ELECTROLYTES IN HIGHSURFACE AREASYSTEMS
97
Electrolytes in High Surface Area Systems. I.
The Reaction between Lithium
Chloride-Acetone Solutions and Silica Gel, an Unusual Addition Reaction‘* by Russell Maatman,lb Abel Geertsema, Harold Verhage, Glenn Baas, and Michael Du Mez Department of Chemistry, Dordt College, Sioux Center, Iowa
(Received M a y 12, 1067)
A reaction between LiCl dissolved in acetone and silica gel is described. General methods for distinguishing between adsorption and exchange are given. Both cation and anion react reversibly without ion exchange. This is an unusual example of salt addition (as contrasted with exchange) to a noncrystalline surface. The amount of reaction is enhanced by the addition of small amounts of water, but with further addition there is less reaction. From Langmuir plots, the equilibrium constant for surface complex formation, K , was found to be 2-6 X lo2 1. mole-’. LiN03 and NaI react less extensively, with K 10 1. mole-’. The site concentration varied from 0.2-0.6 mmole g-’, depending upon the amount of water present. LiCl does not add to the gel surface when n-propyl, n-butyl, or isoamyl alcohol is the solvent. These observations are interpreted to mean that Li+, C1-, or both add to polar surface sites, which involve water added to the surface, that excess water (or alcohol) destroys surface sites by further adsorption, and that excess water stabilizes the ions in solution. Salt addition to the gel surface is similar to molecule addition. It was Shown that LiCl and LiN03 can be removed from acetone by silica gel powder in a chromatographic column.
-
Introduction We showed earlier that alkali metal nitrates dissolved in HzO, CH30H, or CzHsOH react a t the most only slightly with the surface of silica gel.2va An incidental observation in these studies was that LiN03 in acetone reacts extensively with the gel surface. Polar molecules, such as fatty acids, are known to react with surfaces which are more polar than the s01vent.~ Adamson states that if both the positive and negative ions of an electrolyte are adsorbed, “...the situation is essentially the same as for molecular adsorption.”6 Addition of ions to crystalline solids is well known; we decided to determine whether the reaction observed with silica gel, at most much less crystalline than the materials usually used for ion adsorption (e.g., BaS04), is analogous to molecular adsorption. The reaction of both cation and anion with porous oxide surfaces is not unexpected. The reaction may occur because there are present easily exchanged cationic and anionic groups on the surface (e.g., aqueous alkaline earth salt ions exchange with H + and OH- of the alumina imrface*); because the salt is slightly soluble (e.g., aqueous silver salts are adsorbed by silica-alumina, Fe203,and Fe2Oa-SiO27); or because a new phase forms (e.g., CuClz forms Cue(OH)&l on the surface of hydrated Al2O3S). There is not much information on silica gel ion reaction in nonaqueous systems, and the pattern is not
evident. For example, French and Howard observed, at the most, only slight absorption by silica gel of transition metal salts dissolved in dioxane or quinoline.9 Strazhesko and Y ankovskaya noted, using tracer methods, some cation uptake from CH30H, acetone, and dioxane by silica gel.Io (1) (a) Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also supported by Atomic Energy Commission Contract AT(ll-1)-1354, and by the National Science Foundation Program for High School Teachers. (b) Correspondence should be sent to this author. (2) D. L. Dugger, J. H. Stanton, B. N. Irby, B. L. LMcConnell, W. W. Cummings, and R. W. Maatman, J . Phys. Chem., 68, 757 (1964). (3) J. L. Daniel and R. W. Maatman, J . Miss. Acad. Sci., 9, 70 (1963). (4) (a) H. N. Holmes and J. B. McKelvey, J . Phys. Chem., 32, 1522 (1928); (b) Y. Morinaga and I. Nakamori, Kyushu Kogei Seni Daigaku Kogiegakubu Kenkyu Hokoku, 12, 59 (1962). (5) A. W. Adamson, “Physical Chemistry of Surfaces,” Interscience Publishers, New York, N. Y., 1960, p 590. (6) K. C. Williams, J. L. Daniel, W. J. Thomson, R. I. Kaplan, and R. W. Maatman, J . Phys. Chem., 69, 250 (1965). (7) (a) 0. I. Dmitrenko and A. A. Ryabinina, Konoid. Zh., 15, 29 (1953); (b) 0. I. Dmitrenko, Chemisatwn Socialistic Agr. (U.8 . 8 . R.), 9, No. 5, 46 (1940); (c) V. A. Kargin, P. S. Vasil’ev, and 0. I. Dmitrenko, Rum. J.Phys. Chem., 14, 1628 (1940). (8) H. Schafer and W. Neugebauer, Naturwissenschaften, 38, 561 (1951). (9) C. M. French and J. P. Howard, Trans. Faraday SOC.,52, 712 (1956). (10) D. N. Strazhesko and G. F. Yankovskaya, Ukr. Khim. Zh., 25, 471 (1959). Volume 72, Number 1 JanUaTy 1968
98
MAATMAN, GEERTSEMA, VERHAGE,BAAS,AND Du MEZ
To investigate the reaction of nonaqueous salt solutions with silica gel, we treated the gel under various conditions with LiNOa, LiC1, and NaI, dissolved in acetone. Most of the experiments were with LiC1. Acetone was the solvent of choice because of the initial observation and because there is no complicating reaction of acetone with the silica gel surface.” In a few additional experiments, various alcohols were used as solvent for comparison purposes, since alcohols do react with the gel surface.
where B’ is the exchange site and C is the substance released from the surface. (B’ decomposes into B” C.) The adsorption equilibrium constant is (neglecting the deviation of activity coefficients from unity)
Methods Determination of the Reaction Isotherm. When a solution is mixed with a porous solid, the total amount of solute in the pores of one gram of the solid at equilibrium, ytot, is given by CiV vtot
=
- cr(V - W P ) W
(1)
where V ml of solution of concentration ci is mixed with W g of solid of pore volume P (ml g-l), and where cf is the final concentration of the liquid external to the pores. To determine yr, the amount of reacted solute per gram, ytot is diminished by ctP. In the present investigation (except for one case where the assumption is not needed) yr >> ctP, and there is little error involved in neglecting the correction which takes into account the inability of the solute to “see” the whole pore volume.12 The reaction isotherm is a plot of yr us. cy. If there is a time-invariant shoulder in the reaction isotherm, the reaction is reversible.e Determination of the Nature of the Surface Reaction. A distinction between exchange and adsorption reactions can be made by examination of the shape of the reaction isotherm, since the two kinds of reaction do not lead to curves of the same shape. This is a difficult comparison to make, given the usual experimental error. There are other means of distinguishing between these two kinds of surface reaction. First, exchange can sometimes be detected by observing exchange products in solution. Inability to find such products is, however, no proof of absence of exchange, since it is not always certain which materials should be tested for; also, the amounts sought in systems with only slight reaction can be too small to detect. Even when a supposed exchange product is detected, one may actually find a secondary product. Second, a distinction between exchange and adsorption may be made by analyzing the reaction isotherm in detail. We can show this by examining the equations for the two types of reaction. For adsorption A+B=AB
I
+ B’ = AB” + C
The Journal of Physical Chemistry
K,
= [ABl/[A/Vl[Bl =
[ABI/[A/Vl(D - [AB]] (4) where [AB] and [B] are the amounts per unit weight of solid, in moles, of surface sites covered and uncovered, respectively, D is the number of moles of adsorption sites per unit weight of solid, and [A/”] is the equilibrium concentration of solute. The exchange equilibrium constant is Ke
=
[AB”] [C/Vl/ [A/Vl [B’l = [AB”l[C/Vl/JA/Vl{D‘ - [ A w l
(5)
The adsorption or exchange isotherm is a plot of [ABJ or [AB”] us. [A/V]. Equation 4 shows that such a plot for adsorption is independent of V . (For these equ% tions, it is assumed for simplicity that W = 1 g.) On the other hand, eq 5 shows that because of the [C/V] factor, even though species C is not detected, such a plot for exchange does depend upon V . Thus the reaction isotherm is unique for adsorption, but not for exchange, where there is a curve for every value of V . Observing a family of reaction isotherms, therefore, indicates exchange occurs, even if there are also nonexchange reactions occurring. Another way of using the reaction isotherm to distinguish between exchange and adsorption is to attempt the so-called Langmuir plot. Equation 4, for adsorption, can be rearranged to [ABl/[A/Vl
=
-Ka[ABl
+ KaD
(6)
A linear plot of [AB]/[A/V] us. [AB] thus indicates adsorption, providing a similar plot of the same data assuming exchange is not linear. In what follows it is shown that a similar plot in an exchange reaction is not linear. We first obtain an expression for [AB”] from eq 5 [AB”] = KeD’[A/VI/( [C/VI
+ K,IA/Vl]
(7)
If no more than a negligible amount of species C is introduced from other sources [AB”] = [C]
(8)
From eq 5 and 8 [C/Vl
=
+ +
-K,[A/V1/2 {Ke2[A/VI2 4Ke[A/Vl[D’/Vl\ ”*/2 (9)
From eq 7 and 9, the quantity [ C / V ]is eliminated and
(2)
where A is the solute, B the adsorption site, and AB the surface complex. For exchange
A
+
(3)
(11) G. Blyholder and W. V. Wyatt, J . Phys. Chem., 70, 1745 (1966). (12) B. L. McConnell, K. C. Williams, J. L. Daniel, J . H. Stanton, B. N. Irby, D. L. Dugger, and R. W. Maatman, J . Phys. Chem., 68, 2941 (1964).
ELECTROLYTES IN HIGHSURFACE AREASYSTEMS
1.5
I \
99 apparently in this category.6 If, however, the exchange is not coupled, it can be detected. For example, in the case cited nonequivalent exchange would lead to a pH change.
1300
c
b
100
0.5
0 0
0.2
0.6 [AB]/D or [AR”lID’. 0.4
0.8
0 1.0
Figure 1. Attempted Langmuir plots for adsorption and exchange, with amount adsorbed expressed as fraction of saturation value. (a) adsorption, with K , = 1 ; (b) exchange, with A:, = 1 and D‘/V = 0.01; (c) exchange, with either K , = 1 and D ‘ / V = 1 or K , = 10 and D r / V = 10; (d) exchange, with K , = 1 and D’/V = 100. See text.
plots of [AB”]/[A/V] us. [AB”] for varioul; values of the system parameters, K , and D’/V, can be constructed. I n Figure 1, it is shown that such plots are linear for adsorption and definitely nonlinear-easily seen even with normal experimental error-for widely varying values of the parameters in the exchange system. Equation 5 can be rearranged to show that a plot of { [A]/ [AB”] us. [A]/ [AB”lZ/, at constant volume, is linear in exchange systems. The analytical treatment of cases in which the exchanging ions are not of the same valence xs obviously much more complicated. Even with the use of these tests, there is at least one case in which exchange might be thought to be adsorption. Suppose both the cation and anion of the solute exchange for solvent cation and anion of the surface. The over-all effect is that of a “molecule” (“kome”) of salt adsorbing. For the salt TVI,X, mhP+(aq)
+ nXm-(aq) + mnH+(s) + rnnOH-(s)
JcmM”+(s)+ nXm-(s)
+ nznHsO
(10)
where ( 8 ) refers to species of the surface phase. Since in aqueous solutions the production of water will not be observed, reaction 10 will appear to be, if there is equivalent anion and cation exchange M,X,(aq)
+ mn(H+-OH-)(s)
R/I,X,(s)
(10’)
where there is “coupled” exchange and (H+-OH-) (s) denotes a double site which can accommodate both anion and cattion. Since reaction 10‘ is in the form of reaction 2, adsorption will appear to occur where there is actually exchange. The alumina-salt reaction is
Experimental Section Materials. Except where noted, the silica gel was Davison Code 40 silica gel, 6-12 mesh, washed with nitric acid as described elsewhere.Ia After washing, the gel was dried in an oven and calcined for 2 hr in air at 450’) except where other temperatures are specified. The temperature of calcining, surface area (BET, measured in the sorptometer described elsewhere14)in m2/g, and pore volume (amount of liquid acetone taken up by pores) in ml/g for the various gels are 2/50’, 480, 0.43; 450’, 445, 0.37; 6/50’) 401, 0.33. The gel in the chromatographic columns was Baker Reagent silica gel powder, used as received. Reagent acetone was used either as received, with -0.2 vol % H 2 0 (where vol % HzO is obtained by comparing the volume of water with the total solution volume), or after drying over Linde 3A molecular sieve. Except where noted, the dry acetone was used. Isoamyl, n-butyl, and n-propyl alcohols (Eastman White Label) were used as received. Other materials were of reagent grade. Procedure. I n the batch experiments, V/W varied from 1.25 to 5.83 ml/g; in most experiments 20 ml of solution was mixed with 12 g of gel. After an equilibration period of at least 2 days, aliquots were removed for analysis. All experiments were at room temperature, 22 f 3’. Except where noted, the acetone used was dried. I n preparing acetone-water-LiC1 solutions, it was noted that LiCl solubility passes through a minimum as the water content is varied. With 0.05 M LiCl the salt precipitated, as water was added, at 1.0 vol % H 2 0 and it redissolved at 4.7 vol % H2O; the effect was reversible. Precipitation was not observed with 0.01 M LiCl. I n the column experiments, 10 g of silica gel powder was placed on a glass-wool plug in a 100-ml buret of 0.58 cm2 cross section. After the column was just covered with acetone, 50 ml of salt solution was passed through with a flow rate of 0.9 ml/min; liquid flow was continued by elution with acetone. Analytical. For the alkali metal salt solutions, 10-ml aliquots, from the initial and the equilibrated solution, were evaporated until free of liquid, either in the air or in a rotating vacuum evaporator. The residue, after being dissolved in 10 ml of water, was analyzed with the appropriate electrode and a calomel reference electrode (13) (a) S. Ahrland, I. Grenthe, and B. Noren, Acta Chem. Scand.,
14, 1059 (1960); (b) J. Stanton and R. W. Maatman, J . Colloid Sci.,
18, 132 (1963). (14) H. L. Lutrick, K. C. Williams, and R. W. Maatman, J . Chem. Educ., 41, 93 (1964).
Volume 76,Number 1
January 1968
MAATMAN, GEERTSEMA, VERHAGE, BAAS,AND Du MEZ
100 in the Beckman GS pH meter. With all LiCl solutions, chloride was determined using an Ag-AgC1 electrode; in LiN03 experiments, lithium was determined using a general cation electrode ; in NaI experiments, sodium was determined using a sodium ion electrode. Greater precision was obtained with the chloride electrode than with the cation electrodes, and therefore a careful comparison of Li+ and C1- of the same solution was not practical. Water in acetone and in the LiC1-acetone solutions was determined using a 10% ethofat-chromosorb T column in an Aerograph 90-P gas chromatograph. The presence of LiCl did not affect the analysis.
0.3 -
YlOt 9 mmol-q-1
THREE POINTS
01 0
0.001
Results
0,002
0 003
CfDM
Aqueous aluminum nitrate reacts with silica gel by ion exchange.2 To show that varying V / W in exchange systems does indeed lead to a family of reaction isotherms, the results for aqueous Al(N03)3-silica gel are given in Table I. Isotherm dependence upon V / W is seen even though ion exclusion is not separated out. The validity of this test is therefore assumed.
Figure 2. LiCl adsorption for various equilibration times: 0, 9 days; 8,15 days; 8,80 days. All three times at the one place so indicated. The initial solution contained 0.2% H10.
c
‘I-
0.6
/O/j/2.P’
Table I : Dependence of Aqueous Aluminum Nitrate-Silica Gel Reaction upon V / W a -----.Moles Cf, M
,
v/w,
of Al/g of gel, X 106 b---
v/w,
v/w,
Al(NOa)a
1 . 2 5 ml/g
2 . 5 0 ml/g
5 , O O ml/g
0.02 0.04 0.08
1.37 1.85 3.00
1.93 2.85 4.74
3.15 4.18 .
.
I
a Davison Silica gel of 491 ma/g, BET, used as received; 25-ml solution was used with contact for a minimum of 2 days. Total of both reacted and unreacted aluminum in pores per gram of gel; values reported are those rounded off from isotherm.
To investigate reversibility in the acetone-salt-gel systems, the effect on the isotherm of solution-silica gel contact time was studied. The low-concentration results are shown in Figure 2, where there are virtually identical isotherms for contact times of 9, 15, and 80 days. The existence of a shoulder proves reversibility.e The effect of the water of both the calcined gel and the initial solution is of interest. For anhydrous solutions, there are, in Figure 3, isotherms for LiCl reacting with gels calcined at various temperatures, thus varying both water content and surface area. I n Figure 4, the points and dashed curves are for similar experiments, except that the solvent used contained 0.2 vol yGHzO; for reference, the curves of Figure 3 are included. In Figure 5 , the effect of water in the initial solution is given for LiCl solutions of three different initial salt concentrations. The change in the mater content of the salt solutions (and also of solutions containing no salt) was measured, and the results are given in Table 11.
-
The Journal of Physical Chemistry
0 0
0.01
0.02
0.03
Cf, M
Figure 3. LiCl adsorption by gels calcined at various temperatures: 0, 250”; a,450’; e, 650’.
I n Figure 6, there is an isotherm comparison of LiC1, LiN03, and NaI, all in acetone containing -0.2
% HzO.
V O ~
Two experimental approaches to determine whether the reaction in acetone is exchange or addition were used. In Figure 7, the absence of an effect of varying V / W on the LiCl isotherm indicates the reaction involves the two ions in equivalent amounts. (See Methods.) Where equivalent reaction is the same as addition, the Langmuir plot should be linear. Linear plots are seen in Figure 8. A summary of the Langmuir constants (including those for curves not shown) is given in Table 111. There are also given, in Table 111, the ytot values (read from the isotherms) at cy = 2 X 10-4M, thus permitting a direct comparison between systems by the “initial slope’’ method.16 Proof that LiCl and LiN03 can be removed from acetone by the use of a silica gel adsorption column is (15) Reference 5 , p 579.
ELECTROLYTES IN HIQHSURFACE AREASYSTEMS
101
Table I1 : Silica Gel Adsorption of Water from LiC1-HzO-Acetone Solutions" ci, LiCl
Initial vol % Hz0
~ O . O O M - - - - Eq UC HsOb H20
0.5 1.0 2.0 5.0
M-
-0.01 Eq H*Ob
* * .
...
0.6 1.3 3.6
3.7 6.5 13
UC
Hz0
0.20 0.50 1.0 3.4
3.3 5.5 l1 18
t
0.04 M-
7-0.02MEQ HZOb
UC H20
Eq H20b
Hi0
0.20 0.40 1.0 3.3
3.3 6.7 11 19
0.10 0.40 0.90 3.1
4.4 6.7 12 21
r
UC
20-ml solution mixed with 10 or 12 g of gel calcined a t 450'. (Not shown: with no water in initial solution, none appears in ' H20 in equilibrium solution. equilibrium solution.) For the amount of salt reaction in these experiments, see Figure 5. Volume % In mmoles of HzO adsorbed per gram of gel, calculated on basis of difference between initial and final water concentrations.
Table 111: Salt-Gel Adsorption Constants
Salt LiNOs N a1 LiCl LiCl LiCl LiCl LiCl
Treatment," OC
Water, vol % b
450 4tiO 480 6!jO 2!jO 450 650
0.2 0.2 0.2 0.2 0 0 0
Site density
Urr
mmole
K,
g-lc
M-ld
0.28 0.19 0.50 0.36 0.62 0.48 0.23
12 11 610 290 290 260 370
mmole g-l
4
f f 6.0 X loW2 2.7 X lod2 8.0 X 4 . 6 X lo-* 1 . 4 x 10-2
Water in solvent a Temperature a t which gel was calcined. initially. Determined from Langmuir plot; & l o % estimated error. Equilibrium constant for salt adsorption, determined from Langmuir plot; & l o % estimated error. e Amount of salt adsorbed at C Y = 2 X 10-4 M. Too small to measure; see Figure 6.
'
given by the elution curves of Figure 9, The Figure also shows there is no salt adsorption when isoamyl, n-butyl, or n-propyl alcohol is the solvent.
Discussion The Water Efect. The solution and the surface compete for water, even when the surface is the source of the water. Increasing the calcination temperature decreases the amount of water in the g e P and, therefore, it also decreases the amount of water in the equilibrium system. I n Figures 3 and 4, it is shown that the amount of reaction also decreases with increased calcining temperature or addition of a small amount of water to the system. (The decrease in surface area as the calcining temperature is increased (see Experimental) is much smaller than the decrease in the amount of reaction.) With water added to the initial solution, there is a mziximum in the amount of reaction at -2 vol % HzO (Figure 4). (Adsorption by gels calcined above the standard temperature, 450°, would have to be shown in Figure 5 as a function of negative added water values.) The site densities, in Table 111,suggest that a factor contributing to the greater amount of reaction at the
Figure 4. LiCl adsorption by gels calcined a t various temperatures. Experimental points and solid curves for solvent with 0.2% HIO initially: 0, 250'; (D, 450'; 8,650'. Dashed curves for anhydrous solution, taken from Figure 3.
higher water levels (still in the low-water concentration range) is the increased number of sites. I n related experiments, it has been shown that the amount of CH,OH ~ a p o r orl ~methyl ~ ~ ~red in benzene solution17c which adsorbs on silica gel increases with increasing water content of the gel. Adsorption, Not Exchange. In the Results, it was shown that cation and anion react in equivalent amounts. Equivalent exchange of cation and anion cannot, however, be distinguished from adsorption (Le. , addition) when the exchange products are initially present in excess. (See Methods.) This occurs when H+ and OH- are the exchange products and water is the solvent. In the salt-acetone-silica gel experiments, (16) R. K. Iler, "The Colloid Chemistry of Silica and Silicates," Cornel1 University Press, Ithaca, N. Y., 1955,pp 236,237. (17) (a) L. G. Ganichenko, V. F. Kiselev, and K. G. Krasil'nikov, Dokl. Akad. Nauk SSSR, 125, 1277 (1959); (b) M. M. Egorov, T. S. Egorova, V. F. Kiselev, and K. G. Krasil'nikov, Vestn. Monk. Unw. Ser. Mat. Mekh., Astron., Fiz., Khim.,13, 203 (1958); (0) ref 16,p 238. Volume 78, Number 1 January 1068
MAATMAN, GEERTSEMA, VERHAQE, BAAS,AND DTJMEZ
102
0 0 4 M LiCl
0.06
t
0 C,, M.
Figure 7.
LiCl adsorption a t various values of V / W .
0, V / W = 1.67 ml g-'; 01 0
1
,
2
1
4
5
0, V / W = 3.33
ml g-1; 0, V / W = 5.83 ml g-1.
1
VOl% $0, Initial
Figure 5. LiCl adsorption as a function of initial water composition of the solution. V / W = 2.0 ml g-1; salt molarities of initial solutions indicated on curveg. For water uptake in same experiments, see Table 11.
0.3
02 ytot 1 mmol-g'l
01
Ct, M
Figure 8. Langmuir plots: 0, LiCI, 250' gel; 0, LiCl, 450" gel; &f, LiCl, 650' gel; A, LiNOa, 0.2 vol % HgO, 450' gel. 0
0
0 040
ooao Cf,
0,120
0460
M.
Figure 6. LiC1, LiNOa, and Ne1 adsorption; initial solution contained 0.2 vol % HzO.
the solvent cannot be produced by any combination of exchange products, and we deduce that the salt-surface reaction is a true addition reaction. This is an unusual example of salt addition (as contrasted with exchange) to a surface as noncrystalline as silica gel. There is some question, for example, as to whether the reported addition of silver salts to various oxide surfaces is actually an example of equivalent exchange.' Further Remarks. The distribution of LiCl between the gel and liquid phases depends upon the stabilities and concentrations of the species in the reaction (where (sol) refers to the solution) LiCl(so1)
+ site
LiClasite
a reaction formally similar to molecule-gel-liquid systems. Thus, Puri, et al., showed that fatty acid adsorption by silica gel from toluene solution increases as the moisture content of the gel increases, and that the The Journal of Physical Chemistry
adsorption is greater if the acid is more soluble in water.l* The chief complicating factor in the salt experiments is that changing the water content of the system alters the surface in a different way at low water concentration than at high concentration. The LiCl solubility experiment (with a minimum in solubility between 1.0 and 4.7 vol % HzO) probably indicates that varying the water content of the solution also changes the affinity of LiCl for the solution phase in a different way at low water concentration than a t high. Several additional observations are made. (1) The adsorption sites are polar and probably distinct. Discussion: If the Langmuir conditions are met, as they seem to be (Figure S), the sites are distinct. Since ions adsorb, the sites are polar. For the largest maximum in an adsorption isotherm (LiCl, 250" gel) the are 8 X lo1* sites cm-2, which are -35 A apart, if hey are distributed uniformly over the surface. For comparison, Iler16 shows that the silica gel surfac? contains ca. 80 X 10l8bound hydroxyls per om2 when saturated (18) B. R. Puri, R. K. Sud, and M. L.Lakhanpal, J . Indian Chem. Soc., 31, 612 (1954).
ELECTROLYTES IN HIGHSURFACE AREASYSTEMS
0.04 SOlt Cone,. M
t
P
0.02
LICI,
0
0
40
SO
120
140
Val. eluted, rnl,
Figure 9. Elution curves. Salt (with initial concentration) and solvent: curve at left is 0.05 M LiCl in n-butyl, n-propyl, and isoamyl alcohols; A, 0.1 M LiNOs in 0.2 vol % H20 in acetone; 0, 0.05 M LiCl in 0.2vol % Hz0 in acetone.
and 40-60 X 1018 bound hydroxyls per cm2 when calcined a t 300-600". Therefore, the reactive site of interest in the present work could be a certain fraction of the bound hydroxyls. Recently, Snyder and Ward discovered a new kind of reactive site on the gel surface;lQ while these need not be the same sites-as those upon which ionsadsorb, the fact of their recent discovery suggests there may be other sets of heretofore unknown sites. (2) The adsorption order is, with the strongest adsorption first, alcohol > salt ion($) > acetone. Discussion: When the solvent is alcohol, salt adsorption was not detected in the silica gel columns nor was reaction detected in the earlier work when C2H50H and CHaOH were the solvents.s If one salt ion is the primary adsoirbate, the ion of opposite charge also adsorbs to preaerve electrical neutrality. Simultaneous adsorption of cation and anion is much more likely, however. The dielectric constant of acetone is 20.7 (at 25") and salts of the type used are less than 30% dimociated in 0.1 M solution in acetone.20 Thus in the lowest concentrations used (-1 X M) the salts were largely dissociated; in the highest (-0.4 M), associated. Addition of ions, one at a time, is improbable in the associated high-concentration solutions. Since the low concentration and high concentration solutions behaved similarly, we suggest that simultaneous adsorption is probable over the whole concentration range studied. There is also appreciable association in some of the alcohol solutions; such association would not be a factor, since thereis no doubt that the polar alcohol molecules adsorb on the gel surface. (3) At equilibrium LiCl(so1) and the surface compete
103 for water. Discussion: The data of Table I indicate that some of the water reacts either with the surface or with the LiC1-site complex. The amount of water (on a molar basis) which adsorbs is from 5 to 35 times the maximum number of sites recorded in Table 11. Also, salt adsorption apparently enhances water adsorption. (4) Above -2 vol % H20, adsorption of salt diminishes because water affects both LiCl(so1) and the sites of reaction 1. Discussion: Li+ is a structure former in water, and the LiCl solutions are therefore stabilized by the addition of enough water. Even though LiCl adsorption brings water to the surface (Table I), appreciable energy to dehydrate Li+ partially is probably needed for adsorption to occur. Furthermore, addition of water to the surface hydrolyzes the Si-0-Si bond; in this concentration range, water may destroy sites by hydrolysis or addition onto previously existing sites. I n 100% water, salts of the type studied here do not detectably adsorb (Le., add), although there is slight ion exchange.2 ( 5 ) Even though in dilute solution the least soluble molecules tend to adsorb most,21the situation is more complex in the salt-acetonesilica gel system. Discussion : At 22" the solubilities of LiCl and NaI in acetone are, respectively, 0.62 and 0.16 m,22the same as the order of reactivities. To expect more adsorption with lower solubility is an oversimplification, since the individual stabilities of the salt crystals must be considered. This is more of a problem than it is with molecule deposition, since the energies of molecular crystals are much smaller than the energies of salt crystals. While we have not described the nature of the surface complex formed, we have described a reversible addition reaction of salt ions to the gel surface; we have also given evidence which suggests the same surface complex involves the anion, the cation, and some of the surface hydroxyls.
Acknowledgments. We acknowledge with appreciation the preliminary work of Rlr. John Daniel and the surface area measurements of Mr. Arlyn Schaap. (19) L. R. Snyder and J. W. Ward, J. Phys. Chem., 70, 3941 (1966). (20) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," Butterworth and Co., Ltd., London, 1959,p 550. (21) R.8.Hansen and R. P. Craig, J. Phys. Chem., 58, 211 (1954). (22) See 3. 'W. Mellor "A Comprehensive Treatise on Inorganio and Theoretical Chemistry," Vol. 11, John Wiley and Sons, Inc., New York, N.Y., pp 543,600.
Volume 72,Number 1 January 1968
