Molecular Length Dependence of Single Molecule Wobbling within

Article pubs.acs.org/JPCC

Molecular Length Dependence of Single Molecule Wobbling within Surfactant- and Solvent-Filled Silica Mesopores Rajib Pramanik, Takashi Ito,* and Daniel A. Higgins* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-0401, United States S Supporting Information *

ABSTRACT: The confined orientational motions of fluorescent dye molecules diffusing along one dimension (1D) within individual silica mesopores are investigated by simultaneous single molecule tracking (SMT) and single molecule emission dichroism (SMED) methods. Four perylene diimide (PDI) dyes of different lengths are employed as the probe species. Wobbling angles exhibited by the individual molecules are measured within cetyltrimethylammonium bromide-filled mesopores. The results show a clear dependence on probe molecule length, attributable to confinement of the molecular motions to small cavities within the surfactant- and solvent-filled mesopores. These results are used to obtain quantitative estimates of the accessible cavity diameters. Histograms of these data reveal a broad distribution of cavity sizes. The average cavity diameters are shown to be largely independent of molecular length and yield a global mean value of 1.06 ± 0.03 nm, corresponding to ∼1/3 the physical diameter of the silica mesopores, as estimated from X-ray scattering data. The difference in physical and accessible pore diameters is attributed to confinement of PDI orientational motions by nanostructuring of the solvent/surfactant medium filling the pores. It is proposed that the PDI molecules are confined to the most hydrophobic regions of the surfactant micelles and that formation of a water-rich solvent layer at the silica/surfactant boundary may also contribute. These results will facilitate a deeper understanding of solute−solvent interactions in nanoconfined systems and are relevant to applications of mesoporous silica materials in solution-phase catalysis and chemical separations.

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INTRODUCTION Ordered mesoporous silica materials1,2 possess unique attributes such as regular and adjustable pore sizes (ranging from 2 to 50 nm), high surface areas, and tunable surface chemistries that make them attractive for applications in chemical separations and catalysis. Control over pore diameter is readily achieved by use of different length surfactants as structure-directing agents during materials synthesis.2 Enhanced selectivities can be achieved in gas-phase applications of mesoporous materials by simply controlling the steric and/or chemical interactions between incorporated analytes or reagents and the pore walls. In their solution-phase applications, the properties of incorporated solvents play a profound role in governing these interactions. Importantly, the properties of nanoconfined solvents usually differ from those of bulk liquids. Indeed, a wealth of interesting phenomena arise when solvents are confined to nanometer-sized pores, as summarized in recent reviews.3−5 It has long been known that many solvents exhibit depressed melting points and glass transition temperatures under these circumstances.6−8 The translational and orientational motions of confined solvents are also altered. For example, Kittaka et al.9 and Gupta and co-workers10 have shown that the motions of capillary-condensed alcohols become slower as the pore size decreases in mesoporous silica, due to interactions of the alcohols with the pore walls. Vibrational spectroscopy experiments11 and modeling studies12 lead to similar conclusions. In certain cases, ordered solvent © XXXX American Chemical Society

layers appear near the pore surfaces as a result of these interactions.10,12 The properties of solvents in central pore regions have also been reported to differ from those of bulk liquids, due to molecular cooperativity.10 Such effects are particularly important in strongly interacting liquids like water, as reported by Scodinu and Fourkas.13 When mixed liquids are incorporated in the pores, apparent variations in the solvent composition along the pore radius have also been detected.14 The effects of solvent nanoconfinement in turn impact the static and dynamic properties of incorporated solutes. For example, nuclear magnetic resonance and electron spin resonance studies have revealed the slowing of spin probe motions in solvent-filled mesopores.14−16 The slowing of the translational and orientation motions of dyes has been observed by fluorescence methods,17,18 while simulations have provided molecular level models that map the locations and orientations of confined species.19 The latter studies show that probe molecule position and orientation within nanopores depend upon the nature of the solute, solvent, and pore surfaces. These highly complex materials require further study if steric and chemical interactions between solutes and silica pore surfaces are to be effectively manipulated to achieve enhanced selectivities in their solution-phase applications. Received: May 21, 2013 Revised: June 24, 2013

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dx.doi.org/10.1021/jp404991m | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Surfactant-filled mesoporous silica films represent an important model system upon which to base such studies.20,21 They also have important applications of their own in micelleenhanced ultrafiltration,22 and surfactant-controlled transport through nanopores.23 The translational diffusion of fluorescent dye molecules within one-dimensional (1D) surfactant-filled silica mesopores has been studied previously by Bräuchle et al.,17,24,25 Inagaki and co-workers,26 and our group.27−29 These single molecule tracking (SMT) studies have conclusively demonstrated that fluorescent probe molecules diffuse very slowly within the mesopores, yielding diffusion coefficients of 1−10−4 μm2/s, depending on the exact dyes and surfactants employed, and the solvent content of the pores. Interestingly, this series of studies also demonstrates that the orientational motions of the diffusing probe molecules are highly variable, with the dye molecules diffusing in an oriented state in relatively dry materials,25,28,29 and exhibiting free molecular rotation in larger pores17,25 and/or those filled with liquid solvents.27 Investigations of probe molecule orientational motions30 represent an important alternative route to understanding the effects of solvent and solute nanoconfinement. Importantly, such measurements can provide insight into structuring of the medium (i.e., solvent and surfactant) incorporated within the pores. We recently reported a quantitative method for measuring the orientational motions of single molecules diffusing along 1D within silica mesopores.29 This method involved recording SMT data simultaneously in two orthogonal emission polarizations to obtain single molecule emission dichroism (SMED) data.31 The 1D trajectories obtained were fit to linear functions using orthogonal regression methods28 to determine the orientations of the individual mesopores in the film plane. Taken together, the pore orientation and SMED data conclusively demonstrated that the perylene diimide (PDI) dye employed diffused within the pores in a highly oriented state, with its long axis aligned, on average, parallel to the pore axis. These data also showed that the fluorescence from individual molecules was somewhat depolarized. We attributed this depolarization to strongly confined orientational motions (i.e., wobbling) of the dye molecules within the pores, as depicted in Scheme 1. We showed that the apparent wobbling angle and the accessible cavity diameter could be estimated from the results. Unfortunately, the possibility that fluorescence depolarization was instead due to pore tortuosity on short (∼100 nm) length scales could not be discounted. In this report, we expand upon these studies by employing a series of four PDI molecules to probe the mechanisms of molecular confinement within solvent-filled, surfactant-templated silica mesopores. The probe molecules are designated as C11OPDI, C7OPDI, C4OPDI, and C1PDI. All have different lengths, as shown in Scheme 1. Cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent and was left in place after film synthesis. The translational and orientational motions of the dye molecules were induced by exposing the films to ethanol/water vapor mixtures prior to and during widefield fluorescence imaging. The SMT and SMED results obtained conclusively demonstrate that fluorescence depolarization varies with molecular length and, hence, that these measurements probe the wobbling motions of the individual dye molecules. The results depict the confinement of PDI orientational motions to cavities that are much smaller than the physical size of the mesopores and point to the important role played by nanostructuring of the incorporated

Scheme 1. (Top) Representation of Single Molecule Wobbling Motions in a CTAB-Filled Cylindrical Silica Mesoporea and (Bottom) Chemical Structures of the Perylene Diimide (PDI) Dyes Employed, along with Their Average Lengthsb

The parameters θ and d represent the wobbling angle and the diameter of the solvent- and surfactant-filled cavity accessible to the dye.

a

b

The length of C1PDI was determined using freely-available molecular modeling software.36 The length of the alkyl ether tails in C4OPDI− C11OPDI were estimated using a freely-rotating chain model.37 The width of the PDI molecules is estimated to be 99.9% confidence. In our initial report,29 which employed only a single dye, it was noted that submicrometer scale variations in the pore alignment (i.e., pore tortuosity) could explain the observed deviations in LD from the zero wobbling case. The molecular-size-dependent wobbling observed here allows this alternative explanation to be firmly discounted. The measured wobbling angles would be independent of molecular length were pore tortuosity the cause of the observed deviations. It is therefore concluded that the SMT and SMED data provide quantitative measurements of the confined PDI wobbling motions within the surfactantfilled silica mesopores.

Figure 5. Maximum wobbling angle (filled squares) vs mean molecular length. The error bars depict the 90% confidence intervals. The black line depicts a fit of the C4OPDI−C11OPDI data to the model given in eq 7, yielding an apparent mean pore diameter of 1.06 ± 0.03 nm. Mean molecular length was estimated as described in the text.

While Figure 4 shows that the wobbling angle varies with molecular length, the same data suggest that the accessible cavity diameter is largely invariant across the series of dyes employed (see upper axis in Figure 4). To further test this hypothesis, the wobbling angle data plotted in Figure 5 were fit using eq 7, assuming a constant value for d. Only the C4OPDI− C11OPDI data were included in the fit. The fitted line passes through the error bars for these three points, consistent with a constant cavity diameter of 1.06 ± 0.03 nm. The error (±0.03 nm) was obtained from the fitting procedure. The mean and 90% confidence interval determined from the 57 individual trajectories included in this analysis are 1.04 ± 0.07 nm. An invariant cavity diameter for C4OPDI−C11OPDI suggests these molecules diffuse along the same pathways within the pores. In contrast to the three longer molecules, the fitted line misses the error bar on the C1PDI data, suggesting a smaller cavity diameter (0.8 nm) in this case. While C1PDI is expected to follow a similar pathway, the results suggest it may be more F



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effects may lead to the formation of a water-rich layer at the surfactant/silica boundary that is strongly avoided by the hydrophobic dyes. It is concluded that a combination of these factors may confine the PDI molecules to central ∼1 nm diameter regions within the micelles. The cavity diameter variations depicted in Figure 4 are interpreted to arise from variations in the length scale and extent of medium nanostructuring within the pores. The apparent deviation of C1PDI from the length-dependent trend in wobbling angles (Figure 5) is consistent with this explanation. C1PDI is unlikely to permeate the hydrocarbon structure of the inner micelle core as easily, and thus should exhibit a higher level of confinement. Future studies will explore the mechanisms of orientational confinement in greater depth by implementation of charged PDI species and by removing the surfactant from the pores.

sensitive to the factors that limit the orientational motions of these dye molecules. C1PDI may also more readily enter the smallest cavities and thus yield a relatively smaller wobbling angle. Interestingly, no correlations could be detected between the wobbling angles and apparent translational diffusion coefficients, D, determined from individual single molecule trajectories (data not shown). The mean D values for each PDI species, calculated by averaging the data from the single molecules, also showed no detectable trend with molecular length. The mean D values obtained from C11OPDI−C1PDI were 0.15 ± 0.06, 0.16 ± 0.04, 0.07 ± 0.03, and 0.18 ± 0.07 μm2/s, respectively. While D is expected to decrease inversely with molecular size in bulk solution, the trend should be even more significant in nanoconfined systems.46 The lack of a clear trend in D may be due to significant experimental error in the measurements (e.g., as reflected by the deviation of the C4OPDI result, 0.07 μm2/s, from the others). However, these data may also reflect the confinement of all four PDIs to cylindrical cavities of similar diameter within the silica mesopores. In this case, the diffusion coefficient along the cavity axis would be specifically related to the diameter of the wobbling cone, rather than the molecular length. The apparent invariance of the wobbling cone diameter (Figure 5) suggests the PDI molecules should exhibit similar D values, exactly as is observed. It is concluded from the above results that the orientational motions of all PDI probes are confined to relatively small cavities (∼1 nm diameters) within the solvent- and surfactantfilled mesopores. Importantly, these cavities are much smaller than the physical diameter (∼3.7 nm) of the pores themselves, indicating that steric interactions play little or no role in molecular confinement in these materials. While the exact mechanism(s) for orientational confinement are unknown, they likely involve nanostructuring of the medium filling the mesopores. The properties of the solvent- and surfactant-filled pores are expected to vary dramatically along the pore radius, with highly polar environments found at the charged surfactant/silica boundaries and very hydrophobic environments found in the center of the micelles. The hydrophobic PDI dyes are expected to partition to the latter. However, confinement to hydrophobic regions alone also provides an unsatisfactory explanation for the level of confinement observed, because the hydrophobic core diameter of the CTAB micelles is too large (∼3.8 nm). Instead, it is believed confinement of the PDI dyes also involves structuring of the CTAB chains and/or the solvents (water and ethanol) incorporated within the pores. Support for the above model is obtained from recent theoretical and experimental reports, as well as from the known properties of CTAB micelles. For example, Vartia and Thompson have used molecular dynamics simulations to map the locations of nonpolar probe molecules confined within cylindrical, ethanol-filled silica nanopores.47 Their results show that nonpolar solutes are most often found in central pore regions where they are best solvated by the incorporated solvent. Furthermore, it is well-known that the internal properties of CTAB micelles vary along their radial dimension, with alkane chain disorder increasing with distance from the micelle surface and approaching that of bulk hydrocarbon solvents near the center of the micelle.41 Finally, mixed solvents are also expected to exhibit radial composition gradients when confined within nanopores.14 In the present materials, such

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CONCLUSIONS In conclusion, SMT and SMED measurements have been used to investigate the confined orientational motions of PDI single molecules diffusing within solvent- and CTAB-filled silica mesopores. Four different PDI species having different lengths were employed and quantitative measurements of their wobbling angles were obtained. A clear trend toward increased wobbling angles with decreasing molecular length was observed, providing strong evidence that dye wobbling is being probed in these studies. The accessible cavity diameters derived from these measurements were found to be largely independent of probe molecule length, yielding a mean value of 1.06 ± 0.03 nm. This value was shown to be ∼1/3 the physical size of the silica pores, demonstrating that orientational confinement is dominated by factors other than steric interactions between the probe molecules and pore surfaces. Confinement of PDI orientational motions was proposed to result from nanostructuring of the medium (i.e., surfactant and solvent) filling the pores. In this model, the PDI molecules are confined to the most hydrophobic central regions of the CTAB micelles. Because the hydrophobic portion of the micelles is too large to explain the full extent of confinement, it was noted that radial variations in micelle chain order and/or formation of a water-rich solvent layer at the silica/surfactant boundary could also contribute. These results provide valuable new insights into how solutes interact with nanostructured media incorporated within silica mesopores and will aid in the development of nanoporous membranes and monoliths having enhanced selectivities for applications in chemical separations and catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

Single molecule localization data, X-ray scattering results, and representative videos. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.I.); [email protected] (D.A.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support was provided by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences G



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of the U.S. Department of Energy (DE-FG02-12ER16095). Feng Li, Seok Chan Park, and Paul Smith are acknowledged for their help with the synthesis and purification of the C4OPDI dye, X-ray scattering measurements, and the estimation of molecular lengths, respectively.

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Molecular Length Dependence of Single Molecule Wobbling within

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