Dissociation Of Carboxyl Groups Research Articles

Polyelectrolyte polymer properties in relation to male contraceptive RISUG ® action

RISUG ® a polyelectrolytic hydrogel (styrene maleic anhydride and dimethyl sulfoxide) has proven to be efficacious as a contraceptive for a long term when injected into the lumen of vas deferens. Currently it is in advanced phase III clinical trials in India. Present investigation analyzes the swelling characteristics of RISUG ® hydrogel in different pH buffers and various biological fluids to understand its retention in the vas deferens as reported in previous studies. Significant variation in degree of swelling and equilibrium swelling ratio with transformation of Fickian to non-Fickian mode of diffusion was observed with increased pH. This might be due to ionization of carboxylic groups at high pH resulting in increased electrostatic repulsive force and high osmotic pressure inside the hydrogel network affecting its physical cross-linking and increases the free volume. Conversely, at low pH the dissociation of carboxylic group is limited making the hydrogel more stable. Interaction with various biomolecules present in various biological fluids was also studied. SEM, AFM and FTIR were used to analyze the topological and structural parameters of the polymer in different mediums. Loosening of structure and increasing porosity with significant adsorption of various biomolecules was observed. AFM revealed a significant change in overall roughness of polymer surface on interaction with different biological fluids. These observations suggest that the swelling and increased roughness will lead to increased resistance to sperm movement in the vas deferens.

Enantioselective complexation of carbamoylated quinine and quinidine with N-blocked amino acids: vibrational and electronic circular dichroism study

Infrared absorption (IR) and vibrational and electronic circular dichroism (VCD and ECD, respectively) spectra of tert-butylcarbamoylquinine (t-BuCQN) and pseudoenantiomeric tert-butylcarbamoylquinidine (t-BuCQD), denoted as selectors (SO), complexed with chiral (S) and (R)-3,5-dinitrobenzoylleucine (DNB-Leu) and achiral 3,5-dinitrobenzoylglycine (DNB-Gly), denoted as selectands (SA), in methanol and acetonitrile, with the spectra of pure SA and SO are reported. H-D exchange of exchangeable hydrogen atoms of SA and SO in deuterated methanol which occurs in IR and VCD experiments is exploited to identify Amide II and Amide III vibrational modes. The formation of preferentially bound complexes composed of sterically compatible combinations of DNB-Leu and SO are manifested by increased intensity of VCD bands assigned to vibrations of amide, carbamate, quinoline, and dissociated carboxylate group and also by increased ECD signals. The VCD technique revealed similarities between the strongly bound diastereomeric complex of chiral DNB-Leu and SO and the complex of achiral DNB-Gly and SO, highlighting the leading role of SO in the formation of SA-SO complex.

Poly(MAA-co-AN) hydrogels with improved mechanical properties for theophylline controlled delivery

A hydrophobically improved copolymer hydrogel was constructed and developed based on methacrylic acid (MAA) and acrylonitrile (AN) monomers. The swelling investigations indicated that the hydrogels were highly sensitive to the pH and ionic strength of the surrounding environment. The swelling ratios of the gels in stimulated intestinal fluids (SIF, pH 7.4) were higher than those in stimulated gastric fluids (SGF, pH 1.4), and followed a non-Fickian and a Fickian diffusion mechanism, respectively. The results match well with scanning electron microscopy observations showing that mesh sizes in hydrogels in SGF are larger and more patulous than in SIF. It is believed that the phenomena are concerned with the electrostatic repulsion originating from the formation and dissociation of carboxyl groups, as well as a charge screening effect of the cations leading to the reduction of osmotic pressure. A dynamic mechanical analyzer revealed that the addition of hydrophobic AN units notably improved the compression mechanical properties of the control sample. With an increase in AN concentrations, the ability for the copolymers to resist compression deformation enhanced in distilled water and the compression strains have almost the same values in SIF environments. Differential scanning calorimetry was used to analyze the state of water and to determine the amounts of freezing and non-freezing water. The controlled release examination based on the theophylline model drug showed that the release rate in distilled water was faster than that in SIF or SGF. The introduction of the hydrophobic AN decreased the theophylline release rates in both SIF and SGF, and the release rate variation in SIF was notably larger than that in SGF. The drug release behavior of the AN-modified P(MAA-co-AN) copolymer hydrogel deviates from the Fickian diffusion control mechanism. This hydrogel is expected to be used as an excellent material in oral drug controlled release.

Effect of Biopolymers on Structure of Hydroxyapatite and Interfacial Interactions in Biomimetically Synthesized Hydroxyapatite/Biopolymer Nanocomposites

The interfacial interaction and effect of biopolymer on crystal structure of hydroxyapatite in biomimetically synthesized nanocomposites, chitosan/hydroxyapatite (ChiHAP50), polygalacturonic acid/hydroxyapatite (PgAHAP50), and chitosan/polygalacturonic acid/hydroxyapatite (ChiPgAHAP50) have been investigated using atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and Rietveld analysis. AFM phase images show nano-sized hydroxyapatite particles uniformly distributed in biopolymer. FTIR spectra indicate that chitosan interacts with hydroxyapatite through NH(3)(+) groups, whereas in polygalacturonic acid/hydroxyapatite, dissociated carboxylate groups (COO(-)) form unidentate chelate with calcium atoms. A change in lattice parameters of hydroxyapatite in all nanocomposites is observed using Rietveld analysis. The increase in lattice parameters was most prominent along c-axis in ChiHAP50 and ChiPgAHAP50 nanocomposites, which was 0.388% and 0.319%, respectively. Comparison between particle sizes of hydroxyapatite, determined from AFM and Rietveld analysis, indicates presence of amorphous phase in hydroxyapatite particles, which is believed to be present at the interface of hydroxyapatite and biopolymer.

Kinetic study for the extraction of succinic acid with TOA in fermentation broth; effects of pH, salt and contaminated acid

Reactive extraction can be used for the recovery of carboxylic acids from fermentation broth. Through the formation of complex with extractants at the two-phase interface, the carboxylic acids are partitioned into organic solvents. However, the recovery of carboxylic acids is interrupted by the conditions of fermentation broth. In this study, the effects of conditions of fermentation broth on the extraction kinetics were investigated using a microporous membrane-based stirred cell for the extraction of succinic acid with tri- n-octylamine. The interfacial concentrations of species in various systems were correlated and thus the effects of pH, salts and contaminated acid on the intrinsic reaction kinetics were discovered. The reaction rate constants were determined from the forward reaction rate equation reported in our previous work. It was found that the extraction rates were steeply decreased at pH values larger than 3 due to the dissociation of carboxylic group. Competitive extraction of salts, which were HPO 4 2− (or PO 4 3−), CO 3 2− and SO 4 2−, and contaminated acid, which was pyruvic acid, had negative influence on the extraction process of succinic acid and thus the extraction rates were decreased. The interfacial concentrations of succinic acid and TOA in fermentation broth had no difference with those in artificial single acid systems. Therefore, the decrease of extraction rates can be explained by the change of ionic strength in fermentation broth.

Formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide and sodium acrylate observed by ATR-FTIR spectroscopy

In this paper, the formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) were studied using Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR). Aqueous solutions of NIPA and SA monomers with different pHs were prepared, and the ATR-FTIR spectra were obtained immediately after preparing the solution and after having been stored at room temperature for 6 months. It was found that the IR spectra evidently changed after 6 months due to polymerization, and the viscosity of a solution in the lowest pH system increased with time and finally the solution became a gel. The detailed analysis of the IR spectra indicated that the network of the gel was formed by the formation of hydrogen bonds (without crosslinker). Moreover, this physical gel exhibited the re-swelling transition by increasing the pH of solvent. The transition was caused by the destruction of hydrogen bonds due to the dissociation of carboxyl groups, which was also confirmed by the IR spectroscopy.

Effects of hydrophobic interaction on rheological behavior and microstructure of carboxylated core‐shell latex suspension

AbstractWe have investigated the effects of hydrophobic interactions on the rheological behavior and microstructure of suspension of carboxylated core‐shell latex particles with changing hydrophobicity of shell polymer and suspending medium. The carboxylated core‐shell latex particles formed lattice‐like microstructures in aqueous suspension with dissociation of carboxyl groups. With increasing hydrophobicity of the shell polymer, the interparticle distance ξ in the microstructure decreased. However, ξ increased with increasing hydrophobicity of the suspending medium. The effect of hydrophobic interaction on ξ was explained by the steric stabilization theory for particles with grafted polymer on the surface. As the carboxylated core‐shell latex particles overlapped each other in the microstructure, an attractive force was generated between the particles in aqueous suspension. With increasing hydrophobicity of the shell, the attractive force increased, but with increasing hydrophobicity of the suspending medium, the attractive force decreased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4153–4158, 2006

Experiments and simulation of pH‐responsive N‐isopropylacrylamide–acrylic acid copolymer hydrogels

AbstractIn this study, N‐isopropylacrylamide–acrylic acid (NIPAAm–AAc) copolymer hydrogels were synthesized by radical polymerization. The equilibrium and dynamic water uptake of these copolymer hydrogels were investigated as a function of acrylic acid content of copolymer, pH, and temperature. The pH‐dependent swelling ratio of the hydrogels was distinguishable between two pH‐modulating methods. In addition, a modified thermodynamic simulation that is able to predict pH‐dependent swelling of polyanionic gels was developed. In the simulation, the influence of ionic strength on dissociation of carboxyl groups was taken into consideration. Also a new relation between Debye length and volume change of gels was derived from the approximation used in derivation of ionic osmotic pressure. The predictions from this modified equation were reasonably good compared with experiments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 323–330, 2006

Static and Dynamic Mechanical Behavior of Hydroxyapatite-Polyacrylic Acid Composites Under Simulated Body Fluid

In this work, we have investigated mechanical response of hydroxyapatite/polyacrylic composites under dry, wet and simulated body fluid conditions. Hydroxyapatite (HAP) is mineralized under two conditions; one, in presence of polyacrylic acid (in situ HAP), second, in absence of polyacrylic acid (ex situ HAP). Further, in situ and ex situ HAP are mixed with polyacrylic acid to make HAP/PAAc composites. Interfacial interactions between PAAc and HAP have been studied using photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR). The mechanical response of the composites under wet condition is studied by soaking composite samples in simulated body fluid (SBF). Under wet conditions, SBF and water weaken the HAP-HAP interface significantly. PAS-FTIR data suggests that PAAc attaches to HAP through the dissociated carboxylate groups. The water and SBF soaked samples showed creep-like behavior and exhibit large residual strain after unloading. Loading under different strain rates has significant effect on mechanical properties of these composites. Both in situ and ex situ 70:30 composites exhibit highest elastic modulus at strain rate of 0.01 sec-1. XRD study indicates formation of Ca2P2O7 phase in ex situ composite after soaking in SBF and water for 3 hours, whereas in situ composites showed presence of only hydroxyapatite phase after soaking in SBF and water for same duration of time.

Formation and discharge of nematocysts is controlled by a proton gradient across the cyst membrane

Cnidaria catch and kill their prey by means of nematocysts. A nematocyst consists of a capsule containing a coiled tubule. On triggering, the cyst extrudes this tubule in an extremely rapid manner. The mechanisms and driving forces of discharge are still unknown. We found nematocysts of various cnidarians to be acidic inside and propose that the pH difference between cyst matrix and cytoplasm drives the discharge of cysts. For large cysts of Aiptasia we calculated that the internal concentration of protons and protonated carboxyl groups is about 5 M. Cysts contain polyacids in a high concentration. At a low pH several of the carboxyl groups of the polymer are uncharged. The carboxyl groups dissociate when, on triggering, the proton concentration becomes balanced across the cyst membrane. The speed of protons in water is extremely rapid. Thus, the equilibration of the proton concentration initially results in a negative net charge within the cyst and therefore in a sudden electrostatic repulsion between the dissociated carboxyl groups of the polymer. This causes an increase in the pressure of the matrix against the cyst wall. We suggest that this nonosmotic pressure increase causes the first and extremely rapid step of discharge. We propose that in a second step cations and water are taken up, generating an increase in osmotic pressure. A change in the pH value may also facilitate the invagination and evagination, respectively, of the tubule.

Surface treatment and characterization of functionalized latex particles and inorganic pigment particles used in the study of film formation from pigmented latex systems

AbstractThe adsorption of sodium polyacrylate [NaPA] on noncarboxylated and carboxylated poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) [P(BMA/BA)] latexes and ground calcium carbonate (GCC) was studied. The adsorption isotherms of NaPA on P(BMA/BA) latex surfaces showed that NaPA tended to adsorb to a greater extent onto the latex particle surfaces when the carboxyl group surface coverage of latex polymer particles is low, which indicates a repulsive interaction between the dissociated carboxyl groups and NaPA macroions. The electrophoretic mobility of cleaned model P(BMA/BA) latexes decreased with the increasing carboxyl group surface coverage at pH 10 due to the alkali‐swelling characteristics of carboxylated latexes. For GCC, used as extender pigment particles in the pigmented latex blend systems, the size of the GCC pigment particles stabilized with NaPA decreased during a sonification process and their ζ‐potential became increasingly negative with the addition of NaPA to the GCC pigment slurry. Particle size and ζ‐potential measurements showed that NaPA can stabilize GCC particles effectively, and the optimum concentration of NaPA to stabilize GCC is around 1 wt % based on solid GCC. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 398–404, 2006

The influence of pH and solvent polarity on the mechanism and efficiency of folic acid extraction with Amberlite LA-2

Studies on the reactive extraction of folic acid with Amberlite LA-2 in three solvents of different polarity (n-heptane, chloroform, dichloromethane), at various extractant concentrations and pH-values of the aqueous solutions, indicated that the pH-value controls the extraction mechanism through the dissociation of carboxylic groups of folic acid. Thus, the solute and the extractant participate at the interfacial reaction at a molar ratio of 1:2 for pH below pK1, and 1:1 for pK1 < pH < pK2. The solvent polarity and pH-value exhibit a significant effect both on the extraction degree of folic acid (for an Amberlite LA-2 concentration of 80 g/l the extraction degree in dichloromethane was about 2-2.7 times greater than that corresponding to extraction in n-heptane, this difference increased with increasing pH of the aqueous solution) and on the extraction constant (by increasing the pH-value from 3 to 5.2, the extraction constants obtained for dichloromethane increased about 2.7 to 27.1 times compared with those obtained for extraction in n-heptane).

Synthesis, Structure, and Molecular Dynamics of Gallium Complexes of Schizokinen and the Amphiphilic Siderophore Acinetoferrin

A new general synthesis of the citrate-based siderophores acinetoferrin (Af) and schizokinen (Sz) and their analogues is described. The molecular structure of gallium schizokinen, GaSz, was determined by combined (1)H NMR, Hartree-Fock ab initio calculations, DFT, and empirical modeling of vicinal proton NMR spin-spin couplings. The metal-coordination geometry of GaSz was determined from NOE contacts to be cis-cis with respect to the two chelating hydroxamates. One diaminopropane adopts a single chairlike conformation while another is a mixture of two ring pucker arrangements. Both amide hydrogens are internally hydrogen bonded to metal-ligating oxygen atoms. The acyl methyl groups are directed away from each other with an average planar angle of ca. 130 degrees. The kinetics of GaSz racemization were followed by selective, double spin-echo inversion-recovery (1)H NMR spectroscopy over the temperature range of 10-45 degrees C. The racemization proceeds by a multistep mechanism that is proton independent between pD 5 and 12 (k(0) = 1.47 (0.15 s(-1))) and acid catalyzed below pD 4 (k(1) = 2.25 (0.15) x 10(4) M(-1) s(-1)). The activation parameters found for the two sequential steps of the proton independent pathway were DeltaH(++) = 25 +/- 3 kcal M(-1), DeltaS(++) = 25 +/- 7 cal M(-1) K(-1) and DeltaH(++) = 17.1 +/- 0.2 kcal M(-1), DeltaS(++) = 0.3 +/- 2.7 cal M(-1) K(-1). The first step of the proton-independent mechanism was assigned to the dissociation of the carboxyl group. The second step was assigned to complex racemization. The proton-assisted step was assigned to a complete dissociation of the alpha-hydroxy carboxyl group at pD < 4. The ab initio modeling of gallium acinetoferrin, GaAf, and analogues derived from the structure of GaSz has shown that the pendant trans-octenoyl fragments are oriented in opposite directions with the average planar angle of ca. 130 degrees. This arrangement prevents GaAf from adopting a phospholipid-like structural motif. Significantly, iron siderophore complex FeAf was found to be disruptive to phospholipid vesicles and is considerably more hydrophilic than Af, with an eight-fold smaller partition coefficient.

Incorporation of new carboxylate functionalized co-monomers to temperature-responsive polymer-grafted cell culture surfaces

Several cultured cell types are easily detached from poly( N-isopropylacrylamide) (PIPAAm)-grafted surfaces only by reducing culture temperature without traditional proteolytic treatments that might damage certain cell functions. We have exploited these novel surfaces for tissue engineering applications where harvested intact cell sheets are useful for fabricating tissue-like constructs. We now extend the polymer chemistry of such grafted surfaces with new charged co-monomers. Functional carboxylate groups are incorporated into temperature-responsive surfaces with newly designed analogous carboxylate co-monomers, 2-carboxyisopropylacrylamide (CIPAAm) and 3-carboxy- n-propylacrylamide (CNPAAm), which have a small structural difference in the placement of the carboxylate group ( iso or normal to the monomer propyl group). P(IPAAm- co-CIPAAm) demonstrates a similar phase transition behavior to that of pure PIPAAm, and the ionic dissociation of carboxyl groups is suppressed (elevated p K′a) even under physiological conditions. By contrast, P(IPAAm- co-CNPAAm) exhibits a higher charge density (lower p K′a), higher hydration, and reduced temperature-sensitivity under identical conditions. Introduction of 5 mol% CNPAAm into PIPAAm grafted surfaces produces no cell attachment under typical cell culture conditions, while identical introductions of CIPAAm into grafted copolymers functions well for cell attachment. Cultured cell spreading efficiency was essentially similar on PIPAAm-grafted surfaces as on copolymer-grafted surfaces with 1 mol% introduction of either carboxylate co-monomer. Accelerated cell detachment upon reducing culture temperature was observed for the 1 mol% these copolymer-grafted surfaces since polymer hydration and swelling kinetics are enhanced by the increased ionizable moiety in these grafted surfaces.

Effects of continuous water flow on the swelling properties of polyelectrolyte hydrogels.

We report the swelling behavior of a polyelectrolyte copolymer gel in water, which consists of poly(N-isopropylacrylamide) and sodium acrylate. The diameter of the cylindrical gels was measured at room temperature under a continuous flow of solvent water (equivalent to an infinite amount of water; open system). After a sufficient water flow, the diameter of the gel in the limited amount of water (closed system) was measured as a function of the temperature. The gel in the open system was found to shrink as a result of the continuous flow of water, and the gel in the closed system began to swell again at the phase transition point by increasing the temperature. The effects of the degree of initial ion exchange by the water flow on the macroscopic swelling behavior were discussed in terms of the exchange of counterions (the ion dissociation of carboxyl groups) and of the creation and destruction of intermolecular forces (hydrogen bonding). It was concluded that the amount of solvent water determines the environmental variables, such as the pH and ion concentration, which affects the swelling properties of polyelectrolyte hydrogels; intermediate re-swollen states can be observed in a closed system.

Uptake of sodium chloride by mixture of weakly acidic and weakly basic ion exchange resins: equilibrium and kinetic studies

The equilibrium and kinetic behaviour of weakly acidic resin (WAR) with alkali and weakly basic resin (WBR) with acids have been studied by many research teams. But little work is reported in literature regarding equilibrium and kinetic behaviour of mixture of WAR and WBR. In the present study, detailed analysis of equilibrium and kinetic behaviour of mixed resins with sodium chloride has been undertaken. From equilibrium studies, no adsorption is observed taking place, when salt solution is brought in contact with WAR or WBR individually; however adsorption is observed taking place, when salt solution is brought in contact with the mixture of WAR and WBR. The salt uptake depends on the concentration of salt solution, the amounts of individual ion-exchange resins as well as their proportion. Mixture of resins and salt solution has three phases interacting with each other and a number of resistances can influence salt uptake rate either individually or in combination. A series of possible resistances to the salt uptake are identified, namely, diffusion across films surrounding the resin particles, diffusion inside the resin particles, and dissociation of carboxylic group (–COOH) in WAR. To minimize the mathematical complexity, limiting case approach is adopted in this study. The underlying assumption in this approach is that resistance to the entire process lies in that limiting step. It is finally concluded that generation of H + ions (by interaction of WAR particles and salt solution) followed by adsorption of acid by WBR governs the salt uptake process in mixed resin systems.

Hydrogen-bonding-driven spontaneous gelation of water-soluble phospholipid polymers in aqueous medium

It has been found that mixing of two kinds of water-soluble phospholipids polymers, such as poly(2-methacryloyloxyethyl phosphorylcholine-co-methacrylic acid) (PMA) and poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) (PMB), spontaneously forms a hydrogel in aqueous medium at room temperature without any chemical treatment. However, the mechanism of spontaneous gelation has not been clarified yet. The purpose of this study is to investigate the gelation mechanism of the hydrogel. Moreover, effects of ions on gelation and dissolution behavior were observed. We investigated the mechanism of the hydrogel formation by spectroscopic techniques and a rheological method with attention to the interactions between polymer chains. Both Raman spectroscopic analysis and FT-IR analysis revealed that carboxyl groups in methacrylic acid (MA) formed dimer when two polymer solutions were mixed, and the results of the rheological study showed dissociation of carboxyl groups caused dissolution of the hydrogel. Thus, the gelation occurred due to the formation of dimers by hydrogen bonding which acts as a physical cross-linking of polymer chains. The hydrogel dissolved in a large amount of aqueous medium. We also observed the addition of inorganic salts during the preparation of the hydrogel affected the gelation and dissolution behaviors by a rheological and a weight measuring method, respectively. The gelation period became longer in the presence of NaCl and CaCl2 compared with that in the absence of these salts. NaCl and CaCl2 disturbed the formation of hydrogen bonding between carboxyl groups by stabilization of carboxylate anion of the MA units. On the other hand, addition of FeCl3 made the gelation period shorter and stabilized the hydrogel in the aqueous medium. This is because FeCl3 can suppress dissociation of the carboxyl groups by acidic condition of FeCl3 aqueous solution and cross-link the carboxylate anions in the PMA effectively.

Acute toxicity of benzene derivatives to the tadpoles ( Rana japonica) and QSAR analyses

Acute lethal toxicity (the negative logarithm of molar concentrations of 12 h acute median lethal, expressed as 12 h-log 1/LC 50) of 46 benzene derivatives to Rana japonica tadpoles was determined. 1-octanol/water partition coefficient (log K ow)-dependent models were developed to study the toxicity of different categories chemicals. In an effort to model all chemicals, response surface analyses and stepwise multiple regression analyses were performed and successful models were obtained. A general and robust QSAR model was achieved with the combined application of variables reflecting hydrophobicity, electric property, and molecular size respectively ( 12 h-log 1/ LC 50=0.393 logK ow −0.428E lumo +0.0110 Vol.+1.362 n=51, r 2=0.834) using stepwise multiple regression analyses. Because of strong dissociation of carboxyl group greatly decreasing their observed toxicity, using log D ow in instead of log K ow the quality of the models is greatly improved. The conventional r 2 and cross-validation r 2 CV were 0.914 and 0.785, respectively, indicating that QSAR was both internally consistent and highly predictive.

Peculiar change in membrane potential of taste sensor caused by umami substances

Monosodium glutamate (MSG) induces a favorable taste, termed umami. The action of MSG on lipid membranes of a taste sensor was investigated. In general, food constituents monotonously increase the electric potential of the negatively charged membranes. Contrary to this, MSG decreases the potential at lower concentration and increases it at higher concentration, thus the effect of MSG on lipid membrane is biphasic. On the decrease of the membrane potential, a model was proposed in which the dissociated carboxyl group of MSG accelerates the dissociation of phosphate group of the membrane lipid.

Modeling of metal-humate complexation based on the mean molecular weight and charge of humic substances: Application to Eu(III) humate complexes using ion exchange

A general model, the so called Mean Molecular Weight Model (MMWM), of complexation of metal cations (Mez+) with macromolecular polyanions of humic acid (HAp-) is proposed. The model is based on the results of previous studies of the electrophoretic mobility of humate complexes and assumes that the complexation proceeds by consecutive neutralization of the dissociated carboxyl groups of the central polyanion HAp- with Mez+ cations. It reflects the macromolecular character of humic acid, applies molar concentrations of reacting components with equations for stability constants and incorporates also the mean charge of humic macromolecules. The model has been verified with experimental data obtained in the study of complexation of Eu(III) with Aldrich humic acid using ion exchange (Amberlite IR-120), over a broad range of [Eu] to [HA] ratio, at pH 4 and 7.