Quantitative In Situ Attenuated Total Internal Reflection Fourier

Jan 12, 2010 - Department of Chemistry, Central Washington University, 400 East University Way, Ellensburg,. Washington 98926-7539. Received October 7...
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Quantitative In Situ Attenuated Total Internal Reflection Fourier Transform Infrared Study of the Isotherms of Poly(sodium 4-styrene sulfonate) Adsorption to a TiO2 Surface over a Range of Cetylpyridinium Bromide Monohydrate Concentration Mike Hase, Ryan Scheffelmaier, Sarah Hayden, and Dion Rivera* Department of Chemistry, Central Washington University, 400 East University Way, Ellensburg, Washington 98926-7539 Received October 7, 2009. Revised Manuscript Received December 16, 2009 Quantitative in situ attenuated total internal reflection Fourier transform infrared (ATR FTIR) spectroscopy has been used to study the isotherm of poly(sodium 4-styrene sulfonate), PSS, adsorption to a TiO2 surface in aqueous solution at a pH of 3.5. The effect of adding surfactant cetylpyridinium bromide monohydrate (CPBM) on the adsorption isotherm of PSS was investigated at CPBM concentrations of 3.60  10-7, 1.02  10-5, and 1.04  10-4 M. The use of in situ ATR FTIR allowed for the calculation of the concentration of both PSS and CPBM at the TiO2/water interface over the entire course of all experiments. It was found that the addition of a small amount of CPBM, 3.60  10-7 M, to PSS solutions resulted in 23 ( 3% less PSS accumulating at the TiO2/water interface compared to isotherm studies with no CPBM present. The mole ratio of CPBM to PSS varies from 4 ( 1 to 1 to 20 ( 4 to 1 in a stepwise manner as the solution concentration of PSS is increased for solutions with a CPBM concentration of 3.60  10-7. The addition of CPBM at concentrations of 1.02  10-5 and 1.04  10-4 M showed distinct differences in the behavior of the PSS isotherm, but at the highest solution PSS concentrations, the amount of PSS at the TiO2/water interface compared to that of PSS solutions with no CPBM added is indistinguishable within the experimental uncertainties. For these higher concentrations of CPBM, both PSS and CPBM appear to come to the TiO2 surface as aggregates and the mole ratio of CPBM to PSS at the TiO2/water interface decreases as the concentration of PSS is increased. For a CPBM concentration of 1.02  10-5 M, the mole ratio of CPBM to PSS changes from 139 ( 29 to 1 to 33 ( 7 to 1 as the solution PSS concentration is increased. For a CPBM concentration of 1.04  10-4 M, the mole ratio of CPBM to PSS changes from 630 ( 130 to 1 to 110 ( 21 to 1 as the solution PSS concentration is increased. Despite the large differences in the CPBM to PSS mole ratios, the amount of PSS that adsorbs to the surface is statistically indistinguishable for CPBM concentrations of 0, 1.02  10-5, and 1.04  10-4 M, indicating that the structure of the PSS molecules in each of the systems does not significantly change in the presence of CPBM.

Because of their unique properties, aqueous mixtures of polyelectrolytes and surfactants are commonly employed in diverse areas including manufacturing processes involving colloidal suspensions, oil recovery, and potentially in water purification systems.1-4 Because of the wide range of applications of these mixed polyelectrolyte/surfactant systems, there have been a large number of studies on the interaction of these mixed systems under *Author to whom correspondence should be addressed. E-mail: riverad@ cwu.edu. (1) Polymer Surfactant Systems, Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol 77. (2) Myers, D. Surfaces, Interfaces, and Colloids: Principles and Applications; VCH Publishers: New York, 1991. (3) Wang, Y.; Banziger, J.; Dubin, P. L.; Filippelli, G.; Nuraje, N. Environ. Sci. Technol. 2001, 35, 2608–2611. (4) Mishael, Y. G.; Dubin, P. L. Environ. Sci. Technol. 2005, 39, 8475–8480. (5) Asnacios, A.; Langevin, D.; Argillier, J. Macromolecules 1996, 29, 7412– 7417. (6) Zhou, S.; Hu, H.; Burger, C.; Chu, B. Macromolecules 2001, 34, 1772–1778. (7) Ritacco, H. A.; Busch, J. Langmuir 2004, 20, 3648–3656. (8) Deo, P.; Deo, N.; Somasundaran, P.; Moscatelli, A.; Jockusch, S.; Turro, N. J.; Ananthapadmanabhan, K. P.; Ottaviani, M. F. Langmuir 2007, 23, 5906–5913. (9) Dan, A.; Chakraborty, I.; Ghosh, S.; Moulik, S. P. Langmuir 2007, 23, 7531– 7538. (10) Doe, P.; Deo, N.; Somasundaran, P. Langmuir 2005, 21, 9998–10003. (11) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566–1574. (12) Monteux, C.; Williams, C. E.; Meunier, J.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 57–63. (13) Taylor, D. J. F.; Thomas, R. K. Langmuir 2002, 18, 4748–4757. (14) Naderi, A.; Claesson, M. Langmuir 2006, 22, 7639–7645.

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various conditions.3-33 Despite the large number of studies, however, many fundamental questions remain about the nature of the interactions of these mixed systems with the surface, and obtaining direct in situ spectroscopic information about these complex systems interacting at the liquid/solid interface can be quite challenging.19-23,30-32,34 Factors that have been found to (15) Penfold, J.; Tucker, I.; Thomas, R. K. Langmuir 2005, 21, 11757–11764. (16) Trabelsi, S.; Langevin, D. Langmuir 2007, 23, 1248–1252. (17) Claesson, M.; Fielden, M. L.; Dendinaite, A.; Brown, W.; Funden, J. J. Phys. Chem. B 1998, 102, 1270–1278. (18) Talingting, M. R.; Ma, Y.; Simmons, C.; Webber, S. E. Langmuir 2000, 16, 862–865. (19) Neivandt, D.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519–2526. (20) Poirier, J. S.; Tripp, C. P.; Neivandt, D. J. Langmuir 2005, 21, 2876–2880. (21) Li, H.; Tripp, C. P. Langmuir 2005, 21, 2585–2590. (22) Sukhishvili, S.; Granick, S. J. Chem. Phys. 1998, 109, 6861–6868. (23) Sukhishvili, S.; Granick, S. J. Chem. Phys. 1998, 109, 6869–6878. (24) Hansupalak, N.; Santore, M. M. Langmuir 2003, 19, 7423–7426. (25) Hodges, S. C.; Biggs, S.; Walker, L. Langmuir 2009, 25, 4484–4489. (26) Chu, D.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270–6276. (27) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16694–16703. (28) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (29) Almgren, M.; Hansson, P.; Mukhtar, E.; Van Stam, J. Langmuir 1992, 8, 2405–2412. (30) Ahrens, H.; Baltes, H.; Schmitt, J.; Mohwald, H.; Helm, C. A. Macromolecules 2001, 34, 4504–4512. (31) Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 4748–4757. (32) Ishikubo, A.; Mays, J.; Tirrel, M. Ind. Eng. Chem. Res. 2008, 47, 6426–6433. (33) Penfold, J.; Staples, E.; Thomas, R. K. Langmuir 2004, 20, 7177–7182. (34) Lane, T. J.; Fletcher, W. R.; Gormally, M. V.; Johal, M. S. Langmuir 2008, 24, 10633–10636.

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influence the adsorption of polyelectrolytes and surfactants to a surface include relative ratios of the surfactant concentration to the polyelectrolyte concentration, pH, charge density of the polyelectrolyte/surfactant complex, and ionic strength of the solution. Despite the large body of work on polyelectrolyte/ surfactant systems, there is still a great deal of behavior at liquid/solid interfaces that is difficult to predict with these mixed systems and further development of in situ spectroscopic methodologies that can provide both quantitative information and structural information is needed. The challenge of studying surface interactions in the solution phase is that the signal due to interactions at the surface is often masked by an interfering signal in the solution. A technique that has been employed successfully in other surfactant and/or polyelectrolyte systems to get around this problem is attenuated total internal reflection Fourier transform infrared spectroscopy (ATR FTIR).19-23,35-37 With ATR FTIR, an infrared beam is transmitted through an internal reflection element (IRE) whose index of refraction is higher than that of the medium that surrounds the element. At each reflection of the IR radiation within the IRE, some infrared radiation tunnels through the IRE and into the solution phase. The distance over which the amplitude of this evanescent wave decays to 1/e of its original value is usually 2 μm or less for typical mid-infrared ATR analysis involving a ZnSe IRE near 1000 cm-1. Thus, a thin coating can be placed on the solution side of the ATR, and the evanescent wave is sensitive primarily to the solid/liquid interface. The ATR FTIR technique makes it possible to obtain direct structural spectroscopic information about molecular interactions at the surface. In addition, the very small path length of the evanescent wave allows infrared spectra to be collected easily in aqueous media. Whereas many studies of polyelectrolytes and surfactants have employed ATR FTIR, use of this methodology in a quantitative way has not been employed as frequently because of the need to have good estimates of all necessary optical parameters that are a function of wavelength for the equations that govern the depth of penetration of the evanescent wave.35,36,38-40 There have been many studies of the adsorption behavior of either polyelectrolyte, surfactant, or both to a variety of surfaces,3-25,30-37 but studies analyzing the behavior of mixtures of polyelectrolytes and surfactants at surfaces through in situ spectroscopic techniques that can provide structural information about chemical species, such as infrared spectroscopy, are less common.19-23,30-34 Surfactants that have a charge that is opposite that of a polyelectrolyte are known to undergo force aggregation of the polyelectrolyte with the surfactant at concentrations that can be several orders of magnitude below the critical micelle concentration of the surfactant.19,25-29 It is thought that these aggregates contain domains of surfactant micelles that interact with the polyelectrolyte.19,25 Studies that have been done on the coadsorption of polelectrolytes and surfactants to liquid/solid surfaces13,16,17,19-25,32-34 or the air/water interface14,15,18,30,31 indicate that the polyelectrolytes interact with surfactants and are not separate entities but rather a polymer/surfactant complex at the surface. Studies of the structure of the coadsorption of polymers and surfactants to the air/water interface indicate that a (35) Sperline, R. P.; Muralidharan, S.; Freiser, H. Langmuir 1987, 3, 198–202. (36) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 2183–2191. (37) Jiang, C.; Gamarnik, A.; Tripp, C. P. J. Phys. Chem. B 2005, 109, 4539– 4544. (38) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley & Sons: New York, 1967. (39) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587–3597. (40) Hansen, W. H.; Kuwana, T.; Osteryoung, R. A. Anal. Chem. 1966, 38, 1810–1821.

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dense gel-like layer is formed at the interface.14 Other studies at the air/water interface indicate that much more surfactant is brought to the interface in the presence of polyelectrolytes then in the absence of polyelectrolytes.18 Studies of strong negatively charged polyelectrolytes using ATR FTIR have been employed to understand the behavior of the polyelectrolyte in the presence of an oppositely charged surfactant at a silica surface under a range of pH conditions.19 For the adsorption of cationic polyelectrolytes on mica in the presence of anionic surfactant, the ratio of surfactant to polyelectrolyte was found to be crucial in determining whether the polyelectrolyte/surfactant complex adsorbs to the surface in a predictable fashion or undergoes a sudden precipitation of aggregates on the surface.21 Studies that have looked at the competition of like-charged polyelectrolytes and surfactants for an oppositely charged surface indicate that the resulting surface coverage is a subtle interplay between pH, concentrations of the surfactant and polyelectrolyte, and ionic strength.13,17-25 Despite the large amount of work that has been done regarding interactions of surfactants and polyelectrolytes, there is still a need to understand how polyelectrolyte adsorption behavior is influenced by the presence of surfactants and how surfactants and polyelectrolytes interact with polyelectrolytes at the water/solid interface. Both structural and quantitative information about the molecules at the interface are needed. This article presents a quantitative in situ ATR FTIR study of polyelectrolyte adsorption of poly(sodium 4-styrene sulfonate) (PSS) at a TiO2/water interface and how the addition of surfactant cetylpyridinium bromide monohydrate (CPBM) at concentration ranging from 3.60  10-7 to 1.04  10-4 M influences the adsorption isotherm of PSS. The ATR FTIR methodology outlined in this article provides quantitative in situ information about the concentrations of surfactant and polyelectrolyte at the TiO2/water interface as a function of the PSS solution-phase concentration as well as the mole ratios and ratios of CPBM to PSS to monomer units. To our knowledge, a systematic study that varies the amount of surfactant for several adsorption isotherms of polyelectrolyte and gives in situ quantitative and spectroscopic infrared data about the behavior of polyelectrolytes and surfactants at an oxide/water interface is unique.

Experimental Section Nanopowder titanium dioxide (TiO2) anatase was obtained from Sigma-Aldrich with a purity of >99.7%. The TiO2 nanopowder has an average particle size of 12 ( 2 nm for the TiO2 films prepared. This particle size was determined by applying the Debye-Scherrer equation31 to X-ray diffraction (Phillips PW 3400) spectra of the TiO2 films prepared. A ZnSe IRE (50 mm  10 mm  2 mm) was coated with TiO2 by a modified literature procedure.21,39 The XRD also showed that the nanoparticles had a diffraction pattern very similar to that of anatase and showed the nanoparticles to be very crystalline in nature. A solution of 0.500% TiO2 by weight in 200 proof ethanol was prepared. The ethanol solution was sonicated prior to application for several minutes. The sonicated solution was then deposited over the ZnSe IRE in 10 μL drops for a total of 10 drops. After the ethanol evaporated from the surface, a thin film of TiO2 was left behind. A Niclolete Magna 760 FTIR was used in conjunction with the Thermo Omnic software package. The FTIR was fitted with an ATR accessory (Thermo Scientific) with a liquid flow cell, and the TiO2-coated ZnSe elements were placed inside the flow cell. Typically, 5 mL of solution is flowed over the TiO2 film over a period of 1 to 2 min and then allowed to equilibrate. Figure 1 illustrates how the infrared radiation interacts with the TiO2 film. Using this setup, all spectral data were collected using 128 scans with a scan velocity of 0.1581 m/s to maximize the interferogram DOI: 10.1021/la903787t

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(41) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley & Sons: New York, 1991.

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-logðRÞ ¼ A ¼ εbc

ð1Þ

where -log(R) is equivalent to the absorbance A, b is the path length in centimeters, ε is the molar absorptivity, and c is the concentration. As stated in the Experimental Section, calibration curves at various PSS and CPBM concentrations were generated and the slope of the line is equal to εb. For the PSS concentrations, the slope εb can be used to calculate the concentrations of PSS at the TiO2/water interface assuming you can correct for the presence of the TiO2 film (see below). For CPBM concentrations, ε has to be calculated by dividing the observed absorbance by the concentration and path length of the transmission flow cell. To calculate the concentration of CPBM in ATR experiments, the effective depth of penetration needs to be calculated using the equation below38 d e ¼ ðd e^ þ d e Þ=2 )

peak signal. The resolution of the FTIR was set to 4 cm-1. For the ATR FTIR spectra for each isotherm to be confirmed, replicates of each experiment were undertaken. The equilibration time between sample concentrations varied depending on polyelectrolyte concentration, but in general it took 1 to 3 h for equilibration with the TiO2 film to occur. Equilibration was determined by monitoring changes in the IR bands due to PSS and CPBM. A typical adsorption experiment could take anywhere from 10 to 15 h to complete. Chemicals, including cetylpyridinium bromide monohydrate, poly(sodium 4-styrene sulfonate) with an average molecular weight of 70 000, and 37% HCl, were obtained from SigmaAldrich and were used without further purification. The pKa of the acid form of PSS was calculated to be 1.09 from the pH of known weight percent solutions of poly(4-styrene sulfonate) (the acid form of PSS from Sigma-Aldrich). Using this pKa value, we determined that the percentage of sulfate groups that are estimated to be protonated at a pH of 3.5 is 0.39%. This low percentage was confirmed by infrared ATR spectra of the PSS in solution at pH 3.5 and showed no detectable peaks arising from the asymmetric stretching frequency of sulfonic acid, 1350-1342 cm-1,41 indicating that the PSS is present mainly as the sodium salt, as is assumed in other studies.19 Experiments were carried out multiple times with different films on different days, and uncertainties represent the standard deviation from replicate runs. For purposes of clarity in the Figures presented in this article, the average percent relative standard deviation (% rsd) over all of the data points is given in the Figure captions. All solutions of PSS, CBMD, and HCl were prepared using class A volumetric glassware and analytical scales (Mettler). Stock solutions of PSS and CPBM, both with a concentration of 1 mM, were used to create the various solutions required. For ATR experiments, PSS solutions ranged in concentration from 1.25  10-7 to 1.85  10-6 M using the average molecular weight of 70 000 and the CPBM concentration ranged from 3.60  10-7 to 1.08  10-4 M. For a given isotherm experiment, the concentration of PSS was varied but the concentration of CPBM was kept constant. For solutions of CBPM at 1.08  10-4 M and a PSS concentration of 1.25  10-7 to 3.60  10-7 M, CBPB needs to be heated to ∼30 C for the CBPM to dissolve. However, some CPBM does not stay dissolved, especially at the 1.25  10-7 M PSS concentration. Thus, solutions in this concentration range are considered to be saturated with CBPM. For calibration curves, ATR spectra of PSS were obtained for solution-phase concentrations of PSS ranging from 5.00  10-4 to 5  10-3 M on a ZnSe IRE at pH 3.5. Because CPBM is a surface-active compound, calibration curves were generated using a 15 μm flow cell with a

Teflon spacer and CaF2 windows. The pH of the solutions was adjusted by the addition of 85 to 250 μL of 0.1 M HCl to all solutions in order to adjust the pH to 3.50. At this pH, the TiO2 surface has a net positive charge. All experiments were carried out at a pH of 3.5 unless otherwise noted. The pH of all solutions was recorded prior to sample analysis using a digital pH meter (VWR) calibrated with pH 4/7 standards. All water used in these experiments was doubly distilled deionized water. Surface tension measurements were carried out with a tensiometer (Sergeant Welch) using the du No€ uy ring method. Solutions were prepared using doubly reverse osmosis deionized water. In all experiments, doubly distilled deionized water was used as a blank to ensure that no contaminants were present. All readings were taken a minimum of three times to ensure the reproducibility of the surface tension measurements. If the CPBM did not dissolve easily, then the solution was heated to ∼30 C for ∼5 min. The CPBM then stayed in solution for the timeframe of the experiments. Surface tension experiments were carried out at 25 ( 2 C. For all surface tension experiments involving CPBM and PSS, the CPBM dissolved readily at 25 ( 2 C. In this article, the mole ratios of both CPBM and PSS are calculated along with the ratio of CPBM molecules per PSS monomer unit. The mole ratio of CPBM to PSS is an average number based on the average molecular weight of the PSS used in this study, 70 000, but it does provide a useful comparison to mole ratios of surfactant /polyelectrolytes complexes in the literature as well as an average estimate of the number of CPBM molecule aggregated with a PSS molecule. The calculations for the CPMB to PSS monomer are made on the basis of the grams of PSS in each of the solutions described above and the formula weight of the PSS monomer unit and are not dependent on the average molecular weight of the polymer. Calculations. Measurements made with ATR are not a classical absorbance measurement because absorbance refers to the continuous adsorption of light over a continuous path length rather than the summation of discrete reflections. However, in practice the difference between these two quantities is very small and for the attenuation of IR light seen in this study the difference is essentially negligible.42,43 Thus, concentrations are calculated through the application of Beer’s law shown below

ð2Þ

where de is the effective depth of penetration, de^ is the penetration of infrared radiation polarized perpendicular to the surface normal, and de is the penetration of the infrared radiation polarized parallel to the surface normal. This is different from the standard depth of penetration equation that refers to the reduction in amplitude of the infrared radiation at a distance of 1 /e from the surface.38 The quantities in eq 2 can be calculated as )

Figure 1. Schematic of the ZnSe internal reflection element coated with a TiO2 film. Arrows show the path of the infrared beam through the crystal.

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(42) Rivera, D.; Harris, J. M. Langmuir 2001, 17, 5527–5536. (43) Poston, P. E.; Rivera, D.; Uibel, R. H.; Harris, J. M. Appl. Spectrosc. 1998, 52, 1391–1398.

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shown in the equations below de^ ¼

)

de ¼

λðn1 =n2 Þcos θ 2 πð1 -n1 =n22 Þðsin2 θ -n12 =n22 Þ1=2

ð3Þ

λðn1 =n2 Þcosθð2 sin2 θ -ðn1 =n2 Þ2 Þ πð1 -n12 =n22 Þðð1

þ n12 =n22 Þsin2 θ -n12 =n22 Þðsin2 θ -n12 =n22 Þ1=2

ð4Þ where n1 is the refractive index of aqueous media at the wavelength of interest, n2 is the refractive index of the ZnSe, λ is the wavelength of light, and θ is the angle at which infrared radiation enters the ZnSe IRE, which is 45 for all ATR experiments. Once the effective depth of penetration is calculated, the concentration of CPBM at the TiO2/water interface can be estimated by multiplying de by the total number or reflections of the ZnSe IRE, 12 for the IRE used in this work. This path length can then be used in conjunction with ε and the absorbance reading from the ATR spectra to calculate the concentration of CPBM in the ATR FTIR experiments. The polarization ratio of the FTIR is a measure for the wavenumbers of interest and is taken into account in the calculations of de because the magnitude of the electric field with a given orientation does influence the de calculations.36 The refractive indices for water and ZnSe are adjusted for the wavelength of interest by using empirically derived equations in the literature.36 The change in the refractive index of ZnSe with wavelength is less than 1% for the wavelengths studied whereas the change in the refractive index of water is upwards of 20% for the wavelengths used in this study. The refractive index of TiO2 in this study is assumed to be 2.4944 and constant to within 1% as a function of wavelength, which is reasonable given the crystalline nature of the TiO2 nanoparticles.45 The refractive index of TiO2 is just slightly greater than the refractive index of ZnSe, 2.41-2.43, at the infrared wavelengths used in this study. The path length, l, in eq 1 for the ZnSe IRE is calculated by multiplying de by the number of reflections on the IRE, 12 in this case. Some assumptions are implicit in the analysis described above. One major assumption is that the TiO2 surface does not significantly alter the strengths of the vibrational modes of PSS and CPBM. The solution-phase data of PSS and CPBM does not show any significant changes in the spectral signatures relative to the spectra of PSS and CPBM at the TiO2/water interface. Therefore, solution-phase calculations of concentration can be applied to the surface species, as has been done in other studies.35,36 Another major assumption is that the depth of penetration is essentially unaffected by the presence of the TiO2 film. For films that are very thin and uniform, the equations governing the depth of penetration and the magnitude of the electric field have been explicitly worked out.35,36,38,40 Quantitative ATR work has been done with porous films using a volume-weighted methodology and modifications to the electric filed amplitude equations outlined by Harrick.39 However, that approach is useful only for films whose constituents have a lower refractive index than the ZnSe IRE over the wavelength range of interest. Equations 3 and 4 are assumed to be valid for the calculation of de in this study given the high porosity of these types of films39 and a majority of the infrared radiation is likely to couple first with water rather than TiO2. Once the infrared radiation is coupled to water, eqs 3 and 4 apply and any interaction of the evanescent wave with TiO2 would either result in transmission through the particle or Rayleigh scattering. If the infrared radiation in the ZnSe IRE couples with a TiO2 particle, then the radiation is almost completely transmitted to the TiO2 particle because the refractive index of the two materials are within 3% of one another over the wavelength (44) CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1991. (45) Busani, T.; Devine, R. A. B. Semicond. Sci. Technol. 2005, 20, 870–875.

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region of interest. The Fresnel equations governing the reflectance of light in the parallel and perpendicular directions show that only 1.5% or less of the light is going to be reflected across the TiO2/ ZnSe interface. The coupling of the refractive indexes effectively extends the surface boundary of the IRE before the infrared radiation enters the water. Given the small size of the TiO2 nanoparticles, 12 ( 2 nm, it is unlikely that the extension of this boundary significantly effects the depth of penetration of the evanescent wave significantly. Calculations of the PSS concentration are based on the average of four different infrared bands due to the sulfonate functional group and the backbone vibrations of the PSS (see below). This is done as a least-squares multivariate calculation in Matlab and has been described thoroughly elsewhere.33,42,43 The bands that are used to quantitate the PSS concentration are separated from major CPBM bands, and because of the multivariate nature of the calculation, any interference with one band is offset by the use of the other bands. Calculations of the CPBM concentrations are done by subtracting out the peak height contributions of the PSS for the CH2 stretching band at 2924 cm-1. The peak height contributions are calculated by using spectral data at 2924 cm-1 for PSS alone and estimating the εb parameter for the CH2 stretching band at 2924 cm-1. Because the PSS concentrations can be calculated independently of CBPM, this information can be used to estimate the peak height at 2924 cm-1 due to PSS. The remaining absorbance after subtraction of the CH2 due to PSS is due to the CH2 stretching mode of the CPBM, and the CPBM concentration is calculated as discussed above. This subtraction method does lead to a larger error in the resulting data because of the need to propagate the error of two absorbance values in the subtraction. The pyridinium vibrational mode of the CPBM was not used to quantitate the amount of CPBM at the TiO2 water interface because it proved to be a very weak band, and for lower concentrations of CPBM, it was masked by the interference of water vapor bands. Characterization of TiO2 Film. Deposition of the film as described in the Experimental Section produced a film of TiO2 nanoparticles that proved to be robust to the weak shear force of the water injected into the flow cell and is consistent with other work with TiO2 films prepared from this method.21 Four different PSS adsorption ATR FTIR isotherm experiments utilizing different TiO2 films showed that the average amount of PSS that accumulates in the interfacial volume over the course of the experiment is (2.17 ( 0.13)  10-4 M PSS. The 6.2% rsd in the experiments indicates that the TiO2-coated ZnSe IRE elements yield a reproducible surface. Though TiO2 is transparent in the mid-IR spectrum, the effect of the TiO2 film is evident from the fact that the amount of PSS signal in the interfacial volume is 335 times greater for the PSS than what would be seen in an ATR spectrum with no TiO2 film on the ZnSe crystal for a concentration of PSS at 1.85  10-6 M. The robustness of the TiO2 films in the PSS solution over time was tested by leaving the TiO2 film in contact with the 1.85  10-6 M solution of PSS for 72 h, which is much greater than the time required for the experiments described. After corrections for any spectral drift, the overall variance in the PSS signal was quite small, (5% rsd at most. Because of the nature of how the films are prepared (Experimental Section), the thickness is not uniform across the ZnSe IRE and one can see that the films are quite rough just by visual inspection. However, as the data above indicates, the ATR infrared signal is reproducible to within 6.2% rsd for the adsorption of PSS alone. This indicates that the sampling of the film by the infrared beam at the 12 reflections along the IRE averages out the irregularity of the TiO2 surface. The average thickness of the film cannot be more than 0.579 μm because that is the effective depth of penetration of the infrared radiation at 2900 cm-1, which is in the middle of the CH2 stretching vibrational region. Vibrational bands in this region track the behavior of the sulfate bands in the 1100 cm-1 wavenumber region (see the next section) if the infrared radiation has an effective depth of penetration of 1.33 μm DOI: 10.1021/la903787t

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Article according to eqs 3 and 4. If the average thickness of the TiO2 film was greater than 0.579 μm, then the bands from the CH2 stretching region would not be expected to track the bands from the sulfate region. An estimation of the thickness of the TiO2 layer from the concentration of the TiO2 particles applied to the ZnSe surface and assuming a uniform packing geometry gives a thickness of 255 nm. This is an idealized situation because the drop coating method does not yield a uniform thickness but the thickness is on the order of what would be expected given the behavior of the CH2 stretching frequency at 2924 cm-1. Because of the difference in the depth of penetration of the infrared radiation for perpendicular and parallel polarizations described in eqs 3 and 4, there is a difference in the absorbance measured depending on the orientation of PSS and CPBM. This can effect the quantification of infrared bands if either PSS or CPBM adopts a confirmation that is perpendicular or parallel to the surface normal. However, given that the TiO2 films do not have a uniform thickness it is highly unlikely that the conformational effects are seen in the quantitative data. The ridges and valleys of the film would likely average out any orientational effects.39 Although the TiO2 film is porous, the TiO2 nanoparticles occupy volume that the PSS and CPBM cannot occupy. Thus, the volume available for PSS or CPBM molecules to occupy for ATR experiments run with a TiO2 film is less than those run without the film. To estimate the volume occupied by the TiO2 particles, measurements on PSS were performed at or near the isoelectric pH of TiO2, 6.8.21 At the isoelectric point, there is no net positive charge on TiO2 and there is no columbic attraction of PSS to the TiO2 surface so the infrared signal due to PSS would be expected to be that of solution-phase PSS minus the effects of the TiO2 film. The experimental data indicates that the PSS signal with the TiO2 film present is 79.0 ( 1.4% of what is seen for ATR spectra of PSS with no TiO2 film present. The physical interpretation of this number is that the particles of the TiO2 film occupy 21% of the detection volume, and this correlates to an estimated film thickness of 280 nm, which agrees with the estimated film thickness calculated from the amount of TiO2 applied to the ZnSe IRE, 255 nm. The empirically derived value of 0.79 is factored into calculations of PSS and CPBM concentrations at the TiO2/water interface simply to normalize the numbers to a common solution volume. The uncertainty in this empirical correction factor is also propagated in the calculations. Adsorption of PSS and CPBM to TiO2 Films. Figure 2 shows a typical ATR FTIR isotherm data set for the adsorption of PSS to the TiO2 surface over a PSS solution concentration range of 1.25  10-7 to 1.86  10-6 M; in terms of the concentration of PSS, the range would be 8.75-120 ppm. The spectra shown in Figure 2a have four major infrared bands. The bands at 1179 and 1036 cm-1 are due to the asymmetric and symmetric stretching modes, respectively, of the sulfonate group on PSS.41 The bands at 1127 and 1009 cm-1 are assigned to vibrational modes of the carbon backbone of PSS.41 The concentration of PSS that accumulates at the TiO2/water interface in Figure 2b was calculated to vary from 3.63  10-5 to 2.73  10-4 M over the course of the experiment, from 2540 to 19 100 ppm of PSS at the TiO2/water interface. Estimates of the surface area of the TiO2 film based on a flat PSS adsorption at the concentrations used in this study and an area per monomer unit of ∼0.50 nm244 put the surface area at 150 cm2. This puts the amount of PSS adsorbed at the TiO2 water interface in the 0.1-0.7 mg/m2 range, which is well within the range of what has been seen for PSS adsorption by other surfaces and at the air/water interface.30 Figure 2b represents the average behavior of four isotherm adsorption experiments of PSS adsorbing to the TiO2/water interface. Figure 2b shows that there is a roughly linear increase in the amount of PSS at the TiO2/water interface for the first four PSS solution concentrations, 1.25  10-7 to 3.60  10-7M, followed by a leveling off of the amount of PSS at the TiO2 surface as the concentrations of PSS are increased to 1.85  10-6 M. If solution PSS concentrations are increased 5538 DOI: 10.1021/la903787t

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Figure 2. (a) In situ ATR FTIR spectra of PSS accumulating at the TiO2/water interface for solution PSS concentrations ranging from 1.25  10-7 to 1.85  10-6 M (8.75 to 120 ppm). (b) Concentration of PSS at the TiO2/water interface as a function of solution PSS concentration (same as in part a). The concentration of PSS at the TiO2/water interface was calculated using the data from the bands at 1179, 1127, 1036, and 1009 cm-1 from the data in part a. The percent relative standard deviation (% rsd) of the data in part b is (6.2%. above 2.00  10-6 M, then the signal due to PSS increases again with a linear dependence on concentration. This type of linear behavior at the higher concentrations is indicative of solutionphase PSS accumulating at high enough concentrations in the ATR collection volume to be detected. Such linear solution-phase behavior has been seen in ATR experiments with simpler organic molecules adsorbing to sol-gel surfaces.42,43,46 The reason for the rollover in the adsorption isotherm at a concentration near 4.32  10-7 M PSS is likely due to PSS effectively neutralizing the positive charge on the titanium dioxide and effectively creating a negatively charged surface that then repulses other PSS molecules.19,33 Figure 3 shows the average adsorption isotherms for PSS accumulating at the TiO2/water interface with no CPBM surfactant added, with 3.60  10-7 M CPBM added, and with 1.02  10-5 M CPBM added. Each of the four bands mentioned above is (46) Rivera, D.; Poston, P. E.; Uibel, R. H.; Harris, J. M. Anal. Chem. 2000, 72, 1543–1554.

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Figure 3. Concentration of PSS accumulating at the TiO2/water interface as a function of PSS solution concentration ranging from 1.25  10-7 to 1.85  10-6 M (8.75 to 120 ppm) for no CPBM added (circles), 3.60  10-7 M CPBM added (triangles), and 1.04  10-5 CPBM added (squares). The concentration of the PSS at the TiO2/water interface was calculated using the data from the bands at 1179, 1127, 1036, and 1009 cm-1. The % rsd for no CPBM added is 6.2%, and the % rsd for the data with CPBM added is 14.2%. separated from the major bands of the CPBM infrared spectrum so that the PSS concentration can be calculated in the presence of CPBM with no interference. For PSS solutions with 3.60  10-7 M CPBM added, there is no statistical difference compared to PSS solutions with no CPBM added for PSS accumulation at the TiO2/water interface up to PSS solution concentrations of 1.29  10-6 M PSS in solution. Once solution PSS concentrations of 1.29  10-6 M and above are introduced, the differences become statistically significant and the amount of PSS that accumulates at the TiO2/water interface is 23 ( 3% less for PSS solutions with 3.60  10-7 M CPBM added than with PSS solutions with no CPBM added. Information on the accumulation of CPBM at the TiO2/water interface for PSS solutions with 3.60  10-7 M CPBM can be extracted from the ATR FTIR spectra of the CH2 stretching band at 2924 cm-1 shown in Figure 4a. Figure 4b shows the behavior of the CPBM CH2 stretching band at 2924 cm-1 after the data has been corrected to exclude the contribution of the CH2 bands from PSS (Experimental Section). As seen in Table 1 and Figure 4b, the mole ratio of CPBM to PSS ranges from 5.5 ( 1 to 1 to 2.5 ( 0.4 to 1 over the first three PSS solution concentrations. The ratio of CPBM to PSS then increases by a factor of 2 to average 10 (2 to 1 over the next three PSS solution concentrations. The ratio of CPBM to PSS then increases again by a factor of 2 to an average of 20 ( 4 to 1 for the last six concentrations of the PSS isotherm. This stepwise accumulation of CPBM at the TiO2/water interface lags behind the accumulation of PSS at the TiO2/water interface shown in Figure 3, indicating that the CPBM does not come to the surface initially with the PSS and continues to accumulate after the PSS accumulation has started to level off. This indicates that CPBM and PSS are not coming to the TiO2/ water interface as aggregates but as separate species. This is supported by solution-phase du No€ uy ring experiments that show that the addition of CPBM at a concentration of 5  10-7 M and PSS at a concentration of 5.00  10-6 M results in an 8.4% decrease in surface tension relative to that for a solution of 5.00  10-6 M PSS alone. These results suggest that CPBM at a concentration of 3.60  10-7 M is not forming aggregates with PSS. Langmuir 2010, 26(8), 5534–5543

Figure 4. (a) Raw ATR FTIR data of the CH2 and CH3 stretching

region for PSS solution concentrations ranging from 1.25  10-7 to 1.85  10-6 M (8.75 to 120 ppm) and a CPBM concentration of 3.60  10-7 M. (b) Behavior of the CH2 stretching band for CPBM at 2924 cm-1 in part a after subtraction of the contributions of PSS calculated from the PSS concentration data at the TiO2/water interface shown in Figure 3. Within the 4 cm-1 resolution of the experiments, the CH2 stretching bands due to the PSS and CPBM are not separable except by the subtraction methodology. The % rsd for the data in part b is 21%.

This stepwise accumulation of CPBM at the TiO2/water interface indicates that PSS needs to undergo some rearrangement at the interface before CPBM can accumulate at the TiO2/water interface. This could be a kinetic phenomenon because the concentration of PSS at the TiO2/water interface shown in Figure 3 does not change significantly for the concentration range over which the CPBM to PSS mole ratio increases from 10 ( 2 to 1 to 20 ( 4 to 1. The mole ratio of CPBM to PSS at the TiO2/water interface for CPBM concentrations of 3.60  10-7 M is much less than what is seen for solution-phase aggregates of PSS and n-alkyltrimethyl ammonium bromides that have ratios of 75-85 to 1 of the surfactant to PSS.29 Studies of polymethylmethacralyte, polyvinylsulfates, and dextran sulfate with n-alkyltrimethyl ammonium bromides have shown the ratio to be in the in the 100-120 to 1 range.26,28 Table 1 also shows that the number of CPBM molecules to the monomer units of PSS is also quite low, ranging from 0.010 to 1 to 0.062 to 1 compared to literature values DOI: 10.1021/la903787t

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Table 1. Mole Ratios of CPBM to PSS and Ratio of CPBM Molecules per PSS Monomer at the TiO2/Water Interface with Increasing Solution PSS Concentration for PSS Solutions with 3.07  10-7 M CPBMa solution concentration 0.125 0.250 0.300 0.360 0.432 0.518 0.622 0.746 0.896 1.08 2.29 1.548 1.858 of PSS (M)  10-6 mol ratio of CPBM to PSS 5.5:1 3.5:1 2.5:1 8.9:1 9.0:1 12:1 10:1 17:1 23:1 18:1 21:1 23:1 17 ratio of CPBM molecules to 0.016:1 0.010:1 0.0074:1 0.026:1 0.027:1 0.035:1 0.030:1 0.050:1 0.066:1 0.052:1 0.062:1 0.066:1 0.051:1 PSS monomer a The % rsd for the ratios is (21%.

of 3 to 1 to 1.5 to 1.31 The fact that the ratio of CPBM to PSS for a CPBM concentration of 3.60  10-7 M is much lower than what is seen in solution-phase polyelectrolyte/surfactant aggregations provides extra evidence that CPBM and PSS do not come to the surface as aggregates. Figure 3 also shows that for PSS solutions that contain 1.02  10-5 M CPBM there is a statistically significant increase in the amount of PSS that accumulates at the TiO2/water interface at a solution-phase PSS concentration of 2.50  10-7 M compared to the isotherm with no CPBM added, (1.20 ( 0.17)  10-4 M PSS at the TiO2/water interface compared to (8.19 ( 0.05)  10-5 M PSS at the TiO2/water interface. At a solution-phase PSS concentration of 4.30  10-7 M PSS, the concentration of PSS accumulated at the TiO2 surface is (3.15 ( 0.44)  10-4 M compared to a PSS concentration of (2.05 ( 0.13)  10-4 M at the TiO2 surface for solutions with no CPBM added. This represents a 56.0 ( 7.9% increase in the amount of PSS adsorbed to the surface with CPBM present as compared to no CPBM present. The increased accumulation of PSS at the TiO2/water interface is expected as a result of the decreased charge of the PSS in solution due to interactions with CPBM.19 This interaction prevents water from being able to solvate the polyelectrolyte as effectively, and adsorption of the aggregate at the TiO2 surface becomes more energetically favorable. Studies of PSS interacting with n-alkyltrimethylammonium bromides show that the aggregation of PSS and the surfactant can occur at concentrations similar to the PSS and CPBM concentrations described in these experiments.19,27,30-32 Figure 5a shows the spectral data for the behavior of the CH2 stretching band for PSS solutions with 1.02  10-5 M CPBM added. Figure 5b shows the behavior of the CH2 stretching band for CPBM once the contributions of the PSS have been subtracted out of the spectral data shown in Figure 5a. The behavior of the CH2 band shows a rollover in the isotherm that is very similar to what is seen in the isotherm for the PSS bands in the 1000-1300 cm-1 region for this system. The data in Figure 5b also shows that a significant amount of CPBM is present at low PSS concentrations compared to what is seen at a CPBM concentrations of 3.6  10-7 M. Because CPBM is present at the TiO2/water interface at low PSS concentrations and its behavior mimics the rollover of the PSS isotherm seen in Figure 3, the spectral data suggests that CPBM and PSS come to the surface as aggregates. Evidence for the aggregation of CPBM with PSS was also seen in du No€ uy ring experiments that showed no measurable change in the surface tension with the addition of CPBM at concentrations ranging from 1  10-6 M CPBM to 1  10-5 M CPBM in solutions of PSS at 1  10-6 M. The fact that no change in the surface tension was observed when adding CPBM over this concentration range indicates that CPBM is aggregating with PSS at these concentrations. The cmc was measured to be (4.1 ( 0.6)  10-4 M for CPBM, so a CPBM solution of 1.02  10-5 M CPBM would be a reasonable concentration for aggregation to occur in the presences of an oppositely charged polyelectrolyte. The isotherm for PSS accumulation in Figure 3 abruptly levels off at PSS solution concentrations greater than 4.32  10-7 M. The final concentration of PSS at the TiO2/water interface for a CPBM concentration of 1.02  10-5 M CPBM in Figure 3 is statistically indistinguishable from PSS concentrations at the TiO2 surface with no CPBM present, (2.54 ( 0.35)  10-4 and (2.17 ( 0.13)  10-4 M, respectively, at a PSS solution concentration of 1.86  10-6 M. This indicates that the aggregation of PSS with CPBM does not greatly influence the amount of PSS that adsorbs at the TiO2/water interface. 5540 DOI: 10.1021/la903787t

Figure 5. (a) Raw ATR FTIR data of the CH2 and CH3 stretching

region for PSS solutions ranging from 1.25  10-7 to 1.85  10-6 M (8.75 to 120 ppm) and a CPBM concentration of 1.02  10-7 M. (b) Behavior of the CH2 stretching band for CPBM at 2924 cm-1 after subtraction of the contributions of PSS calculated from the PSS concentration data at the TiO2/water interface shown in Figure 3. The % rsd for the data is 21%.

Mole ratios of CPBM to PSS at the TiO2/water interface for a CPBM solution concentration of 1.02  10-5 M are calculated in Table 2 along with the ratio of CPBM molecules to PSS monomers. The initial ratio of CPBM to PSS is 139 ( 29 to 1. This ratio of CPBM to PSS is roughly a factor of 2 greater than what has been reported for n-alkyltrimethylammonium bromides aggregating with PSS in solution, 75-85 to 1,29 but is not significantly outside the range of n-alkyltrimethylammonium bromides aggregating with other polyelectrolytes, 100-120 to 1.26,28 This high ratio of CPBM to PSS is what drives the higher concentration of Langmuir 2010, 26(8), 5534–5543

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Table 2. Mole Ratios of CPBM to PSS and Ratio of CPBM Molecules per PSS Monomer at the TiO2/Water Interface with Increasing Solution PSS Concentration for PSS Solutions with 1.02  10-5 M CPBMa solution concentration of 0.125 0.250 PSS (M)  10-6 mol ratio of CPBM to PSS 139:1 54:1 ratio of CPBM molecules 0.41:1 0.16:1 to PSS monomer a The % rsd for the ratios is (21%.

0.300

0.360

0.432

0.518

0.622

0.746

0.896

1.08

2.29

1.548

1.858

37:1 0.11:1

32:1 0.093:1

33:1 0.096:1

30:1 0.088:1

29:1 0.086:1

31:1 0.091:1

32:1 0.095:1

31:1 0.090:1

33:1 0.096:1

35:1 0.10:1

39:1 0.12

PSS at the TiO2/water interface for the first five PSS solutions compared to the same PSS concentration at the interface with no CPBM present (Figure 3). The large number of CPBM molecules interacting with PSS decreases the effective charge, making adsorption to the TiO2 surface more favorable. Table 2 shows that the mole ratio of CPBM to PSS decreases abruptly as the solution-phase PSS concentration is increased and the CPBM to PSS mole ratio appears to level off to an average value of 33 ( 7 to 1. Figure 5b shows that this decrease in the mole ratio of CPBM to PSS from 139 to 1 to 33 to 1 is not due to a loss of CPBM at the surface because the concentration of CPBM at the TiO2/water interface continues to increase as a function of PSS solution concentration. If there was a significant loss in CPBM to the solution phase, then the number of moles of CPBM at the surface would be expected to decrease as the CPBM went into solution to interact with excess PSS. Although there is a statistical difference between the mole ratios of solutions with 3.60  10-7 M CPBM, 20 ( 4 to 1, compared to 1.02  10-5 M CPBM solution, 33( 7 to 1, the ratio of CPBM to PSS changes only by roughly a factor of 1.6 for a increase in CPBM concentration by a factor of 27. One variable that is consistent in each of the experiments shown in Figure 3 is that the solution-phase PSS concentration, 4.30  10-7 M PSS, at which the PSS starts to roll over is essentially unchanged for each of the three experimental conditions. This is an interesting result, since it does not appear that the addition of CPBM significantly influences the rollover point of the PSS isotherm at the concentrations of the experiments summarized in Figure 3. If aggregates of PSS and CPBM accumulate as the spectroscopic data suggest, then CPBM would be expected to neutralize some of the charge of the PSS and allow more PSS to come to the TiO2 surface. This does appear to be the case for solutions with 1.02  10-5 M CPBM, as 56 ( 8.0% more PSS accumulates at the interface relative to no CPBM present. However, these accumulations occur at concentrations at or below 4.30  10-7 M PSS solution concentrations. Once solution concentrations of 4.30  10-7 M PSS are reached, the rollover shown in Figure 3 for PSS with 1.02  10-5 M CPBM is quite abrupt. For adsorption of PSS to the TiO2 surface in the presence of lower surfactant concentrations, 3.60  10-7 M CPBM, the concentration of PSS in the detection volume is less, on average, compared to no CPBM present, but only by 23%. Figure 6a shows the spectral data of the PSS bands in the 1000-1300 cm-1 region, and Figure 6b shows the spectral data for the CH stretching bands in the 2820-2980 cm-1 region. Figure 7a,b shows the PSS behavior in the 1000-1300 cm-1 spectral region and the CH2 stretching band at 2924 cm-1, respectively, for experiments with CPBM concentrations of 1.04  10-4 M and PSS concentrations ranging from 1.25  10-7 M to 1.86  10-6 M. The data presented in Figure 7b show that an initial signal is present due to the CH2 stretching band at 2924 cm-1 for CPBM and does not significantly change at PSS concentrations from 1.25  10-7 M to 3.60  10-7 M PSS. However, Figure 7a shows that no significant increase in the spectral signal for the 1000-1300 cm-1 region is detected over this concentration range. Since the PSS has a strong signal in the 1000-1300 cm-1 region, the spectroscopic data suggest that the PSS remains in solution and is not attracted to the TiO2 surface over this concentration range. The constant spectral signal observed for the band at 2924 cm-1 in Figure 7b is due to accumulation of the CBPM at the TiO2 surface. Accumulation of the CPBM at the TiO2 surface is surprising given that the Langmuir 2010, 26(8), 5534–5543

Figure 6. (a) In situ ATR FTIR spectra of PSS accumulating at the TiO2 water interface for solution-phase PSS concentrations ranging from 1.25  10-7 M to 1.85  10-6 M (8.75 to 120 ppm) with 1.04  10-4 M CPBM present. (b) Raw ATR FTIR data of the CH2 and CH3 stretching region for PSS and CPBM concentrations identical to those in (a). surfactant carries a positive charge and should be repelled from the positively charged TiO2 surface. However, 1.04  10-4 M CPBM is within a factor of 4 of the cmc for CPBM, so the surfactant would be quite surface-active. It is unlikely that the CPBM is precipitating out of solution onto the TiO2 surface, since the intensity of the infrared signal in the 2800-2980 cm-1 region is essentially the same intensity seen for the CH2 stretching bands when CPBM/PSS aggregates accumulate at the TiO2/water interface for CPBM concentrations of 1.02  10-5 M. Figure 7b shows that once the PSS concentration of 4.32  10-7 M is reached a large increase in the CH2 stretching band at 2924 cm-1 is observed along with an increase in the bands associated with the PSS in the 1000-1300 cm-1 region shown in Figure 7a. DOI: 10.1021/la903787t

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Article However, as Figure 7b shows there is a significant increase in the intensity of the CH2 stretching bands after the PSS contribution to this signal is subtracted out. This observation indicates that a substantial amount of CPBM is accumulating at the TiO2/water interface, as PSS starts to accumulate at the TiO2/water interface, and it is highly probably that PSS and CPBM come to the TiO2 surface as an aggregate. The signal from both spectral regions continues to increase as the PSS concentration is raised to 5.18  10-7 M PSS. Above this concentration, Figure 7b shows that the CPBM concentration decreases with increasing solution-phase

Figure 7. (a) Concentration of PSS accumulating at the TiO2/ water interface as a function of PSS solution concentrations ranging from 1.25  10-7 M to 1.85  10-6 M (8.75 to 120 ppm) with 1.04  10-4 M CPBM added. (b) Behavior of the CH2 stretching band for CPBM at 2924 cm-1 after subtraction of the contributions of PSS at the TiO2/water interface given in (a). The % rsd for the data in (b) is 21%.

Hase et al. PSS concentrations to 53 (10% of the value at 5.18  10-7 M solution-phase PSS. This decrease is not a kinetic phenomenon on the time scale of the experiment, as additional ATR FTIR experiments have shown the bands in the 2800-2980 cm-1 region to be very stable; band intensities vary by (3% over a 15 h period at PSS concentrations of 5.18  10-7 and CPBM concentrations of 1.04  10-4 M. Since Figure 7a shows that the PSS concentration at the TiO2/water interface has essentially leveled off at solution PSS concentrations of 5.18  10-7 and higher, the decrease in the signal in Figure 7b is due to a decrease in the concentration of CPBM at the TiO2/water interface. Table 3 shows the mole ratio of CPBM to PSS and ratio of CPBM molecules to PSS monomer for PSS solutions with CPBM concentrations of 1.04  10-4 M. The CPBM to PSS mole ratio when PSS first starts to accumulate at the TiO2/water interface is quite high, (630 ( 130) to 1, compared to the other experiments in this study. However, the ratio of CPBM molecules to PSS monomers shown in Table 3 is in line with other ratios of n-alkylammonium bromide surfactant molecules to PSS monomer units, 3:1 for surfactant concentrations of 9  10-4 M.31 This ratio drops steadily with increasing PSS concentration until it reaches a mole ratio (110 ( 22) to 1 at a solution PSS concentration of 1.85  10-6 M. This mole ratio is in line with ratios of poly(methyl methacrylate) polyelectrolytes, poly(vinyl sulfonates), and dextran sulfonate with n-alkyltrimethyl ammonium bromides shown to be in the (100-120) to 1 (surfactant to polymer) range.26,28 Given that the PSS is considered a very strong negatively charged polyelectrolyte,19,29-32 it is surprising that it does not force the CPBM off the TiO2 surface until concentrations of 4.32  10-7 M are reached. This indicates that at concentrations at or near the saturation limit of CPBM in solution with PSS (Experimental Section) the PSS is a much weaker negatively charged polyelectrolyte due to the interactions with a relatively large number of positively charged CPBM molecules per PSS molecule, ∼800 to 300 CPBN molecules per PSS. This decrease in infrared signal for the CH2 band at 2924 cm-1 seen in Figure 7b is most likely due to a shift in the equilibrium concentration of CPBM that can be supported in the solution phase as aggregates of PSS and CPBM as the solution concentration of PSS is increased. This is supported by the fact that there is no significant decrease in the surface tension of aqueous solutions of PSS and CPBM over the concentration range discussed above, indicating that most of the CPBM in solution forms micellular aggregates with the PSS under this range of concentrations and is consistent with studies of PSS and n-alkyltrimethylammonium bromides in aqueous solution.19,29-32 As the solution is able to accommodate more CPBM/PSS aggregates, some of the CPBM at the TiO2/ water interface goes into solution and lowers the amount of CPBM at the interface. The amount of PSS that accumulates at the TiO2/water interface is indistinguishable within the uncertainties from what is seen with CPBM concentrations of 1.02  10-5 M and indistinguishable from the PSS isotherm with no CPBM present. This implies that the structure of the PSS at the TiO2/ water interface is not significantly changed from the system with no CPBM present or CPBM at 1.02  10-5 M. Also, the change in the amount of CPBM at the interface is not stepwise as for the solutions with CPBM at 3.60  10-7 but continuous, indicating that the structure of the established at the interface occurs on a faster time scale than what is seen for PSS solutions with CPBM concentrations of 3.60  10-7 M.

Table 3. Mole Ratios of CPBM to PSS and Ratio of CPBM Molecules per PSS Monomer at the TiO2/Water Interface with Increasing Solution PSS Concentration for PSS Solutions with 1.04  10-4 M CPBMa solution concentration of PSS 0.432 0.518 0.622 0.746 (M)  10-6 mole ratio of CPBM to PSS 630:1 440:1 370:1 280:1 ratio of CPBM molecules to 1.9:1 1.3:1 1.1:1 0.83:1 PSS monomer a The first four solution PSS concentrations are not shown, because no detectable concentrations. The % rsd for the ratios is (21%.

5542 DOI: 10.1021/la903787t

0.896

1.08

1.29

1.55

1.86

210:1 0.61:1

160:1 0.47:1

130:1 0.39:1

110:1 0.33:1

110:1 0.33:1

amount of PSS was adsorbed to the TiO2 surface at those

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Figure 8. Interactions of CPBM (short dark lines) with PSS (long wavy lines) at high CPBM to PSS mole ratios (left) and lower CPBM to PSS mole ratios (right) at the TiO2/water interface as viewed from above. The data given in Figures 3, 5, and 7 and Tables 2 and 3 indicate that at high CPBM to PSS mole ratios large numbers of CPBM molecules are interacting with relatively isolated PSS molecules. As this ratio decreases, the amount of PSS at the TiO2/water interface is indistinguishable from what is seen with no surfactant added, indicating that the CPBM may cause local disruption of the PSS structure, but on the whole, the organization of the PSS is likely the same as with no CPBM added. The shapes of the CPBM and PSS are purely for purposes of illustration and not meant to indicate any particular structural confirmation of the molecules. Data from Figures 3 and 7a show that the concentration of PSS that adsorbs at the TiO2/water interface is statistically indistinguishable for PSS solutions with 0, 1.02  10-5 M, and 1.04  10-4 M CPBM at a PSS concentration of 1.85  10-6 M . A larger change in the amount of PSS at the TiO2/water interface would be expected, since surfactant/polyelectrolyte aggregates are assumed from coiled-type structures in solution,29 and if the PSS adopted a coiled-type geometry on the surface, the overall amount of PSS adsorbed at the TiO2/water interface would be expected to increase significantly. While there is some evidence of this in Figure 3 for the lower solution PSS concentrations (see discussion above), it does not hold true at the higher PSS solution concentrations. Even if the uncertainties in the data are ignored and just the average values for the PSS accumulation are taken into account, the maximum increase in concentration of PSS at the TiO2/water interface that is seen is 19% for PSS solution concentrations of 1.85  10-6 M. While the ATR FTIR data in this study cannot confirm structural conformations, it is likely that that the PSS has a similar structure at the interface with CPBM present at ratios given in Tables 2 and 3 to when there is no CPBM present. In the presence of CPBM, the PSS may form small coiled domains with the CPBM and/or the CPBM fills in vacant space between PSS molecules at the interface. A graphical summary of the data in this is presented in Figure 8. As shown in this figure, the initial structure of the CPBM aggregate is likely to involve large numbers of CPBM molecules around relatively isolated PSS molecules on the TiO2 surface. As the ratio of CPBM to PSS decreases with increasing solution-phase PSS concentration, the data in this work suggest that the PSS forms a structure at the TiO2/water interface that is not too dissimilar to the structure it forms with no CPBM present, but the CPBM does form kinks in the structure of the PSS. Once this structure is formed, it is very robust on the time scales of the experiment. Data in Tables 1, 2, and 3 show that stable ratios of CPBM to PSS are reached and that increasing the solution-phase PSS concentration does not significantly influence these ratios. Thus, addition of excess PSS in the solution phase does not force CPBM to leave the TiO2/water interface and interact with the solution

Langmuir 2010, 26(8), 5534–5543

Article phase on the time scales of the experiments carried out in this study. While this could be a kinetic phenomenon as polyelectrolytes are known to form kinetically frozen states at surfaces,23 the data show that the CPBM forms a very stable structure with PSS at the TiO2 water interface. The stability of the CPBM to PSS ratio has been found to be stable for at least 15 h. This result is not completely surprising given the stability of multilayer films of polyelectrolytes and surfactants that are used in a variety of application, but this work does show that a certain ratio of surfactant to polyelectrolyte is reached that appears to be the most stable for the systems studied. The concentration of PSS at the TiO2/water interface is lower for CPBM concentrations of 3.07  10-7 M compared to 0, 1.02  10-5 M, and 1.04  10-4 M CPBM solutions at a statistically significant level. The reason for the smaller accumulation is not entirely clear, but the mechanism by which CPBM comes to the TiO2/water interface for the 3.07  10-7 M CPBM solutions is in a stepwise fashion rather than as an aggregate with the PSS. This difference in the delivery of CPBM to the TiO2 interface may account for the difference in how PSS accumulates at the TiO2 water interface and the lower concentration of PSS at the interface.

Conclusions This work has shown the effect of addition of the surfactant CPBM on the adsorption of PSS to a TiO2 film through the use of quantitative in situ ATR FTIR spectroscopy at a pH of 3.5. Results of experiments outlined in this manuscript show that addition of CPBM to PSS solutions at concentrations that cause aggregation of the CPBM with PSS in solution does not significantly effect the concentration of PSS at the TiO2/water interface relative to experiments with no CPBM present at the higher concentrations of the adsorption isotherm. This suggests that the CPBM does not significantly alter the structure of the PSS at the TiO2/water interface relative to solutions with no CPBM present when the CPBM and PSS come to the surface as aggregates. At PSS solution concentrations below 4.30  10-7 M, the amount of PSS that accumulates at the TiO2/water interface is influenced by the amount of CPBM added for solutions with CPBM concentrations of 1.02  10-5 and greater. For solutions with CPBM at concentrations lower than what is likely to cause aggregation of CPBM and PSS in solution, 3.60  10-7, the concentration of PSS at the TiO2/water interface is lower than for solutions with CPBM at 0, 1.02  10-5 M, and 1.04  10-4 M. Solutions with this lower CPBM concentration show that CPBM comes to the surface in a stepwise manner rather than as an aggregate with PSS, and this difference may influence the amount of PSS that can accumulate at the TiO2/water interface on the time scales of the experiments. All experiments show that a stable ratio of CPBM to PSS is reached and that higher solution concentrations of PSS do not decrease this ratio on the time scales of the experiments. This indicates that the CPBM and PSS form a very stable structure at the TiO2 water interface that is not easily disrupted. Acknowledgment. Funding from Central Washington University office of Undergraduate Research, the Science Honors Program at Central Washington University, the Murdock Foundation, and the ACS-PRF are gratefully acknowledged.

DOI: 10.1021/la903787t

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