Langmuir 1988,4, 572-578
572
What is worth noticing is the difference in the transition temperature, although care should be taken in comparing the transition temperature obtained by different methods of characterization. In the case of CdA monomolecular film, the disordering of hydrocarbon tails begins at room temperature, and a breakdown of the cadmium lattice occurs at the melting point of bulk CdA (110 "C). The stability of the fluorocarbon chain up to 60 OC can be ascribed to the large van der Waals radius of the fluorine atom. The rigidity of the fluorocarbon chain due to steric hindrance should suppress the thermal motion of the chain. A more striking difference, compared to the CdA film, is that the second disordering of perfluorodecanoic acid occurs a t a temperature fairly higher than its bulk melting point. This can be attributed to the strong chemical bond formed between the carboxylic group and the substrate. Thermal characteristics of solids, such as the melting point and enthalpy and entropy of fusion, are determined by intra- and intermolecular factors of constituent molecules. In the film, however, the interaction between molecules and the substrate also affects these characteristics. This interaction should be strong when a chemical bond is formed between molecules and the substrate, as in the present case. The fluorine substitution for the hydrogen atoms in the alkyl group should not be a main factor concerning the increase in the interaction. This is
supported by the fact that perfluorodecanoic acid deposited on the substrate without preheating desorbs easily. Consequently,strong interaction between the f i i and the substrate must be derived from the dehydrated, clean substrate and not from the fluorinated compound itself. It is generally thought that a substance can be endowed with novel characteristics when in an ultra-thin-film state. In this article, we have presented a typical example of this potential, a rise in disordering temperature.
Conclusion The order-disorder transition of a vacuum-deposited film was examined by using Penning ionization electron spectroscopy. A monomolecular f i of perfluorocarboxylic acid deposited on a dehydrated SiOz substrate was shown to cover the substrate surface. Cyclic thermal treatment of the film revealed that the molecular arrangement of the film is maintained even at an elevated temperature as long as the head group of the molecule is directly attached to the substrate. The cleanliess of the substrate surface was ascertained to play an important role in the stability of the molecules assembled on it.
Acknowledgment. We are grateful to Dr. Shigeru Masuda for his valuable discussion. Registry No. SOz, 7631-86-9;CF3(CF2)&02H, 335-76-2; CH3(CH2)14CHS,544-76-3.
Effect of Thermal Pretreatment on the Surface Reactivity of Amorphous Silica C. H. Lochmuller* and M. T. Kersey Paul M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706 Received July 27, 1987. In Final Form: November 13, 1987 The effect of thermal pretreatment of an amorphous silica on the distribution and condensation of surface reactive groups has been examined by using elemental carbon analysis in conjunction with steady-state and time-dependent fluorescence spectroscopy. It is found that the condensation of surface silanols is a reversible process up to heat pretreatment temperatures of 800 OC and that above 800 O C the condensation reaction is no longer reversible. Steady-state and time-dependent results indicate that the distribution of a covalently bound probe, approximating a chlorcdimethyloctadecylsilaneligand, is relatively unaffected by thermal surface modification. Finally, the present work suggests that isolated surface silanols are more reactive toward chlorotrimethylsilane than paired silanols.
Introduction Silica gel is the most commonly used stationary phase support in reversed-phase high-performance liquid chromatography (RPHPLC). Its use has prompted a large number of investigations into the effect of thermal pretreatment on the bulk and surface properties of silica.' Heat pretreatment of silica prior to chemical modification has also been examined to discover what effects, if any, are seen in the ultimate chemical separations.2 For example, the separation of basic solutes appears to be de(1) (a) Scott, R. P. w.;Traiman, S. J. Chromatogr. 1980,196,193. (b) Unger, K. K. Porous Silica; Elsevier: Amsterdam, 1979. (c) Belyakova, L. D.; Kiselev, A. V.; Kivaleva, N. V. Anal. Chem. 1964,36, 1617. (d) HalHsz, I.; Martin, K . Angew. Chem. 1978,90,954. (e) Scott, R. P. W. J. Chromatogr. Sci. 1980, 18, 297. (0Lark, K. D.; Unger, K. K. J . Chromutogr. 1986,352, 199. (9) Scott, R. P. W.; Kucera, P. J. Chromatogr. Sci. 1976, 13, 337. (h) Davydov, V. Ya; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday SOC.1964,60, 2254. (2) Mauss, M.; Engelhardt, H. J . Chromatogr. 1986, 371, 235.
pendent, in part, on the distribution of surface-bound silanes and on the activity of the underlying, residual surface silanols. Thermal pretreatment of amorphous silica has been used to deactivate these irregular, heterogeneous surfaces. Scott'g suggested that silica surfaces have three strongly adsorbed surface water layers. Thermogravimetricanalysis of silica indicated a continuous loss of surface adsorbed water and the possible condensation of surface silanols at temperatures exceeding 450 OC. Further examination of amorphous silica by Mauss and Engelhardt2 showed a general decrease in the surface area and pore volumes of silica with increasing pretreatment temperature. Their results suggested that loss of surface area and pore volume upon dehydration of silica surfaces is reversible up to 800
"C. There is general agreement in the literature that two types of surface silanols are accessible for surface modification. Figure 1 depicts the two silanol types: isolated
0743-7463/88/2404-0572$01.5Q~Q0 1988 American Chemical Society
Thermal Pretreatment of Amorphous Si
Langmuir, Vol. 4, No. 3,1988 573 Derivatization Reaction. A. Preparation of Chemically Modified Heat Pretreated Silica. 1. Heat Pretreatment of Silica. Silica gel samples were heated in a Lindberg furnace
A
B
C
Figure 1. Schematic of three silica surface species: (A) isolated silanol, (B) paired or vicinal silanol, and (C) siloxane bridge.
(free), vicinal (paired), and a nonreactive siloxane bridge. Infrared3 and nuclear magnetic resonance4 spectroscopy both have been employed to determine the presence of isolated and paired silanols and indicate a decrease in the paired silanol population with increasing thermal treatment of silica. These techniques, while powerful, are limited in at least two important ways: (a) the spatial distribution of surface silanols before and after thermal pretreatment of silica cannot be determined; (b) these spectroscopic methods probe both chemically accessible and inaccessible silanols, and the extent to which surface silanols are available for reaction with silanes cannot be deduced from the results. Lochmuller et a1.6 introduced steady-state and timedependent luminescence spectroscopy to examine the distribution of surface-bound species. In the present work, the reaction of chlorotrimethylsilane (TMCS) with thermally pretreated silica has been studied, and the same surfaces modified with [3-(3-pyrenyl)propyl]chlorodimethylsilane (3PPS)were examined by using steady-state and time-dependent luminescence spectroscopy. The intent was to determine the effect of dehydration and hydration on silanol spatial distribution and on the surface reactivity of the thermally modified surfaces. Our results indicate that condensation of surface silanols is a reversible process up to 800 "C, that TMCS reacts preferentially with isolated silanols, and that the condensation of surface silanols up to 800 OC has little effect on the average spatial distribution of monofunctional, surface-bound silanes with cross sectional areas (a) on the order of chlorodimethyloctadecyl (C1J ligands. This work is relevant to the study of amorphous silica, the distribution of alkylsilanes on silica surfaces, and to a greater understanding of the role of surface silanol distribution of stationary-phase materials in chemical separations.
Experimental Section Materials. An amorphous silica, Partisil 10, was used as the support (N2surface area 323 m2/g,mean pore diameter 93 A, and
mean particle size 10 fim). Chlorotrimethylsilane (TMCS) and [3-(3-pyrenyl)propyl]chlorodimethylsilane(3PPS) were used in the derivatization of surface silanols. Synthesis of the pyrenylsilane and purification and subsequent hydrosilylation reactions of this silane have been reported.sb Chloroform (Mallinckrodt) distilled over calcium hydride was used in all derivatization reactions. Spectral grade hexane (Mallinckrodt) was used as the contacting solvent in the spectroscopic experiments. (3) (a) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcell Dekker: New York, 1967; Chapter 4. (b) Davydov, V. Ya; Zhuravlev, L. T.; Kiselev, A. V. Russ. J. Phys. Chem. 1964, 38, 1108. (c) Armistead, C. G.; Hockey, J. A. Tram. Faraday SOC.1967,63, 2549. (d) Annistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S.A,; Hockey, J. A. J. Phys. Chem. 1969, 73, 3947. (e) Bush, S. G.; Jorgenson, J. W.; Miller, M. L.; Linton, R. W. J. Chromatogr. 1983,260, 1. (4) (a) Miller, M. L.; Linton, R. W.; Maciel, G.E.; Hawkins, B. L. J. Chromatogr. 1985, 319, 9. (b) Sindorf, D. W.; Maciel, G. E. J . Phys. Chem. 1982,86, 5208. (c) Kahler, J.; Chase, D. B.; Farlee, R. D.; Vega, A. J.; Kirkland, J. J. J . Chromatogr. 1986, 352, 275. ( 5 ) (a) LochmUer, C. H.; Wilder, D. R. J. Chromatogr. Sci. 1979,17, 574. (b) Lochmdler, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983,55, 1344. (c) Lochmaller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J. Am. Chem. SOC.1984, 106, 4077.
(Model51800 series) at temperatures between 300 and loo0 "C. The temperature of the furnace can be set to within f5.0 "C, and temperatures were measured with a calibrated thermocouple (Fisher no. 13-902). Silica samples of 0.5 g were weighed on a Mettler balance and placed in a quartz tube built by GM associatm (Oakland, CA) consisting of a quartz 14/20 inner joint, flame sealed at one end. The sampleswere then placed in the preheated furnace for 24 h. 2. Derivatization of Heat Pretreated Silica. Silica samples heated to a given temperature for 24 h were removed from the furnace and placed directly into the 14/20 neck of a flame-dried, three-neck, round-bottomflask (RBF)which was purged with dry N2 This procedure provided for minimal water absorption. The 800 and 1000 "C samples were first cooled in a desiccator filled with Drierite before being transferred to a RBF. After the silica had time to cool to room temperature in the N2 purged RBF, 20 mL of distilled chloroformwas added to form a silica slurry. The following silanization reactions were carried out with different silanes: a. Derivatization Using Chlorotrimethylsilane. Heat pretreated silicas were reacted with a 10% TMCS (v/v silane/ chloroform) solution. This ensured complete or exhaustive derivatization of the accessible surface silanols. Silica samples were heated at 120, 300, 400, 500, 600, 700,800, and 1000 "C for 24 h prior to derivatization. Each of these experimentswas repeated twice. Three silica samples heated to 120 "C were reacted with TMCS. b. Derivatization Using [3-(3-Pyrenyl)propyl]chlorodimethylsilane. Silica samples were heated to 120,400,600,800, and lo00 "C prior to reaction with the organochlorosilane,3PPS. c. Combined Derivatization of Silica Using Chlorotrimethylsilane and [3-(3-Pyrenyl)propyl]chlorodimethylsilane. Silica samples were dried at 120 "C and reacted in a
two-step reaction sequence using first TMCS and then 3PPS. Silica samples (0.5 g) were heated to 120 "C in a three-neck RBF for 24 h. The RBF containing the silica and stir bar was removed from the oven and immediately purged with N2. The RBF was then equipped with a flame-dried condenser and a 50-mL addition funnel with side arm for N2 inlet. After the mixture cooled to room temperature, 20 mL of dried chloroform was added to the silica and the silica/solvent slurry was stirred at a medium rate. TMCS was placed into the addition funnel and diluted to 5 mL with chloroform. The silane solution was added dropwise to the silica slurry, and then 0.5 mL of dry pyridine was added to the reaction mixture. The reaction was heated to 60.0 "C for 3 h. After 3 h an aliquot of approximately 0.1 g of the partially silylated silica was pipetted from the N2-purgedreaction flask to a buchner funnel and was washed, dried, and analyzed for elemental carbon and hydrogen. 3PPS was then added to the remainingsilica, and the reaction proceeded for 3 h at which point the silica, derivatized with both TMCS and 3PPS, was washed and dried. Aliquots of the TMCS/3PPS-derivatized silica were analyzed for elemental carbon and hydrogen. 3. Boiling of Heat Pretreated Silica. A number of silica samples were heated to temperatures ranging from 300 to 1000 "C for 24 h. These silica samples were then placed in a desiccator and cooled to room temperature. The silica was then placed in a 2WmL RBF containing75 mL of deionized water and equipped with a stir bar and condenser. The slurry was heated to boiling for 90 min and then filtered in a medium-frit glass filter. The filtered silica was dried at 120 "C and then reacted with either TMCS or 3PPS as previouslydiscussed. No differencesin sample handling or solvation by organic and aqueous solvents were observed for heat pretreated and boiled silica samples. Steady-StateLuminescence Studies. Elementalcarbon and hydrogen analyses (M.H.W. Lab., Phoenix, AZ) were obtained on all bonded phases, and these were diluted with underivatized silica to avoid inner-fiiter Derivatized silica sampleswere (6) Avnir, D. J. Am. Chem. SOC.1987,109, 2931.
574 Langmuir, Vol. 4, No. 3, 1988
Lochmiiller and Kersey
Table I. Heat Pretreatment Temperatures,Percent Carbon," Surface Coverage,b and Average Distance between Bound Ligands for Heat-Pretreated and Rehydrated Silica Derivatized with Chlorotrimethylsilane
8 ,
I
surface
tema. "C 120c 300 400 500 600 700 800 1000
coverage,
%
carbon umol/m2 Heat-Pretreated Silica 5.51 5.57 5.29 5.24 4.50 4.47 3.46 1.80
7.43 7.52 7.12 7.05 6.01 5.98 4.57 2.33
D..... &mol 4.73 4.70 4.83 4.85 5.26 5.27 6.03 8.43
Rehydrated Silica 120 300 400 500 600 700
800 1000
5.51 5.44 5.35 5.22 5.34 5.15 5.26 3.24
7.43 7.34 7.21 7.02 7.19 6.92 7.08 4.27
4.73 4.76 4.80 4.86 4.80 4.90 4.84 6.24
Average % carbon values from two derivatization reactions. Surface coverage values calculated by using equations from Unger.lb cAverage % carbon values from three derivatization reac-
tions.
prepared for data acquisition by adding approximately 20 mg of the derivatized silica to 2 mL of solvent. The solvent/silica slurry was then subjected to three cycles of freeze-pump-thaw (FPT) to minimize O2 quenching of the fluorescence signal. The spectroscopy was performed in a cell with geometry as described in previous work.s All steady-state fluorescence spectra were obtained by using a Perkin-Elmer Model MPF-66 spectrophotometer. Emission spectra were collected from 360 to 530 nm at an excitationwavelength of 315 nm. Excitation spectra were collected from 270 to 370 nm at emission wavelengths of 390 (monomer) and 480 nm (excimer). Luminescence data were collected at 22 "C. Time-Dependent Luminescence Studies. A Photochemical Research Association 2000-ns-lifetimeinstrument was used for time-dependent data acquisition. This instrumentation, data collection, and numerical analysis have been discussedqkData were acquired at an excitation wavelength of 315 nm and an emission wavelength of 390 nm (monomer emission). An average instrument response of 3.8 ns (fwhm)was recorded by using 0.5 atm of N2 (National Specialty Gases) in the flashlamp.
Results and Discussion This section is divided into three parts. Results on the effect of silica heat pretreatment and the reversible nature of this condensation reaction are reported in the first part. The reaction of TMCS with surface silanols was used to provide a quantitative measurement for the total number of accessible surface silanols after heat pretreatment. The reaction of TMCS with heat pretreated and rehydrated silica samples was also used to provide a quantitative value of the reversibility of this surface condensation reaction. The next two parts discuss the use of steady-state and time-dependent fluorescence spectroscopy, respectively, to study the effect of silica heat pretreatment on the distribution of surface silanols and bonded ligands. The ligand 3PPS was employed as a surface-bound probe. In the part on steady-state fluorescence spectroscopy, the steady-state excimer and monomer fluorescence intensities (7) Parker, C. A.; Rees, W. T. Analyst (London) 1962,87, 83. (8) Lochmtiller, C. H.; Kersey, M. T. Anal. Chim. Acta 1987, 200,
143-150.
0
200
400
600
800
1000
1200
T (C)
Figure 2. Plot of TMCS surface coverage vs heat pretreatment temperature of silica. Curve A represents dehydrated silica;curve B represents silica which has been dehydrated and rehydrated.
Figure 3. Condensation reaction of paired silanols induced by thermal treatment producing a siloxane bridge and water. were measured as a function of surface coverage and heat pretreatment temperature. The last part discusses the use of time-dependent fluorescence spectroscopy to provide a more quantitative indication of the distribution of surface silanols as a function of heat pretreatment temperature. Effect of Temperature Pretreatment on Silica Derivatization. The condensation of surface silanols was examined by reacting TMCS with remaining, accessible surface silanols of heat-pretreated silica. Amorphous silica was derivatized with TMCS to insure maximum or exhaustive silanol derivatization. Prior to reaction with TMCS, the silica was heated for 24 h at temperatures ranging from 120 to 1000 OC f 5 "C. Table I lists the percent carbon, surface coverage (pmol/m2),and average distance between trimethylsilane ligands for heat-pretreated silica. Mauss and Engelhardt2 suggest that boiling heat-pretreated silica for 90 min in HzO should rehydrate the silica surface. This was tested in the present work by heating silica for 24 h at temperatures between 120 and 1000 "C and then boiling these samples for 90 min in HzO. The bottom half of Table I lists the percent carbon, surface coverage, and average distance between trimethylsilane ligands for silica samples which underwent a dehydration, hydration, and derivatization process. The reported values are an average of two reactions. Results of the 120 "C bonded phases are the mean of three reactions, and a ca. 1.3% RSD for the percent carbon data was obtained. Figure 2 plots the surface coverage of TMCS ligands vs heat pretreatment temperature and clearly shows that the number of accessible surface silanols decreases with increasing heat pretreatment temperature. The total number of silanols which can be reacted with TMCS decreases from 7.43 to 2.33 pmol/m2. The deactivation of silica surfaces is caused by the condensation of two surface silanols forming a siloxane bridge and waterlb and is depicted in Figure 3. Explicit in Figure 3 is the distance between surface silanols. Condensation may occur if the distance between silanols is within 3.1 A, a maximum distance for hydrogen bonding.8 In addition, the condensation of surface silanols appears to be a reversible process up to pretreatment temperatures of 800 OC. The
Thermal Pretreatment of Amorphous Si
Langmuir, Vol. 4 , No. 3, 1988 575
Table 11. Percent Carbon, Surface Coverage, and Average Distance between Bound Ligands for Monofunctional 3PPS Phases as a Function of Heat Pretreatment Temperature % carbon surface coverage' D.-b 1.11 1.49 3.11 3.76 5.56 9.76
120 O C 0.15 0.21 0.44 0.54 0.81 1.48 400
33.0 28.3 19.4 17.6 14.3 10.6
oc
2.10 2.34 2.48 3.97 5.26 5.66 10.79
0.29 0.32 0.35 0.57 0.76 0.82 1.66
2.49 3.05 3.35 7.92 10.35 12.25
600 O C 0.35 0.43 0.47 1.20 1.58 1.92
1.73 2.72 3.77 5.71 7.09
800 "C 0.24 0.38 0.54 0.83 1.05
23.8 22.7 21.8 17.1 14.8 14.2 10.0 21.8 19.6 18.8 11.9 10.2 9.3 26.2 20.8 17.6 14.1 12.6
lo00 "C 4.44 4.49 7.46 7.87 8.20
0.64 0.66 1.11 1.17 1.23
16.14 16.05 12.25 11.90 11.64
4.30e
0.62
16.4
'Surface coverage values are in units of pmol/m2. bDexpvalues are in units of A/molecule. CThissample was heated to 1000 'C, boiled, and then reacted with silane. observed reversibility may arise from the probable distribution in siloxane bond strengths formed by the condensation of paired, surface silanols. Plot B in Figure 2 illustrates the extent of the reversibility of this condensation reaction. It is evident that the number of surface silanols present before heating above 120 "C is replenished by the boiling procedure. Silica samples heated to 1000 "C and then reacted with TMCS do not exhibit the same degree of reversibility as samples heated to 800 "C. The siloxane bridges formed at 1000 "C appear to be energetically more stable than those formed a t lower temperatures. Our results, in conjunction with those reported by Mauss and Engelhardt,2 support the idea of sintering and possible structural rearrangement of amorphous silica to a more ordered, e.g., quartz, tridymite or ~ristobalite~ crystal structure at loo0 "C. The possible rearrangement of silicon and oxygen atoms at lo00 O C would be more resistant to change under the hydration conditions described than the breaking of strained siloxane bridges. Steady-StateFluorescence Spectroscopy of Silica Chemically Modified with 3PPS. The TMCS elemental analysis data verified that condensation of surface silanols is a reversible process up to 800 "C. To date, no results (9)
Snyder, L. R.; Ward, J. W. J. Phys. Chem. 1966, 70,3941.
Table 111. Surface Coverage, Dilution Factor, and Steady-State Excimer/Monomer Ratio for Heat-Pretreated 3PPS Bonded Phases
-
surface coverage'
dilution
EX/MON
0.15 0.21 0.44 0.54 0.81 1.48
120 o c 1/20 1/20 1/20 1/20 1/1000 1/1OOo
0.15 0.22 0.67 1.20 2.80 3.90
0.29 0.32 0.35 0.57 0.76 0.82 1.66
400 O C 1/20 1/20 1/20 1/20 1/1000 1/1000 1/1000
0.16 0.23 0.34 0.72 1.40 2.10 3.56
0.35 0.43 0.47 1.20 1.58 1.92
600 O C 1/20 1/20 1/20 1/1000 1/1000 1/1000
0.74 1.62 1.76 3.46 4.00 5.14
0.24 0.38 0.54 0.83 1.05
800 O C 1/20 1/20 1/20 1/1000 1/1000
0.39 1.21 2.05 2.31 2.83
0.64 0.65 1.11 1.17 1.23
1000 O C 1/20 1/20 1/1000 1/1000 1/1000
10.29 9.67 14.33 13.75 9.33
0.62b
1/20
7.07
nSurface coverage values are in pmol/m2. bThis sample was heated to lo00 OC, boiled, and reacted with silane. have been reported which examine the effect dehydration and hydration have on the spatial distribution of surface silanols. In the present work, steady-state fluorescence spectroscopy was employed to examine the effect of silica heat pretreatment and rehydration of heat-pretreated silicas on the distribution of accessible surface silanols. Silica gel samples were heated to temperatures of 120, 400,600,800, and 1000 "C for 24 h in the furnace before reaction with 3PPS. Surface coverages of the pyrene ligands range from low to near saturation surface coverage. Table I1 lists the percent carbon, surface coverage (bmol/m2),and "average distance" between 3PPS ligands calculated by the method of Ungerlb for these modified surfaces. Excimer/monomer ratios were determined as a function of silica pretreatment temperature and surface coverage. The dilution factor, surface coverages, and the steady-state excimer/monomer ratios determined from emission spectra of these heat-pretreated silica samples are listed in Table 111. A 4 5 % RSD of the excimer/ monomer ratio was calculated for these thermally and chemically modified silicas studied. Figure 4 is a plot of the data listed in Table I11 showing that little change occurs in the excimer/monomer ratios with increasing silica pretreatment temperature. One might expect the excimer/monomer ratio to decrease with increasing heat pretreatment temperature because fewer surface silanols are available for reaction with the silane. TMCS data indicate a loss of accessible surface silanols, due presumably to silanol condensation in which siloxane
576 Langmuir, Vol. 4, No. 3, 1988
Lochmiiller and Kersey
Table IV. Lifetimes ( T ~ and ) Normalized Preexponential Factors (Ai)as a Function of Heat Pretreatment Temperature for SPPS Bonded Phasesn rmol/m2 temp, O C A1 A2 A3 71 72 73 0.54 0.57 0.47 0.54 0.64
120 400 600 800 1000
0.572 0.517 0.631 0.515 0.842
0.263 0.262 0.250 0.439 0.095
0.164 0.221 0.119 0.046 0.062
15.7 3.3 10.3 10.3 6.5
39.1 19.8 39.6 33.4 24.7
77.6 58.7 126.6 137.9 148.8
"The contact solvent is hexane.
. 0
X
X
A A
"
,
0 0
'
a
0 4
'
,
I
0 8
1 2
.
,
1 6
2 0
pmolesd
Figure 4. Steady-state excimer/monomer ratios vs surface coverage for silica samples heated to 120 O C (a),400 O C (A),600 "C (a),and 800 "C (X).
bridge formation lowers the total number of silanols available for reaction. Dehydration of the surface should increase the distance between remaining surface silanols, and the distance between neighboring, covalently-bound, 3PPS ligands should increase, decreasing the probability of excimer formation and lowering the excimer/monomer ratio. This is not observed, The larger molecular dimensions of bound pyrene ligands, compared with TMCS, may enable pyrene molecules to interact with other pyrene molecules by spanning the distance between underlying silanols or product siloxanes. The volume swept by a bound silane depends, in part, on the constraints on its motion created by the fact that it is bound, the chain length alkyl ligand, and the size of the aromatic ring at the terminus. Berendsen and de GalanQ have calculated the molecular dimensions and area swept out by TMCS and phenylsilane ligands bound to amorphous silica and indicated that TMCS ligands occupy less surface area per ligand than phenylsilane ligands. They also showed that residual, unreacted surface silanols are shielded from reacting further with other silanes by already attached groups. This "screening" of residual silanols lowers the total number of accessible surface silanols, which decreases the total ligand surface coverage and increases the average distance between bound ligand attachment pointa. For example, TMCS and 3PPS bonded phases at saturation surface coverages should have different interligand anchor-point distances. The average distance in the case of TMCS is 4.73 A whereas for 3PPS this distance is more likely 10.6 A. Further comparisons can be made by examining the TMCS data in Table I and the SPPS data in Table II. The equationslbused in these calculations only approximate the irregular surface of amorphous silica; however, on a scale corresponding to the ligand's microenvironment, comparisons between the two phases bound to the same silica can be made. The present work and that by Unger et a1.I0 (which employed chlorodimethyloctyl-
silane (CB)bonded phases) further illustrate the need to consider the molecular dimensions of covalently bound monofunctional silanes when considering the effects of silica heat pretreatment on surface silanols and how that effects bonded-phase ligand distribution. In the work of Unger et al.,l0 silica was heated to temperatures between 127 and 627 "C and then exhaustively reacted with Cs. The total number of accessible surface silanols was determined independently by a NMR method involving the isotopic exchange of deuteriated trifluoroacetic acid with surface si1anols.l' The results indicated a 35% decrease in the total number of surface silanols over the temperature range 127-627 "C; however, the number of silanols reacted with C8 ligands decreased only 13% over the same temperature range. An analogous result was observed in the present work. The TMCS data showed a 38% decrease in the number of accessible surface silanols from 120 to 800 "C; however, over the same temperature range, the steady-state luminescence data of 3PPS bonded phases changed little, which indicates no significant difference in the ligand distribution with thermal pretreatment. A final examination of Table I11 shows that the loo0 "C bonded phases are not distributed about the 120 "C data but rather have much greater excimer/monomer ratios for similar surface coverages. Earlier work2 and the present TMCS data obtained in our laboratory indicate that substantial changes of the silica's bulk and surface properties occur when silica is heated to 1000 "C. The 1000 "C bonded-phase steady-state luminescence data suggest a surface comprised of regions with high ligand density. This might occur if, at higher temperatures, the silica cracks-a process in which siloxane bridges break. The broken bonds would form surface silanolswhen exposed to trace amounts of water, thereby forming regions of high local silanol density. Greater silanol density would be indicated by higher excimer formation, as observed. Time-Dependent Fluorescence Spectroscopy of Chemically Modified 3PPS Silica. An examination of monofunctional 3PPS bonded phases using time-dependent luminescence was initiated to further examine the distribution and organization of 3PPS bound to thermally pretreated silica. Time-dependent luminescence results offer additional data on the effect of dehydration and hydration on surface silanol condensation and formation. Time-dependent fluorescence spectroscopy was used to determine the average lifetimes and mean population distributions for five 3PPS bonded phases. The five pyrene bonded phases were pretreated at different temperatures but have comparable silane surface coverages. A 10% RSD in 3PPS surface coverage was calculated for the five samples. Average lifetimes and normalized preexponential factors (NPF) were determined for each of the samples in contact with hexane. The lifetimes and NPF for these surfaces are listed in Table IV. (10)Berendsen, G. E.; de Galan, L. J.Liq. Chromatogr. 1978,1,403. (11)Unger, K.K.;Lork,K. D.;Kinkel, J. N.J.Chromatogr. 1986,352, 199.
Thermal Pretreatment of Amorphous Si
Langmuir, Vol. 4, No. 3, 1988 577 Table V. Total Percent Carbon, Percent Carbon Due to TMCS and Pyrene Surface Bound to Silica, Surface Coverage (pmol/m*) of TMCS and Pyrene, and the Excimer/Monomer Ratio for TMCS/3PPS Bonded Phases total % carbon
(PY)
iuGace coverage (MTCS)
4.49 4.78 5.54 5.15 4.29 5.50 6.68
0.14 (3.45)o 0.22 (3.20) 0.34 (3.10) 0.41 (2.27) 0.47 (1.00) 0.67 (0.85) 0.87 (0.75)
4.56 (1.04) 4.21 (1.58) 4.08 (2.44) 2.96 (2.88) 1.29 (3.29) 1.09 (4.65) 0.96 (5.93)
surface coverage
0 00
I
0
200
.
,
400
'
l
600
'
I
800
'
I
1000
'
1200
T (C)
Figure 5. Normalized preexponential factors plotted vs heat pretreatment temperature for five silicas which have the same silane surface coverage: ( 0 )AI, (X) A2, and (H) Aa. The TMCS work showed that the total number of surface silanols decreased with increasing silica pretreatment temperature. This suggests a change in the distribution of the silanols, presumably increasing the number of isolated silanols. Time-dependent luminescence spectroscopy can provide an indication of silane population distributionSk Figure 5 plots the NF'F vs treatment temperature for the five BPPS bonded phases. The NPFs in Figure 5 show little change in the three population distributions from 120 to 800 OC. The same lack of change with increasing silica thermal pretreatment was also observed for the steady-state luminescence data. The NPFs A2 and A, may represent isolated and paired silanols, respectively.& It is interesting to note that the A2 population exhibited by these heat-treated silica samples comprises approximately the same percentage (25%) of sites as reported in earlier work for normally activated silicas.sb*c Like the steady-state data, the NPFs for the 1000 "C phase show considerable change. The NF'F A,-the mean of pyrenylsilanes forming excimers-increases substantially a t 1000 "C. This increase is offset by a decrease in the NPFs A2 and A,. The time-dependent luminescenceresulta are consistent with the steady-state luminescence results. Heat pretreatment of silica reduces the total number of accessible surface silanols; however, the condensation of surface groups appears to have little consequence on the distribution of larger molecular weight silanes. This was further investigated by time-dependent luminescence spectroscopy. Silica samples heated to 400, 600, and 800 "C, rehydrated, and reacted with 3PPS showed no difference in the NPFs AI, A2, and A, when compared with the five dehydrated 3PPS samples. These results, like the steady-state and time-dependent resulta of the heat-treated samples, indicate that thermal pretreatment of silica or subsequent rehydration has little effect on the distribution of n-alkylorganochlorosilanes.Throughout the time-dependent studies an isolated pyrene population, A2, was observed. Although there is debate12over the location of regions in which isolated silanols and/or bonded ligands could reside, it is apparent that they are present and are relatively unaffected by increasing surface coverage or by thermal modification of the surface. To better understand the effect of surface deactivation on the distribution of silanes with molecular sizes ap(12) Holik, M.; MatejkovH, B. J. Chromatogr. 1981,213, 33.
EX/MON 0.28 0.10 2.85 5.87 4.88 8.03 7.50
'Values in parentheses represent the percent carbon data determined by elemental carbon analysis.
proximating chlorodimethyloctadecylsilane(Cl& silica was heated to 120 "C and then reacted with a combination of TMCS and 3PPS (TMCS/3PPS). The partial deactivation of surface silanols with TMCS is an analogous situation to the condensation of surface silanols due to thermal pretreatment. The relative distribution of covalently bound BPPS ligands was determined by steady-state excimer/monomer ratios as a function of 3PPS surface coverage. The bonded phases synthesized in these experiments were made by first reacting limited quantities of TMCS with silica and then reacting 3PPS with any unreacted, accessible surface silanols. With this procedure, a range of 0-50% of the accessible (to TMCS) surface silanols was reacted. The degree of surface silanol deactivation with TMCS was determined by assuming a total of 7.3 pmol/m2 of reactive surface silanols. For example, if 3.5 pmol/m2 of surface silanols was derivatized by TMCS ligands, then 50% of the surface would have been deactivated prior to reaction with 3PPS. The degree of silica derivatization was determined by elemental carbon analysis. Table V lists the percent carbon, surface coverage, and the steady-state excimer/monomer ratio for these bonded phases. As in the previous section, the TMCS/3PPS phases were diluted with underivatized silica. The dilution factor, 1/20 or 1/1000, was based on the percent carbon due to the BPPS ligands. The amount of 3PPS bound to the surface was calculated by subtracting the percent carbon value due to the TMCS ligands from the total percent carbon value due to the TMCS/3PPS silane reaction. Examination of the excimer/monomer data in Table V and those in Table 111shows that the TMCS/3PPS phases have considerably higher excimer/monomer values for similar surface coverages than the 3PPS phases. Partial deactivation of the silica surface with TMCS appears to enhance the clustering of 3PPS ligands into regions of higher local surface density than observed for surfaces reacted only with 3PPS. Although there is debate in the 1 i t e r a t ~ r e ' ~over ~ ~the ~ ~reactivity ~ ~ ~ J of isolated and paired silanols, evidence which indicates that TMCS reacts preferentially with isolated or free silanols rather than with vicinal or paired silanols e ~ i s t s . l h ~Kinetic3b ~9~ data indicate that TMCS ligands react more rapidly with isolated silanols. ThermodynamiclhJ3data also favor the reaction of TMCS with isolated silanols. The preferential reaction of TMCS with isolated silanols would leave a greater number of paired silanols available for reaction with 3PPS. Since TMCS is added in limited quantities, under the described reaction conditions, it is reasonable to assume (13) Hertl, W.; Hair, M. L.J.Phys. Chem. 1971, 75, 2181.
578
Langmuir 1988,4, 578-579
that TMCS ligands react with isolated silanols to a greater degree than with paired silanols. Therefore, with the addition of 3PPS, a majority of the isolated silanols have been deactivated and the 3PPS ligands must cluster into regions of high local density. If the present work is indicative of the distribution of C1, ligands, it appears that thermal pretreatment of silica does not change the overall distribution of C18 monofunctional silane ligands. The effect of heat pretreatment may be more pronounced when considering the deactivation of surfaces when highly active solutes, e.g., basic solutes, are separated rather than altering the distribution of surface bound ligands. Although an analogous surface-bound probe is not readily available to mimic C8 and chlorodimethylbutylsilane(C,) bonded phases, the smaller molecular size of these silanes would indicate that thermal pretreatment of the silica surface should affect the n-alkyl
chain distribution and surface coverage to a greater degree than C18phases. The deactivation of silica surfaces toward solutes, which may interact with residual silanols by partially reacting the surface with TMCS and then with C18 ligands, as demonstrated by Lochmuller and Marshall,14may be a more reproducible method to make less reactive, more homogeneous chromatographic surfaces.
Acknowledgment. This work was supported, in part, by a grant from the National Science Foundation, Grant NO. CHE85-00658 (to C.H.L.). Registry No. TMCS, 75-77-4; 3PPS, 86278-57-1; Si02, 7631-86-9. (14)Lochmuller, C. H.; Marshall, D. B. Anal. Chim. Acta 1982,142, 63.
A Simple Method for the Production of Controlled Electroplated Designs on a Metal Surface Ewa S. Kirkor, Donald E. David, Thomas F. Magnera, and Josef Michl* Center for Structure and Reactivity, Department of Chemistry, The University of Texas, Austin, Texas 78712-1167 Received September 14, 1987. In Final Form: November 19, 1987 A procedure for the imaging of an adsorbate macroscopic island on a metal surface covered elsewhere by a single molecular layer of another adsorbate has been developed. Carbon monoxide and iodine were utilized as the adsorbents. The latent image of the islands was created by laser ablation of the original adsorbate (carbon monoxide or iodine) from the illuminated part of the surface in the presence of the vapor of the other adsorbate (iodine or carbon monoxide, respectively). The image can be developed by silver electroplating.
Introduction We report a simple procedure for the production of electrodeposited images on a metal surface. Macroscopic islands of an adsorbate (A) in the layer of another adsorbate (B) were produced by first covering all of the surface with B and then desorbing it in the atmosphere of the adsorbate A by a laser beam passing through a mask defining a spatial pattern. Adsorbates A and B need to adsorb to the support surface strongly enough to prevent exchange with other potential adsorbates and to prevent mutual scrambling before the image is developed by selective electroplating. The latter is induced by choosing one of the adsorbates so that it passivates the surface with respect to metal deposition and the other adsorbate so that it does not interfere with metal deposition. For initial demonstration, we have chosen carbon monoxide and iodine for adsorbates A and B, platinum as the primary metal surface, and silver for the plating metal. Iodine and carbon monoxide are known to interact strongly with many metal surfaces, and each one has the ability to hinder the adsorption of certain other adsorbates.'$ Silver is known to electroplate on a surface covered with iodine but does not deposit on a surface covered with C0.3 It is known that iodine or CO adsorbates on platinum are
stable at silver electrodeposition potentials in oxygen-free ele~trolytes.~,~ It is also known that an iodine adsorbate on platinum can be exposed to AgClO, electrolyte without damage although restructuring of the adsorbed layer occurs during ele~troplating.~-'
Results and Discussion Two ways of preparing the images were tested, in which either CO or iodine was utilized as an initial adsorbate. The desorption from the irradiated area was performed in the atmosphere of the other adsorbate in order to stabilize the image immediately. The latent image was then developed by silver plating (Figure 1). The irradiation resulted in a desorption as demonstrated by the observation of a short-lived RGA signal of the initial adsorbate. The signal intensity was the highest after the first laser flash and decreased to zero after the fifth. The nature of the adsorbates at various stages of the process was characterized by XPS spectroscopy. Since the spatial resolution of the available spectrometer was low, we have formed only one large island on the surface in this series of experiments and did not attempt to follow the (4)Breiter, M. W. J. Electroanal. Chem. Interfacial Electrochem. 1981, 127, 157 and references therein.
(1) Katekaru, J. Y.; Garwood, G. A., Jr.; Hershberger, J. F.; Hubbard, A. T. Surf. Sci. 1982, 121, 396. (2) Sherman, M. G.; Kingsley, J. R.; Dahlgren, D. A.; Hemminger, J. C.; McIver, R. T., Jr. Surf. Sci. 1985, 149(1), L-25. (3) Hubbard, A. T., personal communication.
(5) Stickney, J. L.; Rosasco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Surf. Sci. 1983, 130, 326. (6) Salaita, G. N.; Lu, F.; Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem. Interfacial Electrochem. 1987, 229, 1. (7) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem. Interfacial Electrochem. 1987, 222, 305.
0743-7463/88/2404-0578~01.~0l0 0 1988 American Chemical Societv
