and argon. The medians and variances of the three distributions are presented in Table 11. The agreement between the measured time variance of X and the value of 93.0 sec2 calculated from Equation 7 is excellent.
The interdiffusion coefficient of argon in helium was experimentally found to be 0.65 cmZ/sec. The value for butane in helium was calculated to be 0.33 cm2/sec by the method of Elliot and Watts (13) using data from Reid and Sherwood (14) and Fuller, Schettler, and Giddings (15). R and y were experimentally found to be 0.89 and 1.0, respectively. The time distribution for butane, considering only diffusion, is denoted Y and was found from Equation 2 using the /3 value calculated above and the experimental m and t values of argon. The two curves Y' and Y are presented in Figure 2 . The experimental butane curve, X Y , was subsequently deconvoluted with the diffusion curve Y and the resulting distribution, X , is shown in Figure 3. This distribution is attributed solely to the interaction of n-butane with cellulose acetate. The median time of X was set equal to the difference between the medians of the experimental curve X Y and the diffusion curve Y . Adjusting the time axis of X in this manner causes about 20% of the curve to lie in the negative time zone. This, obviously, has no physical meaning and is attributed to errors arising from the small difference between the experimental elution times of butane
CONCLUSIONS Any total GC elution curve which is due to a combination of independently occurring processes (or processes that are approximately independent) may be resolved into its component time distributions through deconvolution if the time distribution of any one of the constituents is known. The advantage of deconvoluting the parent curve lies in the fact that the distribution of the component curve is obtained without making approximations as to its shape. Thus one can directly measure the statistical moments of the component distribution and determine its functional form (16). The applications of this technique to chromatographic problems are manifold. Not only can diffusion and adsorption effects be isolated but also variables such as the geometry of the stationary phase or the contribution of a substituent group on the adsorbate can be resolved. ACKNOWLEDGMENT The author gratefully acknowledges Ichiro Hasegawa for many helpful discussions concerning the use of Fourier transforms and decofivolution techniques. Thanks are also due to Homer A. Hartung for his advice and encouragement.
(13) R. W. Elliot and H. Watts, Nature Phys. Sci., (London), 234, No, 48. 98 11972). (14) R . C. Reid a'nd T. K. Sherwood. "The Properties of Gases and Liquids," 2nd ed. McGraw-Hill, New York N.Y., 1966, p 536. (15) E. N. Fuller, P. D. Schettler, and J. C. Giddings, lnd. Eng. Chem.. 58, 19 (1966). (16) E. Gruska, J. Phys. Chem., 76, 2586 (1972).
Received for review October 20, 1972. Accepted January 15, 1973.
Role of Tri- and Dimethylsilanes in Tailoring Chromatographic Adsorbents R.
K. Gilpin and M. F. Burke'
Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
The reactions of trimethylchlorosilane (TMCS) and dimethyldichlorosilane (DMCS) with the porous silica adsorbents marketed under the trade name of Porasil (Waters Associates, Farmington, Mass.) are discussed. The effects of pre-reaction surface dehydration and the effective area available for bonding have been examined. A correlation between these bonding areas and nitrogen surface data is presented. In addition, the secondary reactions of short chained alcohols with dimethyldichlorosilane-treated silica adsorbents have been investigated. These reactions have been followed by means of thermal neutron activation and microcarbon analysis. In light of these data, a physical "picture" of the surface as well as a mechanism for the reaction is suggested.
With the advent of lower detection limits, gas-solid chromatography has been shown to be one of the most effective tools for the separation of closely related comA u t h o r t o w h o m correspondence s h o u l d b e addressed
pounds. To achieve such separations, the specific tailoring of solid adsorbate surfaces and subsequent characteriza-' tion is of prime importance. The use of porous silica beads for adsorbents has increased greatly because of the improvements in technology which provide a much greater uniformity of surface characteristics (1). While many approaches for surface modification have been tried in the past, the work by Halasz ( 2 ) and others (3-16) has demonstrated the desirabil(1) C. L. Guillemin, M . LePage and A. J. devries, J. Chromatogr. Sci.. 9, 470 (1971). (2) I . Halaszand I . Sebestian, Angew. Chem., 8 . 453 (1969). (3) E. W. Abel, F. H. Pollard, P. C. Uden, and G. Nickless, J. Chromatogr., 22, 23 (1966). (4) D. J. Moore and V. L. Davison, J. Amer. Oii Chem. Soc.. 44, 362A (1967). (5) C. J. Bossart. / S A Trans., 7, 283 (1968). (6) Walter A. Aue and Corazon R. Hastings, J. Chromatogr.. 42, 319 (1969) (7) V. K. Berg and K. Unger, Kolloid-Z. Z. Polym., 246, 1108 (1969). (8) K. Unger and V. K. Berg, Z. Nafurforsch. B , 24, 245 (1969). (9) J. J. Kirkland and J. J. DeStafane, J. Chromatogr. Sci.. 8 , 309 (1970). (10) J. B. Sorrel1 and R . Rowan, Jr., Ana/. Chem., 42, 1712 (1970). A N A L Y T I C A L C H E M I S T R Y , VOL. 45,
NO. 8,
J U L Y 1973
1383
ity of chemically bonding organic material to the surface. The “bonded” phase not only allows potentially greater thermal stability, but also a greater probability of understanding the actual surface structure which is responsible for adsorbate-adsorbent interactions. Perhaps the most common means of surface modification for silica type adsorbents has been the reaction of the organosilanes with such surfaces. Trimethylchlorosilane (TMCS) and dimethyldichlorosilane (DMCS) are two of the most commonly used compounds (17, 18). DMCS has been thought to have had the potential for subsequent replacement of the second chloro group (19). If we are to take full advantage of the potential selectivity of the surfaces which can be developed, it will first be necessary to more fully understand the nature and extent of such surface reactions. This involves measurements of amounts of bonded material, the chemical form of this material, and the amount (surface) available to a given adsorbate, as well as the interaction of the adsorbate with the bonded phase. The measurement of amounts of material bonded, relative bonding surface areas and chemical forms involves nonchromatographic experiments and will be discussed in this paper for tri- and dimethylsilanes. A chromatographic investigation upon these modified surfaces involving the measurement of thermodynamic quantities for adsorbate-adsorbent interactions will be published separately (20). The reactions of trimethylchlorosilane and dimethyldichlorosilane with porous silica beads are discussed. In addition, the reactions of short chained alcohols with dimethyldichlorosilane treated beads have been investigated. These reactions have been followed by means of thermal neutron activation analysis and elemental carbon analysis. A physical “picture” of the surface as well as a mechanism for the reaction is suggested. EXPERIMENTAL All work described here was carried out on the Porasil series (A-F) controlled surface area and pore size chromatographic supports. Each alcohol and the carrier toluene was dried and redistilled from calcium hydride. Dehydration of Supports. All samples were rinsed and soaked in distilled-deionized water for 1 2 hours prior to drying. Drying was accomplished a t a particular temperature in a dry flowing nitrogen stream for 24 hours. Sample Preparation. The chemically bonded stationary phases were prepared in the following manner: the dried beads were placed in a reaction vessel with a fritted glass bottom which allowed simultaneous agitation of the mixture and expulsion of the HC1 generated by the reactions. For the trimethylchlorosilane (TMCS) reactions, 25 ml of toluene and 7.5 ml of TMCS were added to the reaction vessel. The mixture was refluxed for 8 hours with nitrogen bubbling through the solution. In the case of dimethyldichlorosilane (DMCS), 25 ml of toluene and 7.5 ml of DMCS were added to the reaction vessel containing the dried Porasil beads. The mixture was refluxed for 4 hours while saturating the solution with nitrogen. After this treatment, the beads were rinsed with toluene. For the secondary reactions, 25 ml of toluene and 7.5 ml of the alcohol were added to the reaction vessel which contained initially silanized Porasil C beads. This mixture was again refluxed with nitrogen passing through the solution for 24 hours. (11) R. Rowan, Jr., and J . B. Sorrell, Anai. Chem., 42, 1716 (1970). (12) C. L. Guiilemin, Michel Deleuil, Simone Cirendini, and Jean Vermont, Anal. Chem., 43, 2015 (1971). (13) V . K . Berg and K. Unger, Koiloid-Z. Z. Polym.. 234, 1108 (1969) (14) K . Ungerand P. Ringe,J. Chromatogr. Sci.. 9, 463 (1971). (15) D. C. Locke, J . T. Schmerrnund, and B. Banner, Anal. Chem.. 44, 90 (1972). (16) K . Unger,Angew. Chem., Int. Ed., 11, 267 (1972). (17) D.M . Ottenstein, J. Gas Chromatogr., 6 , 129 (1968). (18) A. M . Filbert and M . L. Hair, J. Gas Chromatogr.. 6 , 150 (19681. (19) J. Bohemen, S. H . Langer, R. G . Perrett, and J. H. Purnell, J. Chem. Soc.. 2444 (1960). (20) R. K. Gilpin and M . F. Burke, to be published.
1384
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, JULY 1973
After completion of all reactions, samples were rinsed with toluene and dried in a dry flowing nitrogen stream at 140 “C for 24 hours. All reactions were carried out under dry nitrogen to maintain controlled moisture conditions. The reaction times used were in excess of the times required to give maximum surface coverage. Characterization Techniques. The number of C1 groups present at each step of the dimethyldichlorosilane reaction was determined by thermal neutron activation. Samples were prepared by placing approximately 100 mg of the reacted silica adsorbents in 1-ml polyvials to which 600 pl of toluene was added. Standards were 600-p1 aliquots from a solution of 30 p1 of carbon tetrachloride in 100 ml of toluene. All vials were heat-sealed immediately upon preparation. Irradiation and counting was performed a t the University of Arizona reactor facility. Vials were placed in an irradiation tube in the order of sample, standard, sample. This orientation was found to be best for minimizing slight variations in neutron flux within the reactor. Samples were irradiated for 1 minute a t full power ( 1012 neutrons/sec). Upon completion of irradiation, the outside of the sample vials was rinsed with water to remove any traces of exterior contamination. Samples were placed in lead receptacles and allowed to cool for 15 to 25 minutes. The purpose of this delay was to allow a slight trace of *SAl, which was observed, to decay to a low level so that coincidence loss during counting would be minimized. The amount of 2SA1 was found to be 0.19 f 0.03% for all Porasils (A-F). Possible egplanation for its presence are slight impurities in the silica or from an (n,P) reaction on 28Si. This latter explanation is favored in light of the relatively constant nature of the observed impurity. The only other detectable impurity was sodium which was found in all samples and probably arises from the process used in making these beads. Counting of the 1.65-MeV gamma peak for 38Cl was performed with Nuclear Diodes Model LGC 3.05-2.9 detector and Northern 4096 computer controlled analyzer system (Northern Scientific). Because of the reactivity of the intermediate samples with water, extreme care was taken to eliminate all possibilities of moisture. All transfers of sample were under toluene, and irradiation was performed immediately after reaction. Samples were dried and weighed after completion of counting. The amount of bound carbon on the silica surface was determined by removing physically adsorbed organic matter in a dry flowing nitrogen stream for 24 hours and analyzing the remaining samples for total carbon. All samples were transferred and weighed in dry nitrogen. These analyses were performed by Huffman Laboratories, Inc., Wheatridge, Colo.
-
THEORETICAL Surface Characteristics. The extent to which chemical reactions occur with a given porous’silica surface are controlled by the types and reactivity of -chemical groups present, and the steric availability of these groups. Much disagreement and confusion has arisen over these points. Earlier workers suggested the existence of three distinct groups: silanol or “bound water” (A), silanol with physically adsorbed water ( B ) , and dehydrated oxides (C) (21, 22). More recently slightly different views of the surface have arisen. Snyder (23, 24) has viewed the silica surface as containing varying proportions of five groups: geminal (D), bound and reactive (E), free (A), and siloxane (C). The amount of each of these groups is dependent upon structural considerations.
-s i -
-Si-
-Si-0-Si-
(21) P. Klein, Anal. Chem.. 34, 7, 733 (1962). (22) R. K. Her, “The Colloid Chemistry of Silicas and Silicates,” Cornell University Press, Ithaca, N.Y., 1955. (23) L. R . Snyder, Separ. Sci.. 1, 191 (1966). (24) L. R . Snyder and J . W . Ward, J. Phys. Chem., 70, 3941 (1966)
(D)
(E)
The silica surface is ordinarily considered to be covered with a monolayer of silanol groups which arise from the tendency of each silicon atom on the surface to maintain tetrahedral coordination (25). Various values which have been reported for the total number of silanol groups per (mF)2 have ranged from approximately 5 (26) to 8 (22, 27). The upper end of the range, eight hydroxyl groups per (mF)2, is set by calculating the area covered by each hydroxyl group from crystal structure data (22). The number determined experimentally will be some fraction of this depending on the size and shape of the pores, the size and shape of the molecular probe being used, as well as the reactivity of the molecular probe with the silanol group. The dehydration of silica has also been a much argued topic. A number of authors (22, 27) have suggested the following: below 150 "C loss of physically adsorbed water, around 150 to 600 "C evolution of bound water without appreciable structural deformation; and above 600 "C an increased possibility of internal alterations. On the other hand, Unger (16) has recently shown evidence to indicate that the hydroxyl group, silanol, concentration decreases only slightly up to 300 "C and that a pronounced decrease in silanol concentration occurs between about 300 to 500 "C. This decrease is attributed to condensation of bound or paired hydroxyl groups. The condensation of free hydroxyl groups occurs only above 600 "C. Evidence in this paper seems to support these latter conclusions. Trimethylchlorosilane. When the silica surface is heated above 150 "C, partial removal of the "bound water" occurs, very slowly u p t o approximately 300 t o 350 "C a t which point a pronounced decrease occurs to about 500 to 550 "C, leaving a dehydrated oxide condition which will not be chemically active to trialkylchlorosilanes. These conditions may be described by the following reactions:
I
I
I
-Si-O- Si-
+
- I
CH3-Si-C1
+
I
CH,
CH3
I CH3-Si-CH3
I
CH3-Si-CH,
I
I
0
0
I
I I
Si -
-Si-0Dehydrated Surface OH OH
I
-ii-o-hiI
I
I I
I
CH3-Si-CH3
I
I
0
A +
/-\
-Si-O-Si-
I
I
+
H~O
I
CH3
(25) P. C. Carman, Trans. FaradaySoc.. 36,964 (1540). (26) A. V . Kiselev and Y. I . Yashin. "Gas-Adsorption Chromatography, Plenum Press, New York, N.Y.. 1569. (27) W . K. Lowen and E. C . Broge, J . Phys. Chem . 65, 16 (1561).
RXU)
0
-0-Si-O-
I I
c1
OH
OH
-Si-0-Si-
I I
+
I
I
-
I
CH,-Si-CH3
I c1
CH3
CH3
I
c1
-Si
Hydrated Surface OH OH
I
Thus, by controlling the pre-reaction dehydration, varying amounts of TMCS may be chemically bonded to the porous silica surface. A second controlling feature of reactions upon a porous surface is the actual area available for bonding. When the porous structure is such that molecular exclusion occurs, the available bonding surface will be reduced. In this respect, only the accessible surface will be available for chemical modification. The surface area as calculated from nitrogen adsorption data may be much larger than effective bonding area. The reaction of trimethylchlorosilane has been used as a molecular probe to investigate this effect on a series of porous silica chromatographic supports. Dimethyldichlorosilane. It has been suggested in the literature (19, 28) that two possibilities exist for the reaction of dimethyldichlorosilane with the silica surface. These possibilities are illustrated in the following equations.
I
-O-Si-
I
If Reaction 1 is the reaction path, then the chlorine and carbon bound to the surface should have a 1:2 ratio and be directly related to the number of OH groups available for reaction. In the event that Reaction 2 is the controlling process, no chlorine will be detectable and the ratio of bound carbon to initially available hydroxyl groups should have a one to one correspondence. However, in view of these reactions, a third complicating alternative must be considered, the possibility that both Reactions 1 and 2 occur simultaneously to some degree. In this event, the number of C1 and OH groups would not show direct agreement nor would the surface be completely devoid of C1. Also, the bound carbon would not show a 2 : l or 1 : l ratio with initially available hydroxyl groups as in the case of purely Reaction 1 or 2, respectively, but some intermediate value. The above reactions assume a totally dry system free from all traces of water. In view of the nature of this assumption, it seems unlikely that this totally water devoid condition could ever truly exist. In the case of trace amounts of water, a different set of reactions may be viewed to take place. These reactions appear on the next page. (28) D
M Ottenstein, J , Gas Chromatogr
, 4, 11 (April 1 . 1963)
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
1385
OH
c1
OH
I
I + CH,-Si-CH, I c1
I
-Si -0- Si-
I
I
CH,
I
I
I
I
0
0
-Si-0
-Si
I
A
CH,
B C D E F
I
I
I
c1
I
Bonding surface area, m’lgb
SupBonded port, carbon. Porasil %“
CH,
CH, -Si-0-Si-
+
Table I . Effective Bonding Area as Measured by TMCS on Porasil Adsorbents
trace
5.70 3.06 1.46 0.48 0.20 0.09
236 127 60.8 25.3 8.1 3.6
CH,-Si-CH,
I I 0 I
-0-s1-0-
I
I
c1
OH
I I
+ CH,-Si
I I c1
I
-CH,
CH,-
Si -CH3 I
I -
I
O-Y-
R
0
-0-Si-O-
I
I
ROH
I
0
CH,-
I
Si-
I 0 I -0-Si-OI
CH:,
RX(6)
In this instance, the reaction of the alcohol with remaining C1 groups would lead to an increase of surface carbon. This bound carbon should be, therefore, directly related to loss in observed C1. 1386
H+
R
I
RX(i)
0
I I
-o-s1-0-
0
If Reaction 3 represents the reaction path, then no chlorine should be detected and the amount of bound carbon to initally available OH groups should be 2 : l . If DMCS reacts with the glass surface via Reaction 4, then the carbon to OH ratio should still be 2:1, but C1 should be detectable and related to the undimerized (CH3)ZClSi-0Si- groups bound to the surface. If long polymeric chains formed as represented by Reaction 5, the amount of bound carbon to initially available hydroxyl groups would be greater than 2:l. In this respect, no direct relation should exist. Also, the surfaces formed would be nonreproducible. After silanization, the surface may be treated with a small chained alcohol. This reaction has been proposed (28) to proceed in the following manner:
-
ROH
RESULTS AND DISCUSSION
I ,-si -
I
-
TMCS because of its unique reaction with the surface has been used as a reference compound to study the effects of surface dehydration and pore structure. From these data, the area available for bonding chloromethylsilanes and the number of reacted groups per unit area have been calculated. In light of these data, the DMCS and DMCS-ROH reactions have been examined and a new reaction model has been proposed. Effects of Molecular Exclusion. The effective bonding surface of Porasil (A-F) series controlled silica chromatographic supports with trimethylchlorosilane (TMCS) and dimethyldichlorosilane (DMCS) has been examined for fully hydroxylated surfaces. A fully hydroxylated surface was one which was dried a t a temperature of 150 “C. Correlation data between the effective areas available for bonding and nitrogen measured surface areas are presented in Table I. A maximum surface coverage of 4 TMCS groups per (mp)2 of surface gave the best agreement between calculated area available for bonding and Tu’z data for the low surface area adsorbents where molecular exclusion is a t a minimum. A value closer to 3 TMCS groups per (mp)2 is probably more reasonable based on steric arguments. Unger ( 1 6 ) has recently reported a value of 2.7 on certain silica gels. Also, through correlation of some preliminary Nz adsorption (29) data the values for surface area of these adsorbents as given in Table I are probably too small. Thus,. the value of 4 TMCS per (mp)2 of surface would approach 3. However, since all arguments presented in this paper are relative, this will not affect the interpretations. All data would be merely shifted with trends remaining unchanged.
0
I I
(72)
CH, -Si- CH,
I
I
CH3- Si- CH,
7
3 (72)
By this reaction, the dimethyldichlorosilane groups are totally displaced from the surface. The net change in total bound carbon is thus dependent upon the difference in carbon between the DMCS and the alkoxy groups. In view of these many possibilities, an extensive investigation has been made with the ultimate goal of understanding these surface modifications and their related ramifications upon the chromatographic process.
CH3-Si-CH3
I CH,-x-CH,
c1
20 (72)
4c 1. 5 c
However, another possibility exists as shown below. CH3-Si-CH3
I
25c
146(30)
I
c1
-Si-0-Si-
5OC
535 ( 2 9 ) d 118 (29) 64 (72)
48OC 200c
a Corrected for background Assuming maximum coverage of 4 groups per (mp)’ of surface Values reported by manufacturer Reference number in parentheses
trace
c1
OH
Nitrogen surface area (m’/g)
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
(29) D G Ackerman private communication (30) S Masukawa and R Kobayashi, J Gas Chromatogr 6 , 3 9 (1968)
Table II. Effective Bonding Area as Measured by D M C S on Selected Porasil Adsorbents Support, Porasil
Bonded carbon, %a
Reactedb DMCS groups/(mF)*
3.50 3.58 3.47
3.30 0.87 0.05
A C
F Corrected for background. TMCS.
* Based
on bonding surface areas of
Table I l l . Effects of Pre-reaction Surface Dehydration for TMCS on Porasil C Drying temp, C
150 250 350 450 550
Bonded carbon, O h n
1.46 1.37 1.30 0.96 0.85
Corrected for background. 60.8 M2/g. a
Total TMCS groups reacted
2.43 X lozo 2.28 X l o z o 2.17 X lozo 1.60 X I O z o 1.41 X 1 O z o
* Based
Reactedb TMCS groups/(mF)*
4.00 3.75 3.56 2.63 2.32
on a bonding surface area
% Of
To resolve this problem completely, work now in progress includes chromatographically measuring available areas with molecules similar in size and shape t o DMCS and TMCS. Bonding data have been summarized in Figures 1 and 2 for TMCS and DMCS, respectively, and indicate a divergence between nitrogen adsorption areas and the areas measured by silylation reactions. In all cases, the general surface homogeneity and total number of silanol groups per (mp)2 were assumed to be constant between adsorbents (31). This assumption was shown to be valid and will be further discussed below. The divergences between nitrogen and bonding data were most dramatically noted in the cases of Porasil A with both TMCS and DMCS. Porasil A, which has the greatest surface area and inherently the smallest pore opening, is most dramatically affected. This may be explained in part by the fact that over 25% of the surface area was contained in pores with diameters under 50A (32). It has long been noted (33) that unless size and shape are considered, erroneous abstractions may be made when extrapolating between sets of surface reactions where molecular size or shape are significantly different. In the case of the TMCS and DMCS, this should not present a problem because of their very similar size and shape. The number of groups reacted per unit surface area, (mp)2, for Porasil A, C, and F are shown in Table 11. A value of 3.52 f 0.06% was obtained which again supports the conclusion that the number of groups available for reaction per unit area may be considered to be constant per (mp)2 of surface for the Porasil series adsorbents. T o check to see if the reactivity of the molecule affected the above results, trichloromethylsilane (TCMS) was bonded to Porasil C under exactly the same reaction conditions as DMCS. Carbon analysis gave a value of 0.45% or 3.70 groups per (mw)2 of surface. The amounts of chlorine as determined by neutron activation agree very well with expected values from the DMCS data which will be discussed later. Apparently the reactivity of the groups has (31) A . V . Kiselev. J . Chromatogr., 49, 84 (1970). (32) D. E. Carter, Private communication. (33) D. E. Martire, R. L. Pecsok, and J. H . Purnell, Nature, 203, 1279 (1964).
CARBON
Figure 1. Surface area as a function of T M C S bonded to the ad-
sorbents
+ Nitrogen area: A bonding area
800
-
0 N &
600
I
I
I
e w
a
e w
400 a v) 3
200
%
CARBON
Figure 2. Surface area as a function of D M C S bonded to select-
ed adsorbents
+ Nitrogen area; A bonding area little effect on number bonded per unit surface area under prolonged reaction conditions. The decreased values of 3.6 and 3.7 groups per (mp)2 for DMCS and TCMS, respectively, on Porasil C as compared to 4.0 groups per (mp)2 for TMCS may be the results of structural differences of these bound species to the silica surface. Effects of Surface Dehydration. Listed in Table I11 are the results of pre-reaction surface dehydration upon the reactivity of trimethylchlorosilane with Porasil C. From a plot of these, Figure 3, a break was observed to occur a t about 350 “C. At this dehydration temperature, the bound silanol groups on the silica surface begin to condense rapidly as a function of temperature (16) resulting in a direct decrease in the number of TMCS which may be bonded to the silica surface. The general shape of this reaction curve is very similar to that reported for the dehydration of A N A L Y T I C A L C H E M I S T R Y , VOL.
45, N O . 8, J U L Y 1973
1387
Table I V . Effects of Pre-reaction Surface Dehydration for DMCS on Porasil C Dryin2 temp, C
Bonded carbon, %"
120
0.96 0.87 0.73 0.61
150 250 350
a Corrected for background.
*
OH groups reacted
Reacted DMCS groups/(mp)2
X X X X
3.95 3.58 3.05 2.52
2.40 2.18 1.83 1.53
1020 1020 1020
1020
Based on TMCS bonding surface area of
60.8 M2j g .
Table V I . Extent of Secondary Reactions of DMCS-Treated Porasil C with Selected Alcohols A Carbon
Reactant H20
0.51
MeOH
0.77
Et0H
0.92 1.19 1.19 1.51 1.02 1.34 1.12 1.24
n-prop0H n-BUtOH n- h exy IO H
iso-propOH iso-ButOH
Table V. Stability of TMCS on Porasil C toward M e O H
Sample
Carbon, %
TMCS T M CS- Me0 H
1.33
Approximate No. of bonded C groups per reacted OH"
between DMCS and ROH per group
Bonded carbon, %"
a Based on a value
-2 -1
0 +1 +2 +4
+1 +2 +2 +4
of 0.87% carbon from initial DMCS reaction.
1.39
(N2)
TMCS-MeOH 0.44
(HCI)
1.7
-
"Assuming TMCS pre-dried to 350 "Cas the reference.
800
1.3 -
I
az m 0
a U
V
.\o
.9
-
.5
-
.1
L
i
i
I
1
-2
0
+2
t4
A CARBON Figure 4. Change in carbon content of of various alcohols reacted with DMCS Linear alcohols; A branched alcohols;
.6
1.0
1.4
% CARBON Figure 3. Fraction of TM'CS and DMCS bonded to the surface as
a function of dehydration temperature
+ TMCS; A DMCS other silica surfaces (16). A similar study for the reaction of TMCS on powdered silicas has been reported (27). Breaks in those data were also observed to occur a t approximately the same dehydration temperatures. The very gradual decrease in bonded material from 150 to 350 "C is attributed to the fact that very little change in the number of silanol groups occurs in this temperature range. The effects of pre-reaction surface dehydration for dimethyldichlorosilane (DMCS) on Porasil C are shown in Table IV. These data have been summarized in Figure 3. The general slope of the pre-reaction dehydration curve for DMCS in the 150 to 350 "C region seems to be greater than that of TMCS, which would indicate that the gradual removal of silanol groups in this region has a more pronounced effect upon DMCS than TMCS. The reason 1388
* ANALYTICAL CHEMISTRY, VOL.
45,
NO.
8, JULY 1973
the surface as a function
cyclic alcohol
for this can be explained in light of data yet to be presented. DMCS is believed to form a dimer with adjacent hydroxyl groups. Any condensation of hydroxyl groups in the 150-350 "C region would result in the loss of two DMCS molecules from the surface. However, TMCS may not react with these adjacent groups for steric reasons since the closest silanol groups are believed to be the first to condense ( 2 4 ) , and thus only one reactive site would be lost. From Figure 3, it therefore can be seen that dehydration a t 150 "C provides for the greatest number of reactive groups while a t the same time minimizing the role of the physically adsorbed water. Reactions with TMCS Treated Surfaces. A study of the stability of bonded TMCS surfaces toward water and methanol was made. In the presence of an Nz saturated solution of either water or methanol and toluene under reflux conditions, no significant change in amounts of bonded carbon occurred. However, upon treatment of the TMCS coated silica surface with a solution of methanol and toluene in the presence of HC1 and under refluxing conditions, the surface bonded carbon was reduced to onethird of its original value as shown in Table V. It would thus appear that the trimethylsilyl groups are totally re-
placed by methoxyl groups in the presence of HC1. Similar results were obtained with water. The surface bonded carbon was reduced to background levels in the presence of HC1. This would indicate that the acid acts to catalyze the transesterification of the silanol groups. These results help to understand the reactivity of the DMCS treated surface. Reactions with DMCS Treated Surfaces. The silanization of the surface of Porasil C with dimethyldichlorosilane followed by treatment with water and short chained alcohols has been examined. The amount of bonded carbon a t each step of these reactions were determined by total carbon analysis. The data thus obtained are summarized in Table VI. Upon initial treatment of the porous silica surface with DMCS, a value of 0.87 f 0.07% bound carbon was obtained, or 2.2 X 1020 groups per gram of bead. This corresponds to a surface coverage of slightly less than two methyl units for each available hydroxyl group assuming a surface reactivity of four silanol groups per (mk)2 area as calculated from the TMCS data. Analysis for chlorine by thermal neutron activation gave a value of 3.6 f 0.4 mg of C1 per gram of reacted beads of approximately 6.3 X 1019 groups of unreacted chlorine. Thus the ratio of carbon to chlorine is 3.5:l. Upon examination of the previously discussed reaction possibilities, the following conclusions were made: If Reaction 1 were the path, the number of carbon to chlorine groups would have had a 2 : l correspondence, and the amount of chlorine directly related, to the number of surface OH groups available for chemical reaction. This, however, was not the case. Reaction 2 does not explain the results because the ratio of carbon to available hydroxyl groups was not 1: 1 but approximately 2 : l and also because of the presence of some chlorine. A combination of Reactions 1 and 2 was rejected because of the 2 : l carbon to reactive hydroxyl ratio and low value of chlorine. Reaction 3 indicates the absence of all chlorine and thus also must be rejected. Reaction 5 was also rejected on the basis that the surfaces formed were quite reproducible. Using Reaction 4 and varying the ratio of dimer to monomer units, a proper combination of chlorine t o carbon can be obtained and yet a 2 : l carbon to hydroxyl group ratio remain. In view of these results and considerations, it would appear that the initial reaction, the silanization of the silica sclrface with DMCS, proceeds in the following manner:
CHI
I CH3-Si-O-Si-CH3
I
CH3
I
I
c1 I
CH,-Si-CH3
I
where the surface formed consists statistically of 1 monomer unit to 1.25 dimer units. Upon subsequent treatment of the initially silanized surface with both water and selected alcohols, only background amounts of C1 (0.28 f 0.08 mg) were observed, indicating essentially complete reactions. Bonded carbon was also determined and the data obtained are summarized in Figure 4. Plots for monomer to dimer unit ratios of 1:1.25 for both reaction possibilities 6 and 7 are represented by lines A and B, respectively. A least squares fit
of our dat,a with no background correction appears as line C. Reaction 7 seems to best describe the overall position of our data; however, the general slope is slightly greater than predicted by chlorine analysis. Explanations which are consistent with these observations are that the surface area available for normal short chained alcohols is larger than that available to DMCS and/or that a greater number of alcohol molecules bond per (mW)2 of surface than the displaced DMCS molecules, caused also by reaction with unreacted surface silanol groups. In either case, an increase in slope would be observed. These possibilities are supported by the data obtained for isopropanol, tertbutanol, and cyclohexanol. Isopropyl and tert-butyl alcohol which are more similar in size and shape to DMCS than the linear alcohols should fall closer to the predicted line. This is shown to be true in Figure 4. In the case of cyclohexanol which is even larger and bulkier than either linear or branched alcohols, a negative deviation was expected. This is also shown to be true in Figure 4. In light of these data, the secondary treatment of the initially silanized surface was viewed to proceed as follows: CH,
I
CH3-Si-O-Si-CH3
c1
CH,
I
I
I
I
0
0
CH3-Si-CH3
I
OH
Y
0
FHJ
CHB
I
I
CHJ-Si-0-Si-CH3
-Si-
I
ROH *
0-Si-
I
I
R I 0-Si-0-
I
Si -
I
Examination of the reaction mechanism involved in this alcohol displacement of the monomer DMCS units is planned as future work. A chromatographic investigation of adsorbate-adsorbent interactions on dimethyldichlorosilane-alcohol modified surfaces (20) supports the above reaction model. CONCLUSION The determination of the roles played by DMCS and TMCS in the modification of the surface character of porous silica beads has been examined. Of particular interest has been the reaction of DMCS silanized surfaces with short chain alcohols. These reactions have allowed the development of a model system which should prove to be extremely useful as a means of investigating the nature of adsorbate-adsorbent interaction. A detailed chromatographic study is now in progress. Also thermal neutron activation analysis has been shown to be useful as a means of monitoring heterogeneous reactions between chlorinating species and the surface of porous silica. A great advantage of this technique is that it does not suffer from the problem of molecular exclusion which may be found in conventional means. ACKNOWLEDGMENT We appreciate the help provided by Morton Wacks in relation to the neutron activation analysis experiments. Received for review September 18, 1972. Accepted January 26, 1973. This work was supported in part by the National Science Foundation under Grant No. 017322. A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
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