Torbjörn Akesson

Lund University, Lund, Skane, Sweden

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Publications (33)87.59 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This work utilizes a combination of theory and experiments, to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces, at pH 9. Both polymers, Poly(diallyldimethylammonium chloride), PDADMAC, and Poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged, and highly soluble in water. Of specific focus in this work is the response of the adsorption to the addition of simple salt. Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes, are evaluated by direct quantitative comparisons with corresponding experimental data, as obtained by ellipsometry measurements. We find that, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops, but retains a finite level even at very high salt concentrations, indicating the presence of non-electrostatic contributions to the adsorption. In the theoretical treatment, the strength of this relatively modest, but otherwise largely unknown, non-electrostatic surface affinity, was estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Given these estimates for the non-electrostatic part, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory.
    Langmuir 08/2013; · 4.38 Impact Factor
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    ABSTRACT: The free energy of interaction between parallel charged platelets with divalent counterions has been calculated using Monte Carlo simulations to investigate the electrostatic effects on aggregation. The platelets are primarily intended to represent clay particles. With divalent counterions, the free energy for two platelets or two tactoids (clusters of parallel platelets) shows a minimum at a short separation due to the attraction caused by ion-ion correlations. In a salt-free system, the free energy of interaction has a long-range repulsive tail beyond the minimum. The repulsion increases for tactoids with larger aggregation numbers, whereas the depth of the free-energy minimum is gradually reduced. For large enough aggregation numbers, the repulsion is dominating and the minimum is no longer a global free-energy minimum. This is an effect of the depletion of counterions free in solution (outside tactoids) as counterions and platelets aggregate into tactoids and the resulting redistribution of counterions in the system changes the effective interactions between platelets and tactoids. The difference in tactoid-tactoid interactions as a function of aggregation number can be removed by adding enough salt to mask the depletion. Adding salt also reduces the repulsive tail of the free energy of interaction and enhances the minimum. No dependence on the aggregation number suggests that an isodesmic model with a monotonically decaying distribution of aggregation numbers can be used to describe a clay system. This may help to explain the experimental observations of low average numbers of platelets in tactoids, although factors not included in the simulation model may also play an important role.
    Langmuir 07/2013; · 4.38 Impact Factor
  • Jenny Algotsson, Torbjörn Akesson, Jan Forsman
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    ABSTRACT: (23) Na magnetic resonance imaging and the delayed gadolinium-enhanced magnetic resonance imaging methods to investigate cartilage can be used to determine the fixed charge density of cartilage. The methods give results that differ by a factor of 2. In this study, we use Monte Carlo simulations on a model system of cartilage and find that the difference originates from the Coulombic intermolecular interactions between the ions in the cartilage, and in the synovial fluid. Those interactions are neglected in the standard Donnan analysis that generally is adopted to evaluate magnetic resonance imaging data. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 08/2012; 68(4):1298-302. · 3.27 Impact Factor
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    ABSTRACT: Aqueous dispersions of pure sodium and calcium smectite clays with platelet sizes on the order of a few hundred nanometers were characterized using a combination of cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). With monovalent sodium counterions the clay is dispersed as individual platelets, as seen by cryo-TEM, that order into a nematic phase. From SAXS a one-dimensional swelling of the clay in water is observed with the characteristic spacing hs = δ/c, where hs is the separation between the platelets, δ = 1 nm is the effective platelet thickness, and c is the clay volume fraction in the sample. In calcium montmorillonite, on the other hand, cryo-TEM images clearly show the presence of tactoids, where the platelets have aggregated into stacks with a periodic spacing of 2 nm. From imaging a large number of tactoids the distribution function f(N) for the number of platelets per tactoid was estimated, and the average number N ≈ 10. The characteristic 2 nm spacing as well as the small number of platelets per tactoid was also confirmed by SAXS. The present study demonstrates that cryo-TEM, with carefully prepared specimen, is a very useful technique to characterize clay dispersions, particularly in aggregated systems.
    The Journal of Physical Chemistry C. 03/2012; 116(13):7596–7601.
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    ABSTRACT: Recent experimental studies show that oppositely charged proteins can self-assemble to form seemingly stable microspheres in aqueous salt solutions. We here use parallel tempering Monte Carlo simulations to study protein phase separation of lysozyme/α-lactalbumin mixtures and show that anisotropic electrostatic interactions are important for driving protein self-assembly. In both dilute and concentrated protein phases, the proteins strongly align according to their charge distribution. While this alignment can be greatly diminished by a single point mutation, phase separation is completely suppressed when neglecting electrostatic anisotropy. The results highlight the importance of subtle electrostatic interactions even in crowded biomolecular environments where other short-ranged forces are often thought to dominate.Keywords: protein−protein interactions; electrostatics; noncentrosymmetric interactions; microsphere formation; whey proteins; coarse graining
    The Journal of Physical Chemistry Letters. 02/2012; 3(6).
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    ABSTRACT: Gathering experimental evidence suggests that bovine as well as human lactoferrin self-associate in aqueous solution. Still, a molecular level explanation is unavailable. Using force field based molecular modeling of the protein-protein interaction free energy we demonstrate (1) that lactoferrin forms highly stereo-specific dimers at neutral pH and (2) that the self-association is driven by a high charge complementarity across the contact surface of the proteins. Our theoretical predictions of dimer formation are verified by electrophoretic mobility and N-terminal sequence analysis on bovine lactoferrin.
    Biophysical chemistry 10/2010; 151(3):187-9. · 2.28 Impact Factor
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    ABSTRACT: Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 A. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one "intra-lamellar" and a second "extra-lamellar" swelling. Montmorillonite in contact with a salt reservoir containing both Na(+) and Ca(2+) counterions will only show a modest swelling unless the Na(+) concentration in the bulk is several orders of magnitude larger than the Ca(2+) concentration. The limited swelling of clay in presence of divalent counterions is a consequence of ion-ion correlations, which reduce the entropic repulsion as well as give rise to an attractive component in the total osmotic pressure. Ion-ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion-ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic, which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the "extra-lamellar" swelling found in dialysis experiments. This idea also finds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.
    Langmuir 03/2010; 26(8):5782-90. · 4.38 Impact Factor
  • Martin Trulsson, Jan Forsman, Torbjörn Akesson, Bo Jönsson
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    ABSTRACT: In this work, we present Monte Carlo simulations of mixtures containing negatively charged latex particles and positively charged dendrimers. We focus on the interaction between two latex particles as salt concentration, dendrimer dose, and generation number are varied. Interaction free energies and corresponding stability ratios are calculated. Minimal stability is found near the isoelectric point, i.e., where the amount of adsorbed dendrimer charge matches the charge of the latex particles. Away from the isoelectric point, the stability increases as the latex particles get more and more under- or overcompensated, an increase that is more steep on the overcompensated side. Increasing the dendrimer generation leads to a more "patchy" surface. This heterogeneity is particularly relevant close to the isoelectric point. Given the relative simplicity of the model, the simulation results are in surprisingly good agreement with the experimental data.
    Langmuir 07/2009; 25(11):6106-12. · 4.38 Impact Factor
  • Andrey V Brukhno, Torbjörn Akesson, Bo Jönsson
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    ABSTRACT: Attractive interactions between like-charged aggregates (macromolecules, colloidal particles, or micelles) in solution due to electrostatic correlation effects are revisited. The associated phenomenon of phase separation in a colloidal solution of highly charged particles is directly observed in Monte Carlo simulations. We start with a simple, yet instructive, description of polarization effects in a "cloud" of counterions around a single charged aggregate and show how the ion-ion correlations can be mapped onto a classical analogue of the quantum-mechanical dispersion force. We then extend our treatment to the effective pair interaction between two such aggregates and provide an analysis of different interaction regimes, based on a simple coupling parameter. By computing the potential of mean force, we illustrate the physics behind the crossover between the regimes of pure repulsion and attraction with increasing counterion valency. Finally, we turn to semi grand NpT simulations of the corresponding bulk systems where mono- and multivalent ions can exchange with an external reservoir. Thus, the coagulation and phase separation phenomena, widely observed and used in real-life applications, are directly studied in these computer simulations.
    The Journal of Physical Chemistry B 05/2009; 113(19):6766-74. · 3.61 Impact Factor
  • Martin Turesson, Torbjörn Akesson, Jan Forsman
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    ABSTRACT: We simulate interactions between charged flat surfaces in the presence of block polymers, where the end blocks carry a charge opposite to the surfaces. Using a recently developed simulation technique, we allow full equilibrium, i.e. the chemical potential of the polyelectrolyte is retained as the separation is changed. In general, the block polyions will adsorb strongly enough to overcharge the surfaces. This results in a double layer repulsion at large separation, with a concomitant free energy barrier. At short separations, the surfaces are pulled together by bridging forces. We make some efforts to theoretically design the polymers to be efficient flocculants, i.e. minimize the free energy barrier and still allow for a long-ranged bridging attraction. Here, we also take into account the possibility of nonequilibrium circumstances, which may be relevant in practice. Our results suggest that short chains, with small charged end blocks and a (relatively speaking) long neutral mid block, are likely to promote rapid flocculation.
    Journal of Colloid and Interface Science 10/2008; 329(1):67-72. · 3.17 Impact Factor
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    ABSTRACT: A new simulation method for nonuniform polymer solutions between planar surfaces at full chemical equilibrium is described. The technique uses a grid of points in a two-dimensional thermodynamic space, labeled by surface area and surface separations. Free energy differences between these points are determined via Bennett's optimized rates method in the canonical ensemble. Subsequently, loci of constant chemical potential are determined within the grid via simple numerical interpolation. In this way, a series of free energy versus separation curves are determined for a number of different chemical potentials. The method is applied to the case of hard sphere polymers between attractive surfaces, and its veracity is confirmed via comparisons with established alternative simulation techniques, namely, the grand canonical ensemble and isotension ensemble methods. The former method is shown to fail when the degree of polymerization is too large. An interesting interplay between repulsive steric interactions and attractive bridging forces occurs as the surface attraction and bulk monomer density are varied. This behavior is further explored using polymer density functional theory, which is shown to be in good agreement with the simulations. Our results are also discussed in light of recent self-consistent field calculations which correct the original deGennes results for infinitely long polymers. In particular, we look at the role of chain ends by investigating the behavior of ring polymers.
    The Journal of Physical Chemistry B 08/2008; 112(32):9802-9. · 3.61 Impact Factor
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    ABSTRACT: We have simulated interactions between charged surfaces in the presence of oppositely charged polyelectrolytes by coupling perturbations in the isotension ensemble to a free energy variance minimization scheme. For polymeric systems, this method completely outperforms configurationally biased versions of grand canonical simulations. Proper diffusive equilibrium between bulk and slit has been established for polyelectrolytes with up to 60 monomers per chain. A consequence of imposing diffusive equilibrium conditions, in contrast to previous more restricted models, is the possibility of surface charge inversion; ion-ion correlation and the cooperativity of monomer adsorption drive the formation of a polyion layer close to the surface, that overcompensates the nominal surface charge. This is observed even at modest surface charge densities, and leads to a build up of a long ranged electrostatic barrier. In addition, the onset of charge inversion requires very low bulk polymer densities. Due to screening effects, this leads to a higher and more long-ranged free energy barrier at low, compared to high, bulk densities. Oscillatory forces, reminiscent of those found in simple hard sphere systems, are resolved in the high concentration regime. As a consequence of a second surface charge inversion, the system "stratifies" to form a stable polyelectrolyte layer in the central part of the slit, stabilized by the adsorbed surface layers.
    The Journal of Physical Chemistry B 05/2008; 112(16):5116-25. · 3.61 Impact Factor
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    ABSTRACT: The interaction of two oppositely charged surfaces has been investigated using Monte Carlo simulations and approximate analytical methods. When immersed in an aqueous electrolyte containing only monovalent ions, two such surfaces will generally show an attraction at large and intermediate separations. However, if the electrolyte solution contains divalent or multivalent ions, then a repulsion can appear at intermediate separations. The repulsion increases with increasing concentration of the multivalent salt as well as with the valency of the multivalent ion. The addition of a second salt with only monovalent ions magnifies the effect. The repulsion between oppositely charged surfaces is an effect of ion-ion correlations, and it increases with increasing electrostatic coupling and, for example, a lowering of the dielectric permittivity enhances the effect. An apparent charge reversal of the surface neutralized by the multivalent ion is always observed together with a repulsion at large separation, whereas at intermediate separations a repulsion can appear without charge reversal. The effect is hardly observable for a symmetric multivalent salt (e.g., 2:2 or 3:3).
    Langmuir 12/2007; 23(23):11562-9. · 4.38 Impact Factor
  • Martin Turesson, Torbjörn Akesson, Jan Forsman
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    ABSTRACT: With grand canonical simulations invoking a configurationally weighted scheme, we have calculated interactions between charged surfaces immersed in a polyelectrolyte solution. In contrast to previous simulations of such systems, we have imposed full equilibrium conditions (i.e., we have included diffusive equilibrium with a bulk solution). This has a profound impact on the resulting interactions: even at modest surface charge densities, oppositely charged chains will, at sufficiently large separations, adsorb strongly enough to overcompensate for the nominal surface charge. This phenomenon, known as charge inversion, generates a double-layer repulsion and a free-energy barrier. Simpler canonical approaches, where the chains are assumed to neutralize the surfaces perfectly, will not capture this stabilizing barrier. The barrier height increases with the length of the polyions. Interestingly enough, the separation at which the repulsion becomes attractive is independent of chain length. The short chains here are unable to reach across from one surface to the other. We therefore conclude that the transition to an attractive regime is not provided by the formation of such "intersurface" bridges. With long chains and at large separations, charge inversion displays decaying oscillatory behavior (i.e., the apparent surface charge switches sign once again). This is due to polyion packing effects. We have also investigated responses to salt addition and changes in polyelectrolyte concentration. Our results are in qualitative and semiquantitative agreement with experimental findings, although it should be noted that our chains are comparatively short, and the experimental surface charge density is poorly established.
    Langmuir 10/2007; 23(19):9555-8. · 4.38 Impact Factor
  • Martin Turesson, Jan Forsman, Torbjörn Akesson
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    ABSTRACT: We simulate interactions between adsorbing and nonadsorbing surfaces immersed in solutions containing monodisperse semiflexible chains. Apart from the nature of the surfaces, we investigate responses to changes of the intrinsic chain stiffness, the degree of polymerization, and the bulk concentration. Our simulations display a sufficient accuracy and precision to reveal free-energy barriers that are small on a typical scale of surface force simulations, but still of the same order as the expected van der Waals interactions. Two different approaches have been tested: grand canonical simulations, improved by configurational-biased techniques, and a perturbation method utilizing the isotension ensemble. We find the former to be preferable when the surfaces are nonadsorbing, whereas the isotension approach is superior for calculations of interactions between adsorbing surfaces, especially if the polymers are stiff. We also compare our simulation results with predictions from several versions of polymer density functional theory. We find that a crucial aspect of these theories, in quantitative terms, is that they recognize that end monomers exclude more volume to the surrounding than inner ones do. Those theories provide satisfactorily accurate predictions, particularly when the surfaces are nonadsorbing.
    Physical Review E 09/2007; 76(2 Pt 1):021801. · 2.31 Impact Factor
  • Torbjörn Åkesson, C E Woodward, Bo Jönsson
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    ABSTRACT: We have used Monte Carlo simulations and an extended mean field theory to study the interaction between two uniformly charged surfaces with an interstitial, neutralising solution of counterions, consisting of flexible polyelectrolytes. The latter are modelled as charged monomers bonded via harmonic potentials. The monotonic repulsion between surfaces, expected from traditional double layer theory, is not realised in this system. Indeed, the force can be attractive over a wide range of parameter values, with a magnitude significant compared to ordinary double layer or van der Waals forces. This attraction is attributed to both an entropically driven bridging, as well as electrostatic correlations, of polyelectrolyte chains across the mid-plane. The former effect is predicted by mean field theory. These results have relevance to numerous experimental systems of technical and biological importance.
    Physica Scripta 01/2007; 1990(T33):32. · 1.03 Impact Factor
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    ABSTRACT: In answer to recent experimental force measurements between oppositely charged surfaces we here reproduce the repulsion in the presence of multivalent salt using Monte Carlo simulations within the primitive model. Our osmotic pressure curves are in good agreement with experimental results. In contrast with Poisson-Boltzmann calculations, both repulsion and charge inversion are seen in the simulations. Repulsion is observed only for conditions under which there is charge inversion at large separations. However, in these cases, the repulsion is present also at intermediate separations, where there is no charge inversion. The charge inversion is thereby not the cause of the repulsion. Instead the repulsion appears to be an effect of the large amount of excess salt in the slit. Both phenomena, however, are closely linked and a consequence of ion-ion correlations, promoted by a strong electrostatic coupling.
    Physical Review Letters 09/2006; 97(6):068302. · 7.73 Impact Factor
  • Martin Turesson, Jan Forsman, Torbjörn Akesson
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    ABSTRACT: The strength and range of surface forces in a system consisting of charged polymers with variable intramolecular stiffness confined between two charged planar surfaces have been investigated by Monte Carlo simulations. The negatively charged surfaces are neutralized by polymers carrying charges of opposite sign. Introducing the intermediate intrinsic stiffness of the chains gives rise to a weaker, but more long-ranged attraction between the surfaces. In the limit of infinitely stiff chains, this bridging attraction is lost, but it is replaced by a strong correlation attraction at short distances. Comparisons with predictions by a correlation-corrected polyelectrolyte Poisson-Boltzmann theory are made. The theory predicts surface attractions that are somewhat too weak, but all qualitative features are correctly reproduced. Given the crudeness of the model, the quantitative agreement is satisfactory.
    Langmuir 07/2006; 22(13):5734-41. · 4.38 Impact Factor
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    ABSTRACT: Both natural and synthetic polyelectrolytes form strong complexes with a variety of proteins. One peculiar phenomenon is that association can take place even when the protein and the polyelectrolyte carry the same charge. This has been interpreted as if the ion-dipole interaction can overcome the repulsive ion-ion interaction. On the basis of Monte Carlo simulations and perturbation theory, we propose a different explanation for the association, namely, charge regulation. We have investigated three different protein-polymer complexes and found that the induced ionization of amino acid residues due to the polyelectrolyte leads to a surprisingly strong attractive interaction between the protein and the polymer. The extra attraction from this charge-induced charge interaction can be several kT and is for the three cases studied here, lysozyme, alpha-lactalbumin, and beta-lactoglobulin, of the same magnitude or stronger than the ion-dipole interaction. The magnitude of the induced charge is governed by a response function, the protein charge capacitance Z2-Z2. This fluctuation term can easily be calculated in a simulation or measured in a titration experiment.
    The Journal of Physical Chemistry B 04/2006; 110(9):4459-64. · 3.61 Impact Factor
  • Mikael Lund, Torbjörn Akesson, Bo Jönsson
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    ABSTRACT: When a protein molecule approaches a charged surface, its protonation state can undergo dramatic changes due to the imposed electric potential. This has a large impact on adsorption strengths that may be enhanced by several kT. Using mesoscopic simulation techniques as well as analytical theories, we have investigated this regulation mechanism and demonstrate how it is influenced by salt concentration and solution pH. Using hisactophilin as a test case, we show how the binding to a lipid membrane is governed by small changes in pH and that this is intimately coupled to the charge regulation mechanism.
    Langmuir 09/2005; 21(18):8385-8. · 4.38 Impact Factor

Publication Stats

115 Citations
87.59 Total Impact Points

Institutions

  • 1999–2013
    • Lund University
      • • Department of Theoretical Chemistry
      • • Department of Physical Chemistry
      Lund, Skane, Sweden
  • 2009
    • University of Leeds
      • School of Chemistry
      Leeds, ENG, United Kingdom
  • 2006
    • University of São Paulo
      • Faculdade de Ciências Farmacêuticas de Ribeirão Preto (FCFRP)
      São Paulo, Estado de Sao Paulo, Brazil
  • 2003
    • Victoria University Melbourne
      Melbourne, Victoria, Australia