Cyclohexane Crystals Research Articles

Hierarchically Macroporous Cryogels of Polyisobutylene and Silica Nanoparticles

Organic-inorganic hybrid materials attract particular interest because of their excellent mechanical properties. Here, we report the synthesis of hybrid cryogels consisting of interpenetrated polyisobutylene and silica networks. The gels were prepared by cross-linking of butyl rubber in cyclohexane containing silica nanoparticles using sulfur monochloride (S(2)Cl(2)) as a cross-linking agent. The microstructure of the hybrid networks formed at subzero temperatures exhibits two generations of pores: 10(1) microm sized large pores due to the cyclohexane crystals acting as a template during gelation and, 10(-1)-10(0) microm sized small pores between the aggregates of the nanoparticles. The nanoparticles in hybrid cryogels accumulate within the large pores where cyclohexane crystals originally resided. Compared to the organogel networks with an elastic modulus of a few kPa, hybrid networks exhibit a modulus of elasticity around 300 kPa. Hybrid cryogels can be converted into organic cryogels by dissolving the silica component in aqueous hydrofluoric acid, while removing the polymer component by calcination results in porous silica networks with 10(-1) microm sized pores.

The phase diagram of cyclohexane–methanol: a challenge in chemical education

Cloud point and cooling curve studies of the cyclohexane–methanol system were ideally suited as a laboratory exercise for a physical chemistry curriculum. Within 3–4 h, students can obtain a sufficient number of relatively precise experimental data (temperature vs. mol fraction of methanol) to construct the major part of the solid–liquid phase diagram. The experimental procedure is based on three different methods: (1) cloud points were detected using an H–Ne laser; (2) temperatures of the methanol-rich part below the monotectic were taken when the first crystals of cyclohexane become visible; and (3) cooling curves were recorded in order to determine the melting point of C 6H 12 and the monotectic temperature. The results of the system exhibiting an upper critical solution temperature, UCST, agreed reasonably with the most reliable data presently available. Assuming a tetrameric methanol species, the experimental T− x data of the freezing-point depression and the sub-monotectic curve were fitted to a sub-regular model with three adjustable parameters for the liquid phase. However, only an extended Ising model agreed with the experimental curve near the critical temperature.

The CIDNP effect in plastic crystals

The CIDNP effects in the photolysis reactions of some aldehydes and ketones (including linear, aromatic and cyclic) in plastic crystals of cyclohexane have been discovered and studied. In going from liquid to solid solutions, the change of polarization sign is observed for some substances investigated. The CIDNP effect for a simple model for a relative motion of atoms of radicals possessing spin density (translational diffusion in a restricted volume) has been calculated. The results observed are interpreted in the framework of the radical pair mechanism of CIDNP effect formation.

Behavior of Aromatic Electron Donor-Acceptor Pairs in Low Temperature Solvent Matrices

Abstract Some cyano and amino naphthalenes form excimers in low temperature cyclohexane solutions. Effects of the presence of naphthalene molecules in the solution and crystalline state of the solvent on the behavior of these molecules in rigid cyclohexane matrices were studied using low temperature high-resolution fluorescence spectroscopy. In the case of the cyanonaphthalenesnaphthalene systems, an exciplex formation is not clearly observed. However, efficient energy transfer from naphthalene molecules to cyanonaphthalene monomers or aggregates is observed when single crystals of cyclohexane are formed in the matrix. In contrast, a physico-chemical interaction between the electron donor and acceptor molecules (exciplex formation) is observed in the case of the 1-aminonaphthalene-naphthalene system.

A light scattering study of the solid-liquid interface layer of cyclohexane crystals

The solid-liquid interface of growing cyclohexane crystals was studied by Rayleigh scattering techniques. If crystal growth reaches a critical growth velocity of υ c ≃ 0.4 μm/s, intense Rayleigh scattering is observed. The enhanced Rayleigh scattering originates in an interface layer with a thickness of some μm located at the liquid side of the interface. Enhanced scattering persists if the crystal growth velocity is reduced below υ c even if the crystal is melting. The interface layer was studied for crystal growth velocities varying from 0.15 μm/s up to 30 μm/s. Dynamic and static light scattering experiments were combined and compared with various model calculations. A model was developed which is compatible with the experiments. The salient features of the model are: A layer of a mesophase is formed at the liquid side of the interface. The mesophase consists of weakly bonded aggregates of molecules. Fluctuations of the index of refraction scatter the light. The Lorentzian spectrum of the scattered light corresponds to a random walk process characterized by a diffusion constant D i ≃ 6 × 10 -9 cm 2/s. This is about 10 6 times less than the thermal diffusivity in liquid cyclohexane which determines the dynamics of entropy fluctuations in the bulk liquid. The fluctuations in the index of refraction can be described by a Debye-Bueche or by an Ornstein-Zernike pair correlation function. The correlation length deduced is about ξ ≃ 0.05 μm for the Debye-Bueche correlation function. The enhanced scattering corresponds to a mean amplitude of the fluctuations of the order of Δ n ≃ 10 -4.

Light scattering at the solid-liquid interface of cyclohexane

Anomalous Rayleigh scattering in the interface layer of cyclohexane crystals has been studied during crystal growth. For the first time we have observed anomalous Rayleigh scattering in the interface layer during melting. Cyclohexane has been chosen because the molecule has no permanent dipole moment. Thus all models based on the existence of long range dipolar interaction to describe these light scattering phenomena can be eliminated. A new crystal growth geometry has been developed to eliminate any possibility of contamination. A liquid sphere inside of a multiple zone refined single crystal is melted. The same light scattering phenomena are observed during inward growth, which we know from experiments during zone refining. Light scattering is also observed during melting of the solid. The solid is not superheated and thus no instabilities can occur at the interface. Thus it is concluded that light is not scattered by surface corrugations, small scale convections or by segregated impurities. Quasielastic light scattering techniques have been combined with elastic light scattering measurements in our studies of the freezing process. Model calculations for the angular dependence of the intensities of the scattered light have been compared with the experimental data. Calculations have been performed on the basis of 3 models: Mie theory for spherical particles, continuous fluctuations with an Ornstein-Zernike type pair correlation function and continuous fluctuations with a Debye-Bueche type pair correlation function. Only continuous fluctuations are compatible with the experiment. Thus models based on the assumption of gas bubbles or crystallites in front of the crystal-liquid interface can also be eliminated. The experimental results with cyclohexane are compatible with earlier experiments with H 2O and salol, indicating that the interface layer is a general phenomenon of growing dielectric crystals.

Matrix induced localization of proton tunneling in malonaldehyde

The infrared spectrum of malonaldehyde (MA) isolated in an argon matrix has been studied in the frequency range 4000–270 cm −1 at temperatures between 15 and 30 K. In addition, the ultraviolet spectrum of MA isolated in cyclohexane crystals has been studied in the frequency range 28100–30700 cm −1 at temperatures between 1.2 and 50 K. In contrast to the previously observed gas phase spectrum of MA, no tunneling splitting due to proton motion in a double minimum potential is apparent in the spectrum of matrix isolated MA. Furthermore, the relative intensities of the MA bands are not temperature dependent in contrast to expectations for a tunneling system. These results strongly suggest that matrix/MA interactions localize the proton on the time scale of the infrared and optical experiments.

Diffusion of nitroxide radicals in plastic solids as studied by electron-electron double resonance

The temperature dependence of the spin packet width in the inhomogeneously broadened EPR line of nitroxide radicals in plastic crystals of cyclohexane and d-camphor was measured through discrete saturation of the line by electron-electron double resonance. The temperature dependence of the homogenization rate of the inhomogeneously broadened EPR line was determined by comparing the experimental and theoretical spectra for discrete saturation; effective activation energies of the translational diffusion of nitroxide radicals in plastic cyclohexane and d-camphor were thus obtained. We discuss a model describing the observed slowing-down of self-diffusion in a plastic crystal containing impurities as well as the low activation energies for translational diffusion of the radicals.

Self diffusion in cyclohexane single crystals—a re-appraisal

Self diffusion in f.c.c. cyclohexane crystals has been re-examined using the radiotracer isotope mass effect technique. The experimental penetration curves show that this highly plastic solid is fundamentally very defective. Experiments for short periods yield overall diffusion coefficients dominated by diffusion along dislocations and mosaic boundaries (pipe diffusion) in the lattice. This line defect structure, which is known to increase with impurity content, is believed to be the cause of the very high values of the self diffusion coefficient found previously. The dominance of pipe diffusion was confirmed by isotope mass effect experiments carried out under the previous conditions. These gave values for the mass factor (ƒΔK)≃ 0.35 characteristic of pipe diffusion. Careful experiments for longer periods using ultra pure, well annealed, samples yielded values of the mass factor consistent with lattice self diffusion by a vacancy process. For these experiments the self diffusion coefficients and activation energy are in good relative agreement with values found for other rotator phase solids. The activation energy was confirmed by plastic deformation measurements.

ESR Study on Transient Radicals in Plastic Crystals under

Abstract An apparatus was constructed for ESR measurements during electron irradiation, and the behavior of transient radicals in the plastic crystals of cyclohexane (CyH) and 2,2,3,3-tetramethylbutane (TMB) was investigated. It was found from the experiment that cyclohexyl radical produced by irradiation of CyH decays with a rate constant of 2×10−8 exp (−7.9 kcal/RT) cm3/s. The rate constant is in good agreement with the results reported by Buben et al. On the other hand, radicals formed in irradiated TMB were assigned to be (Remark: Graphics omitted.) from the present results. It was shown from the results that the assignment made by Marx et al. before should be corrected to some extent. In the plastic crystals of CyH and TMB, it was suggested that the decay reactions of the observed radicals are controlled by the diffusional migration of the radicals. The analysis of radiolytic products of TMB was also made, and the results were compared with the results of ESR observation.

Lattice defects in plastic organic crystals. Part 7.—Isotope mass effects in self-diffusion in cyclohexane

A study has been made of the diffusion of the species 12C514C11H12 and 12C62H113H1 in single crystals of cyclohexane at 242 K. The measured diffusion coefficients are in good agreement with previously published values. An estimate of the isotope mass effect yields a value of the mass factor Eab= 0.003 ± 0.004. This virtually-zero value indicates that self-diffusion in cyclohexane proceeds by a complicated mechanism in which many molecules are involved ( >12). This discovery confirms previous speculations on the defect structure of the low entropy of fusion plastic solids.

Effects of polymorphic transitions on the vibrational spectra of cyclopentane and cyclohexane crystals

The Raman spectra of these compounds in their various crystalline modifications have been examined and compared with those for the amorphous state. Temperature and phase state have marked effects on the frequencies and intensity distribution for some lines. The symmetry of the intermolecular forces can affect the vibrational spectra of naphthenes.

Self Diffusion in Cyclohexane Single Crystals

Abstract Self diffusion has been studied in single crystals of cyclohexane in the plastic crystalline phase of this solid. In the temperature range −70°C to −30°C there appears to be one diffusion mechanism in operation which is best described by an equationIt is proposed that diffusion occurs by the random walk of relaxed vacancies in the lattice. The concentration of these defects is much higher at the melting point than in other types of solid. It is possible that this high defect concentration could account for the extreme plasticity of this solid.

Growth of single crystals of cyclohexane

Single crystals of cyclohexane have been grown in a specially constructed crystallization apparatus by the moving vessel technique. The effects of purity of the material, the temperature gradient in the apparatus and the rate of growth on the production of single crystals were investigated.

Resonance Absorption in Solid Cyclohexane

Measurements of the acoustic absorption in monocrystalline cyclohexane have been made in the frequency range 3 to 9 Mc. Absorption coefficients in this range are unusually large, being of the order of 0.1 cm−1 and the absorption increases as the square of the frequency. Although the absorption is large, its measurement is difficult unless large single crystals affording long path lengths are available. A method for growing single crystals of cyclohexane having linear dimensions of 10 cm has been developed; the method and the absorption measurement technique are described. Both the magnitude of the absorption and its frequency dependence are in good agreement with the resonance absorption theory [L. N. Liebermann, Phys. Rev. 113, 1052 (1959)]. These results for cyclohexane, together with previously reported data for benzene, offer support for the existence of this absorption mechanism. As a further test of the theory, absorption was measured as a function of temperature in the range 0 to −150°C. The results are described in light of the theory.