STAI-coated mica has both the highest water contact angle (is 'most hydrophobic') at the same time as having the highest surface charge relative to STABr and STACl. We use measurements of interfacial tension, asymmetric force measurements, and XPS to lead us towards an interpretation of these results and an understanding of the effect of counterion on the structure of self-assembled monolayers.
    Faraday Discussions 01/2010; 146:309-24; discussion 367-93, 395-401. · 3.82 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We investigate the effective interaction mediated by salt ions between charged nanoparticles (NPs) and DNA. DNA is modeled as an infinite cylinder with a constant surface charge in an implicit solvent. Monte Carlo simulations are used to compute the free energy of the system described in the framework of the primitive model of electrolytes, which accounts for excluded volumes of salt ions. A mean-field Poisson-Boltzmann theory also allows us to compute the free energy and provides us with explicit formulae for its main characteristics (position and depth of the minimum). We intend here to identify the physical parameters that have a major impact on the NP-DNA interaction, in an attempt to evaluate physico-chemical properties which could play a role in genotoxicity or, which could be exploited for therapeutic use. Thus, we investigate the influence on the effective interaction of: the shape of the nanoparticle, the magnitude of the nanoparticle charge and its distribution, the value of the pH of the solution, the magnitude of Van der Waals interactions depending on the nature of the constitutive material of the NP (metal vs. dielectric). We show that for positively charged concave NPs the effective interaction is repulsive at short distance, so that it presents a minimum at distance from the DNA. This short-range repulsion is specific to indented particles and is a robust property that holds for a large range of materials and charge densities.
    Physical Chemistry Chemical Physics 06/2011; 13(27):12603-13. · 3.83 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The development of highly efficient asymmetric-flow field flow fractionation (A4F) methodology for biocompatible PEGylated gold nanorods (GNR) without the need for surfactants in the mobile phase is presented. We report on the potential of A4F for rapid separation by evaluating the efficiency of functionalized surface coverage in terms of fractionation, retention time (t R ) shifts, and population analysis. By optimizing the fractionation conditions, we observed that the mechanism of separation for PEGylated GNRs by A4F is the same as that for CTAB stabilized GNRs (i.e., according to their AR) which confirms that the elution mechanism is not dependent on the surface charge of the analytes and/or the membrane. In addition, we demonstrated that A4F can distinguish different surface coverage populations of PEGylated GNRs. The data established that a change in Mw of the functional group and/or surface orientation can be detected and fractionated by A4F. The findings in this study provide the foundation for a complete separation and physicochemical analysis of GNRs and their surface coatings, which can provide accurate and reproducible characterization critical to advancing biomedical research.
    Analytical and Bioanalytical Chemistry 09/2013; · 3.66 Impact Factor

Full-text (2 Sources)

Available from
Feb 7, 2013