The Investigations Of Water Sorption

Category: VI Ukrainian-Polish Symposium

THE INVESTIGATIONS OF WATER SORPTION

AND POROSITY PROPERTIES OF THE ARGENTINIAN MONTMIRILLONITES USING THE THERMAL ANALYSIS

AND SORPTOMETRY METHODS

P. Staszczuk1, M. Matyjewicz1, J.C. Bazan2, E. Robens3

1Department of Physicochemistry of Solid Surface, Faculty of Chemistry,

Maria Curie-Sklodowska University,

Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland

2Dpto. De Quimica e Ing. Quimica, Lab. De Fisicoquimica Inorganica, Universidad Nacional del Su, 8000 Bahia Blanca, Argentina

3Institute for Inorganic and Analytical Chemistry, Johannes Gutenberg University, D – 55099 Mainz, Germany

A very important problem, which has caused more and more interest is knowledge of the processes of hydration and dehydration of soil, especially montmorillonites [1]. Soil-water interactions is the decesive factor for various processes like for example infiltration of rain water throught soil, destruction of roads and buildings, exploration of crude oil from boring holes, etc.

 In a water-montmorillonite system there exists free water which fills pores and great spaces between the grains and the bounded water. We know at present that bounded water has other properties than free water [2]. The layers of bounded water consists of interpacket water and water adsorbed on the surface of montmorillonite [3]. The structure and other properties of adsorbed water on the surface are not established precisely so far. Water adsorption on soil surfaces has been investigated by various methods, for example gravimetrical (McBain balance) and dynamic (gas chromatography) techniques. We cannot, however, obtain with the help of these methods complete information about the properties of the water layers and the role which its play in surface wettability.

For that reason, special thechnique of thermal analysis in quasi-isothermal conditions has been adopted to study the water-solid systems [4].The effects taking place in the processes opposite the adsorption, i.e. in the programmed thermodesorption of water from solid surfaces were registred. The

montmorillonite-Na and montmorillonite-La from Bahia Blanca (Argentina) were wetted with water vapours in vacuum dessicator, where p/po = 1.

Fig.1. The mass loss Q-TG and differential Q-DTG curves of water thermodesorption from montmorillonite-Na (left side) and montmorillonite-La (right side) samples

Fig. 2. Pore-size ditribution functions of the mintmorillonite-Na (left side) and montmorillonite-La (right side) samples

The thickness of adsorbed liquid layers on the surface can be controlled by the immersion mode of solid sample. The immersing of sample with water vapours in a vacuum dessicator blocks all adsorption active centres, the surface and capillary forces of the studied montmirillonite samples. The studies were made using simultaneous derivatograph Q-1500 D (MOM, Budapest, Hungary). The water thermodesorption measurements were carried out in quasi-isothermal conditions in a temperature range of 20 – 200° C with a furnace-heating rate of 6° C/min. The Q-TG and Q-DTG curves (Fig. 1) were registered digitally under the control of the program Derivat running on PC. Specific surface area, pore size and total pore volume as a function of pore radius (Fig.2) of the montmorillonite samples were calculated from the low-temperature nitrogen adsorption-desorption isotherms measured by means of the sorptomat apparatus type ASAP 2405 V1.01 (Micrometrics Inc., USA).

It can be stated that it is possible to obtain from one single and quick experiment the important information concerning surface capacity (thickness and volume of water films) and wetting phenomena, nature of active centres, discontinuous change of adsorption water layer properties (piks on Q-DTG curves in Fig. 1), mechanism and surface film destruction, kinetic thermodesorption of water films and its stability. Moreover, it is possible to study the effect of surface heterogeneity by ions modification of montmorillonite surfaces on its wettability and formation of water films. Using experimental Q-TG technique the adsorption and porosity (e.g. pore volume) parameters of the tested materials were studied, compared with analogous ones obtained by classical sorptometry technique and good correlation have been obtained.

References

1. S. Lee-Swartzen Allen, E. Matijevic, Chemical Reviews, 1974, 74, 3.

2. P. Staszczuk, Colloids and Surfaces A:, 1995, 94, 213.

3. L. Stoch, Minerały ilaste, 1974, Wyd. Geologiczne, Warszawa.

4. P. Staszczuk, J. Thermal Anal., 1998, 53, 597.



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