C - Lublin

Kloc, 2001]. Nitrogen adsorption isotherms were measured instrumentally at liquid
nitrogen temperature using Sorptomatic 1990 made by Fisons (Petersen et.al.1996).
The surface area of solids were calculated from adsorption data using the
Brunauer-Emmet-Teller (BET) equation (Brunauer et al. 1938):
y/a = 1/(a m C) + x(C-1)/(a m C), (1)
where y=x/(1-x), x=p/p 0 , p 0 (Pa) is the saturated vapor pressure at the temperature
of the measurements T(K), a m (kg kg -1 ) is the statistical monolayer capacity,
C=exp(-(E a -E c )/RT) is the constant related to the adsorption energy, E a (J mol -1 ),
and condensation energy of water, E c (J mol-1) and R(J mol-1 K-1) is universal gas
constant. Having calculated am values from the slope and the intercept of Eq (1),
the surface area of the solid, S(m 2 kg -1 ) can be calculated using:
S = Nωam / Mr, (2)
where N(mol-1) is the Avogadro number, M r (kg) is molecular weight of the
adsorbate and ω (m 2 ) is the area occupied by a single adsorbate molecule, assumed
to be 1.08x10-19 m 2 for water and 1.08x10-19 m 2 for nitrogen.
RESULTS AND DISCUSSION
The surface areas of plant roots measured from water vapor adsorption were
around 10 times higher than these derived from nitrogen adsorption that is
presented in Table 1.
Table 1. Surface areas of the studied plant roots
Shooting stage
Tillering stage
Surface area
Ars Inia Henika Ars Inia Henika
H 2 O (m 2 g -1 ) 147 164 156 150 172 164
N 2 (m 2 g -1 ) 27.4 22.25 35.33 29.1 24.92 24.03
Such large differences may arise from differences in water and nitrogen
molecules polarities: water is adsorbed on polar surfaces while nitrogen on
nonpolar ones. Possibly in the roots polar surfaces dominate. However, principles
of physical adsorption state that surface areas (at least of flat homogeneous
adsorbents should be similar. In this respect one should look for the reason of the
differences mentioned either in complicated geomety of root surfaces or in their
heterogeneous energetic character. Also, the nitrogen adsorption method requires
prior evacuation and heating of the sample, which thins water films and brings the
root tissues closer. The quasi-contact of the root material (e.g cell walls) can extend
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