Reconciling DLVO and non-dlvo forces and their implications for ion rejection by a polyamide membrane

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1 Supporting Information: Reconciling DLVO and non-dlvo forces and their implications for ion rejection by a polyamide membrane Yijue Diao, Mengwei Han, Josue A. Lopez-Berganza, Lauren Valentino, Benito Marinas, Rosa M. Espinosa-Marzal Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 N. Matthews Avenue, Urbana, Illinois 61801, United States. *Corresponding author. Rosa M. Espinosa-Marzal, Tel: , address: rosae@illinois.edu 1/15

2 PA active layer roughness Figure S1. Commercially-available polyamide membranes (ESNA1-LF) were saturated with nanopure water and equilibrated for 24 hours prior to the AFM imaging in Tapping Mode with a sharp AFM tip (spring constant ~0.5 N/m, CSC37/Al, Mikromasch, Estonia) in areas a) 3 µm x 3µm, b) 10 µm x 10 µm, and c-d) 50 nmx50 nm; the circle gives the approximate size of the contact with the silica tip. Cross sectional profiles corresponding to the white lines in the AFM images are shown respectively at the bottom. The roughness of the membranes was estimated by averaging the RMS values reported in JPK Data Processing Software of 10 randomly selected regions in each image. The RMS roughness is 41.5±2.4 2/15

3 nm for a scan size of 3 µmx3µm, it is 48.2±2.9 nm for a scan size of 8µmx8µm, and less than 1.1 nm for a scan size of 50x50 nm. Spectroscopy Analysis (a) Transmission (%) (b) Arbitrary Intensity (c) Arbitrary Intensity Wavenumber cm (6) (5) (4) (3) (2) (1) Wavenumber cm Wavenumber cm -1 Figure S2. a) Representative IR spectrum obtained on a piece of clean membrane dried overnight in the range of 1500 to 1800 cm -1 ; Raman spectra of membranes exposed to various concentrations of b) NaCl solutions and c) CaCl 2 solutions. IR peaks (in cm -1 ): (1)1728 (ν C=O), (2)1668 (ν C=O), (3) 1615 (ν C=C), (4) 1585 (ν C=C), (5) 1545(δ N-H), (6) 1503(ν C=C). Water NaCl 1 mm NaCl 10mM NaCl 100mM NaCl 1M Water CaCl2 10mM CaCl2 1mM CaCl2 100mM CaCl2 1M 3/15

4 A piece of the membrane was cut and rinsed with nanopure water. After overnight drying, the FTIR spectrum of the membrane specimen was collected, using an ATR-IR spectrometer (Perkin Elmer, USA and Pike Technologies, USA). Following AFM measurements in a different ionic strength, specifically, 1 mm, 10 mm, 100 mm and 1 M NaCl and CaCl 2 solutions, and in water, Raman spectroscopy was performed on the specimen. The membrane samples were rinsed with nanopure water and dried overnight before mounted onto a Nanophoton RAMAN-11 Raman spectroscopy system (Nanophoton Raman-11, Japan). The 532 nm laser was deployed as the impinging light and the scattered spectra between 700 cm -1 and 3200 cm -1 were collected to probe changes of the chemical structures of the nanofiltration membranes in each electrolyte solution. Area scans over regions of 15 µm x15 µm were performed at different locations, with laser power ranging from 0.8 mw to 1 mw and exposure time from 2 s to 3 s, respectively. The power and the exposure times were limited to minimize damage to the membrane. The spectra in Figure S2 correspond to averages within the scan areas and are offset arbitrarily for clarity. Figure S2a shows a representative IR spectrum. The peak at 1728 cm -1 is assigned to the C=O stretching mode in carboxylic groups. The peaks at 1668 cm -1 and 1545 cm -1 are believed to represent the C=O stretching and N-H bending modes in solid-state secondary amide groups, usually named amide I and amide II bands. The peaks at 1585 cm -1 and 1503 cm -1 likely originate from the C=C stretching modes in conjugated, potentially aromatic, systems, while the peak at 1618 cm-1 may be attributed to aromatic amides 1. The measurement not only verifies the chemical composition of the polyamide membrane, but also validates the accuracy of the Raman data. Good agreement was found between the Raman and the IR spectra, for the peaks at 1728 cm -1 and 1615 cm -1, with differences of around 5 cm -1. The disappearance of the other IR peaks on the Raman spectra can be ascribed to the penetration depth of the Raman laser, or the vibrational modes being only IR active. As shown in Figure S2 (b) and (c), the Raman spectra taken on membrane specimens immersed in NaCl and CaCl 2 solutions during the AFM experiments are qualitatively similar to those obtained after immersion in nanopure water. This indicates that the chemical composition of the membranes is not sensitive to the ionic strength of the solutions to which the membranes have been exposed. Imaging of AFM tips AFM force measurements on unmodified PA active layers were performed with thermally annealed AFM tips. AFM cantilevers (Mikromasch, CSC37/Al, Estonia) were heated up to 1050 C for 2 hours in a furnace (Lindberg/Blue M, Asheville, USA). The resultant radius of the AFM probe is estimated to be 1100 nm based on SEM images and the RMS roughness is ~0.35±0.28 nm based on AFM images (not shown). 4/15

5 Figure S3. Image of the thermally annealed AFM tip used in force measurements obtained by Scanning Electron Microscopy (Hitachi S4700 High Resolution SEM, Tokyo, Japan). NaCl/CaCl2 [mm] HCO 3 - /CO 3 2- calculated [mm] NaCl HCO 3 - /CO 3 2- calculated [mm] CaCl 2 DL [nm] NaCl DL [nm] CaCl E E E E E E E E E E E E Table S1. CaCl 2, NaCl and carbonate concentrations, and corresponding Debye length of CaCl 2 and NaCl solutions. Ionic concentration values were obtained with Visual MINTEQ version 3.1, assuming the solution is equilibrated with atmospheric carbon dioxide. 5/15

6 Representative force-separation profile fitting considering both DLVO and non-dlvo forces 1 NaCl F/R eff [mn/m] Hydration (non-dlvo) force DLVO fit 0 mm 1 mm 10 mm 100 mm 1 M 4 M Separation [nm] Figure S4. Force-separation curves measured on the PA active layer of ESNA1-LF with a silica probe (R=1.1 µm) in water (black) and in NaCl solutions at the concentrations of 1 mm (blue), 10 mm (red), 100 mm (green), 1 M (yellow) and 4 M (purple); the lines give representative fits of Eq. (3) to measurements on different spots. The Debye length was calculated according to eq. (2), with the ion concentrations in equilibrium to the atmosphere generated by Visual Minteq at ph 6. The fitted parameters are shown below in Table S2. NaCl (mm) Considering F R (Eq. 3) Neglecting F R (Eq. 1) σ PA (mc/m 2 ) ψ tip (mv) S 0 (mj/m 2 ) λ (nm) D 0 (nm) σ PA (mc/m 2 ) ψ tip (mv) Variation of σ PA 0 (Water) -1.4(0.04) (0.03) 0 7% 1-2.4(0.2) -33.7(12.9) (0.1) -19.3(5.7) 8% 10-3(0.2) -65(14) (0.4) -59(17) 7% (2.9) -1.2(2.0) (2.3) -7.3(12) 8% Table S2. Fitting parameters (standard deviation in parenthesis) of the force-separation curves in NaCl solutions using Eqs. 1 and 3. The small discrepancy between the PA surface charge densities fitted by the two equations indicates that the effect of the roughness-induced steric repulsion is statistically insignificant in our system. 6/15

7 Statistical analysis of 2 D histograms with FTTs for PA active layers Figure S5. 2D histograms of the normalized layering force (F*/R) vs. layer thickness (Δ) resolved in the hydration force measured on the membrane surface in NaCl solutions a) for the first transition (T1, closest to the hard wall) and b) for other transitions (T2+) and in CaCl2 solutions c) T1 and d) T2+. Dashed lines in a) show the three characteristic peaks of layer thickness for 4 M NaCl, as an example. The shadowed regions in b) represent the size of water (2~3 Å in grey), and OScounterions with different hydration states (3~4 Å in orange and >4 Å in green). Control force measurements on silica surfaces AFM force measurements were also conducted on a silicon wafer with the AFM silica probe (Figure S3), to compare to the results measured on the supported PA active layer with the same probe. Representative results are shown in Figure S6a. The two surfaces (thermally oxidized blunt silicon tip and naturally oxidized silicon wafer) are assumed to have the same surface potential with dielectric properties corresponding to silica (n=1.448 and ε=3.8). This yields a Hamaker constant equal to 5.85x10-21 J. The lines in Figure S6a give the fits of the DLVO theory to representative surface forces (see parameters in the caption). The surface forces are repulsive at all concentrations 7/15

8 except at 4 M, where a secondary minimum is measured. Hence, we note that an expansion of the EDL is not measured in 4 M NaCl, in contrast to the results observed on the PA active layer. Figure S6. (a) Force-separation curves between the silica probe on a silica surface in water (black), and in NaCl solution (ph 6) at following concentrations: 1mM (blue), 10 mm (red), 100 mm (green), 1 M (yellow) and 4 M (purple); (b) Magnification of selected short-range forces (D< 3 nm) showing steps (see arrows). The lines in (a) give the fits to eq. (1) with following parameters for DL and ψ s: 95 nm/-17 mv (water), 9 nm/-12 mv (1 mm), 2 nm/-22 mv (10 mm), 2.3 nm/-16 mv (100 mm) and 1.4 nm/-14 mv (1M). A ph above 6 has been shown to facilitate gel formation on silica surfaces with time 2-5, thereby leading to long-range (brush-like) forces. In the discussed AFM force experiments (ph 6), no such force profiles were observed, and hence gel formation on silica can be ruled out. When NaCl is added to the solution, a few of the short-range forces exhibit steps (Figure S6b), which are characteristic of the layering of species (ions and water) in the solution film confined between the silica surfaces. However, the frequency of layering is much smaller (less than 15% of the 200 force curves per concentration have steps) than in membrane-silica experiments, which implies that the most part of the resolved layers in the latter system is attributed to the confining effect of the PA active layer. The FTTs in 200 force curves were analyzed and the obtained size of the FTTs and the corresponding layering force are summarized in the following 2D histogram (Figure S7). The small number of data points reflects that the small percentage of curves with FTTs. It is also to be noted that most of the FTTs have a size smaller than 3 Å, and hence significantly smaller than on the polyamide membrane. No distinction is made between T1 and T2+. The results are consistent to those measured between silica particles and reported in ref. 6. 8/15

9 Figure S7. 2D histograms of the normalized layering force (F*/R) vs. layer thickness (Δ) of all FTTs (T1 and T2+) resolved in the hydration force measured between silica surfaces in NaCl solutions. 9/15

10 Multipeak Fitting Parameter NaCl (mm) Peak 1 Peak 2 Peak 3 T1 T2+ Peak 4 Peak 1 Peak 2 Peak 3 Peak ±0.2 (58%) 2.8±0.2 (42%) ±0.2 (46%) 2.9±0.1 (54%) 1 2.2±0.1 (34%) 2.7±0.3 (59%) 3.6±0.2 (7%) - 2.4±0.2 (35%) 2.9±0.3 (35%) 3.6±0.3 (20%) 4.4±0.3 (10%) Film Thickness (Å) ±0.4 (55%) 2.3±0.2 (46%) 3.2±0.5 (37%) 2.8±0.4 (45%) 3.9±0.3 (9%) 3.9±0.2 (9%) ±0.3 (33%) 2.3±0.2 (16%) 3.1±0.4 (29%) 2.9±0.6 (38%) 3.9±0.2 (21%) 3.9±0.3 (25%) 4.6±0.4 (17%) 4.8±0.4 (21%) ±0.4 (69%) 3.1±0.3 (21%) 3.9±0.3 (10%) - 2.6±0.5 (42%) 3.5±0.4 (29%) 4.5±0.4 (19%) 5.6±0.1 (10%) ±0.4 (77%) 3.5±0.3 (18%) 4.7±0.2 (4%) - 2.6±0.4 (48%) 3.3±0.5 (38%) 4.2±0.3 (13%) 5.4±0.1 (13%) 0 0.8±0.5 (94%) 2.8±0.2 (6%) ±0.4 (84%) 1.9±0.1 (16%) 1 0.9±0.5 (65%) 2.3±0.3 (13%) 2.9±0.2 (22%) - 0.4±0.4 (74%) 1.2±0.3 (26%) Layering Force F*/R (mn/m) ±0.6 (50%) 0.9±0.6 (76%) 1.9±0.3 (29%) 2.1±0.3 (20%) 2.8±0.2 (21%) 3±0.2 (4%) ±0.3 (59%) 0.4±0.4 (83%) 1.2±0.3 (41%) 1.1±0.4 (17%) ±0.6 (75%) 1.9±0.4 (25%) ±0.2 (73%) 0.6±0.3 (27%) ±0.6 (69%) 1.9±0.5 (31%) ±0.3 (66%) 0.7±0.4 (34%) Table S3. Peak means, widths and relative frequency (in parenthesis) of the layer thicknesses in NaCl solutions obtained by fitting the histograms (Figures S5a and S5b) of the layer thickness distribution to Multipeak Gaussian distribution by Igor Pro, for the first film thickness transition (T1), and all other transitions (T2+). 10/15

11 Multipeak Fitting Parameter CaCl 2 (mm) T1 T2+ Peak 1 Peak 2 Peak 3 Peak 4 Peak 1 Peak 2 Peak 3 Peak ±0.2 (58%) 2.8±0.2 (42%) 2.4±0.2 (46%) 2.9±0.1 (54%) 1 2.4±0.2 (69%) 2.8±0.2 (24%) 3.5±0.2 (8%) 2.4±0.3 (44%) 3.3±0.3 (45%) 4.2±0.1 (11%) - Film Thickness (Å) ±0.2 (62%) 2.2±0.3 (50%) 2.7±0.2 (21%) 2.9±0.3 (39%) 3.4±0.2 (9%) 3.9±0.2 (11%) ±0.3 (43%) 2.4±0.3 (58%) 3.1±0.3 (43%) 3.3±0.4 (35%) 3.8±0.2 (14%) 4.1±0.3 (7%) ±0.3 (73%) 3.0±0.3 (32%) 3.9±0.2 (5%) - 2.5±0.5 (33%) 3.0±0.4 (25%) 3.6±0.4 (25%) 4.4±0.4 (17%) ±0.4 (57%) 3.1±0.3 (34%) 3.9±0.3 (10%) 4.9±0.1 (3%) 2.7±0.5 (50%) 3.5±0.3 (45%) 4.5±0.3 (5%) ±0.5 (94%) 2.8±0.2 (6%) ±0.4 (84%) 1.9±0.1 (16%) 1 0.9±0.7 (88%) 2.4±0.3 (12%) 0.3±0.3 (74%) 1.5±0.3 (26%) Layering Force F*/R (mn/m) ±0.5 (82%) 0.0±0.5 (77%) 2.4±0.5 (18%) 1.0±0.3 (17%) 2.5±0.5 (6%) - 0.4±0.4 (100%) 0.4±0.4 (73%) - 0.9±0.3 (27%) ±0.5 (79%) 1.2±0.4 (16%) 2.1±0.2 (5%) - 0.1±0.2 (88%) 0.8±0.1 (12%) ±0.6 (92%) 1.9±0.2 (8%) 0.2±0.3 (100%) - Table S4. Peak means, widths and relative frequency (in parenthesis) of the layer thicknesses in CaCl 2 solutions obtained by fitting the histograms (Figures S5c and S5d) of the layer thickness distribution to Multipeak Gaussian distribution by Igor Pro, for the first film thickness transition (T1), and all other transitions (T2+). 11/15

12 Figure S8. Bridging forces resulting from interactions between PAH chains (positively charged) with the negatively charged silica colloid in water before compression of the film. Radius [nm] Figure S9. Radius and zeta-potential of PAH coils in NaCl solution. The ph of the 1 mg/ml PAH solution was adjusted to 6. Various amounts of NaCl (purity 99.0%, Sigma-Aldrich, St. Louis, USA) were added into the PAH solution to achieve the final concentrations of NaCl (1 mm, 10 mm, 100 mm, and 1 M). The solutions were then individually filtered with clean 0.2-µm polytetrafluoroethylene (PTFE) membranes (Fisherbrand, Pittsburgh, USA) before the size and zeta-potential measurements were conducted with a Zetasizer Nano ZS90 (Malven, UK). 12/15

13 Figure S10. Representative frequency (blue) and dissipation (red) changes obtained during PAH adsorption on silica surfaces using QCM-D (11th, and 13th overtones). The arrows indicate the injection of the solutions into the QCM cell. The baseline was obtained in DI water (1). A concentration of 1 mg/ml PAH in 500 mm NaCl solution was used for adsorption. The changes in frequency and dissipation during injection of the NaCl solutions result from both the collapse of the PAH film and the density and viscosity changes of the bulk solutions. The dashed line shows that the mass of the adsorbed PAH film remained constant after incubation in NaCl solutions (3), which indicates that no desorption of the polymer occurred. Standard viscoelastic Voigt s model from Q-tools (Biolin In-strument, USA) was employed to determine the PAH film thickness over 5 measurements. The Voig model yields following thickness (T) of the films as a function of NaCl concentration: ~21.5 nm in water and in NaCl solutions at the concentrations of 1mM and 10mM, 21 nm in 100mM NaCl, and 13 nm in 1M NaCl. The collapse occurs at a higher concentration on silica compared to the membrane likely due to the higher grafting density. 13/15

14 a) Sym-Osmotic b) Sym-Salted F/R eff [mn/m] Bridging F/R eff [mn/m] Figure S11. a) Surface forces between a modified PA active layer with a PAH film and a silica colloid coated with a PAH film (symmetric system, or PAH-PAH) as a function of NaCl concentration in the a) osmotic and b) salted regime. Lines give the fits to Eqs. (4a) and (4b), respectively. The shadowed regions give the range of measured surface forces at each concentration (water in black; 1mM in blue, 10mM in red, 100mM in green and 1 M in yellow mm 1 mm F/R eff [mn/m] Separation [nm] Figure S12. Representative force-separation curves for the symmetric system (PAH-PAH) in water and in 1mM NaCl solution, showing the EDL force at separations beyond the film. The lines give the slope of the EDL force with DL of 30 nm and 9.5 nm, respectively. 14/15

15 References 1. Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O., Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry. Desalination 2009, 242 (1-3), Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J., Interactions of silica surfaces. Journal of Colloid and interface science 1994, 165 (2), Taran, E.; Donose, B. C.; Vakarelski, I. U.; Higashitani, K., ph dependence of friction forces between silica surfaces in solutions. Journal of Colloid and Interface Science 2006, 297 (1), Wang, Y.; Wang, L.; Hampton, M. A.; Nguyen, A. V., Atomic Force Microscopy Study of Forces between a Silica Sphere and an Oxidized Silicon Wafer in Aqueous Solutions of NaCl, KCl, and CsCl at Concentrations up to Saturation. The Journal of Physical Chemistry C 2013, 117 (5), McNamee, C. E.; Higashitani, K., Time Dependence of Silica Surfaces on Their Interactions in Water and Alkaline Solutions. Langmuir 2015, 31 (22), Diao, Y.; Espinosa-Marzal, R. M., Molecular insight into the nanoconfined calcite solution interface. Proceedings of the National Academy of Sciences 2016, /15