Water evaporation rates across hydrophobic acid monolayers at equilibrium spreading pressure

The effect of alkanoic acid [CH(3)(CH(2))(n-2)COOH; HCn] and perfluoroalkanoic acid [CF(3)(CF(2))(n-2)COOH; FCn] monolayers on the water evaporation rate was investigated by thermogravimetry tracing the decrease in amount of water with time. The evaporation rate from the surface covered by a monolayer was measured as a function of temperature and hydrophobic chain length of the acids, where the monolayer was under an equilibrium spreading pressure. From thermal behavior of the crystallized acids, their solid states are C-type in crystalline state over the temperature range from 298.2 to 323.2 K. The dry air was flowed through a furnace tube of a thermogravimetry apparatus at the flow rate of 80 mL min(-1), where the evaporation rate becomes almost constant irrespective of the flow rate. The temperature dependence of the evaporation rate was analyzed kinetically to evaluate the activation energy and thermodynamics values for the activated complex, which demonstrated that these values were almost the same for both alkanoic acids and perfluoroalkanoic acids, although the effect of perfluoroalkanoic acids on the evaporation rate was smaller than that of corresponding hydrogenated fatty acids. The difference in the evaporation rate between FCn and HCn was examined by atomic force microscopy (AFM), Brewster angle microscopy (BAM), surface potential (DeltaV) at equilibrium spreading pressure, and Langmuir curve (pi-A isotherm), and their results were consistent and supported the difference.     

Impurity effects at hydrophobic surfaces

The effective charge of hydrophobic surfaces and in particular of the air–water interface is a crucial parameter for electrochemistry, colloidal chemistry and interfacial science, but different experiments give conflicting estimates. Zeta-potential and disjoining-pressure measurements point to a strongly negative surface charge, often interpreted as being due to adsorbing hydroxide ions. In contrast, surface tension measurements of acids and bases suggest the hydronium ion to be surface active, in agreement with some surface-specific non-linear spectroscopy results. The air–electrolyte interfacial tension exhibits a characteristic minimum at millimolar electrolyte concentration for all salts, the so-called Jones–Ray effect, which points to competitive adsorption mechanisms present in dilute electrolyte solutions. We show that all these puzzling experimental findings can be explained by the presence of trace amounts of surface-active charged impurities, most likely anionic surfactants.     

Molecular dynamics simulations of pressure effects on hydrophobic interactions.

We report results on the pressure effects on hydrophobic interactions obtained from molecular dynamics simulations of aqueous solutions of methanes in water. A wide range of pressures that is relevant to pressure denaturation of proteins is investigated. The characteristic features of water-mediated interactions between hydrophobic solutes are found to be pressure-dependent. In particular, with increasing pressure we find that (1) the solvent-separated configurations in the solute-solute potential of mean force (PMF) are stabilized with respect to the contact configurations; (2) the desolvation barrier increases monotonically with respect to both contact and solvent-separated configurations; (3) the locations of the minima and the barrier move toward shorter separations; and (4) pressure effects are considerably amplified for larger hydrophobic solutes. Together, these observations lend strong support to the picture of the pressure denaturation process proposed previously by Hummer et al. (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 1552): with increasing pressure, the transfer of water into protein interior becomes key to the pressure denaturation process, leading to the dissociation of close hydrophobic contacts and subsequent swelling of the hydrophobic protein interior through insertions of water molecules. The pressure dependence of the PMF between larger hydrophobic solutes shows that pressure effects on the interaction between hydrophobic amino acids may be considerably amplified compared to those on the methane-methane PMF.     

Molecular dynamics at constant pressure and temperature

Methods are discussed for generating by molecular dynamics isobaric-isoenthalpic, NPH, isochoric-isothermal, NVT, and isobaric-isothermal, NPT, ensembles. Andersen's constant-pressure method is reformulated so that the ensemble rather than the scaled system is directly calculated. Four constant-temperature schemes were considered. Two involve the addition of a stochastic collision term to the molecular trajectories. The Andersen method and a stochastic dynamics approach were examined. The latter employed a velocity damping term in addition to the random force. Two other methods employed uniform velocity scaling applied to all molecules. The NPT algorithm induces a transition to the dilute phase for a Lennard-Jones fluid in the spinodal region (p^* = 0.5, T^* = 1.28) of the phase diagram. The thermodynamic equivalence of the ensembles is demonstrated by long calculations of the chemical potential of Lennard-Jones states by the particle insertion method. The internal energy, pressure, constant volume and pressure specific heats, adiabatic compressibilities, pair radial distribution functions and self-diffusion coefficients are also evaluated. Only for second-order thermodynamic quantities is there evidence of an ensemble dependence.     

Altering efficiency of hydrophobic interaction chromatography by combined salt and temperature effects

The coupled effect of salt concentration and temperature on the retention behavior of proteins in hydrophobic interaction chromatography has been studied. The retention data of four model proteins, i.e., myoglobin, lysozyme, α-chymotrypsinogen and bovine serum albumin, have been acquired by isocratic experiments of chromatographic elution within the temperature range 5–25 °C at different ammonium sulphate concentrations in the mobile phase. The retention dependencies quantified as functions of the salt concentration and temperature have been exploited in designing the process of gradient elution. The propagation velocity of proteins under conditions of the step gradient of salt and temperature has been determined by use of the equilibrium theory. To evaluate kinetic effects accompanying the band propagation the transport-dispersive model has been employed. It has been shown that altering the propagation of the salt and temperature waves in a proper manner allows improving the separation efficiency. Moreover, manipulation of specific kinetics effects can also be exploited in protein separations.     

Clustering of Lennard-Jones particles in water: temperature and pressure effects

While the hydrophobic effect is, for many systems, one of the most relevant interactions, it may be said that in the case of biological systems this effect becomes of determinant importance. Although the matter has been analyzed extensively, certain aspects are yet to be elucidated. Hence, the study on the behavior of the hydrophobic effect with temperature, and particularly with pressure deserves further investigation; model systems may help us in the task. We have analyzed the behavior of Lennard-Jones particles in water by means of molecular dynamics simulation under different conditions of size, concentration, temperature, and pressure. Following the formation of particle aggregates we can observe an increase of the hydrophobic effect with temperature and a strong weakening of the effect at high pressures. The results agree with the experimental evidence and show the ability of molecular dynamics simulation to account for the behavior of nonpolar substances under different conditions, provided that the intermolecular interactions used are adequate.     

Loading, stationary phase, and salt effects during hydrophobic interaction chromatography: alpha-lactalbumin is stabilized at high loadings

Amide hydrogen-deuterium exchange labeling has been used to study the effects of salt and protein loading on alpha-lactalbumin (BLA) stability during hydrophobic interaction chromatography (HIC). Stability in the adsorbed phase increased dramatically with increasing loading, and unfolding was nearly undetectable close to the resin saturation capacity. We also found that a butyl surface destabilized BLA more than a phenyl surface, despite the fact that BLA was bound more strongly on the phenyl surface. These observations have important implications for HIC process design and indicate that in some cases column capacity does not have to be sacrificed to preserve protein stability.     

"Glass transition" in peptides: temperature and pressure effects

We report on the origin of the liquid-to-glass transition in a series of oligopeptides of gamma-benzyl-L-glutamate up to the polymer (PBLG), and in Poly-Z-L-lysine (PZLL) and Polyglycine (PGly) using dielectric spectroscopy as a function of temperature and pressure. We show that temperature is the dominant control variable of the dynamics associated with the peptidic "glass transition." This is an intrinsic feature of the peptide dynamics, irrespective of the type of amino acid and of the peptide secondary structure. The influence of the type of secondary structure (alpha helix vs beta sheet) on the liquid-to-glass dynamics is discussed.     

Pressure and temperature dependence of hydrophobic hydration: volumetric, compressibility, and thermodynamic signatures

The combined effect of pressure and temperature on hydrophobic hydration of a nonpolar methanelike solute is investigated by extensive simulations in the TIP4P model of water. Using test-particle insertion techniques, free energies of hydration under a range of pressures from 1 to 3000 atm are computed at eight temperatures ranging from 278.15 to 368.15 K. Corresponding enthalpy, entropy, and heat capacity accompanying the hydration process are estimated from the temperature dependence of the free energies. Partial molar and excess volumes calculated using pressure derivatives of the simulated free energies are consistent with those determined by direct volume simulations; but direct volume determination offers more reliable estimates for compressibility. At 298.15 K, partial molar and excess isothermal compressibilities of methane are negative at 1 atm. Partial molar and excess adiabatic (isentropic) compressibilities are estimated to be also negative under the same conditions. But partial molar and excess isothermal compressibilities are positive at high pressures, with a crossover from negative to positive compressibility at approximately 100-1000 atm. This trend is consistent with experiments on aliphatic amino acids and pressure-unfolded states of proteins. For the range of pressures simulated, hydration heat capacity exhibits little pressure dependence, also in apparent agreement with experiment. When pressure is raised at constant room temperature, hydration free energy increases while its entropic component remains essentially constant. Thus, the increasing unfavorability of hydration under raised pressure is seen as largely an enthalpic effect. Ramifications of the findings of the authors for biopolymer conformational transitions are discussed.     

Polymer coatings on steel substrates: An infrared spectroscopic study of temperature effects

Polymer coatings on steel substrates were analyzed by reflectance/absorbance infrared spectroscopy. Initial studies were performed ex-situ on samples which had been cured at a variety of temperatures on two different substrates. Further studies were done in-situ under both air and nitrogen atmospheres. The two substrates studied were untreated cold-rolled steel and cold-rolled steel with a conversion coating of zinc phosphate. Changes in the spectra of the phosphate conversion layer correlated well with anti-corrosion properties. Mechanisms of degradation and cure could be determined from the in-situ studies (1).     

HgWO4 synthesized at high pressure and temperature

Crystals of mercury(II) tungstate(VI), HgWO₄, were grown in sealed gold tubes under an Ar atmosphere at 300 MPa and 973 K. The monoclinic crystal structure (C2/c) of HgWO₄ consists of zigzag chains of edge‐sharing WO₆ octahedra running along the c axis and layers of very distorted corner‐sharing HgO₆ octahedra in the bc plane. The Hg atom lies on an inversion centre and the W atom is on a twofold axis. No structural effects which can be ascribed to the high pressure used in the synthesis were found.     

Theoretical studies of solid bicyclo-HMX: effects of hydrostatic pressure and temperature

On the basis of density functional theory (DFT) and molecular dynamics (MD), the structural, electronic, and mechanical properties of the energetic material bicyclo-HMX have been studied. The crystal structure optimized by the LDA/CA-PZ method compares well with the experimental data. Band structure and density of states calculations indicate that bicyclo-HMX is an insulator with the band gap of ca. 3.4 eV and the N-NO(2) bond is the reaction center. The pressure effect on the bulk structure and properties has been investigated in the range of 0-400 GPa. The crystal structure and electronic character change slightly as the pressure increases from 0 to 10 GPa; when the pressure is over 10 GPa, further increment of the pressure determines significant changes of the structures and large broadening of the electronic bands together with the band gap decreasing sharply. There is a larger compression along the c-axis than along the a- and b-axes. To investigate the influence of temperature on the bulk structure and properties, isothermal-isobaric MD simulations are performed on bicyclo-HMX in the temperature range of 5-400 K. It is found that the increase of temperature does not significantly change the crystal structure. The thermal expansion coefficients calculated for the model indicate anisotropic behavior with slightly larger expansion along the a- and c-axes than along the b-axis.     

Friction and nanowear of polystyrene against hydrophobic and hydrophilic substrates

The ambition of this study is to analyze the role of interfacial interactions in friction and nanowear of polystyrene, by comparing friction against hydrophobic wafers (methyl‐terminated) and hydrophilic wafers (hydroxyl‐terminated) as a function of sliding velocity and normal force. Friction experiments are performed with a translation tribometer and nanowear investigation is achieved by using atomic force microscopy (AFM) analysis of the wafer surfaces after friction. Experimental results show that the friction coefficients measured on hydrophilic surfaces are always larger than those obtained with hydrophobic surfaces, indicating a relationship between friction and interfacial interactions. Elsewhere, AFM analysis shows that polystyrene transfer appears for a higher normal force in the case of hydrophobic substrates compared to hydrophilic one. However, the corresponding tangential (or friction) force necessary to detect transfer is quite similar for both types of substrates, indicating that the initial wear of polystyrene occurs for a similar threshold interfacial shear. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2449–2454, 2006     

Radiation-induced polymerization of hexafluoropropylene at high temperature and pressure

The radiation-induced polymerization of hexafluoropropylene was studied in the pressure and temperature ranges of 4,500–15,000 atm. and 100–230°C., respectively. Retardation was a serious problem; data thought to apply to the unretarded polymerization are summarized below. At 1,500 rad/hr. the polymerization rate was 15%/hr. at 230°C. and 15,000 atm. The activation enthalpy and volume are 9.5 kcal./mole and ?10 cc./mole, respectively. The rate varies as the square root of the radiation intensity. The largest intrinsic viscosity of the polymer is 2.0 dl./g.; values increase with temperature and pressure. At 130°C. and 10,000 atm. the intrinsic viscosity was the same at two radiation intensities.     

Thermal coefficients and internal pressure of aqueous urea. Concentration,temperature, and isotopy effects

The thermal expansion α, isothermal compressibility β, and internal pressure coefficients of H₂O-(NH₂)₂CO and D₂O-(ND₂)₂CO fy systems at 278 K, 298 K, and 318 K and aquamolality m ≤ 1.5 were calculated. The changes in the isotope differences Δα, Δβ_T, and Δfy at different solute concentrations and temperature are discussed. In contrast to Δα and Δfy, Δβ_T is almost independent of the urea concentration already at 298 K and independent of m at 318 K. The derivative δfy/δT increased in dilute solution, at lower temperatures, and on passing from protium to deuterium system, which corresponded to increased structuring. The isotope difference for the Grüneisen constant at given temperatures and concentrations is shown to be independent of the urea content.     

Self-Reaction of HO2 and DO2: Negative temperature dependence and pressure effects

The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO₂ are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO₂ self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high-pressure limit of the self-reaction of HO₂.     

Degradation effects in the extraction of antioxidants from birch bark using water at elevated temperature and pressure

Experiments with birch bark samples have been carried to enable a distinction between extraction and degradation effects during pressurised hot water extraction. Two samples, E80 and E180, contained birch bark extracts obtained after extraction at 80 and 180 °C for up to 45 min, respectively. Two other samples, P80 and P180, were only extracted for 5 min at the two temperatures and were thereafter filtered and hydrothermally treated at 80 and 180 °C, respectively. During the latter treatment, samples were collected at different times to assess the stability of the extracted compounds. An offline DPPH (2,2-diphenyl-1-picrylhydrazyl) assay, as well as a high performance liquid chromatographic separation coupled to an electrochemical detector, were used to determine the antioxidant capacity of the processed samples. The results obtained with the different techniques were compared to assess the yield of the extraction and degradation processes. In addition, an online hyphenated system comprising high performance liquid chromatography coupled to diode-array; electrochemical; and tandem mass spectrometric detection (HPLC-DAD-ECD-MS/MS) was used to study the compositions of the extracts in more detail. The results for the samples processed at 80 °C showed that the extraction reached a steady-state already after 5 min, and that the extracted compounds were stable throughout the entire extraction process. Processing at 180 °C, on the other hand, gave rise to partly degraded extracts with a multitude of peaks in both the diode array and electrochemical detectors, and a higher antioxidant capacity compared to for the extracts obtained at 80 °C. It is concluded that HPLC-DAD-ECD is a more appropriate technique for the determination of antioxidants than the DPPH assay. The mass spectrometric results indicate that one of the extracted antioxidants, catechin, was isomerised to its diastereoisomers; (+)-catechin, (−)-catechin, (+)-epicatechin, and (−)-epicatechin.     

Generalized Johnson-Nyquist noise: white noise of temperature and pressure at the nanoscale

White noise is expected to show up in both pressure and temperature at the nanoscale if linear transport equations prevail. This prediction is based on the thermodynamic fluctuation theory in analogy to a new derivation of the Johnson-Nyquist voltage noise. The pressure noise in liquid filled nanotubes, and temperature noise in nanoslabs are estimated and experiments are proposed. Measurements might prove useful in the determination of nanoscopic transport coefficients.