Electrostatic interactions to modulate the reflective assembly of nanoparticles at the oilwater

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1 Supplemental Information Electrostatic interactions to modulate the reflective assembly of nanoparticles at the oilwater interface Mingxiang Luo, Gloria K. Olivier, and Joelle Frechette* Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA * Phone: (+1) FT-IR characterization of ion-pair gold nanoparticles The ion-pair AuNPs exhibit evidence of TPeA functional groups in the form of prominent asymmetric and symmetric CH 3 stretching bands at 2954 ± 2 cm -1 and 2870 ± 1 cm -1, respectively. The position of these bands are in close agreement with the asymmetric (2954 cm - 1 ) and symmetric (2870 cm -1 ) C-H stretch in methyl groups present in the spectrum of pure TPeACl salt (Fig S1a), indicating that these bands are derived from the CH 3 groups of TPeA cations. The MHA molecules incorporated into the physical mixture with TPeACl exhibit protonated carboxylic acid tail groups, as can be seen from the strong carbonyl (C=O) stretch appearing at 1713 cm -1 (Fig S1b). This band is absent from the spectrum of the ion-pair NPs. Instead, the ion-pair AuNPs exhibit de-protonated carboxylate bands at 1570 cm -1 (asymmetric COO - stretch) and 1417 cm -1 (symmetric COO - stretch). 1, 2 The presence of carboxylate vibrational modes has been used by others to identify the formation of carboxylic acid salts within acid-terminated SAMs on gold. 3 Similarly, the presence of carboxylate groups on the surface of the ion-pair NPs provides evidence that the MHA tail group is paired with a TPeA + cation. To obtain the correlation between CH 2 / CH 3 infrared absorbance intensity and the molar ratio of TPeA:MHA we created calibration curve by measuring the IR spectra of physical mixtures of dry MHA thiol and TPeACl, in which the molar ratio of MHA to TPeA was varied from 0 to 7. The linear correlation between the CH 2 / CH 3 infrared intensity ratio and the molar ratio of TPeA:MHA was employed to estimate the molar ratio of TPeA:MHA present on the surface of the AuNPs (Fig. S1c). The ratio of the intensity of the asymmetric CH 2 stretch to the intensity of the asymmetric CH 3 stretch (asymmetric CH 2 / CH 3 ratio) is proportional to the molar ratio of CH 2 / CH 3 functionality present within a sample. 4 The same is true for the symmetric CH 2 / CH 3 ratio. The results of this analysis method show that a 1:1 stoichiometric ratio of TPeA:MHA

2 exists on the surface of the ion-pair NPs (Table S1), within the experimental error. If the presence of TPeA that we observe in the IR spectrum of the ion-pair NPs were solely due to physisorption of TPeA cations on the gold surface, then we expect this ratio to deviate much more significantly from 1.0, especially in view of the strong interaction known to exist between thiol and gold. 5, 6 Similarly, if TPeA cations show up in the IR spectrum of the ion-pair NPs due to insufficient removal of the TPeACl phase-transfer agent employed during the nanoparticle synthesis, then we anticipate that this ratio would deviate from 1.0 and possibly vary from one synthesis batch to another. The fact that the IR spectra of the ion-pair AuNPs consistently exhibits TPeA in a molar ratio with MHA that is close to 1.0 supports the conclusion that TPeA remains paired with the terminal carboxylate group of the MHA chain during assembly on the surface of the gold nanoparticles. Moreover, we observe that stoichiometric ratio remains at 1.0 for ion-pair AuNPs once they are dispersed in ph 11.7 electrolyte solutions of both KOH and TPeAOH, indicating the ion-pair complex are stable in base aqueous (Table S1). Table S1. Measured composition of ion-pair NPs as synthesized, and after dispersing in 5 mm TPeAOH and KOH, ph 11.7 electrolytes, respectively. Molar ratio of TPeA:MHA present on the particle surface was obtained by applying the correlation equation shown in Figure S1c to the measured IR asymmetric and symmetric C-H stretching intensities. Dry Ion-pair NPs (as synthesized) CH 2 / CH 3 intensity ratio TPeA:MHA molar ratio C-H stretch (Asym.) 1.6 ± ± 0.3 C-H stretch (Sym.) 1.3 ± ± 0.2 Ion-pair NPs extracted from 5 mm TPeAOH, ph 11.7 electrolyte C-H stretch (Asym.) 1.6 ± ± 0.2 C-H stretch (Sym.) 1.3 ± ± 0.3 Ion-pair NPs extracted from 5 mm KOH, ph 11.7 electrolyte C-H stretch (Asym.) 1.5 ± ± 0.3 C-H stretch (Sym.) 1.4 ± ± 0.2

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4 Figure S1. IR spectra of the (a) C-H and (b) C=O stretching regions for pure TPeACl salt (green), TPeACl salt mixed with MHA thiol in a 1:1 molar ratio (purple), and ion-pair gold NPs (black). (c) Linear regression used to correlate measured ratio of C-H stretching intensity with the molar ratio of MHA to TPeA of a sample. Data points were obtained by measuring the IR spectra of physical mixtures of dry MHA thiol and TPeACl salt, for which the molar ratio of MHA and TPeA + is known.

5 TEM characterization of ion-pair gold nanoparticles at the oil-water interface Here we show the TEM images of ion-pair gold nanoparticles collected from toluene/aqueous electrolyte (TPeAOH, ph 10) interfaces at ionic strength of 0.1 mm and 5 mm. The images in Figure S2 clearly show no aggregation of particles at the interfaces when TPeA cations present in the aqueous phase. However, we also notice the formation of doublet and triplet particle clusters at the interface, which could occur in the drying step. Figure S2. TEM imges of ion-pair gold nanoparticles collected from toluene/aqueous electrolyte TPeAOH interfaces at ph 10 and ionic strength of (a) 0.1 mm and (b) 5 mm.

6 FT-IR characterization of ion-pair gold nanoparticles upon change in ph The effect of ionic strength on the TPeA:MHA stoichiometry of the ion-pair NPs was obtained by comparing the IR spectrum of the AuNPs collected from the interface at toluene/aqueous solution (ph 10) in both TPeAOH and KOH at 0.1 mm (Fig. S3a) and 5 mm (Fig. S3b). When no additional salt is added (0.1 mm), we observe no change in stoichiometric ratio of ion-pairs in both aqueous solutions. In contrast, when additional salt is added to reach a higher ionic strength (5 mm) we observe the appearance of the absorption peak of carboxylic acid dimer when KOH is present, indicating the dissociation of ion-pairs. The decrease in TPeA/MHA ratio from 1.0 to 0.6 also suggests the loss of TPeA cations in KOH 5mM. In comparison, gold nanoparticles still remain 1:1 ratio of ion-pair in TPeAOH. Absorbance (arb. units) Absorbance (arb. units) (a) Asym. CH 3 KOH TPeAOH Wavelength (cm -1-1 )) (b) Asym. CH 2 Asym. CH 3 Asym. CH 2 COOH dimer 1713 cm -1 COO - (asym.) 1560 cm -1 COO - (asym.) 1560 cm Wavelength (cm -1-1 ) ) Figure S3. IR spectra of gold nanoparticles collected from toluene-aqueous solution interface at ph 10. Ionic strengths are kept at 0.1 mm (a) and 5 mm (b) in aqueous KOH and TPeAOH.

7 Table S2. Surface composition of AuNPs collected from interfaces of toluene/aqueous solution (ph 10) at ionic strength of 0.1 mm and 5 mm, respectively. Molar ratio of TPeA:MHA present on the particle surface was obtained by applying the correlation equation shown in Figure S1c to the measured IR asymmetric and symmetric C-H stretching intensities. molar ratio of TPeA:MHA electrolyte TPeAOH KOH 0.1 mm 1.2 ± ± mm 1.1 ± ± 0.2 References 1 M. Luo and J. Frechette, J. Phys. Chem. C, 2010, 114, G. K. Olivier, D. Shin, J. B. Gilbert, L. A. A. Monzon and J. Frechette, Langmuir, 2009, 25, R. Arnold, W. Azzam, A. Terfort and C. Woll, Langmuir, 2002, 18, B. H. Stuart, Infrared Spectroscopy: Fundamentals and Applications, Johns Wiley and Sons, New York J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and G. M. Whitesides, Chem. Rev., 2005, 105, A. Ulman, Chem. Rev., 1996, 96, 1533.