Homologous Series Of Alkylbenzenes Research Articles

Postpolymerization modification of a hydroxy monolith precursor. Part II. Epoxy biphenyl modified poly (hydroxyethyl methacrylate-co-pentaerythritol triacrylate) monolithic capillary columns for reversed-phase capillary electrochromatography based on π-π and hydrophobic interactions.

In this second part of the series of investigations involving the postpolymerization modification of a hydroxy monolith (OHM) capillary column, the surface hydroxyl groups were reacted with epoxy biphenyl thus yielding the so-called Biphenyl OHM capillary column. The modification involved the epoxy ring opening of the 2-biphenylyl glycidyl ether catalyzed by BF3 and its subsequent reaction with the hydroxyl groups on the OHM precursor surface. The Biphenyl OHM capillary column thus obtained exhibited the typical reversed phase behavior by primarily hydrophobic interactions vis-à-vis the homologous series of alkyl benzenes and in addition by π-π interactions toward nitroalkane homologous series via their π-electron rich nitro groups. This dual retention mechanism was very distinctly observed with a set of PAH solutes in the sense that the k values of the PAH solutes were comparable to those obtained on a more non polar stationary phase, namely the Epoxy OHM C-16 reported in the preceding article. Other aromatic solutes showed the dual retention mechanism on the Biphenyl OHM capillary including phenols, anilines derivatives, and phenoxy acid herbicides. The Biphenyl OHM capillary exhibited good reproducibility from run-to-run, day-to-day, and column-to-column.

Understanding mixed-mode retention mechanisms in liquid chromatography with hydrophobic stationary phases.

The chromatographic retention mechanisms of two hydrophobic bonded phases, octadecyl ethyl-bridged organic/inorganic (BEH-C18) and straight-chain perfluorohexylpropyl silica (C6F13), have been investigated by using a homologous series of alkyl-benzenes and perfluoroalkyl acids as test compounds in a variety of acetonitrile/water mobile phases and at different temperatures. On both columns, polar compounds exhibited a characteristic U-shape retention behavior in function of acetonitrile amount in the eluent, whereas retention of neutral molecules decreased continuously, following an increase of organic modifier, over the entire mobile phase range. The dependence of perfluoromethylene selectivity upon eluent composition explains the typical reversed-phase behavior (decreasing in retention following an increase of acetonitrile in mobile phase) initially exhibited by perfluoroalkyl acids, but alone it cannot justify their increasing of retention at organic-rich mobile phases (approximately >90% v/v for acetonitrile with the C6F13 column and acetonitrile >80% v/v for the BEH-C18 one). It actually predicts an opposite trend, indicating thus the presence of mixed-mode retention mechanisms. Indeed it was found that, at organic-rich mobile phases, the transfer from the mobile to the stationary phase of the polar moiety of molecules drives retention. This finding has been correlated to the excess adsorption isotherm of acetonitrile/water binary mixtures and thus to the composition of the stationary phase. At organic-rich mobile phases, in fact, stationary phases are characterized by a positive excess of adsorbed water that creates an "environment" suitable to the transfer herein of polar groups.

Instrumentation for hand-portable liquid chromatography

Liquid chromatography (LC) has lagged behind gas chromatography (GC) in developments related to hand-portable instrumentation. In this work, a new battery-operated (24V DC) nano-flow pumping system with a stop-flow injector was developed and integrated with an on-column UV-absorption detector (254nm) that was reduced in size to an acceptable weight and power usage for field operation. The pumping system, which includes nano-flow pump, stepper motor and high-pressure valve weighs only 1.372kg (3lbs) and can generate up to 110.32MPa (16,000psi) pressure. A major advantage of this pump is that it does not employ a splitter, since it was specifically designed for capillary column use. The volume capacity of the pump is 24μL, and a sample volume as low as 10nL can be injected. Flow rate calibration (300nL to 6.12μL per min) was performed, and an accuracy >99.94% was obtained. The percent injection carry-over was found to be low (RSD 0.31%), which makes it practical for quantitative analysis. The detector linear range and limit of detection (LOD) were determined using sodium anthraquinone-2-sulfonate. A linear regression coefficient (R) of 0.9996 was obtained for a plot of log peak area versus log concentration over the range of 3.2μM to 6.5mM, and the LOD (S/N=3) was found to be 7.8fmol (0.13μM). The short term noise of the detector is comparable to commercially available detectors (∼10−5AU). In this work, the system was tested in the laboratory using regular line power (120V AC) with an AC to DC adapter. Reversed-phase isocratic separations were performed using a 15.5cm×75μm i.d. fused silica capillary column containing a monolithic stationary phase synthesized from 1,6-hexanediol dimethacrylate. Good retention time repeatability (RSD 0.09–0.74%) was obtained for a mixture containing an unretained marker (i.e., uracil) and a homologous series of alkyl benzenes.

THE EFFECT OF THE CHAIN LENGTH OF BONDED PHASE ON RETENTION OF AROMATIC SOLUTES IN RP LC WITH PURE WATER AS MOBILE PHASE

Retention factors (log kw ) in pure water were calculated for a homologous series of alkylbenzenes, PAHs, and aromatic monosubstituted derivatives for column C18, C8, and C4, using the numerical method based on the linear form of the Ościk's equation, which were correlated with the log kw values determined by the linear extrapolation method from the log k dependence on methanol volume fraction in binary eluent. The chromatographic parameters (log kw ), being the hydrophobicity measure, were compared with the experimental partition coefficients in n-octanol/water system (log P). The author examined the contribution of the substituents (‒CH2‒, ‒OH, ‒Cl, ‒NO2, and ‒CHO) in the substance retention in pure water for columns of different hydrocarbon chain length of the bonded phase. Moreover, the transfer thermodynamics of the particular functional groups from water to the stationary phase, as well as the influence of the kind of the stationary phase on the thermodynamics of this process were studied. For this purpose the standard free enthalpy (ΔG o) of the functional groups transfer from water to the stationary phase and from the gas to stationary one was calculated.

Selectivity comparisons of monolithic silica capillary columns modified with poly(octadecyl methacrylate) and octadecyl moieties for halogenated compounds in reversed-phase liquid chromatography

Stationary phase selectivities for halogenated compounds in reversed-phase HPLC were compared using C18 monolithic silica capillary columns modified with poly(octadecyl methacrylate) (ODM) and octadecyl moieties (ODS). The preferential retention of halogenated benzenes on ODM was observed in methanol/water and acetonitrile/water mobile phases. In selectivity comparison of selected analytes on ODM and ODS, greater selectivities for halogenated compounds were obtained with respect to alkylbenzenes on an ODM column, while similar selectivities were observed with a homologous series of alkylbenzenes on ODM and ODS columns. These data can be explained by greater dispersive interactions by more densely packed octadecyl groups on the ODM polymer coated column together with the contribution of carbonyl groups in ODM side chains. For the positional isomeric separation of dihalogenated benzenes ( ortho-, meta-, para-), the ODM column also provided better separation of these isomers for the adjacently eluted isomers that cannot be completely separated on an ODS column in the same mobile phase. These results imply that the ODM column can be used as a better alternative to the ODS column for the separation of other halogenated compounds.

Interaction mechanisms on octadecyl packed columns in subcritical fluid chromatography with CO 2–modifier mobile phases

An experimental design was carried out for describing the interaction mechanisms between solutes and octadecyl bonded silicas in subcritical fluid chromatography (SubFC), with CO 2–methanol and CO 2–acetonitrile mobile phases. The effects of modifier amount, temperature and outlet pressure were studied. The homologous series of alkylbenzenes was mainly used as probe, and results were in part assessed with other series. Curves between the methylene selectivity ( α CH2) and the alkyl chain carbon number ( C n) were plotted, because changes of slope or discontinuity in these curves are yielded by interaction mechanism modifications. Moreover, the linearity of the Van ‘t Hoff curves with CO 2–acetonitrile mobile phases has enabled one to calculate the transfer enthalpy (Δ H) for each homologue. The curves log k=f(−Δ H) allow a discrimination of the retention behaviors between the short and the long homologues for CO 2–acetonitrile mobile phases. Depending on the analytical conditions, different oriented partition mechanisms occur for the long homologues, when the short ones seem to be fully embedded into the grafted chains near the silica surface. With methanol–CO 2 mobile phases the discrimination between the homologues disappears and the methylene selectivity curves correspond to a bulk partition mechanism. The differences in the interaction mechanisms following the modifier nature are related to the adsorption the mobile phase onto the stationary phase, because the amount of adsorbed mobile phase modifies the bonded chain mobility. With methanol, an important adsorption of the mobile phase occurs, when this adsorption is reduced with acetonitrile. In this latter case, an anisotropy in the stationary phase mobility can explain the observed difference in the interaction mechanisms of homologues. Finally, effects of stationary phase chain length (from C 18 to C 22) and bonding density (from 2.5 to 3.4 μmol m −2) were also reported.

Application of the Retention Index Concept in Micellar Electrokinetic Capillary Chromatography

The application of the retention index concept in micellar electrokinetic capillary chromatog. (MECC) was evaluated for the identification of neutral species. Homologous series of alkylbenzenes and alkylaryl ketones were applied as retention index stds. and also for the calcn. of the micelle migration time by an iteration procedure. The relation between retention indexes, I, and octanol-H2O partition coeffs. is discussed, and DI values were calcd. from retention indexes, obtained with both anionic and cationic surfactant systems, to study the sepn. mechanism for solutes with different functionalities in MECC. The influence of the phase ratio is treated theor., and the capacity factor is linearly related to the surfactant concn., whereas the retention index is independent of the surfactant concn. The temp. dependence of the retention index was studied and the std. enthalpy, entropy, and Gibbs free energy for micellar solubilization of the sample compds. were detd., showing that the hydrophobic interaction plays a significant role in MECC anal

Origin of long-chain alkylcyclohexanes and alkylbenzenes in a coal-bed wax

A coal-bed wax was fractionated and analyzed using capillary column GC and combined GC/MS. It was found that the major components in the wax were n-alkanes (55.6%), cyclic/branched alkanes (26.0%), and several homologous series of alkylbenzenes (5.7%). All alkylbenzene isomers (except 6- n-alkyl- m-xylene) were positively identified by comparison with the retention times and mass spectra of newly synthesized authentic standards. 5- n-Alkyl- m-xylene, 2- n-alkyl- p-xylene, 4- n-alkyl- m-xylene, 4- n-alkyl- o-xylene, 2- n-alkyl- m-xylene, and 3- n-alkyl- o-xylene were identified for the first time from geological sources. All of these long-chain alkyl compounds (e.g., n-alkylcyclohexanes, n-alkylbenzenes, n-alkyl- o-toluenes, n-alkyl- p-toluenes, and 5- n-alkyl- m-xylenes) have similar total carbon number distributions and maxima with a slight even over odd carbon number preference between C 28–C 30. Moreover, the carbon number distributions of these compounds resembled those of the n-alkanes found in the same wax with slight odd over even carbon preference between C 27–C 21. This indicates that the alkylcyclohexanes and alkylbenzenes may have the same fatty acid precursors as the n-alkanes. The alkylcyclohexanes and alkylbenzenes could have been formed by direct cyclization and aromatization, while the n-alkanes could have been formed by decarboxylation of the straight chain fatty acids. This explanation is further supported by the identification of homologous series of tetramethyl- n-alkylbenzenes and pentamethyl- n-alkylbenzenes with relatively high abundances at C 15, C 16, and C 18, and a fatty acid distribution with maxima at C 16 and C 18. Based on these findings, mechanisms for the conversion of fatty acids or alcohols to alkylcyclohexanes and alkylbenzenes are proposed.

Copolymer of Di (methacryloyloxymethyl) naphthalene and divinylbenzene as a column packing for high-performance liquid chromatography

A new porous polyaromatic ester packing was synthetized for high performance liquid chromatography. The relationship between retention and chain length of the members of homologous series of alkylbenzenes, N-alkylanilines, alkylarylethers, alkylbenzoates and alkylarylketones on this new stationary phase using different eluents was investigated. Using the alkylarylketone scale the retention indices of the homologues and test compounds were calculated. The results were compared with those obtained for poly (styrene-divinylbenzene) polymers. For both types of packing the first members of each homologous series gave non-linear behaviour. The methylene group index increments are different for the studied homologous series; thus there is no simple additivity of the retention indices. The efficiency of the porous polymeric columns is a function of the capacity factor of the solute and the organic component of the eluent.

Low Interfacial Tensions Involving Mixtures of Surfactants

Abstract The interfacial tension of surfactant mixtures with hydrocarbons obeys a simple scaling rule. Many apparently inert surfactants give low tensions when in mixtures; the scaling rule still applies to these mixtures. The influence of surfactant structure and molecular weight on low-tension behavior is examined, and the application of these results to the optimization of surfactant flooding systems is discussed. Introduction It has been shown that the interfacial-tension behavior of a given crude oil with a surfactant solution of the sulfonate type may be modeled by replacing the crude oil with one particular alkane. The number of carbon atoms in the alkane is referred to as the equivalent alkane carbon number (EACN) of the crude oil, and this EACN is independent of the surfactant used (at fixed standard conditions). This equivalency of a crude oil and an alkane is a result of the simple averaging behavior of hydrocarbons when mixed. Any hydrocarbon may be assigned an EACN value. For instance, when homologous series of alkyl benzenes and alkanes are run against the petroleum sulfonate TRS 10-80 at 2 gm/liter of surfactant with 10 gm/liter NaCl present, heptyl benzene and heptane, respectively, give minimum interfacial tensions, a. The EACN of heptyl benzene is 7, since it is equivalent to heptane. A simple averaging rule will give the EACN of a hydrocarbon mixture : (1) where x is the mole fraction of the ith component. Thus, an equimolar mixture of undecane (EACN 11) and heptyl benzene (EACN 7) has an EACN of 9. If a surfactant gives a low (minimum) sigma against nonane (EACN 9), it will also give a low sigma against the above mixture. Eq. 1 implies that a crude oil, which is a multicomponent hydrocarbon mixture, may be assigned an EACN. This has been verified experimentally. For example, Big Muddy field crude oil has an EACN of 8.5. Therefore, any surfactant phase giving a minimum tension against an equimolar mixture of octane and nonane gives a low tension against Big Muddy crude. All crude oils rested to date have EACN's ranging from 6 to 9. For a given surfactant, the alkane of minimum tension (min) may be affected by the electrolyte concentration or type, the temperature, the surfactant concentration, or the presence of a cosurfactant. These system variables may be adjusted until the nmin for a surfactant matches exactly the EACN of a crude oil. For any particular surfactant, many different combinations of variables will give the same n min value; therefore, there are many possible systems, each with n = EACN, available for crude oil recovery. In practice, however, the system variables may be manipulated to a limited extent only. The temperature of an oil field is fixed, and the surfactant concentration is limited by considerations of solubility and expense. The electrolyte concentration and type is partly determined by oilfield conditions and is limited by the effect on surfactant solubility. These limitations mean that many of the surfactants presently available on a large enough scale for use in low-tension flooding will not give minimum tensions in the range required (n of 6 to 9). This paper shows how minimal sigma's in the required range may be found for some of these "off-scale" surfactants when they are used in surfactant mixtures. The hypothesis tested here is that surfactant mixtures average in a manner analogous to the averaging of hydrocarbons in the oil phase. It will be shown that each surfactant component may be assigned an n value and that the alkane of minimum tension of a mixture of surfactants, (n), is then given by (2) where x is now the mole fraction of the ith component of the surfactant mixture. This greatly extends the number of surfactants that may be considered as candidates for use in low interfacial-tension flooding. SPEJ P. 122

Chromatographic behaviour of alkylbenzenes and alkyl benzoates on porous micro-spherical poly(styrene-divinylbenzene) gel

Homologous series of alkylbenzenes and alkyl benzoates were analyzed by high-performance liquid chromatography employing a porous micro-spherical poly(styrene-divinylbenzene) gel as the stationary phase with various proportions of methanol and water as the mobile phase. In each series of alkylbenzenes and alkyl benzoates, the elution order followed the order of increasing alkyl chain length. The capacity factors increased with increasing water content in the eluent, but decreased as the column temperature increased. The logarithm of the capacity factor for each homologous series was directly proportional to the carbon number. The reciprocal of the column temperature was also directly proportional to the logarithm of the capacity factor. The results might be explainable in terms of the theory of liquid-liquid partition chromatography.

Precambrian organic compounds from the Ketilidian of South-West Greenland. Part III

Ketilidian sedimentary rocks nearly 2000 m.y. old and only slightly metamorphosed have been shown to contain fossils and organic compounds. Results of work on organic material from a 1-3 m thick coal-graphite layer from the Foselv Formation in the Sortis Group in Grænseland are presented. Aliphatic (straight chain, branched and cyclic) hydrocarbons are dominant in the extract from two coal-graphite samples weighing about 4 kg and 10 kg respectively. Further investigations of these compounds have revealed many saturated straight-chain hydrocarbons, with the n-C15 alkane, the most abundant among the normal alkanes, ranging from C12 to about C21, and various isoalkanes and anteisoalkanes. Saturated cyclic hydrocarbons like alkyl cyclohexanes, and homologous series of alkyl benzenes and alkyl naphthalenes are also present. A series of monoterpenoid compounds is present. Six of them are fairly well characterized by their mass spectra. Fatty acids and methyl esters of fatty acids are present, notably a homologous series of straight-chain fatty acids. Branched acids are also present in minor amounts. The organic compounds further indicate that this Precambrian coal-graphite layer is the result of biological activity.