tion detector approaches that of the flame ionization detector. Unlike the flame ionization detector, however, this combination is a catholic one and applicable to analyses of permanent gases. There has in the planetary exploration program long been a need for a sensitive and reliable method of analysis for atmospheric gases. The combination of a transmodulator and reliable catholic detector appears to fulfill this need and, in addition, provides the unexpected bonus of automatically
resolving the analyses of argon and oxygen which otherwise are difficult to separate. RECEIVED for review March 5 , 1969. Accepted May 5 , 1969. This paper presents the results of one phase of research carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract NAS 7-100, sponsored by the National Aeronautics and Space Administration.
Variables in the Preparation of Modified Silica Adsorbents R. E. Majors' and L. B . Rogers Chemistry Department, Purdue University, Lafayette, Ind. 47907
Variables in a procedure for modifying the selectivity and capacity of silica, by precipitating silicic acid in the presence of the intended adsorbate, have been studied so as to optimize those qualities and improve the reproducibility. ' The chief factor affecting the xerogel selectivity for methyl orange relative to ethyl orange was the structure of the dye present during gelation. Capacity was increased by slow gelation rates, longer aging or drying times, and the use of pure methanol instead of acidified methanol or water for washing the gel. The amount of dye that remained in the xerogels after exhaustive extraction had no effect on the capacity or selectivity of the adsorbents. Repeated attempts to prepare stereoselective silica adsorbents for enantiomers, reported in earlier literature, resulted in failure.
THE PRODUCTION of adsorbents with increased selectivity allows the chromatographic separation of compounds in a much shorter time. More selective adsorbents, when combined with high-pressure liquid chromatography ( I ) , should permit the speed of liquid-solid chromatography to approach that of gas chromatography. In 1931, Polyakov ( 2 ) demonstrated that molecular specificity could be tailored into silica gel by the pretreatment of silicic acid with organic adsorbates (tailoring agents) prior to polycondensation. Later, more extensive investigations were carried out by Dickey (3, 4), who showed that the natural selectivity of silica gel for one dye in a series of azo dyes could be reversed by pretreatment. The current status of tailored adsorbents has been reviewed recently by Snyder (5). Use of specific adsorbents for practical separations and the development of a suitable theory of the mechanism of increased adsorbent selectivity has been plagued by the inability to successfully reproduce these adsorbent systems. Gels prepared under supposedly identical conditions have given adsorbents with vastly different adsorption properties. For instance, methyl orange (MO) tailored gel, prepared by the hydrochloric acid technique (4), exhibited concentration ratios 1 Present address, Celanese Research Co., P.O. Box 1000, Summit, N. J. 07901
(1) T. W. Smuts, F. A. Niekerk, and V. Pretorius, J. Gus Chromarogr., 5 , 190 (1967). (2) M. W. Pclyakov, Zh. Fiz. Khim., 2,799 (1931). (3) F. H. Dickey, Proc. Nut. Acad. Sci. U . S., 35, 227 (1949). (4) F. H. Dickey, J. Phys. Chem., 59, 695 (1955).
(5) L. R. Snyder, "Principles of Adsorption Chromatography," Marcel Dekker, Inc., New York, N. Y., 1968, p 177. 1052
ANALYTICAL CHEMISTRY
for MO that ranged from 3.6 (6) to 23.0 (7). Procedures, while usually performed consistently within one laboratory, have not been the same from one laboratory to another so that quantitative comparisons have been very difficult. The present paper deals with variables in the preparation and characterization of tailored adsorbents using azo dyes as tailoring agents. Critical variables were the rates of gelation, aging times, drying methods, and washing solvents. Tailored and control gels have been examined with regard to optimizing reproducibility, capacity, and/or selectivity. In addition, rates of adsorption and preparation of stereoselective adsorbents have been studied. EXPERIMENTAL Reagents. Methyl orange (MO), available as the sodium salt from Mallinckrodt, and ethyl orange (EO), Eastman Kodak Co., were recrystallized twice from hot distilled water. N,N-Dimethyl-p-phenylazoaniline(DMO), Eastman Kodak (DEO), supplied by Co., and N,N-diethyl-p-phenylazoaniline Du Pont as Oil Yellow NB Powder, were recrystallized twice from 95% ethanol. They were further purified by elution with toluene from a 2.0-cm X 20-cm column of Davison Code 950 silica gel followed by a final recrystallization from 95% ethanol. The melting point for DMO was 118-119' C (uncorrected) [reference (8) 117-118°C] and for D E 0 it was 97°C (uncorrected) [reference (9) 97-98' C]. Methyl red, National Aniline Div., Allied Chemical Co., was recrystallized from 1 :I ethanol-water, mp 181-183 "C (uncorrected) [reference (IO) 183" Cl. Stock solutions of 60.0 p M MO and EO were prepared in 5 % (v/v) acetic acid-water. Sodium silicate was obtained from Fisher Scientific Co., 40-42 "BC,Technical, di5 = 1.40. The NanO content was determined by a pH titration of the silicate with standardized HC1 and found to range from 1.39-1.47 moles per kilogram for three different batches. The SiOzcontent was determined from the HC1-insoluble residue after ignition to 1100 "C. The methanol used for extraction was 99.85%. All other chemicals were reagent grade and were used as received. Apparatus. Most spectrophotometric measurements were made with a Beckman Model DU. A Rudolph Photoelectric (6) R. G. Haldeman and P. H. Emmett, J. Phys. Chem., 59, 1034 (1955). (7) G. H. Reed and L. B. Rogers, ANAL.CHEM., 37,861 (1965). (8) R. Mohlau, Ber., 17, 1490 (1884). (9) E. Sawicki and F. E. Ray, J. Org. Chem., 19, 1686 (1954). (10) W. Utermark and W. Schicke, "Melting Point Table of Or-
ganic Compounds," 2nd ed., John Wiley and Sons, Inc., New York, N. Y., 1963, Lfd. Nr. 2455.
Polarimeter was used for the polarimetric measurements. Procedures, GELPREPARATION. Procedures for gel preparation and isotherm evaluation were similar to those in the hydrochloric acid method of Dickey (4). Detailed procedures can be found elsewhere (11); only the modifications relating to the control of critical experimental parameters will be given below. After gelation, the gels were aged exactly 10.0 days. Then, they were dried in a vacuum desiccator at a pressure of 2 mm. The drying time for four gels (from 168 grams of sodium silicate) was normally one week. After drying and sieving, the 60/200 mesh particles were thoroughly extracted with methanol in a chromatographic column. When one portion of the effluent, after acidification by a drop of HCl, did not absorb at 510 nm and another portion showed no chloride upon the addition of 0.1M silver nitrate, the washing process was stopped. Control gels were prepared in the same manner as tailored gels but in the absence of dye. In the study of some variables, however, the standard procedure was somewhat modified for special gel preparations. In the aging study, in order to minimize variations from one preparation to another, a 14-fold batch of the silica was prepared. A six-fold batch was used as a control while an eightfold portion was tailored. DMO was used because its tailoring ability had been found to be similar to that for MO, but the excess dye was much easier to extract (7). To study the effect of aging, smaller portions of gels were tightly covered for specified time intervals before being carried through the normal procedure of desiccation. To study the effect of drying method, a seven-fold batch was used to prepare four DMO-tailored and three control gels. The gels were covered, allowed to age for two days, and a tailored and control gel each were subjected to one of three drying techniques. One pair and a duplicate tailored gel were allowed to remain undisturbed in the hood until dry. The exact drying time was difficult to estimate, but a gel was considered dry when it was brittle and could be crushed easily by a stirring rod. A second pair were rapidly dried by blowing a stream of dry, clean air over the surface of the hydrogels, which had been crushed with a stirring rod. The third pair was dried by the normal vacuum drying technique. The effect of fluoride on gelation was studied by adding sodium fluoride to the sodium silicate solution in sufficient amounts to yield solutions that were 0.005,0.010,0.025,0.050, 0.100, and 0.150M in fluoride. Then the normal amount of hydrochloric acid was added. Relative to fluoride-free preparations, hydrogel formation was quite rapid. The gels were aged for approximately one week, vacuum-dehydrated, and treated further by the normal procedure. To study the effect of slower gelation, gels were prepared in the presence of alcohols. Because sodium silicate solutions are somewhat insoluble in alcoholic media (12), the silicic acid sol was prepared first and alcohol was added until it was 27% by volume. When examining the effect of the extraction solvent, equal portions (15 grams) of several random samples of dried control and tailored gels were placed in 29 X 400 mm chromatographic columns and washed with about 20 liters of the appropriate solvent. Pure methanol, HC1-acidified methanol (0.02M and 0.20M), and distilled water were used as extractants. ISOTHERM MEASUREMENT. Isotherms were obtained by a method similar to Dickey ( 4 ) except that a constant weight of 0.400 gram was used for each equilibrium measurement. Five different concentrations were used to determine an isotherm. Equilibrium time at 25.0 =t0.5 "Cwas 2.0 hr. The results were calculated in terms of adsorption capacity :
-.
-
f MEOn
J 40
Time of Aging (days)
Figure 1. Effect of aging time of hydrogels on concentration ratios for adsorption of MO and EO from 5% acetic acid solution Equilibrium Concentration was 20 p M Upper curves-DMO-tailored gels Lower curves-control gels
Cp = adsorption capacity for adsorbate, y , on a gel, z ,
in pmol/kg.
Ci = initial dye concentration in solution in pmol/l. C, = equilibrium dye concentration in solution in pmol/l. V = volume of solution in ml. w = dry weight in grams of silica gel after correcting for the amount of water, determined on a separate sample by ignition at 1100 "C. Isotherms were plotted as adsorption capacity us. equilibrium concentration. A concentration ratio, Kzy = Cp/C,, was obtained at 20 pM from the isotherm. The selectivity value, cy,", for a particular dye y on adsorbent z is defined as the concentration ratio for y , K p , divided by the concentration ratio, K,., for another dye x . The two dyes used as adsorbates in this study were MO and EO and the selectivity was expressed in terms of MO. For instance, CYC = K c M O / K C E O represents the selectivity of a control gel for MO relative to EO at an equilibrium concentration of 20 pM. The error limits on K values given in the text represent average deviations of the points on the isotherms used to determine the adsorption capacity. When measuring the rates of adsorption, a slightly modified procedure was used. A 0.400-gram sample of gel was weighed into a glass-stoppered 15-ml centrifuge tube and mixed with 10 ml of dye solution in 5 % acetic acid. The tube was quickly stoppered and vigorously shaken by hand for the desired time interval, exactly one or two minutes. Then, the tube was centrifuged on a low-torque centrifuge for 5 sec to remove most of the gel. Before measuring the absorbance, the supernatant liquid was transferred to a second centrifuge tube and centrifuged for a longer period of time to remove all particles of gel. Absorbance of replicates could be reproduced to about f 5 %. DETETERMINATION OF UNEXTRACTABLE DYE. The method was similar to that of Dickey (4). SURFACEAREAAND PACKING DENSITYDETERMINATIONS. The nitrogen surface areas were measured using a flow apparatus similar to that of Nelson and Eggertsen (13). Bulk packing density determinations were obtained by placing incremental amounts of gel into a small narrow-necked
(1 1) R. E. Majors, Ph.D. Thesis, Purdue University, Lafayette,
Ind., 1968. (12) L. A. Munro and J . A. Pearce, Can.J. Res., 16B, 390 (1938).
(13) C. M. Nelson and F. T. Eggertsen, ANAL.CHEM., 30, 1387 (1958). VOL. 41, NO. 8,JULY 1969
1053
Table I. Bulk Properties of Gels Prepared with Different Aging Times Aging UnexPacking density, Nitrogen surface - gram/cm3 area,*m*/gram time, tractable days dyea DMO Control DMO Control 3.9 9.9 17.0 25.2 33.2
2.7 3.4 5.0 7.1 8.4
0.55 0.52 0.45 0.42 0.43 a Expressed in (mol/kg) x lo4. Corrected to a dry basis.
0.55 0.50 0.46 0.43 0.44
820
... ... ...
740
Table 11. Effect of Drying Method Drying Drying time, KcMO, KDMoMO, technique days ml/gram CYC ml/gram Vacuum 3 8 . 4 i 0 . 1 1 . 3 21.5 It 0 . 2 Airblowing 5 7 . 6 i 0 . 3 1 . 3 21.8 f 0 . 2 Slow 21 7 . 0 i 0 . 2 1 . 3 32.0 It 0 . 3 Slowa 21 ... . . . 34.0 zk 0 . 2 a Duplicate.
850
... *..
...
760
:I
~ D M O
1.55 1.48 1.55 1.51
0.10
Fluoride Concentration ,E vial of known volume and vigorously tapping on the table top until the gel did not fall below the mark in one minute of additional tapping. The value of packing density, expressed in grams/cm 3, could be reproduced to a relative standard deviation of about 1%. RESULTS
Effect of Aging Time. Figure 1 shows that there was a distinctly different effect of aging time for the tailored and control gels. For the control gels there was a slight decrease in adsorption capacity that was paralleled by a decrease in nitrogen surface area and packing densities, as reported in Table I. However, the selectivity for MO, relative to EO, was unchanged with time. For the DMO-tailored gels, there was a pronounced increase in adsorption capacity with aging time even after almost five weeks. At that time, there had been an 8001, increase in the concentration ratios compared to the ratios found after a few hours while the nitrogen surface areas decreased. There was little difference between the bulk packing densities for tailored and control gels at a given contact time. The decreases in areas and densities for both types of gels were in agreement with the observations of Neimark et at. (14, and Okkerse (15). One important observation was that the amount of dye, which remained in the silica matrix after methanol extraction, increased with the time of aging. The relative increases in unextractable dye were much greater than the increase in the concentration ratios for dye adsorption. The MO selectivities for the tailored gels remained unchanged with the time of aging.
(14) I. E. Neimark, R. V. Sheinfarn, N. S. Kruglikova, and 0. P. Stas, Kolloid Zh., 26, 595 (1964). (1 5) C. Okkerse, "Submicroporous and Macroporous Silica," Ph.D. Thesis, Delft, 1961; through ref. (14). 1054
ANALYTICAL CHEMISTRY
1
I
0.05
0,15
Figure 2. The effect of fluoride concentration on gel time
Effect of Drying Method. Table I1 shows that the drying method affected the concentration ratios but not the selectivities for MO compared to EO. For longer drying time, capacity increased for tailored gels but decreased for controls. The extracted, slowly-dried tailored gels were more highly colored than their rapidly-dried counterparts. This observation was in agreement with the aging time study where more unextractable dye was associated with longer aging m d , therefore, drying times. Effect of Extraction Solvent. There was no significant effect on the MO selectivity by any of the washing procedures. However, Table 111shows that extraction with either water or acidified methanol lowered the adsorption capacity for all of the tailored gels but had no effect on the control gels. Increasing the acid concentration had the effect of lowering the adsorption capacity still further. In a similar study, control and DEO-tailored xerogels were steeped in 2.OM HCI for periods of time up to a week. There was no effect on capacity or selectivity for the control gel, but for the tailored gel the capacity decreased by 60% after one week with little change in selectivity. Contrary to the work of Morrison et at. (16), the present study indicated that more dye was removed by acidic solvents. Effect of Gelation Rate. The effect on gel time of the addition of large amounts of sodium fluoride to the preparation medium is illustrated in Figure 2. The curved line departs from the linear relationship between the logarithm of the gelation time and fluoride concentration found by Iler (17) at very low fluoride concentrations.
(16) J. L. Morrison, M. Worsley, D. R. Shaw, andG. W. Hodgson, Can. J. Chem., 37, 1986 (1959). (17) R. K. Iler, J. Phys. Chem., 56, 680 (1952).
Table 111. Effect of Extraction Solvent
Gel Control I Control I1 DMOa DE0 MO
EO
CHaOH KgelMO, ml/gram
agel
9.6 zk 0.1 6.3 f 0.1 39.8 =k 0 . 3 25.8 =k 0.2 32.4 f 0.2 34.6 i 0 . 3
1.2 1.2 1.42 0.67 1.70 0.56
KgelMO,
Extractant 0.02M HCl 0.20M HC1 ml/gram agei K g e l M O , ml/gram
9.7 f 0.2
1.2 ... 1.46
3 6 . 8 . i 0.3
... .,. ...
9.6 f 0.2
...
32.8 f 0.2
...
6.3 5 0.1 34.4 2z 0.2 22.0 f 0 . 3 25.2 3= 0 . 5 26.6 f 0 . 3
1.40
... ... ...
age1
...
1.2
...
... ... ...
Hz0 Kgeilfo,ml/gram
agei
...
...
...
1.2 1.45b 0.70* 1.70 0.58
6 The amount of unextractable dye for the DMO tailored gels was 855 pmol/Kg (CH30H), 811 pmol/Kg (0.02M HCI), and 768 pmol/Kg (0.20M HCI). * Because these dyes were quite insoluble in water, these samples were first extracted with water, followed by methanol until the dye was removed, and finally with water.
Table IV. Effect of Rapid Gelation
Control gels NaF Concn, M
KcMO,
0.005 0.010
0.025 0,050 0.100 0.150
ml/gram
ffC
9.9 f 0.2 8 . 0 =k 0.2 7.1 5 0.1 6.0 3z 0.2 5.1 f 0.3c 3.7 =k 0.3d
MO-tailored gels Density," gram/cm3
1.1
1.1 1.1 1.1 1.lc 1. O d
0.50 0.48 0.39 0.36 0.36 0.35
KlrohfO,
ml/gram
24.0 f 0.1 22.2 f 0.6* 21.0 i. 0 . 6 18.0 f 0 . 3 16.7 f O.lc 16.0 =k 0.3
cyuo
Density," gram/cm3
1.44 1.45b 1.42 1.48 1.5oc 1.41
0.50 0.47 0.41 0.38 0.36 0.36
Unextractable dye, (mol/kg)
x
104
1.6 1.9 3.3 4.1 5.5 5.5
Packing density. Estimated from data obtained in 2.OM HCl as a solvent. c Average of two gel preparations. d Extrapolated from isotherm.
a
b
Table V.
Alcohol present (Hz0) CHaOH CzHsOH n-C,H,OH i-C3H?OH
Effect of Slow Gelation
K$O, ml/gram
ffC
7.3 f 0.2 7.9 =t0 . 4 8.9 =k 0.4 12.1 f 0.5 10.9 f 0.4
1.1 1 .o
mligram
K~ohfO,
17.9 23.2 26.2 32.0 30.4
1.o
1.1
1.1
CYMO
1.5 1.4 1.4 1.4 1.4
f 0.5 f 0.2 2z
Unextractable dye, (molikg) X IO4
0.2
zt 0.1 d= 0.2
0.54 1.2 1.2 1.4 1.9
Table VI. Extents of Adsorption of MO and EO.
cgepo cpel~o, f Gel Control Controld MO EO EO (HOAC)~
pmol/kg pmol/kg 138 138 343 363 3 50
108 115
223 488 464
=
Per cent of equilibrium at time tb 1 min t = 2 min t = equil.
MO
EO
MO
EO
MO
EO
min
69 73 75 76 78
69 68 75 70 72
82 78 83 85 86
84
100
80
100 100
100 100 100 100
1.27 1.29 1.54 0.81 0.82
84 81 83
100 100
time fc t = 2 t = 3 min equil.
a110at
t = l
100
1.26 1.17 1.51
0.78 0.79
1.28 1.20 1.54 0.75 0.75
The gels were prepared using the conditions of Reed and Rogers (IO). This represents capacity at time t divided by capacity at equilibrium. Apparent selectivity because normal selectivities are expressed at a defined equilibrium concentration. These equilibrium and nonequilibrium values are given for capacities at time t . d Duplicate. e EO gel treated with 5 acetic acid prior to mixing, a
b
VOL. 41, NO. 8,JULY 1969
1055
Table VII. Preparation Reproducibilitya Gel DMO DE0 MRb Control
KgelMO,
KgelEO
ml/gram
ml/gram
27.9 f 1 . 9 17.9 f 1 . 3 21.9 ==! 0 . 9 28.9 f 1 . 2 19.5 f 1 . 5 13.8 f 1 . 1 8 . 5 f 0 . 5 6 . 7 =t0 . 4
agel
No. of replicates
1.56 f 0.03 0.762 f 0.003 1 . 4 4 f 0.01 1.26 f 0.03
5 3 3 6
Standard deviations given for replicate determinations. bunextractable dye contents of three gels were 135 pmollkg, 295 pmol/kg, and 425 pmol/kg. 5
For both tailored and control gels there was a steady decrease in the concentration ratios as the fluoride concentration-Le., rate of gelation-increased (Table IV). The nitrogen surface areas for the 0.005M and the 0.150M fluoride controls were found to be 680 m2/gram and 260 mZ/gram, respectively, so the loss in adsorption capacity was apparently related to a decrease in surface area. At the same time, there was a steady decrease in the packing densities. The MO selectivity did not change appreciably for either tailored or control gels. However, there was a larger amount of unextractable dye in the tailored gels formed at faster rates of gelation. When the gels were prepared in the presence of water, methanol, ethanol, n-propanol, and i-propanol, the gel times lengthened in accordance with the results of Munro and Pearce (12). The aqueous gel became firm within the normal 8-10 hours, the methanol and ethanol gels in about a day, and the propanol gels in a little over two days. Table V shows that for gels prepared in the presence of the normal alcohols, adsorption capacities increased with carbon numbers. However, there was little change in the selectivities for tailored or control gels. Effect of Adsorption Rate. Table VI shows that there was little effect of adsorption rate on the selectivity of any of the gels, whether dry or prewet. In no case was the selectivity of the control reversed. The chromatographic anomaly referred to by Reed and Rogers (7) would not be expected to occur on the basis of these data. In a similar experiment using 5% acetic acid, D E 0 had a slightly larger Rfvalue (0.12) than DMO (0.08) on a control gel by thin layer chromatography. This is in agreement with the equilibrium isotherms for those dyes in that solvent. Preparation Reproducibility. Gels were prepared from the same batch of sodium silicate but at different times. Table VI1 shows than when an aging time of 10.0 days, the vacuumdrying technique, and extraction with pure methanol were combined, DMO-, DEO-, methyl red- (MR) tailored gels, and control gels had relative standard deviations in the concentration ratios of about 7 x and in the selectivities of about 3%. The gels tailored with DMO and MR, which are structurally related to MO, had MO concentration ratios greater than EO concentration ratios and were, therefore, selective for MO. The selectivity difference between these two gels was large enough to show that DMO was more effective than MR. Also, the D E 0 gel was selective for EO just as when EO was used to tailor a gel (3, 4, 6, 7). A control gel was somewhat selective for MO (1.26), whereas Haldeman and Emmett (6) reported a value of 0.86, and Reed and Rogers (7), a value of 1.50. 1056
ANALYTICAL CHEMISTRY
For MR gels, there was no correspondence between the concentration ratios and tlie unextractable dye content. This illustrated further the relative unimportance of unextractable dye in the adsorption properties of tailored gels. Other Studies. Curti and Colombo (18) prepared a gel in the presence of D-camphorsulfonic acid and reported that a racemic mixture of camphorsulfonic acid in 5% acetic acid could be partially resolved. Work on DL-mandelic acid was also reported, but the resolution was less complete. Both of these systems were studied in the hope of extending the usefulness of the tailoring technique to other systems. Although experimental detail was lacking in the communication, D-camphorsulfonic acid gels were prepared using the acetic acid and hydrochloric acid methods of Dickey (4). The gels were air dried in the dark to avoid any racemization of the D-camphorsulfonic acid by ultraviolet radiation, as claimed by Heveran (19) for D-camphor. The gels were extracted with methanol until the washings showed no optical rotation. When frontal analysis was performed with a 0.02Msolution of DL-camphorsulfonicacid and the effiuent was collected in 20ml portions, in no case was any enrichment of the L-isomer found. This work was repeated with a new series of gels prepared from D-camphorsulfonic acid and also D-mandelic acid without success. Recently, Bartels, Prijs, and Erlenmeyer (20) experienced similar difficulties in attempts to prepare stereoselective adsorbents using D-camphorsulfonic acid as a tailoring agent. More recently, Sinclair and Rogers (21) used not only mandelic acid but other substituted mandelic acids and found virtually no adsorption onto silica from 5% acetic acid. Hence, the work of Curti and Colombo is, at best, difficult to reproduce. However, the preparation of adsorbents specific for quinine and quinidine (22) was confirmed (23). Those compounds are diastereomers, not enantiomers, and their physical properties, such as pK, (24) are quite different. DISCUSSION
Previous investigations of selectivity (5) had shown that the magnitudes of the selectivity changes by tailoring were somewhat random relative to an untreated gel. However, in the present study, once the dye had been added to the silicic acid sol, the increased selectivity imparted to the xerogel relative to an untreated gel was affected very little by any subsequent treatment. The absolute values of the selectivity for some of the tailored gels were somewhat lower than those prepared under different conditions, but the selectivities of control gels prepared at the same time were also correspondingly lower. The consistent differencein selectivities may reflect a difference in composition of the particular batch of sodium silicate used for gel preparation, for Table VI1 showed that very reproducible selectivities could be obtained for gels prepared at different times from the same batch of sodium silicate. In contrast to the relative constancy of the selectivity, capacity was quite sensitive to experimental variables. By far the most important variable in the preparation of these tailored adsorbents was the time of aging the hydrogel prior to drying. (18) R. Curti and U. Colombo, J. Amer. Chem. Soc., 74, 3961 (1952). (19) J. Heveran and L. B. Rogers, unpublished results, 1964. (20) H. Bartels, B. Prijs, and H. Erlenmeyer, Helu. Chim. Acta, 49, 1621 (1966). (21) J. D. Sinclair and L. B. Rogers, unpublished results, 1967. (22) A. H. Beckett and P. Anderson, Nature, 179, 1074 (1957). (23) R. Majors and L. B. Rogers, unpublished results, 1966. (24) D. D. Perrin, “Dissociation Constants of Organic Bases in Aqueous Solution,” Butterworths, London, 1965.
Although aging exerted a relatively unimportant effect on selectivity, the adsorption capacities of tailored gels increased with time, whereas capacities of the control gels decreased slightly. Most of the variations in adsorption capacities for the control gels could be explained in terms of the relative change in the nitrogen surface area. However, the capacities for the tailored gels were not always related to the surface area. For tailored gels, the large increase in dye adsorption with aging time must reflect the influence of the dye in promoting a greater number of tailored sites. At the same time, more dye becomes unextractable. Even so, the total amount of unextractable dye in a gel aged 33.2 days, assuming that each molecule is 100 A z ( 6 ) and was on the surface, would cover only about 0.1% of the total nitrogen surface area. Actually, the fraction of dye on the surface must have been low because of the nonreactivity of the dye to a wide variety of corrosive reagents which destroy it in other environments (4). The role of the unextractable dye will be discussed more fully in a subsequent paper (25). In the past, it has been very difficult to distinguish between aging time and drying method because, with air drying, long and indefinite aging times were necessarily involved. The aging time-drying method interrelationship probably accounts for some of the major discrepancies among the results of other investigators who repeated the original work ( 4 , 6 , 7, 1 6 , 2 6 ) . Clearly a relatively rapid, standardized drying technique is very important. The washing step can also introduce variations. For example, water may be used for extracting slightly soluble organic compounds, but extremely insoluble dyes, such as DMO, require methanol. However, the need for acidified methanol is questionable. Morrison et al. ( 1 6 ) stated that acid washing of the xerogel enhanced adsorption of tailored gels prepared by the acetic acid method but not on those prepared by the oxalate method. Their data did not bear this out; instead, there appeared to be random fluctuations in both capacity and selectivity. Furthermore, the present study showed that the use of water or acidified methanol as extractants and the steeping of gels in 2.OM aqueous hydrochloric acid resulted in tailored gels which had smaller adsorption capacities but nearly the same selectivities. The latter data are in agreement with those of Dickey (4). Very likely, both acidified methanol and water dissolved some of the surface layer of the gel, because silica is somewhat soluble ( 2 7 ) and particles are known to grow, especially in the (25) R. Majors and L. B. Rogers, ANAL.CHEM., 41, 1058 (1969). (26) Z. Z. Vysotskii, L. G . Divinch, and M. V. Polyakov, Proc. Acud. Sci. USSR,Phys. Secr. (English Transl) 139, 637 (1961); see also Chem. Abstr., 56, 13577i(1962). (27) G. B. Alexander, W. M. Heston, Jr., and R. K. Iler, J. Phys. Chem., 58, 453 (1954).
presence of electrolytes (28). Apparently, the dissolution of the surface of tailored gels caused a loss of specific sites, as reflected by the decrease in the concentration ratios, but had little effect on the selectivity of the remaining sites. For the control gels, washing did not affect the surface enough to cause a decrease in dye adsorption. With respect to variations that can arise in the gelation step, the effects of different agents fit a consistent picture. The capacity increase observed for the gels prepared in the presence of the normal alcohols of increasing chain length can be attributed to the decrease in the rate of growth of silica aggregates. The stabilizing effect of the organic media (29) resulted in higher surface areas for longer gelation times. (As in the aging study, the amount of unextractable dye was larger at slower gelation rates. This may reflect either a greater dye solubility in the alcoholic media or an aging effect.) On the other hand, the increased rate of gelation caused by fluoride ion led to decreases in surface area and packing density. Increased aggregation of the primary particles by fluoride is borne out by the results of Kimberlin (30)who prepared silica gels having very wide pores by adding hydrofluoric acid to a silicic acid sol made by hydrolyzing Sic14 in ice water. Of special interest in the fluoride study was the observation that faster rates of condensation gave larger amounts of unextractable dye but significant decreases in adsorption capacity. Thus, the dye was probably trapped mechanically below the surface. This indicates that the role of the unextractable dye may not be as important as previously thought (16). From an analytical standpoint, this study has shown that selectivity depends only on selection of the tailoring molecule and not on the conditions of preparation. In contrast, the adsorbent capacity can be increased or decreased by controlling the preparative conditions. To produce high capacity gels, long aging times and/or the presence of condensation retarders, such as alcohols, are required in the preparation medium. To produce low capacity gels, washing of the xerogel with acidified water and/or the presence of large amounts of fluoride in the preparation medium are needed. To produce gels for separations based on pore size, the presence of rate controlling agents in the preparation medium might be useful. In attempts to use these results, it should be recognized that the preparative conditions for the azo dye-silica system may not be directly applicable to other systems.
RECEIVED for review January 8, 1969. Accepted April 30, 1969. The support of the U S . Atomic Energy Commission under Contract AT(11-1)-1222is gratefully acknowledged. (28) S. A. Mitchell, Chem. Znd. (London), 1964, p 924. (29) R. Yu. Sheinfarn and I. E. Neimark, Kinet. Kutal. 8, 370 (1967). (30) C. N. Kimberlin, US. Patent 2447695 (1949).
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