1,721,041 research outputs found
Semianalytical approximation of Ion Adsorption Layers and Capacitance in Carnahan-Starling-like steric models
Full text linkThe Carnahan–Starling (CS) steric model is the best description of hard-sphere fluids within the mean-field theory. Here we introduce an approximation of the near-linear adsorption concentration profile of a counterion near an electrode for a CS model and derive the subsequent electric field and electrostatic potential profile in a double layer. This enables the derivation of a semianalytical approximation of the electrode charge density, differential capacitance, and total energies (grand potentials) of an electric double-layer capacitor. These semianalytical equations are valid for electrode potentials between 0.2–4 V and converge to the full numerical solutions of the CS model at high potentials of 1V and bulk concentration of 1M with relative errors less than 2% for the electrode charge densities, and less than 5% for the capacitance and total energies. We find the steric contribution comprises approximately one-quarter of the total energy at high electrode potentials, while the contribution from ideal ion entropies becomes insignificant. The model shows very good agreement with experimental measurements of an aqueous electrolyte, and good agreement at high potentials with computer simulations of an ionic liquid. These semianalytical approximations are effective for applications with concentrated solutions or ionic liquids at high applied voltages where the full numerical solution is computationally expensive or in some cases impossible
pH-sensitive spontaneous decay of functionalized carbon dots in solutions
Full text linkCarbon quantum dots have become attractive in various applications, such as drug delivery, biological sensing, photocatalysis, and solar cells. Among these, pH sensing via luminescence lifetime measurements of surface-functionalized carbon dots is one application currently investigated for their long lifetime and autonomous operation. In this article, we explore the theoretical connection between excitation lifetimes and the pH value of the surrounding liquid via the protonation and deprotonation of functional groups. Example calculations applied to m-phenylenediamine, phloroglucinol, and tethered disperse blue 1 are shown by applying a separation approach treating the electronic wave function of functional groups separately from the internal electronic structure of the (large) carbon dot. The bulk of the carbon dot is treated as an environment characterized by its optical spectrum that shifts the transition rates of the functional group. A simple relationship between pH, pKa, and mixed fluorescence lifetime is derived from the transition rates of the protonated and deprotonated states. pH sensitivity improves when the difference in the transition rates is greatest between protonated and deprotonated species, with the greatest sensitivity found where the pKa is close to the pH region of interest. The introduced model can directly be extended to consider multicomponent liquids and multiple protonation states
Thermodynamics beyond dilute solution theory: Steric effects and electrowetting
No full textHere, we address the impact that steric models, introducing finite ion size effects, have on the contact angle for charged
surfaces. We review the two most common steric models, namely the excluded volume (commonly referred to as the
Bikerman model) and the Carnahan-Starling (CS) models. We clarify the thermodynamics of the solid–liquid electrolyte
interface and show that the common case of an electrolyte reservoir characterized by bulk ion concentrations corresponds to
the thermodynamics of the grand potential with fixed ionic chemical potentials. The grand potential gives distinctly different
interfacial energies compared to the free energy, which corresponds to a finite number of ions in an electrolyte solution
(relevant in nanofluidics, for instance). Steric models may be applied to either thermodynamic scenario, and applications to
electrowetting are shown under the grand potential (large droplets with a bulk reservoir in the center). When sufficiently large
potentials are applied to conducting electrodes, the steric models, unlike the classical point charge model, introduce ion
specificity into the electrowetting contact angle via finite ion sizes, which introduces an asymmetry in the contact angle at
positive and negative potentials. For electrowetting on dielectrics (EWOD), the theoretical contact angles match experimental
values well until electrode potentials of 240 V, with CS performing better than Bikerman. We hypothesize that contact angle
saturation at 240 V may arise due to a switch in the thermodynamics of the solid–liquid interfacial energy from grand
potential (bulk reservoir) to free energy (finite ion number) conditions, capping the formation of a counterion adsorption
layer at the electrode surface
Buffer-specific effects arise from ionic dispersion forces
No full textBuffer solutions do not simply regulate pH, but also change the properties of protein molecules. The zeta potential of lysozyme varies significantly at the same buffer concentration, in the order Tris > phosphate > citrate, with citrate even inverting the zeta potential, usually positive at pH 7.15, to a negative value. This buffer-specific effect is a special case of the Hofmeister effect. Here we present a theoretical model of these buffer-specific effects using a Poisson–Boltzmann description of the buffer solution, modified to include dispersion forces of all ions interacting with the lysozyme surface. Dispersion coefficients are determined from quantum chemical polarizabilites calculated for each ion for tris, phosphate, and citrate buffer solutions. The lysozyme surface charge is controlled by charge regulation of carboxylate and amine sites of the component amino acids. The theoretical model satisfactorily reproduces experimental zeta potentials, including change of sign with citrate, when hydration of small cosmotropic ions (Na+, H+, OH−) is included
Cation effects on haemoglobin aggregation: Balance of chemisorption against physisorption of ions
No full textA 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 kBT for Li+ and 3.0 kBT 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+
A theoretical and experimental study of the effects of NaCl and the competitive chemisorption of ions at the surface sites in the context of galena flotation
Full text linkIn this study we have investigated the effects of increasing the NaCl concentration on the flotation of galena. Experiments were carried out using a Hallimond tube in NaCl solutions with concentrations of 1, 10 and 100 mM, at pH 9. It was found that the recovery of galena by collector-less flotation improved for higher NaCl concentrations.
Zeta potential measurements made on galena particles conditioned in NaCl solutions were used to calibrate the parameters for describing a chemisorption model representing charge regulation at the galena binding sites. Our galena chemisorption model presented here is a “two-site/not amphoteric” surface complexation model. A chemisorption model representing the charge regulation process on the surface of an air bubble has also been applied. The zeta potentials calculated using these models were in agreement with the measured values, indicating that these models can be applied to predict the potentials on the surface of galena and an air bubble for a range of NaCl concentrations and pH.
To investigate the mechanisms of particle-bubble interactions for each NaCl concentration, the total interaction free energy as a function of the separation distance between galena and an air bubble was determined. It was found that at a NaCl concentration of 1 mM, due to adsorption of ions at the galena surface, repulsion dominated the interaction, and the lowest galena recovery was reported. With an increase in NaCl concentration in solution to 100 mM, the total interaction between galena and an air bubble was represented by a purely attractive total interaction free energy curve. The theoretical predictions of the models supported the experimental results, with a stronger attraction predicted by the models at higher ionic concentrations, corresponding to a higher galena recovery during flotation
Interactions between coarse and fine galena and quartz particles and their implications for flotation in NaCl solutions
Full text linkIn this study we have investigated the interactions between coarse and fine galena and quartz particles and their implications for flotation in NaCl solutions. Tested were four different particle systems: (CC) Coarse galena/Coarse quartz, (CF) Coarse galena/Fine quartz, (FC) Fine galena/Coarse quartz and (FF) Fine galena/Fine quartz. The flotation experiments were carried out on the four particle systems in NaCl concentrations of 10 mM and 100 mM, at pH 9. It was found that the recovery was the highest for the CC particle system, and the lowest for the FF particle system. The experiments also indicated that the recovery improved for the higher NaCl concentration.
As part of this study, we calculated the total interaction free energy as a function of separation distance for each pairwise combination of particles and bubbles, for each test condition. It was found that for all particle systems, galena/quartz interactions are dominated by repulsion. The interactions between two galena particles indicated attraction as the dominating force, while the interactions between quartz particles showed repulsion in 10 mM NaCl, which changed to attraction with an increase in NaCl concentration. Galena/air bubble interactions were controlled by repulsive electrostatic interactions in 10 mM NaCl, but these interactions became attractive in 100 mM NaCl salt solutions. On the other hand, the quartz/air bubble interactions were always repulsive, irrespective of th
The effects of NaCl addition on the particle-bubble interactions of galena in the presence of xanthate
Full text linkThis work investigated the effects of NaCl addition on galena flotation in the presence of xanthate. The micro-flotation experiments were performed using NaCl solutions which also included xanthate, at pH 9 (±0.1). Our results indicated that galena recovery improved for higher NaCl as well as higher xanthate concentrations.A pH-dependent chemisorption model for the galena surface, with the addition of xanthate adsorption was calibrated using measured zeta potential values. We propose that xanthate adsorption on galena can take place via two separate mechanisms. The first mechanism involves direct xanthate chemisorption to specific surface sites. The second mechanism involves lead/xanthate complexes formed in the bulk solution. These lead/xanthate complexes attach on the galena surface as hydrophobic lead xanthate salts.The galena-air bubble interactions are repulsive in 1 mM NaCl, with or without xanthate, consistent with the lower galena recovery measured experimentally. An increase to 100 mM NaCl, irrespective of the xanthate addition, resulted in attractive galena-air bubble total interaction energies. The agreement with the experimental results shows the effectiveness of the charge regulated model for estimating the galena and air bubble behaviours during flotation in NaCl solutions
The effects of saline water on the recovery of lead and zinc sulfide during froth flotation
Full text linkIn this study, we investigated the effects of water salinity on the flotation performance of pure lead and zinc sulfide mineral samples as well as a Pb/Zn complex sulfide ore by means of micro-flotation and batch flotation experiments. Our results showed higher PbS and ZnS recoveries in more concentrated NaCl salt solutions. The results for the experiments using seawater demonstrated that in the presence of additional ions, such as Ca2+ and Mg2+, the recovery of PbS and ZnS was significantly reduced.
As part of this investigation, we developed and implemented a surface complexation model for ZnS based on the presence of two differently charged surface sites. Zeta potential measurements of ZnS particles were used to optimise the parameters of our model. It was found that the surface potentials calculated using this model were in good agreement with the experimental zeta potentials, validating the model for predicting the zeta potential behaviour of ZnS particles over a broad range of pH and NaCl concentrations.
Additionally, total interaction free energies were determined as a function of separation distance, representing particle–particle and particle-bubble interactions of our study in different NaCl concentrations. The theoretical analyses showed that asymmetric Pb/Zn particle–particle interactions were repulsive at lower NaCl concentrations, before becoming purely attractive at higher NaCl concentrations. For the case of the symmetric particle–particle interactions, attraction controlled all interactions, regardless of NaCl concentration. The calculated PbS-bubble interactions were repulsive in lower NaCl concentrations but became increasingly attractive in higher NaCl concentrations. Strong repulsions controlled all ZnS-bubble interactions, and these interactions remained repulsive with increasing NaCl concentration. The theoretical projections presented in this study were in good agreement with the measured saline water flotation phenomena
Nanodiamond-treated flax: improving properties of natural fibers
Full text linkSynthetic fibers are used extensively as reinforcement in composite materials, but many of them face environmental concerns such as high energy consumption during production and complicated decommissioning. Natural fibers have been considered as an attractive solution for making composites more sustainable. However, they are generally not as strong as synthetic fibers. It is therefore of interest to investigate ways to improve the properties of natural fibers without compromising environmental issues. Here, we present a study of the moisture absorption and mechanical properties of flax that has been exposed to hydrogenated nanodiamonds through an ultrasonic dispersion treatment. Nanodiamonds are known to be non-toxic, unlike many other carbon-based nanomaterials. We show that nanodiamond-treated flax fabric has a lower moisture content (~ −18%), lower moisture absorption rate and better abrasion resistance (~ +30%). Single yarns, extracted from the fabric, show higher tensile strength (~ +24%) compared to untreated flax. Furthermore, we present a theoretical model for the nanodiamond fiber interaction, based on the Derjgauin–Landau–Verwey–Overbeek (DLVO) theory of colloid interactions. The simulations indicate that the mechanical properties improve due to an enhancement of the electrolytic force, dispersion force and hydrogen bonding of nanodiamond-treated fibers, which strengthens the cohesion between the fibers. We also apply the model to nanodiamond-treated cotton. The lower zeta potential of cotton increases the electrolytic force. Comparing the results to experimental data of nanodiamond-treated flax and nanodiamond-treated cotton suggests that the fiber’s zeta potential is critical for the improvements of their mechanical properties. Graphical abstract: [Figure not available: see fulltext.
- …
