Synthesis and Chromatographic Application of Bonded, Monomolecular Polymer Films on Silicic Supports Walter A. Aue, Corazon R. Hastings, and Shubhender Kapila Environmental Trace Substances Center and Department of Agricultural Chemistry, University of Missouri, Columbia, Mo
Through heat-treatment and exhaustive extraction with various solvents, a new type of chromatographic phase can be obtained from common GLC packings. It may be nominally considered a bonded, monomolecular polymer layer on either silica gel or diatomaceous earth supports. The characteristics of these highly deactivated phases are short retention times and the capability to separate polar, closely related substances in GC, and an improvement of resolution in LC.
Monolayers of polymers on adsorbents like carbon black or silica gel have been employed to achieve deactivation of surfaces and improve resolution in both liquid and gas chromatographies ( 1 ) The use of polar substances to suppress surface effects considered detrimental to GLC is a common approach ( e g., Ref. 2 ) , although little is known about their specific interaction with the solid/liquid interface. The deactivating substances can be coated from solution ( 1 ) or from the vapor phase (31 In this paper, we describe the synthesis and application of very thin films of polymers present on chromatographic supports such as silica gel or diatomaceous earth particles. Nominally, these phases can be considered “bonded, monomolecular” layers. The term “bonded” is used to emphasize the fact that protracted, continuous extractions with solvents from benzene to methanol fail to remove the polymer layers. This bonding may result from a large number of Van der Waals and/or hydrogen bonds between the adsorbent surface and the polymer. The long polymer chains could be envisioned in an oriented, “stretched-out” configuration; a t least for a significant part of their chain length. Such an arrangement, in which the polymer layer is well within the range of surface forces, is termed “monomolecular” within the context of this paper. When a polymer is deposited on the surface of an adsorbent, the chain length of the polymer influences considerably the amount needed for monomolecular coverage as determined by measurement of adsorption equilibria. Kiselev et al have shown with polyethylene glycols of disparate molecular weights that only part of the higher polymers can be considered “stretched-out” on a carbon black surface (1) The polymers used in our study are of considerably larger molecular weight and could not be expected to form efficient monolayers. Yet, the monolayer would represent an interesting object of study from both chromatographic and physicochemical points of view. Of several approaches possible, a heat-treatment (akin to the “no-flow conditioning” common in gas chromatography) was chosen t o achieve a more desirable distribution of polymer chains on the adsorbent surface. We had some prior information about the extractability of Carbowax 20M from Chromosorb W after heat (1) A . V. Kiselev, N. V. Kovaleva, and Yu. S. Nikitin, J. Chrornatogr., 58, 19 (1971). (2) M . 6. Evans, Chrornatographia, 4, 441 (1971). (3) N . F. lves and L. Giuffrida, J. Ass. Offic. Anal. Chern., 53, 973 (1970).
treatments; obtained, more or less by chance, during a study of chemically bonded liquid phases (4). There is evidence that a polymer stretched out on a surface may be much harder to remove with organic solvents than is commonly assumed from solution or adsorption data (5). Precise measurements of the amount of polymer remaining after exhaustive extraction are obviously difficult to obtain, especially when the diatomaceous earth supports with their relatively low surface areas are used. Gas chromatography, however, is a very sensitive, though qualitative, indicator of surface coverage, judged by the shape of peaks resulting from polar compounds. Therefore, the gas chromatography of hydrocarbon and alcohol standard mixtures was used as the major test in characterizing the surface properties of various experimental phases. Carbowax 20M, despite our scant knowledge on the precise structure of this polymer (61, was used in most cases because it yielded the best chromatographic performance.
EXPERIMENTAL
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P r e p a r a t i o n of the Phases. A silicic support-Chromosorb W, Celite 545, Silica gel 62 untreated, or Silica gel 62 hydrothermally modified ( e . g . . 71-was extracted exhaustively with hydrochloric acid, washed to neutrality, dried, coated with 6% polymer by rotary evaporation, heated under a very slow flow of nitrogen overnight in a tubular furnace ( e . g . , a t 260-280 “C for Carbowax 20M), and extracted with methanol or other appropriate solvents in continuous extraction equipment. The remaining supports were characterized by test chromatography with n-alkanes and n-alkanols, weight loss incurred by ignition (1000 “C for 3 hours in air), and, occasionally, by elemental analysis. The phases produced by Carbowax 20M on the diatomaceous earth supports were further tested with a variety of analyses which involved polar and/or labile compounds: Chlorinated hydrocarbon and thiophosphate insecticides, the butyl ester derivatives of nitrilotriacetic 181, citric and a number of other hydroxy acids of biochemical importance, the N-trifluoracetyl-n-butyl esters of protein amino acids (91, and a variety of aromatic aldehydes: as well as other mixtures of a more stable nature such as the methyl esters of fatty acids and diesel oil. The same phase was also used as a support for other GLC phases such as Apiezon L, and was tested with chlorinated hydrocarbon insecticides to demonstrate the deactivation of the surface in comparison with untreated support. The silica gel phases were additionally tested by liquid chromatography with a standard phenol mixture, and compared to a chemically bonded material on spherical silica beads (Durapak, Carbowax 400/Porasil C; Waters Associates, Framingham, Mass.)
(4) W. A. Aue. C . R . Hastings. and S. Kapila, IUPAC Congress on Analvtical Chemistrv. Kvoto. Jaoan. Aoril 1972. (5) R . R. Stromberg, D. J. Tutas, and E. Passaglia, J. Phys. Chern. 69, 3955 I 1965). (6) Union‘ Carbide Corp.. “Carbowax Polyethylene Glycols,” Interim Reprint F-4772F, 11/68-5M. (7) A. V. Kiselev, Yu. S. Nikitin, and E. 8 . Oganesyan, Koiloid. Zh., 31, 525 (1969); compare W. A. Aue, S. Kapila, and K. 0. Gerhardt, J.
Chrornatogr., in press.
(8) W . A. A u e , C. R. Hastings, K. 0. Gerhardt, J. 0. Pierce, H. H. Hill, and R. F. Moseman, J. Chromatogr., 72, 259 (1972). (9) M. W. Lamkin and C. W. Gehrke, Ana/. Chern.. 31, 383 (1965).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 4, A P R I L 1973
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RESULTS AND DISCUSSION The gas chromatographic efficiency of the final packing depended strongly on the temperature of the heat-treatment. Carbowax 20M, for instance, improves u p to approximately 280 "C beyond which the polymer apparently deteriorates. Although Carbowax 20M gave the best chromatographic performance, other polyethylene glycols, as well as two polysiloxanes and Apiezon L showed similar effects: When a regular phase was compared with a heat-treated one, the latter proved to be superior in gas chromatographic performance. Elemental and ignition analyses plainly operated a t their lower limit in attempts to determine the amount of Carbowax 20M left on Chromosorb W after extraction. All that can be said is that the true value was most likely lower than 0.270 polymer load-which corresponds approximately to a film thickness of less than 15 A. In a similar determination, a silica gel with 140 m2/gram surface area showed a nominal film thickness of 2 A. The true film thickness of polymer monolayers on a chemically and physically heterogeneous surface is, of course, open to discussion; but the above results indicated that the layer was certainly too thin to be considered a bulk liquid phase. As a typical case, the steps of producing a bonded layer of Carbowax 20M on Chromosorb W were followed by gas chromatography and ignition analysis. Typically, well acid-washed, blank Chromosorb gave a mediocre separation of n-alkanes and no chromatography a t all of n-alkanols. Adding 670 CW-20M by rotary evaporation produced the common GLC phase, with sharp peaks for both types of compounds. The overnight heat-treatment a t 280 "C did not change this chromatographic performance significantly while reducing the polymer load from 6 to 5%. The following exhaustive extraction with methanol reduced the load to less than 0.2% and produced a material with an efficiency comparable to the common 6% GLC phase. Retention temperatures, however, were ca. 30 "C lower for alcohols and 10 "C lower for hydrocarbons in temperature726
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programmed runs. As could be expected, the mass transfer effect in a Van Deemter plot proved comparatively small. In this, as in other cases, the performance of this modified surface was surprisingly good. The test substances listed in the Experimental part all chromatographed well, with sharp, symmetric peaks, and at a low temperature. There was little evidence of decomposition, and high carrier gas flows could be used. Figures 1, 2, and 3 show some examples, of which the last points out yet another characteristic of the phase: The capability to separate closely related substances which were difficult to resolve by bulk liquid phases (IO). Another example is the separation of the butyl esters of nitrilotriacetic and citric acids (8). The Carbowax phase bled, but a t a very low rate. Thus it could be used for a single column, temperature-programmed run with the EC detector set a t high sensitivity for chlorinated hydrocarbons. The bleed itself, like that from a regular Carbowax POM-coated column, showed few fragments above 100 amu in the mass spectrometer. When the Carbowax phase was used as a support for a 1% load of Apiezon L, it suppressed (as could be expected) the decomposition of chlorinated hydrocarbons which was rampant on a 1%Apiezon L on Chromosorb W column made for comparison (1 1). This apparent surface deactivation can also be observed on wide-pore silica gels. Figure 4 shows a chromatograph of n-alkanols, in which the deactivation of the surface by Carbowax 20M is further aided by the well-known method of humidifying the carrier gas. The deactivation aspect, as well as the non-extractability of the bonded layer suggested the use of silica gelbased materials in liquid chromatography. No attempt was made to achieve high-resolution liquid chromatography; rather, we tried to observe any effects which the sur-
(10) J. R. Koonsand E. W. Day../. Chrornatogr., Sci., i o , 176 (1972) (11) W. A. Aue. Toxicol. Environ. Chern. Rev., in press.
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face treatment would entail. Figure 5 shows the improvement which the Carbowax 20M layer adds to the advantages of the hydrothermal treatment. This is further illustrated in Figure 6 with a mixture of phenols. In the introductory part of this paper, we used, as a working hypothesis, the idea that polymer chains a t high temperature would rearrange on a surface (in competition with each other) to form a strongly held (=bonded) layer. Such a layer was presumed non-removable by organic solvents, by virtue of its minimal potential energy configuration, its orientation on the surface ( e . g . . oxygen atoms extending toward the support, methylene groups toward the
gas phase), and/or the large number of Van der Waals and hydrogen bonds between a single polymer chain and the silicic surface. This picture of the interface is, of course, just one out of several ones possible. Our failure to extract completely the chromatographic capabilities from heat-treated GLC phases could be attributed, for instance, to cross-linking of the phase or to the formation of chemical bonds to the surface. It is difficult to see, however, how Apiezon L could have crosslinked or how bonds between the terminal hydroxyls of the Carbowax and the surface silanol groups could have survived days of exposure to refluxing methanol. ANALYTICAL CHEMISTRY, VOL. 45, NO. 4 , APRIL 1973
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Figure 6. Separation of a standard phenol mixture A. Durapak, 8. Silica gel 62 140/200 mesh, HT-260 "C Carbowax 20M coated; mobile phase: chloroform, flow rate 1.5 ml/min., UV detector
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On the other hand, the best phases were produced from Carbowax 20M and the structure of this polymer, especially the diepoxide used in its production (S), represents proprietary information. Carbowax 20M extracted from heat-treated samples showed no significant difference from the original material by IR and GPC; but this finding offers, at best, an indication that the polymer bonded to the surface may have retained its original structure. The thickness of the layer, somewhat arbitrarily termed "monomolecular," is another point in question. It seems unlikely that a total polyethylene glycol chain of a molecular weight close to 20,000 would achieve contact with the surface. If part of the chain is "bonded," the rest would stick out into a mobile liquid phase or form a second or third layer in contact with a mobile gas phase. It will obviously take further experiments to define closer the state of the bonded layer. What appears fairly certain so far is that the layer is extremely thin and chromatographies performed on it are not obtainable either from the bare surface or the bulk liquid phase. Whether one should consider the separation of benefin and trifluralin gas-solid or gas-liquid chromatography can be debated; it bears traits of both concepts. Yet, these mechanistic and structural ambiguities should not detract from the usefulness of these and similar highly deactivated phases in liquid and gas chromatography. They may, furthermore, lead to surface studies all of their own.
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ACKNOWLEDGMENT
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Our sincere expression of gratitude is due to J. M. Augl for his inspiring suggestions, and his lucid explanations of the behavior of polymers on surfaces. as
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Figure 5. Van Deemter plot, p-nitrophenol, mobile phase: chloroform A. Silica gel 62 AW, 8. Durapak, C. Silica gel 62 HT, D. Silica gel 62 HT, Carbowax 20M coated
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
Received for review November 22, 1972. Accepted January 11, 1973. Part of this research is a contribution from the Missouri Agricultural Experiment Station, Journal Series No. 6567. The work was supported by the Environmental Trace Substances Center, by NSF grant GP-18616, and by EPA grant R-801050.
