RESUMEN

Constant-pressure heat capacities (CPs) of crystalline benzoic acid (BAh) and its deuterated analogue (C6H5COOD, BAd) were measured by adiabatic calorimetry, and the phonon density (g(ω)) of states was determined from their CP data using a real-coded genetic algorithm (RCGA) with just generation gap + real-coded ensemble cross-over. The distribution of g(ω) was in reasonable agreement with the spectroscopic one observed for molecular vibration modes, indicating sufficient reliability of g(ω) determined by the RCGA. Based on the fact that CPs reveals an inverse-isotope effect in the temperature range 30-130 K, the determined g(ω)s were used to investigate the molecular mechanism of the effect: g(ω) of BAd revealed blue shifts in the ranges of ω = 80-100 and 150-230 K, as referred to that of BAh. It was suggested from the combined considerations on g(ω) and spectroscopic results that an anticooperative correlation exists between O-H···O hydrogen bonds and interdimer interactions in BA.

RESUMEN

A method was proposed to derive the phonon density [g(ω)] of states of materials from their heat capacity data by using Real-Coded Genetic Algorithm (RCGA) with Just Generation Gap + Real-Coded Ensemble Crossover. The performance of the method was confirmed by testing whether or not the RCGA reproduces a reasonable g(ω) by analyzing the set of heat capacity data evaluated from an initially assumed model g0(ω) composed of Debye and optical modes. As an example, constant-pressure heat capacities (CPs) were measured for soft molecular materials, diphenyl phosphate (DPP) and diphenylphosphinic acid, in the condensed state, and their g(ω)s were determined from the CP data by applying the RCGA. The unusual behavior that the CP value of glass was smaller than the one of the crystal in the temperature range from 10 to 70 K was observed in DPP; the behavior is contrary to that expected ordinarily for the glass as compared with the crystal. The g(ω)s determined by the RCGA demonstrated that the unusual behavior was attributed to the blue shift in g(ω) of ω = 30-240 K in the glass compared with the crystal. The blue shift and other effects were discussed reasonably as originating from the competitive concurrence of strong and weak intermolecular hydrogen bonds in DPP, with the help of determination of their intramolecular vibrations for the isolated molecule by the density functional theory calculation. It was concluded that the method using the RCGA is of value for obtaining the microscopic information of g(ω) from the precise heat capacity data and for investigating any difference between the details of g(ω)s in different phases of materials.

RESUMEN

We constructed an adiabatic calorimeter adapted for the preparation and in situ thermal characterization of vapor-deposited glasses and reported the investigation of the enthalpic states and dynamic properties of ethylcyclohexane (ECH) glasses prepared by vapor deposition in the temperature range of (0.71-0.96)Tg,liq; Tg,liq = (101 ± 1) K is the calorimetric glass transition temperature of the bulk liquid. It was verified that the ECH glasses deposited at temperatures immediately below Tg,liq were characterized by lower enthalpies and higher devitrification temperatures (Tdev), as compared to the glass obtained by supercooling the bulk liquid. The deposition temperature (TD) expected to yield experimentally the entity with the highest Tdev and the lowest enthalpic state was estimated to be 0.93Tg,liq. A model potentially elucidating the fundamental mechanism of formation and devitrification for the glasses prepared via the physical vapor deposition method as a function of TD was proposed. The fundamental point is that the glass is formed by deposition in a molecule-by-molecule fashion and the molecule deposited is frozen in a certain configuration determined by its being itself on the surface. For amorphous entities prepared at a TD much lower than Tg,liq, the surface molecule is frozen mostly as they are deposited. For the entity deposited at TD = 0.93Tg,liq in the case of ECH, the surface molecule is mobile immediately after the deposition to look for its stable configuration only on account of the intermolecular interactions with the molecules beneath and in the same surface layer as itself and freezes in a certain reasonably stable configuration; the molecules below the surface layer have already frozen in and get more stabilization energy through the additional interactions with the surface molecules. As a result, the intermolecular interaction of the molecules accumulated in such a way is stronger than that in the bulk liquid glass. It is argued that this is the fundamental reason why the glass formed immediately below Tg,liq has a lower enthalpy and a higher devitrification temperature than those of the liquid-cooled one.

RESUMEN

We investigated the thermal properties of liquid methylcyclohexane and racemic sec-butylcyclohexane, as representatives of a molecular system with only dispersion-force intermolecular interactions, confined in the pores (thickness/diameter d = 12, 6, 1.1 nm) of silica gels by adiabatic calorimetry. The results imply a heterogeneous picture for molecular aggregate under confinement consisting of an interfacial region and an inner pore one. In the vicinity of a glass-transition temperature T(g,bulk) of bulk liquid, two distinguishable relaxation phenomena were observed for the confined systems and their origins were attributed to the devitrification, namely glass transition, processes of (1) a layer of interfacial molecules adjacent to the pore walls and (2) the molecules located in the middle of the pore. A third glass-transition phenomenon was observed at lower temperatures and ascribed to a secondary relaxation process. The glass transition of the interfacial-layer molecules was found to proceed at temperatures rather above T(g,bulk), whereas that of the molecules located in the inner pore region occurred at temperatures below T(g,bulk). We discuss the reason why the molecules located in different places in the pores reveal the respectively different dynamical properties.

RESUMEN

Heat capacities and spontaneous enthalpy-relaxation effects of the benzene confined in silica MCM-41 and SBA-15 pores with uniform diameters were measured by high-precision adiabatic calorimetry. The fusion temperatures and fusion enthalpies determined were compared with the literature results of benzene confined within pores of CPG glasses. It was confirmed, from the observed spontaneous heat-release or -absorption effects, that there exists a non-crystallizing amorphous component of confined benzene, as reported previously. The pore-diameter dependence of fusion enthalpy observed was inconsistent with the previously proposed model which suggested that the non-crystallizing amorphous component is located on the pore wall in the form of a shell-like structure of a few nm in thickness. A very slow relaxation process corresponding to a translational-diffusion motion of molecule was observed, indicating that the benzene fills the pores incompletely along the pore channel. In addition, we found that the fusion enthalpy as a function of inverse pore-diameter dependence decreases steeply in the range of 60-10 nm in diameter while gradually in the range around 5 nm.

RESUMEN

Glass-transition behaviors concerning the quadruple proton/deuteron rearrangements in the crystalline p-tert-butylcalix[4]arene-toluene (BC4A·T) host-guest compound were studied by adiabatic calorimetry. The glass-transition temperatures (Tg's) of the hydrated BC4A·T-h and deuterated BC4A·T-d were found to be around 90 and 181 K, respectively. The difference of 90 K is too big to be recognized as the ordinary isotope effect, which is about 5 K in classical thermal activation processes. The temperature interval over which the enthalpy relaxation effect was observed was beyond 50 K, being too wide, in BC4A·T-h, while it was 20-30 K in BC4A·T-d; the latter is the one ordinarily observed in the classical processes. The associated equilibration rate in BC4A·T-h was evaluated from the enthalpy relaxation curve observed at 62 K to evidence the non-Arrhenius nature of the temperature dependence. The relaxation effect was detected even at 20 K. These are quite the same behaviors as observed in calix[4]arene, and the quantum tunneling rate in the quadruple proton rearrangement was concluded to be only a little affected by the substitution of the tert-butyl group at the para-position of each phenol moiety.

Asunto(s)

RESUMEN

Heat capacities of crystalline p-tert-butylcalix[4]arene·toluene host-guest compound were measured in a temperature range between 3 K and 345 K by adiabatic calorimetry. The crystal showed one second-order phase transition at 256.4 K accompanied by wide heat-capacity tails on both the high and low temperature sides and two first-order phase transitions with heat-capacity peaks at 127 K and 139 K. The total entropy of a sequence of the phase transitions was assessed experimentally to be 47 J K(-1) mol(-1), indicating that the number of allowed distinguishable molecular configurations is over 100 in the high-temperature disordered state. Many such distinguishable configurations are understood to be produced by a special intermolecular interaction of C-H···(π-electron) bonds formed between p-tert-butylcalix[4]arene and toluene molecules, and the transitions were interpreted as originating from orientational order-disorder of both the guest toluene molecule and the tert-butyl groups of host arene in their combination.

RESUMEN

Proton rearrangement rates in hydrogen bond networks are dominated by classical activation and quantum tunneling at higher and lower temperatures, respectively. Calix[4]arene (C4A) has a square-ring network composed of four hydroxyl groups with the O···O length of ~0.265 nm. Calorimetry and dielectric relaxation measurements were applied to determination of the rates in the crystals of C4A and its deuteron analogue (C4A-d). The rearrangement rates in C4A-d exhibited Arrhenius dependence in the measured temperature range. On the other hand, the rates in C4A showed the same dependence as those in C4A-d above 200 K, deviated from this dependence at around 180 K, and became independent of temperature at around 10(-4) s(-1) below 100 K. This evidenced that the tunneling in the quadruple proton rearrangement proceeds at a very slow rate of 10(-4) s(-1). This is the first determination by calorimetry of the proton tunneling rate.

Asunto(s)

RESUMEN

Enthalpic path and enthalpy-relaxation rates of ethylbenzene glasses prepared by vapor deposition at various temperatures, T(D), were examined on heating intermittently with a high-precision adiabatic calorimeter. It was confirmed that when T(D) is in the range 0.79-0.96T(g), the enthalpies elucidated at their preparation temperatures, i.e., T(D), are lower than those of the liquid-cooled glass. The fictive temperature T(f) at which the enthalpy path of each glass crosses the enthalpy curve expected for the equilibrium supercooled liquid was observed to be lowest when T(D) = 0.92T(g) = 105 K. The glasses revealed two remarkable characteristics: first, the temperature of the peak in the endothermic effect, which corresponds to the temperature T(g,dev) of the steepest devitrification, was observed to increase in correlation with the low-enthalpic nature of the glasses. Second, the devitrification manner was quite different between the glasses with T(D) < 0.92T(g) and T(D) ≥ 0.92T(g), even if the two glasses have the same T(f); the former devitrified gradually and the latter relatively sharply.

RESUMEN

At what temperature between 136 and 165 K the glass transition of water occurs is still controversial, while the crystallization of water prevents the determination. To confine water in nanopores stabilizes its liquid state down to low temperatures. Heat capacities and enthalpy relaxation effects of the water confined within MCM-41 nanopores with diameters in the range 1.5-5.0 nm were measured in this work by using adiabatic calorimetry. No fusion of the confined water was detected up to 2.0 nm, part of the water exhibited fusion in 2.1 nm pores, and the whole internal water which excludes the molecules interacting with the pore-wall atoms crystallized within pores with diameter of 2.3 nm and above. A glass transition of the internal water occurred at a temperature T(g) = 160-165 K for pore diameters in the range 1.5-2.0 nm and at 205-210 K for diameters of 2.0 and 2.1 nm; thus, the T(g) jumped from 165 to 205 K at 2.0 nm. The jump is connected to the development of hydrogen-bond network to a more complete one as the diameter is increased, and is conjectured as caused by the increase in the number, from three to four, of hydrogen bonds formed by each molecule. These imply that the glass transition of bulk water occurs at 210 K, which is much higher than 136 or 165 K debated so far.

RESUMEN

How low-temperature water develops the formation of strong hydrogen bonds with some network structure is still open to a question. Heat capacities of the water confined within silica MCM-41 nanopores with different diameters in the range 1.7-4.2 nm were measured by adiabatic calorimetry. They revealed a hump with its maximum at 233 and 240 K for ordinary and heavy water, respectively. The maximum temperatures were essentially independent of the pore diameter, whereas the maximum values increased only in proportion to the fraction of the internal water molecules within the pores. It was concluded that the manner in which the hydrogen-bond formation progresses in bulk water is essentially the same as that in nanopore water and that strong hydrogen bonds are formed on cooling by arranging the neighboring water molecules at tetrahedral positions but keeping their network structure irregular to make striking contrast with ice structure.

RESUMEN

The movement of water molecules in the limited space present within nanoscale regions, which is different from the molecular motion of bulk water, is significantly affected by strong interfacial interactions with the surrounding outer walls. Hence, most of the water molecules that are confined to nanochannel spaces having widths less than ca. 2 nm can generally be classified together as "structural water". Since the motions of such water molecules are limited by interfacial interactions with the outer wall, the nature of structural water, which is strongly influenced by the interactions, will have different characteristics from normal water. For our investigations on the characteristics of structural water, we have developed a nanoporous crystal with a diameter of ca. 1.6 nm; it was constructed from 1-D hydrophilic channels by self-organization of the designed molecules. A tubelike three-layered water cluster, called a water nanotube (WNT), is formed in each internal channel space and is regulated by H-bonds with the outer wall. The WNT undergoes a glass transition (T(g) = 107 K) and behaves as a liquid; it freezes at 234 K and changes into an icelike nanotube cluster. In this study, the structure of the WNT is investigated through neutron structure analysis, and it is observed to stabilize by a mechanistic anchor effect of structural water. Furthermore, from neutron-scattering experiments, it is seen that a few water molecules around the center of the WNT move approximately with the same diffusion constant as those in bulk water; however, the residence time and average jump length are longer, despite the restrictions imposed by the H-bonding with structural water. The behavior of mobile water within a WNT is investigated; this can be used to elucidate the mechanism for the effect of structural water on vital functions on the cell surface.

Asunto(s)

RESUMEN

Enthalpy relaxation processes proceeding in ethylene glycol (EG) aqueous solutions [(EG)(x)(H(2)O)(1 - x)] within silica-gel nanopores were studied by adiabatic calorimetry. While the x = 0.25 solution within pores with diameter of 52 nm showed a glass transition at T(g) = 139 K, ageing of the solution at 160 K caused a phase separation to reveal glass transitions at T(g) = 145 and 160 K for EG-rich and water-rich regions, respectively: the water molecules are understood to form a more developed hydrogen-bond network, and consequently force the EG molecules in between the water-rich regions. The T(g) = 160 K is in good agreement with the T(g) value of the internal (not interfacial) water confined within pores with thickness of 1.1 nm. The ageing further remarkably diminished the T(g) = 115 K glass transition. This indicates that, while the molecules responsible for the glass transition are the mobile water ones forming a lower number of hydrogen bonds than four, the fraction of such water molecules is reduced in association with the development of the network and the glass transition is absent in bulk pure water. When the same x = 0.25 solution was confined within 1.1- and 12 nm pores, the water molecules developed a hydrogen-bond network in the pore centre due to the presence of the pore wall and pushed the EG molecules onto the pore surface even at higher temperatures: the water-rich region gave T(g) = 155 K close to 160 K. It is concluded that the hydrogen-bond network inherent to water structure is developed/collapsed remarkably in the range near x = 0; consequently, the composition dependence of T(g) in the bulk system deviates sharply in the range from the Gordon-Taylor empirical law followed for large x > 0.2.

Asunto(s)

RESUMEN

Glass transition behaviors of dilute aqueous solutions are currently unclear because the water crystallizes immediately below the fusion temperatures to prevent the determination. The behaviors of methanol aqueous solutions [(CH(3)OH)(x)(H(2)O)(1 - x)] were studied here by confining the solutions within silica-gel pores and following the enthalpy relaxation associated with the glass transitions by adiabatic calorimetry. The dilution of the solutions in the composition range x < 0.3 brought both abrupt increase in the glass transition temperature T(g) as referred to the composition dependence expected from the behavior in x > 0.3 and appearance of a new glass transition at around 115 K. It was conjectured from the results that a hydrogen-bond network inherent to water starts to develop at around x = 0.3, and that molecules on the pore wall cannot join the network by forming tetrahedrally extended hydrogen-bonds so that they should constitute a mobile layer as an interfacial one. Such a special layer is understood as absent above x > 0.3, indicating that no network structure inherent to water is developed in the solutions.

RESUMEN

The thermal properties of crystalline complex [Cr(H(2)bim)(3)](TMA) x 23.5 H(2)O were studied by adiabatic calorimetry to clarify the structural ordering and dynamic freezing-in behaviors of the nanochannel water within the pores possessing crystalline wall structure, where H(2)bim denotes 2,2'-biimidazole and TMA is 1,3,5-benzenetricarboxylic acid. Phase and glass transitions were found to occur at 233 K with the associated entropy of Delta(trs)S = 7.96 J K(-1) mol(-1) and at T(g) = 100 K, respectively, in the hydrated sample. The phase transition was interpreted as attributed to the crystallization-like formation of the hydrogen-bond network of the channel-water molecules. The glass transition was interpreted as a freezing-in phenomenon on the way of the development of the network, and its presence indicates that the network formation achieves no completion even at 100 K. The T(g) value is similar to those found previously in other channel-water systems of [Ni(cyclam)(H(2)O)(2)](3)(TMA)(2) x 24 H(2)O and porous silica. It is noted that the channel water within silica pores with their diameter below 1.8 nm undergoes no structural phase transition while the present one does. The origins of the phase and glass transitions and the implication of their presence are discussed based on the difference in the structures of pore wall interacting with the channel-water molecules.

Asunto(s)

RESUMEN

The dynamic properties of water confined within nanospaces are of interest given that such water plays important roles in geological and biological systems. The enthalpy-relaxation properties of ordinary and heavy water confined within silica-gel voids of 1.1, 6, 12, and 52 nm in average diameter were examined by adiabatic calorimetry. Most of the water was found to crystallize within the pores above about 2 nm in diameter but to remain in the liquid state down to 80 K within the pores less than about 1.6 nm in diameter. Only one glass transition was observed, at T(g) = 119, 124, and 132 K for ordinary water and T(g) = 125, 130, and 139 K for heavy water, in the 6-, 12-, and 52-nm diameter pores, respectively. On the other hand, two glass transitions were observed at T(g) = 115 and 160 K for ordinary water and T(g) = 118 and 165 K for heavy water in the 1.1-nm pores. Interfacial water molecules on the pore wall, which remain in the noncrystalline state in each case, were interpreted to be responsible for the glass transitions in the region 115-139 K, and internal water molecules, surrounded only by water molecules in the liquid state, are responsible for those at 160 or 165 K in the case of the 1.1-nm pores. It is suggested that the glass transition of bulk supercooled water takes place potentially at 160 K or above due to the development of an energetically more stable hydrogen-bonding network of water molecules at low temperatures.

Asunto(s)

RESUMEN

To investigate the glass transition behaviors of a 20% (w/w) aqueous solution of bovine serum albumin, heat capacities and enthalpy relaxation rates were measured by adiabatic calorimetry at temperatures ranging from 80 to 300 K. One series of measurements was carried out after quenching from 300 down to 80 K and another after annealing in 200-240 K. The quenched sample showed a heat capacity jump indicating a glass transition temperature T(g) = 170 K, and the annealed sample showed a smaller jump with the T(g) shifted toward the higher temperature side. The temperature dependence of the enthalpy relaxation rates for the quenched sample indicated the presence of two enthalpy relaxation effects: one at around 110 K and the other over a wide temperature range (120-190 K). The annealed sample showed three separate relaxation effects giving 1) T(g) = 110 K, 2) 135 K, and 3) temperature higher than 180 K, whereas nothing around 170 K. These effects were thought to originate, respectively, from the rearrangement motions of 1) primary hydrate water forming a direct hydrogen bond with the protein, 2) part of the internal water localized in the opening of a protein structure, and 3) the disordered region in the protein.

Asunto(s)

RESUMEN

Low-temperature thermal properties of crystalline [Co(H(2)bim)(3)](TMA)·20H(2)O were studied by adiabatic calorimetry, where H(2)bim is 2,2'-biimidazole, TMA is 1,3,5-benzene tricarboxylic acid, and 20H(2)O represents the water forming nano-channel in the crystal. A glass transition was observed at T(g) = 107 K. It was discussed as a freezing-in phenomenon of a small number of water molecules remaining partially disordered in their positional arrangement. The possibility that some defects really remain in the hydrogen-bond network of channel water was mentioned. Two subsequent phase transitions were observed at 54.8 and 59 K. These were interpreted as being of a (super-structural commensurate)-incommensurate-(normal commensurate) type in the heating direction with respect to the hydrogen-atom positions as referred to the periodicity of the hydrogen-bond network. The transition entropy was evaluated to be 0.65 J K(-1)(H(2)O-mol)(-1) as a total of the two, indicating that the disorder of the hydrogen atoms is present only in part of the water molecules of the channel. Based on the fact that the excess heat capacity due to the equilibrium phase transition is observed down to 35-40 K, the relaxation time for the rearrangement of the hydrogen-atom positions was assumed at the longest to be 1 ks at 35 K. This indicates that the activation energy of the rearrangement amounts to at most 13 kJ mol(-1) and that the transfer of Bjerrum defects is attributed to the rearrangement.