Studies on intrinsic ionization constants of Fe-Al-Mg hydrotalcite-like compounds

The theoretical analysis of the intrinsic ionization constant (K(a2)(int)) of Fe-Al-Mg hydrotalcite-like compounds (HTlc) possessing permanent charges was first performed using the double extrapolation method proposed by James et al. The theoretical permanent charge density (sigma(p,T)) of the HTlc sample was calculated from the crystal structure of HTlc, and the influence of sigma(p,T) on the K(a2)(int) was also examined. From the experimental results, these conclusions can be obtained: the zero point of the charge (pH(ZPC)) of Fe-Al-Mg HTlc increases with decreased Fe3+ content and increases with increased Mg2+ in the HTlc. The pK(a2)(int) of Fe-Al-Mg HTlc also increases with decreased Fe3+ and increased Mg2+ content in the sample; furthermore, the pK(a2)(int) of Fe-Al-Mg HTlc increases with decreased sigma(p,T).     

Intrinsic Surface Reaction Constant in 1-pK Model of Mg-Fe Hydrotalcite-like Compounds

The relationship among intrinsic surface reaction constant (K) in 1-pK model, point of zero net charge (PZNC) and structural charge density (σst) for amphoteric solid with structural charges was established in order to investigate the effect of σst on pK. The theoretical analysis based on 1-pK model indicates that the independent PZNC of electrolyte concentration (c) exists for amphoteric solid with structural charges. A common intersection point (CIP) should appear on the acid-base titration curves at different c, and the pH at the CIP is pHPZNC. The pK can be expressed as pK=-pHPZNC log[(1 2αPZNC)/(1-2αPZNC)], where αPZNC≡σst/eNANs, in which e is the elementary charge, NA the Avogadro‘s constant and Ns the total density of surface sites. For solids without structural charges, pK=-pHPZNC. The pK values of hydrotalcite-like compounds (HTlc) with general formula of [Mg1-xFex(OH)2](Cl,OH)x were evaluated. With increasing x, the pK increases, which can be explained based on the affinity of metal cations for H^- or OH^- and the electrostatic interaction between charging surface and H^- or OH^-.     

The Isoelectric Point and the Points of Zero Charge of Fe-Al-Mg Hydrotalcite-like Compounds

The relation of the isoelectric point (IEP) and the point of zero net charge (PZNC) of the hydrotalcite-like compounds was discussed. It was found that the IEP does not equal to the PZNC and the IEP is higher than the PZNC. The structural positive charges existing in the HTlc,which cause the difference between the IEP and the PZNC. The effects of the structural positive charges of the HTlc on its IEP and PZNC are the same as the specific adsorption of metal cations.     

Modeling Pb(II) adsorption onto sandy loam soil

The adsorption of Pb(II) onto hydrous sandy loam soil was investigated with batch equilibrium adsorption experiments. Results show that the amount of Pb(II) adsorbed increases with increasing pH and surface loading. It was demonstrated that the surface acidity of the soil could be determined using electrophoretic mobility measurements. The surface acidity constants, pK(a1)(int) and pK(a2)(int), were 1.57 and 3.43, respectively. A surface complex formation model (SCFM) was employed to describe the adsorption. The intrinsic stability constants, pK(i)(s), for the surface reaction between the Pb species and the ionized soil surface hydroxyl groups were determined from SCFM fitting. The adsorption free energy of Pb2+ and Pb(OH)+ ions ranges from -5.74 to -6.48 kcal/mol and from -9.68 to -10.00 kcal/mol, respectively, for surface loadings between 1.21 x 10(-5) and 2.41 x 10(-4) mol/g. The adsorption binding calculation indicated that the specific chemical interaction is the major mechanism responsible for the adsorption process.     

Zn-Al类水滑石结构正电荷对内禀电离平衡常数的影响

采用电势滴定(potentiometric titration,PT)法测定了Zn-Al类水滑石(HTlc)的零净电荷点(pHPZNC);利用电势滴定数据直接计算得到Zn-Al HTlc的内禀电离平衡常数(pKa2int)和质子吸附自由能(G0ads,2);研究了结构电荷密度(σp)对pKa2int 和G0ads,2的影响.结果表明,随σp增加, pKa2int 和G0ads,2数值均降低,说明σp越大,带正电荷的HTlc与H＋结合力越低,HTlc去质子能力越强,H＋游离出HTlc表面的趋势越大.研究发现，HTlc的pKa2int与pHPZNC之间符合关系式: pKa2int =1.372pHPZNC－3.328.     

Acidity and alkali metal adsorption on the SiO2-aqueous solution interface

The intrinsic deprotonation constant (pK(a(2))(int)) and the intrinsic ion exchange constants (pK(Me(+))(int)) of Li(+), Na(+), and K(+) on SiO(2) were uniquely determined at 30 degrees C by using the potentiometric titration data, the Gouy-Chapman-Stern-Grahame (GSCG) model for the structure of the electrical double-layer (edl) and the double-extrapolation method. The values of these constants were pK(a(2))(int) = 6.57, pK(Li(+))(int) = pK(Na(+))(int) = pK(K(+))(int) = 5.61. The chemical meaning of intrinsic equilibrium constants and the equality in the values of pK(Li(+))(int), pK(Na(+))(int) and pK(K(+))(int) were discussed.     

Study of valsartan interaction with micelles as a model system for biomembranes

In this study, the interaction of valsartan (VAL), an angiotensin II receptor antagonist, with cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated. The effect of cationic micelles on spectroscopic and acid-base properties of VAL was carried out using UV spectrophotometry at physiological conditions (pH 7.4). The binding of VAL to CTAB micelles implied a shift in drug acidity constant (pK(a)(water)-pK(a)(micelle)=1.69) proving the great affinity of VAL dianion for the positively charged CTAB micelle surface. To quantify the degree of VAL/CTAB interaction, two constants were calculated by using mathematical models: micelle/water partition coefficient (K(x)) and drug/micelle binding constant (K(b)). The decrease of K(x) with VAL concentration, obtained by using pseudo-phase model, is consistent with an adsorption-like phenomenon. From the dependence of differential absorbance at lambda=295 nm on CTAB concentration, by using mathematical model that treats the solubilization of VAL dianion as its binding to specific sites in the micelles (Langmuir adsorption isotherm), the binding constant (K(b)=(2.50+/-0.49)x10(4)M(-1)) was obtained. Binding constant VAL/CTAB was also calculated using micellar liquid chromatography (MLC).     

Kinetic and equilibria studies of the aquation of the trinuclear platinum phase II anticancer agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464)

The hydrolysis profile of the bifunctional trinuclear phase II clinical agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464, 1) has been examined using [(1)H,(15)N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy. Reported are estimates of the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pK(a1) approximately equal to pK(a2) approximately equal to pK(a3)). The equilibrium constants for the aquation determined by NMR at 298 and 310 K (I = 0.1 M, pH 5.3) are similar, pK(1) = pK(2) = 3.35 +/- 0.04 and 3.42 +/- 0.04, respectively. At lower ionic strength (I = 0.015 M, pH 5.3) the values at 288, 293, and 298 K are pK(1) = pK(2) = 3.63 +/- 0.05. This indicates that the equilibrium is not strongly ionic strength or temperature dependent. The aquation and anation rate constants for the two-step aquation model at 298 K in 0.1 M NaClO(4) (pH 5.3) are k(1) = (7.1 +/- 0.2) x 10(-5) s(-1), k(-1) = 0.158 +/- 0.013 M(-1) s(-1), k(2) = (7.1 +/- 1.5) x 10(-5) s(-1), and k(-2) = 0.16 +/- 0.05 M(-1) s(-1). The rate constants in both directions increase 2-fold with an increase in temperature of 5 K, and rate constants increase with a decrease in solution ionic strength. A pK(a) value of 5.62 plus minus 0.04 was determined for the diaqua species [(trans-Pt(NH(3))(2)(OH(2)))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)-NH(2))(2))](6+) (3). The speciation profile of 1 under physiological conditions is explored and suggests that the dichloro form predominates. The aquation of 1 in 15 mM phosphate was also examined. No slowing of the initial aquation was observed, but reversible reaction between aquated species and phosphate does occur.     

Determination of intrinsic bacterial surface acidity constants using a donnan shell model and a continuous pK(a) distribution method

Intrinsic acidity constants (pK(a)(int)) for Bacillus subtilis (Gram+) and Escherichia coli (Gram-) cells were calculated from potentiometric titration data at different salt concentrations. Master curves were generated by replotting charge excess data as a function of pH(S) (pH at the location of surface reactive sites) where pH(S) was determined as a function of Donnan potential, Psi(DON). This potential decreased in magnitude with increasing ionic strength, from -48.5+/-0.2 to -3.5+/-0.0 mV for B. subtilis and -47.9+/-0.3 to -3.5+/-0.0 mV for E. coli at 0.01 and 0.5 M K(+), respectively, indicating an efficient surface charge neutralization by counterions. A fully optimized continuous (FOCUS) pK(a) distribution method revealed four binding sites on B. subtilis and E. coli surfaces from the master curves with pK(a)(int) values of 3.59+/-0.38, 4.33+/-0.57, 5.94+/-0.66, and 8.64+/-0.57 for B. subtilis and 3.73+/-0.44, 4.85+/-0.71, 6.56+/-0.64, and 8.79+/-0.62 for E. coli. These were assigned to functional groups according to reported pK(a) ranges of 2.0-6.0 (carboxylic acid), 3.2-3.5 (phosphodiesters), 5.6-7.2 (phosphoric acid), and 9.0-11.0 (amine groups). Average points of zero salt effect (pH(pzse)) for B. subtilis experiments were 6.63+/-0.21 and 6.42+/-0.08 as a function of pH(bulk) and pH(S), respectively. Under the same criteria, E. coli calculations yielded 5.73+/-0.23 and 5.45+/-0.05. An understanding of metal and proton reactivity on bacterial cell surfaces can be addressed quantitatively through the use of electrostatic and chemical equilibrium modeling techniques proposed in this study. The results are consistent with those of electrical force microscopy studies used to document the intrinsic electrochemical heterogeneity of bacterial cell surfaces.     

Heme-Peptide Models for Hemoproteins. 1. Solution Chemistry of N-Acetylmicroperoxidase-8

An improved method for the preparation of the heme octapeptide acetyl-MP8, obtained by proteolysis of horse heart cytochrome c, is described. AcMP8 obeys Beer's law at pH 7.0 in aqueous solution up to a concentration of 3 x 10(-)(5) M. The self-association constant measured at 25 degrees C (log K(D) = 4.04) is an order of magnitude lower than that for MP8, reflecting the role of the N-acetyl protecting group in abolishing intermolecular coordination. However, AcMP8 does form pi-stacked dimers in aqueous solution with increasing ionic strength. A more weakly packed pi-pi dimer reaches a maximum abundance at approximately 3 M ionic strength, but a more tightly packed dimer is favored at &mgr; > 3 M. An equilibrium model based on charge neutralization by specific binding of Na(+) ions gives a total molecular charge of 3- for AcMP8 at pH 7.0 and a self-association constant log K(D) = 4.20. AcMP8 exhibits six spectroscopically active pH-dependent transitions. The Glu-21 c-terminal carboxylate binds to the heme iron at low pH (pK(a) = 2.1) but is substituted by His-18 (pK(a) = 3.12) as the pH increases. The two heme propanoic acid substituents ionize with pK(a)'s of 4.95 and 6.1. This is followed by ionization of iron-bound water with a pK(a) = 9.59, DeltaH = 48 +/- 1 kJ mol(-)(1), and DeltaS = -22 +/- 3 J K(-)(1) mol(-)(1). The electronic spectra indicate that AcMP8 is predominantly in the S = (5)/(2) state at pH 7.0, while the hydroxo complex at pH 10.5 corresponds to an equilibrium mixture of S = (5)/(2) and S = (1)/(2) states at 25 degrees C. In the final transition, His-18 ionizes to form the S = (1)/(2) histidinate complex with a pK(a) of 12.71. AcMP8 is relatively stable under alkaline conditions, dimerizing slowly at high pH (k = 2.59 +/- 0.14 M(-)(1) s(-)(1)) to form a high-spin &mgr;-oxo-bridged species. The pH-dependent behavior of AcMP8 in the presence of excess 3-cyanopyridine, however, is markedly different. At low pH, AcMP8 simultaneously binds the exogenous ligand and the Glu-21 c-terminal carboxylate with a pK(a) < 2. His-18 replaces the carboxylate ligand at higher pH (pK(a) = 2.60), and both heme propanoic acid groups ionize with a mean pK(a) = 5.10. Unlike AcMP8.OH(-), the axial histidine of the 3-CNPy complex ionizes at near neutral pH (pK(a) = 7.83), prior to being replaced by OH(-) (pK(a) = 10.13). The sixth transition in the AcMP8/3-CNPy system produces the bis(hydroxo) complex (pK(a) > 13).     

Studies on zero point of charge and intrinsic ionization constant of Zn−Mg−Al hydrotalcite-like compounds

The zero point of charge (ZPC) and the intrinsic ionization constant ( % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaadaaakeaacqWG % lbWsdaqhaaWcbaGaeeyyaegabaGaeeyAaKMaeeOBa4MaeeiDaqhaaa % aa!304C! K_a^int K_{\rm a}^{{\rm int}} ) of Zn-Mg-Al hydrotalcite-like compounds (HTlc) with the general formula [(Zn,Mg) _{1- x} Al _x (OH) ₂] ^{x +} [(OH,Cl) _x] ^{x -} were determined by potentiometric titration (PT). The variation of the ZPC and % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaadaaakeaacqWG % lbWsdaqhaaWcbaGaeeyyaegabaGaeeyAaKMaeeOBa4MaeeiDaqhaaa % aa!304C! K_a^int K_{\rm a}^{{\rm int}} with xwas investigated. For the colloidal particles possessing permanent charges, the ZPC determined by the PT method is the zero point of net charge (ZPNC). The ZPNC ( pH _ZPNC) values were 9.63 {[Zn _0.27Mg _0.36Al _0.37 (OH) ₂]Cl _0.13 (OH) _0.24 }, 9.68 {[Zn _0.13Mg _0.58Al _0.29 (OH) ₂]Cl _0.12 (OH) _0.17 }, 9.67 {[Zn _0.17Mg _0.54Al _0.29 (OH) ₂]Cl _0.16 (OH) _0.12 },10.16 {[Zn _0.08Mg _0.67Al _0.25 (OH) ₂]Cl _0.17 (OH) _0.08}, 10.33 {[Zn _0.16Mg _0.60Al _0.24 (OH) ₂] Cl _0.16 (OH) _0.08} and 10.60 {[Zn _0.19Mg _0.60Al _0.21 (OH) ₂] Cl _0.15 (OH) _0.06}; the intrinsic ionization constants % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaadaaakeaacqqG % WbaCcqWGlbWsdaqhaaWcbaGaeeyyaeMaeeOmaidabaGaeeyAaKMaee % OBa4MaeeiDaqhaaaaa!329E! pK_a2^int {\rm p}K_{{\rm a2}}^{{\rm int}} of the same HTlc samples were 10.31, 10.44, 10.44, 11.02, 11.19 and 11.54. With decreasing x, ZPNC and % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaadaaakeaacqWG % lbWsdaqhaaWcbaGaeeyyaeMaeeOmaidabaGaeeyAaKMaeeOBa4Maee % iDaqhaaaaa!3137! K_a2^int K_{{\rm a2}}^{{\rm int}} of HTlc increased, and the acidity of the HTlc samples decreased.     

Acid-Base Properties of the Ground and Excited States of Ruthenium(II) Bis(2,2'-bipyridine) 3-Carboxyl-2,2'-bipyridine

Acid-base properties for ruthenium(II) bis(2,2'-bipyridine) 3-carboxyl-2,2'-bipyridine reveal a ground state pK(a) of 0.82 +/- 0.07 and an excited state pK(a) of 2.31 +/- 0.05, a 1.5 pH unit increase from the ground state. The excited state pK(a) is temperature independent while the ground state pK(a)(0) increases with temperature and has DeltaH(0) and DeltaS(0) values of -990 +/- 149 cm(-)(1) and -4.57 +/- 0.48 cm(-)(1) K(-)(1), respectively. The acidic form of the complex emits at lower energy than the basic form at both 296 and 77 K. The emission energy maxima are solvent dependent and decrease in energy when the solvent changes from 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) to water and when the pH decreases. Changes in excited state lifetimes with emission energy follow the energy gap law with an intercept of 49 +/- 1 and a slope of (2.11 +/- 0.09) x 10(-)(3). Emission quantum yields for protonated and deprotonated species in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) are 0.023 +/- 0.001 and 0.110 +/- 0.002, respectively. The temperature dependence of the emission lifetimes gives energy barriers of 270 cm(-)(1) for the complex in aqueous solution at pH -0.5, and 990 cm(-)(1) in aqueous solution at pH 4.5, and 1920 cm(-)(1) in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2.)     

On derivatives of surface charge curves of minerals

Surface titrations of minerals in aqueous electrolyte solutions are used as building blocks for surface complexation modelling. However, these potentiometric data may contain less model relevant information than previously and presently assumed. In the literature, derivative analyses have been applied to experimental surface charge versus pH curves and four or more pK values were extracted for goethite or aluminium oxide. Derivative analysis of specific surface charge versus pH curves calculated for various published model variants for goethite shows that not more than the net-zero proton surface charge condition can be extracted from computer generated data. Generated data can be produced in density and precision superior to experimental data, but yield only relatively little output from such derivative analysis compared to what has previously been extracted from derivatives of experimental data. For the generated goethite data and for all model variants only the point of zero could be extracted. For the various goethite model variants tested a nearly symmetrical peak appeared at the point of zero charge in the derivative curve. A different pattern could be obtained for generic models, for which two sites with unequal sites densities and different pK values were assumed. Variation of these parameters could result in derivatives of the charging curves with two maxima or one maximum and a broad tailing. In the literature, curves with features nearly identical to these generated curves have been interpreted by up to four pK values (i.e., four different sites within a 1-pK model). It is concluded that the interpretation of the generated data is in all cases hampered by the overwhelming electrostatic contributions to the free energy of proton ad/desorption. In no case except for the one-site 1-pK model was it possible to extract the input pK value(s) from the derivatives. Plausible explanations for the discrepancy between generated data and published experimental data are discussed.     

Surface-charging behavior of Zn-Cr layered double hydroxide

A Zn-Cr layered double hydroxide (LDH) having the formula Zn(2)Cr(OH)(6)Cl(0.7)(CO(3))(0.15)2.1H(2)O was synthesized and characterized by powder X-ray diffraction, infrared spectroscopy, acid-base potentiometric titration, mass titration, electrophoretic mobility, and modeling of the electrical double layer. Adsorption of alizarin was also performed in order to show some particular features of the HDL. Net hydroxyl adsorption, which increases with increasing pH and decreasing supporting electrolyte concentration, takes place above pH 5. The electrophoretic mobility of the particles was always positive and it decreased when the pH was higher than 9. An isoelectric point of 12 could be estimated by extrapolating the data. The modified MUSIC model was used to estimate deprotonation constants of surface groups and different adsorption models were compared. Good fit of hydroxyl adsorption and electrophoresis could be achieved by considering both OH(-)/Cl(-) exchange at structural sites and proton desorption from surface hydroxyl groups. The modeling, in agreement with alizarin adsorption, indicates that most of the structural positive charge of the LDH is screened at the surface by exchanged anions and negatively charged surface groups. It also suggests that only structural charge sites initially neutralized by chloride ions are active for anion exchange. The remaining sites are blocked by carbonate and do not participate in the exchange.     

Pulse radiolysis study on free radical scavenger edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one)

The reactions between edaravone and various one-electron oxidants such as (*)OH, N(3)(*), Br(2)(-), and SO(4)(-), have been studied by pulse radiolysis techniques. The transient species produced by the reaction of edaravone with (*)OH radical shows an absorption band with lambda(max)=320 nm, while the oxidation by N(3)(*), Br(2)(-), SO(4)(-) and CCl(3)OO(*) results in an absorption band with lambda(max)=345 nm. Different from the previous reports, the main transient species by the reaction of edaravone with (*)OH radical in the absence of O(2) is attributed to OH-adducts. At neutral condition (pH 7), the rate constants of edaravone reacting with (*)OH, N(3)(*), SO(4)(-), CCl(3)OO(*), and e(aq)(-) are estimated to be 8.5x10(9), 5.8x10(9), 6x10(8), 5.0x10(8) and 2.4x10(9)dm(3)mol(-1)s(-1), respectively. From the pH dependence on the formation of electron adducts and on the rate constant of edaravone with hydrated electron, the pK(a) of edaravone is estimated to be 6.9+/-0.1.     

Formal Mathematical Analysis of the Existence of the Common Intersection Point in Relation to Determining the Parameters Describing Ion Adsorption at the Oxide/Electrolyte Interface: Comparison of the Triple and Four-Layer Models

For most oxide/electrolyte systems potentiometric titration curves measured for different ionic strengths have a Common Intersection Point (CIP) which corresponds to the Point of Zero Charge (PZC). However, there are systems where a CIP exists but the surface charge at this point does not equal zero (PZC CIP). In this paper theoretical analysis of the systems in which the PZC and CIP do not coincide is presented. It is based on the well-known 2-pK surface charging approach and Triple Layer Model (TLM) as well as the Four Layer Model (FLM) of the electric double layer. The appropriate mathematical criterion for CIP existence was applied with detailed derivations, both for TLM and FLM. Having determined in this manner the parameter values, one can draw proper conclusions about the features of oxide/electrolyte adsorption systems, in which PZC and CIP do not coincide. The values of adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both of the experimental titration isotherms, and to the individual isotherms of electrolyte cation adsorption measured using radiometric methods.     

1pK and 2pK protonation models in the theoretical description of simple ion adsorption at the oxide/electrolyte interface: the analysis of temperature dependence of potentiometric titration curves

Two models of oxide surface charging (1pK and 2pK) were used to describe the potentiometric titration curves measured by Blesa et al. (J. Colloid Interface Sci.101, 410 (1984)) at three temperatures and at three concentrations of electrolyte. Rudzinski et al. (Langmuir13, 483 (1997)) have applied the 2pK Triple Layer Model to analyze the above system earlier. Two calculation procedures based on the 1pK Basic Stern Model were developed to described Blesa's data. Since the experimental curves have the CIP (common intersection point) it was assumed that heat of electrolyte ion adsorption was equal to zero. We assumed two different values of the double layer innermost capacitance on both sides of PZC (point of zero charge), which was followed from asymmetry of surface charge curves relative to PZC. Moreover, it was necessary to take into consideration the dependence of capacitance on ionic strength and temperature. The quality of fit given by two models was comparable. Since the 1pK BSM is simpler than the 2pK TLM and includes not so many best-fit parameters it seems to be a better choice in this case. Discussion of the results obtained by other authors concerning the subject under consideration is also presented.     

Interaction of quinapril anion with cationic surfactant micelles of cetyltrimethylammonium bromide

In this study, the interaction of the anion of quinapril (QUIN), angiotensin converting enzyme (ACE) inhibitor, with cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated. The effect of cationic micelles on the spectroscopic and acid-base properties of QUIN was studied at pH 8. The binding of QUIN anion to CTAB micelles implied a shift in drug acidity constant (pK(a)(water)-pK(a)(micelle)=1.39) proving the great affinity of negatively charged QUIN ion for the positively charged CTAB micelle surface. The strong dependence of the partition coefficient K(x) on QUIN concentration, obtained by using pseudo-phase model, is consistent with an adsorption-like phenomenon. From the dependence of differential absorbance at lambda=272 nm on CTAB concentration, by using mathematical model that treats the solubilization of QUIN anion as its binding to specific sites in the micelles (Langmuir adsorption isotherm), the binding constant K(b)=(2.3+/-0.4)x10(3) mol(-1)dm(3) was obtained. QUIN-CTAB binding constant was also calculated from micellar liquid chromatography (MLC) and this method was found to be not accurate enough for its determination.     

Acid-base chemistry of montmorillonitic and beidellitic-montmorillonitic smectite

Suspension of a Tunisian purified smectite and American montmorillonite are studied by acid-base potentiometric and mass titrations. These experimental methods are used to determine the point of zero net proton charge (PZNPC). A very good agreement is observed between the two kinds of experiments. The two Namontmorillonites, studied at different ionic strengths, present proton adsorption vs. pH curves with a common crossing point. The PZNPC of the edge sites are 8.02 for Tunisian purified smectite and 8.11 for pure American montmorillonite. By analyzing the proton adsorption or desorption (H⁺ vs. pH) curves, one may assume the presence of four active sites at the surface. The montmorillonite surface undergoes two successive protonations and two successive deprotonations. Below pH < PZNPC and in acidic range, the cation exchange at layer sites and protonation of edge sites (>A1OH groups) occur simultaneously. For pH > PZNPC and in alkaline pH range, deprotonation of surface hydroxyl groups exposed at the edge sites (>SiOH, and >A1OH at high pH) of montmorillonite platelets causes an overall negative charge. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 2, pp. 175–187. The text was submitted by the authors in English.     

Determination of Point of Zero Charge of Tunisian Kaolinites by Potentiometric and Mass Titration Methods

This paper deals with determining points of zero charge of natural and Na⁺‐saturated mineral kaolinites using two methods: (1) acid‐base potentiometric titration was employed to obtain the adsorption of H⁺ and OH^? on amphoteric surfaces in solutions of varying ionic strengths in order to determinate graphically the point of zero net proton charge (PZNPC) defined equally as point of zero salt effect (PZSE); (2) mass titration curve at different electrolyte concentrations in order to estimate PZNPCs by interpolation and to compare with those determined by potentiometric titrations. The two methods involved points of zero charge approximately similar for the two kaolinites between 6.5‐7.8, comparable to those reported previously and were in the range expected for these clay minerals. The comparison of potentiometric surface titration curves obtained at 25 °C and those published in the literature reveals significant discrepancies both in the shape and in the pH of PZNPCs values.