Publications by authors named "Drew Parsons"

37 Publications

On the control of dispersion interactions between biological membranes and protein coated biointerfaces.

J Colloid Interface Sci 2021 Sep 11;598:464-473. Epub 2021 Mar 11.

PULS Group, Department of Physics and Interdisciplinary Center for Nanostructured Films, Friedrich-Alexander-Universität Erlangen-Nürnberg, IZNF, Cauerstrasse 3, 91058 Erlangen, Germany; Division of Physical Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia. Electronic address:

Hypothesis: Interaction of cellular membranes with biointerfaces is of vital importance for a number of medical devices and implants. Adhesiveness of these surfaces and cells is often regulated by depositing a layer of bovine serum albumin (BSA) or other protein coatings. However, anomalously large separations between phospholipid membranes and the biointerfaces in various conditions and buffers have been observed, which could not be understood using available theoretical arguments.

Methods: Using the Lifshitz theory, we here evaluate the distance-dependent Hamaker coefficient describing the dispersion interaction between a biointerface and a membrane to understand the relative positioning of two surfaces. Our theoretical modeling is supported by experiments where the biointerface is represented by a glass substrate with deposited BSA and protein layers. These biointerfaces are allowed to interact with giant unilamellar vesicles decorated with polyethylene glycol (PEG) using PEG lipids to mimic cellular membranes and their pericellular coat.

Results: We demonstrate that careful treatment of the van der Waals interactions is critical for explaining the lack of adhesiveness of the membranes with protein-decorated biointerfaces. We show that BSA alone indeed passivates the glass, but depositing an additional protein layer on the surface BSA, or producing multiple layers of proteins and BSA results in repulsive dispersion forces responsible for 100 nm large equilibrium separations between the two surfaces.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.jcis.2021.02.078DOI Listing
September 2021

Specific Buffer Effects on the Intermolecular Interactions among Protein Molecules at Physiological pH.

J Phys Chem Lett 2020 Aug 7;11(16):6805-6811. Epub 2020 Aug 7.

Department of Chemical and Geological Sciences, University of Cagliari, and Centro NanoBiotecnologie Sardegna (CNBS), Cittadella Universitaria, SS 554 bivio Sestu, 09042 Monserrato (CA), Italy.

BSA and lysozyme molecular motion at pH 7.15 is buffer-specific. Adsorption of buffer ions on protein surfaces modulates the protein surface charge and thus protein-protein interactions. Interactions were estimated by means of the interaction parameter obtained from plots of diffusion coefficients at different protein concentrations ( = [1 + ]) via dynamic light scattering and nuclear magnetic resonance. The obtained results agree with recent findings confirming doubts regarding the validity of the Henderson-Hasselbalch equation, which has traditionally provided a basis for understanding pH buffers of primary importance in solution chemistry, electrochemistry, and biochemistry.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.0c01900DOI Listing
August 2020

Re-entrant swelling and redissolution of polyelectrolytes arises from an increased electrostatic decay length at high salt concentrations.

J Colloid Interface Sci 2020 Nov 20;579:369-378. Epub 2020 Jun 20.

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, PR China; Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia. Electronic address:

Hypothesis: A detailed understanding of the influence of electrolytes on the conformation of polyelectrolyte chains is an important goal made challenging by the strong coupling between electrostatic interactions and chain conformation. This challenge is particularly evident at moderate to high salt concentrations where mean-field theories of electrolytes are no longer applicable and are therefore unable to predict the interactions between neutral or like charged surfaces that leads to re-entrant swelling of DNA and other polyelectrolytes at high salt concentrations. Recent developments arising from studies of surface forces in ionic liquids that have been extended to include a wide variety of monovalent electrolytes reveal a hitherto unknown increase in the electrostatic decay length at high electrolyte concentrations. We hypothesise that the re-entrant behaviour of polyelectrolytes is driven by an increasing electrostatic decay length with increasing electrolyte concentration.

Experiments: We survey numerous experiments in the literature on re-entrant swelling and calculate the effect of ion pairing on the electrostatic decay length in concentrated electrolytes.

Findings: Re-entrant solubility is driven by an increasing electrostatic decay length at high salt concentrations and is universal across all polyelectrolytes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.jcis.2020.06.072DOI Listing
November 2020

Full-Spectrum High-Resolution Modeling of the Dielectric Function of Water.

J Phys Chem B 2020 Apr 7;124(15):3103-3113. Epub 2020 Apr 7.

Discipline of Chemistry & Physics, Murdoch University, 90 South St, Murdoch, WA 6150, Australia.

In view of the vital role of water, exact knowledge of its dielectric function over a large frequency range is important. We report on currently available measurements of the dielectric function of water at room temperature (25 °C) across the full spectrum: microwave, IR, UV, and X-ray (up to 100 eV). We parameterize the complex dielectric function of water with two Debye (microwave) oscillators and high resolution of IR and UV/X-ray oscillators. We also report dielectric parameters for ice-cold water with a microwave/IR spectrum measured at 0.4 °C, while taking the UV spectrum at 25 °C (assuming negligible temperature dependence in UV). We employ van der Waals dispersion interactions to contrast our model of ice-cold water with earlier models. Air bubbles in water and dissolved gas molecules show attraction toward interfaces rather than repulsion. The van der Waals interaction promotes complete freezing rather than supporting a thin layer of water on ice. We infer that premelting is driven by charge and ion adsorption. Density-based extrapolation from warm to cold water of the dielectric function is satisfactory in microwave but poor (40% error) at IR frequencies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpcb.0c00410DOI Listing
April 2020

Impact of effective polarisability models on the near-field interaction of dissolved greenhouse gases at ice and air interfaces.

Phys Chem Chem Phys 2019 Oct 24;21(38):21296-21304. Epub 2019 Sep 24.

Centre for Materials Science and Nanotechnology, Department of Physics, University of Oslo, P. O. Box 1048 Blindern, NO-0316 Oslo, Norway and Department of Energy and Process Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.

We present a theory for Casimir-Polder forces acting on greenhouse gas molecules dissolved in a thin water film. Such a nano-sized film has been predicted to arise on the surface of melting ice as stabilized by repulsive Lifshitz forces. We show that different models for the effective polarisability of greenhouse gas molecules in water lead to different predictions for how Casimir-Polder forces influence their extractions from the melting ice surface. For instance, in the most intricate model of a finite-sized molecule inside a cavity, dispersion potentials push the methane molecules towards the ice surface whereas the oxygen typically will be attracted towards the closest interface (ice or air). Previous models for effective polarisability had suggested that O would also be pushed towards the ice surface. Release of greenhouse gas molecules from the surface of melting ice can potentially influence climate greenhouse effects. With this model, we show that some molecules cannot escape from water as single molecules. Due to the contradiction of the results and the escape dynamics of gases from water, we extended the models to describe bubble filled with several molecules increasing their buoyancy force.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c9cp03165kDOI Listing
October 2019

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge.

Phys Chem Chem Phys 2019 Sep;21(36):20055-20064

Department of Applied Mathematics, Research School of Physics, The Australian National University, Mills Rd Acton, Canberra, 2601, Australia.

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c9cp02797aDOI Listing
September 2019

A thermodynamic correction to the theory of competitive chemisorption of ions at surface sites with nonelectrostatic physisorption.

J Chem Phys 2019 Jul;151(2):024701

Department of Chemical and Geological Sciences, University of Cagliari, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato, CA, Italy.

We resolve a thermodynamic inconsistency in previous theoretical descriptions of the free energy of chemisorption (charge regulation) under conditions where nonelectrostatic physisorption is included, as applied to surface forces and particle-particle interactions. We clarify the role of nonelectrostatic ion physisorption energies and show that a term previously thought to represent physisorbed ion concentrations (activities) should instead be interpreted as a "partial ion activity" based solely on the electrostatic physisorption energy and bulk concentration, or alternatively on the nonelectrostatic physisorption energy and surface concentration. Second, the chemisorption energy must be understood as the change in chemical potential after subtracting the electrostatic energy, not subtracting the physisorption energy. Consequently, a previously reported specific ion nonelectrostatic physisorption contribution to the chemisorption free energy is annulled. We also report a correction to the calculation of surface charge. The distinction in "partial ion activity" evaluated from bulk concentration or from surface concentration opens a way to study nonequilibrium forces where chemisorption is in equilibrium with physisorbed ions but not in equilibrium with bulk ions, e.g., by a jump in ion concentrations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.5096237DOI Listing
July 2019

The Role of Citric Acid in the Stabilization of Nanoparticles and Colloidal Particles in the Environment: Measurement of Surface Forces between Hafnium Oxide Surfaces in the Presence of Citric Acid.

Langmuir 2018 02 13;34(8):2595-2605. Epub 2018 Feb 13.

Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University , Canberra, ACT 0200, Australia.

The interactions between colloidal particles and nanoparticles determine solution stability and the structures formed when the particles are unstable to flocculation. Therefore, knowledge of the interparticle interactions is important for understanding the transport, dissolution, and fate of particles in the environment. The interactions between particles are governed by the surface properties of the particles, which are altered when species adsorb to the surface. The important interactions in the environment are almost never those between the bare particles but rather those between particles that have been modified by the adsorption of natural organic materials. Citric acid is important in this regard not only because it is present in soil but also as a model of humic and fulvic acids. Here we have studied the surface forces between the model metal oxide surface hafnia in the presence of citric acid in order to understand the stability of colloidal particles and nanoparticles. We find that citric acid stabilizes the particles over a wide range of pH at low to moderate ionic strength. At high ionic strength, colloidal particles will flocculate due to a secondary minimum, resulting in aggregates that are dense and easily redispersed. In contrast, nanoparticles stabilized by citric acid remain stable at high ionic strengths and therefore exist in solution as individual particles; this will contribute to their dispersion in the environment and the uptake of nanoparticles by mammalian cells.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.langmuir.7b03116DOI Listing
February 2018

Effective Polarizability Models.

J Phys Chem A 2017 Dec 15;121(51):9742-9751. Epub 2017 Dec 15.

Physikalisches Institut, Albert-Ludwigs-Universität Freiburg , Hermann-Herder-Strasse 3, 79104 Freiburg, Germany.

Theories for the effective polarizability of a small particle in a medium are presented using different levels of approximation: we consider the virtual cavity, real cavity, and the hard-sphere models as well as a continuous interpolation of the latter two. We present the respective hard-sphere and cavity radii as obtained from density-functional simulations as well as the resulting effective polarizabilities at discrete Matsubara frequencies. This enables us to account for macroscopic media in van der Waals interactions between molecules in water and their Casimir-Polder interaction with an interface.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.7b10159DOI Listing
December 2017

Measurement of long range attractive forces between hydrophobic surfaces produced by vapor phase adsorption of palmitic acid.

Soft Matter 2017 Dec;13(47):8910-8921

Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia.

Extensive research into the surface forces between hydrophobic surfaces has produced experimentally measured interaction forces that vary widely in range and in magnitude. This variability is attributed to interference from surface nanobubbles and the nature of the hydrophobic surface. Whilst the effects of nanobubbles are now recognised and can be addressed, the precise nature of the surface remains a confounding factor in measurements between hydrophobic surfaces. Here we show that a monolayer coating with hydrophobic properties is formed by exposing metal oxide surfaces to palmitic acid vapour. Surface forces measured between these smooth hydrophobic surfaces exhibited an exponential attraction. Neither patchy surface charges, nor surface nanobubbles could explain the measured forces. However, the observed interaction may be explained by the interaction of a single patch of bilayered palmitic acid molecules interacting with an exposed patch of the hafnia surface. Such an interaction is consistent with the observed exponential nature of the attraction and the agreement between the measured decay of the exponential attraction with the Debye length of the solution.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c7sm01563aDOI Listing
December 2017

Cation effects on haemoglobin aggregation: balance of chemisorption against physisorption of ions.

Interface Focus 2017 Aug 16;7(4):20160137. Epub 2017 Jun 16.

Department of Chemical and Geological Sciences, University of Cagliari-CSGI and CNBS Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato (CA), Italy.

A theoretical model of haemoglobin is presented to explain an anomalous cationic Hofmeister effect observed in protein aggregation. The model quantifies competing proposed mechanisms of non-electrostatic physisorption and chemisorption. Non-electrostatic physisorption is stronger for larger, more polarizable ions with a Hofmeister series Li< K< Cs. Chemisorption at carboxylate groups is stronger for smaller kosmotropic ions, with the reverse series Li > K > Cs. We assess aggregation using second virial coefficients calculated from theoretical protein-protein interaction energies. Taking Cs to not chemisorb, comparison with experiment yields mildly repulsive cation-carboxylate binding energies of 0.48 for Li and 3.0 for K. Aggregation behaviour is predominantly controlled by short-range protein interactions. Overall, adsorption of the K ion in the middle of the Hofmeister series is stronger than ions at either extreme since it includes contributions from both physisorption and chemisorption. This results in stronger attractive forces and greater aggregation with K, leading to the non-conventional Hofmeister series K > Cs ≈ Li.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1098/rsfs.2016.0137DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5474036PMC
August 2017

Roughness in Surface Force Measurements: Extension of DLVO Theory To Describe the Forces between Hafnia Surfaces.

J Phys Chem B 2017 07 26;121(26):6442-6453. Epub 2017 Jun 26.

Department of Applied Mathematics, Research School of Physical Sciences, Australian National University , Canberra, ACT 2614, Australia.

The interaction between colloidal particles is commonly viewed through the lens of DLVO theory, whereby the interaction is described as the sum of the electrostatic and dispersion forces. For similar materials acting across a medium at pH values remote from the isoelectric point the theory typically involves an electrostatic repulsion that is overcome by dispersion forces at very small separations. However, the dominance of the dispersion forces at short separations is generally not seen in force measurements, with the exception of the interaction between mica surfaces. The discrepancy for silica surfaces has been attributed to hydration forces, but this does not explain the situation for titania surfaces where the dispersion forces are very much larger. Here, the interaction forces between very smooth hafnia surfaces have been measured using the colloid probe technique and the forces evaluated within the DLVO framework, including both hydration forces and the influence of roughness. The measured forces across a wide range of pH at different salt concentrations are well described with a single parameter for the surface roughness. These findings show that even small degrees of surface roughness significantly alter the form of the interaction force and therefore indicate that surface roughness needs to be included in the evaluation of surface forces between all surfaces that are not ideally smooth.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpcb.7b03131DOI Listing
July 2017

Volume dependence of the dielectric properties of amorphous SiO2.

Phys Chem Chem Phys 2016 Mar;18(10):7483-9

Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway. and Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden and Department of Physics, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway.

Using first principles calculations, the analysis of the dielectric properties of amorphous SiO2 (am-SiO2) was performed. We found that the am-SiO2 properties are volume dependent, and the dependence is mainly induced by the variation of nanoporosity at the atomic scale. In particular, both ionic and electronic contributions to the static dielectric constants are functions of volume with clear trends. Moreover, using the unique parameterization of the dielectric function provided in this work, we predict dielectric functions at imaginary frequencies of different SiO2 polymorphs having similar band gap energies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c5cp06775hDOI Listing
March 2016

The impact of the competitive adsorption of ions at surface sites on surface free energies and surface forces.

J Chem Phys 2015 Apr;142(13):134707

Department of Chemical and Geological Sciences, University of Cagliari-CSGI and CNBS, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato (CA), Italy.

The relationship between surface charge and surface potential at the solid-liquid interface is often determined by a charge regulation process, the chemisorption of a potential determining ion such as H(+). A subtle ion-specific effect can be observed when other ions compete with the primary potential determining ion to bind to a surface site. Site competition may involve alternative ions competing for a first binding site, e.g., metals ions competing with H(+) to bind to a negatively charged oxide or carboxyl site. Second-binding sites with site competition may also be found, including amphoteric OH2 (+) sites, or anion binding to amine groups. In this work, a general theoretical model is developed to describe the competitive adsorption of ions at surface sites. Applied to the calculation of forces, the theory predicts a 20% increase in repulsion between titania surfaces in 1 mM NaCl, and a 25% reduction in repulsion between silica surfaces in 0.1M NaCl compared to calculations neglecting ion site competition.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4916519DOI Listing
April 2015

Intermolecular Casimir-Polder forces in water and near surfaces.

Phys Rev E Stat Nonlin Soft Matter Phys 2014 Sep 18;90(3):032122. Epub 2014 Sep 18.

Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden and Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway and Department of Energy and Process Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.

The Casimir-Polder force is an important long-range interaction involved in adsorption and desorption of molecules in fluids. We explore Casimir-Polder interactions between methane molecules in water, and between a molecule in water near SiO(2) and hexane surfaces. Inclusion of the finite molecular size in the expression for the Casimir-Polder energy leads to estimates of the dispersion contribution to the binding energies between molecules and between one molecule and a planar surface.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevE.90.032122DOI Listing
September 2014

A continuum solvent model of ion-ion interactions in water.

Phys Chem Chem Phys 2014 Oct;16(40):22014-27

Applied Mathematics Department, Australian National University, Canberra, ACT 0200, Australia.

The calculation of ion-ion interactions in water is a problem of long standing importance. Modelling these interactions is a prerequisite to explaining Hofmeister (specific ion) effects. We here generalize our solvation model of ions to calculate the free energy of two ions in water as a function of separation. The same procedure has previously been applied to calculate ion interactions with the air-water interface successfully. The Conductor like Screening Model (COSMO) is used. This treats the ions on a quantum mechanical level and calculates numerically the electrostatic response of the surrounding solvent. Estimates of the change in the cavity formation energy and the change in the ion-water dispersion energy as the ions approach are included separately. The calculated interaction potentials are too attractive and this is a significant issue. However, they do reproduce the affinity of similarly sized ions for each other, which is a crucial property of these potentials. They are also oscillatory, another important property. We normalize the potentials to reduce the over-attraction, and good correlation with experimental values is achieved. We identify the driving contributions to this like-prefers-like behaviour. We then put forward a plausible hypothesis for the over-attraction of the potentials. An agreeable feature of our approach is that it does not rely on salt specific parameters deliberately adjusted to reproduce experimental values.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c4cp02822hDOI Listing
October 2014

Ion interactions with the air-water interface using a continuum solvent model.

J Phys Chem B 2014 Jul 11;118(29):8700-10. Epub 2014 Jul 11.

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University , Canberra ACT 0200, Australia.

Explaining and predicting the distribution of ions at the air-water interface has been a central challenge of physical chemistry for nearly a century. In essence, the problem amounts to calculating the change in the solvation energy of an ion as it approaches the interface. Here, we generalize our recently developed model of ionic solvation energies to calculate this interaction. The change in the Born energy as well as the static polarization response of the ion is included by using the conductor-like screening model (COSMO), which treats the ions quantum mechanically. Approximate expressions for the dispersion repulsion, cavity attraction, and surface potential contributions are also included. This model reproduces the surface tensions of electrolyte solutions and is consistent with ab initio molecular dynamics (MD) simulation. The model provides clear physical insight into iodide's adsorption. Unlike alternative models, no parameters are deliberately adjusted to reproduce surface tensions, and all of the important contributions to the interactions are included. Solving this problem has important direct implications for atmospheric chemistry and bubble properties. It also has important indirect implications for the more complex interactions of ions with protein and mineral surfaces. These play a fundamental role in a vast number of biological and industrial processes. The model is conceptually simple and has low computational demand, which facilitates its extension to these important applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp502887eDOI Listing
July 2014

Surface forces: surface roughness in theory and experiment.

J Chem Phys 2014 Apr;140(16):164701

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia.

A method of incorporating surface roughness into theoretical calculations of surface forces is presented. The model contains two chief elements. First, surface roughness is represented as a probability distribution of surface heights around an average surface height. A roughness-averaged force is determined by taking an average of the classic flat-surface force, weighing all possible separation distances against the probability distributions of surface heights. Second the model adds a repulsive contact force due to the elastic contact of asperities. We derive a simple analytic expression for the contact force. The general impact of roughness is to amplify the long range behaviour of noncontact (DLVO) forces. The impact of the elastic contact force is to provide a repulsive wall which is felt at a separation between surfaces that scales with the root-mean-square (RMS) roughness of the surfaces. The model therefore provides a means of distinguishing between "true zero," where the separation between the average centres of each surface is zero, and "apparent zero," defined by the onset of the repulsive contact wall. A normal distribution may be assumed for the surface probability distribution, characterised by the RMS roughness measured by atomic force microscopy (AFM). Alternatively the probability distribution may be defined by the histogram of heights measured by AFM. Both methods of treating surface roughness are compared against the classic smooth surface calculation and experimental AFM measurement.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4871412DOI Listing
April 2014

Predicting ion specific capacitances of supercapacitors due to quantum ionic interactions.

Authors:
Drew F Parsons

J Colloid Interface Sci 2014 Aug 24;427:67-72. Epub 2014 Jan 24.

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia. Electronic address:

A new theoretical framework is now available to help explain ion specific (Hofmeister) effects. All measurements in physical chemistry show ion specificity, inexplicable by classical electrostatic theories. These ignore ionic dispersion forces that change ionic adsorption. We explored ion specificity in supercapacitors using a modified Poisson-Boltzmann approach that includes ionic dispersion energies. We have applied ab initio quantum chemical methods to determine required ion sizes and ion polarisabilities. Our model represents graphite electrodes through their optical dielectric spectra. The electrolyte was 1.2 M Li salt in propylene carbonate, using the common battery anions, PF6(-), BF4(-) and ClO4(-). We also investigated the perhalate series with BrO4(-) and IO4(-). The capacitance C=dσ/dψ was calculated from the predicted electrode surface charge σ of each electrode with potential ψ between electrodes. Compared to the purely electrostatic calculation, the capacitance of a positively charged graphite electrode was enhanced by more than 15%, with PF6(-) showing >50% increase in capacitance. IO4(-) provided minimal enhancement. The enhancement is due to adsorption of both anions and cations, driven by ionic dispersion forces. The Hofmeister series in the single-electrode capacitance was PF6(-)>BF4(-)>ClO4(-)>BrO4(-)>IO4(-) . When the graphite electrode was negatively charged, the perhalates provided almost no enhancement of capacitance, while PF6(-) and BF4(-) decreased capacitance by about 15%. Due to the asymmetric impact of nonelectrostatic ion interactions, the capacitances of positive and negative electrodes are not equal. The capacitance of a supercapacitor should therefore be reported as two values rather than one, similar to the matrix of mutual capacitances used in multielectrode devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.jcis.2014.01.018DOI Listing
August 2014

A continuum solvent model of the partial molar volumes and entropies of ionic solvation.

J Phys Chem B 2014 Mar 5;118(11):3122-32. Epub 2014 Mar 5.

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University , Canberra ACT 0200, Australia.

Continuum solvent models of electrolyte solutions are extremely useful. However, before we can use them with confidence, it is important to test them by comparison with a range of experimental properties. Here, we have adapted our recently developed1,2 simple continuum solvent model of ionic solvation free energies to calculate the solvation entropies and partial molar volumes of a group of monovalent and monatomic ions. This procedure gives good quantitative agreement for larger ions, and reproduces key qualitative features, such as the shift to positive entropies of solvation for iodide and the shift to negative partial molar volumes for small cations. Small ions require a correction to account for dielectric saturation effects, which brings them also into good agreement with experiment. We argue that this model does not require ad hoc corrections, and uses parameters that have good external physical justification. This work therefore establishes that our continuum solvent model can provide a satisfactory understanding of ionic solvation. It can thus serve as a foundation for improved models that explain and predict more complex ion specific effects.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp410956mDOI Listing
March 2014

Specific cation effects on hemoglobin aggregation below and at physiological salt concentration.

Langmuir 2013 Dec 25;29(49):15350-8. Epub 2013 Nov 25.

Department of Chemical and Geological Sciences, University of Cagliari-CSGI and CNBS, Cittadella Universitaria , S.S. 554 bivio Sestu, 09042 Monserrato (CA), Italy.

Turbidity titrations are used to study the ion specific aggregation of hemoglobin (Hb) below and physiological salt concentration in the pH range 4.5-9.5. At a salt concentration 50 mM cations promote Hb aggregation according to the order Rb(+) > K(+) ~ Na(+) > Cs(+) > Li(+). The cation series changes if concentration is increased, becoming K(+) > Rb(+) > Na(+) > Li(+) > Cs(+) at 150 mM. We interpret the puzzling series by assuming that the kosmotropic Li(+) will bind to kosmotropic carboxylates groups-according to the law of matching water affinities (LMWA)-whereas the chaotropic Cs(+) will bind to uncharged protein patches due to its high polarizability. In fact, both mechanisms can be rationalized by invoking previously neglected ionic nonelectrostatic forces. This explains both adsorption to uncharged patches and the LMWA as a consequence of the simultaneous action of electrostatic and dispersion forces. The same interpretation applies to anions (with chaotropic anions binding to chaotropic amine groups). The implications extend beyond hemoglobin to other, still unexplained, ion specific effects in biological systems.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la404249nDOI Listing
December 2013

A continuum model of solvation energies including electrostatic, dispersion, and cavity contributions.

J Phys Chem B 2013 Aug 1;117(32):9421-9. Epub 2013 Aug 1.

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia.

Physically accurate continuum solvent models that can calculate solvation energies are crucial to explain and predict the behavior of solute particles in water. Here, we present such a model applied to small spherical ions and neutral atoms. It improves upon a basic Born electrostatic model by including a standard cavity energy and adding a dispersion component, consistent with the Born electrostatic energy and using the same cavity size parameter. We show that the well-known, puzzling differences between the solvation energies of ions of the same size is attributable to the neglected dispersion contribution. This depends on dynamic polarizability as well as size. Generally, a large cancellation exists between the cavity and dispersion contributions. This explains the surprising success of the Born model. The model accurately reproduces the solvation energies of the alkali halide ions, as well as the silver(I) and copper(I) ions with an error of 12 kJ mol(-1) (±3%). The solvation energy of the noble gases is also reproduced with an error of 2.6 kJ mol(-1) (±30%). No arbitrary fitting parameters are needed to achieve this. This model significantly improves our understanding of ionic solvation and forms a solid basis for the investigation of other ion-specific effects using a continuum solvent model.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp403596cDOI Listing
August 2013

A continuum solvent model of the multipolar dispersion solvation energy.

J Phys Chem B 2013 Aug 1;117(32):9412-20. Epub 2013 Aug 1.

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia.

The dispersion energy is an important contribution to the total solvation energies of ions and neutral molecules. Here, we present a new continuum model calculation of these energies, based on macroscopic quantum electrodynamics. The model uses the frequency dependent multipole polarizabilities of molecules in order to accurately calculate the dispersion interaction of a solute particle with surrounding water molecules. It includes the dipole, quadrupole, and octupole moment contributions. The water is modeled via a bulk dielectric susceptibility with a spherical cavity occupied by the solute. The model invokes damping functions to account for solute-solvent wave function overlap. The assumptions made are very similar to those used in the Born model. This provides consistency and additivity of electrostatic and dispersion (quantum mechanical) interactions. The energy increases in magnitude with cation size, but decreases slightly with size for the highly polarizable anions. The higher order multipole moments are essential, making up more than 50% of the dispersion solvation energy of the fluoride ion. This method provides an accurate and simple way of calculating the notoriously problematic dispersion contribution to the solvation energy. The result establishes the importance of using accurate calculations of the dispersion energy for the modeling of solvation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp403595xDOI Listing
August 2013

Hofmeister challenges: ion binding and charge of the BSA protein as explicit examples.

Langmuir 2012 Nov 13;28(47):16355-63. Epub 2012 Nov 13.

Department of Chemical and Geological Sciences, University of Cagliari-CSGI and CNBS, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato (CA), Italy.

Experiments on bovine serum albumin (BSA) via potentiometric titration (PT) and electrophoretic light scattering (ELS) are used to study specific-ion binding. The effect is appreciable at a physiological concentration of 0.1 M. We found that anions bind to the protein surface at an acidic pH, where the protein carries a positive charge (Z(p) > 0), according to a Hofmeister series (Cl(-) < Br(-) < NO(3)(-) < I(-) < SCN(-)), as well as at the isoionic point (Z(p) = 0). The results obtained require critical interpretation. The measurements performed depend on electrostatic theories that ignore the very specific effects they are supposed to reveal. Notwithstanding this difficulty, we can still infer that different 1:1 sodium salts affect the BSA surface charge/pH curve because anions bind to the BSA surface with an efficiency which follows a Hofmeister series.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la3035984DOI Listing
November 2012

Hofmeister series reversal for lysozyme by change in pH and salt concentration: insights from electrophoretic mobility measurements.

Phys Chem Chem Phys 2012 Apr 28;14(13):4343-6. Epub 2012 Feb 28.

Department of Chemical & Geological Sciences, University of Cagliari, Monserrato, CA, Italy.

Hofmeister series reversal can occur with change in pH, or increase in salt concentration. The phenomena are a challenge for any theory of ion specific effects. Recent theoretical work predicts how a complex interplay between ionic sizes, hydration and dispersion forces explains Hofmeister series reversal. Electrophoretic mobility measurements on lysozyme suspensions reported here are consistent with the theory.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c2cp40150aDOI Listing
April 2012

Measurements and theoretical interpretation of points of zero charge/potential of BSA protein.

Langmuir 2011 Sep 23;27(18):11597-604. Epub 2011 Aug 23.

Department of Chemical Science, University of Cagliari-CSGI and CNBS, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042- Monserrato (CA), Italy.

The points of zero charge/potential of proteins depend not only on pH but also on how they are measured. They depend also on background salt solution type and concentration. The protein isoelectric point (IEP) is determined by electrokinetical measurements, whereas the isoionic point (IIP) is determined by potentiometric titrations. Here we use potentiometric titration and zeta potential (ζ) measurements at different NaCl concentrations to study systematically the effect of ionic strength on the IEP and IIP of bovine serum albumin (BSA) aqueous solutions. It is found that high ionic strengths produce a shift of both points toward lower (IEP) and higher (IIP) pH values. This result was already reported more than 60 years ago. At that time, the only available theory was the purely electrostatic Debye-Hückel theory. It was not able to predict the opposite trends of IIP and IEP with ionic strength increase. Here, we extend that theory to admit both electrostatic and nonelectrostatic (NES) dispersion interactions. The use of a modified Poisson-Boltzmann equation for a simple model system (a charge regulated spherical colloidal particle in NaCl salt solutions), that includes these ion specific interactions, allows us to explain the opposite trends observed for isoelectric point (zero zeta potential) and isoionic point (zero protein charge) of BSA. At higher concentrations, an excess of the anion (with stronger NES interactions than the cation) is adsorbed at the surface due to an attractive ionic NES potential. This makes the potential relatively more negative. Consequently, the IEP is pushed toward lower pH. But the charge regulation condition means that the surface charge becomes relatively more positive as the surface potential becomes more negative. Consequently, the IIP (measuring charge) shifts toward higher pH as concentration increases, in the opposite direction from the IEP (measuring potential).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la2024605DOI Listing
September 2011

Possible origin of the inverse and direct Hofmeister series for lysozyme at low and high salt concentrations.

Langmuir 2011 Aug 30;27(15):9504-11. Epub 2011 Jun 30.

Department of Chemical Science, University of Cagliari-CSGI and CNBS, Cittadella Universitaria, S.S. 554 bivio Sestu, 09042 Monserrato (CA), Italy.

Protein solubility studies below the isoelectric point exhibit a direct Hofmeister series at high salt concentrations and an inverse Hofmeister series at low salt concentrations. The efficiencies of different anions measured by salt concentrations needed to effect precipitation at fixed cations are the usual Hofmeister series (Cl(-) > NO(3)(-) > Br(-) > ClO(4)(-) > I(-) > SCN(-)). The sequence is reversed at low concentrations. This has been known for over a century. Reversal of the Hofmeister series is not peculiar to proteins. Its origin poses a key test for any theoretical model. Such specific ion effects in the cloud points of lysozyme suspensions have recently been revisited. Here, a model for lysozymes is considered that takes into account forces acting on ions that are missing from classical theory. It is shown that both direct and reverse Hofmeister effects can be predicted quantitatively. The attractive/repulsive force between two protein molecules was calculated. To do this, a modification of Poisson-Boltzmann theory is used that accounts for the effects of ion polarizabilities and ion sizes obtained from ab initio calculations. At low salt concentrations, the adsorption of the more polarizable anions is enhanced by ion-surface dispersion interactions. The increased adsorption screens the protein surface charge, thus reducing the surface forces to give an inverse Hofmeister series. At high concentrations, enhanced adsorption of the more polarizable counterions (anions) leads to an effective reversal in surface charge. Consequently, an increase in co-ion (cations) adsorption occurs, resulting in an increase in surface forces. It will be demonstrated that among the different contributions determining the predicted specific ion effect the entropic term due to anions is the main responsible for the Hofmeister sequence at low salt concentrations. Conversely, the entropic term due to cations determines the Hofmeister sequence at high salt concentrations. This behavior is a remarkable example of the charge-reversal phenomenon.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la202023rDOI Listing
August 2011

Specific ion effects on adsorption at the solid/electrolyte interface: a probe into the concentration limit.

Langmuir 2011 Jul 14;27(14):8710-7. Epub 2011 Jun 14.

Materials Science Division, North-East Institute of Science & Technology, CSIR, Jorhat, Assam, India.

Adsorption of organic acid at the mineral oxide-electrolyte interface has been explored. The adsorption of 2,4-dihydroxybenzoic acid onto α-alumina illustrates that specific ion effects show up at very low salt concentration (<0.05 mM). These surprising Hofmeister effects occur at salt concentrations an order of magnitude lower than in a previous study ( J. Colloid Interface Sci. 2010, 344, 482 ). Salts enhance adsorption and specifically at ≤0.05 mM. With increasing concentration of ion, the adsorption density decreases. The results are accounted for by incorporating the ion size and dispersion forces in the theoretical modeling based on ab initio calculations of polarizabilities. The order appears to be governed by ion size, determining the maximum concentration that ions can attain near the surface due to close packing.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la2006277DOI Listing
July 2011

Hofmeister effects: interplay of hydration, nonelectrostatic potentials, and ion size.

Phys Chem Chem Phys 2011 Jul 14;13(27):12352-67. Epub 2011 Jun 14.

Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia.

The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloids, and corresponding theories of electrolytes, are unable to explain ion specific forces between colloidal particles quantitatively. The same is true generally, for surfactant aggregates, lipids, proteins, for zeta and membrane potentials and in adsorption phenomena. Even with fitting parameters the theory is not predictive. The classical theories of interactions begin with continuum solvent electrostatic (double layer) forces. Extensions to include surface hydration are taken care of with concepts like inner and outer Helmholtz planes, and "dressed" ion sizes. The opposing quantum mechanical attractive forces (variously termed van der Waals, Hamaker, Lifshitz, dispersion, nonelectrostatic forces) are treated separately from electrostatic forces. The ansatz that separates electrostatic and quantum forces can be shown to be thermodynamically inconsistent. Hofmeister or specific ion effects usually show up above ≈10(-2) molar salt. Parameters to accommodate these in terms of hydration and ion size had to be invoked, specific to each case. Ionic dispersion forces, between ions and solvent, for ion-ion and ion-surface interactions are not explicit in classical theories that use "effective" potentials. It can be shown that the missing ionic quantum fluctuation forces have a large role to play in specific ion effects, and in hydration. In a consistent predictive theory they have to be included at the same level as the nonlinear electrostatic forces that form the skeletal framework of standard theory. This poses a challenge. The challenges go further than academic theory and have implications for the interpretation and meaning of concepts like pH, buffers and membrane potentials, and for their experimental interpretation. In this article we overview recent quantitative developments in our evolving understanding of the theoretical origins of specific ion, or Hofmeister effects. These are demonstrated through an analysis that incorporates nonelectrostatic ion-surface and ion-ion dispersion interactions. This is based on ab initio ionic polarisabilities, and finite ion sizes quantified through recent ab initio work. We underline the central role of ionic polarisabilities and of ion size in the nonelectrostatic interactions that involve ions, solvent molecules and interfaces. Examples of mechanisms through which they operate are discussed in detail. An ab initio hydration model that accounts for polarisabilities of the tightly held hydration shell of "cosmotropic" ions is introduced. It is shown how Hofmeister effects depend on an interplay between specific surface chemistry, surface charge density, pH, buffer, and counterion with polarisabilities and ion size. We also discuss how the most recent theories on surface hydration combined with hydrated nonelectrostatic potentials may predict experimental zeta potentials and hydration forces.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c1cp20538bDOI Listing
July 2011

Charge reversal of surfaces in divalent electrolytes: the role of ionic dispersion interactions.

Langmuir 2010 May;26(9):6430-6

Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia.

Surface potentials of alkali earth nitrates at a mica surface are calculated using a modified Poisson-Boltzmann approach that includes nonelectrostatic ion-surface dispersion interactions. New ab initio dynamic polarizabilities are used to determine dispersion interactions. A hydration model describing the hydration shell of cations is presented. Excellent agreement with experiment is achieved, including charge reversal at high electrolyte concentration without the need for site binding models. This suggests that specific ionic dispersion forces provide the mechanism for ion surface binding. An asymptotic surface potential is found in the limit of very high concentration. A Hofmeister series is predicted according to the strength of charge reversal, with Mg > Ca > Sr > Ba. The ion-surface dispersion adsorption energies of hydrated ions appear to explain the apparent repulsive secondary hydration forces observed experimentally between mica surfaces when taken with a surface hydration layer.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/la9041265DOI Listing
May 2010