Functionalization of surface‐grafted polymethylhydrosiloxane thin films with alkyl side chains

Thin films of crosslinked polymethylhydrosiloxane (PMHS) have been grafted on silica using the sol–gel process allowing further functionalization by effective quantitative hydrosilylation of SiH groups by olefins within the network. Postfunctionalization gives the polysiloxane network with n‐alkyl side chains. The PMHS coating was prepared by room temperature polycondensation of a mixture of methyldiethoxysilane HSiMe(OEt)₂ monomer and triethoxysilane HSi(OEt)₃ (TH) as crosslinker. The surface‐attached films are chemically stable and covalently bonded to the silica surface. Subsequently, films were functionalized without delamination. We showed by FTIR spectroscopy how the crosslinking ratio and the molecular size of the alkenes precursors influence the extent of the hydrosilylation reaction of SiH groups in the PMHS network. We have determined that quasi‐full olefin addition catalyzed by a platinum complex occurred within soft networks of less than 5% TH with 1‐alkenes CH₂?CH(CH₂)_n‐2CH₃ of various alkyl chain lengths (n = 5, 11, 17). Powders of PMHS gel were also modified with 1‐alkenes by hydrosilylation. The SiH groups within the soft gel (5% crosslinked) were fully functionalized as shown by ²⁹Si and ¹H solid‐state NMR. The structure of functionalized polysiloxane with n‐octadecyl and n‐dodecyl side chains was studied by FTIR, wide angle X‐ray diffraction, and DSC showing crystallization of the long n‐alkyl chains in the network. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3546–3562, 2008     

Gas chromatography study of the adsorption of refrigerating fluids on microporous solid adsorbents

Adsorption isotherms for microporous solids have been determined by elution gas chromatography. Adsorbents are of ;molecular sieves' type; their utilization in affinity thermal machines is envisaged. These machines work in a relatively high domain of pressure for the adsorbate, so the chromatographic technique is generally inadequate for their study. The possibility of increasing the maximal pressures reached by chromatography has been investigated in relation with: the carrier gas flow rate, the length and width of columns, the injected adsorbate mass. This study has been made for different solid adsorbents (zeolite 4A and 13X, silica gel), in different shapes (small stick, pellet, powder), and with different adsorbates (water, methanol, ethanol). In the most favorable conditions that have been derived, the qualities of the chromatographic method, rapidity, simplicity and large range of measures, appear well fitted to the search of this kind of isotherms which does not require a great accuracy.     

Retention of long-chain acetylenic hydrocarbons on non-polar stationary phases

The retention indices of methyl and trimethylsilyl esters of octadeca-, eicosa- and tricosa-ynoic fatty acids containing acetylenic bonds were measured on non-polar stationary phase (dimethylsilicone with 5% phenyl groups). An unusually large increase in retention is observed for compounds containing conjugated and methylene interrupted acetylenic bonds. The additional increase in retention index as a result of the presence of one conjugated acetylenic bond is roughly equivalent to the retention increase caused by lengthening of the hydrocarbon chain for one carbon atom. The increase in retention for methylene interrupted bonds constitutes approximately 50% increase for conjugated triple bonds. A further increase in interruption substantially decreases the effect. Based on available literature data and the results of this work, the contributions of conjugated acetylenic and olefinic bonds, and methylene interrupted acetylenic bonds to retention were estimated.     

Effect of surface heterogeneity on adsorption on solid surfaces. Application of inverse gas chromatography in the studies of energetic heterogeneity of adsorbents

The paper presents a literature review of the chromatographic methods used for investigations of the heterogeneity of solid surfaces. Special attention is paid to inverse gas chromatography (IGC). Quantitative characteristics of heterogeneity of real solid surfaces including extreme models on adsorption centre topography of the "patch-wise" and "random" types are described. Analytical and numerical methods used for calculating the adsorption energy distribution function as a quantitative measure of surface heterogeneity are presented. Special attention is paid to the condensation approximation as well as to other approximations based on this assumption. IGC is presented as a quick, precise and effective method to characterise physicochemical properties of different kinds of adsorbents. Advantages of IGC over traditional methods of gas and vapour adsorption are shown.     

Stability of workroom air volatile organic compounds on solid adsorbents for thermal desorption gas chromatography

The storage stability of the occupationally frequently occurring compounds, methylethylketone, methylisobutylketone, benzene, toluene, tetrachloroethylene, n-butylacetate, -pinene, β-pinene, limonene and n-decane, has been investigated on the adsorbents Tenax TA, Chromosorb 106 and Carbotrap using thermally desorbable tube type samplers, commonly utilized in ambient and workroom atmospheric measurements. Fifty and 500 ng of each compound were loaded on the various adsorbents tubes, stored at both ambient (20 °C) and refrigerated (4 °C) temperatures and analysed by means of thermal gas chromatography with mass spectrometric detection on days 0, 7, 14 and 28 after exposure. A 90% storage recovery was chosen as acceptance criteria for storage stability, and statistical testing by Student's t-test, analysis of variance and Bonferroni post hoc tests were employed to investigate the effect of the categorical variables storage time, storage temperature and analyte loading on the different adsorbents. Chromosorb 106 showed the overall best behaviour with recoveries of 90% or better for all analytes during the 28-day test period. Tenax TA and Carbotrap yielded lower recoveries and were more influenced by variations in storage time, storage temperature and analyte loading. Refrigerated temperatures were best avoided for storage on Tenax TA, but may increase the recovery of some compounds on Carbotrap (e.g. n-butylacetate). The blank build-up on the adsorbents was also investigated, and Carbotrap and Tenax TA showed no signs of artefact development over time. Chromosorb 106, however, contained inherently more artefacts that build up over time, which in spite of the excellent storage capability, may limit its use in field studies where long storage times are normal.     

Effects of grafted polymer chains on lamellar membranes

We have investigated the effects of grafted polymer chains [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] on the bending modulus and the intermembrane interactions of lamellar membranes (C(12)E(5) water) by means of a neutron spin-echo and a small-angle x-ray scattering technique. In this study the hydrophilic chain takes the mushroom configuration on the membrane. The bending modulus of the polymer-grafted membranes increases in proportion to the square of the end to end distance of the polymer chain, which agrees well with the theoretical prediction of Hiergeist and Lipowsky [J. Phys. II 6, 1465 (1996)]. From the interlamellar interaction point of view, the mushroom layer is renormalized to the membrane thickness, which enhances the repulsive Helfrich interaction. When the size of the decorated polymer chain increases to the interlamellar distance, however, the mushroom is squeezed so as to optimize the interlamellar potential. Further increase of the grafted polymer size brings a lamellar-lamellar phase separation, where the grafted polymer chains are localized in the dilute lamellar phase and the concentrated lamellar phase forms the onionlike texture.     

Overloaded gradient elution chromatography on heterogeneous adsorbents in reversed-phase liquid chromatography

Overloaded band profiles of phenol were measured on a C18-Kromasil column in gradient elution conditions. The mobile phase used was a mixture of methanol and water. The volume fraction of methanol was allowed to vary between 0 and 0.5. A general adsorption model, which expresses the amount of phenol adsorbed q* as a function of both its concentration C and the composition phi of the organic modifier (methanol) in the mobile phase, was empirically derived from previous independent adsorption experiments based on frontal analysis (FA) and frontal analysis by the characteristic point (FACP). Accordingly, the general model was an extension of the simplest heterogeneous model, the Bilangmuir model, to non-isocratic conditions. The low-energy sites followed the classical linear solvent strength model (LSSM), but not the high-energy sites whose saturation capacity linearly decreased with phi. The general model was validated by comparing the experimental and simulated band profiles in gradient elution conditions, in linear and non-linear conditions, as well. The band profiles were calculated by means of the equilibrium-dispersive model of chromatography with a finite difference algorithm. A very good agreement was observed using steps gradient (delta phi) from 0 to 50% methanol and gradient times t(g) of 20, 25, 30, 40, 60, 80 and 100 min. The agreement was still excellent for steps gradient from 5 to 45% (t(g) = 25 min), 5 to 35% (t(g) = 50 min), 5 to 25% (t(g) = 50 min) and 5 to 15% (t(g) = 50 min). Significative differences appeared between experience and simulation when the slope of the gradient (delta phi/t(g)) became too strong beyond 3.3% methanol per minute. This threshold value probably mirrored the kinetic of arrangement of the G18-bonded chains when the methanol content increased in the mobile phase. It suggested that the chromatographic system was not in a full thermodynamic equilibrium state when very steep mobile phase gradients were applied.     

Liquid—liquid—vapor phase equilibria behavior of certain binary ethane + n-alkylbenzene mixtures

The liquid—liquid—vapor loci for the binary mixtures ethane + n-nonylbenzene, ethane + n-decylbenzene and ethane + n-undecylbenzene were experimentally studied. The pressure, temperature, and compositions and molar volumes of the liquid phases are reported along the loci. n-Nonylbenzene was found to be the first member of the n-alkylbenzene homologous series to exhibit liquid—liquid—vapor immiscibility with ethane. For the three alkylbenzenes studied, the liquid—liquid—vapor loci have the same type of behavior: they extend from a lower critical end point (LCEP) to an upper critical end point (K-point).     

Adsorption from solutions of nonelectrolytes on solid adsorbents

Summary We have calculated the individual adsorption isotherms from solutions of chloroform-carbon tetrachloride on three adsorbents using only the data on adsorption from solutions.     

LCST‐type liquid–liquid and liquid–solid phase transition behaviors of hyperbranched polyglycerol bearing imidazolium salt

A hyperbranched polyglycerol bearing imidazolium tosylate units ( ITHB ) was synthesized through the imidazolium salt‐modification of hyperbranched polyglycerol ( HB ). ITHB was found to possess novel reversible lower critical solution temperature (LCST)‐type liquid–liquid and liquid–solid phase transition behaviors in a methanol/chloroform mixed solution. The phase transition temperatures of the liquid–liquid phase transition (PTT_1to2) and liquid–solid phase transition (PTT_2toSus) increased with increasing the ratio of methanol in the mixed solution and decreasing the concentration of ITHB . Additionally, increasing the molecular weight of ITHB decreased the PTT values. The liquid–liquid phase transition was caused by the aggregation of ITHB , which was proved by dynamic light scattering measurement. In contrast, the liquid–solid phase transition was caused by the solvation cleavage between the imidazolium rings and solvents, which was proved by ¹H NMR measurement. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Mass spectra of steroidal alkyl ethers—I

The mass spectral fragmentation of saturated and unsaturated steroidal methyl and ethyl ethers and ethers of 4,4-dimethyl steroids are studied and compared to the fragmentation of steroidal alcohols, trimethylsilyl ethers and ethylene ketals. Unlike the trimethylsilyl ether the small fragment containing the alkoxy group is neither the base peak nor a very strong peak in the spectrum. A significant peak occurs at [M — ROH]^+·, however, sometimes even constituting the base peak of the spectrum. The fragmentations of the alkyl ethers are largely dependent on their environments. The present results also show the generalization that methyl ethers are better leaving groups than the corresponding alcohols in fragmentation processes, is not always valid.     

Effect of surfactants on the fluorescence spectra of water-soluble MEHPPV derivatives having grafted polyelectrolyte chains

Poly(2-methoxy-5-[2′-ethylhexyoxy]-1,4-phenylenevinylene) (MEHPPV) derivatives with polyacrylic acid (PAA) chains grafted onto their backbone were found to be water soluble, and they exhibited a dramatic increase in their fluorescence intensity in the presence of a variety of surfactants, even at concentrations far below their critical micelle concentrations (CMC). This increase was accompanied by a blue-shift in the emission maximum. These observations are rationalized based on the postulate that the backbone conformation of the conjugated polymer is modulated upon interaction of the surfactant molecules with the polyelectrolytic tethers, which in turn results in a significant depletion of intra-chain interchromophore interactions that are known to cause red-shifted emission bands with significantly lower emission yields.     

Effect of the endcapping of reversed-phase high-performance liquid chromatography adsorbents on the adsorption isotherm

The retention mechanisms of n-propylbenzoate, 4-t ert-butylphenol, and caffeine on the endcapped Symmetry-C(18) and the non-endcapped Resolve-C(18) are compared. The adsorption isotherms were measured by frontal analysis (FA), using as the mobile phase mixtures of methanol or acetonitrile and water of various compositions. The isotherm data were modeled and the adsorption energy distributions calculated. The surface heterogeneity increases faster with decreasing methanol concentration on the non-endcapped than on the endcapped adsorbent. For instance, for methanol concentrations exceeding 30% (v/v), the adsorption of caffeine is accounted for by assuming three and two different types of adsorption sites on Resolve-C(18) and Symmetry-C(18), respectively. This is explained by the effect of the mobile phase composition on the structure of the C(18)-bonded layer. The bare surface of bonded silica appears more accessible to solute molecules at high water contents in the mobile phase. On the other hand, replacing methanol by a stronger organic modifier like acetonitrile dampens the differences between non-endcapped and endcapped stationary phase and decreases the degree of surface heterogeneity of the adsorbent. For instance, at acetonitrile concentrations exceeding 20%, the surface appears nearly homogeneous for the adsorption of caffeine.     

Structure and phase transitions of alkyl chains on mica

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M. A.; et al. J. Phys. Chem. B 2000, 104, 4433-4439. Osman, M. A.; et al. J. Phys. Chem. B 2002, 106, 653-662] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C(18)) ions or 12 dioctadecyldimethylammonium (2C(18)) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380-390 pm away from the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C(18) ions on the mica surface are found, while 2C(18) ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Agreement with experimental data, where available, is quantitative. In C(18)-mica with less than 100% alkali-ion exchange, the disordered C(18) rods in the island structures [Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913-5917] break at 40 degrees C. At 60 degrees C, the headgroups of the C(18) alkyl chains rearrange on the mica surface, and the broken chain backbones assume a coillike structure. The C(18)-mica obtained on fast cooling of this phase is metastable due to slow reverse rearrangements of the headgroups. In 2C(18)-mica with 70-80% ion exchange, the alkali ions are interspersed between the alkyl chains, corresponding to a single phase on the surface. The observed phase transition at approximately 53 degrees C involves an increase of chain disorder (partial melting) of the 2C(18) ions without significant rearrangements on the mica surface. We propose a geometric parameter lambda for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, that is, islands, intermediate, or continuous. lambda allows the calculation of tilt angles in continuous layers on mica or other surfaces. The thermal decomposition seems to be a Hofmann elimination with mica as a base-template.     

Retention indices of 28 polychlorinated biphenyls in capillary gas chromatography referred to 2,4,6-trichlorophenyl alkyl ethers as RI-standards

An homologous series of eight 2,4,6-trichlorophenyl alkyl ethers (TCPE) was synthesized. The TCPE has been conceived to be used as retention index markers in the gas chromatography with the atomic emission detector (AED), the electron capture detector (ECD) and the mass spectrometric detector (MSD). On the basis of the TCPE, the retention indices of 28 polychlorinated biphenyls have been determined using the ECD, a 95% dimethyl 5% phenyl polysiloxane phase and six different temperature programs.     

Effect of polydispersity on liquid–liquid phase equilibrium

The effect of polydispersity on liquid–liquid phase equilibrium in systems containing polymer, solvent, and nonsolvent is investigated numerically in the Flory-Huggins approximation for several special cases. The resulting phase diagrams indicate that, except in the immediate vicinity of the critical region, the degree of swelling of a polymer precipitate phase with a given number-average molecular weight is essentially independent of polydispersity.