Charge Neutralization Mechanism Research Articles

The flocculating properties of chitosan‐graft‐polyacrylamide flocculants (II)—Test in pilot scale

AbstractIn this study, the flocculating properties of chitosan‐graft‐polyacrylamide (PAM) copolymer flocculants have been investigated in pilot scale, and the raw water from Zhenjiang part of Yangtse river in China was used as simulated waste water. The influences of dose, mechanical mixing rate, and sedimentation time to the flocculating effects have been investigated in different seasons: summer and winter, respectively. The optimal experimental conditions have been studied by orthogonal test. In addition, the structure effects of the flocculants: the grafting ratio, on the flocculating properties have been also investigated in pilot scale, which results were fully consistent to those from the beaker experiment in laboratory scale using kaolin suspension as simulated waste water. Compared to poly ferric sulfate, current used flocculants in Jinxi Water Factory, chitosan‐g‐PAM copolymer flocculants showed better flocculating properties, which was ascribed to the cooperative effects of the charge neutralization and bridging flocculating mechanisms of polymer flocculants. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The flocculating properties of chitosan‐graft‐polyacrylamide flocculants (I)—effect of the grafting ratio

AbstractA series of chitosan‐graft‐polyacrylamide copolymers have been prepared as flocculants for waste water treatment. The results indicated that the grafting ratio was one of the key factors for the flocculating effects. The copolymers with various grafting ratios showed different flocculating properties. To observe every detail of this effect, the specific refractive index increment measurement (SRIIM) method was used to determine the grafting ratio at the beginning of this study. Interestingly, it was found that as the grafting ratio increased, the flocculating effect increased until it reached its maximum point. At that point, it decreased. This result was discussed in terms of the cooperative effects of the charge neutralization and bridging flocculating mechanisms. A structure model of chitosan‐g‐polyacrylamide copolymer flocculants has been proposed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

A comparison study on Acid Red 119 dye removal using two different types of waterworks sludge

This study aims to compare the performance of two waterworks sludge samples, ferric chloride sludge (FCS) and polyaluminium chloride sludge (PACS), as coagulants to remove Acid Red 119 (AR119) dye from aqueous solution. A series of batch experiments were performed to investigate the effect of initial pH, coagulant dosage and initial dye concentration on coagulation/flocculation process efficiency. Results showed that the dye removal was strongly pH-dependent and followed the same trend for both coagulants, increasing steadily with decreasing initial pH in the range of 3-12. With the increase of coagulant dosage, the dye removal efficiency increased and no re-stabilization phenomenon was observed. However, the coagulant dosage needed for the maximum dye removal efficiency was much lower for FCS (150 mg dried sludge/L) in compare with PACS (5.5 g dried sludge/L). It seems that both charge neutralization and sweep flocculation mechanisms were involved in removal of AR119 using FCS and PACS. With the increase of initial dye concentration, Q (the amount of the removed dye per unit mass of coagulant) steadily increased and reached to a maximum value of 0.96 mg dye/mg sludge and 0.029 mg dye/mg sludge for FCS and PACS, respectively. FCS was more effective than PACS for AR119 dye removal due to the higher amount of dye removed per unit mass of sludge.

Electro-coagulation–flotation process for algae removal

Algae in surface water have been a long-term issue all over the world, due to their adverse influence on drinking water treatment process as well as drinking water quality. The algae removal by electro-coagulation-flotation (ECF) technology was investigated in this paper. The results indicated that aluminum was an excellent electrode material for algae removal as compared with iron. The optimal parameters determined were: current density=1 mA/cm(2), pH=4-7, water temperature=18-36 degrees C, algae density=0.55 x 10(9)-1.55 x 10(9) cells/L. Under the optimal conditions, 100% of algae removal was achieved with the energy consumption as low as 0.4 kWh/m(3). The ECF performed well in acid and neutral conditions. At low initial pH of 4-7, the cell density of algae was effectively removed in the ECF, mainly through the charge neutralization mechanism; while the algae removal worsened when the pH increased (7-10), and the main mechanism shifted to sweeping flocculation and enmeshment. The mechanisms for algae removal at different pH were also confirmed by atomic force microscopy (AFM) analysis. Furthermore, initial cell density and water temperature could also influence the algae removal. Overall, the results indicated that the ECF technology was effective for algae removal, from both the technical and economical points of view.

Inhibition of bactericidal activity is maintained in a mouse α-defensin precursor with proregion truncations

α-Defensin biosynthesis requires the proteolytic conversion of inactive precursors to microbicidal forms. In mouse Paneth cell pro-α-defensin proCrp4(20–92), anionic amino acids positioned near the proregion N-terminus inhibit proCrp4 activity by an apparent charge neutralization mechanism. Because most pro-α-defensins contain proregions of highly conserved chain length, we tested whether decreasing the distance between the inhibitory acidic residues of the proregion and the α-defensin component of the precursor would alter proCrp4 inhibition. Accordingly, two proCrp4 deletion variants, (Δ44–53)-proCrp4 and (Δ44–58)-proCrp4, truncated in a manner corresponding to deletions between MMP-7 cleavage sites, were prepared and assayed for bactericidal peptide activity. Consistent with the properties of full-length proCrp4(20–92), (Δ44–53)-proCrp4 and (Δ44–58)-proCrp4 were processed effectively by MMP-7, lacked bactericidal activity at high peptide levels over a 3h exposure period, and failed to induce permeabilization of live Escherichia coliin vitro. Thus, bringing the inhibitory proregion domain into greater proximity with the Crp4 component of the precursor did not alter the activity of this pro-α-defensin. Therefore, the conserved distance that separates inhibitory acidic proregion residues from the Crp4 peptide is not critical to maintaining proCrp4(20–92) in an inactive state.

Characteristics of BPA removal from water by PACl-Al 13 in coagulation process

This paper discussed the coagulation characteristics of BPA with polyaluminum chloride (PACl-Al 13) as coagulant, examined the impact of coagulation pH, PACl-Al 13 dosage, TOC (total organic carbon) and turbidity on BPA removal, and analyzed the possible dominant mechanisms in water coagulation process. Formation and performance of flocs during coagulation processes were monitored using photometric dispersion analyzer (PDA). When the concentration of humic acid matters and turbidity was low in the solution, the experimental results showed that the removal of BPA experienced increase and subsequently decrease with the PACl-Al 13 dosage increasing. The optimal PACl-Al 13 dosage was found at BPA/PACl-Al 13 = 1:2.6(M/M) under our experiment conditions. Results show that the maximum BPA removal efficiency occurred at pH 9.0 due to the adsorption by Al 13 aggregates onto BPA rather than charge neutralization mechanism by polynuclear aluminous salts in the solution. The humic acid matters and kaolin in the solution have significant effect on BPA removal with PACl-Al 13 in the coagulation. The BPA removal will be weakened at high humic matters. The removal rate of BPA increased and subsequently decreased with the turbidity increasing.

Turbidity Removal from Wastewaters of Natural Stone Processing by Coagulation/Flocculation Methods

AbstractThe effectiveness of coagulation (at pH values of 6, 7.5, and 9), flocculation (at pH 9), and coagulation plus flocculation (at pH 9) on turbidity removal from natural stone (travertine) processing wastewaters (NSPW) were examined by applying classical sedimentation tests. FeCl3·6 H2O, AlCl3, and Al2(SO4)3·16 H2O were used as coagulants and a polyacrylamide based anionic polymer was used as the flocculant. In this way, it was found that the coagulation method alone was not sufficient to purify NSPW, whereas flocculation and coagulation plus flocculation methods provided superior purification. Among the coagulants used, AlCl3 gave the best result in terms of turbidity removal by coagulation from NSPW at pH 6 and 9, whereas the turbidity removal performances of the three coagulants were almost identical at pH 7.5. In addition, relatively low pH (i. e., pH 6) improved the purification performance of all coagulants. During coagulation of NSPW at pH 6, a charge neutralization mechanism played a decisive role in turbidity removal. However, in neutral (pH 7.5) and slightly basic (pH 9) media, a sweep coagulation mechanism was predominant. For flocculation of NSPW, the basic mechanism comprised of polymer bridging.

Characterization of a Heparin-Binding Site on the Catalytic Domain of Factor XIa: Mechanism of Heparin Acceleration of Factor XIa Inhibition by the Serpins Antithrombin and C1-Inhibitor

Heparin accelerates inhibition of factor XIa (fXIa) by the serpins antithrombin (AT) and C1-inhibitor (C1-INH) by more than 2 orders of magnitude. The mechanism of the heparin-mediated acceleration of fXIa inhibition by these serpins is incompletely understood, as heparin appears to interact with both the catalytic and noncatalytic domains of the protease. We replaced the basic residues of the fXIa 170 loop (Lys-170, Arg-171, Arg-173, Lys-175, and Lys-179; chymotrypsin numbering) with Ala, using an expression system that allows separation of the fXIa catalytic domain (CD) from noncatalytic domains. Heparin-mediated inhibition of 170 loop CD variants with AT was impaired 3-10-fold relative to that of the wild-type (CD-WT). In reactions with C1-INH, Arg-171 was the most critical residue contributing approximately 2-3-fold to heparin-mediated inhibition of CD-WT. A template mechanism did not fully account for the effect of heparin with either serpin, as the second-order inhibition rate constants did not exhibit a characteristic bell-shaped dependence on heparin concentration. Further studies revealed that the C1-INH inhibition of full-length fXIa containing Ala substitutions for basic residues of the 148 loop is not enhanced by heparin. Inhibition by AT of a full-length fXIa variant containing an Ala substitution for Arg-37 in the fXIa CD was approximately 5-fold greater than for wild-type fXIa in the absence of heparin. These results suggest that basic residues of the fXIa 170 loop form a heparin-binding site and that the accelerating effect of heparin on inhibition of fXIa by AT or C1-INH may be mediated by charge neutralization and/or allosteric mechanisms that overcome the repulsive inhibitory interactions of serpins with basic residues on the fXIa 148 and 37 loops.

Formation, Breakage and Reformation of Humic Flocs by Inorganic and Organic Coagulants

The floc formation, breakage and reformation of humic acid by inorganic (alum and PAC) and organic coagulants (cationic polyelectrolytes) at several conditions (pH, ionic strength and floc breakage time) were examined and compared among the coagulants at different conditions using a continuous optical monitoring method, with controlled mixing and stirring conditions. For alum, the shapes of formation, breakage and reformation curves at different pH (5 and 7) were different, but the shapes and the sizes of initial floc and reformed floc were nearly the same in the absence and presence of electrolytes at pH 7. For PAC, similar shapes of the curves were obtained at different pH and ionic strength, but the sizes were different, except for those of reformed flocs at different pH. However, for these coagulants, reformed flocs after floc breakage, occurred irreversibly for all the conditions used in this study. For organic coagulants, the time to attain the initial plateau floc size, the extent of floc strength at high shear rate and reversibility of reformed floes were different, depending floc formation mechanism. Especially, for the cationic polyelectrolyte forming humic flocs by charge neutralization or electrostatic patch effect mechanism, reformed flocs occurred reversibly, regardless of pH and floc breakage time, but occurred irreversibly in the presence of electrolytes.

Acetylation Mimics within Individual Core Histone Tail Domains Indicate Distinct Roles in Regulating the Stability of Higher-Order Chromatin Structure

Nucleosome arrays undergo salt-dependent self-association into large oligomers in a process thought to recapitulate essential aspects of higher-order tertiary chromatin structure formation. Lysine acetylation within the core histone tail domains inhibits self-association, an effect likely related to its role in facilitating transcription. As acetylation of specific tail domains may encode distinct functions, we investigated biochemical and self-association properties of model nucleosome arrays containing combinations of native and mutant core histones with lysine-to-glutamine substitutions to mimic acetylation. Acetylation mimics within the tail domains of H2B and H4 caused the largest inhibition of array self-association, while modification of the H3 tail uniquely affected the stability of DNA wrapping within individual nucleosomes. In addition, the effect of acetylation mimics on array self-association is inconsistent with a simple charge neutralization mechanism. For example, acetylation mimics within the H2A tail can have either a positive or negative effect on self-association, dependent upon the acetylation state of the other tails and nucleosomal repeat length. Finally, we demonstrate that glutamine substitutions and lysine acetylation within the H4 tail domain have identical effects on nucleosome array self-association. Our results indicate that acetylation of specific tail domains plays distinct roles in the regulation of chromatin structure.

Site-specific Binding Affinities within the H2B Tail Domain Indicate Specific Effects of Lysine Acetylation

Acetylation of specific lysines within the core histone tail domains plays a critical role in regulating chromatin-based activities. However, the structures and interactions of the tail domains and the molecular mechanisms by which acetylation directly alters chromatin structures are not well understood. To address these issues we developed a chemical method to quantitatively determine binding affinities of specific regions within the individual tail domains in model chromatin complexes. Examinations of specific sites within the H2B tail domain indicate that this tail contains distinct structural elements and binds within nucleosomes with affinities that would reduce the activity of tail-binding proteins 10-50-fold from that deduced from peptide binding studies. Moreover, we find that mutations mimicking lysine acetylation do not cause a global weakening of tail-DNA interactions but rather the results suggest that acetylation leads to a much more subtle and specific alteration in tail interactions than has been assumed. In addition, we provide evidence that acetylation at specific sites in the tail is not additive with several events resulting in similar, localized changes in tail binding.

The effect of boronic acid-positioning in an optical glucose-sensing ensemble

Abstract The quenching of the anionic dye 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine) with three different boronic acid-substituted benzyl viologens was determined, and the fluorescence signal modulation obtained upon addition of glucose to the dye/quencher system was also studied. The benzyl viologen that contains boronic acids in the ortho-position (o-BBV) was found to display unique behavior, which can be rationalized by a charge neutralization mechanism facilitated by an intramolecular interaction between sp3 boronate and the quaternary nitrogen of the viologen. Potentiometric titration and 11B NMR spectroscopy were used to generate pH profiles for the boronic acids, which provide additional evidence for the proposed mechanism.

Adsorption and flocculation behaviors of polydiallyldimethylammonium (PDADMA) salts: Influence of counterion

Two modified polymer flocculants of PDADMA salts, PDADMA nitrate (PDADMAN) and PDADMA sulfate (PDADMAS), were prepared from the most widely used commercial product of polydiallyldimethylammonium chloride (PDADMAC). Solution properties such as conductivity and viscosity were investigated, showing that the conductivity follows the order of PDADMAS > PDADMAC > PDADMAN, while the reduced viscosity increases in the order of PDADMAS < PDADMAC < PDADMAN. The results indicate that, compared with PDADMAC, PDADMAN has a more cationic density and a more extended polymer chain, whereas PDADMAS exhibits a smaller coiled polymer size because of the stronger charge affinity of sulfate ion to the polycation of PDADMA. Flocculation experiments were performed on kaolin suspensions. It was found that PDADMAN is the most efficient in turbidity removal, while PDADMAS has wider optimum dosages, larger floc size and more rapid settling rate. Atomic force microscopy (AFM) was employed for imaging the adsorption structures of the three polymers on the negatively charged mica surface. Remarkable differences in molecular conformation were observed. Compared with the pearl necklace-like aggregation of hemispheroids of PDADMAC polymer, PDADMAN takes a sponge-like thin layer structure and PDADMAS exhibits a uniform hemispherical structure. The above differences in both the solution property and the adsorption and flocculation behaviors are attributable to counterion effects. Different counterions may lead to different adsorption and flocculation mechanisms that PDADMAN adopts a flat and layer–layer adsorption without loops and tails extended from the surface, implying that a charge neutralization mechanism plays more roles, while adsorbed PDADMAS is in the form of coiled spheres with loops and tails so that a bridging mechanism is more involved. Namely, it is the adsorption structure and morphology of the polymer flocculants that is the main factor influencing the flocculation mechanism.

Sulfanilic Acid: A Novel Consolidation Agent for Al2O3 in Aqueous Media

A new solidifying agent, 4‐aminobenzene sulfonic acid (sulfanilic acid), is reported in this paper. The consolidation process and mechanism were followed using on viscoelastic, FTIR, SEM, and Hg porosimetry measurements. It was shown that the Al2O3 slurries with PAA‐Na as dispersant exhibited a high degree of particle stabilization. After the addition of sulfanilic acid, we observed an exponential increase in the storage modulus (G′) as a function of consolidation time. Correspondingly, Al2O3 slurries exhibited a transition from a viscous to an elastic state (in 62 min). FTIR analysis indicated that the consolidation process might follow two steps: first, the adsorption of sulfanilic acid on the Al2O3 particle surface; second, the acid–base interaction between the adsorbed PAA‐Na molecules and the sulfanilic acid molecules. This interaction could possibly induce the formation of three‐dimensional networks through a bridging or charge neutralization mechanism. The as‐consolidated Al2O3 green samples were homogeneous, with the relative green density being 54.69%. Results showed that it was feasible to introduce sulfanilic acid for the consolidation of Al2O3 slurries in aqueous media.

Electrochemical quartz crystal microbalance study of mass transport during electrochemical oxidation of [Os(bpy) 3] 2+ in clay-modified electrodes

Mass transport in clay films containing [Os(bpy) 3] 2+ cations was investigated by EQCM and crystal impedance spectroscopy. Admittance measurements on 10 μg clay films exchanged with [Os(bpy) 3] 2+ show no change in the width or height of the conductance peaks before and after potential scans or potential steps. The [Os(bpy) 3] 2+ exchanged films could be considered to be rigid with no change in their viscoelastic properties, and the shifts in resonant frequency in the EQCM measurements interpreted as mass changes. In an electrode coated with a 8.5 μg clay film, oxidation of the adsorbed cations resulted in a large increase in frequency, corresponding to a decrease in mass. The mass per mole of electron transferred (MPE) was −205 g/mol, or about one third of the weight of one [Os(bpy) 3] 2+ ion. This was consistent with the ejection of one [Os(bpy) 3] 3+ from the film for each three [Os(bpy) 3] 2+ cations oxidized. The ratio of cathodic charge to anodic charge of 0.7 was also consistent with the loss of one third of the oxidized [Os(bpy) 3] 3+ ions. The mechanism of charge neutralization was dependent of the weight of the clay films. Oxidation of [Os(bpy) 3] 2+ in a 33 μg clay film resulted in a small decrease in frequency, corresponding to an increase in mass. The MPE, +50 g/mol, was consistent with charge neutralization by adsorption of sulfate anions from the electrolyte.

Influence of polyelectrolyte characteristics on pressure-driven activated sludge dewatering

Activated sludge conditioning and dewatering is a complex process involving many different parameters with varying degrees of importance. Due to the biological nature of the sludge, which impedes extensive testing in time on the same sample, a restriction of parameter values for testing has to be made. Based on a well-considered experimental design, this work has thoroughly investigated the influences of polyelectrolyte molecular weight and charge density on the dewatering performance with respect to both filtration and expression of activated sludge. Sludge samples were thereby collected in time with different solids concentrations and surface charge characteristics. Higher molecular weight polyelectrolytes were shown to yield clear flocculation already at low doses, but tend to create elastic cakes presumably due to high internal electrostatic repulsion, as shown by the filter cake rebound after pressure release. As a further consequence of this high internal repulsion, the water retention of the flocs is high, which causes a high susceptibility to compression and limited dewaterability. Lower molecular weight polyelectrolytes require higher doses to become effective flocculants. In sufficient dose, these allow considerable water release from the sludge, enhancing the filtration phase. Thanks to the high degree of charge neutralisation, the formed flocs tend to form less elastic filter cakes. Furthermore, it seemed that a charge neutralization mechanism led to less water retention of the flocs formed and a more solid structure, making these polyelectrolytes also perform best in the expression phase.

Fluorescence sensors for monosaccharides based on the 6-methylquinolinium nucleus and boronic acid moiety: potential application to ophthalmic diagnostics

Continuous monitoring of glucose levels in human physiology is important for the long-term management of diabetes. New signaling methods/probes may provide an improved technology to monitor glucose and other physiologically important analytes. The glucose sensing probes, BMQBAs, fabricated using the 6-methylquinolinium moiety as a fluorescent indicator, and boronic acid as a chelating group, may have versatile applications in glucose sensing because of their unique properties. In this paper we discuss the design logic, synthesis, characterization and spectral properties of three new isomeric glucose sensors (BMQBAs), and a control compound (BMQ) in the presence and absence of sugars. The sensing ability of the new probes is based on a charge neutralization and stabilization mechanism upon sugar binding. The new probes have attractive fluorescence quantum yields, are highly water-soluble, and have spectral characteristics compatible with cheap and portable LEDs and LDs. One of the probes, o-BMQBA, has a sugar bound p K a of 6.1, and a dissociation constant K D of 100 mM glucose. These probes have been designed specifically to respond to tear glucose in a contact lens polymer for ophthalmic glucose monitoring, where the reduced sugar bound p K a affords for sensing, in a lens environment that we have previously shown to be mildly acidic.

Electrochemical quartz crystal microbalance study of mass transport in thin film of a redox active Ni–Al–Cl layered double hydroxide

Films of redox active Ni–Al–Cl layered double hydroxides (LDHs) lost mass on oxidation and gained mass on reduction. The size of the mass changes depended on the composition and on the pH of the electrolyte solutions. In pH 8 potassium borate or phosphate buffers, the decreases in mass on oxidation were small. An apparent mass per mole of electrons transferred (MPE) of only 4–6 g/mol was obtained. This suggests that compensation for the positive charge generated by oxidation of the Ni(II) sites occurred by loss of protons from the LDH lattice accompanied by the loss of some water from the LDH interlayer spaces. The mass changes were two to three times larger when sodium was present in the buffers. For example, the MPE in a pH 8 potassium borate buffer containing 4 mM sodium increased to 19 g/mol. This was attributed to a contribution from the expulsion of sodium ions adsorbed in the LDH films to the charge neutralization mechanism. The MPE increased when the pH of the electrolyte increased. At pH 10 the mass loss was also no longer reversible. This was attributed to the decomposition of the Ni–Al LDH when it was subjected to potential scanning in the more basic solutions. In non-buffered potassium acetate or sodium acetate solutions, the MPEs were found to decrease with decrease in the scan speed, from 20 to 22 g/mol at 50 mV/s to as little as 6 g/mol at 5 mV/s in potassium acetate.

Crystal Structures of QacR-Diamidine Complexes Reveal Additional Multidrug-binding Modes and a Novel Mechanism of Drug Charge Neutralization

The Staphylococcus aureus multidrug-binding protein QacR represses transcription of the plasmid-encoded membrane protein QacA, a multidrug efflux transporter. QacR is induced by multiple structurally dissimilar monovalent and bivalent cationic lipophilic compounds, many of which are effluxed from the cell by QacA via the proton motive force. The multidrug-binding pocket of QacR has been shown to be quite extensive and features several glutamates and multiple aromatic residues. To date, the structure of only one QacR-bivalent cationic drug complex (that of QacR bound to dequalinium) has been determined, and how other longer or shorter bivalent cationic compounds bind is unknown. Here we report the crystal structures of QacR bound to two cytotoxic bivalent diamidines, pentamidine and hexamidine. These compounds are structurally similar, differing by only one methylene carbon in the alkyl chain linker. However, this small difference results in very dissimilar binding modes. Similar to dequalinium, hexamidine spans the multidrug-binding pocket, and its positively charged benzamidine groups are neutralized by residues Glu-57 and Glu-120. Pentamidine binds QacR in a novel fashion whereby one of its benzamidine groups interacts with residue Glu-63, and the other is neutralized by carbonyl and side chain oxygen atoms. Thus, these structures demonstrate that a formal negative charge is not a prerequisite for binding positively charged drugs and underscore the versatility of the QacR and, likely, all multidrug-binding pockets.

Mathematical modeling of polymer-induced flocculation by charge neutralization

A detailed mathematical model for flocculation of colloidal suspensions in presence of salts and polymers is described and validated. In former case, the classical DLVO theory, which accounts for relevant variables such as pH and salt concentration, is incorporated into a geometrically sectioned discrete population balance model. For processes involving polymers, flocculation via simple charge neutralization is modeled using a modified DLVO theory in which the effect of adsorbed polymer layers on van der Waals attraction is included. The fractal dimension of aggregates is obtained by dynamic scaling of experimental data for time evolution of mean aggregate size. The particle surface potential is assumed to be approximately equal to the zeta potential. The model predictions are in close agreement with experimental results for flocculation of colloidal hematite suspensions in the presence of KCl and polyacrylic acid at different concentrations. In particular, given values of model parameters, e.g., Hamaker constant, fractal dimension, surface potential, and thickness of adsorbed polymer layer, the model can realistically describe the kinetics of flocculation by a simple charge neutralization mechanism and track the evolution of floc size distribution. Representative examples of sensitivity of the flocculation model to perturbations in surface potential and fractal dimension and to modification in the DLVO theory for polymer-coated particles are included.