Page 150 - Adsorbents fundamentals and applications

THE SILANOL NUMBER (OH/NM −1 ) 135

Hydrogen bonding is the most important type of bonding for adsorption of
water as well as other compounds. For water to adsorb on the first layer of silanol,
it has been shown that water sits “oxygen down” on the SiOH group (Klier and
Zettlemoyer, 1977). As more water molecules adsorb, hydrogen-bonded clusters
form. The heats of adsorption are (Iler, 1979; Zhuravlev, 1993) the following:

First Adsorbed Layer:

H =−6.0kcal/mol
Si − OH + H 2 O ⇒ Si − OH:OH 2
Higher Layers:

Si − OH:OH 2 + xH 2 O ⇒ Si − OH:OH 2 (OH 2 ) H =−10.5kcal/mol

The value of 10.5 kcal/mol is very close to the latent heat of condensation of
water (10.8 kcal/mol).
Many techniques have been used to identify and quantify the different silanol
groups. The most extensively used is infrared spectroscopy (IR). IR spectroscopy
of the adsorbed water and hydroxyls (as well as other compounds) has been
discussed extensively by Hair (1967) and by Kiselev and Lygin (1975). The
most useful absorption band is the OH stretching frequency. In addition to the
surface OH groups, there also exist internal OH groups in the silica structure.
The characteristic OH stretching vibrational frequencies for the different groups
are (Iler, 1979; Hair, 1967; Kiselev and Lygin, 1975) as follows:

Type of OH Approximate IR
Frequency, cm −1

Isolated OH 3750
Paired or H-Bonded on Surface 3540
Internal OH, H-Bonded 3650
Liquid Water 3400

The stretching frequency band for the isolated OH groups is a very sharp one.
All other bands are broad. This sharp band appears only after dehydration, for
◦
example, >200–400 C in vacuo.

−1
6.3. THE SILANOL NUMBER (OH/NM )
The density of the OH groups on the surface is referred to as the silanol number,
α OH , expressed in OH groups per square nanometer. The silanol number can be
determined by deuterium exchange (Kiselev and Lygin, 1975; Zhuravlev, 1993) or