Electrostatic interactions in strongly coupled soft matter (original) (raw)
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Statics and dynamics of strongly charged soft matter
Physics Reports, 2005
Soft matter materials, such as polymers, membranes, proteins, are often electrically charged. This makes them water soluble, which is of great importance in technological application and a prerequisite for biological function. We discuss a few static and dynamic systems that are dominated by charge effects. One class comprises complexation between oppositely charged objects, for example the adsorption of charged ions or charged polymers on oppositely charged substrates of different geometry. Here the main questions are whether adsorption occurs and what the effective charge of the resulting complex is. We explicitly discuss the adsorption behavior of polyelectrolytes on substrates of planar, cylindrical and spherical geometry with specific reference to DNA adsorption on supported charged lipid layers, DNA adsorption on oppositely charged cylindrical dendro-polymers, and DNA binding on globular histone proteins, respectively. In all these systems salt plays an important role, and some of the important features can already be obtained on the linear Debye-Hückel level. The second class comprises effective interactions between similarly charged objects. Here the main theme is to understand the experimental finding that similarly and highly charged bodies attract each other in the presence of multi-valent counterions. This is demonstrated using field-theoretic arguments as well as Monte-Carlo simulations for the case of two homogeneously charged bodies. Realistic surfaces, on the other hand, are corrugated and also exhibit modulated charge distributions, which is important for static properties such as the counterion-density distribution, but has even more pronounced consequences for dynamic properties such as the counterion mobility. More pronounced dynamic effects are obtained with highly condensed charged systems in strong electric fields. Likewise, an electrostatically collapsed highly charged polymer is unfolded and oriented in strong electric fields. All charged systems occur in water, and water by itself is not a very well understood material. At the end of this review, we give a very brief and incomplete account of the behavior of water at planar surfaces. The coupling between water structure and charge effects is largely unexplored, and a few directions for future research are sketched. On an even more nanoscopic level, we demonstrate using ab-initio methods that specific interactions between oppositely charged groups (which occur when their electron orbitals start to overlap) are important and cause ion-specific effects that have recently moved into the focus of interest.
Long-ranged and soft interactions between charged colloidal particles induced by multivalent coions
Forces between charged particles in aqueous solutions containing multivalent coions and monovalent counterions are studied by the colloidal probe technique. Here, the multivalent ions have the same charge as the particles, which must be contrasted to the frequently studied case where multivalent ions have the opposite sign as the substrate. In the present case, the forces remain repulsive and are dominated by the interactions of the double layers. The valence of the multivalent coion is found to have a profound influence on the shape of the force curve. While for monovalent coions the force profile is exponential down to separations of a few nanometers, the interaction is much softer and longer-ranged in the presence of multivalent coions. The force profiles in the presence of multivalent coions and in the mixtures of monovalent and multivalent coions can be accurately described by Poisson–Boltzmann theory. These results are accurate for different surfaces and even in the case of highly charged particles. This behavior can be explained by the fact that the force profile follows the near-field limit to much larger distances for multivalent coions than for monovalent ones. This limit corresponds to the conditions with no salt, where the coions are expelled between the two surfaces.
Le Journal de Physique IV, 2000
The paper summarizes recent theoretical work on the effective interactions between charge-stabilized, spherical colloidal particles. Despite the purely repulsive nature of the effective pair potential, fluid-fluid phase separation is shown to occur at very low salt concentrations, due to a frequently forgotten "volume" term. A longrange attractive component to the pair interaction is predicted when the colloidal particles are confined by charged surfaces (e.g. in slit or pore geometries), in agreement with recent experimental findings. The importance of excluded volume and solvent effects is briefly discussed.
A pr 2 00 1 Strong electrostatic interactions in spherical colloidal systems
2016
We investigate spherical macroions in the strong Coulomb coupling regime within the primitive model in salt-free environment. We first show that the ground state of an isolated colloid is naturally overcharged by simple electrostatic arguments illustrated by the Gillespie rule. We furthermore demonstrate that in the strong Coulomb coupling this mechanism leads to ionized states and thus to long range attractions between like-charged spheres. We use molecular dynamics simulations to study in detail the counterion distribution for one and two highly charged colloids for the ground state as well as for finite temperatures. We compare our results in terms of a simple version of a Wigner crystal theory and find excellent qualitative and quantitative agreement. PACS numbers: 82.70.Dd, 61.20.Qg, 41.20.-q Typeset using REVTEX ∗email: messina@mpip-mainz.mpg.de †email: holm@mpip-mainz.mpg.de ‡email: k.kremer@mpip-mainz.mpg.de 1
Self-consistent effective interactions in charged colloidal suspensions
The Journal of Chemical Physics, 2002
We use an integral equation scheme to obtain self-consistently the effective interaction between colloids in salt-free charged colloidal suspensions. The colloid-counterion direct correlation function ͑DCF͒ is obtained for the fixed colloid-colloid pair structure by solving the corresponding hypernetted-chain equation ͑HNC͒. This DCF is then used to formulate an effective colloid-colloid pair potential for which the one-component reference hypernetted-chain equation is solved. Both processes are iterated until self-consistency is achieved. Counterion-counterion correlations are considered linear and uncoupled from the rest of the correlations. The method is based on a similar treatment utilized in liquid metals ͓Phys. Rev. B 61, 11400 ͑2000͔͒ and provides equivalent results to those obtained using the standard multicomponent HNC equation for mixtures of charged hard spheres. The theory proves rather accurate when compared with molecular dynamic simulations of charged hard and soft spheres for colloidal charges of up to 300. We study in detail the existence of net attractions between colloids in certain cases ͑especially in the presence of divalent and trivalent counterions͒ and how this attraction may lead to phase instability. The problem of the lack of solution of the integral equation for more realistic cases ͑larger charges͒ is also discussed.
Strong electrostatic interactions in spherical colloidal systems
Physical Review E, 2001
We investigate spherical macroions in the strong Coulomb coupling regime within the primitive model in salt-free environment. We first show that the ground state of an isolated colloid is naturally overcharged by simple electrostatic arguments illustrated by the Gillespie rule. We furthermore demonstrate that in the strong Coulomb coupling this mechanism leads to ionized states and thus to long range attractions between like-charged spheres. We use molecular dynamics simulations to study in detail the counterion distribution for one and two highly charged colloids for the ground state as well as for finite temperatures. We compare our results in terms of a simple version of a Wigner crystal theory and find excellent qualitative and quantitative agreement.
Pair interaction of charged colloidal spheres near a charged wall
Physical Review E, 2001
Although equally charged colloidal particles dispersed in clean water are expected to repel each other, an unexplained long-range attraction has consistently been reported for charged colloidal spheres confined by charged macroscopic surfaces. We present an alternative equilibrium measurement of the pair interaction energy for charged spheres near a single charged wall. Analyzing their radial distribution functions for different concentrations reveals a purely repulsive sphere-sphere interaction that is well described by a screened Coulomb potential.
Like-charge interactions between colloidal particles are asymmetric with respect to sign
Soft Matter, 2009
Two-dimensional dispersions of colloidal particles with a range of surface chemistries and electrostatic potentials are characterized under a series of solution ionic strengths. A combination of optical imaging techniques are employed to monitor both the colloid structure and the electrostatic surface potential of individual particles in situ. We find that like-charge multiparticle interactions can be tuned from exclusively repulsive to long-range attractive by changing the particle surface composition. This behavior is strongly asymmetric with respect to the sign of the surface potential. Collective long-range attractive interactions are only observed among negatively charged particles.
Soft Matter, 2018
In this paper we study the structure and phase behavior of binary mixtures of charged particles at low ionic strength. Due to the large size asymmetry between both species, light scattering measurements give us access only to the partial static structure factor that corresponds to the big particles. We observe that the addition of small charged colloids produces a decrease of the main peak of the measured static structure factor and a shift to larger scattering vector values. This finding is in agreement with theory based on integral equations with the Hypernetted-Chain Closure (HNC) relation. The effective interaction between two big particles due to the presence of small particles is obtained by a HNC inversion scheme and used in numerical simulations that adequately reproduce the experiments. We find that the presence of small particles induces an electrostatic depletion screening among the big colloids, creating around them an exclusion zone for the small charged colloids greater than that caused in the case of neutral small colloids, which in turn augments the depletion effect.
Electrostatic Interactions of Colloidal Particles at Vanishing Ionic Strength
Langmuir, 2008
Electrostatic interactions of colloidal particles are typically screened by mobile ions in the solvent. We measure the forces between isolated pairs of colloidal polymer microspheres as the density of bulk ions vanishes. The ionic strength is controlled by varying the concentration of surfactant (NaAOT) in a nonpolar solvent (hexadecane). While interactions are well-described by the familiar screened-Coulomb form at high surfactant concentrations, they are experimentally indistinguishable from bare Coulomb interactions at low surfactant concentration.