Ion Size Effects Research Articles

Electrokinetic mixing at high zeta potentials: Ionic size effects on cross stream diffusion

The electrokinetic phenomena at high zeta potentials may show several unique features which are not normally observed. One of these features is the ionic size (steric) effect associated with the solutions of high ionic concentration. In the present work, attention is given to the influences of finite ionic size on the cross stream diffusion process in an electrokinetically actuated Y-shaped micromixer. The method consists of a finite difference based numerical approach for non-uniform grid which is applied to the dimensionless form of the governing equations, including the modified Poisson–Boltzmann equation. The results reveal that, neglecting the ionic size at high zeta potentials gives rise to the overestimation of the mixing length, because the steric effects retard liquid flow, thereby enhancing the mixing efficiency. The importance of steric effects is found to be more intense for channels of smaller width to height ratio. It is also observed that, in sharp contrast to the conditions that the ions are treated as point charges, increasing the zeta potential improves the cross stream diffusion when incorporating the ionic size. Moreover, increasing the EDL thickness decreases the mixing length, whereas the opposite is true for the channel aspect ratio.

Size Effect of Ion Translocation through Nanopore by Molecular Dynamics

While the flow rates (in unit of particles/s) of the ions in aqueous solution depend on the cross sections of the channels they pass through, the flux (in unit of particles/(m2s)) is usually independent of them. However, if the diameter of the channel becomes extremely small and comparable to the effective size of the ions, there can be the dependence of the flux on the channel diameter. The finite size effect of solute ions through such small channel is relevant to the physical process of other nanofluidic technologies such as the DNA translocation through nanopore for the sequencing technologies. In this study, we examine the flow of K+ and Cl- ions in water through nanopores made of graphene structure by molecular dynamics method. The results show that there is a transition of zero to nonzero flux when the pore diameter is around the effective size of the ions. Furthermore, there is a dependence of the flux on the nanopore diameter around this regime.

Ion Beam Irradiation-Induced Amorphization of Nano-Sized KxLnyTa2O7-v Tantalate Pyrochlore

Nano-sized (~10-15 nm) tantalate pyrochlores KxLnyTa2O7-v (Ln = Gd, Y, and Lu) were irradiated with 1 MeV Kr2+ beams at different temperatures and their radiation response behaviors were studied by in-situ TEM observations. All of these nano-sized KxLnyTa2O7-v pyrochlores are sensitive to radiation-induced amorphization with low critical doses (~0.12 dpa) at room temperature and high critical amorphization temperatures above 1160 K. The K+ plays a key role in determining the radiation response of tantalate pyrochlores, in which the K+-rich KLuTa2O7 displays greater amorphization susceptibility than K0.8GdTa2O6.9 and K0.8YTa2O6.9 with lower K+ occupancy at the A-site. The reduced amorphization tolerance of the composition with a greater K+ content is consistent with the prominently larger K+/Ta5+ cationic radius ratio, which may result in more structural deviation from the parent fluorite structure and less capability to accommodate radiation induced defects. An empirical correlation between critical amorphization temperature and ionic size was derived, generally describing the dominant effect of ionic sizes in controlling radiation response of a wide range of pyrochlore compounds as potential nuclear waste forms. The results of the tantalate pyrochlore in this work highlight that nanostructured pyrochlores are not intrinsically radiation tolerant and their responses are highly compositional dependent.

Volume phase transition of polyelectrolyte gels: effects of ionic size.

Although the volume transition of the polyelectrolyte gel has been studied for decades, less study on the finite size effects of the mobile ions has been conducted. In the present paper, Tanaka's classical theory of polyelectrolyte gel is extended to the cases of mobile ions of finite volume. In the salt free limit, the theoretical results show that the discontinuous volume transition of the polyelectrolyte gel will become a continuous one for counterionic size larger than a critical value. When a significant amount of salt is added, the critical value for the volume transition increases as a result of electrostatic screening. An increase in salt concentration can also make the polyelectrolyte gel in poor solvent collapse. Poorer solvent is needed to trigger the salt-induced collapse in polyelectrolyte gel with larger mobile ions than that with smaller ones. The effects of ionic size on the critical points and phase diagram of the volume transition are also discussed. The theoretical results suggest that the swelling behavior of polyelectrolyte gel might be tuned with salt of different volumes.

Coarse-grained simulations of an ionic liquid-based capacitor: I. Density, ion size, and valency effects

We introduce a hierarchy of generic coarse-grained models of ionic liquids of increasing complexity. We use them in molecular dynamics simulations to study the differential capacitance of a capacitor consisting of an ionic liquid between two planar electrodes. The primary goal is to explain the complex dependence of the differential capacitance Cd on the electrode potential U in simple terms, e.g. in terms of the size and valency of the ions. For this purpose we introduce the symmetric model A, which qualitatively reproduces the Cd(U) dependence predicted by the mean-field theory but also reveals strong quantitative deviations. We further introduce size asymmetry in model A by increasing the cation size. In model B we vary the cation valency, keeping the sizes of both ions constant. We show that simultaneous increases in size and valency may compensate for each other, leading to a Cd(U) very similar to that for the symmetric case. We interpret distinct features in Cd(U) on the basis of the density profiles of the ions and charge density profiles. We focus on the first two ion layers at the electrode, and demonstrate that the polarization of the ionic liquid proceeds through replacement of one ion type by the other, in contrast to the simple increase in ion concentrations typical for dilute systems. The understanding gained for the simple models serves as a reference for interpretation of complex effects of ion size, valency and shape. This is carried through in part II (a separate article) where we show how the planar shape of ions in model C brings new features to the Cd(U) curve and also to the polarization mechanism.

Ionic asymmetry and solvent excluded volume effects on spherical electric double layers: A density functional approach

In this article, we present a classical density functional theory for electrical double layers of spherical macroions that extends the capabilities of conventional approaches by accounting for electrostatic ion correlations, size asymmetry, and excluded volume effects. The approach is based on a recent approximation introduced by Hansen-Goos and Roth for the hard sphere excess free energy of inhomogeneous fluids [J. Chem. Phys. 124, 154506 (2006); Hansen-Goos and Roth, J. Phys.: Condens. Matter 18, 8413 (2006)]. It accounts for the proper and efficient description of the effects of ionic asymmetry and solvent excluded volume, especially at high ion concentrations and size asymmetry ratios including those observed in experimental studies. Additionally, we utilize a leading functional Taylor expansion approximation of the ion density profiles. In addition, we use the mean spherical approximation for multi-component charged hard sphere fluids to account for the electrostatic ion correlation effects. These approximations are implemented in our theoretical formulation into a suitable decomposition of the excess free energy which plays a key role in capturing the complex interplay between charge correlations and excluded volume effects. We perform Monte Carlo simulations in various scenarios to validate the proposed approach, obtaining a good compromise between accuracy and computational cost. We use the proposed computational approach to study the effects of ion size, ion size asymmetry, and solvent excluded volume on the ion profiles, integrated charge, mean electrostatic potential, and ionic coordination number around spherical macroions in various electrolyte mixtures. Our results show that both solvent hard sphere diameter and density play a dominant role in the distribution of ions around spherical macroions, mainly for experimental water molarity and size values where the counterion distribution is characterized by a tight binding to the macroion, similar to that predicted by the Stern model.

Strong non-classical induction forces in ion-surface interactions: general origin of Hofmeister effects.

Hofmeister effects continue to defy all-encompassing theories and their origin is still a matter of debate. We observed strong Hofmeister effects in Ca2+/Na+ exchange on a permanently charged surface over a wide range of ionic strengths. They could not be attributed to dispersion forces, classical induction forces, ionic size, or hydration effects. We demonstrated that another stronger force was active in the ion-surface interactions and which would create Hofmeister effects in general. The strength of this force was up to 104 times that of the classical induction force and could be comparable to the Coulomb force. Coulomb, dispersion and hydration effects appeared to be interwined to affect the force. The presence of the observed strong non-classical induction force implied that energies of non-valence electrons of ions/atoms at the interface might be heavily underestimated in current theories and possibly just those underestimated energies of non-valence electrons determined Hofmeister effects.

The relationship between occupied sites and Mn2+ quantum efficiency in dual-emission Mg3Ca3(PO4)4: Eu2+, Mn2+ phosphors

Abstract Dual-emitting Eu2+ and Mn2+ codoped Mg3Ca3(PO4)4 phosphors have been synthesized, and energy transfer process from Eu2+ to Mn2+ was studied. The relationship between Eu2+ → Mn2+ energy transfer efficiency (ηT) and Mn2+ quantum efficiency (QE) with different Mn2+ occupied sites were compared and discussed. The ion-size compensation effect when Eu2+/Mn2+ occupying Ca2+ sites leads to the formation of Eu2+–Mn2+ cluster, which benefits the effective Eu2+ → Mn2+ energy transfer and efficient Mn2+ sensitized emission. The emission color of the phosphor can be tuned from blue to near-white and eventually to pink by adjusting the contents of Eu2+ and Mn2+. The Mn2+ emission wavelength red-shifts with increasing Mn2+ content, which is attributed to photon reabsorption process. The results indicated that Mg3Ca3(PO4)4: Eu2+, Mn2+ may serve as a potential color-tunable phosphor for NUV LED.

Ionic size effects to molecular solvation energy and to ion current across a channel resulted from the nonuniform size-modified PNP equations.

Ionic finite size can impose considerable effects to both the equilibrium and non-equilibrium properties of a solvated molecular system, such as the solvation energy, ionic concentration, and transport in a channel. As discussed in our former work [B. Lu and Y. C. Zhou, Biophys. J. 100, 2475 (2011)], a class of size-modified Poisson-Boltzmann (PB)/Poisson-Nernst-Planck (PNP) models can be uniformly studied through the general nonuniform size-modified PNP (SMPNP) equations deduced from the extended free energy functional of Borukhov et al. [I. Borukhov, D. Andelman, and H. Orland, Phys. Rev. Lett. 79, 435 (1997)] This work focuses on the nonuniform size effects to molecular solvation energy and to ion current across a channel for real biomolecular systems. The main contributions are: (1) we prove that for solvation energy calculation with nonuniform size effects (through equilibrium SMPNP simulation), there exists a simplified approximation formulation which is the same as the widely used one in PB community. This approximate form avoids integration over the whole domain and makes energy calculations convenient. (2) Numerical calculations show that ionic size effects tend to negate the solvation effects, which indicates that a higher molecular solvation energy (lower absolute value) is to be predicted when ionic size effects are considered. For both calculations on a protein and a DNA fragment systems in a 0.5M 1:1 ionic solution, a difference about 10 kcal/mol in solvation energies is found between the PB and the SMPNP predictions. Moreover, it is observed that the solvation energy decreases as ionic strength increases, which behavior is similar as those predicted by the traditional PB equation (without size effect) and by the uniform size-modified Poisson-Boltzmann equation. (3) Nonequilibrium SMPNP simulations of ion permeation through a gramicidin A channel show that the ionic size effects lead to reduced ion current inside the channel compared with the results without considering size effects. As a component of the current, the drift term is the main contribution to the total current. The ionic size effects to the total current almost come through the drift term, and have little influence on the diffusion terms in SMPNP.

Comparative Study on the Properties of Hydration Water of Na- and K-Halide Ions by Raman OH/OD-stretching Spectroscopy and Dielectric Relaxation Data

Properties of hypermobile water (HMW) were studied by Raman OH-stretching spectroscopy. Hydration water properties measured by Raman OH-stretching spectra of NaX/KX (X: Cl, Br, I) solutions (0.05-0.2 M) were comparatively analyzed with the data by dielectric relaxation spectroscopy (DRS), NMR, and statistical mechanical studies. The Raman OH-stretching spectra were well-fitted with linear combinations of the spectra of pure water both at the same and the higher temperatures. The fitting analysis determined the "structure temperature" Tstr and mole fraction of the high Tstr water region, giving the hydration number Nhyd, for each electrolyte solution. The determined Tstr was much higher than the solution temperature of 293 K for each tested salt and was higher for larger halide ions, consistent with commonly known "structure-breaking" order Cl < Br < I. No significant differences in Nhyd were observed between NaX and KX and among even halide ion species within the experimental errors. Measured Nhyd values of 25-27 were much greater than the reported numbers by NMR chemical shift and (17)O NMR relaxation studies and comparable to the numbers of hypermobile water reported in the previous DRS studies. The results indicated that the hydration region around NaX or KX measured by the present Raman study was nearly overlapped with the region of HMW by DRS. It was also suggested that differences in the ion size effects on Tstr and the DR frequency resulted from the sensitivity difference to long-range many-body interactions among water molecules. High structure-temperature regions were also detected by the analysis of OH-stretching and OD-stretching bands for 0.2 M NaI in H2O/D2O mixed solvent of 50 mol %, and we found that both OH-stretching and OD-stretching bands have almost equivalent Tstr ≈ 330 K and mole fractions with each other.

Effect of pressure on octahedral distortions in RCrO3 (R = Lu, Tb, Gd, Eu, Sm): the role of R-ion size and its implications

The effect of rare-earth ion size on the octahedral distortions in rare-earth chromites (RCrO3, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic structure has been studied using Raman scattering and synchrotron powder x-ray diffraction up to 20 GPa. From our studies on RCrO3 we found that the octahedral tilts (distortions) increase with pressure. This is contrary to the earlier report which suggests that in LaCrO3, the distortions decrease with pressure leading to a more ordered phase at high pressure. Here, we observe that the rate of increase in distortion decreases with the increase in R-ion radii. This occurs due to the reduction in the compression of RO12 polyhedra with a corresponding increase in the compression of the CrO6 octahedra with increasing R-ion radii. From the Raman studies, we predict a critical R-ion radii, above which we expect the distortions in RCrO3 to reduce with increasing pressure leading to what is observed in the case of LaCrO3. These Raman results are consistent with our pressure dependent structural studies on RCrO3 (R = Gd, Eu, Sm). Also, our results suggest that the pressure dependence of Néel temperature, TNCr, (where the Cr3+ spin orders) in RCrO3 is mostly affected by the compressions of Cr-O bonds rather than the alteration of octahedral tilts.

Electrochemical Double Layers in Electrochemical Energy Storage and Conversion: A Multiscale Simulation Study

The nanometric liquid/solid electrochemical interface is the pivotal scale behind the operation principles of low-temperature batteries and fuel cells. It involves ionic transport by diffusion and electro-migration in the electrolyte, surface multi-step REDOX reactions, among other mechanisms. Developing an understanding of its functional role is thus crucial to help on determining the limiting factors of batteries and fuel cells performance and stability [1-3].A very significant amount of work has been carried out since more than 100 years to understand the structure of such interfaces in electrochemical systems (also known as electrochemical double layers -EDL-) but under ideal conditions (equilibrium conditions, absence of REDOX reactions): this is not representative of the non-equilibrium conditions typically found in electrochemical power generators. In this work a new non-equilibrium EDL multiscale model is presented, able to capture the following features of relevance on designing high performant and stable electrolytes in batteries and fuel cells [4]:- the impact of solvent composition, additives and lithium salt concentrations onto the charge-discharge curves in lithium ion and lithium air batteries (LIBs and LABs), and thus discriminating between thermodynamic and kinetic effects;- the impact of the platinum-based catalysts oxidation state onto the structure of the EDL, the ionomer morphology and the effectiveness of the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells (PEMFCs).The model is supported on an extension of the so-called "Poisson-Nernst-Planck approach" and accounts for solvent orientational polarization effects, finite ion size effects and extended Transition State Theory. REDOX reactions occuring on the surface of the active particles (LIBs and LABs) and catalysts (PEMFCs) are described through an elementary kinetic approach parametrized with DFT-calculated activation energies [5]. The influence of the coverage onto the effective activation barriers is evaluated through on lattice Kinetic Monte Carlo simulations within the Variable Step Size Method (VSSM) describing surface diffusion, reactions between adspecies and adsorption/desorption.Some application cases of this model are discussed: for example, the model reproduces nicely well complex phenomena such as the irreversible Nafion reduction/oxidation peaks observed in cyclic voltammetry experiments in [6] (PEMFC), and it predicts how solvent composition impacts polarization and lithium ion transport properties (LIB/LAB). Finally, a cell device multiscale model integrating these EDL models is presented [7], and the EDL impact onto the overall cell performance dynamics (as function of the applied potentiodynamic and galvanodynamic conditions in PEMFCs, and cycling rate for batteries) is studied. Acknowledgements. Prof. Dominiquer Larcher and Dr. Abdelouahab El Kharbachi (LRCS) are gratefully acknowledged for their experiments devoted to our models validation.

Theoretical study of the multiferroic properties in M-doped (M=Co, Cr, Mg) ZnO thin films

Abstract The origin of multiferroism is still an open problem in ZnO. We propose a microscopic model to clarify the occurrence of multiferroism in this material. Using Green׳s function technique we study the influence of ion doping and size effects on the magnetization and polarization of ZnO thin films. The calculations for magnetic Co- and Cr-ions are based on the s–d model, the transverse Ising model in terms of pseudo-spins and a biquadratic magnetoelectric coupling, whereas in case of nonmagnetic Mg-ions the model takes into account the Coulomb interaction and an indirect coupling between the pseudo-spins via the conduction electrons. We show that the magnetization M exhibits a maximum for a fixed concentration of the doping ions. Furthermore M increases with decreasing film thickness N . The polarization increases with increasing concentration of the dopant and decreasing N . The results are in good agreement with the experimental data.

Study of the Electronic Structure of Various RE-Doped Oxypnictide Superconductors Using X-Ray Absorption Spectroscopy

The X-ray absorption spectroscopy (XAS) measurements on Fe L2,3-edge and O K-edge obtained at ELLETRA synchrotron Trieste, Italy are presented in this manuscript to study the electronic structure of REFeO1−x F x As (RE=Rare Earth elements; La, Ce, Sm) oxypnictide superconductors. The Fe L2,3- and O K-edge XAS for all the samples were found to be similar with varying intensity of the peaks. The results suggest that T c increases with doping of the rare earth elements as we go from a large periodic number to small; it is due to the size effect of rare earth ions with decrement in lattice spacing. Gaussians are fit to estimate the contribution of various spectral components in XAS spectra, and their relative spectral weight is calculated to correlate the density of states with T c’s of various RE doped oxypnictides.

Graphite nanosheets as an electrode material for electrochemical double layer capacitors

In this paper, the effect of ion sizes of cations and anions on the charge storage capability of graphite nanosheets is investigated. Electrochemical properties of prepared electrodes are studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques, in 3M NaCl, NaOH and KOH electrolytes. A scanning electron microscope (SEM) is used to characterize the microstructure and nature of prepared electrodes. SEM images and XRD patterns confirm the layered structure (12nm thickness) of the used graphite with an interlayer distance of 3.36Å. The electrochemical results and the ratio of qO⁎/qT⁎ confirm a better charge storage and charge delivering capability of prepared electrodes in 3M NaCl electrolyte. The charge/discharge cycling test shows a good reversibility and confirms that the solution resistance will increase after 500 cycles.

Effect of finite ion sizes in an electrostatic potential distribution for a charged soft surface in contact with an electrolyte solution

We provide a theory to analyze the impact of finite ion sizes (or steric effect) in electrostatic potential distribution for a charged soft surface in contact with an electrolyte solution. The theory is based on a free energy model that appropriately accounts for the contribution of finite ion sizes as well as the structural characteristics of a soft interface, represented by a combination of a rigid surface and a fixed charge layer (FCL), with the FCL being in contact with an electrolyte solution forming an electric double layer (EDL). This FCL contains a particular kind of ion which is impermeable to the electrolyte solution, and this impermeability is quantified in terms of the corresponding Donnan potential of the "membrane" represented by the FCL-electrolyte interface. We find that consideration of the finite ion size increases the magnitude of this Donnan potential, with the extent of increase being dictated by three length scales, namely, the thickness of the FCL, the thickness of the electrolyte EDL, and the thickness of an equivalent EDL within the FCL. Such regulation of the Donnan potential strongly affects the distribution of the permeable electrolyte ions within the FCL, which in turn will have significant implications in several processes involving "soft" biological membranes.

The structural classification of the highly disordered crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6] salts (Nn(+) = tetraalkylammonium and Pn(+) = tetraalkylphosphonium).

The structures of 16 symmetric tetraalkylammonium (Nn(+)) and tetraalkylphosphonium (Pn(+)) salts ([Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6], where n = 1 to 4, and denotes the number of carbon atoms in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size on the disordered structure of organic salts. All the salts exhibit one or more solid-solid phase transitions in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The structures are classified into 5 different types: CsCl', NaCl, NaCl', inverse NiAs, and TBPPF6. The CsCl'-type whose octant corresponds to the original CsCl unit cell is observed for [N1][PF6] owing to the orientational difference for the cation or the anion. The NaCl-type structure is observed for the N2(+) and P2(+) salts while the NaCl'-type structure is observed for [N3][PF6], where the configuration of ions is based on the NaCl-type but the four equivalent positions in the original NaCl lattice split into two sets of equivalent positions (three and one). The inverse NiAs structure is observed for [P3][PF6]. Single-crystal X-ray diffraction reveals that the disordering of ions in [P4][PF6] becomes more significant with increasing temperature. The new structure of a cubic phase, the TBPPF6-type structure, is found for the salts with long alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.

Effect of ionic size of dopants on the lattice structure, electrical and electrochemical properties of La2−xMxNiO4+δ (M = Ba, Sr) cathode materials

La2−xMxNiO4+δ (M = Ba, Sr; x = 0.1, 0.3), with a formula of A2BO4, has been prepared and investigated as cathode for solid oxide fuel cells to understand the influence of A-site dopants on the lattice structure, electrical conductivity and electrochemical properties of La2NiO4+δ. All the compositions belong to tetragonal I4/mmm space group. La2−xBaxNiO4+δ shows larger lattice parameters than La2−xSrxNiO4+δ due to the large ionic radius of Ba2+ compared with Sr2+. For both Ba and Sr, the parameters a and b decrease while the c increases with increasing doping level. Rietveld refinement demonstrates that the increase in c parameter is partially originated from the increase in rocksalt layer thickness (La–O2( × 1) bond), which makes the adsorbed oxygen on particle surface much easier to enter the lattice and form interstitials, and thereby promoting the electrode reaction. The electrical conductivity of La2−xMxNiO4+δ increases with doping level but decrease with increasing ionic radius of dopants. Both Ba and Sr doping decrease the electrode polarization and increase the power density of single-cell. La1.7Ba0.3NiO4+δ exhibits superior electrochemical properties than La1.7Sr0.3NiO4+δ. The La1.7Ba0.3NiO4+δ electrode exhibits the best performance with an ASR of 0.13 Ω cm2 and a maximum power density of 310 mW cm−2 at 800 °C under electrolyte (La0.8Sr0.2Ga0.83Mg0.17O3−δ, 300 μm) – supported configuration.

Ionic size effects: generalized Boltzmann distributions, counterion stratification and modified Debye length

Near a charged surface, counterions of different valences and sizes cluster; and their concentration profiles stratify. At a distance from such a surface larger than the Debye length, the electric field is screened by counterions. Both recent studies using a variational mean-field approach that includes ionic size effects and Monte Carlo simulations suggest that counterion stratification is determined by the ionic valence-to-volume ratios. Central in the mean-field approach is a free-energy functional of ionic concentrations in which the ionic size effects are included through the entropic effect of solvent molecules. The corresponding equilibrium conditions define the generalized Boltzmann distributions relating the ionic concentrations to the electrostatic potential. This paper presents a detailed analysis and numerical calculations for such a free-energy functional to understand the dependence of the ionic charge density on the electrostatic potential through the generalized Boltzmann distributions, the role of ionic valence-to-volume ratios in the counterion stratification and the modification of Debye length due to the effect of ionic sizes.

Effects of ionic strength, particle size, flow rate, and vegetation type on colloid transport through a dense vegetation saturated soil system: Experiments and modeling

Summary Colloids are widely distributed in agricultural runoff, especially from croplands with manure applications. Dense vegetation has been suggested to be effective to reduce surface runoff contaminants, including colloidal particles. In this work, small scale laboratory experiments were used to determine the influence of physical and chemical factors (i.e. solution ionic strength, particle size, surface flow rate, and vegetation type) on the surface transport and removal of colloids in a dense vegetation system without drainage. Conservative tracer studies of bromide were conducted as a control to quantify the deposition of colloids onto grass surfaces and the mass exchange of colloids between the overland flow and soil underneath under various experimental conditions. The deposition of colloids enhanced with increases in solution ionic strength and particle size, and with decreases in flow rate. We also found vegetation type played an important role on colloid transport with more deposition onto Ryegrass than onto Bahia grass under the same experimental conditions. A mathematical model combining overland flow, convection–dispersion equations and exchange layer theory was developed to simulate the transport of colloids in overland flow through the dense vegetation. Simulations of the model fitted the experimental data well and helped to understand the effect of ionic strength, particle size, flow rate, and vegetation type on colloid transport and removal in dense vegetation. Although additional investigations are still needed, findings from this study can inform the installation and maintenance of dense vegetation systems, such as vegetative filter strips, to reduce the loading of colloids in surface runoff.