SURFACE CHARGEDEVELOPMENT ON POROUS SILICA Integration of eq 22 can now be carried out for that particular case with no difficulty, and results similar to the previous ones are obtained. For instance, the drying time is still given qualitatively by eq 25, replacing xop2by R/2. This again shows that the drying time for z(0) greater than 10 p is of the order of a few hours. The drying of lyophilized food product^^^*^ also takes place in a few hours and may well correspond to the case discussed here. On the contrary, the drying of nonlyophilized products is much s l o ~ e rand ~ ~ the ,~ controlling factor is clearly the very slow transport of liquid water from the core to the surface of the sample.
2547 I n conclusion, the present study justifies Philip’s theory of condensation in sharp wedges initially dry. The theory is then generalized to condensation and evaporation in any wedge under arbitrary initial conditions. The results are of importance whenever moisture movement in a porous medium occurs in the vapor phase.
AcknowEedgment. The author is grateful to Dr. J. R. Philip for many helpful discussions. (9) B.Makower and E. Nielsen, A w l . Chem., 20, 856 (1948).
Surface Charge Development on Porous Silica in Aqueous Solution by R. P. Abendroth Corporate Research Laboratories, Owens-Illinois Technical Center, Toledo, Ohw 43661 (Received March 22, 1972) Publication costs assisted by Owens-Illinois, Incorporated
Charge densities on three porous silicas of varying pore parameters suspended in aqueous solution were determined by potentiometric titration. Comparison with results obtained on nonporous silica showed that the porous silica charge densities were less, and the deviations were a function of pore size, electrolyte ionic strength, and pH. The extent of diminution of the porous silica charge densities could be qualitatively accounted for by consideration of diffuse layer interactions in the pores. In this study, surface charge density determinations were made on porous silicas of varying pore sizes to determine whether the presence of porosity significantly alters the behavior of these silicas in aqueous solutions. Comparison with results obtained in a previous study’ on nonporous silica (Cab-0-Sil) showed the porous silica surface charge densities to be less. The extent of the deviations appeared to be a function of pore size, electrolyte ionic strength, and pH. Lyklema2 has described a porous surface that can account for charge development far in excess3of that to be expected strictly from the density of surface groups. Owing to porosity, potential determining ions and counterions are allowed to penetrate past the surface into the solid, excluding the solution phase. Ions associated with the diffuse double layer play little part in compensating for the total charge, their effects being limited to the fraction of charge located on the surface. The results of this study show that these porous silica charge densities are less than those for nonporous silica, and Lyklema’s model apparently is not applicable here. Instead, the porosity of the silicas studied here is considered to be an extension of the external surface into
the interior of the solid. Charged groups are located only on these surfaces and are compensated by ions in the solution phase that enters the pore structure. The experimental results will be interpreted in terms of diffuse double layer interactions in the pore structure. Konporous silica surface charge densities will be used as a reference level to correlate the relative diminution of observed porous charge densities. It has been shown‘ that widely different methods of preparation do not lead to significantly large differences in nonporous charge densities; precipitated and pyrogenic silicas have similar charge densities in simple electrolytes. It is assumed then that the method of silica preparation will not significantly affect the conclusions reached in this work. It will be shown that the experimentally observed reductions in charge density can be qualitatively accounted for by using theory developed for interactions of diffuse double layers originating at plane surfaces. (1) R. P. Abendroth, J . Colloid Interface Sei., 34, 591 (1970). (2) J. Lyklema, J . Electroanal. Chem., 18, 341 (1968). (3) Th. F. Tadros and J. Lyklema, ibid., 17, 267 (1968). The Journal of Phusical Chemistry, Vol. 76, No. 18, 1972
2548
R. P. ABENDROTH
Experimental Section The porous silicas studied were formed4 by addition of a solution of sulfuric acid and isopropyl alcohol to an aqueous solution of sodium silicate. Addition of sodium chloride causes liquid phase separation, and the precipitated porous silica particles are removed, washed, and dried at about 300". The three porous silicas used were washed in distilled, deionized water until the efluent pH became constant and then were dried at 120". The properties, reported in Table I, did not change as a result of this treatment. The pore parameters were estimated from the hysteresis portion of a gas adsorption-desorption plot using Ar. Potentiometric titration was used to determine charge densities, and the experimental details are as previously rep0rted.l The usual silica concentration was 2.0 g/400 ml of electrolyte. Results were found to be independent of the amount of silica present. Thc silica was introduced approximately 18 hr before titration commenced. Equilibrium, as evidenced by constant pH readings in a 30-min period, was established in 2 hr or less when approached via base additions, but required at least 24 hr from the acid side. No hysteresis in the equilibrium quantities adsorbed was lO-l, observed. The electrolytes used were and loo M KC1 for silicas A, B, and C, and additionally, lo-' M LiCl and CsCl for silica A. The sequence of counterion adsorbability on silica A was established as Cs+ > K + > Li+.
Table I A
Surface area, m2/g Pore volume, ml/g Pore radii, A Largest Sinallest Average Particle size,
B
C
400
450
861
0.55
0.55
0.55
>50
>50
27
10 21 75-150
12 36 75-150
9 11
