J. Phys. Chem. B 2001, 105, 173-181
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Thermal Stability of Octadecylsilane Monolayers on Silica: Curvature and Free Volume Effects Rongwei Wang† and Stephanie L. Wunder* Department of Chemistry 016-00, Temple UniVersity, Philadelphia, PennsylVania 19122 ReceiVed: July 5, 2000; In Final Form: October 11, 2000
The thermal stability and dynamic behavior at elevated temperature (120 °C) of octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on fumed silicas [Aerosil 380(A380), 7 nm primary particle size, and Aerosil OX50 (OX50), 40 nm primary particle size] and nonporous silica beads (106-nm) were investigated using vibrational spectroscopy and calorimetry. In the case of well-ordered SAMs, formed by OTS adsorption on “superhydrated” silica, thermal disordering of the alkyl chains increased with increased surface curvature and thus increased free volume. A melt endotherm was observed only for the high curvature A380 fumed silica. In-situ Raman data at 120 °C supported these results and showed increased conformational disorder and mobility for A380 compared with OX50. The conformational structure of OTS SAMs was found to be largely reversible up to 200 °C, except for “superhydrated” A380. This suggests that, for close-packed SAMs, silanol condensation only occurs when the resulting Si-O-Si bond does not interfere with the hydrophobic interactions of the alkyl chains; the Si-O-Si bond distance is smaller than the van der Waals distance between alkyl chains. In the case of “superhydrated” A380, the increased free volume available due to the high surface curvature, as well as the void space left in the interfacial region after evaporation of the adsorbed water, resulted in some unfavorable cross-linking at high temperature and irreversible disordering of the chains. When triethylamine (TEA) was added to already-close-packed SAMs, in the presence of excess OTS, no further OTS adsorption occurred, and the conformational order remained high. For lower coverage fumed silica, the addition of TEA resulted in increased OTS coverage at the expense of chain order.
Introduction Well-organized monolayers of long chain alkyl molecules on different solid substrates have been extensively studied for nearly twenty years.1-5 In particular, OTS SAMs on silica surfaces have attracted much attention because of their similarity to biological membranes and for potential material applications such as improved friction and wear resistance of surfaces.6-8 The driving force for the spontaneous formation of OTS SAMs includes chemical bonding of the headgroup of OTS molecules with the surface and hydrophobic intermolecular interactions of the alkyl chains.9,10 It has been emphasized11,12 that the headgroup immobilization plays an important role in the thermal stabilization of monolayer structures. There are two factors that contribute to headgroup immobilizationscovalent bonding in the intralayer and layer-to-substrate bonding. Angst et al.13 pointed out, and Brzoska et al.14 later confirmed, that OTS adsorption on either hydrated or dry oxide silicon wafers (giving complete or incomplete OTS coverage, respectively) increased the amount of water adsorbed on the surface compared with the bare silica. They further claimed that water adsorption took place at the organosilane-oxide interface and attributed their results to the fact that the OTS layer itself had silanol groups present that provide sites for water adsorption. This suggested that the condensation between alkyl silanols on the surface was incomplete. Thus, after curing their samples at 150 °C, they found a reduction in water adsorption by the OTS * Author to whom correspondence should be addressed. Fax: 215-2041532. E-mail:
[email protected]. † Current Address: Department of Polymers, Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, P. R. China.
layers, which was attributed to cross-linking and reduction in the number of silanols present. Although thermal curing has been suggested to improve the quality of OTS SAMs,13 the word, “quality” has referred to macroscopically observed physical properties of OTS monolayers such as wettability and water adsorption, and was not directly connected to molecular characteristics such as chain packing and conformation. The packing and conformational order of alkyl chains in OTS SAMs depends on the underlying structure of the surface silanols and adsorbed water.5,15 Due to a lack of direct microscopic evidence, the effect of the curing process on the structure of OTS SAMs on silica substrates has not been unambiguously determined. Tripp and Hair,16 using FTIR, showed that no chemical attachment of OTS to the silica surface (i.e., Sis-O-Si bond formation, where Sis is a surface silicon atom) occurs at room temperature. However, the cross-linking reaction (condensation between adjacent silanols with formation of a Si-O-Si bond) of OTS has been shown to occur at room temperature using FTIR5 and 29Si CP/MS NMR spectroscopy.17 Curing, at temperatures from 120 to 200 °C,16,18,19 has been used to promote further condensation reactions. Tripp and Hair,18 using differences in the IR frequencies between the Si-O-Si and Sis -OSi vibrations, concluded that the majority of the hightemperature condensation reactions of hydrolyzed trichloromethylsilanes were the result of cross-linking. Our previous studies20 have shown that additional cross-linking reactions in polyoctadecylsiloxane (POS) occur at 170-180 °C,21 consistent with the temperature used by Tripp and Hair. Addition of amines, such as triethylamine (TEA), is sometimes used to improve coverage and attachment of OTS to silica substrates.22-24
10.1021/jp002404j CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000
174 J. Phys. Chem. B, Vol. 105, No. 1, 2001 Using solid-state NMR and FTIR, Gao et al.25 studied the thermal behavior of OTS SAMs on fumed silica at low surface coverage (5 times and kept under a positive pressure of argon. The reaction mixture was stirred for 4 h at a temperature of 25 ( 2 °C and transferred to centrifuge tubes in the glovebox. The stoppered tubes were centrifuged outside the glovebox and moved back to the glovebox to decant the supernate. This procedure was repeated 4 times until no OTS-based products were detected in the supernate. The fumed silica samples were further rinsed with pentane in a Bu¨chner funnel and finally dried under reduced pressure at room temperature overnight. SAMs were also prepared with TEA after excess OTS was removed, and with TEA added before removal of excess OTS. In the latter case, the silica/OTS solutions were kept at 25 ( 2 °C for 4 h before the addition of TEA. The solution was held for an additional 0.5 h. It was then centrifuged and rinsed with pentane and methanol; the latter was used to remove the TEA salt. Finally, samples were dried under reduced pressure at room temperature overnight. Raman Spectroscopy. Raman spectra were collected using a computer-controlled double monochromator (Spex 1403) with 1800 grooves/mm grating and a thermoelectrically cooled
Thermal Stability of Octadecylsilane on Silica
J. Phys. Chem. B, Vol. 105, No. 1, 2001 175
TABLE 1: Weight Loss from TGA Analysis in the Temperature Region from 100 to 800 °C A380
OX50 “superhydrated”
sample water density H2O/nm2 weight loss % surface coverage of OTS %c
“dehydrated”
“as is”
17
4.7 21.1 34
8.7 32.8 61
“dehydrated”
TEAa
TEAb
8.7 33.8 64
4.6 46
“as is”
“superhydrated”
106-nm beads
TEAa
TEAb
“superhydrated”
4.0 7.1 73
7.4 76
∼65 5.1 93
TEAb 2.3 3.2 32
5.5 55
4.0 7.0 71
a The samples were treated with TEA after rinsing. b The samples were treated with TEA before rinsing. c The theoretical mass of adsorbed OTS per unit area for 100% monolayer coverage is calculated to be 2.1 mg/m2 assuming a maximum value of 5 adsorbed OTS molecules/nm2.
photomultiplier tube. The samples were excited with ∼30 mW of the 514.5 nm line of a Coherent Innova 90 argon-ion laser. All of the spectra were collected at 2 cm-1 steps with 1-s integration interval, and in backscattering geometry with an 80× microscope objective, 5 cm-1 resolution, and signals from >20 scans were averaged. FTIR Spectroscopy. FTIR spectra were recorded with 10 scans and a resolution of 1 cm-1 using a Mattson Research Series FTIR spectrometer. The sample compartment was purged with dry air, and spectra were referenced to a background dry air spectrum. Measurements were performed at room temperature with KBr pellets. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). TGA measurements were performed on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer using a ramp rate of 10 °C/min. For weight-loss analysis, the samples were held at 100 °C for 0.5 h and the temperature then was raised at a rate of 10 °C/min up to 800 °C. For isothermal treatment, the samples were held at 150 or 200 °C for 1 h. DSC measurements were performed on a TA Instruments DSC 2920 differential scanning calorimeter using a heating rate of 10 °C/min. Results Thermal Analysis. The results from TGA analysis for samples used here and reported previously are summarized in Table 1. Water density was measured before the preparation of the OTS SAMs. The amount of physisorbed water on A380 under both hydration conditions was higher than that on OX50. In the absence of added TEA, OTS coverage on both “as is” silica surfaces were comparable, but the surface coverage on OX50 was higher than that on A380 in the case of “superhydrated” fumed silica. These differences in OTS coverage have been attributed to differences in the effective surface area (larger for OX50) and mobile water content (larger for A380) on the particles.32 The highest water density and surface coverage of OTS were observed for the 106-nm beads. This can be attributed to the nonaggregated/agglomerated nature of these particles as well as to their high water content. When TEA was added to the silica after excess OTS had been removed, the surface coverages were the same, as expected. When TEA was added after the silica/OTS solutions had been held for 4 h but before excess OTS had been removed, the amount of additional OTS attached to the surface depended on the conditions of the underlying substrate. Large increases in OTS coverage were observed for “dehydrated” and “as is” fumed silica, whereas there was only a very slight increase for “superhydrated” OX50. DSC scans of SAMs on “superhydrated” OX50 and A380 are shown in Figure 1. Only the latter sample exhibited a melting endotherm at 70 °C. DSC traces for POS, presented previously26 also exhibited a melting endotherm, at 63 °C, but DSC traces for all other samples were featureless. Effect of Curing Temperature on OTS Structure. The peak positions of the symmetric, d+, and antisymmetric, d-, CH2
Figure 1. TGA plots of OTS on “superhydrated” (a) OX50 and (b) A380.
stretching modes in the IR spectra of long alkyl chains depend on the state of conformational order. Frequencies in the range of 2846-2850 and 2915-2920 cm-1, for d+ and d-, respectively, have been reported for all-trans extended chains and shift upward to ∼2856 and ∼2928 cm-1 for liquidlike disordered chains.36,37 The IR data presented in Table 2 compare results obtained for POS20 with the results for silanated fumed silica. The frequencies of the d+ and d- modes for the “as prepared” (i.e., not thermally cured) POS sample indicate that the alkyl chains are predominantly in the all-trans, extended chain conformation. The OTS chains on the initial “superhydrated” A380 and OX50 samples also adopt a mainly all-trans conformation, whereas for the “as is” samples the chains appear conformationally disordered. The IR data in Table 2 indicates that the 150 °C treatment did not result in frequency shifts of the stretching modes except for a 1 cm-1 upward shift of the d+ mode for “superhydrated” OX50. Thermal treatment at 200 °C induced increases of 1-2 cm-1 for all the samples. From the results in Table 2, differences in thermal stability between the Aerosil samples are difficult to distinguish. Raman spectra in the CH2 stretching (2800-3000 cm-1), C-C skeletal stretching (1000-1150 cm-1), and CH2 twisting/ bending (1250-1500 cm-1) regions are shown in Figures 2-4, respectively, for “superhydrated” A380, “superhydrated” OX50, and POS as a function of curing temperature. The CH2 stretching region consists of a sharp d- antisymmetric stretching mode as well as the symmetric d+ stretching mode; the latter interacts via Fermi resonance with overtones of the bending mode so that three broad features centered at 2850, 2900, and 2930 cm-1 are observed. The frequency of the d- mode did not shift as a function of thermal treatment for any of the samples, indicating a predominance of all-trans chains. However, it would be difficult to detect a small population of disordered chains with a d- mode shifted up in frequency. The change in the d+ mode as a result of chain disordering is more easily detectable. The intensity distribution shifts to higher frequency and is most easily observed as an increase in the 2930 cm-1 band at the expense
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TABLE 2: The Effect of Curing Temperature on IR CH2 Stretching Bands of POS and OTS Adsorbed on A380 and OX50 temperature, °C initial sample 150 200
“as is” OX50 sample d+ cm-1 d- cm-1 2855
2926
“as is” A380 sample d+ cm-1 d- cm-1 2853
2923
POS d+ cm-1
d- cm-1
2850 2850 2851
2919 2919 2921
“superhydrated” OX50 sample d+ cm-1 d- cm-1 2851 2852 2852
2921 2921 2923
“superhydrated” A380 sample d+ cm-1 d- cm-1 2852 2852 2853
2922 2922 2924
d+: CH2 symmetric stretching mode. d-: CH2 antisymmetric stretching mode.
Figure 2. Raman spectra of CH2 stretching region before and after curing at 150 and 200 °C for 1 h. Top spectra (c, f, i) stand for OTS on “superhydrated” A380; middle (b, e, h) for OTS on “superhydrated” OX50, similar for 106-nm beads; bottom (a, d, g) for POS.
Figure 3. Raman spectra of C-C region before and after curing at 150 and 200 °C for 1 h. Top spectra (c, f, i) stand for OTS on “superhydrated” A380; middle (b, e, h) for OTS on “superhydrated” OX50, similar for 106-nm beads; bottom (a, d, g) for POS.
of intensity at 2850 and 2900 cm-1. Since the d- mode sits on top of the symmetric stretching band at 2900 cm-1, relative decreases in the peak height of d- often are related to decreases in intensity of d+. Irreversible disordering of the chains, as indicated by changes in the CH2 stretching spectra, is not significant for curing at 150 °C for any of the samples. For the 200 °C treatment, the most significant changes were observed in the case of “superhydrated” A380. The peak frequency of the antisymmetric stretching mode has increased to 2885 cm-1, and there is considerable increase in intensity of the symmetric stretching modes around 2900 and 2930 cm-1. This can be attributed to increases in gauche conformational isomers, which have higher frequency d- modes and intensity distribution increases for d+ also at higher wavenumber. These results are
Figure 4. Raman spectra of CH2 twisting mode at 1294 cm-1 and deformation mode at 1436 cm-1 before and after curing at 150 and 200 °C for 1 h. Top spectra (c, f, i) stand for OTS on “superhydrated” A380; middle (b, e, h) for OTS on “superhydrated” OX50, similar for 106-nm beads; bottom (a, d, g) for POS.
consistent with the IR data where the highest frequencies observed for the d- and d+ modes occur for the 200 °C thermally treated “superhydrated” A380 sample. Raman spectra in the C-C skeletal stretching and CH2 bending/twisting regions show similar trends. The increases in intensity at 1090 cm-1 indicate the presence of gauche conformational isomers as does the band broadening at the high wavenumber side of the 1300 cm-1 CH2 twisting vibration. The shoulder band at 1460 cm-1 arises from Fermi resonance interactions between overtones of the CH2 wagging mode and the CH2 bending vibration at 1445 cm-1 when the chain is in an all-trans conformation. The decrease of this shoulder band therefore is also an indicator of the loss of all-trans structures. The Raman data indicate that in all spectral regions the irreversible disordering after thermal treatment at 200 °C is in the order “superhydrated” A380 > “superhydrated” OX50 > POS. Previous results20 show that OTS on “superhydrated” OX50 behaves similarly to OTS-modified 106-nm silica beads with 93% surface coverage. Raman spectra were also obtained for samples that had been treated with TEA. When the TEA was added to silanated “superhydrated” OX50 after excess OTS was removed, the room-temperature spectra, and those obtained at thermal treatments of 150 and 200 °C were the same as those observed when no TEA was added. In the case of “superhydrated” A380, similar results were obtained, except that the 200 °C spectrum was slightly more ordered for the TEA treated sample. When TEA was added to “dehydrated” OX50 before excess OTS was removed, the morphology remained liquidlike even though the surface coverage increased. When TEA was added to “as is” OX50 before excess OTS was removed, additional OTS attached to the silica and the Raman spectra was liquidlike. However, when TEA was added to “superhydrated” OX50, no additional OTS attached to the silica surface and the Raman spectra were unchanged.
Thermal Stability of Octadecylsilane on Silica
Figure 5. In-situ Raman spectra at 120 °C for (a) octadecane; (b) POS; (c) OTS on “superhydrated” A380; (d) OTS on “superhydrated” OX50 or 106-nm glass beads.
In-Situ High-Temperature Raman Spectra. To further interpret the relationship between the structure of OTS SAMs and temperature, in-situ Raman measurements at 120 °C were conducted. This temperature is above that at which the alkyl chains would melt if not cross-linked or anchored to the surface, but below the temperature at which further silanol condensation is known to occur.20,21 As shown in Figure 5a, the Raman spectrum of octadecane was totally liquidlike in the CH2 stretching region and virtually identical with its spectra just above its melting point of 28-30 °C. As discussed previously,26 octadecane has an additional methyl group, compared with the octadecyl moiety, so that its Raman spectrum is not identical to higher molecular weight liquid alkanes or molten polyethylene. In the case of POS (Figure 5b) and “superhydrated” A380 (Figure 5c), the 2880 cm-1 antisymmetric CH2 stretching band broadened and decreased in intensity relative to the CH2 symmetric stretching band compared with their initial spectra (shown in Figure 2). This trend is consistent with that expected for the melting of confined alkane chains. The Raman spectra26 are not as disordered as observed for molten alkanes or polyethylene and line widths obtained from solid-state proton NMR are similar for C30 SAMs and the rotator phase of (C19) alkanes, indicating similar mobilities.38 For the “superhydrated” OX50 sample (Figure 5d) and OTS-modified 106-nm silica beads with 93% surface coverage (not shown), the broadening of the 2880 cm-1 band was also observed, but was not as great as for “superhydrated” A380 (Figure 5c). The broadening of the 2880 cm-1 band has been shown to be due to coupling of this mode with rotational and torsional oscillations of alkyl chains.39 These results indicate increased free volume for torsional motions of the alkyl chains for “superhydrated” A380, and hindrance of these motions for “superhydrated” OX50 and 106-nm beads. The shoulder band at 2930 cm-1 increased for all the samples, as expected when the number of gauche conformational isomers increases. It is interesting to point out that the high-temperature Raman spectra for OTS on “super-
J. Phys. Chem. B, Vol. 105, No. 1, 2001 177
Figure 6. In-situ Raman spectra at 120 °C for (a) OTS; (b) OTS on “dehydrated” A380; (c) OTS on “as is” A380; (d) OTS on “as is” OX50.
hydrated” OX50 and the 106-nm silica beads are similar to that of dipalmitoyl phosphatidylcholine (DPPC) dispersions (larger, lower curvature bilayers) at 50 °C while the spectra of POS and “superhydrated” A380 are similar to that of dipalmitoyl phosphatidylcholine vesicles (smaller, higher curvature bilayers) at 50 °C.40 The temperature of 50 °C is above the “melt” or gel/liquid crystal transition for DPPC bilayers. For comparison, Raman spectra of OTS, OTS on “dehydrated” A380, “as is” A380, and “as is” OX50 measured at 120 °C are shown in Figure 6; the spectra of the “as prepared” samples have been reported previously.32 The spectra for OTS (Figure 6a) and OTS on “dehydrated” A380 (Figure 6b) are liquidlike and similar to spectra observed for liquid alkanes and polyethylene. The spectra for OTS on both “as is” samples look distinctly different. In particular, there is an increase in frequency for the antisymmetric CH2 stretching mode and a marked increase in intensity to higher wavenumbers for the symmetric CH2 stretching mode; both the 2900 and 2930 cm-1 bands increase relative to the 2850 cm-1 band so that the intensities of all three are comparable. As reported previously,26 the melting was reversible, and when cooled to room temperature, the spectra were the same as for the “as prepared” samples. POS Formation To rule out the possibility that polymer (POS) formation occurred in solution and affected the results, the following experiments were performed using the same conditions as for the other silanization reactions. OTS was placed in anhydrous pentane and no polymer formation, i.e., precipitation, was observed. In addition, the 106-nm beads, which had the highest water content, were silanated, centrifuged ,and decanted, but not rinsed, and enclosed in a sealed capillary tube. The Raman spectra showed a strong chloride stretching vibration, indicating the lack of sufficient water to result in complete hydrolysis of OTS; the sample could be kept in this condition (i.e., anhydrous,
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chloride present) for several weeks. Addition of AgNO3 yielded a black precipitate, also indicative of the presence of the chloride ion. When the sample was exposed to the atmosphere, the chloride peak disappeared. TGA analysis indicated that there was enough OTS adsorbed on the surface to be 2-3 layers thick and that enough water was present to hydrolyze more than this amount of OTS. This experiment clearly showed that water adsorbed on the silica surface did not migrate to the top layers of OTS and hydrolyze the chlorosilane there, and thus did not migrate into the solution to form a polymer. Discussion OTS SAMs are often cured at high temperature in order to enhance OTS monolayer stability. Comparison of the Raman spectra before and after curing indicates that the change in the conformational structure of OTS SAMs is largely reversible up to 200 °C except for the case of “superhydrated” A380. Three effects contribute to the enhanced stability: (i) removal of the interfacial water layer;14 (ii) cross-linking between alkylsilane headgroups;41 and (iii) formation of permanent covalent bonds between the alkylsilane hydrolyzed headgroups and surface silanols.14 All three effects are temperature dependent. Interfacial water is removed at ∼100 °C,21 creating void space, so that in the absence of intervening water molecules there is increased possibility for the interaction between the silanol groups of neighboring OTS molecules. Condensation of silanols occurs at 200 °C. The difference in temperature used for curing may lead to different relative contributions of these effects. One concern in this study is whether the conformational structure of OTS SAMs will be affected by cross-linking between adjacent OTS molecules or covalent bonding to surface silanols after thermal treatment. The structure of the SAMs before and after thermal cure and the possibility for cross-linking between neighboring OTS molecules depend on the available free volume. In the case of fumed silica, the available free volume is affected not only by the surface coverage of OTS, but also by two other factors, namely, the state of aggregation/agglomeration and the curvature of the primary particles. Both the curvature and aggregation/agglomeration are greater for the smaller primary size particles.32,42 The aggregation/agglomeration phenomena simply result in decreased surface area available for silanation. However, the curvature of the silica particles creates free volume between the chains and the resultant differences in free volume can thus affect the packing of the OTS alkyl chains and their ability to thermally disorder and further cross-link at elevated temperature. The alkyl chains can be viewed as occupying a truncated wedge, in which the attachment of OTS to the silica surface occurs at the truncated portion. While the packing at the truncated portion is determined by the volume requirements of the alkyl chains, the free volume toward the other end increases with increasing curvature. To evaluate the effect of particle curvature in this study, a simple calculation was made of the efficiency of space occupied by all-trans octadecyl chains at full surface coverage, based on OTS molecular parameters found in the literature.6,43 If it is assumed that the all-trans octadecyl chain is a rigid rod with a length of 2 nm and cross-section area of 0.2 nm2, then the number of octadecyltrichlorosilane molecules, N, which can be accommodated on a d-nm spherical particle, is πd2/0.20 and the total volume, V, occupied by the N molecules, is 2πd2. The volume available for occupation by the OTS monolayers, Vs, is π[(d + 4)3 - d3]/6. Therefore, the efficiency, with which the OTS molecules occupy the available volume, V/Vs, is 12d2/[(d
Figure 7. Efficiency of space occupied versus particle diameter.
Figure 8. Schematic of aggregation states of OTS chains on the silica surface.
+ 4)3 - d3], and this is inversely related to the amount of void space. Figure 7 is a plot of V/Vs as a function of the diameter, d, of the particles. In the case of the 7-nm A380 particles, only 60% of the available volume is in fact occupied by the OTS molecules, whereas 90% and 96% is occupied for the 40-nm OX50, and 106-nm beads, respectively. This model can be used to explain the experimental observation that increased free volume for smaller particles results in increased possibility of chain packing through either intercalation/interdigitation of chains between neighboring particles44 or in the bilayers on the particles.45 In the present study, the thermal behavior of the alkyl chains in OTS SAMs was found to depend on the underlying siloxy substrate characteristics such as the radius of curvature and degree of hydration, as well as the OTS coverage. A schematic of the proposed structures on the silica surface, discussed in more detail below, is shown in Figure 8. In case of “dehydrated” samples, where OTS surface coverage is low, there is excess free volume available for the disordering of chain segments. The similarity between the spectra of OTS on “dehydrated” silica and liquid OTS both at room and elevated temperature implies that the alkyl chains on the “dehydrated” silica surface possess a random and melt-like conformation. Chain-chain interactions can be neglected, indicating that the alkyl chains are sparsely distributed on the surface. Thermal annealing results in no further ordering of the chains, which might be expected if the chains were mobile. In the case of “as is” samples, in which there are islands of alkyl chains,32 the Raman spectra in the CH2 stretching region at room temperature are similar to those of disordered chains
Thermal Stability of Octadecylsilane on Silica observed in lipid vesicles (high curvature bilayers) above their melt transitions (similar to Figure 5b and c). The chains at the center of the islands are expected to have less free volume compared with the chains at the edge. Since the mobility of the alkyl chains in the interior is restricted both by their attachment to the surface and the surrounding peripheral chains, not much increase in free volume is available for the chains upon heating. The chains therefore are even more disordered, that is, have a greater population of gauche conformational isomers, as indicated by the shift to higher frequency in the Raman spectra, than expected based upon Boltzmann statistics at the measured temperature (120 °C). Alternatively, it is possible that the frequency shifts observed in the high-temperature CH2 stretching region result from tetrahedral bond angle changes that result when the alkyl chains thermally disorder in a confined space. In the case of DPPC vesicles, heating above the melt transition can result in a more truly liquidlike melt, since the headgroups can move laterally, creating free volume. For the “superhydrated” samples, the alkyl chains are the most closely packed and all have the same room-temperature Raman spectra; the amount of attached OTS for the “superhydrated” samples is as great as possible consistent with the aggregation/ agglomeration characteristics of each silica. There is therefore limited volume available for the disordering of chain segments. The in-situ high-temperature Raman spectra are more disordered, with a broader antisymmetric stretching vibration, for “superhydrated” A380 (Figure 5c) than for “superhydrated” OX50 or the OTS on the high coverage 106-nm silica beads (Figure 5d), and therefore this observation supports the idea that the “superhydrated” A380 has more free volume due to its higher curvature and thus can accommodate more conformational disorder, and increased torsional motion, at high temperature than “superhydrated” OX50 or the 106-nm silica beads. In the case of POS, the increased free volume and ability to disorder results from the POS morphology, which is less rigid than for the silica particles, and in which the alkyl chains point in the same direction on alternate Si atoms.20 The DSC data support the above model. In order for a melting endotherm to occur, there must be thermal disordering; the width of the transition will be narrower the smaller the temperature range over which the disordering occurs, and the change in enthalpy will increase the greater the difference between the initial and final states. In the case of a “dehydrated” surface, the OTS molecules are already disordered and so no endotherm is observed. For well-ordered SAMs, thermally disordering is constrained by the free volume limitations. Only in the case of “superhydrated” A380, where the high curvature of the particles creates the free volume necessary for the chains to disorder, does an endotherm occur; no endotherm was observed for “superhydrated” OX50 where this free volume was not available. The interfacial region between the A380 surface and the OTS monolayer held more adsorbed water than that for OX50. Therefore, after removal of this water at elevated temperatures, the OTS on “superhydrated” A380 sample may be in a favorable situation to further cross-link with neighboring OTS molecules at 200 °C, the temperature at which the cross-linking reaction occurs. Upon cooling to room temperature, chains cross-linked unfavorably at their bases must conformationally disorder in the free volume available, possibly at the chain ends or in regions previously occupied by adsorbed water. The crosslinking thus results in the permanent irreversible disordering of the chain conformation as observed in the stretching, bending, and skeletal regions (Figures 2-4). The OX50 surface has less free volume at full surface coverage, as shown in Figure 7, and
J. Phys. Chem. B, Vol. 105, No. 1, 2001 179 also has less initial adsorbed water. There is therefore decreased probability that the cross-linking reaction will occur. After thermal treatment at 200 °C, there is in fact very little irreversible disordering of OTS on the “superhydrated” OX50 sample or the 106-nm silica beads.20 These results suggest that crosslinking at elevated temperatures for high coverage, well-packed OTS SAMs does not occur for low curvature or planar surfaces, and occurs for high curvature surfaces at the expense of disorder in the alkyl chain packing and conformation. Recent atomic force microscopy studies of SAMs of octadecylphosphonic acid on mica surfaces have suggested that thermal annealing results in migration and rearrangement of the octadecyl chains.46 Cohen et al.11 also observed a striking difference in thermal stability between OTS SAMs on aluminum (OTS/Al) and self-assembled monolayers of arachidic acid on aluminum. The OTS/Al systems had the highest stability. These results suggest that the ionic substrate-monolayer interactions (i.e., between acids and surface OH groups) is not sufficient to maintain orientational stability. For OTS SAMs formed on silica substrates investigated here, the reversibility of the molecular morphology strongly suggests that the alkyl chains are not mobile on the silica surface. Instead, the two-dimensional network set up by the occasional attachment of OTS to the surface and adjacent cross-links results in translationally immobile chains. The order of preparation of the monolayers, not just the OTS coverage, was found to be a critical factor in the formation of SAMs when TEA was used. In the case of well-ordered “superhydrated” OX50, the percent OTS coverage, the chain morphology and packing, and temperature dependence for OTS adsorbed (i) without the addition of TEA, (ii) with the addition of TEA after excess OTS had been removed, and (iii) with the addition of TEA in the presence of excess OTS (after the OTS/ silica had equilibrated for 4 h) were nearly identical. The similarity of morphology of SAMs prepared in (i) and (ii) suggests that, as was the case for thermally annealed samples, once the optimal packing morphology is set up by the hydrophobic interactions of the alkyl chains, the chains are too far apart to permit a condensation reaction between adjacent OTS molecules, except if there is occasional fortuitous proximity. The similarity of OTS coverage in (i) and (iii) further suggests that once closely packed SAMs are formed, there is no room for addition of more OTS, and also that there is no polymer formation in solution that subsequently adsorbs to the silica surface as has been suggested in the literature.47 In the case of well-ordered “superhydrated” A380, the similar morphology of (i) and (ii), also supports the above conclusions. In addition, the enhanced thermal stability at 200 °C of the TEAtreated sample (w/o additional OTS) may be due to additional reaction of the chlorosilane with the silica surface at room temperature. In the case of OTS on “as is” OX50 fumed silica, if excess OTS is available when TEA is added, additional OTS attaches (32% f 55%) to the surface. However, the OTS morphology is more disordered for the higher coverage sample. In the case of OTS adsorbed on “dehydrated” OX50 prepared with TEA and excess OTS, there is a large amount of attached OTS whose conformation is liquidlike. Although the amount of adsorbed OTS is greater (46%) for “dehydrated” OX50 prepared with TEA than for the “as is” silica prepared without TEA (32%), the conformational order is greater for the latter sample. These results suggest that for “dehydrated” or “as is” fumed silica, the originally attached OTS is randomly distributed on the surface, or in islands, respectively. Further addition of OTS is
180 J. Phys. Chem. B, Vol. 105, No. 1, 2001 catalyzed by TEA and occurs by adjacent condensation of silanols with chlorosilanes; the latter reaction is faster than interchain ordering. Since there is substantial available free volume, the condensation can occur and the attached alkyl chains can conformationally disorder in the free volume around the points of attachment. Thus, addition of TEA to dehydrated or “as is” fumed silica does not improve alkyl chain packing, although the amount of OTS attached to the silica increases. These results suggest that during the formation of SAMs hydrophobic chain interactions, which favor ordering of the chains, compete with silanol condensation reactions. The latter only occur, in the case that the alkyl chains are already favorably packed, if there is no disruption of chain packing, since as pointed out by Stevens29 the Si-O-Si bond distance is smaller than the van der Waals distance between all-trans alkyl chains. If the condensation reaction takes place before the alkyl chains form well-ordered structures, chain disorder can occur, with the caveat that, since the volume occupied by molten chains is greater than that for the crystal, there must be enough free volume available for splaying and disordering of the chains. Conclusions Using FTIR and Raman Spectroscopy, TGA, and DSC, the thermal behavior of OTS SAMs was investigated as a function of surface coverage and curvature. OTS coverage was varied by changing the amount of adsorbed surface water and through the addition of TEA. Curvature effects appeared only at the highest surface coverage, where the OTS chains were wellpacked. The dynamic behavior of the alkyl chains in OTS SAMs at high temperature was greatly dependent on the surface coverage and curvature of the underlying silica substrate. The in-situ Raman spectra of OTS on “dehydrated” silica with low surface coverage were liquidlike at room temperature and at an elevated temperature of 120 °C. The OTS SAMs on “as is” fumed silicas, with higher surface coverage in island formation, were packed at room temperature in a disordered state similar to lipid vesicles above their melt transition. At 120 °C, the alkyl chains exhibited even greater disorder than expected based on a free random chain with conformations based on Boltzmann statistics. In the case of “superhydrated” OX50 and for 106nm silica beads, with the highest possible OTS coverage, the alkyl chains appeared at room temperature intermediate between lipid vesicles and dispersions below the melt transition, and at 120 °C to lipid dispersions above the melt transition. The OTS on the higher curvature “superhydrated” A380 fumed silica, also with high surface coverage, behaved as bilayer vesicles at room temperature and 120 °C. In all cases, these transitions were reversible at 120 °C, and also to 200 °C for all samples except for “superhydrated” A380 fumed silica. In the case of OTS on “superhydrated” OX50 and the 106-nm beads, the OTS chains were well-packed and had good in plane organization. The high OTS surface coverage prevented complete disordering of the alkyl chains at high temperature. Since the volume occupied by a disordered chain is greater than that for an all-trans chain, the chains cannot get close enough for further cross-linking to occur through condensation of the silanol groups; this is sterically prohibited without additional available free volume, except for fortuitous cases in which the reacting silanols do not require additional free volume. For OTS on “superhydrated” A380, the free volume available due to the high surface curvature and voids left by removal of water allowed for some additional crosslinking of adjacent OTS molecules at elevated temperature. Since this was unfavorable in terms of chain packing at room
Wang and Wunder temperature, some irreversible disordering of the chains occurred. There is therefore good thermal stability of OTS chains on silica with high surface coverage, low surface curvature and low free volume. These results are consistent with model calculations29 that point out that the Si-O-Si bond distance is less than that required for accommodating an all-trans alkane chain. Under ambient conditions, the self-assembly of OTS may be guided by the hydrophobic association of the long alkyl chains, with cross-linking occurring only when this favorable packing is not disrupted. The subsequent temperature dependence of the chains depends on the constraints, such as anchoring of the chains to the surface and packing density, imposed by the structures formed at room temperature. The effect of the addition of TEA to the silanation reaction confirmed this process of self-assembly. When TEA was added to silica to which only equilibrium amounts of OTS were adsorbed (and no excess OTS was available), no changes in chain packing or morphology were observed. Condensation competed unfavorably with chain packing. When TEA was added to silica solutions with excess OTS, previously annealed to allow for equilibrium adsorption and ordering of OTS, the amount of additional OTS adsorbed depended on the available free volume. For well-packed monolayers, no additional adsorption occurred and chain packing was unaffected, while for silica with low initial OTS adsorbed, the OTS subsequently adsorbed was disordered. Mobility of the OTS, mediated by a film of adsorbed surface water, has been suggested as a mechanism that assists in-plane organization of the monolayer.14 The results of the present investigation suggest that during thermal annealing the OTS adsorbed on the silica is no longer mobile, either because of attachment to the underlying substrate, to other OTS molecules, restrained by surrounding OTS chains, or due to removal of adsorbed water. For “dehydrated” silica surfaces with no film of adsorbed water, the chains are not mobile even during their preparation, and are disordered before and after temperature treatment. Acknowledgment. The authors gratefully acknowledge the support of NIH Grants AR45472 and DE09530. References and Notes (1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (3) Meuse, C. W.; Niaura, G.; Lewis, M. L.; Plant, A. L. Langmuir 1998, 14 (4), 1604. (4) Plant, A. L. Langmuir 1999, 15, 5128. (5) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (6) Parikh, A. N.; Beers, J. D.; Shreve, A. P.; Swanson, B. I. Langmuir 1999, 15, 5369. (7) Ulman, A. AdV. Mater. 1990, 2, 573-582. (8) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868. (9) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (10) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 11471152. (11) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (12) Iimura, K.-I.; Kato, T.; Morita, S.-I.; Ozaki, Y. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1999, 337, 113-116. (13) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (14) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (15) Wang, R. Ph.D. Dissertation, Temple University, 2000. (16) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (17) Pursch, M.; Sander, L. S.; Albert, K. Anal. Chem. 1996, 68, 41074113. (18) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (19) Blitz, J. P.; Murthy, R. S. S.; Leyden, D. E. J. Colloid Interface Sci. 1988, 121, 63.
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