straight chain alkanes bonded to silica gel are 4.5 μπιοΐ/m2 for trimethylsilane and 3.5 μπιοΐ/m2 for octadecyldimethylsilane. Keep in mind that the mea sured surface area can decrease drasti cally by bonding long chain moieties to the support (8). Reporting moles of the bonded phase per meter of sur face area of the support seems like a logical choice. c) The surface coverage can also be reported in terms of the percent sur face silanol groups replaced by the bonded phase. Several assumptions, however, must be made here. The first assumption is that four surface OH groups per 100 A2 (or 1 ran2) are avail able for reaction. The second assump tion is that of the four available OH groups, at maximum only 70-80% will actually react with the organosilane. If a trichloro- or trialkoxysilane is used, it is further assumed that each molecule interacts only with two sur face hydroxyls and that no polymer ization takes place. Similar assump tions might have to be made in the case of bifunctional organosilanes. As an example, if 5% w/w carbon is the result of reacting 10 g of silica gel hav ing a surface area of 400 m 2 /g with trimethoxyoctadecylsilane, then by use of the assumption mentioned above, it can be shown that the surface cover age is 23% of the maximum possible coverage. The detailed calculations
are shown in the insert on this page. This type of calculation gives a good estimate of brush formation, although one must be cognizant of assumptions made. Our own work (13,14) seems to in dicate that when reacting trimethoxyorganosilanes with silica gel, under an hydrous conditions the resultant bonded phases consist of areas of "brushes", i.e., monomeric organosilane, and patches of polymeric materi al. To avoid the formation of polymer ic parts in the bonded phases, it is ad vised that the silica gel be dried, the reaction take place in dry solvents and glasswares, and that the organosilane be monofunctional. Further helpful suggestions can be found in the works of Unger et al. (8) and Karch et al. (15).
can your integrator do this? Multiple Tailing Peak Analysis
Alkyl-Bonded Phase Applications
Work investigating the retention mechanism on alkyl-bonded phases has been carried out in GC (16-20) and more extensively in HPLC (8, 14, 15, 21-27). The most recent data in gas chromatography seem to show that nonpolar solutes interact mainly with the bonded alkane, and the re tention seems to be due to adsorption on the bonded phase (19). Undoubt edly, unreacted surface OH groups can also affect the chromatographic be havior of the solutes (17). Important
Sample Calculation of Surface Coverage by Bonded Phase 1) Facts: Octadecyltrimethoxysilane reacted with 5 g of silica gel having a surface area of 400 m 2 /g. Elemental analysis indicated 5 wt % carbon. 2) Assumptions: (a) Four accessible OH groups on the silica gel surface, (b) Only 7 0 - 8 0 % of these silanol groups are available for reaction with the organosilane. (c) Only two of the trimethoxy groups react with the surface silanol. (d) No polymerization of the silane takes place. 3) Objective: Find the percent surface coverage of the support by the orga nosilane. The calculations are as follows:
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I) Number of accessible surface OH's: 2 (400 m 2 /g) .ή21 „ . 0 v a (4 OH/nm ) (5 g) —— —„ , = 8 Χ 10 21 OH groups 18 2 2 1.0 X 10~ nm /m II) Number of moles of OH groups in 5 g of silica gel = 0.013. III) Number of moles of the organosilane. Note that because of assumption (c), the number of carbon atoms is 19; 18 from the octadecyl chain plus the remaining methoxy. Five wt % carbon corresponds to 0.25 g C per 5 g of silica gel or 1.1 X 10~ 3 mol C in 5 g silica gel. IV) By use of the two site reactions assumption, the percent of OH involved in the reaction is
(1.1 X 10-3 mol C ) X 2 X 1 0 0 = 1 7 % 0.013 mol OH Thus, 17% of all accessible OH groups have reacted with the organosilane. V) By use of assumption (b) for 75% available OH group, the surface coverage by the octadecylsilane (ODS) is about 23 %. This example shows that large areas of the silica gel surface are bare and that more ODS could have been attached to the support.
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ANALYTICAL CHEMISTRY, VOL . 49, NO. 12, OCTOBER
1977 ·
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