Article pubs.acs.org/Langmuir
Hydrophobic Polydimethylsiloxane (PDMS) Coating of Mesoporous Silica and Its Use as a Preconcentrating Agent of Gas Analytes Eun Ji Park,† Youn Kyoung Cho,† Dae Han Kim,† Myung-Geun Jeong,† Yong Ho Kim,*,†,‡ and Young Dok Kim*,†,§ †
Department of Chemistry and ‡SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea § Korea Research Institute of Korea Technology, Daejeon 305-600, Republic of Korea S Supporting Information *
ABSTRACT: Mesoporous silica with mean pore size of ∼14 nm was coated by polydimethylsiloxane (PDMS) using a thermal deposition method. We showed that the inner walls of pores larger than ∼8 nm can be coated by thin layers of PDMS, and the surfaces consisting of PDMScoated silica were superhydrophobic, with water contact angles close to 170°. We used the PDMS-coated silica as adsorbents of various gasphase chemical warfare agent (CWA) simulants. PDMS-coated silica allowed molecular desorption of various CWA simulants even after exposure under highly humid conditions and, therefore, is applicable as an agent for the preconcentration of gas-phase analytes to enhance the sensitivities of various sensors.
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INTRODUCTION The synthesis and characterization of porous materials have attracted much attention recently due to their wide range of application. For example, adsorbents with high surface area and porosity such as activated carbon and mesoporous silica can be useful for removing pollutants from air and wastewater.1−4 Materials with high surface area and porosity can also be exploited as catalytic supports and low-k (dielectric constant) materials for use in electronic devices, respectively.5−9 In the present work, we used mesoporous silica as adsorbents of chemical warfare agent (CWA) simulants in atmospheric air. By choosing proper adsorbents, which allow facile adsorption and molecular desorption of analytes (in our case, CWA simulants), dilute analytes can be highly concentrated. Preconcentration units equipped with such proper adsorbents can be used for enhancing sensitivity in instrumental analyses such as gas chromatography (GC) and ion mobility spectroscopy.10−14 Various porous materials have been considered for use as agents for the preconcentration of vapor analytes, materials including mesoporous silica and porous polymers such as Tenax. Silica has a relatively high surface area; however, its hydrophilic surface nature preferentially adsorbs water molecules, lowering the uptake of analytes under highly humid conditions.15−17 Hydrophobic polymers do not suffer from this problem, but exhibit relatively low surface area. Therefore, the development of advanced materials for superior preconcentration ability is needed. Superhydrophobicity refers to the extreme wetting resistance of a surface characterized by both a water contact angle of >150° and a contact angle hysteresis of 16 h. The temperature of the chamber was controlled by a temperature control system consisting of a heating band, a k-type thermocouple, a temperature controller, and a power supply. During the heating, PDMS evaporated and was deposited onto the silica surface, forming a thin film. It was possible to prepare large amounts of PDMS-coated silica in a single batch by using this method; we carried out kilogram-scale coating in the laboratory. Sample Characterization. Water contact angle on silica was measured before and after PDMS coating. Bare and PDMS-coated silica samples were fixed onto double-sided adhesive tape on glass slides, and a 3 μL drop of distilled water was deposited onto each sample. Contact angles were measured by using a Theta optical tensiometer (KSV Instruments, Ltd.) equipped with a digital camera connected to the computer; Young−Laplace curves were employed as a fitting method. Water contact angles were averaged among five measurements at different positions on the surface of each silica sample. Contact angle hysteresis was determined by subtracting the receding contact angle from the advancing contact angle. Advancing contact angles were measured by continuously adding small amounts of water to the droplet on the surface; the maximum values were recorded. Similarly, the receding contact angles were measured by slowly removing water from the droplet, and the minimum value was used. In addition, Fourier transform infrared (FT-IR, Bruker, Optics/ vertex 70) spectral analysis and thermogravimetric analysis (TGA, SEICO Inst., TG/DTA 7300) of the silica samples were carried out before and after the PDMS coating. Brunauer−Emmett−Teller (BET) surface areas of bare and PDMScoated silica were also determined by nitrogen adsorption isotherms measured at 77 K, and pore volumes were estimated by using Barrett− Joyner−Halenda (BJH) analysis. For each sample, three nitrogen adsorption isotherms were collected, and their results were averaged. In addition, three different samples of PDMS-coated alumina spheres (Sasol, 610110, diameter = 1 mm, average pore size = 15 nm, BET surface area = 170 m2/g) were prepared by controlling the deposition time at 300 °C. The PDMS-coated alumina spheres were cut in half and fixed on a SEM holder with carbon tape, and their cross sections were analyzed by using a scanning electron microscope (SEM, JEOL, JEM-2100F) equipped with an energy dispersive spectrometer (EDS). Preconcentration of CWAs. To evaluate the preconcentration ability of bare and PDMS-coated silica, samples were exposed to CWA
Figure 1. Schematic description of experimental procedure for (a) adsorption and (b) desorption of CWA simulants under dry and humid conditions. silica samples were prepared for the experiment by outgassing them under atmosphere at 500 °C for 3 h and at 430 °C for 1 h, respectively. For each experiment, an outgassed adsorbent and a CWA simulant (dimethyl methylphosphonate (DMMP), methyl salicylate (MS), or dipropylene glycol methyl ether (DPGME)) were placed separately into open vials, and the vials were placed together inside a sealed plastic bottle. The plastic bottle was kept at 50 °C for 24 h on a hot plate. In the plastic bottle, CWA simulants were vaporized and then adsorbed onto the adsorbents; after 24 h, 1.4 g of each adsorbent was transferred to a reactor for a TPD experiment. The reactor was composed of a quartz holder in a quartz tube equipped with a furnace and was connected to a GC system. All TPD experiments were carried out using a constant flow of nitrogen gas of 5 sccm, which was controlled by a mass flow controller (MFC). The temperature of the reactor was increased from 30 to 630 °C at the ramping rate of 2 °C/ min, and the thermally desorbed molecules were analyzed by GC. The GC system used was equipped with a methanizer, a flame ionization detector (FID), and a 5% phenyl methyl siloxane column (Agilent 19091J-413). Every 15 min during the temperature ramp, 250 μL of the gas sample was injected into GC over a 15 s period using a six-way valve. After the injection, the GC oven temperature was held at 60 °C for 1 min and then raised from 60 to 200 °C at the ramping rate of 20 °C/min. Before the TPD experiment, the GC retention time and concentration of each CWA simulant were calibrated by directly injecting solutions of CWA simulants of known concentration dissolved in ethanol into the GC and correlating the resulting peak area to the known amount of injected CWA simulants. The effect of water vapor on preconcentration efficiency was also studied. Outgassed bare and PDMS-coated silica adsorbents were exposed to DMMP vapor in the presence of water vapor at 50 °C. The relative humidity inside the bottle was determined to be about 80% using a precision humidity meter (Lutron Electronic Enterprise Co., Ltd., HT3009 model). After 24 h, each silica sample was placed in a reactor for a TPD experiment.
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RESULTS AND DISCUSSION FT-IR spectra were collected for samples of bare and PDMScoated silica particles. In the FT-IR spectrum for bare silica, two B
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Figure 2. (a) FT-IR spectra of silica before and after PDMS coating. (b) Optical pictures of bare (left) and PDMS-coated (right) silica on water. (c) Static water contact angle of a surface consisting of PDMS-coated silica. (d) Water contact angle as a function of water drop volume in advancing and receding modes.
absorption bands observed at 1100 and 810 cm−1 were attributed to asymmetric stretching of Si−O−Si; after PDMS coating, additional bands corresponding to the PDMS framework appeared (Figure 2a).34 These additional bands included an absorption band at 1230 cm−1, attributed to symmetric deformation of CH3 groups, and a 1400 cm−1 band, attributed to asymmetric deformation of CH3 groups. Absorption bands at 2980 and 2905 cm−1 corresponded respectively to asymmetric and symmetric stretching of CH3 groups. This result shows that the siloxane frameworks of PDMS were successfully deposited on the silica surface. The PDMS treatment made the silica particles hydrophobic, as can be clearly seen in photographs comparing bare and PDMS-coated silica particles placed in vials containing distilled water (Figure 2b). The bare silica surface is hydrophilic and thus easily miscible with water, and silica is denser than water; thus, the bare silica particles sank in water immediately after they were placed on the surface of the water. Contrastingly, PDMS-coated silica did not sink but rather floated on water, even after heavy stirring, indicating that the PDMS coating imparted hydrophobicity to the silica surface. Furthermore, surfaces consisting of PDMS-coated silica distributed on adhesives showed a water contact angle of >160° and contact angle hysteresis of ∼5°, corresponding to superhydrophobicity (Figure 2c,d). As mentioned above in the Introduction, this superhydrophobicity is the combined result of the hydrophobic surface property of the PDMS coating and the dual surface roughness of the mesoporous silica. Pore size distributions of bare and PDMS-coated silica were estimated by nitrogen sorption experiments and BJH analysis (Figure 3). The mean pore size of the mesoporous silica before PDMS coating was about 14 nm, with a broad pore size distribution; the upper limit of the pore size in the bare silica used was determined to be about 30 nm. Upon PDMS coating,
Figure 3. Pore size distribution of mesoporous silica with and without PDMS coating, obtained from nitrogen sorption experiments and BJH analysis.
the number of pores larger than ∼8 nm was considerably reduced and the number of pores smaller than ∼8 nm was slightly increased. This implies that PDMS coated the inner walls of relatively large pores; still, one cannot exclude the possibility that smaller pores were not fully covered by PDMS. As shown in Table 1, the PDMS coating reduced the BET surface area of the mesoporous silica from 294 to 248 m2/g, the pore volume from 1.02 to 0.86 cm3/g, and the mean pore size from 14 to 11 nm. Even though the surface area, pore volume, and mean pore size of mesoporous silica were reduced by the PDMS coating, the silica largely maintained its porous nature Table 1. Mean Pore Sizes, Pore Volumes, and BET Surface Areas of Bare and PDMS-Coated Silica
bare SiO2 PDMS-coated SiO2 C
BET surface area (m2/g)
pore volume (cm3/g)
BJH pore size (nm)
293.94 (±0.39) 247.60 (±0.81)
1.02 (±0.001) 0.86 (±0.001)
13.92 (±0.04) 10.96 (±0.06)
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after hydrophobic coating, suggesting that thin films of PDMS coated the inner walls of many of the silica pores, providing hydrophobicity while maintaining the original material’s high porosity. To verify that PDMS layers could be deposited on the inner walls of pores, PDMS deposition was carried out on alumina spheres (provided by Sasol) of 1 mm average particle diameter and 15 nm average pore size. PDMS-coated alumina spheres were prepared at 300 °C using different deposition times (1, 3, and 16 h), and their cross sections were analyzed using SEM/ EDS. PDMS contains Si; thus, the atomic percentage of Si in the various coated products was used to track the amount of PDMS deposited. Cross-sectional elemental analysis of the coated spheres showed that Si atomic percentage increased as a function of deposition time, whereas Al atomic percentage decreased (Supporting Information, 2b). In both the shell and core of the alumina spheres, Si atomic percentage continuously increased as the deposition time increased, indicating that PDMS vapor slowly but continuously diffused into the alumina spheres, forming a PDMS layer on the pores’ inner walls (Supporting Information, 2c,d). It is notable that PDMS vapor could diffuse into the alumina spheres even up to the depth of 500 μm from the particle’s outermost shell. Comparing a crosssectional SEM image of alumina coated with PDMS for 16 h against corresponding EDS mapping images (Figure 4) showed
coated silica, likely because most of the thermally unstable species on PDMS-coated silica were removed by the outgassing process, which included heat treatment at 430 °C for 1 h. The outgassed PDMS-coated silica was hydrophobic, having a water contact angle of 144.05° (±0.74°). It is worth mentioning that although the water contact angle on PDMS-coated silica was decreased after the outgassing process, the outgassed PDMScoated silica still exhibited hydrophobicity and thus could selectively adsorb gaseous CWA simulants from humid air. Bare and PDMS-coated silica samples were used as adsorbents for the preconcentration of CWA simulants. Each adsorbent was exposed for 24 h to DMMP vapor in dry or humid air, and TPD spectra were subsequently collected for each sample (Figure 5). When bare silica was used as an
Figure 4. (a) SEM image of a PDMS-coated and subsequently crosssectioned alumina sphere; (b−d) corresponding EDS mapping images of (b) Al, (c) O, and (d) Si.
Figure 5. (a) TPD spectra of DMMP desorption from bare silica. The silica sample was exposed to DMMP vapor under dry and highly humid conditions, respectively. (b) TPD spectra of DMMP desorption from PDMS-coated silica. The coated sample was exposed to DMMP vapor under dry and highly humid conditions, respectively. To facilitate comparison, both panels a and b display the same intensity range.
that Si, which can be correlated to the PDMS layer, was well distributed throughout the cross section. On the basis of the results shown in the Supporting Information (2) and Figure 4, we can conclude that the thermal deposition method did not clog the outer pores of the alumina spheres with PDMS; rather, the PDMS diffused into the pores and formed a hydrophobic layer on the inner walls. Such an experiment could not be carried out using mesoporous silica, because the Si of the silica particles and the PDMS coating could not be discriminated. Bare silica, PDMS-coated silica, and outgassed PDMS-coated silica were subjected to TGA, carried out from 30 to 620 °C at the ramping rate of 10 °C/min in a nitrogen atmosphere (Supporting Information, 3). Bare silica maintained its original weight throughout TGA, whereas PDMS-coated silica lost about 5% of its mass, most likely due to thermal degradation both of the deposited PDMS and of impurities that can be deposited during the PDMS coating process. However, this mass loss was considerably suppressed in the outgassed PDMS-
adsorbent of DMMP under the dry condition, desorption of DMMP took place between 200 and 450 °C; this broad desorption range resulted from the differences in DMMP desorption energy among the pore surfaces of different sizes and structures. Contrastingly, after bare silica was exposed to DMMP under humid conditions, no desorption of DMMP could be detected, indicating that the water in the humid air competed with and completely suppressed the adsorption of DMMP on the bare mesoporous silica surfaces. This result was expected because silica surfaces are generally terminated by hydroxyl groups, making them highly hydrophilic.35−37 When PDMS-coated silica was exposed to DMMP, molecular desorption of DMMP was observed after exposure D
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increased to 12 h, a single desorption peak at 240 °C appeared; when the exposure time was increased further to 24 h, two distinct states at 240 and 350 °C were observed. The lower temperature desorption peak was attributed to DMMP molecules in larger pores, and the higher temperature desorption peak was attributed to smaller pores. The following scenario can explain the TPD spectra shown in Figure 6a: during the initial stage of DMMP adsorption, DMMP adsorbed only on the surfaces of larger pores, and as the exposure time increased, the DMMP on the larger pores had sufficient time to diffuse into the smaller pores, resulting in the additional desorption peak observed at higher temperatures. Contrastingly, molecular DMMP desorption from PDMS-coated silica was observed for all of the exposure times of 6, 12, and 24 h (Figure 6b). After the 6 h exposure, broad desorption peaks from 200 to 400 °C were observed. In the case of the 12 h exposure on PDMS-coated silica, two peaks at 240 and 350 °C were identified. When the DMMP exposure time was increased to 24 h, an additional peak at 450 °C appeared. As the DMMP exposure time was increased, the amount of desorbed DMMP increased, and the desorption temperature shifted to higher temperature. This result agrees with the results for bare silica: DMMP first adsorbed on larger pores and then diffused into small pores when sufficient time was available. It is notable that the amount of DMMP that molecularly desorbed from PDMScoated silica was much greater than that desorbed from bare silica after the same exposure time. The amount of DMMP that was adsorbed on the surface of silica under the dry condition was increased considerably by coating it with PDMS (Figure 5, inset), despite the fact that this coating reduced the surface area, as seen in Table 1. The amount of DMMP that was molecularly desorbed from bare silica after exposure in the dry condition was 24.93 μmol; the corresponding amount desorbed from PDMS-coated silica was 182.3 μmol, about 7 times that of the respective value for bare silica. The amount of DMMP adsorbed on the PDMS-coated surface was estimated to be 90% in GC intensity). We also tested adsorption/desorption of two CWA simulants other than DMMP on PDMS-coated silica. In the case of MS, molecular desorption was observed at about 300 °C, and MS desorption peaks were almost identical in experiments including exposure to both humid and dry air (Figure 8).
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CONCLUSIONS Mesoporous silica with a mean pore diameter of ∼14 nm was coated with PDMS by using thermal evaporation of PDMS and subsequent deposition on the surface of mesoporous silica. Inner walls of pores >∼8 nm were successfully decorated by hydrophobic PDMS, whereas smaller pores were not fully coated by PDMS. PDMS-coated silica was completely immiscible with water, with a water contact angle close to 170°. We studied adsorption and desorption of three CWA simulants (DMMP, MS, and DPGME) on PDMS-coated silica; all three CWA simulants adsorbed on the surface of PDMScoated silica desorbed molecularly from the surface during thermal desorption. Even when the CWA simulant vapors were exposed to the PDMS-coated silica under highly humid conditions, their adsorption and molecular thermal desorption activity persisted. PDMS-coated silica can be used as preconcentration agents of CWA simulant vapor for better detection of these CWA simulants in the atmosphere.
Figure 8. TPD spectra of MS desorption from PDMS-coated silica. The sample was exposed to MS vapor under dry and highly humid conditions, respectively.
Water molecules do not compete with MS for adsorption on PDMS-coated silica, and we suggest that MS adsorption on PDMS-coated silica takes place only in large pores having inners walls decorated by PDMS. DPGME desorption from PDMS-coated silica showed a broad desorption peak between 150 and 600 °C, and the amount of DPGME desorbed was significantly suppressed by exposure in the humid condition (Figure 9). It is worth mentioning that most of the DPGME molecules on PDMScoated silica underwent decomposition during adsorption/ desorption, and therefore only a small amount of molecular desorption was found in the experiments under both dry and
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ASSOCIATED CONTENT
* Supporting Information S
Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(Y.D.K.) Phone: +82-31-299-4564. Fax: +82-31-290-7075. Email:
[email protected]. *(Y.H.K.) Phone: +82-31-299-4162. Fax: +82-31-290-7075. Email:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Agency for Defense Development through the Chemical and Biological Defense Research Center.
Figure 9. TPD spectra of DPGME desorption from PDMS-coated silica. The sample was exposed to DPGME vapor under dry and highly humid conditions, respectively. F
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