Solvent Isotopic Effects on a Surfactant Headgroup at the Air-Liquid Interface

S1 SUPPORTING INFORMATION Solvent Isotopic Effects on a Surfactant Headgroup at the Air-Liquid Interface Uvinduni I. Premadasa, a Negar Moradighadi, b Kondalarao Kotturi, a Jeeranan Nonkumwong, a,c Md. Rubel Khan, a Marc Singer, b Eric Masson *, a and Katherine L. A. Cimatu *, a a Department of Chemistry and Biochemistry, Ohio University, 100 University Terrace, 136 Clippinger Laboratories, Athens, Ohio 45701, United States b Institute for Corrosion and Multiphase Technology, Ohio University's Research and Enterprise Park, 342 West State Street, Athens, Ohio 45701, United States c Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand *Corresponding author e-mails: cimatu@ohio.edu; masson@ohio.edu 1. Generalities... 2 2. Preparation of benzyldimethyloctylammonium bromide (Quat 1) 1, 2... 2 3. Sample Preparation and Experimental Details... 3 4. Fitting Equation... 4 5. SFG Fitting parameters for Quat 1 solutions (8.0 mm)... 5 6. Supporting SFG spectra... 8 7. CMC Measurements... 12 8. Orientation analysis... 16 9. References... 25

S2 1. Generalities Reagents and solvents were purchased from Acros Organics (Pittsburgh, PA), Fisher Scientific (Pittsburgh, PA), Sigma-Aldrich (St. Louis, MO), and Cambridge Isotope Laboratories, Inc. (Tewksbury, MA). Quat 1 was characterized by nuclear magnetic resonance spectroscopy (NMR) using a Bruker 500 MHz spectrometer (Billerica, MA). 2. Preparation of benzyldimethyloctylammonium bromide (Quat 1) 1, 2 N, N-dimethylbenzylamine (15 ml, 14 g, 0.10 mol) and acetonitrile (0.10 L) were heated to reflux. Bromooctane (17 ml, 19 g, 0.10 mol) was added slowly, and the reaction was kept at reflux for 24 h. The solvent was then removed by rotary evaporation. The obtained product was recrystallized using deionized water to afford the title compound as colorless crystals (21 g, 63%). 1 H NMR (CDCl 3 ): δ = 7.65 (Ph, m, 2H), 7.42 (Ph, m, 3H, J = 7.1 Hz), 5.09 (Ph-CH 2, s, 2H), 3.54 (NCH 2, t, 2H, J = 8.2 Hz), 3.29 (N-CH 3, s, 6H), 1.77 (CH 2, s, 2H), 1.29 (CH 2, d, 14H, J = 14 Hz), 0.84 (C-CH 3 ), t, 3H, J = 6.6 Hz). Figure S1. 1 H NMR spectrum of Quat 1.

S3 3. Experimental Details Sample preparation: The eicosanoic acid (EA; >99% purity) monolayer on water method was used for aligning the beams at the air-liquid interface as a reference sample. 0.5 mm, 8 mm, and 20 mm Quat 1 solutions were prepared in deionized H 2 O (purified by Milli-Q plus at 18 MΩ) or in D 2 O, and sonicated for 15 minutes. The samples were placed in a glass petri dish and allowed to equilibrate for 20 minutes under N 2 atmosphere. Spectroscopic analysis was carried out under positive N 2 pressure. Instrumentation: The femtosecond sum frequency generation spectroscopy was attained using Solstice laser (Spectra-Physics) generating 100 fs pulses at 795 nm. 50% of the 795 nm beam was then passed through an automated optical parametric amplification system and the difference frequency crystal to generate mid-infrared beam ranging from 4000 cm -1 to 1000 cm -1.The other half was sent towards a Fabry Pérot Etalon (SLS Optics) to produce a time-asymmetric picosecond visible pulse. The beams are focused to the sample stage with the IR beam aligned at an angle of 60 o and the visible (795 nm) beam at an angle of 50 o from the surface normal. When the incident IR beam is centered at 2900 cm -1, the broadband SFG beam is generated at an angle of 52 from the surface normal. The SFG signal is then collected with lenses, polarizer, spectrograph, and a detector. The energies of the incident beams on the sample stage are ~7µJ (IR) and ~25 µj (795 nm), respectively. SFG Experiment: All data were collected with three trials and a background. Acquisition time of each spectrum is 9 minutes (180 accumulations: 3 s acquisition time per accumulation). The spectra of Quat 1 in H 2 O were acquired at different IR centers positioned from 2800 cm -1 to 3700 cm -1. Quat 1 in D 2 O was characterized from 2400 cm -1 to 3000 cm -1 ; in a 1:1 mixture of H 2 O and D 2 O, Quat 1 was characterized from 2400 cm -1 to 3700 cm -1 every 100 cm -1.

S4 4. Fitting Equation SFG intensity is proportional to which consists of and, the resonant and nonresonant components, respectively. is a term related to hyperpolarizability, the product of the IR dipole moment and Raman polarizability tensor. + (1) N is the number density of vibrational transitions and is the damping constant of the q th vibrational mode. and are the resonance and the incident IR frequencies, respectively. is the phase of the non-resonant response. The non-resonant contribution is considered in the fitting equation, in order to account for any contribution from the bulk. The simplified version of the fitting equation is shown below. + + (2) Where, = + The contribution from the broadband width of the IR beam profile is considered by including the Gaussian function with the spectral width of centered at. 3, 4 The amplitude factors, and, are proportional to as shown in Equation 1. A CH and A OH and and are amplitude and frequency positions arising from CH and OH vibrational modes, respectively.

S5 5. SFG Fitting parameters for Quat 1 solutions (8.0 mm) Table S1. Fitting parameters for 8 mm Quat 1 in H 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 1.7 10-8 2.9 10-2 2 10-2 0.4 3 10-2 0.7 A2 8.0 0.3 0.1 0.1 0.3 0.1 A3 1.4 1.2 0.1 0.1 0.1 0.1 A4 4.0 2.5 0.1 0.1 0.3 0.1 A5 0.2-1 10-2 - 3 10-2 - A6 4.0-0.2-0.3 - A7 3.8-0.2-0.3 - Г1 18.6 22.0 7 10 6 1 10 2 1 10 7 3 10 2 Г2 25.0 19.7 0.3 4 0.6 9 Г3 8.4 12.7 0.2 0.4 0.4 0.7 Г4 20.4 12.6 0.4 0.1 0.8 0.2 Г5 4.4-0.2-0.4 - Г6 30.0-0.7-1 - Г7 30.0-0.8-2 - ω1 2719 2600 2 10 7 2 10 2 3 10 7 3 10 2 ω2 2854 2853 0.1 3 0.3 5 ω3 2876 2890 0.1 0.2 0.2 0.5 ω4 2914 2960 0.1 0.1 0.3 0.2 ω5 2935-0.1-0.2 - ω6 2986-0.3-0.6 - ω7 3040-0.4-0.8 - n 0.1 0.1 4 10-3 3 10-3 7 10-3 0.01 p 1.3 10.5 9 10-3 4 10-2 2 10-2 0.1 n1 0.0 1.5 10-7 4 10-4 3 10-4 8 10-4 6 10-4 R 2 0.98 0.97

S6 Table S2. Fitting parameters for 8 mm Quat 1 in a 1:1 H 2 O/D 2 O mixture. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 9.0 10-7 1.5 3 10-2 0.5 0.1 1 A2 2.2 0.5 0.2 0.2 0.4 0.3 A3 2.8 1.2 0.3 0.1 0.5 0.2 A4 3.0 2.2 0.5 0.1 1.0 0.2 A5 0.7-0.2-0.3 - A6 1.6-0.4-0.9 - A7 1.7-0.4-0.9 - Г1 4.6 0.01 2 10 5 2.1 3 10 5 4 Г2 14.4 25.0 0.9 6 1.8 1 10 2 Г3 12.8 12.6 0.6 0.7 1.2 1 Г4 19.9 11.8 2 0.2 3.7 0.4 Г5 8.1-1 - 2.2 - Г6 25.0-5 - 9.0 - Г7 25.0-4 - 7.8 - ω1 2721 2580 9 10 4 9 2 10 5 2 10 2 ω2 2852 2865 0.4 4 0.8 8 ω3 2875 2893 0.2 0.3 0.5 0.6 ω4 2916 2956 0.9 0.2 1.8 0.4 ω5 2935-0.5-0.9 - ω6 2990-2 - 4.1 - ω7 3040-3 - 5.6 - n 4.6 10-2 0.1 1 10-2 5 10-3 2 10-2 1 10-2 p 1.8 10.5 0.1 0.1 0.2 0.1 n1 4.8 10-4 1.2 10-5 1 10-3 5 10-4 2 10-3 1 10-3 R 2 0.99 0.96

S7 Table S3. Fitting parameters for 8 mm Quat 1 in D 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 0.1 4.5 1 10-2 0.6 3 10-2 1 A2 1.3 0.5 0.1 0.3 0.1 0.5 A3 3.0 1.6 0.1 0.2 0.2 0.5 A4 1.8 3.2 0.2 0.1 0.4 0.3 A5 1.2-0.1-0.2 - A6 0.1-0.1-0.1 - A7 1.4-0.1-0.3 - Г1 2.98 66.2 0.8 5 2 1 10 2 Г2 12.2 25 0.4 8 0.8 2 10 2 Г3 13.5 15.4 0.3 1 0.5 2 Г4 17.2 13.3 0.8 0.2 2 0.4 Г5 10.4-0.4-0.8 - Г6 8.9-4 - 8 - Г7 25.0-2 - 4 - ω1 2729 2620 0.7 4 1 7 ω2 2851 2865 0.2 6 0.4 1 10 2 ω3 2875 2895 0.1 0.6 0.2 1 ω4 2917 2956 0.4 0.2 0.7 0.4 ω5 2937-0.2-0.4 - ω6 2990-2 - 4 - ω7 3040-2 - 5 - n 2.0 10-2 4.5 10-2 4 10-2 7 10-3 9 10-3 2 10-2 p 8.27 10.5 0.1 0.2 0.2 0.3 n1 5.1 10-2 0 7 10-4 3 10-4 1 10-3 R 2 0.99 0.98

S8 6. Supporting SFG spectra 10000 2500 SFG Intensity (a.u.) 8000 6000 4000 2000 SFG Intensity (a.u.) 2000 1500 1000 500 0 2800 2900 3000 3100 3200 Wavenumber (cm -1 ) 0 2800 2900 3000 3100 3200 Wavenumber (cm -1 ) (a) (b) Figure S2. The SFG spectrum of 0.1 mm CTAB in H 2 O at (a) SSP and (b) PPP polarization combinations at 3000 cm -1.

S9 0.12 0.5 mm Quat 1 in H2O SFG Intensity (a.u.) SFG Intensity (a.u.) 0.08 0.5 mm Quat 1 in H2O 0.5 mm Quat 1 in D2O 0.5 mm Quat 1 in D2O 2600 2700 2800 2900 3000 3100 3200 2600 2700 2800 2900 3000 3100 3200 Wavenumber (cm-1) Wavenumber (cm-1) (a) (b) 0.12 20 mm Quat 1 in H2O 20 mm Quat 1 in H2O SFG Intensity (a.u.) SFG Intensity (a.u.) 0.08 0.08 20 mm Quat 1 in D2O 20 mm Quat 1 in D2O 2600 2700 2800 2900 3000 3100 3200-1 Wavenumber (cm ) (c) 2600 2700 2800 2900 3000 3100 3200 Wavenumber (cm-1) (d) Figure S3. SFG spectra of Quat 1; (a) 0.5 mm at SSP polarization combination, (b) 0.5 mm at PPP polarization combination, (c) 20 mm at SSP polarization combination, (d) 20 mm at PPP polarization combination in H2O (top panels) and in D2O (bottom panels). The red lines indicate the fitted spectra.

S10 0.15 0.075 0% salt 0% salt 0.050 0.05 5 SFG Intensity (a.u.) 0.10 SFG Intensity (a.u.) 1% salt 0.03 10% salt 0 0.050 1% salt 5 0 0.050 10% salt 5 0.03 2600 2700 2800 2900 3000 3100 3200 0 2600 2700 2800 2900 3000 3100 3200 Wavenumber (cm-1) Wavenumber (cm-1) (a) (b) Figure S4. SFG spectra of 8 mm Quat 1 at different ionic strengths at (a) SSP polarization combination and (b) PPP polarization combination. The salt is sodium chloride (NaCl). OTAB in H2O 9 SFG Intensity (a.u.) 6 3 0 OTAB in D2O 8 0.032 0.016 0 2600 2800 3000 3200 3400 3600 3800-1 Wavenumber (cm ) Figure S5. SFG spectrum of octyltrimethylammonium bromide in H2O (44 mm; top panel) and in D2O (bottom panel) at SSP polarization. The N-CH3 SS vibrational mode at ~2980 cm-1 and N-CH3 AS vibrational mode at ~3050 cm-1 are highlighted in green.

S11 SFG Intensity (a.u.) 0.12 0.10 0.08 OTAB QUAT 1 SFG Intensity (a.u.) 0.08 0.08 2900 2950 3000 3050 3100 3150 Wavenumber (cm -1 ) 2600 2800 3000 3200 3400 3600 3800 Wavenumber (cm -1 ) Figure S6. The comparison of SFG spectra of 8 mm QUAT 1 and 1 mm OTAB in H 2 O at SSP polarization combination. An inset is provided for the clearer view of the ~3050 cm -1 peak.

S12 1200 Pure H2O 0.12 0.07 Quat 1 in H2O 1000 Quat 1 in HOD 0.10 800 0.05 0.08 600 0.03 400 200 0 2600 2800 3000 3200 3400 3600 0.01 3800 Wavenumber (cm-1) (a) 1400 Pure D2O 0.07 0.07 1200 Quat 1 in D2O Quat 1 in HOD 1000 0.05 0.05 800 600 0.03 0.03 400 200 0.01 0.01 0 2200 2400 2600 2800 3000 3200 3400 3600 3800 Wavenumber (cm-1) (b) Quat 1 in D2O 0.12 0.07 Quat 1 in H2O 0.07 Quat 1 in HOD 0.10 0.05 0.05 0.08 0.03 0.03 0.01 2400 2600 2800 3000 3200 3400 3600 0.01 3800 Wavenumber (cm-1) (c) Figure S7. (a) Comparison of SFG spectra of pure H2O (black), Quat 1 in H2O (red) and Quat 1 in a 1:1 mixture of H2O and D2O (blue). (b) Comparison of SFG spectra of pure D2O (green), Quat 1 in D2O (pink) and Quat 1 in a 1:1 mixture of H2O and D2O (blue). (c) Comparison of SFG spectra of Quat 1 D2O (pink), Quat 1 in H2O (red) and Quat 1 in a 1:1 mixture of H2O and D2O (blue) at SSP polarization combination.

S13 SFG Intensity (a.u.) 0.12 0.10 0.08 10% Salt 1 % Salt 0% Salt 2600 2800 3000 3200 3400 3600 3800 Wavenumber (cm -1 ) Figure S8. The comparison of SFG spectra of 8 mm Quat 1 at different ionic strengths at SSP polarization combination.

S14 7. CMC Measurements The surface tension of H 2 O and D 2 O solutions at 25 C were measured as a function of the concentrations of Quat 1 using the Du Noüy ring method. All measurements were performed using a Krüss- K20 tensiometer. Surface Tension (mn/m) 75 70 65 60 55 50 45 Quat 1 in H 2 O Quat 1 in D 2 O -1 0 1 2 3 4 5 6 7 8 9 Concentration (mm) Figure S9. Surface tension measurements of H 2 O and D 2 O in the presence of Quat 1.

S15 The CMC of Quat 1 in D 2 O was determined using isothermal titration calorimetry. Changes in heat were monitored upon consecutive additions of a Quat 1 solution in D 2 O (0.30 M) to pure D 2 O. The cell volume was 0.26 ml; aliquots (1.0 µl) were injected 38 times at 150-second intervals. Figure S10. Enthalpogram recorded upon addition of a 0.30 M solution of Quat 1 in D 2 O to pure D 2 O.

S16 8. Orientation analysis Table S4. Fitting parameters for 0.5 mm Quat 1 in H 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 3.4 10-10 6.2 10-12 1 10-2 0.1 3 10-2 0.3 A2 4.4 0.3 0.6 0.1 1 0.2 A3 4.5 1.1 0.8 0.1 2 0.2 A4 4.9 2.1 0.6 5 10-2 1 0.1 A5 0.2-0.1-0.2 - A6 5.2-0.4-0.7 - A7 5.4-0.3-0.5 - Г1 17 58.9 1 10 8 1 10-13 2 10 8 2 10-13 Г2 25 20 1 7 3 1 10 2 Г3 25 13 2 0.7 5 1 Г4 25 11.6 2 0.1 4 0.3 Г5 5.4-2 - 3 - Г6 25-1 - 2 - Г7 25-1 - 2.1 - ω1 2719 2603 5 10 8 9 10-12 9 10 8 2 10-11 ω2 2846 2865 1 5 2 1 10 2 ω3 2876 2890 0.8 0.3 2 0.7 ω4 2918 2960 0.6 0.1 1 0.2 ω5 2937-0.5-0.9 - ω6 2986-0.5-1 - ω7 3045-0.7-1 - n 2.8 10-2 0.1 7 10-3 4 10-3 1 10-2 8 10-3 p 1.6 10.8 4 10-2 2 10-2 0.1 5 10-2 n1 9.9 10-12 1.4 10-3 3 10-4 4 10-4 6 10-4 8 10-4 R 2 0.95 0.97

S17 Table S5. Fitting parameters for 0.5 mm Quat 1 in D 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 7.4 10-4 1.7 5 10-2 0.8 0.1 2 A2 1.1 5.0 10-11 4 10-2 2 10-2 0.1 4 10-2 A3 2.5 0.8 0.1 4 10-2 0.1 0.1 A4 2 2.1 0.2 0.1 0.3 0.1 A5 1.2-0.1-0.2 - A6 0.2-0.1-0.1 - A7 1.7-0.1-0.2 - Г1 22.1 40.9 1 10 3 6 3 10 3 1 10 2 Г2 10.3 12.9 0.2 4 10-13 0.5 8 10-13 Г3 13.1 11.3 0.3 0.5 0.5 0.9 Г4 17.4 11.7 0.8 0.1 2 0.3 Г5 10.5-0.4-0.9 - Г6 8.2-2 - 4 - Г7 25-2 - 3 - ω1 2730 2580 7 10 2 2 10 2 2 10 3 3 10 2 ω2 2854 2857 0.1 2 10-9 0.3 4 10-9 ω3 2878 2895 0.1 0.4 0.2 0.8 ω4 2918 2961 0.3 0.1 0.7 0.2 ω5 2939-0.2-0.4 - ω6 2990-1 - 2 - ω7 3040-2 - 4 - n 3.1 10-2 4.5 10-2 3 10-3 4 10-3 5 10-3 7 10-3 p 2.3 10.1 0.1 0.1 0.2 0.1 n1 2.5 10-4 2.0 10-10 1 10-4 2 10-4 3 10-4 5 10-4 R 2 0.99 0.97

S18 Table S6. Fitting parameters for 20 mm Quat 1 in H 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 1.2 10-6 0.7 1 10-2 0.1 3 10-2 0.2 A2 6.1 5.9 10-7 0.2 3 10-2 0.4 0.1 A3 2.1 0.6 0.1 4 10-2 0.2 0.1 A4 3.8 2.1 0.2 5 10-2 0.3 0.1 A5 0.3-2 10-2 - 5 10-2 - A6 4.1-0.2-0.4 - A7 4.0-0.2-0.3-1 6.0 9.0 8 10 4 2 2 10 5 3 Г2 25.0 7.6 0.5 5 10 5 1 1 10 6 Г3 11.0 8.5 0.3 0.5 0.6 1 Г4 20.1 12.1 0.5 0.1 1 0.2 Г5 4.5-0.3-0.6 0.6 Г6 30.0-0.8-2 2 Г7 30.0-0.7-1 1 ω1 2723 2590 5 10 4 2 9 10 4 4 ω2 2854 2858 0.3 3 10 5 0.5 6 10 5 ω3 2876 2890 0.1 0.3 0.2 0.6 ω4 2914 2962 0.2 0.1 0.4 0.2 ω5 2935-0.1-0.2 - ω6 2986-0.3-0.6 - ω7 3040-0.4-0.8 - n 0.1 0.1 4 10-3 3 10-3 8 10-3 5 10-3 p 1.4 3.8 8 10-3 4 10-2 2 10-2 0.1 n1 3.3 10-8 3.1 10-4 4 10-4 3 10-4 7 10-4 5 10-4 R 2 0.99 0.98

S19 Table S7. Fitting parameters for 20 mm Quat 1 in D 2 O. Parameters Estimated Value Standard Error Error with 95% Confidence Level SSP PPP SSP PPP SSP PPP A1 0.1 3.0 2 10-2 0.5 4 10-2 1.1 A2 1.1 0.3 4 10-2 0.1 0.1 0.1 A3 2.5 1.4 0.1 0.1 0.1 0.1 A4 1.4 2.1 0.1 0.1 0.3 0.1 A5 1.4-0.1-0.1 - A6 0.1-4 10-2 - 0.1 - A7 1.3-0.1-0.2 - Г1 11.3 2.1 4 1 10 2 7 2 10 2 Г2 10.9 88.5 0.3 7 0.6 1 10 2 Г3 12 25.0 0.2 0.4 0.4 0.7 Г4 16.2 14.7 0.9 0.1 2 0.3 Г5 10.4-0.3-0.6 - Г6 8-3 - 6 - Г7 23.3-1 - 3 - ω1 2730 2620 2 9 5 2 10 2 ω2 2854 2845 0.2 3 0.3 6 ω3 2878 2895 0.1 0.3 0.2 0.6 ω4 2919 2959 0.4 0.1 0.9 0.3 ω5 2940-0.2-0.3 - ω6 2989-2 - 4 - ω7 3051-2 - 3 - n 3.1 10-2 3.9 10-2 2 10-3 3 10-3 3 10-3 6 10-3 p 9.6 10.3 0.1 0.1 0.3 0.2 n1 1.0 10-3 7.5 10-4 6 10-5 2 10-4 1 10-4 4 10-4 R 2 0.99 0.95

S20 Table S8. Parameter values for generating the simulated SFG curves used to obtain the orientation distribution for Quat 1. Parameters Refractive indices: n 1 (air), n 2 (liquid) and n 1,SFG = n 1,vis = n 1,IR = 1.0 n i (interface) n 2,SFG = 1.3312 n 2,vis = 1.3292 n 2,IR = 1.4260 n i,sfg = n i,vis = n i,ir = 1.1656 R-value 3.4(-CH 3 ),, 3.4,, =,, =,, 1.0 N s (number density) 1.0 (cm -1 ) 12578.6 (cm -1 ) 2900 (cm -1 ) 15478.6 ( ) 50 ( ) 60 ( ) 51.7, 0.6132, 0.6284, 0.5207, 0.7313, 0.7435, 0.6124, 0.5837, 0.5764, 0.6029 Example: 0.163926 (4.4 Cos[ ]+2.4 Cos[ ] 3 ),,

S21 Simulated SFG Intensity 0.4 0.2 0.5 mm, 8 mm, and 20 mm Quat 1 in H 2 O 33.2 o 34.3 o 40.5 o 0.0 0 20 40 60 80 Tilt Angle, θ (a) A BC Simulated SFG Amplitude Ratio 1 0.5mM, 8mM, and 20mM Quat 1 in H 2 O 0 0 10 20 30 40 50 60 70 80 90 Distribution Angle,σ (c) θ=20 o θ=30 o θ=40 o θ=50 o A B C Simulated SFG Intensity 0.5 0.5 mm, 8 mm, and 20 mm Quat 1 in H 2 O 0.4 0.3 0.2 0.1 33.2 o 34.3 o 40.5 o 0.0 0 20 40 60 80 Tilt Angle, θ (b) A BC Simulated SFG Amplitude Ratio 0.7 0.6 0.5 0.4 0 10 20 30 40 Distribution Angle,σ (d) θ=20 o θ=30 o A B Simulated SFG Amplitude Ratio 0.4 0.3 0.2 0 10 20 30 40 50 60 70 80 90 θ=30 o θ=40 o θ=50 o C Distribution Angle,σ (e) Figure S11. Simulated curves of the SFG intensity ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of tilt angle (a and b) and simulated curves of the SFG amplitude ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of distribution angle (c-e) are shown for 0.5 mm, 8 mm, and 20 mm Quat 1 in H 2 O (labeled as A, B, and C, respectively).

S22 Simulated SFG Intensity 1.0 0.8 0.6 0.4 0.2 8 mm Quat 1 in 1:1 H 2 O and D 2 O 32.5 o 0.0 0 20 40 60 80 Tilt Angle, θ (a) B Simulated SFG Amplitude Ratio 8mM Quat 1 in 1:1 H 2 O and D 2 O 1.0 θ=20 o B 0.5 0.0 0 10 20 30 40 50 60 70 80 90 Distribution Angle,σ (c) θ=30 o θ=40 o θ=50 o Simulated SFG Intensity 1.0 0.8 0.6 0.4 0.2 8 mm Quat 1 in 1:1 H 2 O and D 2 O 32.5 o 0.0 0 20 40 60 80 Tilt Angle, θ (b) B Simulated SFG Amplitude Ratio 1.0 8 mm Quat 1 in 1:1 H O and D O 2 2 0.5 0.0 0 10 20 30 40 50 60 70 80 90 Distribution Angle,σ (d) θ=10 o θ=20 o θ=30 o θ=40 o B Figure S12. Simulated curves of the SFG Intensity ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of tilt angle (a, b), and simulated curves of the SFG amplitude ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of distribution angle (c, d) are shown for 8 mm Quat 1 in a 1:1 mixture of H 2 O and D 2 O (labeled as B).

S23 Simulated SFG Intensity 1.0 0.5 mm, 8 mm, and 20 mm Quat 1 in D 2 O 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 Tilt Angle, θ (a) 30.8 o 34.2 o 37.6 o A BC Simulated SFG Amplitude Ratio 0.5 mm, 8 mm, and 20 mm Quat 1 in D 1.0 2 O θ=10 o 0.5 0.0 0 10 20 30 40 50 60 70 80 90 Distribution Angle,σ (c) θ=20 o θ=30 o A B C Simulated SFG Intensity 0.5 mm, 8 mm, and 20 mm Quat 1 in D 1.0 2 O 0.8 0.6 0.4 0.2 30.8 o 34.2 o 37.6 o 0.0 0 20 40 60 80 Tilt Angle, θ (b) A BC Simulated SFG Amplitude Ratio 0.5 mm, 8 mm, and 20 mm Quat 1 in D 1.0 2 O 0.5 0 10 20 30 40 Distribution Angle,σ (d) θ=10 o θ=20 o θ=30 o A B C Figure S13. Simulated curves of the SFG Intensity ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of tilt angle (a, b) and simulated curves of the SFG amplitude ratios simulated curves of the SFG amplitude ratios (CH 3 SS of PPP/CH 3 AS PPP) as a function of distribution angle (c, d) are shown for 0.5 mm, 8 mm, and 20 mm Quat 1 in D 2 O (labeled as A, B, and C, respectively).

S24 Table S9. Orientation analysis of the terminal methyl group of Quat 1 using the amplitude ratio between the methyl symmetric stretch (CH 3 SS) and the methyl asymmetric stretch (CH 3 AS) at PPP polarization combination. Sample Intensity Tilt angle Amplitude Distribution angle ratio ratio 0.5 mm H 2 O 0.3 ± 0.1 33.2º ± 12.4 0.5 ± 0.1 20.4º ± 3.8 (Tilt angle 20º) 11.1º ± 2.1 (Tilt angle 30º) 0.5 mm D 2 O 0.1 ± 0.03 37.6º ± 8.4 0.4 ± 37.7º ± 4.2 (Tilt angle 10º) 30.0º ± 3.4 (Tilt angle 20º) 20.5º ± 2.3 (Tilt angle 30º) 8 mm H 2 O 0.2 ± 0.1 34.3º ± 7.1 0.5 ± 0.1 22.1º ± 2.3 (Tilt angle 20º) 13.2º ± 1.4 (Tilt angle 30º) 8 mm D 2 O 0.3 ± 0.2 34.2º ± 20.6 0.5 ± 0.2 27.7º ± 8.4 (Tilt angle 10º) 21.6º ± 6.5 (Tilt angle 20º) 12.9º ± 3.9 (Tilt angle 30º) 8 mm 0.3 ± 0.1 32.5º ± 13.1 0.6 ± 0.1 25.1º ± 5.1 (Tilt angle 10º) H 2 O/D 2 O 20 mm H 2 O 0.1 ± 40.5º ± 10.2 0.3 ± 28.9º ± 3.6 (Tilt angle 30º) 62.3º ± 7.8 (Tilt angle 30º) 4.4º ± 0.6 (Tilt angle 40º) 66.9º ± 8.4 (Tilt angle 40º) 70.3º ± 8.8 (Tilt angle 50º) 20 mm D 2 O 0.4 ± 0.1 30.8º ± 6.2 0.7 ± 0.1 22.7º ± 2.3 (Tilt angle 10º) 17.2º ± 1.7 (Tilt angle 20º) 5.6º ± 0.6 (Tilt angle 30º) *Experimental amplitude and intensity ratios do not match the simulated curves of SFG amplitude and intensity ratios.

S25 9. References 1. Sommer, H. Z.; Lipp, H. I.; Jackson, L. L., Alkylation of Amines. General Exhaustive Alkylation Method for the Synthesis of Quaternary Ammonium Compounds. The Journal of Organic Chemistry 1971, 36 (6), 824-828. 2. Gibson, M. S., The Introduction of the Amino Group. The Amino Group (1968) 1968, 37-77. 3. Chan, S. C.; Jang, J. H.; Cimatu, K. A., Orientational Analysis of Interfacial Molecular Groups of a 2-Methoxyethyl Methacrylate Monomer Using Femtosecond Sum Frequency Generation Spectroscopy. Journal of Physical Chemistry C 2016, 120 (51), 29358-29373. 4. Mukherjee, P.; Lagutchev, A.; Dlott, D. D., In Situ Probing of Solid-Electrolyte Interfaces with Nonlinear Coherent Vibrational Spectroscopy. Journal of Electrochemical Society 2012, 159 (3), A244-A252.