New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. III. Determination of the acid-base properties of some solid substrates (polymers,oxides and carbon fibres): a new model

HPLC experiments to separate butyl-terminated polystyrene (B-PS) oligomers have been mimicked by equilibrium self-consistent-field calculations based upon the Scheutjens Fleer formalism for polymers at interfaces. The adsorption-desorption transition as a function of the fraction of good solvent in a non-solvent (water)-solvent (tetrahydrofuran) mixture has been analysed and correlated to corresponding experiments. Much attention is paid to keeping the modelling as realistic as possible; for example, the effects of the solvent mixture on the C18-alkyl tails that are grafted on the silica surface are retained in the calculations. It is shown that the butyl end groups affect the elution properties up to chains with approximately 30 styrene units. Excellent semi-quantitative comparison is found with experiments for a realistic set of interaction parameters. Molecular-level information is available for the adsorption layer as a function of the solvent quality. Going from poor to good solvent, it is typical to find that the B-PS is fully absorbed inside the alkyl brush, then adsorbed on top of it, and finally depleted from it. The depletion effect in good solvents increases with increasing molecular mass.     

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. II. Study of the transition temperatures of poly(methyl methacrylate) at various tacticities and of poly(methyl methacrylate) adsorbed on alumina and silica

A simple device was developed for in-vial liquid-liquid extraction using a polymer membrane (nonporous polypropylene) to separate an aqueous sample from an organic extractant. The membrane consisted of tubing with an internal diameter of 6 mm and a wall thickness of 0.05 mm, which was heat-sealed at the lower end and filled with 500 microl hexane. This membrane bag was incorporated into a conventional 20 ml headspace vial suitable for a multi-purpose sampler (MPS 2, Gerstel, Mülheim, Germany) directly interfaced to a gas chromatograph with a mass-selective detector. The sampler enabled the extraction vial to be mixed at a defined temperature with subsequent large-volume injection of the organic extract taken from the membrane bag. The method was evaluated using several triazines, 2,4-dichloroaniline, alpha-hexachlorocyclohexane and phenanthrene as model compounds. Extraction parameters such as temperature, agitation speed, and extraction time were optimised. Recoveries of 60-90% were achieved after 30 min extraction. By increasing the injection volume to 100 microl, detection limits of 1-10 ng/l were determined.     

Coupling of inverse gas chromatography at infinite dilution (IGC-ID) with a controlled modification of a solid surface

This paper presents a new method for studying the surface of a heterogeneous solid by inverse gas chromatography at infinite dilution (IGC-ID). After saturating the high-energy sites by impregnation with a suitable polymer, the chromatographic probes visit the low energy sites, which are not visible by conventional IGC-ID. This method has been used to study the two types of surface of talc: lateral and basal surfaces. In the second part of the paper the influence of the structure of the polymer on the impregnation is examined. In particular it is shown that a polymer with a linear structure is more able to fit a rough surface than is a branched or a cyclic polymer.     

Determination of the surface characteristics of particulate fillers by inverse gas chromatography at infinite dilution: a critical approach

CaCO3 fillers were investigated by inverse gas chromatography (IGC) to determine the dispersion component of their surface tension as well as their acid-base character. Because of the high energy of the filler surface, it readily adsorbs water, thus the parameters measured by IGC depend on the conditioning temperature, as well as on the measurement conditions. As a consequence, the determined surface characteristics are not material constants; different fillers or the effect of coating can be compared only under standard conditions. The use of the same conditioning and measurement temperature eliminates the effect of measurement time. Under appropriate standard conditions the acid-base characteristics of the filler can be determined reliably. However, the accuracy of the determination and the value of the derived parameters depend very much on the selected approach and on the acid-base constants used for the probe molecules. A critical analysis of the approaches used in the current literature pointed out those that yield the most reasonable and accurate values. The results prove that the surface of CaCO3 is strongly basic in character. Coating significantly reduces basicity. Surprisingly, the filler coated with an amount of stearic acid resulting in minimum surface tension showed relatively strong acidity, which indicates a coating exceeding monolayer coverage and/or the uneven distribution of the surfactant on the surface.     

Hold-up time in gas chromatography I. New approach to its estimation

A new procedure to reach a good estimate of the hold-up time value (t_M) in gas chromatography is presented. The value of t_M obtained lies close to the experimental retention time of neon, the gas which has shown the shortest retention time in the columns studied. The new method, based on the retention of n-alkanes, is easily applied with any personal computer and may be used with detectors which do not respond to permanent gases. The procedure is reliable, produces very reproducible hold-up times and the value of t_M obtained may be safely used to calculate both chromatographic and thermodynamic parameters.     

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.