Gas-Solid Chromatography with Salt-Modified Porous Silica Beads Arthur F. Isbell, Jr., and Donald T. Sawyer Department of Chemistry, Uniuersity of California, Ricerside, ea[$. 92502 The differential enthalpies, entropies, and free energies of adsorption for the functional groups of substituted hydrocarbons on salt-modified porous silica beads have been determined using elution gas chromatography. The effect of adsorbent surface area and of surface modification on these thermodynamic parameters has been studied to establish optimum conditions for selective separations. Various coating salts are compared and data are presented for the functional group contributions to adsorption on Porasil C modified with Na,S04, NaCI, LiBr, Na3P04,NazMo04, NiSO,, CoS04, A12(S04)a,and Cr2(S04)3. Surface treatment of the porous silica with hexamethyldisilazane eliminates the specific interactions due to olefinic and aromatic pi electron systems. This provides a basis for evaluation of substituent contributions to aromatic pi electron density. Examples of optimized separations of difficult mixtures are presented.
USEOF SALT-MODIFIED activated aluminas in gas-solid chromatography, which was discussed initially by Scott ( I ) , has been the subject of several detailed studies (2, 3). Recent investigations (4-7) have established that the interaction between a sorbate molecule and a salt-modified alumina is a combination of nonspecific and specific contributions. This previous work has shown that the overall adsorbentadsorbate interaction is dependent upon the nature of the modifying salt and the adsorbent. As a part of this earlier work, salt-modified porous silica beads have been compared with modified aluminas. The porous silica beads have desirable properties for gas-solid chromatography and provide selective interactions that are different from those for aluminas. Their use with aromatic hydrocarbons has permitted correlation of retention volumes with aromatic substituent effects (8). The types of separations that are possible with unmodified porous silica beads also have been discussed (9). The potential usefulness of salt-modified porous silica beads in gas-solid chromatography has led to a detailed investigation of their adsorption thermodynamics as a function of surface area, surface modification, and coating salt. The present paper summarizes the results and indicates the conditions for optimizing difficult separations. The general approach for evaluating the specific interactions of various functional groups in terms of enthalpies and entropies of adsorption has been discussed in detail in a previous paper (6). (1) C. G. Scott, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworths, Washington, 1962, p 36. (2) C. G. Scott and C. S. G. Phillips, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965, p 266. (3) C. S. G. Phillips and C. G. Scott, “Progress in Gas Chromatography,” J. H. Purnell, Ed., Interscience Publishers, Inc., 1968. p 121 (4) D. J. Brookman and D. T.Sawyer, ANAL.CHEM., 40,106 (1968). ( 5 ) G. L. Hargrove and D. T. Sawyer, ibid., p 409. ( 6 ) D. T,Sawyer and D. J. Brookman, ibid., p 1847. (7) D. J. Brookman and D. T.Sawyer, ibid., p 2013. (8) Zbid., p 1368. (9) C. L. Guillemin, M. LePage, R. Beau, and A. J. de Vries,
ibid., 39,941 (1967).
EXPERIMENTAL
The modified absorbents were prepared from porous silica spheres (Porasil) obtained from Waters Associates, Framingham, Mass. A weighed portion of adsorbent was added to an aqueous solution of reagent grade salt to form a heavy slurry. The slurry was quickly dried by rotary vacuum evaporation and the resulting material was sieved to 100-150 mesh. All salt coatings on the porous glass bead adsorbents were 10% by weight. Silazaned adsorbents were prepared by drying 5 grams of unmodified or salt-modified adsorbent for 12 hours at 120 “C. A mixture of the dried adsorbent, 3 ml of hexamethyldisilazane (Peninsular Chemresearch, Inc., Gainesville, Fla.), and 35 ml of 60-70 “C petroleum ether were allowed to reflux in a dry environment for 24 hours, or until the evolution of N H 3 ceased. The mixture was allowed to cool, 2 ml of 1-propanol were added, and the mixture was allowed to reflux for 2 additional hours. After cooling, the adsorbent was filtered and air dried. The hexamethyldisilazane reacts with the acidic surface hydroxyl groups to give a surface of trimethylsilyl groups. Column preparation, sample handling, surface area determinations, and retention volume measurements were the same as described previously (6) except column activation was at 230 “C. All columns were 3-fOOt X l,’S-inch stainless steel tubing. The system dead volume (void space) was assumed to be equal to the methane retention volume at high temperature (-275 “C). Dead volumes measured in this manner agreed within experimental error with those determined by the injection of methane onto an initial base line of hydrogen and methane. Instrumentation included a Varian Aerograph Model 1200 gas chromatograph equipped with a flame ionization detector and an isothermal oven temperature controller. By replacing the one-turn temperature controlling potentiometer with a tenturn potentiometer, precise temperature selection became possible (10.3 “C). A port for measuring inlet pressure was installed between the injector and the column. A Leeds and Northrup Speedomax H recorder with chart speeds of 0.5and 3-inch/min was used to record the chromatograms; an electronic signal recorded the time of sample injection on the chart. RESULTS
The physical characteristics of the various unmodified and modified Porasils are summarized in Table I. Materials and coatings have been selected to establish the effect of surface area and surface modification on the specific and nonspecific interactions of various functional groups. Porasil C has been used to determine the results of surface niodification because its surface area offers a compromise in selectivity, operating temperature, and analysis time. The effect of surface area on the adsorptive thermodynamic parameters has been determined using a series of Porasils modified with a 10% (by weight) Na2S04coating; the results are summarized in Tables I1 and I11 (data for Porasil C are included in Tables IV and V and are similar to those for Porasil D). In general, interactions increase with surface area. However, at 500 OK the free energy contributions for VOL. 41, NO. 11, SEPTEMBER 1969
1381
Table I. Physical Characteristics of Various Modified and Unmodified Porous Silica Beads, 100-150 Mesh Specific surface Av. pore area, diameter, Operating Column packing m2/g. A temp., "C Uncoated Porasil C 53 200-400 175-225 Silazaned Porasil C 62 200-400 125-1 75 Silazaned 10% Na2S04Porasil C 54 200-400 125-1 75 10% Na2304-Porasil A 455 100 200-250 10% NasS04-Porasil C 61 200-400 175-225 10% Na2S04-PorasilD 33 400-800 150-200 I O :/:NarSO.i-Porasil E 19 800-1500 125-175 I O NaCI-Porasil C 63 200-400 175-225 10% NasMo04-Porasil C 60 200-400 175-225 l o x Na3P04-Porasil C 57 200-400 175-225 10% LiBr-Porasil C 62 200-400 175-225 10% NiS04-Porasil C 82 200-400 175-225 10% CoS04-Porasil C 70 200-400 175-225 10% AI~(SOr)3-PorasilC 68 200-400 175-225 10% Cr~(SO4)a-PorasilC 67 200-400 175-225
Table 111. Effects of Surface Area of 10% NarSOdModified Porous Silica Beads on Free Energies of Adsorption for Various Functional Groups [4(-AG),, 500 "K, in Kilocalories] X
0
C Tterrn Tcis-trans
Tooni
Tarom
GCHa
-Et -iPr -tBU
-CF3
Porasil A -7.63 0.42 0.44 0.45 0.53 0.38 0.57 1.07 1.40 1.78 -
-0.05 0.48 0.85 1.33
-F
-c1 -Br --I
-
-0CHs
Porasil D -7.16 0.35 0.25 0.26 0.39 0.51
0.46 0.84 1.08 1.37 0.18 -0.01
0.38 0.67 1.01 -
Porasil E -7.64 0.41 0.34 0.33 0.42 0.32 0.44
0.83 1.06 1.34 0.26 0.03 0.46 0.75 1.11 2.00
Table 11. Effects of Surface Area of 10% NazS04-Modified Porous Silica Beads on Thermodynamic Parameters for Various Functional Groups (Enthalpies in Kilocalories; Entropies in Entropy Units) Porasil D Porasil A Porasil E A( -4mz
X
0
C Tterm Tcis-trans Twuj
Tarum
a-CH3 -Et -iPr -tBu -CF3 -F
-CI -Br --I -0CH3
-0.31 1.22 1.64 2.07 2.03 1.22 1.99 3.45 4.33 5.26
-
-0.56 0.73 1.66 2.86
-
4( - 4&
14.62 1.60 2.40 3.23 3 .OO
1.67 2.82 4.75 5.85 6.95 -
-1.00 0.49
1.62 3.05
-
4(-4H)*
2.41 0.81 1.45
1.90
1.84 1.37 1.86 2.97 3.61 4.30 -0.83 -1.39 -0.81 -0.19 0.64
Porasil E are greater than those for Porasil D. For the halothe free energy contributions are algenated benzenes (@-X) most identical on Porasil C and D at 500 OK; the smaller entropy contributions on Porasil D cause larger free energy contributions at higher temperatures. For pi electron systems (a),the entropy contributions are much larger on Porasil A and D than on Porasil C and E. Tables IV and V summarize the variations of the thermodynamic parameters for porous silica caused by adding a salt coating and by silazaning the surface. Coating Porasil C with 10% Na2S04 increases the aliphatic carbon (CH), a, and @ - X free energy contributions. In contrast, silazaning decreases the CH, a, and aromatic alkyl substituent ( h 4 l k ) free energy contributions while increasing the a - X contributions. These changes in free energy contributions are due to increases in C H ,a-AIk, and a - X enthalpy contributions and to large increases in the 9-Alk entropy contributions. The change in entropy contribution per halogen is increased by silazaning. Combined silazaning and Na2S04coating has a negligible effect on the @-Alk enthalpy contributions but increases the 1382
ANALYTICAL CHEMISTRY
-
A(-As)z
19.17 0.92 2.39 3.26 2.90 2.12 2.80 4.26 5.05 5.98 -2.03 -2.75 -2.40 -1.73 -0.74 -
4( -4Hh
3.27 0.74 1.39 1.75 1.85 1.25 1.75 2.90
3.67 4.61 -0.40 -0.84 -0.09 0.71 1.69 6.25
A( -A s h
21.84 0.66 2.08 2.82 2.85 1.86 2.62 4.13 5.21 6.54 -1.33 -1.76 -1.12 -0.08 1.15 8.51
enthalpy contributions for Cl,,a, and a-X. The entropy contributions for a-Alk, @ X , and aliphatic ?r electron systems also are increased. The reduction of the free energy contributions for a electrons with silazaning supports the theory that ?r systems interact with surface hydroxyl groups because silazaning eliminates the hydroxyl bond. The effects of silazaning and salt-coating are reduced as the temperature is lowered t o 60 "C. While inorganic salt-I-oated Porasil surfaces are generally stable, silazaned surfaces decompose with time to such an extent as to make them impractical for most applications of gas-solid chromatography. In general, reduction of column temperature enhances the free energy contributions of a systems relative to those for CI,. Consideration of the data in Tables VI and VI1 establishes that the inorganic salts used for salt-coating Porasil C have a significant effect on the various specific interactions. Although Na?S04, NaCI, and LiBr act similarly at 500 OK, qualitative differences in enthalpies and entropies exist as indicated by Table VIII. Na3P04 and Na?Mo04 are similar to the other Group I salts with the exception that a interactions are enhanced
~~~
~
~~
~~
Table IV. Effects of Salt Modification and Silazaning of Porasil C on Thermodynamic Parameters for Various Functional Groups (Enthalpies in Kilocalories; Entropies in Entropy Units) 10% NazSOl Uncoated Silazaned Silanazaned A(-AH>z
X 0
C Aterm
Tois-trans Tconj
Tarom
Qi-CH3 -Et -iPr
-?Bu -CF3
-F -c1 -Br -1
-0CHs -NO1 -CN -CHO
Table V.
X 0
C TtWm
Toin-trans Tconj Asram
@
