Secondary Salt Effect Research Articles

Stabilization of diketo tautomer of curcumin by premicellar cationic surfactants: A spectroscopic, tensiometric and TD-DFT study

The interaction of aqueous curcumin with cationic surfactants of varying chain lengths and head group, stabilizing its β-diketo tautomer and exhibiting an UV band around 355nm, has been studied in buffered aqueous solutions in the pH range of 2.00–7.00 by UV–visible and fluorescence spectroscopy, and surface tensiometry under optimal experimental conditions of submicellar concentration of the surfactants. Time Dependent Density Functional Theory (TD-DFT) has also been used to predict the molecular structure and excitation energies of the interaction product in the ground state and the nature of the interaction. As the surfactant concentration increases, the curcumin–cationic complex forms premicellar aggregates where the curcumin exists in a highly polar microenvironment before shifting its location to the nonpolar core of the surfactant micelles. The strength of the interaction increases with the chain length of the surfactant and also changes on changing the head group of the surfactant. An observed secondary salt effect on the interaction indicates an unusual involvement of a proton in the interaction. Based on the experimental and theoretical evidences the following mechanism has been proposed for the stabilization of the β-diketo form: on the approach of the cationic surfactant, a protonation of the methylenic central carbon atom of curcumin takes place simultaneously breaking the π-conjugation and facilitating the detachment of the enolic proton paving the way for an ion dipole binding between the cationic head group and the electron-rich β-diketo oxygen atoms. Hydrophobic interaction between the tail of the surfactant and the nonionic curcumin molecule assists the electrostatic interaction.

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (eaq−) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L−1) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L−1) values. At high pH values, where the HPO42− ions dominate, the eaq− decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·105 L mol−1 s−1). The H2PO4− ions react with eaq− at a rate constant of 2.8·106 L mol−1 s−1 (I = 0), which increases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) in accordance with the Debye-Huckel theory. The rate constant for quenching of eaq− by H3PO4 at pH ≤ 4 decreases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) due to the secondary salt effect and equals 1.6·109 L mol−1 s−1 at I = 0. The logarithm of the rate constant for quenching of eaq− by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule.

The pH-stability and acid degradation of the macrolide antibiotic, josamycin

The influence of pH, ionic strength and buffer concentration on the stability of josamycin has been investigated. The pH-rate profile for josamycin over the pH range 1–12 was characterised and the equation describing the profile determined. Results indicated that josamycin is subject to specific acid catalysis whilst catalysis in alkali media appeared to be more complex. The rate constant for catalysis by hydronium ion ( k H) was 54.11 M −1 · h −1 and the rate constant for catalysis by hydroxide ion ( k OH) was 60.35 M −1 · h −1. Catalysis due to water was insignificant and the water catalysed rate constant was found to be 3.37 × 10 −5 h −1. The pH of maximum stability was determined as pH 6.5 whilst degradation at pH 1.0 and 12.0 is about five orders of magnitude greater than at pH 6.5. The degradation of josamycin in acid is subject to a significant primary salt effect; however, no secondary salt effect was evident. Concentration vs. time profiles for josamycin in acidic media were biphasic which indicated that the degradation reaction did not follow a simple pathway whereby josamycin degrades directly to products. Further investigations suggest that josamycin undergoes a reversible isomerisation step, with subsequent degradation of josamycin and possibly its isomer, by cleavage of the mycarose moiety to desmycarose compounds. Studies to determine the stability of josamycin in simulated gastric fluids demonstrated that acid degradation could be appreciable after oral administration. However, extensive degradation in vivo will only occur at the most acidic gastric pH's of about pH 1.0 to 2.0.

Living cationic polymerization of vinyl ethers

AbstractThe cationic polymerization of vinyl ethers initiated by CH3‐CH(OR)(I) / R4N+A− (R = Alkyl, A− = ClO4−, BF4−, PF6−, I−, NO3−) shows the characteristics of a living polymerization. The rate of polymerization is a function of the solvent polarity, the temperature, the type and concentration of the ammonium salt. The experimental data can be explained on the basis of the secondary salt effect overlapped by some dipol‐dipol interactions of the chain end and the added salt. Functionalization of the chain end with thermolabile azo functions yields polymeric initiator which was applied for the synthesis of blockcopolymers. Vinyl ethers functionalized with furylacrylic ester groups were polymerized and crosslinked via [2+2] cycloaddition.

Aqueous Conversion Kinetics and Mechanisms of Ancitabine, a Prodrug of the Antileukemic Agent Cytarabine

The kinetics of conversion of the prodrug ancitabine to the anticancer drug cytarabine have been studied in aqueous solutions in the pH range of 1.5–10.7, temperature range of 19.5–80.0°C, ionic strength range of 10−4 to 1.5, and in the presence of several general-base catalysts. Under all conditions ancitabine was quantitatively converted to cytarabine. The pH-rate profiles were linear with slope = 1 in alkaline pH, becoming pH independent in the region of maximum stability at pH ± 4, where buffer catalysis was found to be insignificant and kobs ± (1.12 × 1011 h−1)-exp {-10121 deg/T. At 30°C, pH ± 4, it is calculated that an aqueous ancitabine solution will maintain 90% of its initial concentration for 12 d. A novel method for measuring general-base catalysis in competition with predominating specific-base catalysis and in the presence of secondary salt effects at constant ionic strength was developed. Three mechanisms of hydrolytic prodrug conversion are proposed: nucleophilic hydroxide addition, general base-assisted nucleophilic water attack, and spontaneous water attack.

The reactivity of different iron(III) species in the formation of monoacethydroxamatoiron(III) complex

The dependence of the rate constant of the monoacethydroxamatoiron(III) complex formation on ionic strength in acid solution 25 °C has been studied. The observed ‘secondary salt effect’ strongly supports the assumption that an iron(III) hydroxo complex is the reactive species. Evidence is presented showing that the iron(III) hydroxo dimer (Fe 4(OH0 2(H 2O) 4+ 8 exhibits approximately equal reactivity toward acethydroxamic acid as the monomer does. This fact, and lack of evidence for ‘primary salt effect’ strongly confirms the previous findings that the reactive form of ligand is the undissociated rather than dissociated hydroxamic acid. The rate of the reaction of hydroxamic acid with iron(III) hydroxo ion is estimated to be of the same order of magnitude as the rate of water exchange of the same ion.

Complex formation of poly(4‐vinyl‐N‐alkylpyridinium) salts or polymer containing flavin mononucleotide with indole derivatives

AbstractA charge‐transfer‐type complex formation between poly(4‐vinyl‐N‐propylpyridinium bromide) (C3PVP), poly(4‐vinyl‐N‐butylpyridinium bromide) (C4PVP) or poly(4‐vinyl‐N‐benzylpyridinium chloride) (BzPVP), and indole derivatives or between polymer containing flavin mononucleotide residues and indole derivatives was studied in the presence of simple and polyelectrolytes. The association constant (K) of the complex formation with indole acetate increased in the order BzPVP > C4PVP > C3PVP, which indicated an important contribution by hydrophobic interaction. The addition of simple and polyelectrolytes decreased the association constants. This was explained by the “secondary salt effect” of the salts. The importance of the electrostatic interactions in the complexation systems was obvious. The influence of simple electrolytes on the K values was discussed theoretically according to Manning's theory.

Polyelectrolyte effects on the charge transfer complex formation between FMN and indole derivatives

AbstractThe complex formation of flavin mononucleotide (FMN) with tryptamine, indoleacetate, and trytophan was investigated in the presence of polyelectrolytes; that is, sodium polyethylene sulfonate (NaPES), sodium polystyrene sulfonate (NaPSS), and a copolymer of diethyldiallylammonium chloride and sulfur dioxide (DECS). The complexation of FMN and tryptamine was strongly retarded by the macrocations and macroanions, whereas that of FMN and indoleacetate was enhanced by the macrocations. Furthermore, the equilibrium constants of FMN‐tryptophan complex were insensitive to the addition of polyelectrolytes. These results suggest that the complexation of FMN was strongly influenced by the electrostatic interactions between the reactant ions and macroions, in addition to those between the reactant ions, in conformity with the secondary salt effect.

Determination of the acid dissociation constant for diethyldithiocarbamic acid. Primary and secondary salt effects in the decomposition of diethyldithiocarbamic acid

Determination of the acid dissociation constant for diethyldithiocarbamic acid. Primary and secondary salt effects in the decomposition of diethyldithiocarbamic acid

Catalysis in the polymerization of silicic acid

AbstractSome aspects of the condensation polymerization of silicic acid have been discussed in a systematic manner. Empirical relationships between the time of gelation and the catalyzer concentration, with both H+ and OH− catalysis, have been suggested. These are found to be consistent with the observations of various authors. Also the activities of both the ionized and un‐ionized species present in the system account for some anomalous observations and the pH optimum for the maximum time of gelation. The idea of a secondary salt effect, introduced by Brönsted, has been found to be helpful in understanding catalyst function.

Sedimentation and diffusion of polyelectrolytes. Part II. Experimental studies with poly‐L‐lysine hydrohalides

AbstractThe sedimentation, diffusion, and osmotic behavior of poly‐L‐lysine hydrochloride is examined over a wide range of ionic strengths and in solutions which range in relative composition from a swamping excess of added salt (suppression of charge effect) to salt‐free solutions (maximum effect). The charge effect is found to essentially obey the theoretical treatment proposed in Part I of this work. The frictional coefficient of the macroion in salt‐free solutions is comparable to that of the fully stretched chain. In excess of salt the friction is appreciably smaller, its magnitude not varying greatly with ionic strength. The molecular weight is determined osmotically and also from diffusion‐sedimentation, with the charge effect suppressed by addition of salt or eliminated through the use of non‐ionized forms of the polymer; a reasonable agreement is obtained between the methods. The influence of small ions on sedimentation is also examined. The secondary salt effect is found to be much smaller than that predicted theoretically; on the other hand, the substitution of different counterions in the absence of secondary salt effect gives rise to changes of the expected magnitude.

Ultracentrifugation of 1:1 Th(IV)-Diethylenetriaminepentaacetic Acid and 1:1.5 Th(IV)-Pyrocatechol-3,5-disulfonate Chelates1,2

The application of ultracentrifuge measurements to the determination of molecular weights of charged metal chelates was investigated using the 1 : 1 Th(IV)-diethylenetriaminepentaacetate (DTPA) and 1: 1.5 Th(IV)-pyrocatechol-3,5- disulfonate (Tiron) systems. The Archibald nonequilibrium method of determining molecular weights as described bv Klainer and Kegeles was applied to the methods of dealing with charged polymers as outlined by Johnson, et al. The calculated values of the molecular weight of the monomeric Th(IV)-DTPA chelate (which was employed as a model system) were approximately 10 per cent lower than the actual value, presumably because of the fact that a nonideal supponting electrolyte, 1 M NaNO/sub 3/, was employed. The substitution of KNO/sub 3/ for NaNO/sub 3/ resulted in molecular weight values which were only 3 per cent below the theoretical value. Investigation of the 1: 1.5 Th(IV)-Tiron system of 1 M NaCl showed that a binuclear chelate is the predominant species. The effects of varying type and concentration of the supporting electrolyte were studied and discussed in terms of the primary and secondary salt effects. (auth)

STUDIES OF RDX AND RELATED COMPOUNDS: XI. THE CONVERSION OF PHX TO AcAn

The formation of AcAn from PHX, nitric acid, and acetic anhydride has been investigated at various temperatures. The effects of added acetic acid and added salts have been determined. While the reaction is first order with respect to PHX, it is of an order 2.6 with respect to nitric acid. The reaction rate is significantly decreased by small concentrations of sodium nitrate. The reaction is also characterized by a secondary salt effect and a low apparent activation energy of about 2 kcal. mole−1. A mechanism has been suggested in qualitative, and to some extent quantitative, agreement with the experimental data. It is postulated that the conversion of PHX involves a rate-controlling ionic reaction between PHX and nitric acid, and that this is followed by a rapid acetylation of the acidic intermediate to AcAn.

The Salt Effect in the Aniline-Formaldehyde Condensation

Abstract In the hydrochloric or sulfuric acid-catalyzed condensation of aniline with formaldehyde, it was found that the addition of inorganic salt, e. g., sodium chloride or potassium sulfate, increases the rate and this fact is ascribed to the so-called primary and secondary salt effects.

THE KINETICS OF THE REACTION WHICH TAKES PLACE BETWEEN IODOACETIC ACID AND GLYCINE

1. The kinetics of the reaction which takes place between glycine and iodoacetic acid was studied by means of the polarographic method. 2. On the basis of kinetic equations, evidence was obtained that (a) The reaction proceeds in two steps in which the hydrogens of the amino group are consecutively replaced by the acetyl radicals, the velocity constants being in the ratio 2:1. (b) Only the anionic form of glycine is able to react since the velocity constants at any pH are proportional to the concentration of glycine anion. (c) The reaction is of the ionic type, showing a positive salt catalysis, which, according to Brönsted's hypothesis, involves the primary and the secondary salt effects. 3. The fact that only the glycine anion is able to react was explained as being due to the existence of an unbonded pair of electrons on the nitrogen in the NH(2) group. The NH(3) (+) group, however, in which these electrons are shared by H(+), must, therefore, be inactive.