Protolytic equilibria in aqueous sodium deoxycholate solutions

Protolytic equilibria taking place in aqueous solutions of sodium deoxycholate (DCNa) have been studied at 25°C using 0.5M NaCl as ionic medium. Electromotive force measurements of a galvanic cell were carried out by means of a glass electrode.The reagent necessary to change the acidity of the solutions was produced in situ by supplying a constant small current.Solubility and acid constant of deoxycholic acid (HDC) have been determined for the chosen experimental conditions. Experimental data obtained in less acid solutions have been explained by assuming the presence of the species H(DC)₂. The relative stability constant has been determined. At higher deoxycholate concentration the presence of a polymeric micellar species has been assumed.

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Protolytische Gleichgewichte in wäßrigen Natriumdesoxycholat-Lösungen

Zusammenfassung Die Protonierung von Natrium-Desoxycholat (DCNa) in wäßrigen Lösungen mit 0.5M NaCl wurde bei 25°C mit Hilfe von E.M.K. Messungen mit einer Glaselektrode untersucht.Das notwendige Reagens für die Umwandlung der Säure in den untersuchten Lösungen wurde in situ durch einen konstanten schwachen Strom erzeugt.Löslichkeit und Dissoziationkonstante von Desoxycholsäure (HDC) wurden unter den gewählten experimentellen Bedingungen bestimmt. Die experimentellen Daten in schwach sauren Lösungen konnten mit der Annahme der Existenz von H(DC)₂ erklärt werden. Die entsprechende Konstante wurde bestimmt. Zur Erklärung der Daten in stärker konzentrierten Lösungen von Desoxycholat ist die Annahme einer polynuklearen Spezies nötig.

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Symbols H analytical excess of hydrogenions, if negative it corresponds to OH^–; - h free concentration of hydrogen ions; - A total concentration of deoxycholate; - a free concentration of deoxycholate; - K _a acid constant of deoxycholic acid (HDC) defined as follows: [HDC]K _a =ha; - _q,p stability constant of a speciesH _p (DC) _q defined as follows: [H _p (DC) _q ]= _q,p h ^p a ^q ; - C ⁰ solubility of HDC; - formation function representing the average number of H⁺ bonded to deoxycholate.     

Protolytic reactions of 3,3'-dimethylnaphthidine and 3,3'-dimethoxybenzidine

The dissociation constants of diprotonated 3,3'-dimethylnaphthidine (DMN) and 3,3'-dimethoxybenzidine (DMB) have been determined spectrophotometrically. They are: pK(a1) = 2.62 +/- 0.03, pK(a2) = 3.33 +/- 0.09 for DMN: pK(a1) = 2.83 +/- 0.07, pK(a2) = 4.05 +/- 0.12 for DMB. The molar absorptivities (l.mole(-1).cm(-1)) of all forms of the indicators have been also determined: epsilon(B) = 1.68 x 10(4), epsilon(BH(+)) = 9.34 x 10(3), epsilon(BH(2+)(2)) = 1.80 x 10(3) at 300 nm for DMB; epsilon(B) = 7.33 x 10(3), epsilon(BH(+)) = 3.73 x 10(3), epsilon(BH(2+)(2)) = 0 at 330 nm for DMN.     

Protolytic equilibria on the surface of carboxyl-containing silica

Potentiometric titration has been used to study the protolytic equilibria on the surface of carboxyl-containing silica (CS) prepared by the reaction of silica (Silokhrom S-80, S_sp = 80 m²/g) with Cl₃SiCH₂CH₂COOCH₃, followed by hydrolysis with 30% sulfuric acid. The titration curve of the vacuum-dried sample is irreversible. The titration curve of its Na⁺ form with hydrochloric acid proceeds lower than the titration curve of its H⁺ form and coincides with the titration curve of the air-dried sample (the last curve is reversible). The titration curve of CS coincides with the titration curve of butyric acid at pH < 6. At pH > 6 the titration curve of CS passes below the titration curve of butyric acid; this is due to the participation of silanol groups on the silica surface in the protolytic equilibria. The pK_a of the grafted CS groups is equal to 4.80 which is close to the pK_a value of butyric acid (4.78). A method has been proposed for the determination of the amount of weak acid groups grafted to the silica. It has been shown that in the titration of CS the equilibrium is established much faster than in the case of the unmodified silica.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 243–247, March–April, 1986.     

Protolytic equilibria of picric acid in a mixture of protophobic and protophilic polar solvents non-donors of H-bonds

A new differentiating medium for the study of prototropic equilibria is proposed: acetonitrile with a small addition of DMSO. From the dependence of the picric acid pK _a on the content of DMSO in this medium the optimum composition of the mixture was found ensuring the stability of the pK _a values with minimal addition of DSMO: 4 wt %. It was experimentally confirmed that the addition of water in an amount up to 0.1 wt % has virtually no effect on the measured pK _a value in a mixture with the optimum composition.     

NMR data on surface solvation of dispersed silica particles in frozen aqueous suspension

Liquid phase freezing-out combination with relaxation measurements was used for studying the surface solvation of dispersed silica particles. The structure of water-dimethyl sulfoxide complexes near the surface was obtained by measuring the temperature-dependent cross relaxation time of water protons.     

Protolytic equilibria in tetrazoles

Published data on protolytic equilibria involving tetrazole hetero ring are summarized. The information is systematized according to the types of protolytic equilibria.     

Spreading of a suspension drop on a horizontal surface

Experimental studies were performed on the contact line motion of a suspension of PS particles on a glass surface. The base liquids were silicone oil and glycerin. The particle size was in the range of 1-6 μm and the particle loading was 0.5-5 vol %. The drop shape was determined by using a drop image and its reflection and the drop outline was traced to the subpixel level. The Tanner-Voinov-Hoffman relation was valid for suspensions as well as for pure liquids. Silicone oil suspensions showed almost no noticeable change compared with the pure fluid. Glycerin suspensions showed an increase in contact line speed at low particle loading. The difference was due to the microstructure change at the contact line region, and the microstructure change was originated from the wetting characteristics of particles. Particle alignment occurred during the spreading stage for partially wetting particles. The contact line showed a stop-and-go fashioned motion due to surface irregularities. This result can be used as the boundary condition at the contact line in the numerical simulation of suspension spreading.     

Protolytic stability of palladocycles

Protonolysis of ortho-palladated asymmetric metallacycles

(3: Z_A = NMe₂, Z_B = PPh₂; 4: Z_A = NMe₂, Z_B = AsPh₂; 5: Z_A = AsPh₂, Z_B = PPh₂) by acetic acid leads to the departure of PhCH₂NMe₂ (in the case of 3 and 4) and PhCH₂AsPh₂ (for 5) and formation of corresponding dimeric acetato bridged complexes 6 and 7, respectively. The relative stability of the palladocycles is determined by the nature of the donor atom and decreases in the order: P > As > N.     

Protolytic properties of 2-acylthioacetamides

Concentration acidity of 2-acylthioacetamides dissolved in DMSO-H₂O was measured by the method of pH-metric titration. The data obtained were extrapolated for the determination of pK _a values of 2-acylthioacetamides in water and DMSO. We found that in aqueous solutions the acidity of thioacetarylamide (pK _a = 5.75?C6.01) was almost independent of the nature of substituents in their N-phenyl rings, whereas in DMSO solution the nature of substituents significantly affects their ionization constants (pK _a = 7.88?C11.70). The graphs were displayed and the acidity of N-aryl-3-oxobutanethioamides dependence on the Hammett constants of meta- and para-substituents in the phenyl rings derived. Water is shown to be a leveling, and DMSO a differentiating solvent for 2-acylthioacetamides.     

Photocatalysed degradation of a herbicide derivative, Dinoterb, in aqueous suspension

The photocatalytic degradation of a herbicide derivative, Dinoterb (1), has been investigated in aqueous micellar solution in the presence of titanium dioxide (TiO₂) and air as a function of irradiation time under a variety of conditions using UV and HPLC analysis techniques. The degradation kinetics was studied under different conditions such as different types of TiO₂, catalyst concentration, substrate concentration and reaction pH in the presence of air. The photocatalyst Degussa P25 was found to be more efficient catalyst as compared to other photocatalysts tested. The model compound was found to degrade more efficiently under neutral pH as compared to acidic and alkaline pH. GC/MS analysis of the irradiated samples indicate the formation of 2-isopropyl-4,6-dinitro-phenol as by-product which has been characterized on the basis of molecular ion and mass fragmentation pattern. A probable pathway for its formation has been proposed.     

Diffusion-controlled reactions in vinyl chloride suspension polymerization

The vinyl chloride suspension polymerization is kinetically modeled with a general approach for the independent calculation of diffusion effects on polymerization reactions. For the initiator decomposition, propagation and termination an apparent rate coefficient is determined, built up from two contributions: the intrinsic rate coefficient and a diffusional contribution. The diffusional contribution is calculated with the Smoluchowski model, the diffusion coefficients being determined from the free volume theory. When applying the free volume theory no adjustable parameters are used. The intrinsic rate coefficients are taken from the literature. Hence, a model without any adjustable parameters is obtained. Calculations show that the glass effect appears only at (very) high conversions. Due to the cage effect the initiator efficiency decreases strongly as soon as the monomer phase has disappeared. The gel effect always occurs in the polymer-rich phase and results in a decrease of the termination rate coefficient at the start of the third stage in the polymerization process. There is a good agreement with experimental results.     

‘On water’: unprecedented nucleophilic substitution and addition reactions with 1,4-quinones in aqueous suspension

Unique nucleophilic substitution and addition reactions of 1,4-quinones in aqueous suspension with aromatic amines, primary aliphatic amines, amino acid, ester of amino acid, heterocyclic amines, hydrazine, amide, and thioethers are described in absence of catalyst against the traditional synthetic routes of these reactions in non-aqueous medium in presence of catalyst.     

Longitudinal mixing in fast liquid-phase chemical reactions in a two-phase mixture

The possibility of diminishing the diffusion limitations in the course of fast chemical reactions in liquid-gas systems by varying the reactant ratio and the rate of longitudinal mixing in the liquid phase and at the phase boundary (via changes in the geometry and hydrodynamic parameters of flow in tubular apparatus) was examined.     

Preparation of stable aqueous suspension of a hydrophobic drug with polymers

Pharmaceutical preparation of a hydrophobic aldose reductase inhibitor 5-(3-ethoxy-4-pentyloxyphenyl)-2,4-thiazolidinedione (CT112) was investigated. CT112 dissolved in a basic solution with different kinds of polymers was neutralized by acid to obtain a suspension preparation. In particular, the addition of a polymer, hydroxypropyl methyl cellulose (HPMC) provided a stable CT112 suspension with a homogeneous particle size, and there seemed to be an optimal concentration of HPMC for the stable suspension. The addition of polysorbate 80 brought higher CT112 solubility in water, but did not provide a stable suspension. X-ray diffraction, IR spectrum, and thermal analysis revealed that the particles in the suspension with HPMC had lower degree of crystallinity, less hydrophobic particle surface, and lower melting point and decreased fusion enthalpy than the suspension without HPMC. These results suggested that the highly stable CT112 suspension could be attained by the adsorption of the polymer.     

Protolytic properties and reactivity of aminomethylated calix[4]resorcarenes in reactions with esters of phosphorus acids

The acid-base properties and the kinetics of reactions of aminomethylated calix[4]resorcarenes (AMC) withp-nitrophenyl esters of phosphorus acids in aqueous solutions of propan-2-ol (80 vol.% PrⁱOH) were studied by potentiometry, UV spectrophotometry, and³¹P NMR spectroscopy. The effect of the length of the hydrocarbon radical and substituents at the nitrogen atom on the protolytic properties and reactivity of AMC was studied. The reactions studied occur in two stages. At the first stage, phosphorylated AMC are formed, which are hydrolyzed to the corresponding acids at the second stage. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 456–461, March, 1999.     

Isolation and pre-concentration of basic proteins in aqueous mixture via solid-phase extraction with multi-walled carbon nanotubes assembled on a silica surface

Multi-walled carbon nanotubes (MWNTs) were assembled on a silica surface using a polyelectrolyte-assisted layer-by-layer (LBL) assembly technique. The surface-assembled silica spheres with MWNTs (MWNTs/SiO(2)), which serve as a novel solid-phase extraction sorbent for separation/pre-concentration of basic proteins, was investigated. The adsorption behavior of cytochrome c (cyto-c) by MWNTs/SiO(2) spheres agrees well with the Langmuir adsorption model. A thorough scrutiny of the experimental parameters affecting the adsorption of cyto-c from aqueous solution onto the MWNTs/SiO(2) spheres and its subsequent desorption was carried out. A maximum adsorption capacity of cyto-c was derived as 112 mg (cyto-c) g(-1) (MWNTs). A distinct feature of the MWNTs/SiO(2)-packed micro-column provides clear advantages of minimized flow impedance when operated in a flow system, in addition to better separation efficiency as well as the favorable enrichment capability of proteins, characterized by an enrichment factor of 30 by using 2.0 mL of aqueous solution. The practical applicability of the MWNTs/SiO(2) spheres as a sorbent for the isolation of basic proteins from acidic protein species was demonstrated by effective separation of cyto-c from bovine serum albumin (BSA).     

Nanoscale composition of biphasic polymer nanocolloids in aqueous suspension

The molecular distribution in nanocolloids of poly(dimethyl siloxane) (PDMS) and an organic copolymer (methyl acrylate co-methyl methacrylate co-vinyl acetate) preserved in a frozen aqueous solution was investigated using cryovalence electron energy-loss spectroscopy (EELS) coupled with a scanning transmission electron microscope. Low energy-loss spectra depend upon valence electron structure, and we show that they are substantially different for the PDMS, the copolymer, and the vitrified water studied here. Combining a high efficiency detection system and the use of high-signal low-loss spectra in EELS, we achieved a spatial resolution of 8 nm without serious beam-induced specimen damage in this radiation-sensitive soft-materials system. To obtain quantitative phase maps of silicone and copolymer composition within individual nanoparticles, spectrum datasets were processed via multiple least squares fitting. Quantitative line profiles from the resulting compositional maps indicate that the PDMS lobe of biphasic nanoparticles contained a significant amount of the copolymer and a diffuse interface was formed. Since the nanoparticle synthesis involves polymerization of acrylate monomer dissolved in PDMS nanoparticle precursors, these results suggest that the evolution of the nanocolloid morphology during synthesis is kinetically frozen as the acrylate copolymer achieves some critical molecular weight.