Determination of the Specific Surface Area of Solids by Means of Adsorption Data Rossella Brina and Achille De Battisti The University, via L. Borsari 46. 1-44100 Ferrara, Italy Dye adsorption data haw been largely used for the determination of iperific areas ( I , 21. This method Ct,\'Qrsa wide nmge of valucs. from as luw as 0.01 m' g, and is simple and versarile. although its applicaldity for ahsolute et,aluatiuns has 13). - been ~ ~ uuestimed ~ . . Cenerallv. ". difficulties. like micelle formationAinthe bulk of the solutions and a t the surfaces, somewhat hinder the straiehtforward use of ex~erimental " adsorption data (4). The average number of dye ions per micelle. known as the averaee aezreeation number, n,rises -- with the cube of the ionic wGght of the dye and is apparently, for a wide number of solids, independent of the nature of the surface as well as of the dye-ion charge, provided no specific bonds are formed between thelatter and the surface. We describe here an experiment that brings to the student'sattention the basic problems connected with the measurement of soecific surface areas of solids. A brief analvsis of the adsorition isotherms of dyes a t the solid/aque&s solution interface is also .. eiven.. toeether with the flow dia.. gram of a program that alluwi a quick test of the adsorption isotherm and the obtainmt.nt ofthe fundnmentdl adsorution parameters. ~
Some Properties of Ionic Dyes that Have Been Used for Surface Area Determination (from ref I ) Area covered per molecule flat
Aggregation
Dye
(A2]
numbers
Methylene Blue Crystal
120
2.0
6.1
661
225
3.6
9.3
589
~
Determlnatlon of the Type of Adsorption Isotherm Two different supports have been studied in the present work: Pyrex microheads (80 mesh) and Tan06 powder (Fluka puriss.). Some properties of Methylene Blue and Crystal Violet, the cationic dyes tested in our case, are reported in the table. A more complete list of cationic and anionic dyes suitahle for surface area determination. is available in reference I. Sets of 10-20 different adsorbate concentrations have been used. Five milliliters of each aqueous solution of the dye were added to 0.05 to 0.5 g of the chosen support, depending on the expected surface area. The samples were gently tumbled for 30 min in order to reach the equilibrium conditions for the adsorption process. The solution were tested spec1n)~~hotometricnlly before and after the equilibration.Typical adsorption isotherms are reported in F~gure 1. Provided the solurions used are in the ranve ot'vnliditv of ~ e dlaw, s the amount M of dye adsorbed isgiven by
M=-.A, f
(mollg, &'
Molar ext. coeff. (XIO")
h (nml
havic,r could he of the k'rutnkin or uf the I.nngmuir type 16. 71, depending OII whether iinite lnteral interactions exist among a d s d a t e molerulvi. .Acn)rdinply,adopting thi>simplifying asbumptim, the izorherm test can he rertricted to t l ~ enwntionrd posiililitics. The. general expression of the Frumkin isotherm is
where 0 = M/Mad, !M representing the amount of dye adsorbed at the equ~l~brium when the molar fraction in solution is x ) and 0 = exp -[AG,,,/RT] (AGad,representing the standard free energy of adsorption). The term a is the lateral interaction parameter. Equation 2 can be written in terms of molar concentration c instead of molar fraction x , and, if the adsorbate concentration in solution is small, eq 2 becomes, in logarithmic form, Plotting the first member against 0, a straight line is obtained if the Frumkin isotherm is obeyed. From the slope, the lateral interaction parameter can be easily obtained. From the intercept with the ordinate, 0 is obtained. In particular, Methylene Blue, for which detailed calculations
(1)
where A,, and Al indicate the initial and final absorbance of the dve solution, res~ectivelv,r is the molar extinction coef;epresents the volume of soluficient of the soiutioi, and ; tion (in liters) equilibrated with the weight g of the solid (in grams). The abscissa in Figure 1 is All€. The maximum amount adsorbable for the formation of the first monolayer has been obtained by plotting 11M against l l c and extrapolating the linear part of the plots to i l c = 0 (5).The maximum amount adsorbed could also be deduced from the M values at the plateau of the experimental isotherm. The extra~olationnrocedure, however. minimizes the vossible errors due to <ilayer formation prior to the attainment of the first monolaver. Once the maximum amount adsorbed. M,,., has been obtained, 0 values can in turn be evaluated. The available literature on organic adsorption suggests that forsul~stanrcsofthe type dealr with it) th present w r k , the adsorption tsothrrnls that better describe the interfacial be-
Figure 1. Adsorption isotherms for Methylane Blue (water solutions)on dilferen1 suppons. A: Pyrex microbeads (100 mesh): 0 :Ta,06 powder.
Volume 64
Number 2
February 1987
175
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