Frequency shifts of piezoelectric quartz crystals immersed in organic liquids

A piezoelectric quartz crystal oscillates in organic liquids. The frequency change (ΔF) depends on the density (d) and viscosity (η) according to ΔF = ad^1/2 + bη^1/2 — c, where a, b and c, are constants depending on the crystal.     

Effects of laterally heterogeneous slip on the resonance properties of quartz crystals immersed in liquids

A formalism is presented which predicts the influence of laterally heterogeneous slip (for instance, induced by nanoscopic air bubbles) on the shift of the resonance frequency and bandwidth of quartz crystal resonators immersed in liquids. The lateral heterogeneities are decomposed into their Fourier components. The distribution of slip lengths provides a boundary condition, giving rise to a small, secondary flow field. The mean stress exerted by this secondary field induces a shift in resonance frequency and bandwidth. If the slip length is much smaller than the penetration depth of the shear waves and smaller than the lateral correlation length, one finds that the effects of the heterogeneities scale as n(3/2), with n being the overtone order. The frequency, f, and half-band-half-width, Gamma, decrease by the same amount. These calculations match experimental results obtained with a gold-coated resonator in contact with various hydrophilic liquids.     

Rotation of small molecules in liquids. A free-volume model combining collective and collisional effects

The rotational correlation times of C₂D₂ and CO₂ in liquid n-heptane were determined by infrared spectroscopy as a function of temperature. On the basis of the present and other preceding data we propose an interpretation of the rotational motion of small molecules in inert solvents. In this free-volume model the probe molecule is able to rotate freely during a time interval x and is in hydrodynamic interaction with the solvent during the rest (1 ? x). The correlation time of the angular momentum τ_J is, under these conditions, simply given by a linear relation τ_J = x(τ_J)_coll + (1 ? x)(τ_J)_hydro. This model fits very well the experimental data with a value of x consistent with its physical definition.     

Laser two-photon ionization spectroscopy of molecules in liquids

A two-photon lonization technique has been developed and applied to determine the photoionization threshold of a molecule in liquid solution. The photoionization of pyrene in n-pentane was studied by monitoring the photocurrent of an≈ 10^?6 M solution as a function of the laser wavelength in the region 360–530 nm, corresponding to two-photon, transition in the region 180–265 nm. The photoionization threshold thus determined is 4.80 ± 0.02 eV. Resonances were observed in the two-photon ionization spectrum which are tentatively ascribed to preionization of one-photon forbidden transitions to states energetically degenerate with the continuum, reached via two-photon absorption.     

Vibrational population lifetimes of polyatomic molecules in liquids

CH-stretching modes were first excited by picosecond infrared pulses and the generated excess population was monitored by anti-Stokes scattering of subsequent ultrashort probe pulses. Experimental data are reported on five molecules: CHCl₃, CH₂Cl₂, CH₃CCl₃, CH₃CH₂OH, and CH₃I in the neat liquid and/or in solutions of CCl₄. The observed time constants vary between 1 and 100 ps depending upon the individual molecule and surrounding. Theoretical calculations show that rotational coupling, Fermi resonance, Coriolis coupling, and resonance energy transfer can strongly effect the vibrational population lifetime. The relevance of these processes is quite different for the various molecules investigated.     

Environmental and excitonic effects on the first hyperpolarizability of polar molecules and related dimers

We investigate the second-order nonlinear optical properties of a push-pull chromophore in different external and supramolecular environments, through a combined experimental and theoretical approach. In particular, we compare the first hyperpolarizability (beta) of a model dipolar and polarizable chromophore with that of a charged analogue and of a molecular dimer based on the chromophore itself. We find that the beta value of the model chromophore in solutions of low-polarity solvents is strongly affected by association effects, already at concentrations of 10 (-3) M. The presence of a positive charge in close proximity to the chromophore is found to lead to a 100% increase of the beta response of the model push-pull chromophore. This effect is of major importance for biological applications, in particular when chromophores are used as markers in charged anisotropic environments. Finally, excitonic effects, beyond the Frenkel exciton approximation, are discussed for the dimer and found to be more important the higher the order of nonlinearity is.     

Laser multiphoton ionization of aromatic molecules in nonpolar liquids

The laser multiphoton ionization (MPI) of fluoranthene in tetramethylsilane (TMS) and of azulene in n-tridecane, n-pentane, 2,2,4,4-tetramethylpentane, TMS and tetramethyltin is reported. Three distinct types of MPI mechanisms have been identified: two-photon ionization, stepwise three-photon ionization and mixed two- and three-photon ionization. The stepwise three-photon process consists of two-photon excitation, relaxation to a lower lying excited state with a lifetime comparable to the laser pulse duration (for azulene this state is the S₂ while for fluoranthene both the S₁ and S₂ states) and subsequent ionization with the absorption of a third photon. The ionization threshold of azulene in each liquid has been determined and found to vary linearly with the V₀ of the liquid.     

Collision-induced absorption in non-polar molecular liquids: Tetrahedral molecules

The far infrared absorption spectra of liquid CH₄, CF₄ and CCl₄ were measured in the range between 15 and 200 cm^?1 The octupole induced dicole moment correlation functions of these liquids and of liquid CBr₄, which was recalculated from literature data, were compared to the corresponding theoretical correlation functions obtained by molecular dynamics simulation and by analytical theory. At short times(0.05 ps in CH₄ and 0.2 ps in CCl₄) the experimental correlation function follows the model reorientational correlation functions. At longer times this is no more the case and the dynamics coherence observed is qualitatively interpreted in terms of a structural correlation in the liquids.     

Laser two-photon ionization of organic molecules in dielectric liquids

The ionization threshold of fluoranthene and of TMPD in n-pentane was determined by laser two-photon ionization (TPI) and found to be 4.50 ± 0.05 and 3.88 ± 0.05 eV respectively. For both molecules the TPI spectra show distinct structure due to autoionization. For fluoranthene the TPI spectrum suggests that the molecule dissociates via the first excited singlet state.     

Anion effects on gas solubility in ionic liquids

This work presents the results of solubility measurements for a series of gases in 1-n-butyl-3-methyl imidazolium tetrafluoroborate and 1-n-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) imide. The gases considered include benzene, carbon dioxide, nitrous oxide, ethylene, ethane, oxygen, and carbon monoxide. Carbon dioxide and oxygen solubilities are also reported in methyl-tributylammonium bis(trifluoromethylsulfonyl) imide, butyl-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide, and tri-isobutyl-methyl phosphonium p-toluenesulfonate. We report the associated Henry's constants and enthalpies and entropies of absorption. In general, benzene, followed by carbon dioxide and nitrous oxide, have the highest solubilities and strongest interactions with the ionic liquids, followed by ethylene and ethane. Oxygen had very low solubilities and weak interactions. Carbon monoxide had a solubility below the detection limit of our apparatus. Ionic liquids with the bis(trifluoromethylsulfonyl) imide anion had the largest affinity for CO(2), regardless of whether the cation was imidazolium, pyrrolidinium, or tetraalkylammonium. These results suggest that the nature of the anion has the most significant influence on the gas solubilities.     

Gelation of fluorinated liquids by non-fluorinated low-molecular-mass molecules

A family of tetrahydroxy diesters has been synthesised and observed to gel a range of fluorinated solvents and their mixtures; the phase behaviour and gel microstructure are reported for a homologous family of these diesters in blends of 1H,1H-heptafluorobutanol (HFB) and 2H,3H-perfluoropentane (HPFP).     

Coulombic dragging of molecules on surfaces induced by separately flowing liquids

We demonstrate that ions or polar molecules can be driven by fluctuating Coulombic forces induced by flowing polar liquids at nanometer separations. We simulate this intriguing phenomenon on small ions and polar molecules driven on the surfaces of carbon nanotubes through which a flow of water is passing. Our simulations show that the average velocities of the driven molecules are close to those of the passing liquid. These transport phenomena open the door for many potential applications.     

Vibrational energy relaxation of small molecules and ions in liquids

A theoretical/computational framework for determining vibrational energy relaxation rates, pathways, and mechanisms, for small molecules and ions in liquids, is presented. The framework is based on the system—bath coupling approach, Fermi’s golden rule, classical time-correlation functions, and quantum correction factors. We provide results for three specific problems: relaxation of the oxygen stretch in neat liquid oxygen at 77 K, relaxation of the water bend in chloroform at room temperature, and relaxation of the azide ion anti-symmetric stretch in water at room temperature. In each case, our calculated lifetimes are in reasonable agreement with experiment. In the latter two cases, theory for the observed solvent isotope effects illuminates the relaxation pathways and mechanisms. Our results suggest several propensity rules for both pathways and mechanisms.     

On the nature of low-frequency vibrational modes in globular protein molecules immersed in water

Recent vibrational normal mode analysis of globular protein molecules (in vacuum) has shown that they have characteristic low-frequency (≈ 2–5 cm⁻¹) modes. It is shown that when the molecules are immerséd in water the modes are strongly red-shifted and damped. The implications of this result are briefly discussed.     

Environmental effects of chitosan as an immobilization medium for electrochemically active small molecules

Abstract

A number of challenges arise when using ferrocene as a component of electrochemical biosensors, including solubility in aqueous solutions. Therefore, entrapment of the biotin-ferrocene molecules within the chitosan film provides a route for immobilization on an electroactive surface such as an electrode while making the system water compatible. The use of the chitosan-ferrocene bioconjugate thin film on the electrode surface produces a signal that can be monitored in aqueous media. Herein, we discuss a series of modified ferrocene molecules that contain various linkers that provide non-covalent entanglement points to the chitosan medium. The electrochemical analysis and electron microscopy results show marked differences in the ferrocene loaded chitosan polymers when the termini of the ferrocene-linker vary between –SH and –NH₂. The –SH modified systems showed increased reversible and robust electrochemical signals relative to the –NH₂ congeners. Further studies showed that non-covalent impregnation strategy used is robust to degradation and less than 1% of the ferrocene molecules were leached over time. These results indicate that there are specific considerations needed when using chitosan-ferrocene systems as components in sensor arrays in future studies.     

Stable water and glycerol marbles immersed in organic liquids: from liquid marbles to Pickering-like emulsions

Water and glycerol marbles coated with various powders and immersed in organic liquids gave rise to water-in-oil and glycerol-in-oil Pickering-like emulsions. Non-polar oils such as polydimethylsiloxane, toluene, xylenes and chlorinated solvents supported the formation of emulsions, whereas polar liquids such as dimethylsulfoxide, N,N,-dimethylformamide, acetone and ethanol did not. It is demonstrated that there is a direct contact between a liquid filling the immersed marble and the surrounding liquid. A phenomenological theory of the marbles' sinking into emulsion is proposed.     

Basis set approach to solution of poisson equation for small molecules immersed in solvent

The problem of how to calculate the electrostatic interactions between molecules and a solvent is a very important one in theoretical chemistry and biophysics. One of the more commonly used methods has been to represent the solvent by a dielectric continuum and then to solve the Poisson (or the Poisson-Boltzmann) equation for the potential due to the charge distribution of the solute. The solution of the equation has, up to now, been largely carried out using finite-difference grid-based methods. In this article, we investigate the use of an alternative method, based on a basis set expansion of the potential. The choice of basis functions, the representation of the dielectric function and the accuracy that can be obtained are discussed and illustrated by example calculations on small molecules. © 1997 by John Wiley & Sons, Inc.     

Contact angle measurements with liquids consisting of bulky molecules

Well-measured contact angles with different solid-liquid systems fall approximately on smooth patterns when plotted versus liquid surface tension. However, there are deviations of 1 degrees -3 degrees , which are outside the error limits. It is the purpose of this paper to elucidate the reasons for such deviations. Two types of liquids were selected for advancing contact angle measurements on Teflon AF 1600 coated surfaces: a series of n-alkanes ranging from n-hexane to n-hexadecane and five liquids consisting of bulky molecules, octamethylcyclotetrasiloxane (OMCTS), methyl salicylate, tetralin, cis-decalin, and octamethyltrisiloxane (OMTS). It was found that contact angles of the liquids with bulky molecules fall on a perfectly smooth curve corresponding to a solid surface tension of 13.64 +/- 0.1 mJ/m2. However, contact angles of n-alkanes deviated from this curve by up to 3 degrees in a complicated fashion. The observed trend suggests that more than one mechanism is responsible for the deviations. Substrate-induced rearrangement of liquid molecules in the close vicinity of the surface in the case of long-chain n-alkanes and adsorption of vapor onto the solid surface in the case of short-chain n-alkanes are the most likely explanations. The results suggest that liquids with bulky molecules appear to be suitable for contact angle measurements to characterize energetics of polymeric surfaces.     

Collective effects in molecular motions in liquids

The collective effects in water were studied by investigating the spatial distribution of long-living hydrogen bonds and revealing correlations in molecular motions. The existence of extended clusters, whose molecules are linked by long-living bonds, suggests the existence of correlations between the motions of its molecules. The mean scalar products of the shift vectors of two molecules were calculated using the narrow ranges (DP) of intermolecular distances in the initial configuration. The average correlation coefficients (the cosines of angles between the shift vectors of two molecules) were also calculated. The DP and cosine values were averaged over all pairs with this intermolecular distance. The DP values increased with time and formed a plateau after a few hundred picoseconds. The plateau was attributed to the existence of molecular vortices that cover large (several nanometers) volumes of the liquid. The conclusion was drawn that hydrophobic species, for example, noble gas atoms incorporated in the water net could be involved in collective motions.