The thermochemistry of solution of L-α-alanyl-L-α-alanine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution of L-α-alanyl-L-α-alanine in water-ethanol, water-n-propanol, and water-isopropanol mixtures were measured calorimetrically at alcohol concentrations x ₂ ranging from 0 to 0.4 mole fractions. The standard enthalpy of peptide solution Δ_sol H ^o and transfer Δ_tr H ^o from water into a mixed solvent were calculated. The effect of the structure and properties of peptides and mixture composition on the enthalpy characteristics is discussed. The enthalpy coefficients of pair interactions h _xy between L-α-alanyl-L-α-alanine and alcohol molecules were calculated; these coefficients were positive and increased in the series ethanol, n-propanol, isopropanol. The analysis performed allowed the differences in the thermodynamic characteristics of solution of L-α-alanyl-L-α-alanine and DL-α-alanyl-DL-α-alanine in water-alcohol mixtures to be determined.     

New Insights into the Effects of Tetrahydrofuran and Dioxane on the Selectivity of Ternary Eluent Compositions in RP-HPLC Systems

The present paper discusses the tendencies in the retention changes of a diverse set of organic model compounds on HPLC C18 stationary phases in aqueous ternary eluent mixtures containing tetrahydrofuran or 1,4-dioxane, and one short chain aliphatic alcohol (ethanol, n-propanol or isopropanol). The set of compounds consisted of steroid and non-steroid molecules with different hydrogen bond donor and/or acceptor abilities. Every eluent mixture contained 75 V/V% water and 25 V/V% organic solvent(s). The composition of the mixture was changed in 5 V/V% steps, starting with the binary alcohol–water mixtures and finishing with the binary ether–water mixtures. The results show clearly the dependency of retention times on the eluent composition, the size of the molecules, and the occurence of the hydrogen bond donor/acceptor groups. The isopropanol–tetrahydrofurane–water mixtures resulted in selective changes in the retention times of the compounds with acidic groups or with other non-acidic OH or NH protons in the neighborhood of electron-withdrawing groups. Every compound has shown elevated retention times in the isopropanol–dioxane–water, n-propanol–tetrahydrofuran–water, or n-propanol–dioxane–water mixtures. Clear trends could not be observed in the eluents with ethanol. The probable reason for the retention enhancement is the adsorption of the organic components of the mobile phase on the surface of the stationary phase. The different effects of the alcohols may originate from the interaction of their varying aliphatic alkyl chains with the C18 chains. This phenomenon may result in different availability of the C18 chains for dioxane, tetrahydrofuran and the model compounds.     

The stability of suspensions of multiwalled carbon nanotubes in organic solvents in the presence of triton X-165

The stability of suspensions of multiwalled carbon nanotubes in isopropanol, toluene, and p-xylene has been studied. Suspensions of multiwalled carbon nanotubes in isopropanol (unlike those in toluene and p-xylene) display high aggregation stability, which is explained by the formation of repulsive solvation layers as a result of interactions between the polar solvent and nanotube surface. The addition of a nonionic surfactant, Triton X-165, noticeably increases aggregate sizes and accelerates sedimentation processes in nanotube suspensions in isopropanol, while, in toluene and p-xylene, the aggregate sizes decrease and the nanotube suspensions are stabilized.     

The enthalpy of solution of DL-α-alanyl-DL-α-valine depending on the composition of water-alcohol binary solvents at 298.15 K

The integral enthalpies of solution of DL-α-alanyl-DL-α-valine in water-ethanol, water-n-propanol, and water-isopropanol mixtures at alcohol concentrations x ₂ = 0–0.4 mole fractions were measured calorimetrically. The enthalpies of solution of the peptide Δ_sol H° and transfer from water to a mixed solvent Δ_tr H° were calculated. The effect of the structure and properties of the peptide and mixture composition on the enthalpy characteristics of the peptide are discussed. The enthalpy coefficients of pair interactions h _xy of DL-α-alanyl-DL-α-valine with alcohol molecules were calculated. It was found that they were positive and increased in the series ethanol, n-propanol, isopropanol. An analysis of the results allows the general features of changes in the thermodynamic parameters of solution of peptides of the DL-α-alanine series with different amino acid residues in water-alcohol mixtures to be established.     

Isotherms of adsorption of aromatic compounds on cellulose from heptane-isopropanol solutions

The isotherms of adsorption of benzene, anisole, benzyl alcohol, and benzaldehyde on the surface of microspherical mesoporous crosslinked cellulose were calculated from the chromatographic peaks of these compounds with consideration given to the longitudinal smearing of the analyte zone under the action of adsorption forces. The solvents were n-heptane-isopropanol mixtures containing various amounts of isopropanol (up to 2 vol %). The adsorption isotherms were described using equations of the displacement adsorption theory. The mechanism of the sorption of aromatic compounds and the physical meaning of the constants entering into the adsorption isotherm equation were discussed.     

Qualitative analysis of excess dielectric properties of binary mixtures, ternary mixtures and mixing dynamics measurement using surface plasmon resonance

The analysis of the excess dielectric properties for various binary mixtures and a ternary mixture is demonstrated using a surface plasmon resonance (SPR) sensor. Strong deviations from ideality are seen using SPR to monitor deviations in the dielectric properties following mixing. Binary mixtures with similar refractive index were measured: hexanes/isopropanol, n-heptanes/propanol, 1-acetoxy-2-methoxyethane/2-methoxyethanol, butanol/dipropylamine, hexanes/ethylacetate, and ethylacetate/isopropanol binary mixtures. The ternary mixture was composed of 60 different proportions of hexanes, isopropanol, and ethylacetate. Using SPR, mixing dynamics is easily accessible. The mixing of hexanes and isopropanol in static solution was monitored.     

Quantum Chemical Study of Water Adsorption on the Surfaces of SrTiO3 Nanotubes

We have studied the adsorption of water molecules on the inner and outer surfaces of nanotubes generated by rolling (001) layers of SrTiO₃ cubic crystals. The stability and the atomic and electronic structures of the adsorbed layers are determined by using hybrid density functional theory. The absorption energy and the preferred adsorbate structure are essentially governed by the nature of the surface of the nanotube. Dissociative adsorption prevails on the outer nanotube surfaces. The stability of the adsorbed layers on the inner surfaces is related to the possibility of the formation of hydrogen bonds between water molecules and surface oxygen atoms, and depends on the surface curvature. The presence of water molecules on the inner surface of the nanotubes leads to an increase of the electronic band gap. Externally TiO₂‐terminated nanotubes could be used for the photocatalytic decomposition of water by ultraviolet radiation.     

Modification of the electronic properties of zigzag (n = 5–10) and armchair (n = 3, 5) carbon nanotubes by K atom adsorption

The adsorption of the potassium atom onto the surface of (n,0) zigzag nanotube (n = 5–10) and (n,n) armchair nanotubes (n = 3, 5) has been studied by density functional theory. The local density approximation calculation of adsorption energy (E _ads) emphasized on the dependency of E _ads to the diameter and chirality of the nanotube. E _ads decreases when the diameter increases. So the (5,0)-K system has the highest adsorption energy among all structures. Furthermore, a significant change was observed in the electronic properties of potassium-adsorbed single-walled carbon nanotube (SWCNT) and the metallic behavior of the nanotube improved. Therefore, our results showed that such modified SWCNTs can be applied in nanodevices such as transistors.     

Enthalpy characteristics of solution of L-cysteine and L-asparagine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution Δ_sol H ^m of L-cysteine and L-asparagine in mixtures of water with ethanol, n-propanol, and isopropanol at a mole fraction of alcohol of up to 0.32 were determined by calorimetry of solution. The standard enthalpies of solution (Δ_sol H ⁰) of L-serine and of its transfer (Δ_tr H ⁰) from water to a mixed solvent were calculated. The dependences of Δ_sol H ⁰ and Δ_tr H ⁰ on the composition of water-alcohol mixtures pass through a maximum. The calculated enthalpy coefficients of pair interaction of amino acids with alcohol molecules are positive and increase in the order ethanol, n-propanol, isopropanol. The data obtained were interpreted from the viewpoint of various types of interaction in solution and effect of the amino acid residue on the thermochemical characteristics of solution.     

The influence of intramolecular H-bond on chromatographic behavior of phenylalkylamines

The thermodynamic characteristics of adsorption on the surface of graphitized thermal carbon black at 300 K were determined by the molecular statistical method for three phenylalklylamines. The influence of the intramolecular H-bond on the conformation of the molecules compared with structurally related n-alkyl-benzenes was considered. It was shown that the conformations of the molecules could influence chromatographic retention. Conformational isomers stabilized by intramolecular H-bonds were found to retain their structure in adsorption on graphitized thermal carbon black.     

Viscosity of heavy n-alkanes and diffusion of gases therein based on molecular dynamics simulations and empirical correlations

The viscosity of pure n-alkanes and n-alkane mixtures was studied by molecular dynamics (MD) simulations using the Green–Kubo method. n-Alkane molecules were modeled based on the Transferable Potential for Phase Equilibria (TraPPE) united atom force field. MD simulations at constant number of molecules or particles, volume and temperature (NVT) were performed for n-C₈ up to n-C₉₆ at different temperatures as well as for binary and six-component n-alkane mixtures which are considered as prototypes for the hydrocarbon wax produced during the Gas-To-Liquid (GTL) Fischer–Tropsch process. For the pure n-alkanes, good agreement between our simulated viscosities and existing experimental data was observed. In the case of the n-alkane mixtures, the composition dependence of viscosity was examined. The simulated viscosity results were compared with literature empirical correlations. Moreover, a new macroscopic empirical correlation for the calculation of self-diffusion coefficients of hydrogen, carbon monoxide, and water in n-alkanes and mixtures of n-alkanes was developed by combining viscosity and self-diffusion coefficient values in n-alkanes. The correlation was compared with the simulation data and an average absolute deviation (AAD) of 11.3% for pure n-alkanes and 14.3% for n-alkane mixtures was obtained.     

The thermochemical characteristics of solution of DL-α-alanylglycine and DL-α-alanylalanine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution of DL-α-alanylglycine and DL-α-alanylalanine in water-ethanol, water-n-propanol, and water-isopropanol mixtures were measured calorimetrically at alcohol concentrations x ₂ = 0?0.4 mole fractions. The standard enthalpies of solution (Δ_sol H°) of the peptides and their transfer (Δ_tr H°) from water into the mixed solvents were calculated. The influence of the structure and properties of the solutes and mixture composition on the enthalpy characteristics were considered. The Δ_sol H° = f(x ₂) and Δ_tr H° = f(x ₂) dependences were found to have extrema. The enthalpy coefficients of pair interactions (h _xy ) between the peptide and alcohol molecules were calculated. The coefficients were positive and increased in the series ethanol, n-propanol, isopropanol.     

Adsorption from solutions of alicyclic thiophene derivatives on porous graphitic carbon

Adsorption of alicyclic thiophene derivatives from water-acetonitrile and n-hexane-dichloromethane solutions on porous graphitic carbon was studied by HPLC. Retention factors, the Henry adsorption constants, and the standard molar Gibbs energies were determined. The influence of the molecular surface area of the thiophene derivatives on the Gibbs energy of adsorption was discussed. A linear correlation between the Gibbs energy of adsorption from water-acetonitrile and that of adsorption from n-hexane-dichloromethane was found. It was assumed that the retention mechanisms for the thiophene heterocycles eluted with the mixed solvents water-acetonitrile and n-hexane-dichloromethane could be similar when HPLC on porous graphitic carbon is used.     

Intermolecular interactions in liquid adsorption chromatography

Summary For the investigation of intermolecular interactions in adsorption from solution, which are the basis of selectivity in molecular liquid chromatography (LC), it is convenient to use the LC method itself. Using this method the Henry's constants, K₁, and other thermodynamic adsorption characteristics of hydrocarbons and of a series of polar substances on hydroxylated silica surface were determined from aqueous solutions. On the basis of the adsorption of hydrocarbons from water solutions the structure of the chemically modifying layers formed by different hydrocarbon groups on the silica surface is considered. The role of conformation ability of straight-chain bonded phases is demonstrated. Hydrocarbons are adsorbed on the hydroxylated silica surface more strongly from aqueous solutions than from solutions in saturated hydrocarbons and their retention increases with the increase in the number of carbon atoms in the molecule. The retention in LC is determined by the intermolecular interaction of the solute and solvent molecules with the adsorbent, as well by the contribution of the intermolecular interaction, between the solute and the solvent.The thermodynamic characteristics of adsorption of cymarin from water-ethanol solutions on hydroxylated silica gel and on silica gel surface modified by diphenylsilyl groups is compared. The solubility of silica gel modified by diphenylsilyl groups at different composition of water-ethanol eluent at different temperatures is investigated.Enlarged text of a paper presented at the Sixteenth International Symposium on Advances in Chromatography, Barcelona, Spain, September 28–October 1, 1981.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

Adsorption of isopropanol on a nickel catalyst

The adsorption isotherms of isopropanol on a Ni catalyst (15 wt % on SiO₂) in the temperature range of 273–303 K are determined. An increase in the isosteric heat of adsorption and entropy of adsorption after treating the catalyst with high frequency plasma in hydrogen and adding 1.5 wt % of Ce is detected, with treatment involving glow discharge plasma in Ar and O₂ having virtually no impact on these values. At a low degree of surface filling, the adsorption isotherms are described by the equation of induced adsorption. It is concluded that adsorbed isopropanol molecules are present in two forms: positively and negatively charged.     

Nerve agents interacting with single wall carbon nanotubes: Density functional calculations

First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of nerve agent DMMP on typical zigzag (semiconducting) and armchair (metallic) single wall carbon nanotubes (SWCNTs). The adsorption energies for DMMP molecule on different adsorption sites on SWCNTs are obtained. The results indicate that DMMP is weakly bound to the outer surface of both the considered SWCNTs and the obtained adsorption energy values and binding distances are typical for the physisorption. We find that DMMP adsorptive capability of metallic CNTs is about twofold that of semiconducting one. The adsorption of DMMP on the higher chiral angle nanotubes was also investigated and the results indicate that nanotube’s chirality increases the adsorption capability of the tube but however the adsorption characteristic is typical for the physisorption. Furthermore, co-adsorption of two DMMP molecules on the SWCNTs as a single-layer/bi-layer of adsorbed molecules as well as the adsorption of one DMMP molecule on the CNT bundles consisting of three SWCNTs has also been examined. The obtained results reveal that for both the considered systems the binding energy was increased for the DMMP adsorption but it’s still typical for the physisorption, consistent with the recent experimental result. The study of the electronic structures and charge analysis indicate that no significant hybridization between the respective orbital takes place and the small interaction obtained quantitatively in terms of binding energies.     

Adsorption of aromatic compounds from solutions in <Emphasis Type="Italic">n</Emphasis>-heptane by modified nanoporous silica

The adsorption of anisole, anthracene, benzoic acid, benzene, naphthalene, phenanthrene, phenol, and carbon tetrachloride from dilute solutions in n-heptane on the modified nanoporous silica sorbents, Silasorb C18 and Phenyl-Silasorb, at 298 K is investigated by developing liquid chromatography. The Henry law constants and Helmholtz energy of adsorption for these aromatic compounds are calculated from their retention values. The adsorption isotherms of the aromatic compounds are calculated from their chromatographic peaks with allowances made for longitudinal smearing. A correlation is made between the Henry law constants calculated from the retention values of the aromatic compounds and from their adsorption isotherms. The influences of the surface modification of the silica adsorbents and the molecular structure of the aromatic compounds on their elution from the chromatographic column with n-heptane as the eluent are discussed. Contributions of functional groups in the molecules of the aromatic compounds to the Helmholtz energy of adsorption on different surfaces are estimated.     

Solvent effects on structural properties of SiO2 gel using n-octylamine as a catalyst

We have investigated the effect of solvent(ethanol, acetone, isopropanol) on the sol–gel process of tetraethylorthosilicate (TEOS) in which case, n-octylamine is used as polycondensation catalyst. Two sets of materials were prepared in the laboratory by using ethanol, acetone and isopropanol as solvents respectively: (1) xerogels from TEOS/n-octylamine, and (2) composites from TEOS/polyhedral oligomeric silsesquioxanes /n-octylamine. The chemical structure of xerogels was studied by FTIR, TG–DSC, scanning electronic microscope and nitrogen adsorption–desorption isotherms techniques. The results have shown that the solvents directly influence the gel time. Gel time is longer for acetone than that of ethanol and isopropanol. In the case of TEOS xerogels, the materials are essentially mesoporous when n-octylamine is used as a catalyst. Silica, when ethanol is used as a solvent, exhibits a more narrow pore size showing textures with a microstructure uniformly distributed with different porosity levels. However, when the aged-isopropanol gel is used, silica shows textures with a microstructure nonuniformly distributed.     

The behaviour of surfactants in concentrated acids 6. Micellization and interfacial behaviour of n-hexadecyltrimethylammonium bromide in mixtures of methane sulphonic acid and concentrated sulphuric acid

The surface and interfacial activity of cationic surfactants depends on the polarity of the bulk phase. If concentrated sulphuric acid is mixed with water, physical properties such as density, and surface and interfacial tension go through a maximum as the molar composition is changed. Also the properties of surfactants, i.e. surface activity and CMC, do not vary linearly in such aqueous mixtures. However, if concentrated sulphuric acid is mixed with methane sulphonic acid we find a linear relationship.The adsorption isotherms of n-hexadecyltrimethylammonium bromide at the surface (interface) of mixtures of H₂SO₄ and CH₃SO₃H/air (n-hexane) and the CMC were determined for various compositions of the acid phase and approximated by the Von Szyskowski equation. The change in the energy of micelle formation, — Δ_MG, and the energy of adsorption, — Δ_AG°, were calculated as a function of solvent composition. The surface concentration as a function of bulk concentration, the maximum surface coverage and the percentage surface concentration were also calculated.We found a linear dependence of the CMC, and of the lowering of σ and γ at the CMC, on the mole fraction of the acid phase. Also the constant A of the Von Szyskowski equation decreases and the logarithm of B increases linearly with the mole fraction of methane sulphonic acid. Therefore it is possible to calculate all the data needed for the characterization of systems of different acid composition. A special pattern was found for acid mixtures with X_CH3SO3H < 0.1. This can be explained by the transformation of micelle structure and the possibility that micelles can solubilize hydrocarbons in this region.