Nonionic surfactants as ligands for the complexation of alkali and alkaline-earth cations in methanol

The stability constants and thermodynamic values of complex formation of nonionic surfactants with alkali and alkaline-earth cations were determined by calorimetric titration in methanol. The purity of the ligands examined was checked by calorimetric end point titration. This method gives good results only if 1 : 1 complexes are formed. For the smallest ligands a ligand:cation ratio of x : 1 is found. In contrast the surfactant with the longest oligoethylene glycol chain forms 1 : x complexes. With increasing number of donor atoms the reaction enthalpies become more negative but the stability constants do not change very much due to compensating changes in entropies. Compared with the monocyclic ligand 18-crown-6 it is possible to obtain higher values of the reaction enthalpies and the high stability constants for crown complexes are caused mainly by favourable entropic factors.     

Komplexierungseigenschaften von Pyridinokronenethern gegenüber Alkali- und Erdalkaliionen in Methanol

Complexing Properties of Pyridino Crown Ether with Alkali and Alkali-Earth Cations in Methanol The stability constants and thermodynamic values of the complex formation of several pyridino crown ethers with alkali and alkaline-earth cations were determined by calorimetric titration in methanol. The stability of the complexes is lower than with crown ethers. This is mainly caused by a decrease of reaction enthalpies. Compared with other mono substituted crown ethers the complexes with pyridino crown ethers are fare more stable due to entropic effects. With increasing cavity size the stability constants, enthalpies and entropies for the reaction of crown ethers and pyridino crown ethers approach similar values.     

Kinetic evidence for hydrophobically stabilized encounter complexes formed by hydrophobic esters in aqueous solutions containing monohydric alcohols.

The pH-independent hydrolysis of four esters, p-methoxyphenyl 2,2-dichloroethanoate (1a), p-methoxyphenyl 2,2-dichloropropanoate (1b), p-methoxyphenyl 2,2-dichlorobutanoate (1c), and p-methoxyphenyl 2,2-dichloropentanoate (1d), in dilute aqueous solution has been studied as a function of the molality of added cosolutes ethanol, 1-propanol, and 1-butanol. The rate constants for the neutral hydrolysis decrease with increasing cosolute concentration. These kinetic medium effects respond to both the hydrophobicity of the ester and of the monohydric alcohol. The observed rate effects were analyzed using both a thermodynamic and a kinetic model. The kinetic model suggests a molecular picture of a hydrophobically stabilized encounter complex, with equilibrium constants K(ec) often smaller than unity, in which the cosolute blocks the reaction center of the hydrolytic ester for attack by water. The formation of these encounter complexes leads to a dominant initial-state stabilization as follows from the thermodynamic model. Decreases in both apparent enthalpies and entropies of activation for these hydrolysis reactions correspond to unfavorable enthalpies and favorable entropies of complexation, which confirms that the encounter complexes are stabilized by hydrophobic interactions.     

Charge-transfer complex formation between o-chloranil and a series of polynuclear aromatic hydrocarbons

The equilibrium constants, enthalpies and entropies of formation of molecular electron donor-acceptor (EDA) complexes of o-chloranil with a series of aromatic hydrocarbons have been determined spectrophotometrically. Spectroscopic and thermodynamic aspects of these complexes have been analysed.     

Complexation of adamantane-ammonium substrates by beta-cyclodextrin and itsO-methylated derivatives

A spectrophotometric method is used to determine formation constants of complexes of-cyclodextrin, the 2,6-di-O-methylated and 2,3,6-tri-O-methylated derivatives of-cyclodextrin as hosts with adamantan-1-ylammonium, adamantan-2-ylammonium and adamantan-1-ylmethylammonium as substrate species. The spectrophotometric method uses methyl orange anion and acid forms as indicator species. Complexes of the cyclodextrins with these species are determined as well as with the adamantane derivatives. Standard enthalpies and entropies of formation of all complexes are calculated from the temperature variation of the equilibrium constants.-Cyclodextrin and its 2,6-O-methyl derivative have comparable complex strengths with adamantaneammonium substrates and these strengths are about two orders of magnitude stronger than the corresponding complexes of the permethylated derivative. Thermodynamic parameters are interpreted in terms of differing intramolecular properties of the cyclodextrin complexes.     

Equilibria Between Molecular and Ionic Complexes with an NH⋯N Hydrogen Bond. Fluorinated Aromatic Amine as a Proton Donor

Ultraviolet absorption spectra in the range 300-200 K were used to study the composition and structure of complexes formed in solutions by the fluorinated amine (4-CF3C6F4)2NH as a proton donor with dibutylamine. A program is developed for quantitative analysis of sets of the spectra of equilibrium multicomponent systems, that allow to find spectral and thermodynamic characteristics of the individual components. In the systems in study, 1:1 and 1:2 fluorinated diamine-dibutylamine complexes are formed. The first of them in a molecular complex with an NHN hydrogen bond, and the second is an ionic proton-transfer complex. The equilibrium constants between the complexes and the free molecules are found, the enthalpies and entropies of formation of both complexes are found, and the spectra of the complexes are measured.     

Kinetic and thermodynamic studies on molecular interaction of antipyrine donor and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone as an electron acceptor in different solvents

The charge–transfer (CT) complex of donor antipyrine with Π‐acceptor 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) has been investigated spectrophotometrically in different halocarbon and acetonitrile solvents. The results indicated immediate formation of an electron donor–acceptor complex (DA), which is followed by two relatively slow consecutive reactions. The pseudo–first‐order rate constants for the formation of the ionic intermediate and the final product at various temperatures were evaluated from the absorbance–time data. The activation parameters, viz. activation energy, enthalpy, entropy, and free energy of activation, were computed from temperature dependence of rate constants. The stoichiometry of the complex was found to be 1:1 by Job's method of continuous variation. The formation constants of the resulting DA complexes were determined by the Benesi–Hildebrand equation at four different temperatures. The enthalpies and entropies of the complex formation reactions have been obtained by temperature dependence of the formation constants using Van't Hoff equation. The results indicate that DDQ complexes of antipyrine in all solvents are enthalpy stabilized but entropy destabilized. Both the kinetics of the interaction and the formation constants of the complexes are dependent upon the polarity of the solvents. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 81–91, 2013     

Stabilité des carboxylates de terres rares,IV N-acétylglycinates

The stability constants of N-acetylgycinates of some rare earths in aqueous solution have been determined by a potentiometric method, and by a solvent extraction technique. The ligand forms 1:1 complexes which are stabilized by relatively large positive entropies of formation, the corresponding enthalpies being endothermic. This backs up the assumption that a conjugate base of an amino acid forms innersphere complexes and acts as a unidentate ligand towards a rare earth cation, like a simple carboxylate.     

Acetohydroxamatoiron(III) Complexes: Thermodynamics of Formation and Temperature Dependent Speciation

Studies of the thermodynamics of formation of the acetohydroxamatoiron(III) complexes were carried out in acidic media at temperatures ranging from 293 to 323 K. Through the isolation of the unique UV–visible spectra of all three complexes, it was possible to determine their formation constants and deduce enthalpies and entropies of formation as well as their molar absorptivities. The enthalpies of formation of the mono-, bis- and trisacetohydroxamatoiron(III) complexes were found to be −56.4, −17.09 and +19.74 kJ⋅mol⁻¹, respectively. Following the determination of the enthalpy and entropy of formation of these complexes, speciation diagrams were calculated for the complexes at temperatures ranging from 293 to 323 K.     

Study of charge transfer complexes of menadione (vitamin K3) with a series of anilines

Menadione (vitamin K(3)) has been shown to form charge transfer complexes with N,N-dimethyl aniline, N,N-dimethyl p-toluidine and N,N-dimethyl m-toluidine in CCl(4) medium. The CT transition energies are well correlated with the ionisation potentials of the anilines. The formation constants of the complexes have been determined at a number of temperatures from which the enthalpies and entropies of formation have been obtained. The formation constants exhibit a very good linear free energy relationship (Hammett) at all the temperatures studied.     

Effects of methyl substituent on the charge-transfer complexations of dicarbazolylalkanes with p-chloranil, tetracyanoethylene and tetracyanoquinodimethane

Series of 1,n-dicarbazolylalkanes and 1,n-di(3-methylcarbazolyl)alkanes (where n=1-5) were synthesized and the molar extinction coefficients, equilibrium constants, enthalpies, and entropies of their charge-transfer (CT) complexes with the π-acceptors p-chloranil, tetracyanoethylene, and tetracyanoquinodimethane were investigated. 1,n-Di(3-methylcarbazolyl)alkanes formed CT complexes with higher equilibrium constants, more negative enthalpies and entropies than 1,n-dicarbazolylalkanes. Vibrational spectra of CT complexes of one of the donor molecules (1,4-dicarbazolylbutane) with all three acceptors were compared.     

Nitrosation of 2-acetylcyclohexanone. 2. Reaction in water in the absence and presence of cyclodextrins

The kinetic study of the nitrosation of the enol of 2-acetylcyclohexanone (ACHE) has been performed in aqueous acid media in the absence and presence of alpha- and beta-cyclodextrin. The reaction is first-order with respect to both reactants concentration: [nitrite] and [ACHE]; but, unexpectly, the dependence of both [H(+)] or [X(-)] (X(-) = Cl(-), Br(-), or SCN(-)) is not simple first-order. The experimental findings have been explained on the basis of a reaction mechanism that considers the formation of a chelate-nitrosyl complex intermediate in steady-state. Addition of both alpha-cyclodextrin (alpha-CD) or beta-cyclodextrin (beta-CD) diminishs strongly the observed rate constant, k(o), measured either for enol-nitrosation or for enol-ketonization reactions. In the case of beta-CD, the inhibition effect is explained through the formation of nonproductive inclusion complexes between the enol (EH) and beta-CD of 1:1 stoichiometry. Nevertheless, the quantitative interpretation of k(o)-[alpha-CD] profiles requires the assumption that the inclusion complexes formation of both 1:1 (EH/alpha-CD) and 1:2 (EH/alpha-CD(2)) stoichiometries. In the case of enol-ketonization, the EH/alpha-CD complex is nearly as reactive as the uncomplexed enol.     

Catalyse multifonctionnelle—VI : Isomerisation de la Δ-5 cholestenone par les melanges phenols-amines

Kinetic study of Δ-5 cholestenone isomerisation in benzene, catalysed by phenols-triethylamine mixtures indicates a transition state with a mole of phenol and a phenol-triethylamine complex. Catalytic activity of phenols-triethylamine mixtures according to versus those complexes association constants in benzene shows a clear maximum for m-nitrophenol. Besides, activation enthalpies are decreasing with phenols acidities, while activation entropies become more negative.     

Linear free energy relations and solvent effects for complexes ofm-cresol and various bases

The enthalpies, entropies, and equilibrium constants for the hydrogen bonded complexes of m-cresol with ten bases in cyclohexane solvent have been determined by calorimetric and spectroscopic methods. The logarithm of the equilibrium constant correlates well with the dipole moment of the base and the solvatochromic parameter which measures the electron donating ability of the base. The enthalpy and entropy data show that the dipole term does not enter into the log K correlation as a consequence of electrostatic interactions between acid and base in the complex. The free base-solvent interaction, which appears to be dipolar in origin, reduces the entropy of the free base and hence contributes to a favorable entropy change for complex formation. The present data are compared to previously reported data obtained in CCl₄ solvent. Solvent effects on the thermodynamic parameters in CCl₄ and cyclohexane appear to be related to dipolar interactions by m-cresol and the bases with the two solvents.     

Thermodynamics of Amino Acid Methyl Ester Hydrochlorides—Crown Ether Complexes in Methanol at 25°C

Stability constants, free energies, and enthalpies and entropies of the complexation of L-alanine methyl ester hydrochloride (L-Ala-HCl), L-phenylalanine methyl ester hydrochloride (L-Phe-HCl), and valine methyl ester hydrochloride (L-Val-HCl) with 15-crown-5 (15C5), benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), dicyclohexano-18-crown-6 (DC18C6), and dicyclohexano-24-crown-8 (DC24C8) in methanol are reported for 20°C. No significant variation in the stability constants and free energies of complexation is observed, indicating that the various crown ethers are poorly selective in binding the amino acids. However, the nature of the crown ether and the amino acid and their pattern of substitution cause a remarkable variation in the enthalpies and entropies of complexation. This indicates a strong enthalpy–entropy compensation effect. The enthalpy–entropy compensation effect for the crown ether complexes of the amino acid methyl ester hydrochlorides reported herein is compared with that of the crown ethers complexes of the amino alcohols and the free amino acid. It is found that the enthalpy–entropy compensation effect holds equally for the three classes of complexes.     

Interactions between n-octyl and n-nonyl beta-D-glucosides and alpha- and beta-cyclodextrins as seen by self-diffusion NMR

In this work (1)H NMR self-diffusion experiments have been performed to determine the self-diffusion coefficients of n-octyl beta-d-glucoside and n-nonyl beta-d-glucoside in alpha-cyclodextrin and beta-cyclodextrin solutions at 25 degrees C. Two questions are addressed. The first concerns the general influence on nonionic surfactant transport properties when cyclodextrins are present in solution. The second question concerns the influence of surfactant-chain length and cyclodextrin cavity volume on the association constant. The self-diffusion coefficients of the alkyl glucosides, in cyclodextrin-containing solutions, depend on the cyclodextrin concentration on account of the formation of complexes. The cyclodextrin diffusion is only mildly influenced, since the complex has similar diffusion coefficients as the free cyclodextrin. There are some obstruction effects at the highest surfactant concentrations which decrease the cyclodextrin diffusion. A set of equations to model the self-diffusion coefficients of the surfactant and cyclodextrin was developed and is presented. On the basis of such equations, properties such as association constants, and micelle and complex diffusion coefficients can be estimated.     

Étude thermodynamique de la complexation de `l'argent par la pipérazine et quelques-uns de ses dérivés en milieu aqueux

The stability constants of the complexes formed by Ag⁺ ion with piperazine and its 2-methyl-, N-methyl-, and N-phenyl-derivatives were determined in aqueous 0.1 M KNO₃ solution at 25°C, by means of the corresponding metal—complex electrodes.The direct calorimetric study of these reactions in the same conditions of temperature and medium made it possible to calculate the standard enthalpies and standard entropies of formation of the complexes.On the basis of a comparison of all the thermodynamic functions of these systems, the ability of each ligand to coordinate is discussed.     

Thermodynamics of complexation of monovalent metal cations by the ionophore monensin free acid in acetonitrile

The standard Gibbs functions, enthalpies, and entropies for the formation of the complexes of monensin acid with monovalent metal cations in acetonitrile were obtained using various methods (conductometry, potentiometry, and calorimetry). Complexation is appreciable (from 2.0 to 4.5 in terms of association constant logarithm) and controlled mainly by the enthalpic effect. Comparison with formation of the corresponding neutral complexes permits a comparison to the process of monensin mediated transport of cation in membranes. A two-step process involving first the formation of the acidic complex is suggested.     

Kinetics of formation and dissociation of aquocobalt(III) complexes with some carboxylic acids in acid perchlorate solution

The rates of formation and dissociation of monocarboxylic complexes of aquocobalt(III) cations with propionic, malonic, and 2-ethylmalonic acids have been measured with the stopped-flow method over a range of concentrations and temperatures in acid perchlorate media at an ionic strength 3.0 M. Although the rate constants for reactions of CoOHaq2+ with neutral ligands cover only a small range, indicating a dissociative mechanism, the associated activation parameters change cooperatively. These variations are discussed in terms of differences in the structure, proton distribution, and rates of water loss in the ion-pair precursors for the different ligands. Similar activation enthalpies of dissociation indicate a common mode of coordination, and the positive activation entropies for dissociation are consistent with a neutral leaving group.