Solvation and Enthalpy Coefficients of Interaction of Carboxylic Acid Amides with 1,2-Diols in Water

The enthalpies of solution of formamides, acetamides, and propionamides with various extents of N-substitution in aqueous solutions of propylene glycol and 1,2-butanediol (concentration 4 mol kg^{- 1}) at 298.15 K were measured. The enthalpies of transfer of the amides from water to mixed aqueous-organic solvents were determined and discussed in combination with data on the enthalpies of transfer of amides from water to aqueous ethylene glycol solutions. The enthalpy coefficients of pair interactions of amides with 1,2-diols in water were calculated and used for evaluating the group components on the basis of the principle of group additivity of contributions. As the hydrophobic properties of polyfunctional nonelectrolytes are enhanced, the coefficients increase, which is due to the growth of the hydrophobic component of the interaction.     

Thermochemical Study of Solvation of Aliphatic Carboxamides in Aqueous Formamide Solutions

The entahlpies of solution of formamide, acetamide, and propionamide in aqueous formamide solutions (formamide concentration 8 mol kg^-1) at 298.15 K were measured. The entahlpies of transfer of amides from water to the mixed aqueous-organic solvent were calculated and compared with published data on the enthalpies of transfer of amides into aqueous urea solutions. The opposite trend in variation of the enthalpies of transfer of amides in these systems with increasing concentration of the nonaqueous component is due to different proton-donor power of formamide and urea relative to water. The enthalpy coefficients of pair interaction of amides with formamide in ternary aqueous solutions were calculated. Their positive values are due to endothermic interaction with formamide of the alkyl groups of amide molecules. The relative hydrophobicity of amides with different degrees of substitution of the amide group was estimated.     

Solvation Properties of Aliphatic Alcohol–Water and Fluorinated Alcohol–Water Solutions for Amide Molecules Studied by IR and NMR Techniques

Solvation properties of aliphatic alcohol–water and fluorinated alcohol–water solutions were probed by amide molecules as solutes using infrared (IR) and ¹H and ¹³C NMR techniques. These include four alcohols: ethanol (EtOH), 2-propanol (2-PrOH), 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and three amides: N-methylformamide (NMF), N-methylacetamide (NMA) and N-methylpropionamide (NMP). The hydrogen bonds of the amide carbonyl oxygen with water are gradually weakened as the alcohol content increases. This decreases in the order of HFIP > TFE ≈ 2-PrOH > EtOH. In TFE– and HFIP–water solutions, the hydrogen bond between the amide amino hydrogen and water is also gradually broken with increasing x _A. This trend is more notable in the order of NMP > NMA > NMF. The hydrophobic moieties of the amide methyl and ethyl groups are solvated by the fluoroalkyl groups of fluorinated alcohols due to the hydrophobic interaction among them. Thus, the steric hindrance generated by the solvated alkyl group of amides promotes the breaking of the hydrogen bonds between amide and water.     

Interaction of crosslinked polyvinylpyrrolidone with a homologous series of methyl orange derivatives in aqueous and nonaqueous solutions: Thermodynamics of the binding equilibria

The extent of binding of methvI orange, ethyl orange, propyl orange, and butyl orange by crosslinked polyvinylpyrrolidone was measured in all aqueous Solution. The first binding constants and the thermodynamic parameters accompanying the binding were evaluated. These values were compared with those of water-soluble polyvinylpyrrolidone. The first binding constant, the absolute magnitude of ΔF°, and the value of ΔS° of the crosslinked polyvinylpyrrolidone are substantially larger than those of the water-soluble product for any particular dye. These behaviors can be accounted for in terms of increased hydrophobic domains in the former and enhanced hydrophobic contribution in the binding process. Also the binding of the dye by the crosslinked polymer in a nonaqueous solvent, ethylene glycol, was measured to assess the contribution of hydrophobic interaction to the dye-polymer complex formation in aqueous medium. It was found that the binding of butyl orange by the crosslinked polymer is suppressed in ethylene glycol and the contribution of entropy term to the free energy change in the aqueous environment is large compared with that in ethylene glycol. The significance of the hydrophobic of the hydrophobic interaction in the dye-polymer association process is described.     

Characterization of Heterogeneous Interaction Behavior in Ternary Mixtures by a Dielectric Analysis: Equi-Molar H–bonded Binary Polar Mixtures in Aqueous Solutions

The heterogeneous associating behavior of the aqueous binary mixtures of ethyl alcohol, ethylene glycol, glycerol and mono alkyl ethers of ethylene glycol, and aqueous ternary mixtures of equi-molar binary systems (i.e., mono alkyl ethers of ethylene glycol with ethyl alcohol, ethylene glycol and glycerol) have been investigated over the entire concentration range using accurately measured dielectric constants at 25 ^∘C. The concentration dependent values of the excess dielectric parameter ε^E and effective Kirkwood correlation factor g ^eff were determined using the measured values of the static dielectric constant, ε_o, at 1 MHz and the high frequency limiting dielectric constant ε_∞ = n _D ². The observed ε^E values in aqueous binary and ternary mixtures are negative over the entire concentration range, which implies the formation of heterogeneous complexes between these molecules that reduces the effective number of dipoles. The stoichiometric ratio corresponding to the maximum interaction in alcohol + water mixtures increases with an increase in the number of hydroxyl groups of the alcohol molecules, but for mono alkyl ethers of ethylene glycol + water mixtures it decreases with the increase in the molecular size of the mono alkyl ethers. In aqueous ternary mixtures the stoichiometric ratio for the maximum extent of heterogeneous interaction is governed by the molecular size of the mono alkyl ethers. It was also found that the strength of the heterogeneous H–bond connectivities in the water + alcohol systems decrease with an increase in the number of hydroxyl groups of the alcohol molecules. However in the case of water + mono alkyl ether binary mixtures and in ternary mixtures, the strength of H–bond connectivities increases with the increase in the molecular size of the mono alkyl ether. An analysis of the g ^eff values confirms that the heterogeneous interaction involves the orientation of molecular dipoles in the studied systems.     

The random contact point model for pair interaction coefficients of unlike solute molecules

The random contact point model described in previous papers is extended to include like-unlike pair interaction coefficients. On the basis of this extension we present the thermodynamics of group interactions involving alkyl, hydroxyl and amide (peptide) groups and water molecules. The values obtained for the Gibbs energy of group interaction (absolute values ranging from 1.5 to 9.8 kJ-mol^–1) indicate that all these groups attract or repel each other in aqueous solutions with comparable strength. Group interaction parameters obtained from aqueous and non-aqueous systems, and based on interaction coefficients and other thermodynamic quantities, agree well. The model also allows for the quantification, though not for the prediction, of the cooperativity of hydrophobic interaction.     

Enthalpy characteristics of dilute solutions of amides in acetonitrile

The enthalpy of mixing of formamide,N-methylformamide,N,N-dimethylformamide, and hexamethylphosphoric triamide with MeCN was measured in the 283–328 K range. The enthalpic coefficients of the binary and ternary interactions between the amide molecules are calculated within the framework of the McMillan-Mayer theory. The contributions to the enthalpy of dissolution due to cavity formation in the solvent (Δ_cav H ⁰) and due to solute-solvent interaction (Δ_int H ⁰) were determined. The enthalpies of specific and nonspecific solvation of amides in MeCN were calculated. The main contribution to the enthalpy of solvation of formamide andN-methylformamide is from specific interactions, while forN,N-dimethylformamide and hexamethylphosphoric triamide it is from nonspecific interactions. The values obtained are compared with those for solutions of the amides mentioned in water and methanol. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1730–1735, October, 1993.     

Thermodynamic Study of Oxyethylated Glycols and Their Mixtures with Tertiary Carboxamides

The contributions into the total energy of intermolecular interactions in oxyethylated ethylene glycol derivatives were estimated in terms of a model approach that uses inner pressure as a measure of nonspecific interactions in a liquid. Increased number of ether groups in ethylene glycols increases the nonspecific contribution and decreases specific contributions. Unlike diethylene glycol, triethylene glycol and tetraethylene glycol contain H-bond networks in the range 298.15–308.15 K. The enthalpies of mixing of tertiary amides with tetraethylene glycol were measured and compared with those for ethylene glycol, diethylene glycol, and triethylene glycol. The effect of the structural and thermodynamic properties of the components on the integral and differential thermochemical characteristics of mixtures of glycols with N,N-disubstituted amides was discussed.     

Thermochemical Study of Ethylene Glycol Mixtures with N,N-Disubstituted Amides of Carboxylic Acids

The enthalpies of mixing of N'N-disubstituted amides of formic, acetic acids with ethyleneglycol were considered in view of the data on the enthalpies of mixing of the same compounds with form-amide and water. The exothermicity of amide-ethylene glycol interactions results from the formation of heterocomponent H bonds stronger than the H bonds in ethylene glycol. Solvophilic solvation and solvophobic effects are much weaker in nonaqueous solutions with low concentrations of amides than in water.     

微量热法研究烟酰胺和异烟酰胺在KCl水溶液中的焓对自缔合作用

利用等温滴定微量热法(ITC)分别测定了298.15 K时烟酰胺(NA)和异烟酰胺(INA)在纯水及不同浓度KCl(m=0~0.3 mol/kg)水溶液中的稀释焓,根据McMillan-Mayer理论计算得到相应的焓对自缔合作用系数(h xx),发现两者的h xx都是很大的负值,且在较高KCl浓度时都随KCl浓度的增大而减小.从溶质-溶质、溶质-溶剂相互作用的观点出发,对这2种吡啶羧酸酰胺异构体的疏水和亲水作用平衡进行了讨论:NA和INA都是亲水-亲水作用占优势,芳香性的吡啶环与酰胺基团羰基的π电子共轭使得对位异构体的共振结构分子电荷分离而更具亲水性;吡啶环之间的π-π堆叠对焓对自缔合作用的贡献几乎可忽略;溶液离子强度(I)的增大有利于加强亲水-亲水作用,从而使h xx的绝对值都随溶液中KCl浓度的增大而逐渐增大.     

Correlation between the temperature dependence of apparent specific volume and the conformation of oligomeric propylene glycols in aqueous solution

The apparent specific volumes, ?₂, of a series of poly(propylene glycol) and poly(ethyleneglycol) oligomers in aqueous solution were determined as a function of temperature from 4 to 25°C. The slope, d?₂/dT, was taken as a measure of the extent of interaction between the hydrophobic portions of the oligomer and water, higher values of d?₂/dT representing diminished hydrophobic interaction. It is suggested that the observed increase in d?₂/dT with chain length for the poly(propylene glycol) oligomers can be attributed to the previously proposed disk coiled conformation of the chain which reduces the degree of contact between the side-chain methyl groups and water as the chain length increases. This interpretation is supported by (1) the direct relationship between the difference in the thermal expansion behavior of the two oligomer series and the accessibility of the methyl groups in the poly(propylene glycol) disk-coil, and (2) the agreement between the calculated volume changes on mixing for the methyl groups and the values predicted for the disk-coil model from the Némethy and Scheraga theory.     

Enthalpies of solvation of ethylene oxide oligomers CH3O(CH2CH2O)nCH3 (n = 1 to 4) in different H-bonding solvents: Methanol, chloroform, and water. Group contribution method as applied to the polar oligomers

The enthalpies of solution and solvation of ethylene oxide oligomers CH₃O(CH₂CH₂O)_nCH₃ (n = 1 to 4) in methanol and chloroform have been determined from calorimetric measurements at T = 298.15 K. The enthalpic coefficients of pairwise solute–solute interaction for methanol solutions have been calculated. The enthalpic characteristics of the oligomers in methanol, chloroform, water and tetrachloromethane have been compared. The hydrogen bonding of the oligomers with chloroform and water molecules is exhibited in the values of solvation enthalpy and coefficient of solute–solute interaction. This effect is not observed for methanol solvent. The thermochemical data evidence an existence of multi-centred hydrogen bonds in associates of polyethers with the solvent molecules. Enthalpies of hydrogen bonding of the oligomers with chloroform and water have been estimated. The additivity scheme has been developed to describe the enthalpies of solvation of ethylene oxide oligomers, unbranched monoethers and n-alkanes in chloroform, methanol, water, and tetrachloromethane. The correction parameters for contribution of repeated polar groups and correction term for methoxy-compounds have been introduced. The obtained group contributions permit to describe the enthalpies of solvation of unbranched monoethers and ethylene oxide oligomers in the solvents with standard deviation up to 0.6 kJ · mol⁻¹. The values of group contributions and corrections are strongly influenced by solvent properties.     

Molecular dynamics simulations of PNIPAM‐co‐PEGMA copolymer hydrophilic to hydrophobic transition in NaCl solution

Classical molecular dynamics simulations were carried out to investigate the hydrophilic to hydrophobic transition of PNIPAM‐co‐PEGMA close to its lower critical solution temperature (LCST) in 1 M NaCl solution. PNIPAM‐co‐PEGMA is a copolymer of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) methacrylate (PEGMA). The copolymer consists of 38 monomer units of NIPAM with two PEGMA chains attached to the PNIAPM backbone. The PNIPAM‐co‐PEGMA was observed to go through the hydrophilic?hydrophobic conformational change for simulations at temperature slightly above its LCST. Na⁺ ions were found to bind strongly and directly with amide O, even more strongly with the O atoms on PEGAMS chains, whereas Cl^? ions only exhibit weak interaction with the polymer. Significantly a novel caged stable metal‐organic complex involving a Na⁺ ion coordinated by six O atoms from the copolymer was observed after the PNIPAM‐co‐PEGMA copolymer went through conformational transition to form a hydrophobic folded structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Hydration of simple amides. FTIR spectra of HDO and theoretical studies

The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides.     

Volumetric properties of the water-ethylene glycol mixtures in the temperature range 278–333.15 K at atmospheric pressure

Density of the water-ethylene glycol binary mixtures was measured in the entire range of compositions in the temperature range 278–333.15 K (6 values) at atmospheric pressure using a vibration densimeter. Mixtures with low concentrations of ethylene glycol were studied at 15 temperatures in the range of 274–333.15 K. Excess molar volumes V _m ^E , the partial molar volumes of water -V ₁ and ethylene glycol, -V ₂, the coefficients of thermal volume expansion α of the mixture, the partial molar volume coefficients of thermal expansion of water $ \bar V_1 $ \bar V_1 and ethylene $ \bar V_2 $ \bar V_2 were calculated. Excess molar volumes were described using the Redlich-Kister equation. The density ρ of the mixture was found to increase with the increasing ethylene glycol concentration at all temperatures, but at low content of ethylene glycol the dependence ρ = f(T) of the mixture at ∼276.5 K passed through a maximum. The coefficient α increases sharply in the composition range 0 < x < 0.2, in the range 0.5 < x <1 remains almost unchanged, and at T > 277 K is positive for all compositions. The dependences $ \bar \alpha _1 $ \bar \alpha _1 = f (x) and $ \bar \alpha _2 $ \bar \alpha _2 = f (x) are complex in whole temperature range and are characterized by the presence of an extremum. V _m ^E values are negative at all temperatures, and upon increase in the temperature the deviation from ideality decreases (x is the mole fraction of ethylene glycol).     

Enthalpies of interaction of N-acetyl amides of sarcosine and N-methyl-L-alanine dissolved in N,N-dimethylformamide at 25°C

Enthalpies of dilution of N-acetyl amides of sarcosine and N-methyl-L-alanine dissolved in N,N-dimethylformamide have been measured calorimetrically at 25°C. The enthalpic pairwise interaction coefficients calculated there from are negative, indicating a energetically favorable interaction. The results were used to make a comparison with other peptides with regard to the methylation of amide groups. Substituting a primary amide hydrogen by a methyl group gives a smaller positive change of the pairwise interaction coefficient than substituting a secondary amide hydrogen.     

Thermochemical study of the temperature effect on the l-α-alanine-l-α-alanine interactions in aqueous solutions of urea and ethylene glycol

The dilution enthalpies of l-α-alanine (Ala) solutions in aqueous solutions of urea and ethylene glycol were measured at 298.15 and 313.15 K. The enthalpy (h _xx ) and heat capacity (c _xx ) coefficients of pair interaction were used for characterization of the Ala-Ala interaction in solutions. The h _xx values are presented by the sum of contributions from the interactions of the nonpolar side chains (h _R-R) and polar groups (h _FG-FG) of the amino acid. The h _xx value of Ala in water increases with the temperature increase due to an increase in the contribution of h _R-R. The increase in h _xx of Ala in an aqueous-carbamide solvent with an increase in the urea concentration is determined by an increase in the contribution of h _FG-FG. The temperature rise and urea additives exert various denaturing effects. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1364–1368, July, 2008.     

How Strong is Hydrogen Bonding to Amide Nitrogen?

The protonation of the carboxamide nitrogen atom is an essential part of in vivo and in vitro processes (cis-trans isomerization, amides hydrolysis etc). This phenomenon is well studied in geometrically strongly distorted amides, although there is little data concerning the protonation of undistorted amides. In the latter case, the participation of amide nitrogen in hydrogen bonding (which can be regarded as the incipient state of a proton transfer process) is less well-studied. Thus, it would be a worthy goal to investigate the enthalpy of this interaction. We prepared and investigated a set of peri-substituted naphthalenes containing the protonated dimethylamino group next to the amide nitrogen atom (“amide proton sponges”), which could serve as models for the study of an intramolecular hydrogen bond with the amide nitrogen atom. X-Ray analysis, NMR spectra, basicity values as well as quantum chemical calculations revealed the existence of a hydrogen bond with the amide nitrogen, that should be attributed to the borderline between moderate and weak intramolecular hydrogen bonds (2–7 kcal ⋅ mol⁻¹).     

Thermochemical investigation of interaction of L-serin with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

By the method of dissolution calorimetry integral enthalpies of dissolution Δ_sol H ^m of L-serine are measured in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of the organic solvent up to 0.42 mole fraction. The standard values of enthalpies of dissolution (Δ_sol H ⁰) and transfer (Δ_tr H ⁰) of amino acids from water to mixed solvents are calculated. The calculated values of the enthalpy coefficients of pair interactions of L-serine with the molecules of co-solvents are positive. The data obtained are interpreted in terms of prevalence of different types of interactions in solutions and the influence of nature of co-solvents on the thermochemical characteristics of the dissolved amino acids.