Direct quantification of negatively charged functional groups on membrane surfaces

Journal of Membrane Science 389 (2012) 499–508

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Direct quantification of negatively charged functional groups on membrane surfaces Alberto Tiraferri, Menachem Elimelech ∗ Department of Chemical and Environmental Engineering, Yale University, P.O. Box 208286, New Haven, CT 06520-8286, USA

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Article history: Received 30 July 2011 Received in revised form 9 November 2011 Accepted 9 November 2011 Available online 20 November 2011 Keywords: Surface charge Thin-film composite membranes Carboxylic groups Titration Uranyl Uranyl cation binding Charge density Polyamide Water purification Toluidine blue O Charge quantification

a b s t r a c t Surface charge plays an important role in membrane-based separations of particulates, macromolecules, and dissolved ionic species. In this study, we present two experimental methods to determine the concentration of negatively charged functional groups at the surface of dense polymeric membranes. Both techniques consist of associating the membrane surface moieties with chemical probes, followed by quantification of the bound probes. Uranyl acetate and toluidine blue O dye, which interact with the membrane functional groups via complexation and electrostatic interaction, respectively, were used as probes. The amount of associated probes was quantified using liquid scintillation counting for uranium atoms and visible light spectroscopy for the toluidine blue dye. The techniques were validated using selfassembled monolayers of alkanethiols with known amounts of charged moieties. The surface density of negatively charged functional groups of hand-cast thin-film composite polyamide membranes, as well as commercial cellulose triacetate and polyamide membranes, was quantified under various conditions. Using both techniques, we measured a negatively charged functional group density of 20–30 nm−2 for the hand-cast thin-film composite membranes. The ionization behavior of the membrane functional groups, determined from measurements with toluidine blue at varying pH, was consistent with published data for thin-film composite polyamide membranes. Similarly, the measured charge densities on commercial membranes were in general agreement with previous investigations. The relative simplicity of the two methods makes them a useful tool for quantifying the surface charge concentration of a variety of surfaces, including separation membranes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Liquid separation by polymeric membranes is used routinely in a variety of applications, including water and wastewater treatment [1,2], seawater desalination [3], liquid food processing [4], industrial separation processes [5,6], and more recently in energy production and storage systems [7]. Polymeric membranes often possess surface moieties and consequently acquire surface charge when in contact with an aqueous solution. The charged functional groups at the surface affect the interactions of solutes with the membrane surface, thus impacting the membrane performance. In particular, most commercial reverse osmosis (RO) and forward osmosis (FO) membranes have a thin-film composite (TFC) structure, whereby a thin, selective polyamide layer is cast on top of a polysulfone support [8]. The polyamide layer possesses innate carboxyl- and amino-groups when immersed in aqueous solution due to incomplete cross-linking of the polymer during fabrication.

∗ Corresponding author. Tel.: +1 203 432 2789; fax: +1 203 432 4387. E-mail address: [email protected] (M. Elimelech). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.11.018

For “loose” nanofiltration (NF) and ultrafiltration membranes, the separation of charged solutes by electrostatic (Donnan) exclusion is directly related to the density of surface charges [9–12]. In addition, the dissociation of charged groups also affects the “openness” of the pores and therefore, the separation by sieving or size exclusion [10]. In “tighter” NF, RO, and FO membranes, membrane separation is governed by a solution-diffusion mechanism [13]. Generally, the presence of functional groups in these membranes is an indication of a lower extent of polymer cross-linking due to incomplete reaction of the monomers during interfacial polymerization. Membrane surface charge also plays a role during the fouling of tight and loose membranes by charged macromolecules and colloidal matter by governing the electrostatic interactions between the foulants and the membrane surface or pore walls [14]. Furthermore, surface moieties can be exploited as reactive sites for binding of surface coatings [8,15] or nanomaterials [16–18] for membrane surface modification. For these reasons, the development of simple methods for direct quantification of membrane surface charge density is of paramount importance. Direct quantitative measurement of membrane surface charge has remained challenging due to limitations associated with the characterization techniques. Titration methods cannot be readily

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applied because the measured charge density is affected by the bulk of the membrane material below the surface, which is often composed of different material than that of the active layer. Furthermore, the relatively low membrane surface area requires more sensitive characterization techniques. The charge of TFC NF and RO membranes has been investigated using streaming potential and tangential streaming potential [19,20]. These analyses only provide indirect or qualitative measurement of the membrane charge characteristics. More quantitative studies have been carried out using contact angle titration [21–24]. However, this method does not provide characterization of the entire amount of functional groups, but rather of the ionized charge groups [24]. In addition, contact angle titration is highly sensitive to surface physical and chemical properties, and often underestimates the charge density [24,25]. More recently, Rutherford backscattering spectrometry (RBS) has been successfully employed to determine the amount and distribution of ionized charges in polyamide thin films [26–28]. This technique allows for the quantification of the concentration of both the deprotonated carboxylic and protonated amine groups in the polyamide layer as a function of pH. Based on RBS data, modeling of the ionization behavior of membrane moieties was performed for commercial thin-film composite membranes [27]. RBS is reliable and insightful, but entails significant experimental and analytical efforts, as well as the availability of elaborated and expensive instrumentation. Moreover, RBS gives information on the entire probed film, rather than the membrane surface [27]. The ideal experimental technique to quantify the density of surface charges is not influenced by the underlying layer, is independent of ionic strength, and is capable of providing information about the degree of ionization of the functional groups. Uranyl cation binding (UCB) and the use of toluidine blue O (TBO) dye are both straightforward techniques that can be deployed for successful quantification of surface moieties. The UCB method entails the complexation of uranyl ions to the membrane functional groups and subsequent quantification of the number of bound uranyls. Uranyl ions are employed to stain the carboxyl-rich portion of polyamide films for TEM imaging [29–31]. UCB has also been used for quantification of carboxylic groups in cellulose [32] and to determine the relative surface charge due to free sulfonate [33,34] and carboxylic [35] groups in water separation membranes. TBO is a cationic compound that interacts electrostatically with negatively charged groups and can be easily detected by light absorption in the blue region. It is currently employed in a variety of applications as a labeling agent, photosensitizer, or mediator for chemical reactions, but is used especially in the medical field to mark or target acidic compounds using high pH solutions [36]. These features of TBO may suggest its suitability for evaluating the charge density of polymeric membranes. The objective of this study is to evaluate the suitability of the UCB and TBO techniques for the quantification of surface charge density of TFC polyamide membranes. After validation of the techniques using charged self-assembled monolayers (SAMs), both methods were employed to quantify the surface charge of hand-cast as well as commercially available membranes. Protocols for the application of these techniques are established and the advantages and limitations of the techniques are discussed. These straightforward experimental approaches can be further extended to determine the charge density of a variety of other surfaces and materials.

2. Materials and methods 2.1. Chemicals Toluidine blue O (TBO, technical grade), 1,3-phenylenediamine (MPD, >99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%),

(TEA, >99.5%), (1S)-(+)-10-camphorsulfonic triethylamine acid (CSA, 99%), dodecyl sulfate sodium salt (SDS, >99%), 1dodecanethiol (>98%), and 11-mercaptoundecanoic acid (95%) were used as received (Sigma–Aldrich, St. Louis, MO). Uranyl acetate (UA, ACS reagent) was purchased from Electron Microscopy Sciences (Hatfield, PA) and employed as received. Sodium chloride (NaCl, crystals, ACS reagent) from J.T. Baker (Phillipsburg, NJ) was used to adjust ionic strength of the reacting solutions and for the membrane performance tests. The pH of the reacting solutions was adjusted using hydrochloric acid (HCl) and sodium hydroxide (NaOH). During interfacial polymerization of polyamide, TMC was dispersed in Isopar-G, a proprietary non-polar organic solvent (Univar, Redmond, WA). Chemicals used for post-treatment of polyamide for seawater membranes were sodium hypochlorite (NaOCl, available chlorine 10–15%, Sigma–Aldrich) and sodium bisulfite (NaHSO3 , Sigma–Aldrich). To form self-assembled monolayers (SAMs), different concentrations of 1-dodecanethiol and 11-mercaptoundecanoic acid were dissolved in ethanol (anhydrous ACS/USP grade, purchased from Pharmco) and adsorbed onto gold surfaces. Ethanol was deoxygenated by bubbling ultra-high pure Ar for 1 h before use. Unless specified, all chemicals were dissolved in deionized water (DI) obtained from a Milli-Q ultrapure water purification system (Millipore, Billerica, MA). 2.2. Membranes 2.2.1. Fabrication of thin-film composite (TFC) polyamide membrane The polyamide active layer was formed on top of commercial polysulfone (PSf) ultrafiltration membranes (PS20, Sepro Membranes, Oceanside, CA) via interfacial polymerization. Two different procedures were employed for seawater or brackish water TFC polyamide, to form TFC membranes with different surface carboxyl group density. Seawater polyamide (SW-PA) was fabricated using the procedure described in our previous publication [37]. For brackish water polyamide (BW-PA), the support membrane was initially immersed in an aqueous solution of 2.3 wt% MPD, 1.48 wt% CSA, 0.77 wt% TEA, and 0.02 wt% SDS for 120 s. An air knife was used to remove the excess MPD solution from the surface. The saturated support membrane was then immersed into a 0.134 wt% TMC in ISOPAR-G solution for 60 s, resulting in the formation of an ultrathin polyamide film. The composite membranes were cured in an oven at 82 ◦ C for 600 s. All membranes were rinsed thoroughly and stored in DI at 4 ◦ C. Pure water permeability, A, and NaCl permeability, B, of the hand-cast and commercial membranes were evaluated in a laboratory-scale crossflow RO test unit, following the procedure described in our previous publication [37]. 2.2.2. Commercial membranes Commercial asymmetric cellulose triacetate membranes (CTA, Hydration Technology Inc., Albany, OR) were employed for comparison. Thin-film composite seawater reverse osmosis membranes (SW30-HR, Dow Chemical Company, Midland, MI) were also acquired and their surface charge evaluated. These membranes have a similar structure to our hand-cast polyamide membranes, with a fully aromatic polyamide thin film cast on top of a PSf support. 2.3. Fabrication of self-assembled monolayers (SAMs) Mixed monolayer films were formed by spontaneous assembly of 12-carbon organic thiols with different tail groups from solution onto gold (SAMs). The 1-in.2 gold surfaces were cleaned prior to use in a UV-O3 chamber for 20 min and then by thermal desorption at 280 ◦ C for 3 h [38,39]. This treatment was followed by dissolution of a mixture of CH3 -terminated alkanethiols (1-dodecanethiol) and

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COOH-terminated thiols (11-mercaptoundecanoic acid) at varying proportions of 0, 10, 50, and 100 wt% of 11-mercaptoundecanoic acid to the total thiol amount. A total thiol concentration of 1 mM was employed. The gold surfaces were immersed in the reacting solution for 15 h at 25 ◦ C, and constant stirring was provided. By contact of the gold surfaces with these solutions, binary systems of different charge were obtained by co-adsorption of the thiols [40]. The resulting SAM composition is expected to reasonably compare to the initial thiol proportion in solution. The tail acid groups of the SAMs exist predominantly as the fully deprotonated and negatively charged carboxylate ion at high pH [41], and their concentration was quantified using both UCB and TBO techniques.

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for each surface and placed into a scintillation vial with the active side facing the vial walls. Scintillation fluor was added into the vial (15 mL) and the uranium activity was analyzed. The blank consisted of unreacted membrane surfaces of the same size as those used for the experiments. For all the measurements, three energy range channels were recorded by the scintillation analyzer, namely 70–250 keV, 50–350 keV, and 50–2000 keV. The energy peak for the ␣-particles was observed to fall within the smaller range of 70–250 keV. Each vial was analyzed twice and the counting performed for 45 min each time with no quenching. The analyzer was calibrated before every experiment using standards. Output data was recorded in CPM format.

2.4. Uranyl cation binding (UCB) technique The membrane surface charge was determined by quantifying the amount of uranyl cation binding (UCB) [32–34]. Uranium UVI is an alpha ␣-particle emitter with a radioactive decay energy of 4.20 MeV [42]. Its short-lived daughters are 234 Th, 234 Pa, and 231 Th, which are beta emitters [42]. The number of emitted ␣-particles in solution or from the investigated surface can be counted using liquid scintillation counting. In this study, a low activity liquid scintillation analyzer (Tri-Carb 2900TR, PerkinElmer, Waltham, MA, USA) was employed. Although this technique is generally used for ␤-particles, its use for alpha emitters is attractive because it offers counting efficiencies of nearly 100% [43]. The peaked distribution produced by the monoenergetic alpha particles can be easily distinguished from the broad continuum of the beta-particle background, even at very low levels of ␣-particle activity [44]. To construct the calibration curves, uranyl acetate solutions at varying concentrations were prepared by dissolving the dry powder of uranyl acetate in the scintillation fluor (SafeScint AB01937, American Bioanalytical, Natick, MA, USA). A total of 15 mL solution was pipetted into scintillation vials for counting. The obtained calibration curves were used to determine the surface charge concentration from the counts per minute (CPM) raw data. Clean gold surfaces used for the SAMs experiments were placed into the vials to construct the related calibration curve. This procedure was adopted to account for the photoelectric effect, whereby the high energy ␣-particles can cause emission of UV light from the gold surface, thus influencing the counting by the photo cathode in the instrument. No photoelectric effect was observed in the presence of membranes. For the evaluation of SAMs charge, 10-mM uranyl acetate solutions were freshly prepared at unadjusted pH of approximately 4.5. The SAM-covered gold surfaces were immersed in the solution for 3 min at room temperature (23 ◦ C), followed by a thorough rinse with DI water. Next, the surfaces were left in DI water for an extended time (>1 h) and then rinsed one more time with DI water. Subsequently, the reacted surfaces were placed into scintillation vials with 15 mL of scintillation fluor and analyzed. Blanks consisted of unreacted gold surfaces. To measure the membrane surface charge, the membranes were attached to a glass plate using laboratory tape with only the active side accessible for contact with the uranyl acetate solution. The solution was pipetted onto the membrane surface using approximately 0.5 mL for 1 cm2 of membrane surface. Different surfaces were contacted for varying times, ranging from a few seconds to several hours at room temperature (23 ◦ C). Three different concentrations of uranyl acetate were explored, namely 1, 10, and 50 mM. After contact, the surface was detached from the glass plate and rinsed thoroughly with DI water. Next, it was placed in DI water for several hours (>4 h) to allow the diffusion of the unreacted uranyl ions from the film to the bath. Scintillation counting of the rinsing solution did not show any detectable radioactivity. After another rinse with DI water, two 1-in. diameter samples were punched

2.5. Toluidine blue O (TBO) technique The membrane surface charge was also determined by quantifying the amount of bound toluidine blue O molecules. TBO is a cationic blue dye derivative of phenothiazine (C12 H9 NS) [45]. Its molecular weight is around 270 g/mol and it shows a peak of light absorption at a wavelength of approximately 600 nm [46]. To construct the calibration curves, TBO solutions at varying concentrations were prepared by dissolving the dry TBO powder in 0.2 M NaCl solutions at pH 2, and their light absorbance at 630 nm was measured using a microplate reader (SpectraMax 340PC, Molecular Devices). The obtained calibration curve was used to determine the surface charge concentration from the optical density raw data. For the evaluation of SAMs charge, 2 mM TBO solutions were freshly prepared in 0.015 M NaCl at pH 11. The SAM-covered gold surfaces were immersed in the solution for 10 min at room temperature (23 ◦ C), during which the dye bound via electrostatic interaction to the ionized acidic charges. Following a thorough rinse with a solution of 0.015 M NaCl at pH 11, the surfaces were placed in the same solution for an extended time (>1 h) to wash away the unbound dye molecules. After another thorough rinse, the reacted surfaces were placed in 10 mL of 0.2 M NaCl solution at pH 2 for 30 min while stirring. During this step, the TBO molecules bound to the acidic groups of the SAMs were eluted from the analyzed surface and diffused into solution, coloring it blue. The light absorbance of the solutions at 630 nm wavelength was measured. The blank consisted of a 0.2 M NaCl solution at pH 2 that was put in contact with an unreacted gold surface. To measure the surface charge of the investigated membranes, a similar procedure was adopted. The membranes were attached to a glass plate using laboratory tape and care was taken that only the active surface was accessible. The TBO solutions containing 0.015 M NaCl at target pH were pipetted onto the membrane surface, using approximately 0.5 mL for each square centimeter of membrane, and different surfaces were contacted for varying lengths of time ranging from a few seconds to several hours at room temperature (23 ◦ C). Three different concentrations of TBO were explored, namely 0.5, 2, and 10 mM. After contact, the surface was detached from the glass plate and rinsed thoroughly with a dye-free solution of the same pH and ionic strength. The same solution was used to soak the membranes for several hours (>4 h) to allow the diffusion of the unbound dye molecules from the film to the bath. After another rinse, two 1-in-diameter samples were punched for each surface and placed into 10 mL of 0.2 M NaCl at pH 2 for 30 min while stirring. One low pH step was sufficient, as the subsequent solutions were observed to undergo negligible color change once in contact with the membranes. The solutions were analyzed using optical density at 630 nm wavelength. The blanks consisted of 0.2 M NaCl solution at pH 2 in contact with unreacted membrane surfaces of the same size as those used for the experiments.

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Fig. 2. Calibration curve for the concentration of toluidine blue dye using optical density at 630 nm wavelength and 1-cm path length. The light absorbance of 0.3-mL solutions containing a known concentration of dye was measured using a 96-plate well analyzer. Linear fit is shown for the linear region of the experimental data.

Fig. 1. Calibration curves for the number of uranium atoms using a liquid scintillation counter. Known amounts of uranyl acetate were diluted in 15 mL of scintillation fluor in the (A) absence and (B) presence of a gold surface used for self-assembled monolayers. Data points represent results for the 70–250 keV channel window, after subtraction of the blank (no uranyl acetate molecules). Detection limit is around 1 count per minute (CPM), corresponding roughly to (A) 1–3 × 10−8 and (B) 2–5 × 10−11 moles of uranium atoms in solution.

3. Results and discussion 3.1. Calibration curves and detection limit The calibration plots in Fig. 1A show the counts per minute (CPM) obtained by liquid scintillation (70–250 keV energy range) of solutions containing a known amount of uranium atoms. CPM increased linearly with the concentration of uranyl acetate, allowing the use of a linear equation to relate CPM raw data to the concentration of uranium atoms. The lower detection limit was approximately 1 CPM, corresponding to around 1–3 × 10−8 moles of uranium atoms. Fig. 1B presents the CPM data when a clean gold sheet (used for self-assembled monolayer to be discussed later) is immersed in the calibration solutions. The resulting photoelectric effect in the presence of gold [47] significantly enhanced the instrument response and altered the relationship between CPM and the uranium concentration. The enhanced sensitivity in the presence of gold resulted in a lower detection limit of approximately 2–5 × 10−11 moles of uranium atoms in solution. The measured data in Fig. 1B are described by a power law, which was used to convert CPM to moles of uranium atoms. The UCB technique relies on a number of assumptions, some of which can result in underestimation of the density of functional groups, while others in their overestimation. First, we assume that the complexation of uranyl ions with nearly all carboxylic groups occurs fast, meaning that the process is diffusion-limited.

This assumption is corroborated by observations showing that the UVI -carboxyl stability constants for surface carboxylates are high and significantly higher than those determined in bulk solutions under the same conditions [34,48–50]. Second, we assume that bidentate complex formation is more probable than monodentate complexation, giving rise to a 1:1 stoichiometric ratio of uranyl ion to carboxylic group. Bidentate complexation has been observed to be favorable when uranyl ions form complexes with carboxylic groups associated with a solid surface [49,51–53]. Third, we assume that complexation is maximized at pH 4.5. Protonation of carboxylates at lower pH can decrease the complexation kinetics [51,54], while at high pH, association of uranyl ions with hydroxyl groups and subsequent precipitation can occur [54,55]. All the assumptions discussed above can lead to underestimation of functional groups. Overestimation can result from the following assumptions. First, the constructed calibration curves allow for the quantification of uranium atoms from scintillation data, even without ␣/␤ emission discrimination. Second, nonspecific adsorption of uranyl ions in the thin film is negligible. Third, the support side of the membrane comes in contact with no or a negligible amount of uranyl acetate; radioactivity of the rinsing solutions was undetectable, thereby corroborating this assumption. Fourth, we assume that a statistically minor amount of alpha particles are completely emitted into the polymer and thus, are undetectable by the scintillation counter. The fraction of alpha particles emitted into the polymer is a complex function of the surface roughness and of the morphology of surface features. Understanding this process would require a thorough statistical analysis that is beyond the scope of this paper. We note, however, that this fraction of radioactive particles could be significant, which results in underestimation of the total number of uranyl ions at the surface. Finally, we assume complete dissolution of uranyl acetate in solution. Filtering the solution through a 0.1-␮m filter prior to use is recommended to avert the presence of unbound crystals at the surface of the materials characterized. Fig. 2 shows the light absorbance of solutions containing a known amount of TBO at a 630 nm wavelength and a path length of 1 cm. The linear portion of the graph was confined in the region between approximately 0.3 and 100 ␮M TBO, which represent the lower and upper detection limits in our study. All experimental data obtained in this study were within the linear response region. The sensitivity of the instrument corresponded to variations in absorbance on the order of 10−4 units, equivalent to 0.24 nM of TBO in solution.

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performed on SAMs proved that the proposed procedures can be used to adequately estimate the surface charge density of carboxylic groups. 3.3. Surface charge density of hand-cast TFC polyamide membranes

Fig. 3. Carboxylic group density of the prepared self-assembled monolayers (SAMs) as measured by using both the UCB and TBO techniques. Carboxylic group density is plotted as a function of the relative concentration of –COOH-terminated n-alkanethiols to the total amount of n-alkanethiols in the solution used to form the SAMs on the gold surface. A range of SAM packing density was obtained from literature and is reported as a red solid line on the right axis; this range is equivalent to the surface charge density of 100% –COOH SAMs. TBO technique was carried out by contacting the SAMs with a 2-mM solution of toluidine blue O for 10 min. UCB data was obtained by contacting the SAMs with a 10-mM solution of uranyl acetate for 3 min. Functional group density was calculated from the corresponding calibration curves in Fig. 1B and 2.

The TBO technique also relies on a few assumptions. It is assumed that all the carboxylic groups of the examined surfaces are ionized at pH 11 and protonated at pH 2, and that the dye molecules interact quickly with the ionized charges and remain bound until the elution step at pH 2. This assumption could result in underestimation of the density of surface functional groups. In this study, the dye was dissolved in a 15 mM NaCl solution and the same ionic strength was employed for the rinsing solution. Control experiments were also undertaken in the absence of NaCl and compared to experiments carried out using the procedure depicted above. Comparable results were obtained with and without NaCl, thereby suggesting that there is no competition between the Na+ and the toluidine blue O molecules under the conditions used in our study (data not shown). However, competition might arise at higher NaCl concentrations, and users should minimize the amount of positively charged ions in solution that could compete with the TBO dye for binding sites. Finally, non-specific adsorption of TBO on the membrane surface was assumed to be negligible in the charge density calculations. This assumption could lead to overestimation of the density of surface functional groups. 3.2. Carboxylic group density of a reference surface (SAMs) The measured carboxyl group densities of SAMs with different amounts of –COOH terminated alkanethiols are presented in Fig. 3. The surface density of self-assembled alkanethiol molecules on gold has been reported to be in the range of 4–6.5 molecules/nm2 for a complete monolayer coverage [38,56–58]. This range is indicated in the figure by the vertical red bar. The measured charge density was expected to fall within this range when all the thiols were –COOH terminated. Our measurements resulted in overestimation of this predicted charge coverage, possibly due to the high sensitivity of the calibration curve (Fig. 1B) and/or to unaccounted mechanisms described in Section 3.1 where we discuss the assumptions associated with the techniques. As expected, the measured –COOH groups increased monotonically as the –COOH terminated thiols in the solution used to prepare the SAMs increased. When all the thiol molecules were neutrally charged, the UCB technique correctly determined the absence of charges, while the results with the TBO technique suggested a very low density of surface charge. The correct assessment of no or low charge density for the methyl-terminated SAMs can be regarded as a negative control for these experiments. The measurements

The physicochemical properties of thin-film composite polyamide membranes formed by interfacial polymerization of MPD and TMC have been extensively studied [15,28,29,59–62]. Some studies propose that for these membranes, a thin dense selective layer of low net charge separates two more porous layers [29–31,61]: an outer layer of relatively high, negative fixed charges resulting from incomplete reaction and hydrolysis of the acyl halides, and an inner layer with low concentration of positive charges originating for the incomplete reaction of the functional groups of the amine-containing monomers [24,29]. Other studies suggest the existence of a depth-homogeneous polyamide layer with properties nearly equal at all depths [28,62]. Uranyl and TBO molecules can interact with the negatively charged groups and inform us about the density of negative charges on the membrane surface. We note that significant complexation of uranyl with amine groups was ruled out in previous studies [63]. For practical purposes, we define the membrane surface as the entirety of polyamide layer sites available for interaction with the uranyl or TBO probes upon contact with the active side of the membrane, before the probe molecules diffuse into the polymeric thin film. The hand-cast seawater polyamide membranes had a water permeability coefficient, A, of 0.57 ± 0.28 L m−2 h−1 bar−1 and a salt (NaCl) permeability coefficient, B, of 0.084 ± 0.086 L m−2 h−1 . For our hand-cast brackish water polyamide membranes, we obtained an A and B of 2.42 ± 0.23 L m−2 h−1 bar−1 and 0.12 ± 0.01 L m−2 h−1 , respectively. The higher permeability of the brackish water polyamide membranes suggests that these films were less crosslinked than the respective seawater films [8]. Therefore, a higher negative surface charge density would be expected for brackish water membranes. 3.4. Uranyl cation binding (UCB) The densities of charged moieties of hand-cast seawater polyamide membranes measured by the UCB technique are shown in Fig. 4. Functional group density was calculated by dividing the amount of functional groups determined from CPM data by the membrane planar area. Functional group densities are plotted against the square root of contact time of uranyl acetate with the membrane surfaces in order to evaluate the diffusion of uranyl molecules within the dense polyamide thin film. Three different concentrations of uranyl acetate – 1, 10, and 50 mM – were contacted with membrane active layer to explore the dependence of the UCB technique on uranyl concentration. The three plots in Fig. 4 show a similar behavior. A measure of the surface functional group density was obtained for the lowest contact time of 5 s, confirming that the complexation of uranyl with the carboxyl moieties at the membrane surface was fast [34]. For short contact times, the functional group density data remain nearly constant. This initial plateau is more clearly presented in the inset graphs for each uranyl acetate concentration. At larger contact times, we observed an increase in the density of moieties before the data reached a final plateau for the two higher concentrations of uranyl acetate (Fig. 4B and C). A final plateau was not attained for the experiments with 1 mM uranyl acetate due to the low concentration of the compound. The constant value of charge density obtained at short contact times (initial plateau presented in the inset graphs) is associated with the functional groups at the outer layer, i.e. the active

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Fig. 4. Functional group density as a function of the square root of contact time of uranyl acetate with the hand-cast polyamide seawater membrane surface for: (A) 1 mM, (B) 10 mM, and (C) 50 mM of uranyl acetate. Measured points (circles) represent data for the 70–250 keV energy window. Functional group density was calculated from CPM data using the calibration curve shown in Fig. 1. Data points are plotted on a log scale in the y axis and linear scale in the x axis. Note that the x axis scale is different for the three plots. Linear fits are shown for the linear regions of the experimental data along with the corresponding slopes (when plotted as linear–linear curves). Inset plots are shown for each uranyl acetate concentration, presenting the first data points at low contact time with the membrane surface.

surface of the thin film. Charge densities in this region range from 20 to 35 charges/nm2 of planar area. This density of surface moieties was comparable for the three concentrations, confirming that the surface measurements were independent of uranyl acetate concentration. The measured values were slightly higher than those obtained from contact angle titration in previous studies (Table 1) [24]. However, contact angle experiments do not account for all the surface functional groups detectable by other techniques [25]. The total contact time associated with the initial plateaus decreased as uranyl concentration increased and corresponded to approximately 500, 120, and 30 s for 1, 10, and 50 mM uranyl acetate, respectively. The second portion of the curve, showing an increment in functional groups with increasing contact time, is attributed to the diffusion and complexation or uranyl molecules within the polyamide film. The near linear increase in charge density versus square root of contact time (when plotted on a linear-linear scale) in this region of the profiles suggests that the process is diffusion-controlled. As expected, the slope of the diffusion profiles increased with uranyl concentration. The advancing uranyl front can be represented as a diffusion mechanism across a dense thin film with infinitely fast depletion at the front boundary. In this case, the slope of the linear increase should scale with the square root of initial uranyl [64]. Therefore, a √ concentration √ theoretical relationship of 1 : 10 : 50 for the three different concentrations was expected. However, the theoretical relationship was not confirmed experimentally. This discrepancy may be attributed to confounders such as the heterogeneous distribution of charges across the film [28,29] and the inaccessibility of polyamide small network voids to the uranyl molecules [26]. The comparable values of approximately 300 charges/nm2 for the final plateaus are attributed to the total amount of functional groups across the entire film. These results confirm that the technique allows quantification of both surface and film charge densities, independently of uranyl concentration. The amount of

functional groups for the entire film measured by UCB is on the same order of magnitude as the negative charge concentration of commercial polyamide membranes determined by Rutherford backscattering spectroscopy [27]. The functional group density of hand-cast brackish water membranes was measured using 10 mM uranyl acetate. The experimental behavior of the measured density of moieties as a function of contact time was analogous to that observed with seawater polyamide membranes described above. However, the values of surface charge, total charges in the film, and the slope of the profile were higher, as expected for the less cross-linked brackish water films (data not shown) [8]. The density of negative charges at the surface was measured as 35–45 charges/nm2 of planar area (Table 1).

3.5. Toluidine blue O (TBO) The surface charge densities of hand-cast seawater polyamide membranes measured by binding TBO molecules are presented in Fig. 5. Similar to the UCB experimental data plotted in Fig. 4, charge densities are plotted as a function of the square root of contact time of TBO with the membrane surface. Three different concentrations of dye were employed, 0.5, 2, and 10 mM. The dye was bound to the surface at pH 11 and was eluted at pH 2, thus accounting for virtually all the ionizable functional groups. The three plots in Fig. 4 show near constant values of charge density as a function of TBO contact time. TBO is a bulkier molecule than uranyl and its diffusion within the polyamide film is hindered. Therefore, the constant values of the measured charge density are attributed to the charge of the exposed carboxyl groups at the membrane surface. The three TBO concentrations gave similar results in the range of 10–25 charges per nm2 of planar area (Table 1), proving that the technique is independent of agent concentration.

Table 1 Summary of the surface charge density of different water purification membranes as measured using the techniques discussed in this study and other techniques reported in the literature. Values represent the measured or reported charges per unit of projected area (in 1/nm2 ). The values in parentheses represent the calculated average planar surface area per measured charge (in Å2 ). Membrane

Uranyl cation binding

Toluidine blue O 2

Other techniques

2

1/nm (Å ) Hand-cast seawater TFC polyamide (Total) Hand-cast seawater TFC polyamide (pH 7) Hand-cast seawater TFC polyamide (pH 4) Hand-cast brackish water TFC polyamide (Total) Commercial seawater TFC polyamide (SW30-HR) (Total) Commercial asymmetric cellulose acetate (CTA) (Total)

20–35 (3–5)

35–45 (2–3) 45–60 (1.5–2) ∼2 (∼50)

10–25 (4–10) ∼6 (∼15) ∼2 (∼50) 20–30 (3–5) 1–3 (30–100) PVA coating 1.5–3 (30–65)

15 (7) [24]

0.432 M entire layer [27]

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505

Fig. 5. Functional group density as a function of square root of contact time of toluidine blue dye with the hand-cast polyamide seawater membrane surface for: (A) 0.5 mM, (B) 2 mM, and (C) 50 mM of dye. Group density was calculated from absorbance data using the equation from the calibration curve shown in Fig. 2.

The binding of TBO to the membrane charges appeared to be slower than that of uranyl molecules. Consistently lower charge density values were observed for contact times lower than 20 s (data not shown), before reaching the observed constant plateau for TBO contact times larger than 60 s. The obtained surface charge values were also smaller compared to those measured by the UCB technique (Table 1 and Fig. 4). This observation is explained by the inaccessibility of some surface groups by the larger TBO molecules and by steric effects as many molecules of this compound amass at the surface while interacting with the functional groups. By using a 2-mM solution of TBO, the surface charge density of hand-cast brackish water polyamide membranes was measured to be between 20 and 30 charges/nm2 (Table 1). The higher value compared to the hand-cast seawater polyamide membranes is consistent with the results obtained by the UCB technique. Because of the electrostatic nature of the binding, the TBO protocol allows the determination of the degree of charge ionization. Additional experiments were carried out by contacting 2 mM of dye solution to membrane surfaces under different pH conditions. The measured ionized surface charges are shown in Fig. 6. Very few negative charges were measured when the solution in contact with the membrane had a low pH of approximately 3.5. A larger charge density was obtained at pH 6 and 8, which was lower than that obtained when all the charges were virtually ionized at pH 11.

These measurements were consistent with previous studies that suggested the existence of two dissociation constants (pKa ) for carboxylic groups of thin-film polyamide membranes, namely 5.2 and 9.0 [27], indicated in Fig. 6 by vertical lines.

Fig. 6. Functional group density as a function of pH, measured after contact of 2 mM toluidine blue O with the hand-cast polyamide seawater membrane surface for 30 min. Group density was calculated from absorbance data using the calibration curve in Fig. 2. Vertical dash lines indicate expected pKa values for the carboxylic groups of the active polyamide layer, as calculated by Coronell et al. [27]. Group density was calculated from absorbance data using the calibration curve shown in Fig. 2.

Fig. 7. Functional group density as a function of the square root of contact time of uranyl acetate (green circles) and TBO (blue triangles) with commercial membrane surfaces: (A) a thin-film composite polyamide membrane (SW30-HR) and (B) an asymmetric cellulose acetate membrane (CTA). Functional group density was calculated from CPM data using the calibration curves shown in Fig. 1 (UCB) and Fig. 2 (TBO). Data points are plotted on a log scale in the y axis and linear scale in the x axis.

3.6. Surface charge density of commercial membranes The UCB and TBO techniques were also used to quantify the charge density of two commercial membranes, a thin-film composite polyamide membrane (SW30-HR) and an asymmetric cellulose acetate membrane (CTA). The measured surface charge densities for these membranes are presented in Fig. 7.

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Table 2 Comparison of the techniques described in this paper with other techniques for the measurement of surface charge density of water purification membranes. Key advantages and disadvantages of each technique are also indicated. Technique

Quantitative

Advantages

Disadvantages

Uranyl cation binding (UCB)

Yes

Toluidine blue O (TBO)

Yes

No information about degree of ionization Requires training and instrumentation for radioactive materials Not as sensitive as UCB Some non-specific sorption observed

Rutherford backscattering spectroscopy (RBS)

Yes

Contact angle titration

Yes

More sensitive than TBO Information of charge density at the surface and in the entire film Information of charge density at the surface and the degree of ionization No specific instrumentation or skills required Precise and analytical Information on the degree of ionization Information on the density of both positively and negatively charged groups No specific instrumentation or skills required

Streaming potential

No

Information on the degree of ionization

A different behavior of functional group density as a function of contact time with uranyl acetate was observed for the SW30-HR membranes (Fig. 7A) compared to the hand-cast polyamide membranes (Fig. 4). An initial plateau of functional group density was not observed in this case. Instead, an increase in measurement values was detected for very few seconds of contact time. The data stabilized after approximately 4 min of contact and remained stable for at least 30 min, reaching values of 45–60 negative charges/nm2 of planar area (Table 1). These values are assumed to be the density of charges at the surface of the polyamide thin film, while the initial increment is attributed to uranyl diffusion across a low surface charge coating layer. This finding is corroborated by recent studies suggesting the presence of a proprietary oxygen-rich layer coating on the surface of the commercial polyamide [60,65]. TBO is a bulkier molecule than uranyl and its diffusion across this coating layer was significantly delayed (Fig. 7A). For this reason, only 1–3 surface charges/nm2 were measured, even after 30 min of contact time with the membrane surface (Table 1), presumably associated with the oxygen-rich coating. The commercial CTA membranes are prepared through a phase inversion process and the polymer material is the same throughout the membrane. Based on their presumed composition, the membranes should have no charge because the polymer functional groups are converted to esters during the formation process. UCB data suggested the existence of approximately 2 surface functional groups per nm2 of planar area (Table 1 and Fig. 7B), possibly due to incomplete esterification and hydrolysis of the polymeric functional groups, forming alcoholic and carboxylic groups [66–68]. Higher functional group values were observed when the contact time of uranyl acetate with the membrane was significantly increased. However, these measures are ascribed to deep penetration of uranyl molecules within the relatively porous and hydrophilic cellulose acetate dense layer. Cellulose acetate is known to degrade at both high and low pH, rendering the TBO procedure difficult to perform at large contact times. Hence, the TBO technique was performed on the CTA membranes by minimizing the contact time of dye solution with the membranes. Degradation and significant coloration of the polymer was observed for contact times greater than 30 min. For appropriate contact times (<15 min), a constant value of 1.5–3 charges/nm2 was obtained (Table 1 and Fig. 7B), consistent with the UCB method. We measured an average zeta potential of −14.8 mV for these membranes using a streaming potential analyzer [19] at 1 mM KCl and pH 9. This relatively large negative value indicates that

Requires highly specialized and expensive instrumentation Information about entire film, not of the charge density at the surface Indirect quantification Underestimates charges Surface-sensitive Measures zeta potential, not charge density Influenced by both negative and positive charges Negative zeta potential values observed for nearly neutral surfaces

streaming potential measurements tend to overestimate the fixed charge characteristics of nearly neutrally charged surfaces [69,70].

3.7. Advantages, limitations, and comparison to other techniques The advantages and disadvantages of the UCB and TBO methods are summarized in Table 2 along with a comparison to other techniques. UCB is a sensitive technique, which can provide information on both the charge density at the surface and in the entire thin film. However, the possibility to gain insight on the distribution of charge throughout the film should be further investigated. Our experiments further suggest that the UCB technique is reliable and highly reproducible. Because of the nature of the interaction between uranyl acetate and probed functional groups, UCB allows measurement of the total density of ionizable charges and does not provide information about the speciation or degree of ionization. One limitation of the technique is that specific instrumentation to measure radioactivity and personnel training to handle uranyl acetate are needed. Unlike the UCB technique, TBO can provide information on the degree of ionization of functional groups. In our case, the TBO method allowed for the quantification of the true surface charge density, not the entire film, because the membrane structure prevented the bulkier dye molecules from diffusing inside the film. The use and detection of the cationic TBO require basic instrumentation and no highly specialized user skills. This feature makes the TBO technique the most straightforward method for surface charge quantification reported to date. However, the results obtained by this method were not as highly reproducible and sensitive as the UCB technique, and some non-specific adsorption of dye to the membrane surface could be observed. The UCB and TBO techniques rely on a number of assumptions whose validity and implications are discussed earlier in the paper. Awareness of these premises is important for successful deployment and for further improvement of the two methods. The reliability and the sensitivity of both techniques can be significantly maximized by increasing the membrane area in contact with the binding compounds. The reproducibility of the quantification will also be enhanced by parallel measurement of various membrane samples to account for inherent membrane surface heterogeneity. Both the UCB and TBO techniques require particular attention to contacting the binding compound solely with the membrane active layer side. Contact and adsorption of the probe molecule to the

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membrane support layers would result in overestimation of the charge density. [16]

4. Concluding remarks Two methods to measure the surface charge density of polymeric membranes were examined and evaluated. Uranyl cation binding (UCB) exploits the fast complexation of uranyl molecules to the surface moieties and scintillation counting to enumerate the complexed uranyl molecules. Toluidine blue O (TBO) is a cationic dye that interacts electrostatically with the ionized, negatively charged functional groups and can be quantified by visible light absorption. Both techniques confirmed their potential to directly quantify the surface charge density of membranes. The investigated techniques can complement other established methods, such as Rutherford backscattering spectroscopy (RBS) and contact angle titration, especially in cases where rapid charge quantification is needed. UCB and TBO were evaluated for the quantification of the charge density at the surface of polymeric membranes and of self-assembled monolayers of alkanethiols on gold. These methods show potential for use on other surfaces and materials as well. Acknowledgments This publication is based on work supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST); by the NWRI-AMTA Fellowship for membrane technology; and by the WaterCAMPWS, a Science and Technology Center of Advanced Materials for the Purification of Water with Systems under the National Science Foundation Grant CTS-0120978. We thank Eric Hoek at UCLA and Baoxia Mi at the University of Maryland for useful guidance on protocols for interfacial polymerization. We also thank Roni Kasher at Ben-Gurion University, Israel, for discussion and advice on the TBO technique.

[17]

[18]

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