DLVO Analysis Research Articles

Experimental insight into the silica nanoparticle transport in dolomite rocks: Spotlight on DLVO theory and permeability impairment

Nanoparticles (NPs) have been proposed as a promising agent for groundwater aquifer remediation, greenhouse geological storage, and enhanced oil recovery (EOR) applications. Nevertheless, in many of these cases, NPs applications require long transport distances, which may be limited by NPs retention in porous media, especially at high saline conditions as is the case with some oil reservoirs, causing a reduction in rock permeability and subsequent formation damage. Therefore, understanding of NPs transport through porous media and their stability behavior are significant challenges before their large-scale applications, which has been less explored in previous works. This paper investigates silica NPs transport through dolomite rocks, which are the host rocks of many sub-surface reservoirs. To this end, the stability of silica nanofluids with different concentrations in the presence of different ionic strength and ion types was explored by zeta potential measurement and particle size analysis. NPs adsorption on rock surface was examined by SEM images. The DLVO analysis was conducted to determine the interaction energies between NPs and the rock, and investigate the reversibility of NPs adsorption on the rock. Single-phase coreflooding experiments aided by NPs effluent analysis were performed to obtain breakthrough curves. Results indicated that increasing silica NPs concentration from 0.1 to 0.5 wt% leads to a reduction in nanofluid stability in which the average size of silica NPs increased from 110 to 220 nm. Higher ionic strength (50,000 ppm) decreased nanofluids stability and this negative effect was more pronounced for nanofluids containing MgCl2 salt. It was found that less stable nanofluids showed higher NPs retention through porous media. Highly stable nanofluids exhibited a higher probability of NPs to be detached during the brine post-flushing stage. As to the results of silica nanofluid injection in dolomite rock, increase of the silica NPs concentration and decrease of their stability in suspension were two causes of the rock permeability impairment; a 10% reduction in rock permeability was observed by 0.1 wt% salt-free silica nanofluid, whereas it decreased by 87% using 0.5 wt% of saline nanofluids prepared by 50,000 ppm MgCl2. According to the interpretations based on the deep bed filtration model, the higher salinity of nanofluid is, the higher filtration coefficient would be, meaning that the higher rate of particle capture at higher salinities. Also, no internal/external filter cake was formed and instead, the pore filling process was found to be the major mechanism of the permeability impairment by silica NPs.

Thermodynamic study on the prevention of B30 biodiesel wax crystallization by γ-Al2O3 nanoparticles and sorbitan monooleate

This paper reports a thermodynamic investigation on the role of γ-Al2O3 nanoparticles and sorbitan monooleate (SMO) as cold-flow improver additives for B30 biodiesel blend. Here, the two additives were able to improve fuel’s cold-flow properties and responsible for reducing wax crystallization temperature as low as 6.42 °C. Based on DCS analysis, the addition of individual SMO or γ-Al2O3 nanoparticles gave rise to the increase in crystallization enthalpy and entropy. However, the opposite effect was observed when they were used in combination. This synergistic effect was found to be responsible for making them effective in the prohibiting the spontaneity of wax crystallization, which was proven by the increase in crystallization Gibbs free energy (ΔGxtal) value from −72.9 to −35.7 J/g. This is primarily due to that the ability of SMO to stabilize wax nuclei and the role of γ-Al2O3 nanoparticles as an additional nucleation site. Furthermore, DLVO analysis also revealed that γ-Al2O3 nanoparticles exhibited high colloidal stability, enabling them to facilitate stable Pickering emulsion formation. Finally, microscopic observations demonstrated that both additives could suppress the formation of insoluble paraffinic “house of cards” crystals.

Rational design of solid-acid catalysts for cellulose hydrolysis using colloidal theory.

Solid-acid catalysts functionalized with catalytic groups have attracted intense interest for converting cellulose into soluble products. However, design of solid-7 acid catalysts has been guided by molecular level interactions and the actual mechanism of cellulose-solid-acid catalyst particles adsorption remains unknown. Here, colloidal stability theory, DLVO, is used to rationalize the design of solid acids for targeted cellulose adsorption. In nearly all cases, an energy barrier, arising from electrostatic repulsion and much larger than the energy associated with thermal fluctuations, prevents close contact between the solid acid and cellulose. Polymer-based solid-acid substrates such as polystyrene and Nafion are especially ineffective as their interaction with cellulose is dominated by the repulsive electrostatic force. Carbon and metal oxides have potential to be effective for cellulose-solid-acid interaction as their attractive van der Waals interaction can offset the repulsive electrostatic interaction. The effects of reactor temperature and shear force were evaluated, with the finding that reactor temperature can minimize the catalyst-cellulose interaction barrier, promoting coagulation, but that the shear force in a typical laboratory reactor cannot. We have evaluated strategies for enhancing cellulose-catalyst interaction and conclude that raising reaction temperature or synthesizing acid/base bifunctional catalysts can effectively diminish electrostatic repulsion and promote cellulose-catalyst coagulation. The analysis presented here establishes a rational method for designing solid acid catalysts for cellulose hydrolysis.

Investigating the Effect of Antibody-Antigen Reactions on the Internal Convection in a Sessile Droplet via Microparticle Image Velocimetry and DLVO Analysis.

The evaporation of antigen-laden sessile droplets on antibody-immobilized PDMS substrates could be used in place of microwells for detection purposes owing to the lesser requirements of analytes and a reduced reaction time. To develop such techniques, the effects of different parameters on the reaction efficiency and on the resulting deposition patterns of antigens on the surface after evaporation need to be well understood. While the resultant deposition patterns from the evaporation of droplets of biological fluids on surfaces are being studied for various biomedical applications, systems where the analyte of interest in the droplet binds to the surface have not been investigated until now. While the effect of temperature on the internal convection within sessile droplets has been studied, the effect of the analyte (antigen in this work) concentration and the analyte-surface (antigen-antibody in this work) binding on the internal convection has not been studied until now. Therefore, to gain insight, the evaporation dynamics of sessile droplets with different concentrations of antigens along with polystyrene microspheres (used as tracers) in phosphate-buffered saline (PBS) on antibody-immobilized PDMS substrates were experimentally studied using microparticle image velocimetry (PIV). It was found that Marangoni flow due to concentration gradients and surface reactions was responsible for the observed velocity field. The antibody-antigen reaction (as compared to the control case of no surface reaction) and higher concentrations of prostate specific antigen (PSA) resulted in increased strength of Marangoni convection. To obtain further insight into the different deposition patterns obtained, the contributions of different particle-particle and particle-substrate forces were determined, and it was observed that the Marangoni forces along with surface tension and DLVO forces create a uniform deposition of the particles present within the droplet. This learning could be used to design biosensors.

Responses of Exogenous Bacteria to Soluble Extracellular Polymeric Substances in Wastewater: A Mechanistic Study and Implications on Bioaugmentation.

Compared with the chemically defined synthetic wastewater (SynWW), real wastewater has been reported to exhibit distinct effects on microbial community development. Whether and how soluble microbial products in real wastewater contribute to different effects of synthetic and real wastewater on the fate of exogenous bacteria remains elusive. In this study, using a model wastewater bacterium Comamonas testosteroni, we first examined the influences of microfiltration filter-sterilized real wastewater (MF-WW) and SynWW on the retention of C. testosteroni in established wastewater flocs during bioaugmentation. In bioreactors fed with MF-WW, augmentation of C. testosteroni to wastewater flocs resulted in a substantially higher abundance of the augmented bacterial cells than those fed with SynWW. To identify the soluble microbial products in MF-WW contributing to the observed differences between bioaugmentation reactors fed with MF-WW and SynWW, we examined the effect of MF-WW and SynWW on the growth, floc formation, and biofilm development of C. testosteroni. When C. testosteroni grew in MF-WW, visible flocs formed within 2 h, which is in contrast to cell growth in SynWW where floc formation was not observed. We further demonstrated that the observed differences were mainly attributed to the high molecular weight fraction of the soluble extracellular polymeric substances (EPS) in MF-WW, in particular, proteins and extracellular DNA. The DLVO analysis suggested that, in the presence of soluble EPS, the bacterial cell surface exhibits an increased hydrophobicity and a diminished energy barrier, leading to irreversible attachment of planktonic cells and floc formation. The RNA-seq based transcriptional analysis revealed that, in the presence of soluble EPS, genes involved in nonessential metabolisms were downregulated while genes coding for Cco (cbb3-type) and Cox (aa3-type) oxidases with different oxygen affinities were upregulated, facilitating bacterial survival in flocs. Taken together, this study reveals the mechanisms underlying the contribution of soluble EPS in real wastewater to the recruitment of exogenous bacteria by microbial aggregates and provides implications to bioaugmentation.

Steric Stabilization of γ-Fe2O3 Superparamagnetic Nanoparticles in a Hydrophobic Ionic Liquid and the Magnetorheological Behavior of the Ferrofluid

Hydrophobic ionic liquid ferrofluids (ILFFs) are studied for use in electrospray thrusters for microsatellite propulsion under nonatmospheric and in high-temperature environments. We synthesized a hydrophobic ILFF by dispersing sterically stabilized γ-Fe2O3 nanoparticles (NPs) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A diblock copolymer, C4-RAFT-AA10-DEAm60, was synthesized to facilitate multipoint bidentate anchoring to the NP through the acrylic acid block. The DEAm60 layer was incorporated to generate steric repulsion between particles to protect against the aggregation of magnetized particles arising from dipole-dipole attraction. The effect of shearing and variation in the magnetic field strength on the steric repulsion was examined using the DLVO theory. The effect of varying the magnetic field strength and particle concentration on the viscoelastic properties of the ferrofluid was evaluated using rheometry. The viscosity of the ferrofluid increased with the magnetic field strength, indicating that the magnetized particles assembled into a structure. The level of straining required to break down the structure formed by the magnetized particles increased with the magnetic field strength and particle concentration. The absence of particle interlocking during shearing was indicated by the smooth viscosity versus shear rate traces. The DLVO analysis showed that increasing the magnetic attraction between the particles causes the DEAm60 brush layers on the particles to overlap more, resulting in an increase in the steric repulsion. As overlapping increases, osmotic repulsion is caused before progressing to a strong elastic repulsion. The effect of the polymer solubility and particle interaction due to hydrodynamic forces on the steric repulsion was also analyzed.

In situ ion induced gelation of colloidal dispersion of Laponite: Relating microscopic interactions to macroscopic behavior

Aqueous dispersion of Laponite, when exposed to CO2 environment leads to in situ inducement of magnesium and lithium ions, which is, however absent when dispersion is exposed to air. Consequently, in the rheological experiments, Laponite dispersion preserved under CO2 shows more spectacular enhancement in the elastic and viscous moduli as a function of time compared to that exposed to air. By measuring concentration of all the ions present in a dispersion as well as change in pH, the evolving inter-particle interactions among the Laponite particles is estimated. DLVO analysis of a limiting case is performed, wherein two particles approach each other in a parallel fashion – a situation with maximum repulsive interactions. Interestingly it is observed that DLVO analysis explains the qualitative details of an evolution of elastic and viscous moduli remarkably well thereby successfully relating the macroscopic phenomena to the microscopic interactions.

Probing the contribution of extracellular polymeric substance fractions to activated-sludge bioflocculation using particle image velocimetry in combination with extended DLVO analysis

Extracellular polymeric substances (EPS) play a crucial role in the flocculation of activated sludge during biological wastewater treatment. However, as EPS may be classified into sub-fractions, the exact roles of different EPS fractions in sludge flocculation remain to be further defined. This study investigated the contribution of EPS fractions to the sludge flocculation based on a refined EPS classification considering the function and composition of the EPS sub-fractions, including loosely-bound extra-microcolony polymers (LB-EMPs), tightly-bound extra-microcolony polymers (TB-EMPs) and extra-cellular polymers (ECPs). The flocculation ability of the sludge before and after the extraction of the EPS fractions was determined using the particle image velocimetry (PIV) technique. Combined with the flocculation test and sludge characterization, the effects of the EPS fractions on the interaction energy and attachment of microbial cells were evaluated by extended DLVO analysis. The PIV results indicate that sludge flocculation ability decreases with the extraction of EPS. The interaction-energy curves obtained from the sludge before and after the stepped EPS extraction suggest that different EPS fractions contribute to sludge flocculation to different degrees. The interaction energy of LB-EMPs and TB-EMPs is always negative, indicating their important and positive roles in sludge flocculation. In contrast, the contribution of ECPs to sludge flocculation is not always positive, depending on the separation distance between the sludge cells. The quantitative findings will help develop more effective approaches to enhance sludge flocculation by regulating the EPS contents.

Investigation of the effect of nanoparticle exposure on the flocculability of activated sludge using particle image velocimetry in combination with the extended DLVO analysis

With the application of nanotechnologies, nanoparticles (NPs) may enter the wastewater collection system, threatening the operation and stability of the treatment process. The present study investigated the effect of short-term exposures of activated sludge (AS) to NPs on the sludge flocculation ability. Three types of NPs, ZnO, TiO2 and SiO2, were used at a concentration level of 100mg/L for the 24-h exposure, and the particle image velocimetry (PIV) technology was employed to track the sludge flocculation dynamics during the tests. The results showed that a short-term exposure of the AS to NPs did not cause deterioration of the sludge in treatment performance and flocculation. According to the PIV detections, the flocculability of the AS actually was enhanced to some extent after the short-term NPs exposure. Flocs formed after the NPs exposure became more fractal with an elongated shape and a rougher surface, as a result of the improved cell attachment. The extended DLVO theory in combination with the thermodynamic analysis can be well applied to describe the enhanced AS flocculation after the NPs exposure. It is apparent that the attachment of NPs onto cells and flocs can effectively reduce the interaction energy barriers, leading to their improved flocculation ability.

Adsorption of silica nanoparticles onto calcite: Equilibrium, kinetic, thermodynamic and DLVO analysis

Recently, application of silica nanoparticles (SNP) for enhancing oil recovery during water flooding has been much attended. However, understanding how rock and nanoparticles (NP) interacts through adsorption onto the carbonate reservoir rocks is not well discussed. In this work, adsorption behavior of SNP onto the calcite had been characterized, through kinetic, equilibrium, thermodynamics and electrokinetic studies as well as interaction energy analysis by DLVO theory. Also, field emission scanning electron microscopy (FESEM) was utilized to visualize the adsorption process. It had been found that kinetic behavior of SNP–calcite system followed the pseudo-second order model. Equilibrium study revealed that Langmuir model yielded a better fit which implied that monolayer coverage of SNP onto the calcite surfaces was more probable. Analysis of kinetic data indicated that the intraparticle diffusion mechanism was not the sole rate-controlling mechanism and the boundary layer diffusion affected the adsorption to some extent. Thermodynamic study showed that physical/electrostatic adsorption was expected to be the prevailing mechanism for the adsorption with a spontaneous and endothermic nature. Also, adsorption process was enhanced by lowering the pH and increasing ionic strength of the solutions. These findings were argued based on the equilibrium reactions of the SNP and calcite in water and DLVO theory. Results revealed that repulsive DLVO interaction energies predominated from which an unfavorable attachment condition was deduced. In such situation, ionic strength of solution controlled the extent of the SNP–calcite interaction. Reduction in the height of energy barrier and the formation of a secondary minimum were responsible for enhancing the adsorption process at higher ionic strengths. Furthermore, FESEM-observations depicted a spherical morphology for the adsorbed SNP that uniformly distributed on the calcite surface.

Impact of particle size and surface charge density on redispersibility of spray-dried powders

To understand the impact of surface charge density on the redispersibility of spray dried powders, the redispersibility of styrene-butadiene (SB) powders prepared from latexes with various particle sizes and acid contents was investigated. The redispersibility of these powders showed a step function from fully redispersible to non-redispersible. The phase boundary between these two regimes is determined by the product of the latex particle size and acid content, which is strongly related to the surface charge density of the latex particle. Results indicate that to achieve fully redispersible polymer powder, a proper balance of latex particle size and surface charge is needed. For the class of SB latexes studied in this work, with a certain acid level, it is possible to predict the minimum latex particle size that enables redispersibility of its spray dried powder. This phenomenological observation is unique and it may help us to reveal the fundamentals which govern the redispersibility. To maintain the redispersibility of a spray dried powder, a colloidal stabilizer that adsorbs onto the surfaces of latex particles is required. When latexes are brought to close proximity, strong van der Waals forces may deplete the surface stabilizers, causing irreversible latex agglomeration at spray drying temperatures. A qualitative DLVO analysis was employed to study the impact of various colloidal interactions in this system, including van der Waals, electrostatic, and steric interactions. Zeta potential measurements of the mixture of SB latex and polyvinyl alcohol (PVOH, used as a colloidal stabilizer) suggest that the surface charge density plays a significant role on redispersibility, and strong electrostatic repulsive interactions are needed to overcome the attractive van der Waals interaction. Size exclusion chromatography of serum phase analysis indicated that PVOH adsorption onto latex particles did not change significantly at different acid levels, suggesting low impact of steric repulsion on redispersibility. Conclusions from this work provide additional means to control the performance of redispersible materials.

Harvesting of microalgal biomass using MF membrane: Kinetic model, CDE model and extended DLVO theory

In this paper, the Carman–Kozeny (C–K) model is proposed as a means of predicting flux decline for every stage of membrane cleaning, which can be strongly affected by pore blockage and by the growth of a cake layer. A combination of a kinetic model and the C–K model was utilized to determine specific parameters that can be used to estimate the decline in flux. The model was shown to be in excellent agreement with the experimental data obtained during the crossflow microfiltration of microalgal biomass over several cleaning cycles. The mean flux, given suitable transmembrane pressure (TMP), was also predicted by the model. The cleaning kinetics of the process suggested that a higher energy barrier Ec and a higher rate constant k0 were present for the final cycle of cleaning process, resulting from the extra energy required to break the bonds between the deposited cells and the membrane. This paper also focused on the electrostatic interaction between the microalgal cells and the membrane surface. The concentration polarization (CP) thickness of microalgae on a cellulose acetate (CA) membrane was calculated by the CDE model based on the balance of three transport mechanisms, namely, convection, diffusion and electrophoretic migration, with consideration being given to the negative charge of the microalgae and the membrane surface. The CP thickness was found to be strongly dependent on the membrane surface charge and the number of membrane cleaning cycles. The full interaction between the microalgae and the membrane was characterized according to a DLVO and an extended DLVO approach based on the strength of three separate interactions, namely, the van-der-Waals interaction (vdW), electrostatic (ES) repulsion and acid–base (AB) interactions. Both DLVO and XDLVO analysis provide useful methods for predicting the potential role of these interactions in the adhesion of microalgae to membranes. The XDLVO analysis determined a higher repulsion than did the DLVO analysis as a result of the high AB interaction energy in aqueous systems.

Effect of Carboxylation on Carbon Nanotube Aqueous Dispersibility: A Predictive Coarse-Grained Molecular Dynamics Approach

Functionalized single-walled carbon nanotubes (SWNTs) are widely applied in biomedical science. To understand the interaction between SWNTs and biological systems, various studies have attempted to use coarse-grained molecular dynamics (CGMD). However, there is limited validation of the existing CG models of SWNTs. Here, we present CG models for both pristine and carboxylated SWNTs which are validated against experimental dispersion data. In addition, we present the first ever DLVO analysis of the colloidal stability of parallel SWNTs and establish that the solvent-induced repulsion between fullerenes, which is not considered in DLVO theory, is crucial to obtain a correct physical picture of SWNT dispersibility. The results presented here provide physical insight into the colloidal stability of SWNTs and can be applied to large-scale MD studies of biological systems.

Direct observation of bacterial deposition onto clean and organic-fouled polyamide membranes

Nanofiltration (NF) and reverse osmosis (RO) membranes are commonly applied to produce highly purified water from municipal wastewater effluents. In these applications, biofouling limits overall process performance and increases the cost of operation. Initial bacteria adhesion onto a membrane surface is a critical early step in the overall process of membrane biofouling. However, adsorption of effluent organic matter onto the membrane may precede bacterial deposition and change membrane surface properties. Herein we employed direct microscopic observation to elucidate mechanisms governing bacterial cell deposition onto clean and organic-fouled NF and RO membranes. Bovine serum albumin (BSA) and alginic acid (AA) were used as models for protein and polysaccharide rich organic matter in secondary wastewater effluents. In all experiments, organic fouling increased membrane hydraulic resistance and salt rejection, in addition to interfacial hydrophilicity and roughness. Even though surface hydrophilicity increased, the rougher surfaces presented by organic-fouled membranes produced nano-scale features that promoted localized bacterial deposition. An extended DLVO analysis of bacterial cells and membrane surface properties suggested that bacterial deposition correlated most strongly with the Lewis acid–base free energy of adhesion and root mean square (RMS) roughness, whereas van der Waals and electrostatic free energies were weakly correlated. This was true for both clean and organic-fouled membranes. Bacterial deposition rates were clearly influenced by an antagonistic interplay between macroscopic surface hydrophilicity and nano-scale surface roughness.

Solvent effect on microstructure of yttria-stabilized zirconia (YSZ) particles in solvothermal synthesis

Using methanol or methanol/2-propanol mixtures as reaction media, yttria-stabilized zirconia (YSZ) particles were synthesized with a solvothermal route. The particles were characterized with X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser diffraction technique, and nitrogen adsorption isotherms. Results indicated that cubic/tetragonal YSZ nanocrystals with crystal size lower than 5 nm were obtained and the crystal size depends on solvent composition, reaction temperature and reaction time. For the same reaction temperature and reaction time, the solvent composition also controls YSZ crystal agglomeration behaviour. According to the DLVO theory and analysis of experimental results, the solvent effect on microstructures of YSZ particles in solvothermal synthesis has been discussed. In addition, the mechanism of particle microstructure evolution during solvothermal synthesis has been suggested.

Colloidal adhesion to hydrophilic membrane surfaces

Colloidal adhesion to membrane surfaces is an important parameter in determining membrane fouling propensity and in optimizing membrane cleaning strategies. It has previously been demonstrated that acid–base interactions can significantly affect colloid–membrane interaction as a colloid approaches a membrane surface, however, the effect of acid–base interactions on adhesion has received less attention. In this investigation, the approach and adhesion of a silica and polystyrene colloid was measured on three commercially available hydrophilic water treatment membranes using an atomic force microscope and the colloid probe technique. It was found that the hydrophobic polystyrene colloid adhered more weakly to each membrane compared to the hydrophilic silica colloid. These results could not be resolved through classic DLVO analysis alone and were in direct contrast to the expected interaction based on the strong hydrophobic character of the polystyrene colloid. However, the results could be explained by considering the magnitude of the surface’s electron-acceptor ( γ +) and electron-donor ( γ −) components. It is hypothesized that through hydrogen bonding with surface γ + and γ − groups, structured water layers exist to varying extents at the surfaces of the silica colloid and the hydrophilic membranes, and that their removal results in the formation of strong adhesive bonds between reciprocal γ + and γ − groups. Furthermore, even when surface roughness is substantial, γ + and γ − groups appear to play some role in determining the magnitude of the measured adhesion. The lack of such groups on the polystyrene colloid, and thus the lack of hydrogen bonding capacity, was responsible for its weaker adhesion with the membranes.

Macroscopic-scale surface properties of streptavidin and their influence on aspecific interactions between streptavidin and dissolved biopolymers

To avoid aspecific attractions between carrier surfaces for either ligand or receptor molecules in, e.g., immunoassays, or kinetic rate constant measurements, it has long been established that a background consisting of an aspecific, very hydrophilic carrier surface is generally quite effective. However, it is not often realized that one achieves such a non-reactive background, even with electrostatically neutral materials, at the price of creating a strong (polar) hydrophilic repulsion between dissolved biopolymer (e.g., protein) molecules and the non-adsorbing carrier surface. To investigate the quantitative effects of this type of repulsion in systems involving streptavidin, the surface properties of a streptavidin-coated glass plate were determined by contact angle measurements, from which the aspecific, macroscopic-scale free energies of repulsion between a streptavidin-coated surface and dissolved proteins such as immunoglobulin-G (IgG), and human serum albumin (HSA), could be derived. Streptavidin, even at neutral pH (at which it has virtually no electric surface charge as determined by electrophoresis) is very hydrophilic and strongly repels both IgG and HSA molecules. At neutral pH, molecules such as IgG and HSA, in aqueous solution, cannot approach a streptavidin layer more closely than to approximately 3.0 nm, which suffices to prevent IgG or HSA from any aspecific adherence to the streptavidin layer (as determined by extended DLVO analysis). This aspecific repulsion however also has the (usually unsuspected) effect of causing a decreased specific attachment between ligand and receptor molecules. In addition, it decreases the measured kinetic on-rate constants, often by about two decimal orders of magnitude. However, once the surface-thermodynamic properties of all the aspecific (macroscopic-scale) and specific (microscopic-scale) entities, as well as the specific equilibrium binding constant are known, the real kinetic on-rate constant between just the ligand and the receptor determinants can be determined, yielding the value it would have if the measurement of that constant were unhindered by the repulsive interactions exerted by the background of hydrophilic carrier molecules.

Influence of macroscopic and microscopic interactions on kinetic rate constants. III. Determination of von Smoluchowski's f-factor for HSA adsorption onto various metal oxide microparticles, using the extended DLVO approach

The adsorption of human serum albumin (HSA) onto hydrophobic talc and various types of hydrophilic silica has been studied. The kinetic adsorption constants for each case were determined experimentally, using time-resolved fluorescent spectroscopy. Von-Smoluchowski's f-factor, a measure of the dominance of the macroscopic protein repulsion by the solid surface, was determined for each protein-surface system. The free energies of both macroscopic (repulsive) and microscopic (attractive) interactions between HSA and solid substrata were calculated via extended DLVO (XDLVO) analysis. The combination of such kinetic and thermodynamic data can lead to the estimation of a number of other parameters which are characteristic for each system and contain significant information, such as the mean microscopic and macroscopic-scale interaction energies between protein and solid substratum and the ratio of unfavorable to favorable protein orientations during adsorption, as well as the average net total ratio of repulsion over attraction. The latter was found to be inversely proportional to the amount of HSA adsorbed onto the various mineral particles, after 1 h exposure. Furthermore, detailed examination of all kinetic adsorption parameters leads to the conclusion that the equilibrium affinity constant, extrapolated to zero time K aff t → 0 is the parameter which rather accurately defines the specific interactions for a given protein-solid substratum system. However the most precise characterization of a given protein-substratum system is defined by both the specific (microscopic) adsorption k a mic and the specific (microscopic) desorption k d mic rate constants, which are obtained after elimination of the influence of the aspecific (macroscopic) factors which strongly influence the overall rate constants ( k a and k d) .

Influence of macroscopic and microscopic interactions on kinetic rate constants: I. Role of the extended DLVO theory in determining the kinetic adsorption constant of proteins in aqueous media, using von Smoluchowski’s approach

In the case of adsorption in an aqueous medium of a hydrophilic protein (e.g. human serum albumin (HSA)) onto a hydrophilic solid substratum such as a clean glass surface, one has to deal with a macroscopic-level repulsion between HSA and glass at (generally) the majority of orientations of the protein molecules, and also a microscopic-level attraction between HSA and glass at (generally) the minority of orientations of the protein molecules. The first phenomenon represents von Smoluchowski’s improbability of adhesion or adsorption and the second represents the probability of adhesion or adsorption [1]. Both contingencies have to be taken into account in determining von Smoluchowski’s net probability factor, f of the kinetic association constant, k a, pertaining to protein adsorption. In the exceptional case where both the protein and the solid substratum are hydrophobically/hydrophilically and electrostatically neutral, f=1, and the k a-value is only proportional to the diffusion coefficient of the protein [2]. In order to determine the contributions of both the macroscopic repulsion and the microscopic attraction pertaining to the kinetics of protein adsorption, an extended DLVO analysis (XDLVO) needs to be done on these interactions at all distances and at all protein orientations. The XDLVO analysis comprises the Lewis acid–base interaction energies as a function of distance, in addition to the Lifshitz–van der Waals and the electrokinetic interaction energies [2–5].

Role of acid–base interactions on the adhesion of oral streptococci and actinomyces to hexadecane and chloroform—influence of divalent cations and comparison between free energies of partitioning and free energies obtained by extended DLVO analysis

Calcium is an abundantly present, divalent cation in the oral cavity and plays a crucial role in the adhesion of oral microorganisms to tooth surfaces as well as in coaggregation and coadhesion among the oral microflora. The aim of this study was to determine the effects of divalent cation (Ca 2+, Mg 2+, Ba 2+) adsorption on the adhesion of two actinomyces and two streptococcal strains to hexadecane (MATH) and chloroform (MATS) in order to detect changes in acid–base character of the cell surfaces. Initial removal rates of the organisms by hexadecane, lacking an acid–base interaction with the organisms, were always smaller than those by chloroform. Furthermore, adsorption of divalent cations generally increased the initial removal rates of the microorganisms, but no statistically significant differences among different cations were observed. Gibbs energies of partitioning calculated from the stationary end-point adhesion of the organisms ranged from −2 to −4 kT for adhesion to hexadecane and were about twofold more negative for adhesion to chloroform. Contact angles on lawns of microorganisms with and without adsorbed divalent cations were similar. Zeta potentials of all microorganisms were slightly negative under the conditions of MATH and MATS and became only 4 mV more positive upon divalent cation adsorption. Hexadecane had a zeta potentials of −21 mV in the potassium phosphate solution used, which became 13 mV less negative upon Ca 2+ adsorption. An extended DLVO approach of microbial adhesion to hexadecane, based on microbial contact angles and zeta potentials, taking into account Lifshitz–van der Waals, acid–base and electrostatic interactions did not show any potential energy barrier and demonstrated a deep primary interaction minimum at close approach due to acid–base attraction. As the Gibbs energy of partioning was only −2 to −4 kT, it is concluded that for the collection of organisms studied here, the final contactable surface area is small and structural features on the cell surfaces like fibrils and fimbriae, maintain a distance of ca. 10–15 nm between the hexadecane and the overall cell surface and therewith prevent acid–base interactions to become operative to a significant extend. Furthermore, from the lack of influence of divalent cations on macroscopic cell surface contact angles and zeta potentials, it is suggested that cation adsorption is minor and localized to the fibrils and fimbriae.