J. Phys. Chem. 1981, 85, 2706
2706
COMMENTS Long-Range Structurlng of Water by Quartz and Glass Surfaces As Indicated by the Infrared Continuum and Dlffuslon Coefficient of the Excess Proton. A Reevaluatlon of the Thlckness of the Structured Layer from BET Surface Area Measurements
Sir: In a previous publication1 the minimum thickness of the structured layer of water (and 1M HC1) at quartz and glass surfaces was estimated from the depression of the infrared continuum and diffusion coefficient of the excess proton. The surface area of the porous glass and quartz in contact with the liquid was estimated from the manufacturer's (Heraeus) stated pore size range (-4-15 pm), using a mean pore size of 10 pm for the calculation. Electron microscopy of the porous quartz appeared to support this assumption.' However, the specific surface area of the porous quartz determined by the BET method was very much larger than that assumed by using a mean pore size of 10 pm. The original porous quartz disks from which the thin, 100-130 pm, disks were cut for the infrared measurements had a diameter of 30 mm and a thickness of 4 mm. This disk was used for the surface area measurements. The disk was outgassed overnight at room temperature under a nitrogen atmosphere. The surface area of the porous quartz disk was measured by the BET method using a molecular area for nitrogen of 16.2 A2. The specific surface area, the mean of four measurements, was 6.20 m2 g-l. The experimental specific surface area of the porous quartz used in the infrared measurements, 6.20 m2g-l, may be compared with that calculated in the previous paper1 in which an average pore size of 10 pm was assumed. Since the volume of water adsorbed by the porous quartz equals the volume of the pores we have ml
V = - = Nnr21 P
(1)
where V is the volume of water absorbed = volume of pores, r the mean radius of pores, 1 the mean length of pores, ml the mass of water absorbed, p the density of water, and N the number of pores, assuming that the pores in the porous quartz are cylindrical channels. Since the total internal surface area of the channels S = N(2nrl)
(2)
then (3)
Therefore, the specific surface area per gram of porous quartz is
s
- = - -2- m 1 1 m2 r P m2
(4)
where m2 is the mass of porous quartz containing a mass (1)N. K. Roberts and G . Zundel, J. Phys. Chem., 84, 3655 (1980). 0022-365418112085-2706$01.25/0
ml of water. Experimentally ml/mz = 0.2076 and p = lo6 ~ m - ~ . In the original paper1 an average pore size of 10 pm was assumed, Le., the specific surface area from eq 4 was 0.08304 m2 g-l, assuming r = 5 X lo* m compared with the experimentally determined BET value of 6.20 m2 g-l-a large discrepancy! If the experimental specific surface area of 6.20 m2 g-l is inserted in eq 4, r = 0.0670 pm (compared with r = 5 pm assumed previously). Obviously there are a large number of very fine channels in the porous quartz which are not apparent in the electron micrographs (X1600). Infrared Continuum of the Excess Proton. In the original paper a porous quartz disk containing an effective thickness of 10 km of 1 M HCl was considered. A 14% depression of the infrared continuum means that at least 1.4 pm of 1 M HC1 does not contribute to the continuum. If we assume that the absorbed 1M HC1 was distributed in channels of mean diameter 0.134 pm, and not 10 pm as previously assumed, then the total surface area of quartz in contact with the 1 M HC1 can be calculated from eq 3
x 1 x 10-5 s = -2r v = 20.067 = 298.5 m2 x for a film of 1M HCl of 1-m2surface area and 10 pm thick, compared with 4 m2 reported previously for 10-pm channels. Therefore the minimum thickness of structured 1 M HC1 at a quartz surface is 1.41298.5 = 0.0047 pm or 47 A, compared with 0.35 pm calculated in the original paper. Tracer DiffusionCoefficientof the Excess Proton. In this case it was noted that the effective cross-sectional area of the pore was decreased owing to the structuring of the liquid by the glass surface. If the mean channel radius is the same as that for the porous quartz of the same pore size range, i.e., r = 0.067 pm, then the mean cross-sectional area of a pore is ar2,or 0.0141 pm2. A 13% reduction in the limiting diffusion coefficient of the hydrogen ion can be ascribed to a 13% reduction in the effective cross-sectional area of the mean pore size, i.e., the effective crosssectional area available for the Grotthus diffusion of the hydrogen ion is decreased from 0.0141 to 0.0123 pm2or the mean effective radius is 0.0625 pm. The minimum thickness of the structured liquid is, therefore, (0.0670 - 0.0625) pm = 0.00448 pm or 45 A compared with the previously reported value of 0.35 pm. It should be noted that the minimum thickness of the structured layer depends on the shape assumed for the pores.
Acknowledgment. I am very grateful to Professor T. Healy? University of Melbourne, for stressing the need for accurate surface area measurements of the porous quartz and glass, and for carrying out the BET measurements on the porous quartz. (2) T. Healy, private communication. Chemistry D8partm8nt University of Tasmania Hobart, Tasmania, Australia 700 7
Noel K. Roberto
Received: March 24, 198 1; In Final Form: Aprll 15, 198 1
0 1981 Amerlcan Chemical Society
