Vibrating-tube Densitometry Research Articles

Thermodynamic properties of sodium deoxycholate at the gel-sol transition

Sodium deoxycholate forms supramolecular aggregates in aqueous solution that are strongly dependent on temperature and pH. Upon fine tuning of these physicochemical parameters, two supramolecular aggregates can be achieved: a nanotubular phase having viscoelastic, gel-like, behavior and a sponge liquid phase. In this work, the thermodynamics of the transition between the two phases is characterized. Temperatures, volumes, and enthalpies of such transition have been experimentally determined using vibrating tube densitometry and differential scanning calorimetry. In addition, electron microscopy is used to follow the morphological transition between the two supramolecular structures. The dependencies of the thermodynamic properties with the pH and surfactant concentration of the solution are characterized, and the micrographs obtained from electron microscopy is used to explain the thermodynamics of the system.

A calorimetric, volumetric and combined SANS and SAXS study of hybrid siloxane phosphocholine bilayers

Siloxanes are molecules used extensively in commercial, industrial, and biomedical applications. The inclusion of short siloxane chains into phospholipids results in interesting physical properties, including the ability to form low polydispersity unilamellar vesicles. As such, hybrid siloxane phosphocholines (SiPCs) have been examined as a potential platform for the delivery of therapeutic agents. Using small angle X-ray and neutron scattering, vibrating tube densitometry, and differential scanning calorimetry, we studied four hybrid SiPCs bilayers. Lipid volume measurements for the different SiPCs compared well with those previously determined for polyunsaturated PCs. Furthermore, the different SiPC’s membrane thicknesses increased monotonically with temperature and, for the most part, consistent with the behavior observed in unsaturated lipids such as, 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine, and the branched lipid 1,2-diphytanoyl-sn-glyerco-3-phosphocholine (DPhyPC).

Methylene volumes in monoglyceride bilayers are larger than in liquid alkanes

The densities as a function of temperature of four fully hydrated saturated monoglycerides with even chain lengths ranging from eight to fourteen were determined by vibrating tube densitometry and their phase transition temperatures were determined by differential scanning calorimetry (DSC). We find the volume of a methylene group in a monoglyceride bilayer is 2% larger than in liquid alkanes at physiological temperatures, similar to the methylene group volumes found in phosphatidylcholine (PC) bilayers. Additionally, we carefully consider the traditional method of calculating component volumes from experimental data and note potential difficulties in this approach. In the literature, the ratio of terminal methyl volume (CH3) to methylene (CH2) volumes is typically assumed to be 2. By analysis of literature alkane data, we find this ratio actually ranges from 1.9 to 2.3 for temperatures ranging from 0 °C to 100 °C. For a rough sense of scale, we note that to effect a 2% reduction in volume requires of order 200 atmospheres of pressure, and pressures of this magnitude are biologically relevant. For instance, this amount of pressure is sufficient to reverse the effect of anesthesia. The component volumes obtained are an important parameter used for determining the structure of lipid bilayers and for molecular dynamics simulations.

Structure and thermodynamics of aqueous urea solutions from ambient to kilobar pressures: From thermodynamic modeling, experiments, and first principles simulations to an accurate force field description

Molecular simulations based on classical force fields are a powerful method for shedding light on the complex behavior of biomolecules in solution. When cosolutes are present in addition to water and biomolecules, subtle balances of weak intermolecular forces have to be accounted for. This imposes high demands on the quality of the underlying force fields, and therefore force field development for small cosolutes is still an active field. Here, we present the development of a new urea force field from studies of urea solutions at ambient and elevated hydrostatic pressures based on a combination of experimental and theoretical approaches. Experimental densities and solvation shell properties from ab initio molecular dynamics simulations at ambient conditions served as the target properties for the force field optimization. Since urea is present in many marine life forms, elevated hydrostatic pressure was rigorously addressed: densities at high pressure were measured by vibrating tube densitometry up to 500 bar and by X-ray absorption up to 5 kbar. Densities were determined by the perturbed-chain statistical associating fluid theory equation of state. Solvation properties were determined by embedded cluster integral equation theory and ab initio molecular dynamics. Our new force field is able to capture the properties of urea solutions at high pressures without further high-pressure adaption, unlike trimethylamine-N-oxide, for which a high-pressure adaption is necessary.

Methylene Volumes in Monoglyceride Bilayers are Larger than in Liquid Alkanes

Abstract The densities as a function of temperature of four fully hydrated saturated monoglycerides with even chain lengths ranging from eight to fourteen were determined by vibrating tube densitometry and their phase transition temperatures were determined by differential scanning calorimetry (DSC). We find the volume of a methylene group in a monoglyceride bilayer is 2% larger than in liquid alkanes at physiological temperatures, similar to the methylene group volumes found in phosphatidylcholine (PC) bilayers. Additionally, we carefully consider the traditional method of calculating component volumes from experimental data and note potential difficulties in this approach. In the literature, the ratio of terminal methyl volume (CH3) to methylene (CH2) volumes is typically assumed to be 2. By analysis of literature alkane data, we find this ratio actually ranges from 1.9 to 2.3 for temperatures ranging from 0 °C to 100 °C. For a rough sense of scale, we note that to effect a 2% reduction in volume requires of order 200 atmospheres of pressure, and pressures of this magnitude are biologically relevant. For instance, this amount of pressure is sufficient to reverse the effect of anesthesia. The component volumes obtained are an important parameter used for determining the structure of lipid bilayers and for molecular dynamics simulations.

Volume of Hsp90 Protein-Ligand Binding Determined by Fluorescent Pressure Shift Assay, Densitometry, and NMR.

Human heat shock protein 90 (Hsp90) is a key player in the homeostasis of the proteome and plays a role in numerous diseases, such as cancer. For the design of Hsp90 ATPase activity inhibitors, it is important to understand the relationship between an inhibitor structure and its inhibition potential. The volume of inhibitor binding is one of the most important such parameters that are rarely being studied. Here, the volumes of binding of several ligands to recombinant Hsp90 were obtained by three independent experimental techniques: fluorescent pressure shift assay, vibrating tube densitometry, and high-pressure NMR. Within the error range, all techniques provided similar volumetric parameters for the investigated protein-ligand systems. Protein-ligand binding volumes were negative, suggesting that the protein-ligand complex, together with its hydration shell, occupies less volume than the separate constituents with their hydration shells. Binding volumes of tightly binding, subnanomolar ligands were significantly more negative than those of weakly binding, millimolar ligands. The volumes of binding could be useful for designing inhibitors with desired recognition properties and further development as drugs.

Thermodynamic properties of indan: Experimental and computational results

Measurements leading to the calculation of thermodynamic properties in the ideal-gas state for indan (Chemical Abstracts registry number [496-11-7], 2,3-dihydro-1H-indene) are reported. Experimental methods were adiabatic heat-capacity calorimetry, differential scanning calorimetry, comparative ebulliometry, and vibrating-tube densitometry. Molar thermodynamic functions (enthalpies, entropies, and Gibbs energies) for the condensed and ideal-gas states were derived from the experimental studies at selected temperatures. Statistical calculations were performed based on molecular geometry optimization and vibrational frequencies calculated at the B3LYP/6-31+G(d,p) level of theory. Computed ideal-gas properties derived with the rigid-rotor harmonic-oscillator approximation are shown to be in excellent accord with ideal-gas entropies derived from thermophysical property measurements of this research, as well as with experimental heat capacities for the ideal-gas state reported in the literature. Literature spectroscopic studies and ab initio calculations report a range of values for the barrier to ring puckering. Results of the present work are consistent with a large barrier that allows use of the rigid-rotor harmonic-oscillator approximation for ideal-gas entropy and heat-capacity calculations, even with the stringent uncertainty requirements imposed by the calorimetric and physical property measurements reported here. All experimental results are compared with property values reported in the literature.

Molecular and component volumes of N,N-dimethyl-N-alkylamine N-oxides in DOPC bilayers

The volumetric properties of fluid bilayers formed of dioleoylphosphatidylcholine (DOPC) with incorporated N,N-dimethyl-N-alkylamine N-oxides (CnNO, n=6, 10–18 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry in the temperature interval from 20°C to 50°C. It was found that the DOPC and CnNO mixed ideally in the investigated composition range and hence the molecular volumes of DOPC (VDOPC) and incorporated CnNO (VCnNO) were constant and additive within error limits. From the temperature dependencies of the molecular volumes of DOPC and CnNO their coefficients of isobaric thermal expansivities in the investigated temperature interval were obtained. The VCnNO volumes of CnNO incorporated into DOPC bilayers showed linear dependencies on the CnNO alkyl chain length at all measured temperatures. This allowed to calculate the component volume of the CnNO methylene group (VCH2) at several temperatures and its coefficient of isobaric thermal expansivity. Using the assumption that the component volume of the CnNO methyl group VCH3=2VCH2 we also calculated the component volume and the coefficient of isobaric thermal expansivity of the hydrophilic group of CnNO (VNO). We found that the VCH2 volume increases in the whole temperature interval whereas the VNO volume decreases.

Thermodynamic properties of 1-phenylnaphthalene and 2-phenylnaphthalene

Measurements leading to the calculation of thermodynamic properties in the ideal-gas state for 1-phenylnaphthalene (Chemical Abstracts registry number [605-02-7]) and 2-phenylnaphthalene (Chemical Abstracts registry number [612-94-2]) are reported. Experimental methods for 1-phenylnaphthalene were adiabatic heat-capacity calorimetry, differential scanning calorimetry, inclined-piston manometry, comparative ebulliometry, vibrating-tube densitometry, and combustion calorimetry. For 2-phenylnaphthalene, the experimental methods were adiabatic heat-capacity calorimetry, differential scanning calorimetry, and comparative ebulliometry. Critical properties were estimated for both compounds. Molar thermodynamic functions (enthalpies, entropies, and Gibbs free energies) for the condensed and ideal-gas states were derived from the experimental studies at selected temperatures. Statistical calculations were performed based on molecular geometry optimization and vibrational frequencies calculated at the B3LYP/6-31+G(d,p) and B3LYP/cc-pVTZ levels of theory. Ideal-gas entropies derived with two the independent methods are shown to be in good accord for 1-phenylnaphthalene, but significant differences are apparent for 2-phenylnaphthalene. These differences are likely due to a disorder of unknown type in the crystals of 2-phenylnaphthalene at low temperatures, as evidenced by the presence of a glass-like transition in the measured heat capacities for the solid state. All experimental results are compared with property values reported in the literature.

Molecular and component volumes of saturated n-alkanols in DOPC + DOPS bilayers

The volumetric properties of fluid bilayers consisting of dioleoylphosphatidylcholine (DOPC, 96 wt%) and dioleoylphosphatidylserine (DOPS, 4 wt%) with incorporated saturated n-alkanols (CnOH, n = 10–16 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry. The mixing of DOPC and DOPS was found to be ideal and the molecular volumes of pure lipids V DOPC and V DOPS are additive in mixed bilayers. The increase of V DOPS with temperature was steeper than that of V DOPC as quantified by significantly higher coefficient of isobaric thermal expansivity γ. This difference is supposed to be related to the high thermal expansivity of the serine headgroup. The molecular volumes of lipids and CnOH ( V CnOH) in fluid DOPC + DOPS bilayers are constant and additive within error limits for n = 12 and 16 and for the CnOH mole fractions in the lipid bilayer x AL ≤ 0.5 and x AL ≤ 0.3, respectively. In homologous series of CnOHs incorporated into DOPC + DOPS bilayer, the V CnOH volume increases linearly with the CnOH chain length n at x AL = 0.29. From this dependence obtained at several temperatures, the component volume of the CnOH hydroxyl group V OH was calculated and found to decrease between 20 °C and 51 °C while the component volume of the CnOH methylene group V C H 2 increases with temperature. The excess molecular volume, Δ V, for the transfer of CnOH into the DOPC + DOPS bilayers display a biphasic dependence on the alkyl length, n, with a minimum for n = 12. When calculated with respect to the pure CnOH, Δ V is negative for homologs with n > 8.

Thermodynamic properties of 1,2-dihydronaphthalene: Glassy crystals and missing entropy

Measurements leading to the calculation of the standard thermodynamic properties for gaseous 1,2-dihydronaphthalene (Chemical Abstracts registry number [447-53-0]) are reported. Experimental methods include oxygen combustion-bomb calorimetry, adiabatic heat-capacity calorimetry, vibrating-tube densitometry, comparative ebulliometry, and inclined-piston gauge manometry. 1,2-Dihydronaphthalene decomposed significantly when heated to temperatures above T = 480 K. Consequently, the critical temperature, critical pressure, and critical density were estimated. Standard molar entropies, standard molar enthalpies, and standard molar Gibbs free energies of formation were derived at selected temperatures between T = 250 K and 500 K. The standard state is defined as the ideal gas at the pressure p = p ∘ = 101.325 kPa. Standard entropies are compared with those calculated statistically on the basis of assigned vibrational spectra from the literature for the vapor phase. A large and near constant difference between the entropies calculated statistically and those determined calorimetrically was observed over the entire temperature range studied. Two glass-like features are observed in the heat capacity against temperature curve for the solid state, indicating that the crystals are disordered. A quantitative accounting for the entropy discrepancy is proposed based on possible molecular orientations of 1,2-dihydronaphthalene. Results are compared with experimental values reported in the literature.

Densities and heat capacities of aqueous arsenious and arsenic acid solutions to 350 °C and 300 bar, and revised thermodynamic properties of [formula omitted], [formula omitted] and iron sulfarsenide minerals

Densities and heat capacities of aqueous arsenious and arsenic acid solutions of 0.1–0.6 mol/kg were measured using the flow vibrating tube densitometry and differential calorimetry at temperatures to 350 °C and pressures to ∼310 bar. The standard partial molal volumes V ∘ and heat capacities C p ∘ of the neutral aqueous As III and As V (oxy)hydroxide species, As(OH) 3 and AsO(OH) 3, were obtained from these data, via corrections for partial dissociation and extrapolation to infinite dilution. The generated V ∘ and C p ∘ values, together with the existing data on As III oxide and sulfide minerals solubilities and low-temperature As III–As V aqueous solution equilibria, were used to refine the thermodynamic properties of As hydroxide complexes over a wide temperature–pressure range, in the framework of the revised HKF equation of state and using correlation algorithms recently proposed for aqueous neutral species. These revised properties were combined with solubility data for arsenopyrite (FeAsS) and direct calorimetric heat capacity and enthalpy measurements reported in the literature for arsenopyrite, loellingite (FeAs 2), and westerveldite (FeAs), to generate a consistent set of thermodynamic parameters for these iron sulfarsenides. The new Gibbs free energy values of arsenopyrite and loellingite resulting from these properties imply lower solubilities of iron sulfarsenides in aquatic environments than have been assumed. The thermodynamic properties of arsenic aqueous species and solid phases obtained in this study provide quantitative constraints on As-bearing mineral stabilities and arsenic transport by geological fluids.

Viscosity-induced errors in the density determination of room temperature ionic liquids using vibrating tube densitometry

The effect of sample viscosity in the determination of the density of room temperature ionic liquids by means of vibrating tube densitometry was analysed in detail. By measuring the densities and viscosities of density standards from low to high viscosity, the viscosity-induced or damping errors were quantified. This allowed us to correctly determine the density of 1-butyl-3-methyl-imidazolium tetrafluoroborate [bmim][BF 4], 1-hexyl-3-methyl-imidazolium tetrafluoroborate [hmim][BF 4] and 1-octyl-3-methyl-imidazolium tetrafluoroborate [omim][BF 4]. Measurements were carried out at atmospheric pressure and within the temperature interval (288.15–323.15) K using an Anton-Paar DSA-48 vibrating tube densimeter and an Anton-Paar AMVn viscometer, respectively. From these data, the effect of viscosity on the isobaric thermal expansivity was evaluated. In the case of the density, it is found that the viscosity correction can be as high as 0.0007 g cm −3 and it decreases with temperature. In the case of the isobaric thermal expansivity, it is found that the viscosity correction passes through a maximum as temperature is increased. It is observed that α p does not increase with T for the studied room temperature ionic liquids (RTILs), thereby clarifying previous contradictory results.

Standard Thermodynamic Properties of H3PO4(aq) over a Wide Range of Temperatures and Pressures

The densities and heat capacities of solutions of phosphoric acid, 0.05 to 1 mol kg-1, were measured using flow vibrating tube densitometry and differential Picker-type calorimetry at temperatures up to 623 K and at pressures up to 28 MPa. The standard molar volumes and heat capacities of molecular H3PO4(aq) were obtained, via the apparent molar properties corrected for partial dissociation, by extrapolation to infinite dilution. The data on standard derivative properties were correlated simultaneously with the dissociation constants of phosphoric acid from the literature using the theoretically founded SOCW model. This made it possible to describe the standard thermodynamic properties, particularly the standard chemical potential, of both molecular and ionized phosphoric acid at temperatures up to at least 623 K and at pressures up to 200 MPa. This representation allows one to easily calculate the first-degree dissociation constant of H3PO4(aq). The performance of the SOCW model was compared with the other approaches for calculating the high-temperature dissociation constant of the phosphoric acid. Using the standard derivative properties, sensitively reflecting the interactions between the solute and the solvent, the high-temperature behavior of H3PO4(aq) is compared with that of other weak acids.

Component volumes of unsaturated phosphatidylcholines in fluid bilayers: a densitometric study

The specific volumes of six 1,2-diacylphosphatidylcholines with monounsaturated acyl chains (diCn:1PC, n = 14–24 is the even number of acyl chain carbons) in fluid bilayers in multilamellar vesicles dispersed in H 2O were determined by the vibrating tube densitometry as a function of temperature. From the data obtained with diCn:1PC ( n = 14–22) vesicles in combination with the densitometric data from Tristram-Nagle et al. [Tristram-Nagle, S., Petrache, H.I., Nagle, J.F., 1998. Structure and interactions of fully hydrated dioleoylphosphatidylcholine bilayers. Biophys. J. 75, 917–925.] and Koenig and Gawrisch [Koenig, B.W., Gawrisch, K., 2005. Specific volumes of unsaturated phosphatidylcholines in the liquid crystalline lamellar phase. Biochim. Biophys. Acta 1715, 65–70.], the component volumes of phosphatidylcholines in fully hydrated fluid bilayers at 30 °C were obtained. The volume of the acyl chain CH and CH 2 group is V CH = 22.30 Å 3 and V C H 2 = Å 3, respectively. The volume of the headgroup including the glyceryl and acyl carbonyls, V H, and the ratio of acyl chain methyl and methylene group volumes, r = V C H 3 : V C H 2 are linearly interdependent: V H = a − br, where a = 434.41 Å 3 and b = −55.36 Å 3 at 30 °C. From the temperature dependencies of component volumes, their isobaric thermal expansivities ( α X = V X − 1 ( ∂ V X / ∂ T ) where X = CH 2, CH, or H were calculated: α C H 2 = 118.4 × 10 − 5 K − 1 , α CH = 71.0 × 10 −5 K −1, α H = 7.9 × 10 −5K −1 (for r = 2) and α H = 9.6 × 10 −5 K −1 (for r = 1.9). The specific volume of diC24:1PC changes at the main gel–fluid phase transition temperature, t m = 26.7 °C, by 0.0621 ml/g, its specific volume is 0.9561 and 1.02634 ml/g at 20 and 30 °C, respectively, and its isobaric thermal expansivity α = 68.7 × 10 −5 and 109.2 × 10 −5 K −1 below and above t m, respectively. The component volumes and thermal expansivities obtained can be used for the interpretation of X-ray and neutron scattering and diffraction experiments and for the guiding and testing molecular dynamics simulations of phosphatidylcholine bilayers in the fluid state.

Packing properties of 1-alkanols and alkanes in a phospholipid membrane

We have used vibrating tube densitometry to investigate the packing properties of four alkanes and a homologous series of ten alcohols in fluid-phase membranes of dimyristoyl phosphatidylcholine (DMPC). It was found that the volume change of transferring these compounds from their pure states into the membrane, Δ V m(pure → mem), was positive for small (C4–C6) 1-alkanols while it was negative for larger alcohols and all alkanes. The magnitude of Δ V m(pure → mem) ranged from about + 4 cm 3/mol for alcohols with an alkyl chain about half the length of the fatty acids of DMPC, to − 10 to − 15 cm 3/mol for the alkanes and long chain alcohols. On the basis of these observations, previously published information on the structure of the membrane–solute complexes and the free volume properties of (pure) phospholipid membranes, we suggest that two effects dominate the packing properties of hydrophobic solutes in DMPC. First, perturbation of the tightly packed interfacial zone around the ester bonds and first few methylene groups of DMPC brings about a positive contribution to Δ V m(pure → mem). This effect dominates the volume behavior for alcohols like 1-butanol, 1-pentanol and 1-hexanol. More hydrophobic solutes penetrate into the membrane core, which is loosely packed. In this region, they partially occupy interstitial (or free-) volume, which bring about a denser molecular packing and generate a negative contribution to Δ V m(pure → mem).

Heat Capacities and Volumes of Aqueous Dicarboxylate Salt Solutions of Relevance to the Bayer Process

Apparent molar volumes (Vφ) and heat capacities at constant pressure (Cpφ) of aqueous solutions of sodium oxalate (Na2Ox), sodium succinate (Na2Suc), sodium malonate (Na2Mal), and potassium oxalate (K2Ox) were determined at 25 °C up to their saturation limits using vibrating tube densitometry and flow calorimetry. These data were fitted using the Redlich−Rosenfeld−Meyer and Pitzer models. It was established using literature data that Vφ and Cpφ of Na+ and K+ salts with a common anion are additive within well-defined limits even up to high concentrations. This was used to estimate hypothetical values of Cpφ and Vφ at high ionic strengths for the sparingly soluble Na2Ox, which are required for the thermodynamic modeling of concentrated electrolyte mixtures such as Bayer process solutions. These values were in reasonable agreement with those estimated using a Pitzer model with parameters for Na2SO4(aq).

Thermodynamic Properties of 2-Methylquinoline and 8-Methylquinoline

Measurements leading to the calculation of the standard thermodynamic properties for gaseous 2-methylquinoline (Chemical Abstracts registry number [91-63-4]) and 8-methylquinoline (Chemical Abstracts registry number [611-32-5]) are reported. Experimental methods include oxygen bomb calorimetry, adiabatic heat capacity calorimetry, vibrating-tube densitometry, comparative ebulliometry, inclined-piston gauge manometry, and differential scanning calorimetry (dsc). The critical temperature was measured with a differential scanning calorimeter. The critical pressure and critical density were estimated. Standard molar entropies, standard molar enthalpies, and standard molar Gibbs free energies of formation were derived at selected temperatures between 298.15 K and 700 K. All results are compared with experimental values reported in the literature.

Possible precursors and products of deep hydrodesulfurization of gasoline and distillate fuels IV. Heat capacities, enthalpy increments, vapor pressures, and derived thermodynamic functions for dicyclohexylsulfide between the temperatures (5 and 520) K

Measurements leading to the calculation of the standard thermodynamic properties for gaseous dicyclohexylsulfide (Chemical Abstracts registry number [7133-46-2]) are reported. Experimental methods include adiabatic heat-capacity calorimetry, and inclined-piston gauge manometry combined with earlier reported measurements of combustion calorimetry, vibrating-tube densitometry, comparative ebulliometry, and differential-scanning calorimetry (d.s.c.). Critical properties are estimated for dicyclohexylsulfide. Standard molar entropies, standard molar enthalpies, and standard molar Gibbs free energies of formation are derived at selected temperatures between (298.15 and 520) K.

Possible precursors and products of deep hydrodesulfurization of gasoline and distillate fuels III. The thermodynamic properties of 1,2,3,4-tetrahydrodibenzothiophene

Measurements leading to the calculation of the standard thermodynamic properties for gaseous 1,2,3,4-tetrahydrodibenzothiophene (Chemical Abstracts registry number [16587-33-0]) are reported. Experimental methods include combustion calorimetry, adiabatic heat-capacity calorimetry, vibrating-tube densitometry, comparative ebulliometry, inclined-piston gauge manometry, and differential-scanning calorimetry (d.s.c.). Critical properties are estimated for 1,2,3,4-tetrahydrodibenzothiophene. Standard molar entropies, standard molar enthalpies, and standard molar Gibbs free energies of formation are derived at selected T between (298.15 and 600) K.