Anal. Chem. 1998, 70, 4730-4735
Detection of Local Density Distribution of Isolated Silanol Groups on Planar Silica Surfaces Using Nonlinear Optical Molecular Probes Ying Dong, Sastry V. Pappu, and Zhi Xu*
Department of Chemistry and the Center for Molecular Electronics, University of MissourisSt. Louis, St. Louis, Missouri 63121
Crystal Violet has been used as a nonlinear optical molecular probe for the detection of local density distribution of isolated silanol groups on planar silica surfaces. Because of its large size (∼120 Å2) and nearly flat adsorption geometry, Crystal Violet has successfully separated the truly isolated silanol groups (i.e., the silanol groups that cannot form a hydrogen bond with their neighbors by any means, ∼9.3 ×1013 cm-2) into two major classes. The first class includes those isolated silanol groups; each is surrounded by a large empty surface area (g120 Å2) in which a Crystal Violet cation can be placed. The surface density of this type of silanol groups is ∼1.1 ×1013 cm-2. The second class includes the rest of the isolated silanol groups with a surface density of 8.2 × 1013 cm-2. The adsorption behavior of organic molecules on silica surfaces has been the center of intensive investigations for a long time due to the widespread use of silica either as a stationary phase or as a support for a stationary phase in liquid chromatography.1-5 The key elements that determine the adsorption behavior of organic molecules on silica surfaces are the silanol groups (SiOH).1-7 During the past thirty years, a great deal of theoretical and experimental studies have been conducted on the physical and chemical properties of silonal groups present on the surface of silica sol particles that have high surface areas ranging from 10 to 500 m2/g.1,4,7,8 In contrast, only a few studies have been reported on the physical and chemical properties of silanol groups present on planar silica surfaces.9-11 The pioneering work by Ong (1) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979; Chapter 6. (2) Snyder, L. R. Principles of Adsorption Chromatography; Dekker: New York, 1968. (3) Guiochon, G.; Shirazi, S. G.; Katti, A. M. Fundamentals of Preparative and Nonlinear Chromatography; Academic Press: New York, 1994. (4) Nawrocki, J. J. Chromatogr., A 1997, 779, 29-71. (5) Tan, L. C.; Carr, P. W.; Abraham, M. H. J. Chromatogr., A 1996, 752, 1-18. (6) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processes; Academic Press: New York, 1990. (7) The Colloid Chemistry of Silica; Bergna, H. E., Ed.; Advances in Chemistry Series 234; American Chemistry Society: Washington, DC, 1994. (8) Chuang, I. S.; Maciel, G. E. J. Phys. Chem. B 1997, 101, 3052-3064 and references therein. (9) Ong, S. W.; Zhao, X. L.; Eisenthal, K. B. Chem. Phys. Lett. 1992, 191, 327335. (10) Xu, Z.; Li, J. W.; Dong, Y. Langmuir 1998, 14, 1183-1188. (11) Huang, X.; Kovaleski, J. M.; Wirth, M. J. Anal. Chem. 1996, 68, 41194123.
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et al. has shown that there are two types of silanol groups at the silica/water interface, with different pKa values, 4.9 and 8.5, and different surface populations, 19 and 81%, respectively.9 The silanol groups with the lower values of pKa (4.9) and surface population (19%) are believed to be isolated silanol groups, because isolated silanol groups can dissociate more readily compared to the silanol groups coupled to each other through hydrogen bonds.1,9 The silanol groups with the higher values of pKa (8.5) and surface population (81%) are believed to be those connected to each other through hydrogen bonds. It is well understood that the hydrogen bonding between silanol groups could manifest either directly or through the mediation of a bridging water molecule.8,9 Similar results were obtained for silanol groups on the surface of silica sol particles, where the two types of silanol groups have pKa values of 5.5 and 9.0 and surface populations of 15 and 85%, respectively.12 The average surface density of silanol groups on the surface of silica is ∼4.9 nm-2,1,7,8,13 which corresponds to an average surface area of 20.4 Å2 per silanol group. The local density of silanol groups, however, changes from place to place. Some silanol groups are very close to each other so that they may form hydrogen bonds directly with their neighbors, while some are completely isolated. Previous studies indicate that ∼46% of the silanol groups form hydrogen bonds directly with their neighbors on the surface of silica sol particles and that the distance between two adjacent silanol groups is less than 3.3 Å.7,8 Assuming that the same percentage of silanol groups form hydrogen bonds directly with their neighbors on a planar silica surface, it is clear that ∼35% of the silanol groups form hydrogen bonds with their neighbors via the mediation of bridging water molecules.14,15 The separation distance between two adjacent silanol groups under such a situation should be in the range of 3.5-5.5 Å.16-20 The remaining 19% of silanol groups on a planar silica surface should be in a truly isolated configuration with no chance to form a (12) Allen, L. H.; Matijevic, E.; Meites, L. J. Inorg. Nucl. Chem. 1971, 33, 1293. (13) Zhuravlev, L. T. Langmuir 1987, 3, 316-318. (14) For planar silica surfaces, 81% of the silanol groups have hydrogen bonds with their neighbors.9 Assuming that 46% of the silanol groups form hydrogen bonds directly with their neighbors,8 this leaves ∼35% of the silanol groups having hydrogen bonds with their neighbors through bridging water molecules. For high surface area particles, the surface population predicted would be 39% for silanol groups having hydrogen bonds through bridging water molecules. This latter evaluation agrees very well with the results obtained by microcalorimetry study of adsorption of water on silica surfaces.15 (15) Bolis, V.; Cavenago, A.; Fubini B. Langmuir 1997, 13, 895-902. 10.1021/ac9805697 CCC: $15.00
© 1998 American Chemical Society Published on Web 10/20/1998
Figure 1. Schematic drawing of structures of three types of silanol groups on planar silica surfaces: (A) isolated silanol groups (19%); (B) silanol groups that can form hydrogen bonds through a bridging water molecule (35%); (C) silanol groups that can form hydrogen bonds directly with each other (46%). The surface density of silanol groups on silica is ∼4.9 × 1014 cm-2.8
hydrogen bond with their neighbors. No quantitative information is available, however, about the separation distance of those truly isolated silanol groups from their neighbors on planar silica surfaces, because of the lack of sensitive techniques to gather such information. Figure 1 shows the schematic structures of the three types of silanol groups on a planar silica surface. During the past two years, significant progress has been made in our laboratory in regard to the application of nonlinear optical molecular probing (NOMP) technique for obtaining accurate quantitative information about the interfacial density and adsorption equilibrium constant of organic molecules and cations at solid/liquid interfaces.10 Recently, we applied this new probing technique to investigate the local density distribution of isolated silanol groups on a planar silica surface. In the initial investigation, we have used a system comprising a silica/CH3CN interface and a linear molecular probe, trans-4-[4-(dibutylamino)styryl]-1-(3sulfopropyl)pyridinium (DP), which forms a hydrogen bond with silanol groups through its head group (SO3n-).10 Although each DP molecule occupies a surface area near 50 Å2, its small head group allows it to interact with different silanol groups rather indiscriminately, as was revealed by the perfect single-site Langmuir adsorption isotherm.10 To achieve a better understanding of the local chemical environment of those truly isolated silanol groups, we have chosen a planar molecular probe, Crystal Violet cation (CV+). The rationale behind the choice of CV+ is as follows: (1) The CV+ cations are expected to interact only with surface charged sites (SiO-) formed either by the dissociation of silanol groups (SiOH T SiO- + H+) or by an ion-exchange process at the silica/ CH3CN interface. It is expected that the CV+ cations will have to (16) The Si-O bond length is ∼1.62 Å,7,17 the bond length of O-H is ∼0.95 Å,18 the angle of Si-O-H is 113°.8,19 When the two adjacent silanol groups are separated by a distance larger than 5.5 Å, the O-H-O distance between each surface silanol group and the bridging water molecule would be larger than 3.0 Å for structure B in Figure 1. As a result, the bridging water molecule will become much closer to one silanol group in order to lower the total system energy,20 the distance between the bridging water molecule and the other silanol group would be larger than 3.3 Å, and the hydrogen bond between them can be ignored.8,20 (17) Labouriau, A.; Higley, T. J.; Earl, W. L. J. Phys. Chem. 1998, 102, 28972904. (18) CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994; Section 9, p 19. (19) Chuang, I. S.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 401-406. (20) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997.
Figure 2. Molecular structure of Crystal Violet chloride (CV+Cl-).
lie nearly flat on top of the SiO- sites in order to lower the potential energy due to charge-charge attraction. (2) The interaction between CV+ cations and SiO- sites will be strong at those SiOsites surrounded by a large empty surface area, because only then can the probing cation get really close to the charged SiO- site. On the other hand, such an interaction would be relatively weak at those SiO- sites where the steric effect of nearby surface silanol groups does exist. (3) The different electrostatic interaction energies are expected to cause differences in the enthalpy of adsorption (∆Had) and in the adsorption equilibrium constant of CV+ on the above two types of SiO- sites. As a result, the adsorption of CV+ cations on the two different SiO- sites can be differentiated easily. (4) By converting the isolated silanol groups into a SiO- site with the ion-exchange process and subsequently measuring the adsorption isotherm of CV+, the surface density of those isolated silanol groups can be extracted. This strategy has been successful, and we found that the surface density is ∼1.1 × 1013 cm-2 for those isolated silanol groups surrounded by a large surface area in which a CV+ cation can be placed. Our result indicates that, by choosing molecular probes with different sizes, the local density distribution of isolated silanol groups on planar quartz surfaces could be mapped. EXPERIMENTAL SECTION Crystal Violet chloride (CV+Cl-) was obtained from Fisher Scientific and triethylamine (TEA) was obtained from Aldrich. Figure 2 shows the molecular structures of CV+Cl-. These chemicals were used without further purification. HPLC-grade CH3CN (Fisher) was used as solvent for all experiments. Conductivity measurements indicated that, in CH3CN, Crystal Violet chloride dissociates completely into CV+ cations and Cl- anions in the concentration region studied (C < 1.0 mM). The structure of the optical cell for reflection SHG measurements is shown in Figure 3. The glass cell with a Teflon stopper (internal dimension, 10 × 40 × 45 mm3) was obtained from Starna Cell, Inc. The silica surface terminated with silanol groups was prepared by immersion of the internal surface of the cell in concentrated sulfuric acid for 10 min, followed by washing the cell with Millipore water (18 MΩ cm) and HPLC-grade CH3CN. The surface of the cell was completely hydrophilic after this treatment. The excitation light at 1064 nm was coupled onto the silica/ CH3CN interface by a pair of right-angle fused-silica prisms. Optical matching liquid was filled into the space between the prism and the optical cell. The angle of incidence at the silica/CH3CN interface was 45°. The power density in all SHG measurements was kept below 15 MW/cm2. The reflected SHG signal was Analytical Chemistry, Vol. 70, No. 22, November 15, 1998
4731
Figure 3. Structure of the optical cell and the excitation geometry for reflection SHG measurements. Ii,ω is the intensity of excitation laser light at 1064 nm; Ir,2ω is that of reflected SHG light from the interface. Ri, Rr, and Rt are the angle of incidence, angle reflection, and angle of transmission, respectively.
varies rather slowly. This kind of behavior is typical of an adsorption isotherm for an interface with two types of adsorption sites.3 In sharp contrast, our previous study using a linear probing cation showed only single-site-type Langmuir adsorption isotherm,10 thus demonstrating rather convincingly that the observed profile of the isotherm is due to the planar shape and adsorption geometry of CV+ cations. We designate the SiO- sites responsible for the fast-rising portion of the isotherm shown in Figure 4 as type I, and those responsible for the slowly varying portion as type II. This classification could very well be extended to the silanol groups initially present on the planar silica surface, because the SiO- sites are formed as a result of the chemical interaction between the solvent (e.g., CH3CN) and the silanol groups. It is quite obvious that the interaction between CV+ cations and type I SiO- sites is much stronger than that between CV+ cations and type II SiOsites. The overall adsorption isotherm of CV+ at the interface can be expressed by the following Langmuir equation3
N ) N 0I
Figure 4. Adsorption isotherm of CV+ at the silica/CH3CN interface.
collected with a PMT, analyzed with a boxcar averager, and recorded with a computer.10,21-23 In a typical adsorption isotherm 2ω 2ω 2ω measurement, four SHG intensity values I P-P , I S-P , I 45°-P , and 2ω I 45°-S are recorded as a function of the solute concentration. The first letter in the subscript indicates the excitation polarization (45° means that the excitation light is polarized at 45° from the plane of incidence), and the second one indicates the detection polarization. The absolute surface density was calibrated by complementary SHG and UV-visible measurements.10 Solution UV-visible measurements were carried out using a computercontrolled double-beam UV-visible spectrometer (Perkin-Elmer, Lambda-14). For each spectrum, the scan speed was 60 nm/min, the spectrometer resolution was 2 nm, and the data interval was 0.2 nm. RESULTS AND DISCUSSION Adsorption of CV+ on Different SiO- Sites. It is known that silica/liquid interfaces become negatively charged due to the formation of SiO- sites when they are immersed in organic or aqueous solutions.1,9-11 These negatively charged sites attract cations such as CV+. Figure 4 shows the adsorption isotherm obtained with CV+ at the silica/CH3CN interface. The interfacial density of CV+ increases very fast with its solution concentration in the very low concentration region (
