Continua of interactions between pairs of atoms in molecular crystals

The electron density distributions in crystals of five previously studied DMAN complexes and five Schiff bases (two new ones) have been analysed in terms of various properties of bond critical points (BCPs) found in the pair-wise interactions in their lattices. We analysed the continua of interactions including covalent/ionic bonds as well as hydrogen bonds and all other types of weak interactions for all pairs of interacting atoms. The charge density at BCPs and local kinetic and potential energy densities vary exponentially with internuclear distance (or other measures of separation). The parameters of the dependences appear to be characteristics of particular pairs of atom types. The Laplacian and the total (sum of kinetic and potential) energy density at BCPs show similar behaviour with the dependence being of the Morse type. The components lambda1, lambda2, lambda3 of the Laplacian at BCPs vary systematically with internuclear distance according to the type of atom pair. For lambda1 and lambda2 the distribution is of the exponential type, whereas lambda3 does not seem to follow any simple functional form, consistent with previous theoretical findings. Analytical nonlinear dependences of Laplacian on charge density have been found. They agree reasonably well with those obtained by least square fit of the Laplacian to charge density data. There are four distinct regions of the [symbol: see text]2rho(BCP)/rho(BCP) space, generated by E(BCP) = 0 and G(BCP)/rho(BCP) = 1 conditions. Two regions clearly correspond to the shared-shell and closed-shell interactions and the other two to some intermediate situation.     

A method for simultaneous estimation of inhomogeneous zeta potential and slip coefficient in microchannels

In the microchannels made of hydrophobic materials, the fluid velocity is determined by the zeta potential and velocity slip, both of which may be inhomogeneous due to the adsorption of protein to the channel wall. The inhomogeneity of zeta potential and slip coefficient sometimes causes recirculating flows which in turn affect the transport and mixing of solutes through the microchannels. In the present investigation we devise a method for the simultaneous estimation of inhomogeneous zeta potential and inhomogeneous slip coefficient using velocity measurements. A conjugate gradient method supplemented by the adjoint variable method is adopted in the solution of the relevant inverse problem to reduce the computational burden. The present method is found to estimate the inhomogeneous zeta potential and the slip coefficient simultaneously even with noisy velocity measurements. This method is expected to contribute to the optimal design and robust operation of various microfluidic devices, where the flow patterns and the volumetric flow rates are critically influenced by the profiles of inhomogeneous zeta potential and inhomogeneous slip coefficient.     

A review of the zone broadening contributions in free-flow electrophoresis

The broadening of analyte streams, as they migrate through a free-flow electrophoresis (FFE) channel, often limits the resolving power of FFE separations. Under laminar flow conditions, such zonal spreading occurs due to analyte diffusion perpendicular to the direction of streamflow and variations in the lateral distance electrokinetically migrated by the analyte molecules. Although some of the factors that give rise to these contributions are inherent to the FFE method, others originate from non-idealities in the system, such as Joule heating, pressure-driven crossflows, and a difference between the electrical conductivities of the sample stream and background electrolyte. The injection process can further increase the stream width in FFE separations but normally influencing all analyte zones to an equal extent. Recently, several experimental and theoretical works have been reported that thoroughly investigate the various contributions to stream variance in an FFE device for better understanding, and potentially minimizing their magnitudes. In this review article, we carefully examine the findings from these studies and discuss areas in which more work is needed to advance our comprehension of the zone broadening contributions in FFE assays.     

Cavities in molecular liquids and the theory of hydrophobic solubilities

Thermal configurational data on neat liquids are used to obtain the work of formation of hard spherical cavities of atomic size in six molecular solvents: n-hexane, n-dodecane, n-undecyl alcohol, chloroform, carbon tetrachloride, and water. These results are used to test a recent suggestion that the differences between nonaqueous solvents and liquid water in solvation of inert gases are not principally due to the hydrogen-bonded structure of liquid water but rather to the comparatively small size of the water molecule. The frequencies of occurrence of cavities in liquid water can be meaningfully distinguished from those in the organic solvents. Liquid water has a larger fractional free volume, but that free volume is distributed in smaller packets. With respect to cavity work, water is compared to a solvent of the same molecular density and composed of hard spheres of the same size as the water molecule. That comparison indicates that the hard-sphere liquid finds more ways to configure its free volume in order to accommodate an atomic solute of substantial size and thus, would be more favorable solvent for inert gases. The scaled particle model of inert gas solubility in liquid water predicts cavity works 20% below the numerical data for TIP4P water at 300 K and 1.0 g/cm3 for cavity radii near 2.0 angstroms. It is argued that the sign of this difference is just the sign that ought to be expected and that the magnitude of this difference measures structural differences between water and the directly comparable hard-sphere liquid. In conjunction with previous data, these results indicate that atomic sized cavities should be considered submacroscopic.     

Solute-solvent interactions between a range of solutes and trifluoropropyl siloxane stationary phases in terms of gas-liquid chromatography activity coefficients

Summary Several poly(3,3,3-methyltrifluoropropyl siloxane) stationary phases with a low percentage of trifluoropropyl have been recharacterised by means of activity coefficients at temperatures in the range 60–140°C. The temperature effect of activity coefficients was studied. Thermodynamic magnitudes: excess Gibbs energy, excess enthalpy and excess entropy for 44 solutes on these polymers were calculated, and their relationships with solutes’ molecular connectivity indexes were tested. Solute-polymer interactions were calculated at 120°C according to the solvation parameter model, and several correlations for selected solutes and polymers were investigated, mainly the effect of solutes’ structure on the non-polar interactions and the effect of the solute dipole moment on the polar interactions. In addition, the influence of polymer polarity on the different polar and non-polar interactions was investigated.     

Relative peak broadening and pore size distribution of the stationary phase in exclusion chromatography

Summary It is shown experimentally that in size exclusion chromatography theh-values of a homologous series versus the relative molecular mass of the samples and the pore size distribution of the packing material determined by exclusion chromatography exhibit the same maximum.Part of the Ph.D. Thesis of W. Werner, University of Saarbrücken, Saarbrücken 1976.     

Relationship between the internal pressure and cohesive energy density of liquids

The concepts of internal pressure and cohesive energy density of liquids are considered. Conditions of equality between the internal pressure and the cohesive energy density are revealed.     

An analytical solution of a newly proposed peak broadening equation and extension of polydispersities to higher molecular weight averages in size exclusion chromatography (SEC)

Summary Herein is reported an analytical solution to the peak broadening or peak dispersion/flattening equation based on the recently proposed Instrumental Spreading Shape Function and its application to correction for imperfect resolution (inadequate peak separation and/or excessive peak broadening) for higher molecular weight averages. The relationship of these higher MW averages with the familiar Weight Average and number average molecular weights is also discussed. Criteria for perfect resolution are specified and a true molecular weight calibration curve is accordingly defined.     

The importance of the three-particle correlation function in the theory of monatomic classical liquids like argon

The present study was motivated by the work of Fortov et al. (2008) on the use of experimental methods of analysis of spatial correlation of dust particles in plasma. Here, we therefore focus on the importance of the three-particle correlation function g ₃ in the theory of monatomic classical liquids like argon, in the framework of two-body central forces. We cover (i) the asymptotic relation of g ₃ to the Ornstein–Zernike direct correlation function c(r); (ii) the use of g ₃ to determine the specific heat difference c _P ? c _V ; and (iii) g ₃ related to the pressure dependence of the pair correlation function.     

Experimental investigation of the band broadening originating from the top and bottom walls in micromachined nonporous pillar array columns

We report on the experimental investigation of the effect of the top and bottom wall plates in micromachined nonporous pillar array columns. It has been found that their presence yields an additional c-term type of band broadening that can make up a significant fraction of the total band broadening (at least if considering nonporous pillars and a nonretained tracer). Their presence also induces a clear (downward) shift of the optimal velocity. These observations are, however in excellent quantitative agreement with the theoretical expectations obtained from a computational fluid dynamics study. The presently obtained experimental results, hence, demonstrate that the employed high aspect ratio Bosch etching process can be used to fabricate micromachined pillar arrays that are sufficiently refined to achieve the theoretical performance limit.     

Chromatography in pore networks II — The role of structure and adsorption in the band broadening

Summary The complex intraparticle structure typical of chromatographic column packings has been analyzed by use of an equivalent network model which emphasizes pore size distribution and connectivity. Special attention is given as to the way in which diffusion and adsorption interact and display modified peak spreading characteristics according to the morphology of the pore space. This study reveals a very significant increase in the column band broadening over that expected from physical adsorption which can arise from particular distributions of pore sizes. This has implications for designing packings which take advantage of the separating power due to adsorption but do not compromise the resolution of the chromatographic system.     

Computationally-assisted approach to the vibrational spectra of molecular crystals: study of hydrogen-bonding and pseudo-polymorphism.

A new computationally-assisted methodology (PiMM), which accounts for the effects of intermolecular interactions in the crystal, is applied to the complete assignment of the Raman and infrared vibrational spectra of room temperature forms of crystalline caffeine, theobromine, and theophylline. The vibrational shifts due to crystal packing interactions are evaluated from ab initio calculations for a set of suitable molecular pairs, using the B3LYP/6-31G* approach.The proposed methodology provides an answer to the current demand for a reliable assignment of the vibrational spectra of these methyl-xanthines, and clarifies several misleading assignments. The most relevant intermolecular interactions in each system and their effect on the vibrational spectra are considered and discussed. Based on these results, significant insights are obtained for the structure of caffeine in the anhydrous form (stable at room temperature), for which no X-ray structure has been reported. A possible structure based on C((8))--H...N((9)) and C((1,3))--H...O intermolecular interactions is suggested.     

A first principles explanation for the experimentally observed increase in A-term band broadening in small domain silica monoliths and other chromatographic supports

The present computational study illustrates how the existence of a residual lower limit on the variance of the skeleton and through-pore size of monolithic columns can be expected to severely compromise the possibility to prepare well-performing small domain monolithic columns. Adopting rather conservative estimates for the minimal standard deviation on the pore and the skeleton size (0.2 and 0.04 microm, respectively), the presented calculations show that, if such a fixed lower limit on the size variance exists, it will be impossible to decrease the A-term band broadening below a given critical value, no matter how small the domain size is made. From a given critical domain size value on, any attempt to further decrease the domain size without being able to co-reduce the size variance can be expected to be counterproductive and leads to an increase instead of to a further decrease of the plate heights.     

Phase control of the competition between electronic transitions in a solvated laser dye

Excited state population can be manipulated by resonant chirped laser pulses through pump–dump processes. We investigate these processes in the laser dye LD690 as a function of wavelength by monitoring the saturated absorption of chirped ultrafast pulses. The resulting nonlinear absorption spectrum becomes increasingly complex as the pulse is tuned to shorter wavelengths. However, fluorescence measurements indicate that the excited state population depends weakly on chirp when the pump wavelength is far from the lowest order electronic transition. Using a learning algorithm and closed-loop control, we find nonlinear chirp parameters that optimize features in the transmission spectrum. The results are discussed in terms of competition between excited state absorption and stimulated resonant Raman scattering.     

A verification by FTIR spectroscopy of the effect of some local chain conformations on the specific molecular interactions of solvents,esters, and polyesters with PVC

The molecular interactions of poly(vinylchloride) (PVC) with some solvents [cyclohexanone (CH), methyl ethyl ketone (MEK) and N-methylpirrolidone (MP)], esters [dioctylphthalate (DOP) and butyl stearate (BuSt)], and polyesters [poly(ethylene adipate) (PEA) and poly(ε-caprolactone) (PCL)] have been investigated by FTIR spectroscopy. In all cases the band of the carbonyl group is found to shift to lower frequencies, but significant differences between the solvent and the esters, whether polymeric or not, are evidenced. For PVC-solvent systems, the shift proves to increase linearly as PVC/solvent ratio increases, what suggests that only a definite number of polymer sites is involved. From the slopes of the straight lines this effect of composition is shown to increase in the order MP < MEK < CH, i.e., as the basicity of the solvent decreases. In contrast, for the PVC-esters or polyester blends, a nonlinear behavior consisting of two distinct interaction processes, is obtained. The increase of shift as PVC/ester ratio increases is faster in the first process for all PVC-ester systems and it is particularly enhanced for BuSt and, to a lesser extent, for DOP. Instead, during the second process, that increase is of little significance for BuSt relative to DOP and PCL. These results account for the saturation of the polymer structures that are capable of interacting, at different rates depending on the type of ester. Besides, the whole number of those structures appears to be lower than in the case of solvents. The results are discussed on the ground of, on one side, the mechanism of nucleophilic substitution on PVC, in the same solutions and blends, which, as found previously, is of a stereospecific nature, and, on the other, the electron-donor-acceptor concept (EDA) and the hard-soft-acid-base concept (HSAB) as applied to both the interacting agents (solvents and esters) and the isotactic GTGT and GTTG^? triad conformations as well as the heterotactic GTTT. In the light of the resulting conclusions it is suggested that: (i) the linear behavior shown by the solvents obeys the solvent ability to ensure a dynamic equilibrium between the two possible conformations of - mmr - sequence, i.e., GTGTTT and GTTG^?TT, through the preferential interaction with the little likely GTTG^? conformation, the content of which happens so to be constant as long as there are - mmr - sequences in solution; (ii) the nonlinear behavior of PVC-ester or polyester binary systems reveals a nonequilibrium situation and so the conformational change GTGTTT ? GTTG^?TT, which is highly hindered, will occur occasionally depending on the ester nature. This enables one to attribute the fast and the slow interaction processes to the permanent GTTG^?TT conformations derived from the polymerization and to the same conformations formed as the result of the conformational changes, respectively. Strong support for the above novel finding that PVC … O?C interaction is of a local conformational nature is given by two additional investigations. First, a similar study with a PVC sample prepared at ?50°C, shows that the carbonyl band shifts of CH and PCL are appreciably lower than those of PVC prepared at 70°C. The same holds for the blendof PCL with the latter PVC sample after substitution reaction (0.6%) at ?15°C in CH with sodium benzenethiolate (NaBT). Since the PVC obtained at ?50°C and the 0.6% substituted polymer exhibit a lower content of both permanent GTTG^?TT conformations ad -mmr- sequence, these results agree with expectatins and confirm the above suggestions. Secondly, the changes in the C? Cl stretchign frequencies of PVC with increasing amounts of solvent or ester, as extensively studied, clearly indicate the occurrence of the aforementioned conformational change, and so they are consistent with our proposals as to the actual conformational nature of PVC…O? C interactions. © 1995 John Wiley & Sons, Inc.     

Spectroscopic,thermal studies and molecular modelling of charge transfer complexation between 5-amino-2-methoxypyridine with chloranilic acid in different polar solvents

Charge transfer (CT) interaction between 5-amino-2-methoxypyridine (5AMPy), as electron donor (proton acceptor), with 3,6-dichloro-2,5-dihydroxy-p-benzoquinone (chloranilic acid, H₂CA), as electron acceptor (proton donor), has been investigated spectrophotometrically in the polar protic solvents ethanol (EtOH) and methanol (MeOH) and the aprotic one acetonitrile (AN). Pink-coloured solution is formed instantaneously upon mixing 5AMPy with H₂CA solutions in all solvents, which is the hallmark evidence of CT complex formation. Based on Job’s method of continuous variations, as well as spectrophotometric titrations, the stoichiometry of the complex was found to be 1:1 [(5AMPy) (H₂CA)] in all solvents. Benesi–Hildebrand equation has been applied to estimate the formation constant of the produced CT complex (K_CT) and its molar absorptivity (ε), they reached high values, confirming the complex high stability. Solid CT complex has been synthesised and analysed by elemental analyses and FTIR, ¹H NMR spectroscopies, where 2:1 [(5AMPy)₂ (H₂CA)] CT complex was obtained.     

Relation between immersion enthalpies of activated carbons in different liquids,textural properties,and phenol adsorption

The immersion enthalpies in benzene, cyclohexane, water, and phenol aqueous solution with a concentration of 100 mg L^?1 are determined for eight activated carbons obtained from peach seeds (Prunus persica) by thermal activation with CO₂ at different temperatures and times of activation. The results obtained for the immersion enthalpy show values between ?4.0 and ?63.9 J g^?1 for benzene, ?3.0 and ?47.9 J g^?1 for cyclohexane, ?10.1 and ?43.6 J g^?1 for water, and ?11.1 and ?45.8 J g^?1 for phenol solution. From nitrogen adsorption isotherms, the surface area, micropore volume, and average pore diameter of the activated carbons were obtained. These parameters are related with the immersion enthalpies, and the obtained trends are directly proportional with two first parameters in the nonpolar solvents, which is a behavior of microporous activated carbons with hydrophobic character. Phenol adsorption from aqueous solution on activated carbons is proportional to their surface area and their immersion enthalpy in the solution.