Particulate Systems Analysis 2008, Stratford-upon-Avon, UK
085 Influence of humidity on the surface properties of alumina
determined by Inverse gas Chromatography
Rachel Calvet1, Christelle Tisserand1, John Dodds1, Henri Balard2
1
Ecole des Mines d’Albi-Carmaux, Route de Teillet, 81013 ALBI Cedex CT 09 – calvet@enstimac.fr
2
Ecole Nationale Supérieure de Chimie de Mulhouse, Lab. Chimie Physique, ENSISA-W, 11, rue
Werner, 68093 Mulhouse cedex (France) – h.balard@wanadoo.fr
ABSTRACT:
Inverse gas chromatography (IGC) is a very sensitive method for the determination of the surface
properties of divided solids. In this work, it has been used to show the influence of relative humidity on
the surface properties of two alumina samples. Many papers on this subject are available in the
literature, pharmaceutical powders (Buckton 2007), food powders (Zhou 2006, Boutboul 2000) or
mineral fillers (Balard 1997-1,Voelkel 2000). There are two techniques of IGC : infinite dilution (IGCID) (Balard 2000) or finite solute concentration (IGC-FC) (Balard 1997-2). In the first case, very small
amounts of probe molecules are injected leading to a symmetrical chromatographic peak. The
retention times of linear alkanes and polar probes allow respectively the calculation of the dispersive
and specific components of the surface energy (sd and Isp). But in IGC-ID, only the highest energy
sites contribute significantly to the retention time, so the values measured on an heterogeneous
surface solid have no absolute signification and do not represent the average surface energy. The
second technique, IGC-FC, consists in injecting a large amount of probe to cover the surface with a
monolayer which on elution, and leads to a deformed chromatographic peak. The method called the
“ elution by characteristic point ” (ECP) is used because it has the advantage of only needing a single
injection to determine the whole of the adsorption isotherm from the Conder’s equation (1) (1979):
N 
1 J.D.t r Vn

  
m
P L,t r RT m
(1)
Here N is the number of absorbed molecules, P the pressure of the probe at the output of the column,
L the column length, tr the net retention time of a characteristic point on the rear diffuse profile of the
chromatogram, Vn the corresponding net retention volume, J the James and Martin’s coefficient, D the
output flow rate,
m the mass of adsorbent and R the gas constant.
The isotherm can give several useful values: BET specific surface area, BET and Henry’s constants.
The analysis of the isotherm shape also gives access to the distribution function of the adsorption
energies.
The influence of humidity on the surface properties of two alumina powders, (  and  crystallographic
forms) is examined both by ICG-ID and IGC-CF with chromatograph fitted with a humidity generator to
provide a controlled humidity carrier gas.
Firstly, IGC-ID parameters are determinated showing the decrease of the sd and Isp with the increase
of the relative humidity. This evolution is due to shielding of the highest energy sites by water
molecules.
The IGC-FC technique confirms the influence of the humidity on the surface properties of the two
alumina samples. The distribution functions of energy sites are calculated for three probes, octane a
hydrophobic probe, toluene an aromatic probe and dioxanne a basic probe. Only the distribution
functions obtained with the basic probe are sensitive to the relative humidity of the carrier gas (figure
1). The high energy sites, near 40 kJ/mol disappear when relative humidity increases, the molecule of
dioxanne released can access to lower energy sites near 25 kJ/mol. The water molecules adsorb
themselves on acid sites no more accessible to the basic probe.
1
Particulate Systems Analysis 2005, Stratford-upon-Avon, UK
 alumina
 alumina
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Figure 1: Distribution functions of the adsorption energies of dioxanne on samples of  and 
crystallographic forms of alumina.
Keywords alumina, inverse gas chromatography, humidity, surface properties
REFERENCES
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