Partial Molar Compressibility Research Articles

Partial molar isentropic and isothermal compressibilities of some N-acetyl amino acid amides in aqueous solution at 298.15 K

The partial molar isentropic compressibilities at infinite dilution, K°S,2, have been determined for eight N-acetyl amino acid amides and four N-acetyl-N′-methyl amino acid amides in aqueous solution at 298.15 K. The partial molar isothermal compressibilities at infinite dilution, K°T,2, were derived from the K°S,2 results using partial molar expansibilities and heat capacities determined in previous work. The K°T,2 and K°S,2 results were used to calculate the contributions to the compressibility of the amino acid side-chains. These results are compared with those derived using alternative small solutes that have been proposed as models for the amino acid side-chains in proteins. The applicability of the various side-chain contributions in group additivity schemes for the calculation of the compressibilities of poylpeptides and proteins is discussed.

Thermodynamic study of solvation of some amino acids, diglycine and lysozyme in aqueous and mixed aqueous solutions

Apparent molar adiabatic compressibilities and viscosities of glycine, dl-α-alanine, dl-α-amino- n-butyric acid, l-valine, l-leucine and diglycine have been determined in aqueous and mixed aqueous solutions of m B =1.0, 2.0, 3.0, 4.0 and 5.0 aqueous n-propanol solutions at 298.15 K. From these data the partial molar adiabatic compressibilities and viscosity B-coefficients have been evaluated to calculate the corresponding transfer functions. The partial molar adiabatic compressibilities of transfer at infinite dilution Δ tr K° 2,S for all the studied model compounds are positive and increase with the concentration of n-propanol. Positive and negative B-coefficients of transfer Δ tr B have been observed for the studied amino acids in lower and in higher concentration of n-propanol, respectively. The activation free energy for viscous flow in aqueous and mixed aqueous n-propanol solutions has been calculated from B-coefficient and partial molar volume data. Hydration numbers and interaction coefficients have also been calculated from these data. These parameters have been discussed in terms of solute–cosolvent interactions. Thermal denaturation of lysozyme has also been studied using UV-visible spectrophotometer in aqueous and in mixed aqueous solutions of n-propanol, 1,2-propandiol and glycerol. The thermodynamic parameters accompanying the thermal denaturation have been evaluated. The results have been explained on the basis of competing patterns of interactions of the cosolvents with the native↔denatured reaction. The preferential interaction parameters have been calculated from these thermodynamic data and by correlating the surface tension data of n-propanol and 1,2-propandiol to the surface area of the protein. Some parallelism in the patterns of interactions has been observed for the studied model compounds and protein in the aqueous solutions of these solvents.

Volumetric characterization of homopolymeric amino acids.

We have determined the partial molar volumes, expansibilities, and adiabatic compressibilities for poly(L-alanine), poly(L-proline), and poly(L-threonine) within the temperature range of 18-55 degrees C. In addition, we have determined at 25 degrees C changes in volume, DeltaV, and adiabatic compressibility, DeltaK(S), associated with the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) and the alpha-to-beta transition of poly(L-lysine). We have interpreted our volumetric data as suggesting that poly(L-alanine) and poly(L-proline) are not fully unfolded and, probably, retain some solvent-inaccessible core. Further, we propose that poly(L-threonine) is fully unfolded with the majority of its atomic groups being solvent-exposed. Near zero changes in volume and compressibility accompanying the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) suggest that, in the absence of fortuitous compensations, the coil-to-helix transitions of the polypeptides do not result in any significant enhancement of solute hydration. By contrast, the alpha-to-beta transition of poly(L-lysine) causes slight but statistically significant increases in volume and compressibility, an observation that may suggest that the beta-sheet conformation of poly(L-lysine) is slightly less hydrated than its alpha-helical conformation. In general, our results provide a quantitative volumetric description of the hydration properties of the homopolymeric polypeptides investigated. Such characterizations should prove useful in developing an understanding of the role that solvent plays in the stabilization/destabilization of folded protein structures and protein complexes.

Partial molar volume and compressibility of a molecule with internal degrees of freedom

Statistical mechanical expressions for the partial molar volume (PMV) and the partial molar compressibility (PMC) of a solute molecule with internal degrees of freedom are derived. The equilibrium PMV is expressed simply by the conformational ensemble average of PMV. The equilibrium PMC consists of two terms: the conformational ensemble average of PMC and the mean square fluctuation of PMV along conformational axis. As an illustrative application of the theory, the equilibrium PMV and PMC of butane in aqueous solution are calculated from the relations, in which the thermodynamic quantities of each conformer appearing in the relations are obtained by the reference interaction site model theory. The conformational ensemble average of PMC is the predominant contribution to the equilibrium PMC in this particular example. Possibility of shedding light on the protein conformation in terms of PMV and PMC is discussed.

Partial Molar Volumes and Partial Molar Adiabatic Compressibilities of Fe(III) Tetrafluoroborate Complexes with DMSO, Pyridine, and Pyridine Derivatives

The apparent and limiting apparent molar volumes of dilute aqueous solutions of KBF4, and the complexes [Fe(DMSO)6](BF4)3, [Fe(Py)4(H2O)2](BF4)3, [Fe(4-Mepy)2(H2O)2](BF4)3, and [Fe(4-Etpy)2(H2O)2](BF4)3 were determined from density data measured at 5, 15, and 25°C. The apparent and limiting apparent molar adiabatic compressibilities of these complexes were determined from ultrasonic sound velocities measured at the same temperatures in dilute aqueous solutions. The volume change associated with complex formation is discussed in terms of the nature of the coordinate bond and also the role of the central metal atom and ligands in the solvation behavior of these complexes.

Volumetric effects of ionization of amino and carboxyl termini of α,ω-aminocarboxylic acids

We have determined the partial molar volumes and adiabatic compressibilities of a homologous series of six α,ω-aminocarboxylic acids over a broad pH range at 25 °C. We interpret the resulting data in terms of the changes in hydration associated with neutralization of amino and carboxyl termini. By combining our volumetric results with pH-dependent data on 1-anilinonaphthalene-8-sulfonic acid fluorescence we propose the following explanation to the long-standing observation that changes in volume and compressibility accompanying neutralization of a carboxyl group depend on the type of the solute in contrast to solute-independent changes in these parameters accompanying neutralization of an amino group. Unlike amino groups, neutralized carboxyl groups are capable of forming hydrogen-bonded structures stabilized by hydrogen bonds between the carbonyl oxygen of one solute molecule and the hydroxyl group of another molecule. Formation of such hydrogen-bonded structures causes an additional decrease in solute hydration with concomitant increases in volume and compressibility. Furthermore, solutes with large aliphatic moieties may form larger associates stabilized, in addition to intermolecular hydrogen bonds, by hydrophobic interactions which will result in further increases in volume and compressibility. In the aggregate, our results emphasize the need for further studies focused on developing an understanding of the role of electrostatic interactions in stabilizing/destabilizing proteins and protein complexes.

The acoustical and structural changes of aqueous solutions due to transition to molten supercooled salts

The analysis of concentration dependencies of acoustical properties of aqueous electrolyte solutions has been carried out in a wide range of concentration. Among the large number of systems, including binary and multicomponent aqueous solutions of salts, some systems (∼20) are singled out where linear concentration behaviour of the square of sound velocity (u 2) is observed down to saturation. The linear extrapolation of u 2 up to 100% of saline component for solutions of halogens and nitrates of alkali metals yields a value of u 2 for the molten, supercooled salt. The interpretation of such acoustical dependencies is provided on the basis of a structural scheme including bulk water (1), water in hydration shells (2) and ionic clusters (3) in two concentration bands. The main principle of connection of these subsystems is the complementary organization of configuration (1), (2), (3). In the framework of this model the analytical expressions were deduced for a sound velocity of water in hydration shells in the first and second concentration bands. The condition of linearity of u 2 from 100% of water to 100% of salt was obtained. The deviations from linear concentration dependence are explained by the concentration variations of hydrophobic and hydrophilic hydration, or formation of ionic complexes. The calculated magnitudes of partial molar adiabatic compressibility of salts at infinite dilution ( k 2x 0 ) are in a good agreement with results obtained from concentration extrapolation of experimental data of compressibility coefficients and densities in dilute solution. The expression for the calculation of nonlinear acoustic parameters of solution and molten, supercooled salt (B/A) L was deduced using experimental data of dilute solution and density of molten supercooled salt. The packing factor of pure water and salt in molten, supercooled state was estimated.

Partial Molar Volume and Compressibility of Alkali−Halide Ions in Aqueous Solution: Hydration Shell Analysis with an Integral Equation Theory of Molecular Liquids

The partial molar volume and partial molar compressibility of alkali−halide ions in aqueous solution at infinite dilution are calculated based on the RISM−Kirkwood−Buff theory. The theoretical results are in qualitative agreement with the corresponding experimental data. The volume and compressibility values are decomposed into the volume-exclusion and electrostriction contributions. The volume-exclusion effect qualitatively determines the dependence of the partial molar volume on the ion size, whereas the electrostriction effect dominates in the size dependence of the partial molar compressibility. The partial molar volume and compressibility are further analyzed by using the hydration shell model which enables us to distinguish a contribution from each hydration shell. For the primary hydration shell, we can make contact with the classical models of ion hydration proposed by Frank−Wen and Samoilov. Water molecules in the immediate vicinity of an ion always give a negative contribution to its partial molar volume. The first hydration shell makes a negative contribution to the partial molar compressibility for the ions classified with the “positive hydration” in terms of Samoilov's model and does the opposite contribution for those with the “negative hydration”. The reason the water structure around a negatively hydrated ion is more compressible is explained in terms of density fluctuations around the ions from a viewpoint of the Landau fluctuation formula for the thermodynamic response functions.

Volumetric Properties of Surfactant in Water and in Mixed Solvent from Sound Velocity and Density Measurements

Abstract A simple, automatic and economic apparatus was constructed for a precise measurement of sound velocity using a resonance method. It gives the sound velocity value with an accuracy of less than 3×10-3% (±2 cm s-1). Sound velocity and density measurements were carried out in aqueous solutions of several compounds of alkyltrimethylammonium bromide (C1TAB, C8TAB, C10TAB, C12TAB, C14TAB, and C16TAB, where Ci is the number of carbons in straight-chain hydrocarbon groups) at 25 °C. Also, the measurements for C12TAB in a mixed solvent with water and 2-propanol were performed. The critical micelle concentrations (CMC) were determined from the inflection point of the compressibility and electric conductivity. The partial molar adiabatic compressibility (κm* ) and the partial molar volume (Vm*) for the surfactant were determined below and above CMC in water and in the mixed solvent from the apparent molar quantities. The partial molar quantities were also found to be dependent on the concentrations of the added alcohol. These results were interpreted from the points of view of the surfactant conditions and solvent structures.

Effect of molecular structure on thermodynamic properties of carbohydrates. A calorimetric study of aqueous di- and oligosaccharides at subzero temperatures

For aqueous solutions of di- and oligosaccharides thermodynamic properties have been investigated at subzero temperatures using differential scanning calorimetry. The amount of unfrozen water observed is found to increase linearly with the glass transition temperatures of anhydrous carbohydrates. Furthermore, the amount of unfrozen water shows a linear relationship with known solution properties of aqueous carbohydrates, such as partial molar compressibility and heat of solution. The different effectiveness among various di- and oligosaccharides to avoid ice formation is associated with the combination of constitutive monosaccharides and attendant molecular structure features including the position and type of the glycosidic linkage between the constituent units. More unfrozen water is induced in the presence of a carbohydrate having a poorer compatibility with the three-dimensional hydrogen-bond network of water. A series of these results obtained imply that there is a common key of carbohydrate stereochemistry governing several different thermodynamic amounts of a given system involving carbohydrates. In this context, a modified stereospecific-hydration model can be used to interpret the present results in terms of stereochemical effects of carbohydrates.

Complex formation between alkane-α, ω-diols and cyclodextrins studied by partial molar volume and compressibility measurements

The binding of a series of alkane-α,ω-diols, HO(CH2)nOH, n = 4 to 7, to α- and β-cyclodextrin (CD) has been studied by measurements of partial molar volumes (PMVs) and isentropic partial molar compressibilities (PMCs) at 25°C. From the PMV and PMC data, changes in the partial molar quantities upon going from a free state in aqueous solution to a complexed state were evaluated for the diols. Negative changes in PMV and PMC were observed for complex formation with α-CD, while positive values were obtained for the β-CD complexes. Equilibrium constants for the different complexes, assuming the formation of 1:1 complexes, were evaluated from the PMV and/or PMC data, and were found to increase with increasing chain length of the included diol for both α- and β-CD complexes. The equilibrium constant for complex formation is generally higher for the β-CD than for the α-CD complexes.

Adiabatic Compressibility of Aqueous Solutions of Amphiphiles with an Ammonium Group as the Hydrophilic Domain.

The apparent molar volume and the apparent molar adiabatic compression of aqueous solutions of n-alkylammonium bromide (CnH2n+1NH3Br; n=1-8, 10) and n-alkyltrimethylammonium bromide (CnH2n+1N(CH3)3Br; n=1, 8, 10) were measured using a vibration-tube density meter and a sing-around ultrasound velocity meter at 5°C, and the partial molar volume and the partial molar adiabatic compression at infinite dilution (φV0 and φK0) were evaluated. It was confirmed that φK0 linearly decreased with increasing alkyl chain length while φV0 linearly increased. Differences of φK0 between CnH2n+1NH3Br and CnH2n+1N(CH3)3Br with the same alkyl chain were negligibly small, which was in good agreement with the fact that the successive N-methylation of ammonium cation had only a little effect on compression. These results could be interpreted in terms of the compensation between the increase of hydrophobic hydration and the decrease of electrostriction of water around the ammonium group, leading the net compression being almost constant. When the amphiphiles with an octyl or a decyl group form micelles above their critical micelle concentration, on the contrary, the adiabatic compression of CnH2n+1NH3Br was distinctly larger than that of CnH2n+1N(CH3)3Br. The reason for the difference in compression of these micelles was discussed concerning the counter ion binding and the aggregation number.

Study on intermolecular interactions in supercritical fluids by partial molar volume and isothermal compressibility

The apparent densities of H 2O–CO 2, H 2S–CO 2, H 2–CO 2, and CH 4–CO 2 binary systems were determined at 308.15 K and different pressures. The concentration of the solutes was 0.3 mol%. The partial molar volumes (PMVs) of the solutes and isothermal compressibility of mixed fluids were calculated using the experimental data. The PMVs of the two polar solutes (H 2O and H 2S) in supercritical CO 2 are negative, and a minimum can be observed in each PMV versus pressure curve at the pressure where isothermal compressibility of the fluid is largest. The absolute value of the minimum of H 2O is smaller than that of H 2S, probably because of the hydrogen bonding between water molecules. However, the PMVs of the two non-polar solutes are positive, and the maximum value of H 2 is larger than that of CH 4. The effect of molecular size and polarity on the intermolecular interactions between solutes and solvents were discussed.

Thermodynamics of Micellization of Sodium Alkyl Sulfates in Water at High Temperature and Pressure

Apparent molar volumes VΦ,S were determined for sodium octyl, decyl, and dodecyl sulfates in water at 2 and 19 MPa from 25 to 130 °C. The shapes of VΦ,S vs the surfactant concentration curves depend on the surfactant alkyl chain, temperature and pressure. The standard partial molar volumes were calculated from data in the premicellar region whereas the partial molar volumes of the surfactant in the micellar phase were obtained from data in the postmicellar region. The partial molar expansibility and compressibility were evaluated from the dependence of the partial molar volume on temperature and pressure, respectively. Attention was focused to the expansibility and its pressure coefficient since studying the pressure effect on the expansibility is equivalent to studying the temperature effect on the compressibility. The hydrophilic and the hydrophobic (methylene group) contributions to the expansibility were evaluated. The comparison between the present and the alkyltrimethylammonium bromides data evidenc...

Volumetric characterization of the hydration properties of heterocyclic bases and nucleosides

We have determined the partial molar volumes, expansibilities, and adiabatic compressibilities of six heterocyclic nucleic acid bases, five ribonucleosides, and six 2′-deoxyribonucleosides within the temperature range 18–55°C. We interpret the resulting data in terms of the hydration of the component hydrophobic and polar atomic groups. From our temperature-dependent volumetric studies, we found that the total contraction of water caused by polar groups of each individual heterocyclic base and nucleoside depends on the proximity and chemical nature of other functional groups of the solute. In addition, the compressibility contributions of polar groups vary greatly in sign and magnitude depending on the surrounding functional groups. In agreement with previous studies, our results are suggestive of little or no interaction between the sugar and base moieties of a nucleoside. In general, our data shed light into the hydration properties of individual heterocyclic bases and nucleosides, which may have significant implications for the sequence-dependent hydration of nucleic acids. We discuss the potential importance of our results for developing an understanding of the role that solvent plays in the stabilization/destabilization of nucleic acid structures.

Solubilization of alcohols in SDS and TTAB from isentropic partial molar compressibilities and solubilities

Partition coefficients of some medium chain alcohols between water and micelles have been determined at infinite alcohol dilution from measured isentropic partial molar compressibilities, at intermediate alcohol concentrations by NMR self-diffusion measurements and at the phase boundary from total solubilities. The surfactants studied were sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). The agreement between the different methods of measurement is poor. Inspection of the data show, however, that the fraction of alcohol solubilized is constant with respect to alcohol concentration at constant surfactant concentration, and this means that the partition coefficient must be dependent on the alcohol concentration. The partial molar volumes and compressibilities of the alcohols in the solubilized state have been evaluated at infinite dilution, showing increased volumes and compressibilities compared to the aqueous state, as is normal for alcohols in non-aquoeus environments. As alcohol is added at constant surfactant concentration, a significant increase in the partial molar compressibility is observed above a certain alcohol concentration. This happens when the alcohol to surfactant ratio is in the region of 1.7–2.0. The increased compressibility suggests that the alcohol molecules, on average, experience a more hydrophobic environment. A reasonable interpretation is that some of the solubilized alcohol molecules will be solubilized in the micellar interior, once the alcohol to surfactant ratio exceeds a critical value.

Thermodynamic and Spectroscopic Study of a Molecular Rotaxane Containing a Bolaform Surfactant and β-Cyclodextrin

A thermodynamic and proton NMR spectroscopy study of a bolaform type surfactant, docosane 1,22-bis(trimethylammonium bromide), has been carried out in water and in the presence of β-cyclodextrin (β-CD) at 298 K. Density and sound velocity data for the aqueous solutions of the bolaform in both systems were analyzed to calculate the molar apparent and partial volumes and adiabatic compressibilities. For the binary system, the molar partial compressibilities and volumes of the bolaform in water as a function of concentration have been obtained. Compressibility data indicate that the surfactant, both in monomer or in micelle form, is partially folded. For the ternary system, a remarkable increase of the thermodynamic properties of the surfactant is observed at infinite dilution with respect to the value in water and a shift of the critical micelle concentration in an extension that points to complexes of predominantly 2:1 stoichiometry. The values of the transfer properties of the bolaform at infinite dilutio...

Aqueous Guanidinium Salts: Part I. Densities, Ultrasonic Velocities, and Apparent Molar Properties

The densities and ultrasonic velocities in aqueous solutions of guanidinium chloride, bromide, acetate, perchlorate and sulfate have been measured at 25°C. The infinite dilution partial molar volumes and compressibilities of the salts were estimated and analyzed by the Pitzer equations. The concentration dependence of densities and sound velocities of the guanidinium salts can be correlated by the Pitzer equations with an average root-mean-square deviation of 0.025 kg-m-3 and 0.31 m-s-1, respectively.

Apparent Molar Volumes and Adiabatic Compressibilities of Crown Ethers and Glymes in H2O and D2O at 25°C

The apparent molar volumes and adiabatic compressibilities of 18-Crown-6, 15-Crown-5, 12-Crown-4, tetraglyme, and triglyme were measured at 25°C in H2O and D2O. The contribution of the -CH2CH2O- group to the limiting partial molar volumes and compressibilities of cyclic and open-chain ethers in both solvents are compared and solvent isotope effects calculated. It is concluded, based on the compressibility results, that there is a subtle difference between the hydration of the ethene oxde group in cyclic and open-chain ethers and that this difference persists in D2O. These results indicate that the calculation of limiting apparent molar compressibilities using additivity schemes will have to account for whether the group is in a cyclic or open-chain compound.

Ultrasonic velocities and isentropic compressibilities of electrolytes in 2-methoxyethanol from 15 to 35° C

The ultrasonic velocities of lithium tetrafluoroborate (LiBF4), sodium tetrafluoroborate (NaBF4), tetraphenylphosphonium chloride (Ph4PCl), tetraphenylphosphonium bromide (Ph4PBr), and tetraphenylarsonium chloride (Ph4AsCl) in 2-methoxyethanol (ME) have been measured at 15, 25, and 35°C. Apparent molar isentropic compressibilities κφ of these electrolytes were derived from these data supplemented with their densities. Infinite dilution partial molar compressibilities κφo were obtained by extrapolation from the plot of κφ vs. the square root of the molarity. The κφo values of the electrolytes were split into approximate limiting ionic compressibilities κφ±o on the basis of the assumption that κφo (BF−4) = 0. The results have been interpreted in terms of specific constitutional and structural factors of the solvent molecules and of solute ions.