193
Anal. Chem. 1982, 5 4 , 193-202 0.7 ng
0.03-
Il l
Flgure Q. Detectlon limlt tracings for a I-ppb BI standard. Coll length (cm): a = 100, b = 0, c = 4. Sodium borohydride concentration is 0.25% In 0.01 M sodium hydroxide.
With selenium, e.g., it should be very advantageous to complex the interfering ions with chloride since the optimum conditions for selenium hydride formation occur in regions of high acid concentration. Sample Throughput, Sensitivity, Detection Limit,and Precision. Figure 8 shows a typical series of triplicate standard injections. The relative standard deviation (n = 6) for a 30 ppb bismuth solution changed from 0.2% to 0.8% when the sample throughput increased from 90 samples/h to 180 samples/h. The sensitivity (1% Abs) was about 0.1 ng for the optimum conditions with respect to the suppression of interference effects. Detection limit, taken as 3X the standard deviation of the noise or approximately three-fifths the peak-to-peak noise which gives about 0.08 ppb from the tracings of Figure 9 (17). For comparison it can be mentioned that the best sensitivity and detection limit f i i e s for bismuth known to the author are those obtained by Hon et al. (18) and Thomson and Thomerson (19). The former obtained 0.4 and
1.5 ng and the latter 0.43 and 0.2 ng for a 1-mL sample, respectively. The detection lilfiit can probably be extended to lower values for the discrete sampling systems by increasing the sampling volume. However, as pointed out by Verlinden et al. (20) the signal at the same time decreases by ca. 50% (Perkin-Elmer MHS- 1and MHS- 10, hydride forming element selenium) for a 5-fold increase in sample volume. It is thus rather difficult to obtain a detection limit below 0.1 ppb.
LITERATURE CITED (1) Smith, A. E. Analyst (London) 1975, 700, 300-306. (2) Pierce, F. D.; Lamoreaux, T. C.; Brown, H. R.; Fraser, R. S. Appl. Spectrosc. 1976, 30, 38-42. (3) Pierce, F. D.; Brown, H. R. Anal. Chem. 1976, 48. 693-695. (4) Plerce, F. D.; Brown, H. R. Anal. Chem. 1977, 49, 1417-1422. (5) Ruzlcka, J.; Hansen, E. H. “Flow Injection Analysis”; Wiley-Intersclence: New York, 1981. (6) Betterldge, D. Anal. Chem. 1978, 50, 832 A-846 A. (7) Ranger, C. B. Anal. Chem. 1981, 53, 20 A-32 A. (8) Wolf, R. W.; Stewart, K. K. Anal. Chem. 1979, 57, 1201-1205. (9) Vljan, P. N.; Wood, Q, R. At. Absorpt. Newsl. 1974, 13, 33-37. (IO) Gouiden, P. D.; Brooksbank, P. Anal. Chem. 1974, 46, 1431-1436. (11) Chapman, J. F.; Dale, L. S. Anal. Chim. Acta 1979, 1 7 7 , 137-144. (12) Knechtel, J. R.; Fraser, J. L. Analyst (London) 1978, 703, 104-105. (13) Slemer, D. D.; Koteel, P.; Jariwala, V. Anal. Chem. 1976, 48, 836-840. (14) Fernander, F. J. At. Absorpt. News/. 1973, 12, 83-97. (15) BBdard, M.; Kerbyson, J. D. Can. J . Spectrosc. 1976, 21, 64-68. (18) Verlinden, M.; Deelstra, H. fresenius’ 2. Anal. Chem. 1979, 296, 253-258. (17) IUPAC, Anal. Chem. 1976, 48, 2294-2296. (18) Hon, P. K.; Lau, 0. W.; Cheung, W. C.; Wong, M. D. Anal. Chim. Acta 1980, 775, 355-359. (19) Thompson, K. C.; Thomerson, D. R. Analyst (London) 1974, 99, 595-601. (20) Verlinden, M.; Baart, J.; Deelstra, H. Talanta 1980, 27, 633-639.
RECEIVED for review July 6,1981. Accepted October 19,1981.
Determination of the Composition of Organic Layers Chemically Bonded on Silicon Dioxide Jean-FranGois Erard and Ervin sz. KovBts” Laboratolre de Chimie Technlque de I’Ecole Polytechnlque F M r a l e de Lausanne, 10 15 Lausanne, Switzerland
An analytical method for the determination of the composition of chemlcaily bonded organoslloxy layers Is presented and verified. I t consists of the following steps: (I) dissolution of the surface modified silicon dioxide sample in a solution of hydrogen fluoride in diethyl ether; (11) elimination of the main part of the slllcon tetrafiuorlde formed from the bulk slllcon dioxide, displacing it by nitrogen; (111) analysis of the fiuorosilanes formed quantltativeiy from the organosiioxy substituents or their butyl derivatives, by gas chromatography. The precision of the method Is about *2.0% (95% confidence level) for the surface concentration for samples having about 20 m2 of surface area. The lower limit of applicability is estimated to about 1 m2 samples corresponding to a 5 pmoi silane mixlure but with a precision of 10%.
Little to nothing is known of composite chemically bonded organic layers though they can readily be prepared by treating silicon dioxide preparations with a mixture of silylating agents. 0003-2700/82/0354-0193$01.25/0
This astonishing lack of concern logically implies the lack of an analytical method for the determination of their composition. To remedy this situation, we present a method for the determination of the composition of layers prepared with mixtures of monofunctional organosilanes at the surface of silicon dioxide preparations. The method is essentially an application of Booth’s suggestion for the analysis of silicones: the sample is dissolved in hydrogen fluoride where Si-C bonds are not attacked and the volatile organofluorosilanes are then analyzed (1-3)- The procedure involves several disadvantages, one of them being the handling of anhydrous hydrogen fluoride. This can be avoided by using boron trifluoride instead of hydrogen fluoride, but with both reagents an undesired side reaction, the cleavage of certain types of Si-C bonds, is observed. Especially fragile are aromatic silane bonds which are split even under mild experimental conditions ( 3 , 4 ) . To suppress this reaction we largely profited from literature reports describing the removal of organosilyl protective groups (5-7). These studies show that the reactivity of hydrogen fluoride 0 1982 Amerlcan Chemical Society
104
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
can be varied between extremes in binary systems ranging from superacid to nearly neutral reaction medium (8,9).From the present point of view mixtures of weak basic liquids and hydrogen fluoride were of special interest; actually such reagents are often used for the detachment of protective groups. (Physical properties of hydrogen fluoride and its binary mixtures with organic solvents are not well-known; we even doubt the value of the density reported for pure hydrogen fluoride (IO).) These considerations account for the essential features of the analytical method presented in this paper. The surface-modified silicon dioxide preparation is dissolved in hydrogen fluoride/diethyl ether, and the resulting fluorosilanes (or their derivatives) are analyzed by gas chromatography. Application of the method is demonstrated by an example, the study of the composition of a chemically bonded surface layer as a function of the composition of the reagent used for the surface treatment. We would like to emphasize the utility and versatility of hydrogen fluoride in ethyl ether as a reagent which, for some reason, does not seem to have been exploited. This mixture of a weak acid and a weak base is stable for months (the vapor pressure of the mixture is lowered by the formation of a 1:l complex) so that it can easily be stored and handled at room temperature. (For calorimetric and spectroscopic evidence for the formation of the complex, see ref 11 and 12.) During preparation of the manuscript another analytic procedure was described by Verzele, Mussche, and Sandra (13) which could also be adapted to a quantitative evaluation of the surface concentration of composite layers.
EXPERIMENTAL SECTION General Procedures. The structure of all synthetized substances was confirmed by UV, IR, and 'H NMR spectra as well as by elemental analysis. The elemental analyses were made by N. L. Ha in our laboratory. All melting points are uncorrected. The densities (corrected for vacuum) are averages of at least two determinations at 20.0 & 0.1 "C; the confidence limits of the . average at the 95% confidence level is Ag5 = 0.002 g ~ m - ~The refraction index was measured in a Zeiss refractometer (system Abb6) thermostated at 20.0 f 0.1 OC; Ae5 = 0.0003. Materials. Butyllithium (pract; 1.5 M in hexane)(purum) was from Fluka AG (Buchs, Switzerland). The stationary phase Apolane-87 (24,24-diethyl-19,29-dioctadecylheptatetracontane, CmH1,6)was synthesized in our laboratory (14). The methylsilicon OV-101 was from Ohio Specific Chemicals Co. (Marietta, OH). The solvents diethyl ether, isopentane, and cyclohexane (all purum, Fluka) were distilled over NaH before use. Hydrogen fluoride in steel cylinders with purity of better than 99.8% was from Merck-Schuchardt (Hohenbrunn, Germany). (In these steel cylinders we always found hydrogen after a few months of storage. Therefore, before utilization the cylinder was cooled at 10 "C and the hydrogen was eliminated through a column filled with NaOH pellets in order to retain HF vapors.) Fume silica was from Cabot Corp. (Cab-0-Sil-M5). Preparation of the Hydrogen Fluoride in Ethyl Ether Reagent. The cylinder containing HF was connected to a 500-mL Teflon flask containing 185 g of dry ethyl ether; the outlet was protected with a NaOH filled tube. The flask was cooled to -70 "C and the cylinder slightly heated on a water bath after condensation of 241 g of HF (mole fraction of HF, XHF = 0.828) the molarity of the solution was determined by titration to be about 27 M. This reagent is stored at 0 "C. Further solutions were prepared by dilution. Preparation of Surface Hydrated Fume Silica. All surface treatments were made with a carefully prepared fume silica sample. Following the directions of Hockey and Pethica (15) a sample of fume silica (Cab-0-Si1 M5) was heated at 950 "C for 48 h. The material was then rehydrated at the surface by heating it with distilled water for 5 h at reflux. After evaporation of the water, the surface hydrated Cabosil was heated during 48 h at 115 0C/10-3 torr and stored in a drybox in an argon atmosphere (1 ppm of O2 and 1 ppm H20) before use. The Cabosil was
weighed in the drybox and contact with humid air was carefully avoided. The specific surface area was determined from the Nz isotherm by the BET method with points between relative pressures 0.05 pml 0.23 (surface occupied by an N2 molecule: 16.2 A') and gave 175 f 2 m2g-' (average of five determinations). Properties of HF/Ethyl Ether Mixtures. Density and Molar Volume. For the manipulation of this reagent it was necessary to know the molar volume (or density) of the mixtures between 0 and 30 "C. For ethyl ether the values of ref 16 were accepted: V",*(o "c)= 100.2 mL mol-' (doeth= 0.740 g cmS) and Keth = -0.00162 K-' (Meth = 74.123 g mol-'). The molar volumes for a given temperature can be calculated with eq 1,where V"i = V"i(0 "C) exp[Ki ATI (1) Vi is the molar volume, K is the coefficient of thermal expansion, and AT is the temperature in "C. The superscript " refers to the pure substance. For hydrogen fluoride we could not find satisfactory data in the literature. For the coefficient of thermal expansion an average value from ref 17 was accepted KHF = 0.0022 K-'. The value P m ( 0 "C) = 19.33mL mol-' (dom = 1.035 g cm-3 ~ ) taken from ref 18. calculated from d19'54HF= 0.991 g ~ m - was These figures have been used to give the molar volume of the mixtures
v,
= [XethV"eth(O
"c)+ XHFV'HF(O"c)-t kX,thx~~Iexp[K,AT]
(2)
with Km
= XethKeth
+ XHFKHF
and vm
= (XethMeth + XHFMHF)/dxn
where x is the mole fraction. The density of mixtures, d,, was determined with home-made polyethylene picnometers to a precision of about &0.3% at 20 "C. The evaluation of the results gave a value of k = -13.0 mL mol-'. This value is relatively high and confirms the results of ref 11and 12 suggesting the existence of a 1:l complex. Equation 2 gives molar volumes between 0 and 30 "C with a precision of about 0.6%. Apparatus. For gas chromatographic analysis a PackardBecker (Delft, Holland; Model 419) research chromatograph was used equipped with flame ionization detector. The spectrometers used were an IR spectrometer (Model 700, Perkin-Elmer),a UV spectrophotometer (Model 635, Varian-Techtron), and an NMR instrument (Model WP-80, Bruker). The elemental analyses were made with an "Elemental Analyzer Model 240B" from PerkinElmer. Nitrogen adsorption isotherms were determined in the automatized apparatus of Carlo Erba (Milano, Italy), Model Sorptomatic 1800. Samples of silica were stored in a drybox from Mecaplex (Grenchen,Switzerland), Model GB-80, f i e d with argon (1 ppm of O2 and 1 ppm of H20). Reference Compounds for Gas Chromatography. The organofluorosilanes were prepared by dissolving about 1g of the corresponding disiloxane in 15 M HF in diethyl ether (The disiloxanes were byproducts in the preparation of the corresponding dimethylaminosilanes. The synthesis of this class of compounds will be described in a forthcoming paper (19).) The reaction mixture was then diluted with diethyl ether, the solution washed with water and concentrated NaHC03 and dried over Na2S04, and then the solvent evaporated. Vacuum distillation of the residue gave in general the fluorosilane in the desired purity. About half of the fluorosilanewas dissolved in diethyl ether and was made to react with the necessary amount of butyllithium (1.5 M in hexane). After similar workup and vacuum distillation, the corresponding butyl derivatives had in general the desired purity. Mixtures of these silanes and of pure alkanes of known composition were prepared for the determination of gas chromatographic data including the relative specific response, f (g/g), of the flame ionization detector of the compound in question (f(decane) E 1). Table I shows the physical and gas chromatographic properties of a few triorganofluorosilanes and the corresponding butyl derivatives. Having these data at hand one can calculate the amount of these compounds with respect of an internal standard as follows: n(RJ n(DJ = m(Di)/M(Di) = m(St)[A(Di)/A(St)l/M(Di) (3)
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
m
:El
!2
L-
rlrl
195
198
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
t
5cm
n:
U Flgure 2. Diagram of the apparatus for the dissolution of silicon dioxide samples: reactor, R, see Figure 1; F, flow meter (rotameter); M, manometer; C, chromatographic column with squalane on Chromosorb (1585= g/g); B, bubble counter; C1 and C2 three-way cocks; V, vent. Figure 1. Diagram of the Teflon reactor for the dissolution of silicon
dioxlde samples In HFIdiethyl ether: Sl-Sg, sealing screws (monel metal with Teflon O-ring seals); R, reactor; Sp, reaction space (can be varied by manipulating S3);C, gas chromatographic column. where n is the number of moles, m is the mass of a compound,
M is the molecular weight, and A is the reduced surface area of the peak ( A = A*f-'; A* is the surface area in the chromatogram and f is the specific response of the detector). The index i refers to the ith doxy substituent, 5. The symbol D is for the derivative of the substituent R and St is for the standard (St can be decane; other n-alkanes can be used as secondary standards; see footnote to Table I). For a C-component silane mixture evaluation of the chromatogram gives C
ntot = Cn(Rj) and yi= n(Ri)/ntot j=1
(4)
where nbt is the total number of mole substituent at the surface of the silicon dioxide preparation and yi is the mole fraction of the ith substituent. If there was no standard (n-alkane) in the mixture only the mole fraction, yi, can be calculated as
where the symbols have the same meaning as for eq 3. Note that the mole fraction of the substituent in the surface layer is the same as the mole fraction of its derivative, yi. Reactor for the Analysis of Composite Surfaces. All parts of the apparatus in contact with liquid HF or its vapors are of Monel-400 (Ni:Cu = 7030 g/g) or Teflon. Figure 1 shows the reactor itself. The opening, closed by S1, serves to introduce the HF/ethyl ether reagent to dissolve the sample placed in the reaction space, Sp. The size of this space can be changed by manipulating screw 53. The nut, S4, is to fix the Teflon reactor, R. Figure 2 shows a schematic diagram of the apparatus. The reactor and coiled column C are connected to a nitrogen supply by the way of two three-way stopcocks, C1 and C2. The functioning with different positions of C1 and C2 is as follows: normal position (NL, taps as shown in Figure 2: C 1 , l ; C2, T ) with N2 passing through R C B V; back-flush position (BF, T, I)with inversed flow C R B V; bypass position (BP, T, +);short circuit position (SC,+ ,+);leak test position (LT, I, 1). The gas chromatographic column is a Monel metal tube of 0.5 cm i.d. and 108 cm length with 18.0 i 0.5 g column f i g , squalane on Chromosorb-G (15:85 g/g). This quantity is double the necessary amount needed 'to retain methyltrifluorosilane in the column at 0 "C after the passage of 60 mL of an inert gas. The column and the reactor are mounted on a panel in such a way that they can be cooled or heated by means of baths. Dissolution of a Sample To Be Analyzed a Typical Example (See Figure 2). Into the detached Teflon reactor were placed 110.7 mg of a surface-modified Cabosil sample with 4.153 mg of decane
- -- -- -
and a small Teflon coated magnetic stirrer. The original Cabosil had a specific surface area =175 m2g-' and was surface modified with a mixture of (A) trimethyl(dimethy1amino)silaneand (B) decyldimethyl(dimethy1amino)silane. (The sample had a surface area of about 20 m2Si02with about 0.080 mmol of siloxy substituent at its surface.) Dissolution (abbreviationsfor tap positions see above): (i) regulate N2 flow at 10 f 0.5 mL m i d with taps SC and fix the reactor on the apparatus with nut S4. (At this stage of the manipulation it is easy to control the airtightness of the system.) Turn taps to NL, wait 5 min, and cool the reactor at -70 "C (dry ice) and the column to 0 "C (ice bath) then turn the stopcocks to BP. (ii) Open screw S1 and introduce 1.2 mL of a 12 M solution of HF (14.4 mmol corresponding to -100% excess) in ethyl ether then replace the dry ice with a magnetic stirrer and agitate for 30 min. After about 6 min of the warming period, the reaction is seen to begin and is complete after 10 min more (observe SiF4bubbles in B). (iii) Again cool the reactor to -70 "C and then turn stopcocks to NL for 5.0 rnin to vent SiF,. Turn taps to BP, open S1, and introduce 1.0 mL of hexane. (iv) Replace the ice bath of column C with a warm water bath of 50 "C, back-flush with taps BF for 10.0 rnin then turn back to BP. (v) Disconnect the reactor and add 1.5 mL of saturated NaCl to the reaction mixture. After agitation by hand, 2.0 mL of the organic phase was separated via pipet then kept in a 2-mL stoppered flask at -5 "C for further analysis. Calculation of the Surface Coverage from Analytical Data. Average Substituent. A substituent, Ri, is defined as the radical replacing the proton of the surface silanol. In order to simplify the calculation_sof surface coverage let us define an "average substituent", R, with the molecular weight of
where yj is the mole fraction of the jth substituent at the surface. An analogous equation gives the stoichiometric proportion, (z, of an element, X, in the elemental formula of the average substituent: C
li
= cyjaj
(7)
1-1
Example: R(') = SiC3H9and R(2)= SiCloH20; with y1 = 0.3 and y2 0.7. M(R(l))= 73.195 g mol-l and M(RP2))= 203.382 g mol-'; M(R) = 0.3 X 73.195 + 0.7 X 203.382 = 164.326 g mol-1 and similarly R = SiC7.gH18.801.4. The stoichiometry of the surface treatment reaction now becomes
I
sikOzk-r~(HzO)l-??l
+ zmH,O 4
t HY (8)
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 -
197
~~
Table 11. Rate of the Reaction of Phenyldimethylfluorosilane, (sil), with Hydrogen Fluoride To Give Benzene and Dimethyldifluorosilane a
xHF
cm, mol L-'
[HFI, mol L-'
[silI"i mol L-
0.723 0.768 0.808
25.0 27.3" 29.8
18.1
0.051 0.051* 0.051
21.0" 24.0
at 30.0
at 20.0 "C
at 10.0 "C k,
mol L-' min-' 1.98 X
2.07 2.10
X X
cm, mol
[HFI, [sill", mol mol L-I L-
L-I
24.5 26.7 29.2
17.7 20.5 23.6
0.051 0.050 0.049
k, mol L-' min-'
7.94 3.16 5.04
X X X
cm, mol L-I
[HFI, mol
24.0 26.2 28.6
17.3 20.1 23.1
* 0.1 "C
[sill", mol L-
L-I
0.051 0.050 0.050
k,
mol LW1 min-' 2.07 X 1.21 X 1.95 X
loy3
Effect of Additives on the Rate at Conditions Marked by Asterisk additive KF KF BF3
[add], mol LV1 0.001 0.1 0.0002
klk*
[sill", mol L-I 0.045 0.050 0.069
0.67 0.45 1.51
a Symbols: mole fraction of HF in the HF ethyl ether mixture, XHF; mean molar concentration of the solvent, c,; molar concentration of HF, [ HF]; initial concentration of the starting silane, [sill" ; concentration of additive, [add]; zeroth-order rate constant, k .
where the expression between bars refers to the solid not taking part in the reaction. Formula 1 is that part of the silica which has just one hydroxyl at the surface and has a total of "I" water, either physically adsorbed at the surface or as inclusion. There will be, m water molecules lost in the course of the reaction. The molar mass of the reaction product, 3, is M(3) = zM(1) + M(R) - 6
(9)
where 6 = (18zm + 1)is a correction for desorbed water during reaction and for the substitution of the hydrogen of the surface silanol. The value of this correction is roughly 6 = 5 for our carefully prepared and stored fume silica. This correction being small, a very rough estimate of 6 is sufficient. (A 50% error of 6 would introduce an error of 0.1% in the value of I"(R). For samples weighed in laboratory atmosphere use 1 = 1giving 6 c 50; see also ref 20.) Surface Concentration of the Average Substituent. The number of moles of the average substituent, n, referred to unit weight of the starting material, 1, is given by nbt/m(t) = 106/zM(1) pmol g-l
(11)
where s (m2g-l) is the specific surface area of 1. In order to use eq 11,one needs information about M(3) and M(R). For calculation of M(R) the mole fraction of the substituents has to be known from gas chromatography (see eq 4 and 5). The value of M(3) can be calculated from the result of the elemental analysis of element X in 3 given as weight percent, P(X) M(3) = lOOdM(X)/P(X)
C
P(X) = Pl(X) + CPj(X) j-2
(12)
where M(X) is the atomic mass of element X and d is the stoichiometric ratio of X in the average substituent, R. Alternatively, it can be calculated as if ntot was known from gas chromatography with an internal standard. Finally, with the knowledge of r(R),one gets for the individual surface concentrations (14) The calculation of surface concentrations from gas chromatograms with internal standard is as follows. (A) Calculate first the number of moles of the substituents, n(RJ, and the mole fractions, ye from the chromatogram (eq 3 and 4) which allows the calculation of
(15)
where Pj(X) (I' # 1)can be calculated from the gas chromatographic analysis Pj(X) = loon(Rj)ajM(X) / m(3)
(16)
Combination of eq 15 and 16 gives for the unknown, n,
(10)
Combination of eq 9 and 10 gives the surface concentration of R
r(R) = -s1 M(3) - 106 M(R) + 6 pmol m-2
M(R) and M(3) (eq 6 and 13). Use eq 11 and 14 to get the individual surface concentrations, I'+ Other variants are possible. (B) Gas chromatography without internal standard together with the result of the elemental analysis of the product, 3, also allows the calculation of the individual ri - s. (C) If one of the derivatives is of very low volatility gas chromatography with internal standard gives the number of moles of all substituents minus this one (I' f 1). Elemental analysis, P(X), of the element X is the sum of the analyses of the individual substituents
n1 = P(X)m(3)/100a~M(X)
(17)
Kinetic Measurements: Phenyldimethylfluorosilaneand Hydrogen Fluoride (See Eq 22). Three solutions of HF in ethyl ether were prepared: with mole fractions of xHF = 0.723,0.768, and 0.808. The molar concentration of HF and the diethyl ether can be calculated as follows: ,C
= 1000/Vm
[HF] = XHFC, [eth] = xmc, mol L-l
(18)
The corresponding molar concentrations were calculated by using the molar volume of eq 2 for 10,20, and 30 "C as indicated in Table I1 giving solutions of about 18, 21, and 24 mol L-l HF at 10 "C. An aliquot of 5.0 n& of these solutions was thermostated in a Teflon test tube (magnetic stirrer) at a desired temperature (10.0,20.0, and 30.0 k 0.05 "C) and at t = 0 min a defined amount of decane and phenyldimethylfluorosilane was _added (l/l, mol/mol) to give a solution of about 0.05 mol L-' (see Table 11). At appropriate intervals, samples of about 50 p L were taken and added to a small test tube containing a saturated solution of NaJ3COs. The tube was briefly agitated by hand qnd the organic phase analyzed by gas chromatography to give the quantity of unchanged phenyldimethylfluorosilane. The kinetics observed were strictly 0th order in phenyldimol L-' as shown methylfluormilane down to concentrations of in Figure 3 for a few examples. Finally, a few experiments were made with KF and BF, as additives. The experimental conditions and observed rate constants (from a linear regression) are summarized in Table 11. The surface treatment was made with a mixture of (A) trimethyl[ dimethylamino]silane and (B) decyldimethyl[di-
108
ANALYTICAL CHEMISTRY, VOL.
54,
NO.
2,
FEBRUARY 1982
10
0 0
500
1000 min
t
Figure 3. Decrease of the concentration of phenyklimethylfluorosilane (see eq 22) In 18 (41, 21 (O), and 24 (+) mol L-' hydrogen fluoride at 10,20, and 30 O C as a function of the time. For experlmental details see Table I1 and text.
methylaminolsilane. The expected surface concentration was of the order of 4 pmol m-2. A Typical Experiment. In a glass vessel of 5 mL (reflux condenser; lateral introduction closed by septum; magnetic stirrer; N2 atmosphere with
