Dielectric properties of the adsorbate at low surface coverages

Abstract

The main purpose of the work initiated in this paper is the investigation of the dielectric properties of adsorbed vapours and gases at low surface coverages where no adsorbate-adsorbate interaction will hinder interpretation of the results. Changes in capacity were measured with an accuracy of 2 x 10 -4 μμ F by means of a high-precision capacitance meter of high stability. Ammonia adsorbed on a plate of porous Vycor glass, 1 mm thick, was studied and changes of capacity, ∆ C , as a function of the amount adsorbed, V , were measured down to a surface coverage of Ɵ = 0.005. Even at low coverages, hysteresis not only in the adsorption isotherm but also in the curves of ∆ C against V was found. This phenomenon confirms previous results which show the existence of different types of adsorption sites on Vycor glass differing in their energy. Time effects in ∆ C on admission of NH 3 indicate a redistribution of the adsorbate molecules before final equilibrium is reached. The structure of the adsorbent in which the pores and capillaries have an average size of about 15 to 20 Å allows the adsorbent-adsorbate system to be treated as a solution. With Onsager’s equation, molar polarizations P m and molar differential polarizations P diff. of the adsorbate as a function of V were calculated. The curves of P diff. against V show maxima at Ɵ ≈ 0.15 at P diff. of about 150— much higher than for gaseous or liquid NH 3 . P diff. rises with temperature at small surface coverage ( Ɵ ≈ 0.10), whereas at higher coverages P diff. decreases with temperature. This is explained by restriction of rotation of the NH 3 adsorbed on high-energy sites and the increase in their freedom of rotation with temperature. Exchange of surface OH groups by OCH 3 changes the curves of P dilf. against V in such a way as to confirm the assumption that high values of P diff. originate in NH 3 adsorbed on OH by hydrogen bonding. From the maximum P diff. the dipole moment of adsorbed NH 3 is found as 2.42D as compared to 1.46D for gaseous NH 3 . This corresponds to a shift of the centroid of negative charge of the lone pair of 25 %.