Adsorption, Ordering, and Local Environments of Surfactant

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Adsorption, Ordering, and Local Environments of Surfactant Encapsulated Polyoxometalate Ions Probed at the Air-Water Interface Benjamin Doughty, Panchao Yin, and Ying-Zhong Ma Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01643 • Publication Date (Web): 23 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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Adsorption, Ordering, and Local Environments of Surfactant Encapsulated Polyoxometalate Ions Probed at the Air-Water Interface Benjamin Doughty1*, Panchao Yin2,3*, Ying-Zhong Ma1 AUTHOR ADDRESS: 1. Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2. Chemical and Engineering Materials Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3. Shull Wollan Center, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Abstract: The continued development and application of surfactant encapsulated polyoxometalates (SEPs) relies on understanding the ordering and organization of species at their interface and how these are impacted by the various local environments to which they are exposed. Here, we report on the equilibrium properties of two common SEPs adsorbed to the air-water interface and probed with surface specific vibrational sum-frequency generation (SFG) spectroscopy. These results reveal clear shifts in vibrational band positions, the magnitude of which scales with the charge of the SEP core, which is indicative of a static field effect on the surfactant coating and the associated local chemical environment. This static field also induces ordering in surrounding water molecules that is mediated by charge screening via the surface bound surfactants. From these SFG measurements we are able to show that Mo132 based SEPs are more polar than Mo72V30 SEPs. Disorder in the surfactant chain packing at the highly curved SEP surfaces is attributed to large conic volumes that can be sampled without interactions with neighboring chains. Measurements of adsorption isotherms yield free energies of adsorption to the air-water interface of −46.8 ± 0.4 and −44.8 ± 1.2 kJ/mol for the Mo132 and Mo72V30 SEPs, respectively, indicating a strong propensity for the fluid surface. The influence of intermolecular interactions on the surface adsorption energies is discussed.


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Introduction: Research on polyoxometalates (POMs) has blossomed in recent years largely due to their seemingly limitless scope of practical and fundamental uses, including serving as catalysts,1,2 photo-electronic3/magnetic materials,4 biologically active materials,5 and in the emergent fields of spintronics and quantum computing.4

Polyoxometalates, as one of the main classes of

perfectly well-defined monodisperse nanoparticles, comprise a large group of metal-oxide clusters consisting of early transition metals (usually Mo, W, V, Nb, and Ta) in high oxidation states with oxo ligands.6-10 Most POMs are anions that consist of three or more transition metaloxo polyhedra linked together through shared oxygen atoms to form a large, closed 3dimensional framework ranging in size from 1 to 6 nm.7,9,10 Due to the multiple valences and coordination of the central metal ions and the variety of connections between the metal oxide polyhedron units, POMs can demonstrate an extremely rich distributuon of topological structures, sizes, and charges that can be grouped into several different classes.6,9,10 However, due to their high crystalline energies and low solubility in organic media, the full extent of their potential has not yet been realized.11-13 To circumvent these limitations and create a new class of designer surfactant species based on these highly charged ions, cationic surfactants have been used to modify the POM surface.14 These surfactant encapsulated POMs (SEPs) have found applications in catalysis,1,15 chemical separations,15,16 medicine,5 and materials science1,17,18 as a highly tunable moiety capable of self-assembling into novel superstructures and structural films. For instance, recent reports have demonstrated tunable morphologies of self-assembled structures built from SEPs at the air-water interface.1,17-19

SEPs have also shown promise for transporting/separating

materials/chemical species through phase separated media, as demonstrated recently using


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graphene oxide nanosheets coated SEPs at the water-oil interface.16

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While these reports

represent important advances in the application of SEPs, the subtle molecular level mechanisms underlying their organization, solvation, and adsorption to surfaces remains limited. Specifically, the underlying processes governing self-assembly and reactivity of these species is critically dependent on the passivation of the surfaces via electrostatically bound surfactants. However, little is known about the ordering of the surfactant and surrounding solvent molecules, the influence of the highly charged POM core, and how these factors drive the chemistry and organization in various chemical environments. This is especially true in understanding how SEPs or POMs organize at the boundaries of phase separated media, such as at aqueous-organic interfaces or the aqueous-vapor interface.20,21 The limited information regarding SEPs surfaces and related functionality is likely in part due to challenges in probing equilibrium and dynamic processes at interfaces without dominant contributions from bulk species overwhelming weak signals from the surface species.

To

overcome this limitation we apply surface specific vibrational sum-frequency generation (SFG) to selectively probe the SEP surfaces. Vibrational SFG is the nonlinear analog of traditional vibrational spectroscopies (i.e. absorption and Raman), but selectively reports on the interfacial species where local symmetry is broken. This provides an avenue to selectively characterize molecular ordering, assembly, and structure at surfaces.22,23 While the centrosymmetric structure of an SEP might imply that SFG is forbidden, differences in local symmetry and defects, allow for appreciable SFG signals from few nanometer sized particles in solution or at a macroscopic interface.24-40 This report describes the first study using SFG to directly probe the ordering, local chemical environment, and adsorption of SEPs at the air-water interface. As described below,


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SFG provides essential insight into the molecular species, including the organic surfactant and interfacial water, at the SEP interface and how they behave at the air-water interface. This work focuses on two giant POMs: the 3-nm (NH4)42[Mo132O372(CH3COO)30(H2O)72] POM

(abbreviated

here

as

Mo132)

and

the

2.5-nm

Na8K14(VO)2{K10[Mo6O21(H2O)3(SO4)]12(VO)30(H2O)20} POM (Mo72V30).41,42 Both POMs are spherical, stable in solution, and are highly negatively charged and thus, provide a perfectly symmetrical 3D surface for the electrostatic binding of cationic surfactants through ion exchange protocols. The POM cores are decorated with trioctylmethylammonium (TOMA) surfactant to form SEPs, as shown in Figure 1. Details of the synthesis and characterization of these POMs and associated SEPs are given in the supporting information.

Figure 1: The SEPs studied at the air-water interface are shown in a) Mo132 and b) Mo72V30. The structure of the TOMA surfactant is given in c). Results and Discussion: Experimental details regarding the SFG measurements can be found in the supporting information. Figure 2 shows SFG spectra for the Mo132 SEP at the air-water interface measured


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with two different polarization combinations. The abbreviations used to describe the polarization of ingoing and outgoing light fields (e.g., SSP) describes the polarization of the radiated SFG (S), incident near-infrared (S-), and mid-IR pulses (P), respectively. Generally, the SSP spectrum reports on symmetric stretching (ss) modes, whereas the PPP, SPS, and PSS spectra show strong signals corresponding to asymmetric (as) modes.43 This allows for an immediate categorization of many of the spectral features observed. To correctly assign the features observed in Figure 2 the approach proposed by Wang and coworkers43 is adopted, where data is collected with several polarization combinations and globally analyzed using SFG selection rules governing the prevalent signals allowed in each spectrum. Through a detailed analysis of the resulting spectra based on both the observed transition frequencies and polarization selection rules for SFG, the labeled bands in Figure 2 can be assigned. To extract band transition frequencies, the SFG data from all collected polarization combinations are globally fit, sharing transition frequencies where appropriate, to the model equation:38 (1)

= + − + Γ

where ISFG is the measured SFG signal intensity for a given polarization combination. ANR and φ are the nonresonant amplitude and phase, respectively.

The summation is made over n-

resonances observed in the spectral range investigated, where Aq is the amplitude of the qth resonance, ωIR and ωq are the frequencies of the excitation IR and the IR resonance, respectively, and Γq is the linewidth. By sharing parameters between the various data sets, the overall fit


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quality is improved and reduces the number of freely fit parameters needed to describe the data. The extracted bands frequencies and their assignments are summarized in Table 1.

Figure 2: SFG spectra acquired with two different polarization combinations for the Mo132 SEP at the air-water interface. Labels a-g denote the band assignments as summarized in Table 1. On the basis of the extracted resonant frequencies and associated signal strengths for a given polarization combination, several band assignments can be readily made. First, band a in the SSP spectrum show in Figure 2 is assigned to the CH3-symmetric stretch of the methyl group bound to the central nitrogen in TOMA (see structure in Figure 1c). This is in agreement with the methyl ss-band observed in the SFG spectrum of methanol.43,44 As the carbon chain length increases the terminal methyl symmetric stretch approaches higher frequencies and plateaus near 2880 cm-1.43,44

Indeed, band d is readily observed near 2887 cm-1 in the SSP spectrum and is

thus assigned to the methyl group at the end of the octyl chain. Next, band b observed near 2853 cm-1 is assigned to the CH2-ss mode, in good agreement with previous work on a range of systems.37,38,43-47 Notably, this band is quite intense compared to other SFG studies on wellordered monolayers, where gauche conformations in the hydrocarbon chains are minimized, and


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therefore the contribution of the corresponding CH2 stretches to the SFG signals were small.45,47 Given the structure of the TOMA shown in Figure 1c it is not surprising to observe such a large CH2 signal in the SFG spectra since the coordination of the carbon chains to the central nitrogen atom and its arrangement at a surface to minimize interactions with water will likely force gauche conformations in the chains, thereby breaking the symmetry of the methylene backbone. Band e observed near 2912 cm-1 is assigned to a Fermi resonance of the CH2-ss with a bend overtone.43,44 Assignment of the remaining bands is somewhat less straightforward. Based on energetics, the intense band at ~2931 cm-1 (band f) might be assigned to a CH3-ss Fermi resonance typically found at 2930 cm-1.43 However, due to the SFG selection rules, this is assignment is unlikely, as this band negatively interferes with the signals in the SSP spectrum and its strong intensity is observed in the SPS, PPP, and PSS spectra, which would indicate this band is due to an as-stretch. Based on the observation of similar bands in the SFG measurements on alkoxycarbonyl terminated monolayers, we assign band f to the CH2-as stretch of the hydrocarbon chain with a possible contribution from a Fermi resonance.47 The feature near 2868 cm-1 (band c) is then assigned to the CH2-as stretch for the methylene group near the central nitrogen in the TOMA structure, in agreement with previous reports showing analogous transitions at 2873 cm-1.47 Although band g at ~2978 cm-1 can be readily assigned to the CH3asymmetric stretch, it should be noted that the intensity of this band is surprisingly low. This might be due to cancelation of the SFG signals resulting from a disordered assembly of terminal methyl groups, leading to an overall weak signal for this band. This consideration is in line with the observation of strong CH2 vibrations in the SFG spectra observed in this work.


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Observed Frequency (cm-1)

Frequency Shift (cm-1)

Band

Mo132

Mo72V30

TOMA

Mo132 − Mo72V30

Mo132 − TOMA

Mo72V30 − TOMA

Assignment

a

2826.0 ± 0.3

2823.8 ± 0.4

2820.6 ± 0.4

2.3 ± 0.5

5.5 ± 0.5

3.2 ± 0.5

CH3-ss (Me-N), r+

b

2853.3 ± 0.3

2850.6 ± 0.5

2847.7 ± 0.4

2.8 ± 0.6

5.6 ± 0.5

2.8 ± 0.6

CH2-ss, d+

c

2867.8 ± 1.3

2874.0 ± 1.4

2874.6 ± 2.1

-6.2 ± 1.9

-1.8 ± 1.8

4.4 ± 1.9

CH2-as,

d

2887.2 ± 0.7

2884.7 ± 0.5

2881.3 ± 0.7

2.5 ± 0.9

5.9 ± 1.0

3.3 ± 0.9

CH3-ss (octyl), r+

e

2911.9 ± 1.4

2908.9 ± 1.7

2905.2 ± 1.1

3.1 ± 2.2

6.7 ± 1.8

3.6 ± 2.0

CH2-FR,

f

2930.8 ± 0.7

2928.0 ± 0.9

2929.3 ± 0.7

2.8 ± 1.1

1.5 ± 1.0

-1.3 ± 1.1

CH2-as, d-

g

2977.9 ± 1.8

2971.2 ± 3.1

2968.8 ± 3.6

6.7 ± 3.6

9.0 ± 4.0

2.3 ± 4.8

CH3-as, r−

Table 1: Summary of the bands observed in the SFG spectra and their assignments. The labels ag are indicated in Fig. 2.

Frequency shifts between the different measurements are also

tabulated. The designations given in the assignments follow convention.48 The observation of weak CH3 signals and strong CH2 signals is indicative of disordered hydrocarbon chains at the POM interface.37,38

This is likely due to the following two

contributions: (1) There is limited orientational freedom for the hydrocarbon chains to tightly pack owing to their connectivity to a central nitrogen atom in the TOMA surfactant. This will inevitably force gauche conformations in the carbon backbone, especially in the presence of water, which drives the hydrophobic chains toward the interface to minimize their interactions with water. (2) At highly curved surfaces the chains are able to sample a larger conic volume without interactions with neighboring chains thereby increasing the contributions from gauche conformations observed in the SFG spectrum.37,38 The question is then how do the POM cores influence the ordering of the surfactant chains as compared to the pure surfactant at the air-water interface? To address this question, we turn attention to the SFG spectra collected from different SEP samples at the air-water interface as shown in Figure 3a. At first glance, these spectra


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appear very similar; however, a closer inspection reveals several notable differences, which indicate the presence of unique species at the air-water interface as well as variations in the ordering of TOMA hydrocarbon chains. For instance, visible shifts in the vibrational band positions are observed in these SFG spectra, as highlighted by dashed lines in Figure 3a and 3b, indicating the presence of distinct chemical species at the air-water interface. The shifts in the peak positions are summarized in Table 1. Since the POM cores are highly charged (the maximum charge on the Mo132 POM is −42, whereas for Mo72V30 it is −26), the local chemical environment experienced by the surfactants and surrounding water molecules will be different for the two SEPs studied.41,42 The unique local environments will result in shifts of the observed transition frequencies. These shifts can be due to differences in the surface charge density and associated static electric fields (i.e., Stark shifts49,50) or chemical composition.51 Notably, the magnitude of the band shifts differ for the two SEPs studied here; Mo132 shows a typical shift on the order of 6 cm-1 relative to TOMA, whereas corresponding shifts for Mo72V30 are on the order of 3 cm-1. The larger charge on the Mo132 SEP core should result in larger shifts as compared to the Mo72V30 SEP or TOMA, which is in agreement with the present observations. This result indicates that the charge on the POM core influences the surfactant chains and that the vibrational transition energies can be potentially controlled by tuning the charge of the POM core.


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Figure 3: Vibrational SFG spectra acquired using the SSP polarization combination for the two SEPs studied as well as the pure surfactant at the air-water interface (a and b). These spectra are vertically offset for ease of comparison. The PPP spectra are shown in c, and share the same intensity scale. Next we consider the intensity of CHn stretching bands relative to the broad feature observed at ~3200 cm-1 that corresponds to the –OH stretches of interfacial water.52 Previous studies have shown that in the presence of a charged surface, solvent molecules further away from the surface can be polarized due to the static field projected into the bulk, and consequently contribute to the overall nonlinear response.27,34,51,53,54 Thus, for the case of TOMA, we would expect, and indeed observe, a substantial contribution from polarized water species due to the unpaired positive charge on TOMA. It is expected that the SEPs are generally non-polar, as evidenced by the way in which SEPs are extracted from aqueous media when they are synthesized, and as such the amount of –OH signal intensity relative to the CHn band intensities is expected to be less for SEPs than for TOMA. Indeed, from the data in Figure 3a it is found that the relative amount of ordered water (as evaluated from the –OH bands at ~3200 cm-1) relative to the intensity of the CHn stretching vibrations follows: TOMA>Mo132>Mo72V30. This has several implications: First, the TOMA coating is very complete on the POMs, even when added to nano-pure water (i.e., does not strongly dissociate), and thereby passivates/shields much


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of the charge on the POM core. If a significant number of charges were left unpassivated, one would expect to observe large contributions from –OH stretching vibrations due to the high surface charge density and associated static field that is projected into the bulk. The observed trend also indicates that more charge is ‘seen’ by the surrounding water molecules for Mo132 than for Mo72V30. SEP characterization data included in the SI of this manuscript show that the surface coating is quite complete (surface coverage ~ 1) and that there is negligible free surfactant at the surface in the presence of SEPs. This is supported by the SFG measurements, which do not show signals from the free TOMA that would be shifted several cm-1. This difference is likely dominated by the total charge on the POM core that serves to align surrounding water molecules. The addition of electrolytes to the solution could be used to tune the extent that water is ordered in the bulk and extract the surface charge density of the SEPs. This result provides important insight into the interplay between charge and surfactant packing in dictating the local chemical environment at the SEP interfaces. Specifically, the surface charge density on the POM core plays a prominent role in shifting vibrational band positions but also a more functional role in orienting solvent molecules several molecular layers beyond the interface. Next, to assess the differences in surfactant chain ordering at the POM surfaces vs. for the neat surfactant at the air-water interface; we turn attention to the PPP spectra shown in Figure 3c. This polarization combination is chosen over others since the CH2-as stretch is prominent in these spectra and contributions from water vibrations and other bands are limited. This allows for a direct comparison of the spectra from different samples without normalization or baseline removal. Based on the intensities of the CH2-as stretch (i.e., band f, ~2930 cm-1) in Figure 3c, it is clear that the ordering of methylene groups in the SEPs is poorer than for TOMA at the air-


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water interface. This is because the intensity of the CH2 bands, which characterize disorder in the hydrocarbon chains, is larger for both SEPs than for TOMA. This is in line with previous reports showing that ligands on small nanoparticles have more gauche defects due to the increased conic volume that the chain can sample without interacting with a neighboring chain.37,38 With the present signal to noise ratio, it is not possible to differentiate between ordering in the two SEPs. This new insight shows that the charge on the SEP surface does not dramatically impact the ordering of the hydrocarbon chains on their surfaces when at the airwater interface. If the static field were critical to the chain ordering, we would have expected to see more ordering in Mo132 SEPs vs. Mo72V30 SEPs given the differences in charge on the POM cores. This result instead indicates that the ordering on the surface of the SEPs at the air-water interface is likely dominated by hydrophobic interactions that force gauche conformations in the chains to minimize interactions with water. The presence of both marcoscopic and nanoscopic interfaces probed here limit the use of common orientational analysis methods.55-59 To extract orientational information one would need to combine the traditional approaches with SFG scattering theories,35,36 which to the best of our knowledge has not been reported. To evaluate the propensity of the SEPs to reside at the air-water interface, adsorption isotherms were measured, as presented in Figure 4a and 4b that show the band integrated CHnstretching SFG signals vs. the concentration of SEP added to solution. These results show that the measured SFG field ( ~ ) plateaus with increasing SEP concentration, indicating that the available adsorption sites at the air-water interface are filled. Noting that the surface adsorbed species are in equilibrium with bulk SEPs, the relevant reaction can be expressed as (SEP)bulk ⇌ (SEP)surface


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Noting that the SEP population at the air-water interface is proportional to the radiated field,29,30,32,51,53 the relationship between the observed signal and the equilibrium constant takes the form of the well-known Langmuir isotherm: (2) ∝

!"#$ % 1 + !"#$ %

where Kads is the adsorption equilibrium constant and C is the concentration of SEP in solution. Fitting the concentration dependent SFG data to Equation 2 allows for the determination of the adsorption equilibrium constant and the change in the Gibbs free energy for adsorption. While more complicated models could be used to describe the adsorption of the SEPs to the air-water interface,60 the simplicity of the Langmuir isotherm and its ability to adequately describe our data with a minimal number of adjustable parameters makes it well suited to describe the isotherms shown here. The Mo132 SEP is found to have an adsorption equilibrium constant of 177.5 ± 29.4 µM-1, which corresponds to a free energy of adsorption of −46.8 ± 0.4 kJ/mol at 296.7 K. This is clearly different than Mo72V30, which has an adsorption equilibrium constant of 77.4 ± 36.4 µM-1 and associated free energy of adsorption of −44.8 ± 1.2 kJ/mol at 296.7 K. Since the equilibrium constants for the POMs studied differ by more than a factor of two it is clear that the species at the air-water interface are unique, and in conjunction with previously discussed results, it becomes clear that they behave differently at the air-water interface.


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Figure 4: Isotherms for the adsorption of SEPs to the air-water interface. Considering only that the Mo132 SEP is more polar than the Mo72V30 SEP, one might expect that Mo72V30 should adsorb more strongly to the air-water interface, which is not observed in the present measurements. However, if one considers intermolecular interactions, such as size dependent van der Waals interactions21 or hydration forces61-63, it becomes apparent that a balance of charge and other interactions is necessary to describe SEP adsorption to the airwater interface. This has been demonstrated in studies of salts with varying size and charge at the air-water interface64-68 as well as in the assembly of colloidal crystals.61-63 For the work here, given that the Mo132 SEPs are larger in size, the van der Waals attractions between them are correspondingly larger, as compared to Mo72V30 SEPs, which might explain the relative difference in anticipated adsorption free energies. Ideally, one would change the size of the particle to study aggregation, which is strongly size dependent. However, since the POMs are a


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stoichiometrically defined cluster, the only way to change the particle size would be to change the POM itself.

As such, we cannot comment on the nature of the interactions as being

dominated by the size of the particles or from other contributions. The interplay between these competing phenomena cannot be resolved with the present work alone, though similar arguments have been made for the strong adsorption of bare POMs to various interfaces.21 Additionally, the aggregation of chemical species at interfaces is commonly observed, even for charged species,69 which results from hydrophobic interactions. These interactions are likely important for the SEPs studied here. It should be noted that there was no observable change in band positions with increasing concentration over the range reported here. This implies that either the carbon chains do not strongly interact or that the shifts in the band positions are small enough that they are not resolved with our instrument. In general, a unified and complete understanding of surface adsorption and hydration to interfaces remains a topic of extensive and active investigation; as such, the detailed mechanism surrounding the adsorption SEPs to the air-water interface remains an important topic for future work from both an applied and fundamental research perspective. Conclusion: In this work, we report an experimental study on the adsorption, ordering, and local chemical environment surrounding SEPs at the air-water interface using vibrational SFG. We identified and assigned several spectral signatures of the SEPs, which exhibit characteristic shifts in vibrational band positions that are dependent on the SEP studied. The magnitude of the shifts parallels the maximum charge on the POM core, indicating a notable effect of charge on the surfactant coating and local chemical environment surrounding the SEP assembly. Based on relative band intensities, it was deduced that Mo132 SEPs are more polar than Mo72V30 SEPs, which more strongly orders the surrounding water for the former. It was further shown that the


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packing of the surfactant chains are disordered at the SEP surface as compared to the neat surfactant at the air-water interface, which is likely due to the larger conic volumes that can be sampled at the highly curved POM surfaces without interactions with neighboring chains. The differences in the adsorption free energies for the different SEPs reflects the influence of unresolved intermolecular interactions taking place at the air-water interface, which cannot be accounted based on purely electrostatic effects. It is expected that a deeper understanding of charge and ligand tailored SEPs at the air-water interface can help in gaining insight into ionic adsorption at interfaces on a broader scale. These poorly understood interactions remain a topic for future investigations as does elucidating subtle differences in water organization and assembly of these and other SEPs at macroscopic interfaces.

ASSOCIATED CONTENT Supplemental figures including NMR spectra and SFG spectra for all SEPs at different polarization combinations are given in the supporting information section. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Benjamin Doughty, Email: [email protected] *Panchao Yin, Email: [email protected] Author Contributions


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B.D. carried out the SFG measurements, data analysis and interpretation. P.Y. synthesized and characterized the POMs. Y.Z.M. contributed to data analysis and interpretation. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources ACKNOWLEDGMENT B.D. and Y.-Z.M. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. P. Y. is grateful for the support of the Clifford G. Shull Fellowship from the Neutron Sciences Directorate of Oak Ridge National Laboratory. The sample preparation was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Oak Ridge National Laboratory is supported by the Office of Science of the US Department of Energy under Contract No. DE-AC05-00OR22725. ABBREVIATIONS POM – polyoxometalate, TOMA – trioctylmethylammonium, SEP – surfactant encapsulated polyoxometalate, SFG – Sum-frequency generation, ss – symmetric stretch, as – asymmetric stretch REFERENCES (1) Nisar, A.; Wang, X. Dalton Transactions 2012, 41, 9832. (2) Kim, Y.; Shanmugam, S. ACS Applied Materials & Interfaces 2013, 5, 12197. (3) Gao, J.; Miao, J.; Li, Y.; Ganguly, R.; Zhao, Y.; Lev, O.; Liu, B.; Zhang, Q. Dalton Transactions 2015, 44, 14354. (4) Clemente-Juan, J. M.; Coronado, E.; Gaita-Arino, A. Chemical Society Reviews 2012, 41, 7464.


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Table of Contents Figure


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

a)

b)

c)

N


+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SFG Intensity (arb. units)

Langmuir

1400 1200

a

300

b

SSP

1000 d

800 600 400

c

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250

e

200

f

150 g

SPS

100 50

200 2800 2900 3000 3100 3200 3300 Wavenumber (cm -1 )


b)

TOMA TOMA TOMA 72

2800

3000 Wavenumber (cm-1)

3200


a) SFG Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

2700

c)

TOMA TOMA TOMA 72

2900 2800 Wavenumber (cm-1)


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3000


TOMA TOMA TOMA 72

2860 2880 2900 2920 2940 2960 2980 Wavenumber (cm-1)

Langmuir

1.0 0.8 0.6 Integrated SFG Field (arb. units)

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0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 0

5

10 15 20 [POM] (nM)

25

30


Adsorption, Ordering, and Local Environments of Surfactant

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