Colloidal adhesion to hydrophilic membrane surfaces

Transcription

1 Journal of Membrane Science 241 (2004) Colloidal adhesion to hydrophilic membrane surfaces Jonathan A. Brant, Amy E. Childress Department of Civil Engineering, University of Nevada, Reno, NV 89523, USA Received 22 October 2003; received in revised form 21 April 2004; accepted 26 April 2004 Abstract Colloidal adhesion to membrane surfaces is an important parameter in determining membrane fouling propensity and in optimizing membrane cleaning strategies. It has previously been demonstrated that acid base interactions can significantly affect colloid membrane interaction as a colloid approaches a membrane surface, however, the effect of acid base interactions on adhesion has received less attention. In this investigation, the approach and adhesion of a silica and polystyrene colloid was measured on three commercially available hydrophilic water treatment membranes using an atomic force microscope and the colloid probe technique. It was found that the hydrophobic polystyrene colloid adhered more weakly to each membrane compared to the hydrophilic silica colloid. These results could not be resolved through classic DLVO analysis alone and were in direct contrast to the expected interaction based on the strong hydrophobic character of the polystyrene colloid. However, the results could be explained by considering the magnitude of the surface s electron-acceptor (γ + ) and electron-donor (γ ) components. It is hypothesized that through hydrogen bonding with surface γ + and γ groups, structured water layers exist to varying extents at the surfaces of the silica colloid and the hydrophilic membranes, and that their removal results in the formation of strong adhesive bonds between reciprocal γ + and γ groups. Furthermore, even when surface roughness is substantial, γ + and γ groups appear to play some role in determining the magnitude of the measured adhesion. The lack of such groups on the polystyrene colloid, and thus the lack of hydrogen bonding capacity, was responsible for its weaker adhesion with the membranes Elsevier B.V. All rights reserved. Keywords: Fouling; Reverse osmosis; Thermodynamics; Adhesion; AFM 1. Introduction Adhesion of colloidal particles to membrane surfaces is a significant problem that must be overcome in order to reduce the effects of membrane fouling. Colloid deposition and attachment (adhesion) result in the formation of a cake layer that may not be easily removed during hydraulic cleaning. Removal of colloidal cake layers may be facilitated by hydrodynamic shear and changes in aqueous solution chemistry [1 4]. Recent models describe the detachment of colloidal particles from collector surfaces as being dependent on both the hydrodynamic and thermodynamic properties of the system [2 5]. An accurate assessment of both hydrodynamic and thermodynamic interactions is necessary to minimize colloid attachment to membrane surfaces and to facilitate colloid detachment from membrane surfaces. In order to reduce membrane fouling, membrane surfaces must Corresponding author. Tel.: ; fax: address: amyec@unr.edu (A.E. Childress). provide an unfavorable environment for adhesion to occur, thus minimizing the critical hydrodynamic force [6], or the minimum force required for particle detachment from the surface [1]. Furthermore, a comprehensive understanding of the thermodynamic mechanisms necessary for particle detachment from a surface allows for the optimization of membrane cleaning processes by providing an avenue in which the critical hydrodynamic force may be reduced through adjustment of feed water chemistry (e.g., ph, ionic strength, and surfactant addition). It is widely accepted that adhesion is controlled by short-range interfacial interactions [7 9]. The summation of these interfacial interactions is the thermodynamic work of adhesion (W 132 ), which quantifies the energy available for adhesion based on the energetic properties of two interacting surfaces [9 11]. Although many researchers have found qualitative agreement between W 132 values and direct adhesion force measurements, calculated values are in many cases one to two orders of magnitude less than measured values [9,12]. This is undoubtedly due to the complex nature of the mechanisms affecting adhesion and /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.memsci

2 236 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) the difficulties associated with accurately characterizing the interfacial interaction energies. Traditionally, membrane colloid interactions have been characterized within the framework of the classical Derjaguin Landau Verwey Overbeek (DLVO) theory, which describes the total interfacial interaction energy as the summation of electrostatic (EL) and van der Waals (LW) interactions. Recently, it was demonstrated by several investigators [13 15] that polar or acid base (AB) interactions can significantly alter the interaction between a membrane surface and an approaching colloidal particle from that expected from DLVO theory. This has led to the adoption of an extended DLVO (XDLVO) approach where interfacial interactions are described in terms of EL, LW, and AB interactions [10]. Purportedly, the XDLVO approach provides a more comprehensive accounting of the interactions present in aqueous media, and in turn, may be able to resolve common discrepancies between experiment and theory [15]. This is particularly the case when short-range interactions (e.g., hydration forces) are prominent. For example, when repulsive hydration or structuring forces are considered in the evaluation of hydrophilic membrane surfaces, predicted interactions are more representative of the measured interactions [15]. Although the effects of AB forces have been well-documented for interfacial interactions on approach [15 17], their role in colloid attachment (adhesion) to membrane surfaces remains poorly understood. AB interactions are most accurately defined as interactions resulting from the sharing of electrons (or protons) between surface functional groups and other polar molecules (e.g., water). The functional groups may serve as either electron acceptors (γ + ) or electron donors (γ ) [10]. The presence or absence of γ + and γ groups on membrane surfaces, in relation to the magnitude of the LW component, determines the surface s affinity for water (i.e., its hydrophobicity/hydrophilicity) [10]. A membrane characterized by a large AB surface free energy, in relation to its LW component, tends to be hydrophilic, while a membrane characterized by a small AB component tends to be hydrophobic. At hydrophilic surfaces, water structuring is induced as water molecules form hydrogen bonds with surface γ + and γ groups. Thus, when two hydrophilic surfaces are brought into contact, a hydrophilic repulsion results and energy must be put into the system to remove the adsorbed water layers. At hydrophobic surfaces, water structuring also occurs. However, this structuring is due to the inability of water molecules to form hydrogen bonds with hydrophobic surfaces [10,18]. The structuring at a hydrophobic surface is thus weaker and less profound than that at a hydrophilic surface and results in a hydrophobic attraction through the formation of a density gradient. Although hydrophobic surfaces are known to foul at higher rates than hydrophilic surfaces [16,19], fouling of hydrophilic membranes also occurs through colloid attachment and subsequent cake layer formation [16,20]. Various theories, such as roughness effects, have been put forth to explain the fouling of hydrophilic membranes, however, none have been able to address the exact mechanism of adhesion. For this reason, an in-depth analysis of the role played by γ + and γ groups in surface adhesion is warranted. Previous investigations on the role of AB interactions in adhesion have been conducted for a variety of systems [7,9,12,14,21 23]. Durán et al. [7] found that the adhesion of colloidal calcium carbonate to glass was most correctly characterized by including γ + and γ interaction components in the DLVO model. This conclusion was reiterated in work performed by Gotoh et al. [24] where the adhesion between polyethylene and nylon particles to modified silica plates was studied. Mora and Cassinelli [22] studied the adhesion of fibroblast cells (L-929), chosen for their strong adhesion characteristics, to polyanionic alginic acid modified polyethyleneimine surfaces. It was proposed that hydration forces, a result of AB interactions, were responsible for the inability of the L-929 cells to strongly adhere to the polyethyleneimine surfaces. However, the authors also found that this behavior could not simply be related to surface wettability as determined by water contact angle alone, but that the specific nature of AB interactions must be considered. In an investigation of adhesion between thin-film polymers, Funasaka et al. [23] concluded that the strength of adhesion, quantified by peel strength, was proportional to the magnitude of the polar (i.e., γ + and γ ) components of the polymer surface energy. As the polar components of the modified polymer surface increased, adhesion also increased. Most recently, Nalaskowski et al. [12] found that for hydrophilic and slightly hydrophobized silica particles, a thin film of structured water molecules orients itself at the silica surface through interactions with silanol (γ ) groups and that this significantly affects its adhesion with hydrocarbons. Adhesion was found to increase when the water film was removed either through thermal treatment of the silica particle or physical removal by the interacting surface. These results highlight the two key components leading to the formation of strong polar adhesive bonds: the presence of surface γ + and γ groups and the ability of the interacting surface to remove the adsorbed water layers in order to expose such groups for bond formation to occur with reciprocal groups [10]. In a previous investigation [16], the role of AB interactions on the fouling of RO membranes by silica and polystyrene colloids was studied. It was found that hydrophobic polystyrene colloids had an attractive interfacial free energy at contact, while that for the hydrophilic silica colloids was repulsive. However, these calculations were based on the existence of interfacial water layers and thus, did not give a complete picture of the adhesion likely to occur. The presence of water layers results in a reduced adhesion for the hydrophilic silica colloid; an increase in entropy would be required to remove the adsorbed water layers from the interface and allow actual surface contact to occur. If the adsorbed water layers are removed through the introduction of an outside force (e.g., permeation drag

3 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) forces), other adhesion mechanisms may become significant. One aim of the current investigation is to mechanistically assess the role of γ + and γ interactions on the adhesion of colloidal particles to membrane surfaces. The overall goal of this investigation is to comprehensively evaluate the role of AB interactions on the approach of colloids to hydrophilic membrane surfaces and the role of γ + and γ components at contact. This requires that both the membrane and colloid surfaces be thoroughly characterized in terms of their relevant surface properties (i.e., roughness, EL, LW, and γ + and γ (AB) components) using AFM, electrokinetic, and goniometric analyses. These surface properties are used to theoretically evaluate and model the approach interaction and adhesion between the membranes and colloids. AFM force curves are then used to measure the interaction on approach and the adhesion at contact. 2. Materials and methods 2.1. Representative membranes This investigation studied three commercially available reverse osmosis (RO) and nanofiltration (NF) membranes. The RO membrane selected for this investigation was the Desal CD (Osmonics, Minnetonka, MN); the two NF membranes were the Osmonics HL and the Filmtec NF70 (Filmtec, Minneapolis, MN). All membranes were supplied as dry sheets and stored in ultrapure water at 5 C. Rendered 3D topographic images of each of the three membranes investigated are shown in Fig. 1. Each image represents a 100 m 2 (10 m 10 m) area on the membrane surface. Five 100 m 2 scans were collected on each membrane surface so that the reported roughness statistics and images are more representative of the surface as a whole. Changes in height across the area are denoted using a color scale with increases in height represented by lighter colors. Key roughness statistics calculated for each membrane are summarized in Table 1. Average roughness (R A ) is defined as the average deviation of surface asperities or features from the mean plane; root mean square (RMS) roughness (R q ) is the standard deviation of surface features from the mean plane. The surface area difference describes the difference in surface area due to roughness over a perfectly flat plane with the same projected area. Table 1 AFM measured surface roughness statistics for the membrane surfaces investigated (I = 0.01 M NaCl; ph = 5.65; T = 20 C) Membrane Average roughness R A (nm) RMS roughness (nm) R q CD HL NF Surface area difference (%) Fig. 1. Representative AFM topographic images of the (a) CD; (b) HL; and (c) NF70 membranes acquired in water (I = 0.01 M NaCl; ph = 5.6; T = 20 C). Five separate 100 m 2 scans were acquired on each membrane to ensure that the reported image was representative of the entire surface. The CD membrane surface (Fig. 1a) is characterized by varying degrees of broad ( 60 nm wide) dimple features that are 20 nm deep on average. The distribution of these surface formations is random, but they exist to some degree in each area that was imaged on the CD membrane surface. Otherwise, the surface of the CD membrane is very smooth with an R q = 6.84 nm and a surface area difference of 0.1% (Table 1). The HL membrane surface (Fig. 1b) is also relatively smooth (R q = nm) but is characterized

4 238 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) by particulate-like features (max height 80 nm) with a low periodicity and an apparent random distribution. The presence of these features results in a minimal surface area difference (0.7%) for the HL membrane. Finally, the NF70 membrane (Fig. 1c) is the roughest of the three membranes investigated with an R q = nm. The substantial roughness of the NF70 membrane surface results in a large surface area difference (30.7%) compared to the other membranes investigated Representative colloids The two colloids selected for this investigation were silica and polystyrene microspheres. Silica colloids (SS05N, Bangs Laboratories Inc., Fishers, IN) were supplied dispersed in deionized water; the dispersion was stored at room temperature. According to the manufacturer, the silica colloids have an average diameter of 5 m and a density of 1.96 g/cm 3. The polystyrene colloids (PP , Spherotech, Libertyville, IL) were also supplied dispersed in deionized water; the dispersion was stored in a refrigerator at 10 C. According to the manufacturer, the polystyrene colloids have an average diameter of 5.26 m and a density of 1.05 g/cm Surface analysis Contact angle measurements Membrane and colloid surface energy properties were determined using the method outlined by van Oss [10]. Contact angles on the surfaces were measured for three well-characterized probe liquids: doubly deionized water (DDW), formamide, and bromonaphalene. For the membranes, contact angles were measured using the captive bubble technique with a Rame-Hart (Mountain Lakes, NJ) goniometer. Triplicate contact angles were measured on no less than five membrane coupons for each sample, for a minimum of fifteen contact angles per membrane. Colloid contact angle was measured using the thin-layer wicking technique [25]. The thin-layer wicking technique is based on the Washburn equation, which describes capillary rise through a packed colloidal bed: H 2 = tr effγ l cosθ (1) 2η where t is the time required for the solvent to rise a distance H through the packed colloidal bed; R eff is the effective interstitial pore radius of the colloid bed; γ l is the total surface tension of the solvent; and η is the viscosity of the solvent. An in-depth description of the procedures used to characterize each colloid may be found elsewhere [15]. An exception to the previously published procedure for determining contact angle for the silica colloid was that the silica colloid was allowed to equilibrate with water vapor in order to hydrate it prior to the wicking measurements. This was done because it has been demonstrated that the contact angles for dry and hydrated silica surfaces are substantially different [12]. Surface energies were calculated from the measured contact angles using the Young Dupré equation [10]: γ l (1 + cos θ) = 2( γ LW s γ LW l + γ + s γ l + γ s γ + l ) (2) where θ is the measured contact angle; γ LW is the van der Waals free energy component; γ + is the electron-acceptor component; γ is the electron-donor component; and subscripts s and l designate the solid and liquid phases, respectively Electrokinetic measurements The electrostatic (EL) contribution to the interfacial interaction energies was calculated based on measured zeta potentials of the membrane and colloid surfaces. Membrane zeta potential was determined using a streaming potential analyzer (BI-EKA, Brookhaven Instruments Corp., Holtsville, NY). The streaming potential was measured with a background electrolyte of 10 mm NaCl over a ph range of 3 to 9 and at 25 C. Zeta potential was calculated from the measured streaming potential using the Helmholtz Smoluchowski equation with the Fairbrother and Mastin substitution [26]. Colloid zeta potential was determined using microelectrophoresis (Laser Zee Meter, Model 500, Pen Kem, Bedford Hills, NY). The electrophoretic mobility was measured with a background electrolyte of 10 mm NaCl over a ph range of 3 11 and at 25 C. Zeta potential was calculated from the measured electrophoretic mobility using the Smoluchowski equation. The EL interaction energy was calculated from the surface zeta potentials according to the method outlined by Hogg et al. [27] assuming constant surface potential JKR theory of adhesion The adhesion force between each membrane colloid pair was determined using the Johnson Kendal Roberts (JKR) theory for adhesion mechanics [28]. The JKR theory is appropriate for describing surface adhesion when small elastic deformations occur on contact due to an externally applied load or attractive interfacial forces. According to the JKR theory, the pull-off (adhesion) force required to separate a spherical tip from a planar surface can be determined from the following relationship: F AD = 3 2 πrw 132 (3) where F AD is the pull-off force; R is the radius of the spherical tip; W 132 is the work of adhesion between the two surfaces; and the subscripts 1, 2, and 3 represent the membrane, colloid, and liquid phases, respectively. The work of adhesion for each membrane colloid pair was calculated using the Dupre equation [10]: W 132 = γ 13 + γ 23 γ 12 (4)

5 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) where γ 13 and γ 23 are the interfacial free energy components of the membrane and colloid with water, respectively, and γ 12 is the interfacial interaction energy between the membrane and colloid [10,29] AFM surface characterization AFM measurements were carried out using a Nanoscope IIIa multimode atomic force microscope (Digital Instruments, Santa Barbara, CA). Force versus distance curves were measured using colloid probes. Colloid probes were prepared by attaching a single colloid to the end of a tipless silicon nitride cantilever following the method developed by Ducker et al. [30]. The cantilevers had a spring constant of 0.12 N/m, as reported by the manufacturer (Digital Instruments, Santa Barbara, CA). This value was used for converting cantilever deflection to a force (F) value using Hooke s law (F = kd, where k is cantilever spring constant and d is cantilever deflection) [31]. Force curves were collected at no fewer than fifteen randomly selected locations in a 100 m 2 area on each membrane surface. Five scans (i.e., approach and retract cycles) were collected at each location and averaged. Force curves were converted to force as a function of separation distance (h) plots by identifying the contact point and subtracting that value from all scanner position values [31]. Contact was identified as the onset of compliance between piezo scanner movement and cantilever deflection [15]. Measurements were conducted in a fluid cell with a background electrolyte concentration of 10 mm NaCl and at ph 5.6. All measurements were conducted at room temperature (20 C). The methodology for measuring adhesive interactions using AFM force measurements has been documented in detail elsewhere [31] and will only be briefly covered here. In AFM force measurements, the colloid probe is brought towards the membrane surface where the interfacial interaction is measured based on the deflection of the cantilever. Upon contact, the probe is loaded to a preset force value, and then withdrawn. Once the elastic energy in the cantilever exceeds the adhesive force, separation occurs and the probe returns to a position of zero deflection. The adhesion force is then calculated from the maximum deflection in the retract portion of the force curve. Table 3 Theoretical force of adhesion calculated using the JKR theory for contact mechanics (I = 0.01 M NaCl; ph = 5.6; T = 20 C) JKR adhesion (mn/m) Silica Polystyrene CD r a HL NF70 r a a A repulsive interaction was predicted at contact for the CD and NF70 membranes with the silica colloid. 3. Results and discussion Zeta potential and surface energy data calculated for each of the membrane surfaces and colloid probes is summarized in Table 2. The interfacial free energy of interaction with water ( G 131 ) is also included in Table 2 for each surface. The sign and magnitude of G 131 is a quantitative measure of surface wettability [10]. A positive value indicates a hydrophilic surface while a negative value indicates a hydrophobic surface. All of the membrane and colloid surfaces are negatively charged at the ph and ionic strength investigated. The membranes are weakly charged while the colloidal particles are strongly charged. All three membranes are hydrophilic based on positive values of G 131. The CD membrane is strongly hydrophilic, the HL membrane is weakly hydrophilic, and the NF70 membrane was in between. Based on the G 131 values for each colloid, the silica colloid is strongly hydrophilic and the polystyrene colloid is strongly hydrophobic Theoretical adhesion values Table 3 shows the calculated adhesion, normalized to colloid radius, for the membrane colloid pairs investigated using the JKR theory. JKR calculations for the silica colloid predicted adhesion to occur only on the HL membrane, with no adhesion predicted on either the CD or NF70 membrane surfaces. Calculations for the polystyrene colloid predicted the greatest adhesion to occur on the HL membrane, followed by the CD and NF70 membranes, respectively. The strong hydrophobicity of the polystyrene colloid (Table 2) resulted in a two order of magnitude difference between Table 2 Measured surface energy properties for the membranes and colloids investigated Surface Zeta potential (mv) γ LW (mj/m 2 ) γ + (mj/m 2 ) γ (mj/m 2 ) G 131 (mj/m 2 ) CD HL NF Silica Polystyrene a Zeta potential is reported at I = 0.01 M NaCl, ph = 5.6, and T = 20 C. Surface energy properties are reported at ph = 5.6, and T = 20 C. a γ LW, γ +, and γ data for polystyrene was taken from van Oss [10].

6 240 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) Table 4 XDLVO adhesion components calculated for each membrane colloid pair using the JKR theory for contact mechanics (I = 0.01 M NaCl; ph = 5.6; T = 20 C) LW (mn/m) EL (mn/m) AB (mn/m) Total (mn/m) Silica CD HL NF Polystyrene CD HL NF the JKR calculated adhesion values for the polystyrene colloid and the hydrophilic silica colloid. To fully explain the calculated adhesion values it is necessary to examine the individual components of the total adhesion force. The contribution of each XDLVO interaction component to the total JKR calculated adhesion force for each membrane colloid pair is reported in Table 4. Based on the magnitude and sign for each of the interaction components (LW, EL, and AB), the contact interaction is attractive for the HL silica pair due to its smaller AB component relative to those for the CD and NF70 membranes. This condition exists despite the weaker LW attraction component for the HL silica pair. For the polystyrene colloid, interactions at contact were dominated by the strongly attractive AB component representing hydrophobic attraction between the colloid and each membrane. Therefore, from a theoretical perspective, AB interactions play a significant role in determining the magnitude of adhesion between membrane and colloid surfaces Adhesion on smooth membrane surfaces The mean and standard deviation (S.D.) of the measured adhesion force for each membrane colloid pair is shown in Table 5. The goodness of fit to a normal distribution is quantified for each pair using the Shapiro Wilk test, which has been proven to accurately describe the normality of small Table 5 AFM measured adhesion normalized to the colloid radius (I = 0.01 M NaCl; ph = 5.65; n = 15) Mean (mn/m) S.D. (mn/m) W W c Silica CD HL NF Polystyrene CD HL NF (n < 50) data sets [32]. Briefly, the Shapiro Wilk test tests the hypothesis that the data values are random samples from a normal distribution against an unspecified alternative distribution. The Shapiro Wilk statistic (W) is calculated from the data set and is compared to a five percent critical value (W c ) reported for the respective sample size. If W is greater than W c then the data may be said to follow a normal distribution, the degree of which is determined from the difference of the two values. For the silica colloid, the strongest adhesion was measured on the HL membrane, followed by that measured on both the NF70 and CD membranes (Table 5). The largest adhesion for the polystyrene colloid was also measured on the HL membrane. With the exception of the data for the HL membrane and the polystyrene colloid, all membrane colloid adhesion measurements were characterized by a normal distribution (W > W c ). For the CD polystyrene pair, the most frequently measured adhesion value was zero, or no adhesion, so the distribution of results could not be characterized as normal or nonnormal using the Shapiro Wilk test. The particularly high standard deviation for the CD polystyrene pair resulted from the fact that the majority (77%) of the measured adhesion values was zero and the remaining 23% had adhesion values ranging from 2 to 12 mn/m. In any case, the high standard deviation of the measured adhesion for all of the membrane colloid pairs is evidence of the effect of physical and chemical heterogeneities on surface adhesion [31,33,34] Comparison of theoretical and measured adhesion values One to two orders of magnitude difference existed between the calculated (Table 3) and measured (Table 5) adhesion values for the membrane colloid pairs. A substantial difference between calculated and measured adhesion values has been observed in previous investigations employing both the DLVO theory [35 37] and the XDLVO approach [14] for calculating adhesion. Recalling that the JKR theory was developed for ideally smooth and chemically homogeneous surfaces, differences in predicted and measured values may be attributed to physical and chemical heterogeneities on the membrane and colloid surfaces [31,33,34]. These heterogeneities create a complex interaction at contact that is not considered by the characterization techniques and theories employed in this study. The general effect of surface roughness is addressed when results for the NF70 membrane are discussed. A further consideration is whether the role played by the interfacial interaction components, principally the γ + and γ components, are properly assessed at contact. For the hydrophilic silica colloid, the AB interaction at contact is repulsive for each of the hydrophilic membranes (Table 4). This repulsion results from the strong affinity of hydrophilic surfaces for water and the seemingly monopolar (γ ) nature of each of the surfaces studied (Table 2). In fact, only

7 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) the HL membrane had a sizable γ + component capable of bonding with the reciprocal γ component of the silica colloid upon removal of any adsorbed water layers. This resulted in the significantly lower AB repulsion predicted for the HL silica pair (Table 4) and may, in fact, be responsible for the high adhesion measured in this case (Table 5). For the hydrophobic polystyrene colloid, the AB interaction is predicted to be strongly attractive at contact (Table 4). Here, the predicted hydrophobic attraction results from the net repulsion between the hydrophobic colloid and the bulk water through the formation of density gradients [10]. The largest adhesion for the polystyrene colloid was predicted (Table 3) and measured (Table 5) on the HL membrane, where the attractive AB component (Table 4). The substantially higher magnitude of the AB component for the HL polystyrene pair, relative to the other membranes, is likely due to the greater magnitude of the γ + component for the HL membrane (Table 2), which is absent for the CD membrane and significantly lower for the NF70 membrane. Therefore, it appears that on contact it is the ability of the colloid to form bonds with reciprocal γ + and γ groups on the membrane surface that dictates the magnitude of adhesion. While the colloid s interaction with the water medium is significant on approach, it may not be important at contact when the water has been removed from the interface. Based on this observation, it is necessary to reevaluate methods for predicting interactions; if intimate contact is established through the removal of adsorbed water layers from the interface, the existence of γ + and γ groups must be considered Interactions on approach Fig. 2 is a representative approach curve for the silica colloid to the HL membrane surface. Also included in Fig. 2 is the calculated XDLVO force curve for comparison with the measured interaction on approach. As the silica colloid approaches the HL surface it encounters a weak repulsion (F = 0.57 mn/m) at h = 3 nm before the interaction becomes attractive at approximately h = 1 nm prior to contact. The weak repulsion was measured on approach at all but two locations on the HL surface and had an average strength of 0.71 ± 0.55 mn/m (n = 13). The short-range attraction was detected on all but 3 locations. The HL surface is characterized as weakly hydrophilic ( G 131 = 1.61 mj/m 2 ); therefore, it is likely to induce structuring of water molecules through the formation of hydrogen bonds with polar surface groups [10,29]. And theoretically, other water molecules from the bulk solution would form hydrogen bonds with the adsorbed surface waters. The result would be a series of structured water layers that propagates outward to form an adsorbed water film. On approach, a force within the range of nn will displace the outer layers of the water structure [29,31]. As the loading force used in this investigation was 20 nn, the outer water layers are likely to have been displaced. Removal of the adsorbed surface waters, however, is dependent on the bond strength Fig. 2. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the silica colloid to the HL membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). The AFM measured force curve represents a single curve from a population of 15 measured in a 100 m 2 sample area. between the water and the membrane surface. As the HL membrane is only weakly hydrophilic, its bond strength with water is expected to be weak. The theoretical XDLVO prediction in Fig. 2 shows the short-range (h < 3 nm) interaction resulting from repulsive hydration forces. The XDLVO prediction does not consider removal of adsorbed surface waters, and therefore, does not show the attraction that is experienced prior to intimate surface contact (i.e., contact between the two solid phases at the primary energy minimum) occurring. Referring back to Table 5, the strong overall adhesion observed for the HL silica pair may be unexpected considering that both surfaces are hydrophilic [12,38]. Adhesion would be expected to be minimal due to hydrophilic repulsion resulting from the adsorption of water molecules at the hydrophilic interfaces. Specifically, for silica surfaces, such water structuring has previously been characterized as a gel layer that contributes to the high stability of silica particles even under high electrolyte concentrations [29]. Therefore, adhesion can only occur once the water layers are removed. Characteristically, hydrophilic membranes have an abundance of γ + and γ groups that produce a high interfacial interaction energy with water [10]. On the other hand, hydrophobic membranes typically lack such groups, resulting in a low (negative) interfacial energy for water. Here, the HL membrane is characterized by the presence of amide, amino, and carboxylate groups, while the silica surface is characterized by silanol groups. The presence of these groups results in the γ + and γ values determined for each surface (Table 2). When the bound water molecules are removed from the interface between the two solids, the γ + and γ groups on the membrane and colloid surface become

8 242 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) Fig. 3. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the polystyrene colloid to the HL membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). The AFM measured force curve represents a single curve from a population of 15 measured in a 100 m 2 sample area. exposed allowing them to hydrogen bond with reciprocal γ + and γ groups on the other surface. Therefore, for the HL silica pair, removal of adsorbed water from the HL and silica surfaces results in strong adhesion through γ + and γ bonds. It is possible that removal of the adsorbed water film is responsible for the short-range attraction following the repulsive barrier see in Fig. 2. However, to definitively make this point, further study is required. Nevertheless, this provides interesting evidence of the importance of adsorbed water layers as the magnitude of the measured adhesion is greater in those cases where a short-range attraction, as opposed to completely repulsive interaction, was measured. The measured and calculated force curves on approach for the hydrophobic polystyrene colloid to the HL membrane surface are shown in Fig. 3. In contrast to the weak long-range repulsion experienced by the silica colloid (Fig. 2), the polystyrene colloid experienced no long-range repulsion prior to short-range attraction and contact with the HL surface. The XDLVO approach also predicted an attractive interaction on approach. The lack of long-range repulsion results from the hydrophobicity of the polystyrene colloid. In contrast to the silica colloid, the polystyrene colloid does not have adsorbed water layers that must be removed in order for intimate surface contact to occur. The result is an attractive interaction on approach for the polystyrene colloid (Fig. 3) compared to the repulsion encountered by the silica colloid (Fig. 2). This observation is also supported by Table 5, which shows that the mean adhesion for the polystyrene colloid is considerably weaker than the mean adhesion for the silica colloid to the HL surface. The substantial difference in adhesion values for the HL polystyrene pair versus the HL silica pair may be attributed to the differences in the magnitude of the γ + and γ components for each colloid. Whereas the silica surface possesses a sizable γ component in the form of silanol functional groups, the presence of such groups on the polystyrene surface was negligible (Table 2). Therefore, few groups exist on the polystyrene surface to which γ + and γ groups on the HL surface may form strong adhesive bonds through electron acceptor/donor interactions. This resulted in the weak adhesion measured for the polystyrene colloid despite its large LW component and hydrophobic nature. Thus, for the hydrophilic HL membrane, colloid adhesion appears to be dependent on the presence or absence of reciprocal γ + and γ groups on the colloid surface. This demonstrates the importance of the surface characteristics of the potential foulant (i.e., the colloid), when assessing particle adhesion to membrane surfaces [16]. Interestingly, these results cannot be explained solely within the framework of classic DLVO theory. If LW attraction is considered to be the controlling factor, then adhesion should have been greatest for the polystyrene colloid, not the silica colloid, as the polystyrene colloid possessed a larger LW surface energy component (Table 2) that resulted in a stronger LW adhesion component (Table 4). Also, surface charge cannot account for the substantial differences in the adhesion values, as both colloids possessed similar zeta potentials (Table 2) and had minimally different EL adhesion components relative to the substantially different total adhesion (Table 4). This agrees with results presented by Freitas and Sharma [14] and Skvarla [11] who found that although EL interactions play a significant role in the approach of a particle to a surface, their role in determining the strength of adhesion between the two surfaces is minimal. Therefore, non-dlvo interactions, and specifically AB interactions, must be considered in order to describe the adhesion results measured here. The measured and calculated force curves on approach for the silica colloid to the CD membrane are shown in Fig. 4. Unlike with the HL membrane, the silica colloid experienced only a strong short-ranged repulsion prior to contacting the more hydrophilic CD surface. The XDLVO model also predicted a repulsive interaction. In the approach interaction shown in Fig. 4, the silica colloid experienced a repulsion of 0.5 mn/m before contact occurs. Over the entire 100 m 2 sample area, the average strength of this repulsion was 4.55 ± 1.45 mn/m (n = 15) and was measured at each location on the CD membrane surface where force curves were collected. The presence of a short-range repulsion in the CD silica approach curves likely indicates that not all water layers were removed from the interface, and therefore, intimate surface contact was prevented from occurring. This is reasonable because the CD membrane had a greater affinity for water than the HL membrane. Furthermore, the functional groups present on the CD surface (hydroxyl and acetate groups) may produce stronger hydrogen bonds with adsorbed water molecules than those present on

9 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) the HL surface (amide, amino, and carboxylate groups). For strongly hydrophilic surfaces, a monolayer of water molecules and/or hydrated ions have been found to remain bound to the surface if enough energy is not imparted to facilitate their removal, thus preventing intimate surface contact from occurring [14,39 42]. When confined between two hydrophilic surfaces, this monolayer acts like a solid structure allowing for constant compliance (i.e., apparent surface contact) to be measured in the AFM force curve. Conversely, for the more weakly hydrophilic HL membrane, adsorbed water layers were likely to be completely removed as a result of the weaker bond between the HL surface and adsorbed water molecules and as evidenced by the lack of a short-range repulsion. In a similar investigation using AFM force measurements, Senden and Drummond [40] measured a weak adhesion between two hydrated surfaces where presumably bound water molecules were not displaced. In another study, Vakarelski et al. [42] found that the adhesion between silica and mica surfaces increased with contact time, where contact time was defined as the period between the contact of the surfaces on approach and their separation on retraction. It was concluded that disruption, or lack thereof, of adsorbed water molecules and hydrated counter ions could be the only explanation for the dependence of adhesion strength on contact time when a constant loading force (i.e., trigger value) was applied. Even when the loading force was increased, the measured adhesion only increased with an increase in contact time. For strongly hydrophilic surfaces the contact time for intimate surface contact to occur was found to range upwards of s. This may explain the relative independence of the measured adhesion on the loading force used in this investigation. It is conceivable that the contact time (<1 2 s) and loading force (approximately 20 nn) could easily have been insufficient to remove the adsorbed water and counter ion layers. With increased contact time, removal of the adsorbed water molecules and subsequent exposure of the γ + and γ groups would likely increase the adhesion between the CD and silica surfaces. Previously, Grabbe and Horn [38] measured a similar short-range repulsion, between two hydrophilic silica sheets using a surface force apparatus, which is similar in principle to the AFM. They concluded that this repulsion was due to solvation forces occurring because of the perturbation of structured water layers present at the two interacting surfaces. Undoubtedly, the short-range repulsion measured between the hydrophilic CD and silica surfaces results from a similar mechanism as both surfaces promote hydrophilic Fig. 4. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the silica colloid to the CD membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). The AFM measured force curve represents a single curve from a population of 15 measured in a 100 m 2 sample area. Fig. 5. Theoretical force as a function of separation distance calculated using the XDLVO theory for the silica and polystyrene colloids on approach to the (a) HL and (b) CD membrane surfaces (I = 0.01 M NaCl; ph = 5.6; T = 20 C).

10 244 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) water structuring [15]. The inability of two interacting surfaces to come into intimate surface contact can have a substantial impact on the magnitude of the measured adhesion between the two surfaces [42]. For instance, Vakarelski et al. [42] found that the adhesion between two hydrophilic surfaces (mica and silica) decreased by approximately 73% when separated by one to two water layers. The corresponding separation distances were approximately 0.32 and 0.64 nm for one and two water layers, respectively. Examining the XDLVO interaction plots for the HL and CD membranes with the silica and polystyrene colloids (Fig. 5a and b, respectively) a similar trend may also be seen. Intimate surface contact is said to occur at h = nm, which is defined as the minimum equilibrium separation distance [10]. Additionally, the molecular diameter of a water molecule is approximately 0.32 nm. Examining the calculated interaction force curves for the HL and CD membranes, the potential impact of adsorbed water layers on the magnitude of available adhesion energy can be assessed. For the HL membrane and the polystyrene colloid (Fig. 5a), the XDLVO calculated interaction force decreased by 40, 65, and 79% when the surfaces are separated by one (h = 0.48 nm), two (h = 0.80 nm), and three (h = 1.12 nm) water layers, respectively. This same trend was observed for the CD membrane and polystyrene colloid (Fig. 5b) with decreases of 55, 73, and 84% at distances of 0.48, 0.80, and 1.12 nm, respectively. For both membranes, however, the interaction with the silica colloid was calculated as repulsive even at contact. This demonstrates the significant impact of interfacial water layers on surface contact and subsequent surface adhesion. The substantial reduction in calculated interaction forces at short separation distances is based on assumptions of surface idealities (i.e., physical and chemical homogeneity), however, surface roughness would only reduce adhesion further as water would be confined between the colloid and valley-like structures on the membrane surface. The measured and calculated force curves for the polystyrene colloid on approach to the CD surface are shown in Fig. 6. Two plots are shown in this case because the CD polystyrene interaction on approach could not be generally characterized by a single interaction regime as for the previous systems. In the first case (Fig. 6a), the polystyrene colloid encountered a long-range (h = 20 nm) attraction leading to contact. Although the attraction was predicted by the XDLVO calculation, the predicted range of the interaction was substantially shorter (h = 8 nm). It is possible that bridging interactions between adsorbed counter ions occurred in these instances as demonstrated by the relatively flat plateaus in the attractive portion of the Fig. 6. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the polystyrene colloid to the CD membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). For the total population of force curves measured for this pair (n = 15), the measured interaction on approach was characterized by both (a) attractive and (b) repulsive interactions prior to contact.

11 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) force curve and resulting in an extended interaction. In the second case (Fig. 6b), a repulsive interaction was measured on approach, in direct contrast to the modeled interaction. This reversal of the expected interaction may be attributed to the chemical heterogeneity of the CD surface whereby the sample area characteristics are substantially different than those determined for the bulk surface. An increase in the bond strength between the CD membrane surface and interfacial water molecules could prevent the attraction observed in some cases. The dashed line in Fig. 6b represents a modeled interaction between the CD membrane and the polystyrene colloid where the γ + and γ parameters for the CD membrane have been changed from their measured values of 0 and 38 mj/m 2 (Table 2) to 18 and 48 mj/m 2. This change in the CD membrane surface properties results in a repulsive interaction on approach that prevents intimate surface contact from occurring. Consideration of differences between local and global surface characteristics is an issue that warrants further investigation Adhesion on a rough membrane surface The membranes previously discussed represented relatively smooth surfaces (R q < 10 nm). However, a large number of commercially available NF and RO membranes, particularly polyamide membranes, have significant surface roughness [20,43]. For example, the R q for the NF70 membrane was approximately 62 nm (0.062 m) (Table 1). Previous studies have demonstrated that surface roughness reduces adhesion through a reduction in the available contact area when particle dimensions are larger than those of the surface asperities [31,33,34,43]. Previously, Bowen and Doneva [43] found that the magnitude of the measured adhesion differed by a factor of 20 between that measured on a peak (i.e., a single asperity) and that measured in a valley or depression. Because the diameter of the colloids used in this investigation (5 and 5.26 m) are approximately two orders of magnitude larger than the NF70 surface features, it is likely that adhesion would be lower than that expected from theoretical predictions as the colloids will predominantly be interacting with peak features. Theoretical quantification of such an interaction at a rough interface is extremely complex and requires that the exact surface geometry and local separation distances be accounted for [43 46]. The surface element integration (SEI) technique [44 46] accounts for surface roughness in modeling the interaction on approach between two surfaces but has not yet been applied to surface adhesion. Therefore, only a qualitative Fig. 7. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the silica colloid to the NF70 membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). For the total population of force curves measured for this pair (n = 15), the measured interaction on approach was characterized by both (a) repulsive and (b) attractive interactions prior to contact.

12 246 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) interpretation of the adhesive interaction at the rough NF70 surface with respect to AB interactions was pursued here. Fig. 7 shows two characteristic approach curves for the silica colloid to the moderately hydrophilic NF70 membrane in addition to the modeled interaction profiles. The XDLVO force curve is characterized by a weak repulsive barrier at h = 8 nm and a shorter-ranged, exponentially increasing repulsion beginning at h = 4 nm. This short-range repulsion is chiefly due to hydrophilic repulsion. In the first measured force curve (Fig. 7a), the silica colloid experiences a weak repulsion prior to contact. This is in agreement with the XDLVO prediction yet acts over a longer range. The average repulsion for the silica colloid with the NF70 membrane is 0.26 ± 1.01 mn/m (n = 7). The interaction is qualitatively similar to that observed for the CD silica pair, although the repulsion is considerably weaker (F R = 4.55 mn/m for the CD silica pair). This may be explained by the weaker hydrophilicity, and thus weaker hydrophilic repulsion, of the NF70 membrane compared to the CD membrane (Table 2). Alternatively, the reduced repulsion may be due to roughness effects that reduce the contact area and thus, the magnitude of interfacial interactions [31,43 46]. In the second scenario (Fig. 7b), the interaction is purely attractive before surface contact occurs. As with the CD polystyrene pair, the divergence in the two types of interactions observed here may be attributed to the physical and chemical heterogeneity of the NF70 membrane surface. Surface roughness in particular is a likely source in this case as the NF70 membrane surface is more rough than the HL or CD surfaces. Fig. 8 is a representative approach curve for the polystyrene colloid to the NF70 surface. The interaction between the NF70 polystyrene pair is characterized as weakly repulsive until contact occurs in contrast to XDLVO expectations. Regardless, from Fig. 8 no hydrophobic attraction is measured for this pair. Similar to results for both the HL and CD membranes, adhesion is lower for the polystyrene colloid (average F ad = 0.52 mn/m) compared to the silica colloid (average F ad = 2.82 mn/m). Because of the similar size and charge of the two colloids, this indicates that even for a rough surface, such as the NF70 membrane, γ + and γ interactions may play some role in determining the magnitude of the measured adhesion. The impact of surface asperities, considered as hemispheres, on the adhesion between two surfaces may be considered in the JKR theory by assuming the interaction to be between two spheres of radii R 1 and R 2 [29]. The radial function for the interacting system may then be calculated Fig. 8. Measured and calculated force as a function of separation distance plotted on a log scale on approach of the polystyrene colloid to the NF70 membrane surface (I = 0.01 M NaCl; ph = 5.6; T = 20 C). Fig. 9. (a) Full scale and (b) magnified view of the JKR calculated adhesion force for a polystyrene colloid in contact with a smooth surface having the thermodynamic properties of the NF70 membrane surface and as a function of asperity radius for a rough surface having the same surface energy properties.

13 J.A. Brant, A.E. Childress / Journal of Membrane Science 241 (2004) Fig. 10. AFM measured height profile across the NF70 surface with a superimposed sphere representing a polystyrene colloid in contact with the membrane. according to the following relationship: R i = R 1R 2 (5) R 1 + R 2 where R i is the radial term for the system and R 1 and R 2 are the radii of the interacting spheres. This equation can then be substituted into Eq. (3) (with R i = R) in order to calculate the theoretical adhesion for a colloid on a rough surface. Fig. 9a is a plot of the JKR calculated adhesion for the polystyrene colloid in contact with the NF70 surface. The impact of surface asperities on the calculated adhesion may be seen by comparing the adhesion calculated for a smooth surface with the adhesion calculated as a function of asperity radius, simulating a rough surface. Asperity radius may represent a single asperity or the collective radius of multiple asperities. From Fig. 9a it may be observed that adhesion for a rough surface is substantially less than for a smooth surface and that as the available contact area decreases, represented by a decreasing asperity radius, adhesion further decreases. For the polystyrene colloid on the NF70 membrane surface, an average adhesion of 0.52 mn/m was measured. Based on the theoretical curve shown in Fig. 9a and magnified in Fig. 9b, this would correspond to an asperity radius of approximately 9 nm. Fig. 10 is a representative height profile measured on the NF70 membrane showing the deviation of asperity features from a mean plane. A scaled sphere has been drawn to represent the polystyrene colloid in contact with the membrane surface, as would occur during the AFM force measurements. Surface deformation is neglected in Fig. 10. Based on Fig. 10, the colloid comes into contact with only three asperity peaks; this illustrates how substantial the reduction in contact area is for the NF70 membrane as a result of surface roughness. Furthermore, it can be seen how easily a water film could become confined in the valley features between the membrane surface and the colloid causing increased repulsion at contact. Based on AFM analysis, each of these asperities had a radius of approximately 3 10 nm, which generally corresponds to the asperity radius predicted by the JKR analysis. Although this is only a rough correlation, it demonstrates that, like EL and LW interactions, AB forces are significantly reduced by surface roughness. 4. Conclusions This investigation examined the role of γ + and γ surface energy components on the adhesion of a hydrophilic and hydrophobic colloid to several commercially available hydrophilic membranes. From AFM force measurements, it was found that removal of structured water layers, and subsequent exposure of γ + and γ groups, resulted in a greater adhesion than when surface water layers were not removed. Additionally, adhesion was substantially larger in the presence of reciprocal γ + and γ groups on the colloid surface, in agreement with JKR calculations. Thus, colloid surface chemistry must be taken into consideration when assessing membrane fouling propensity. The role played by AB interactions, and implicitly γ + and γ components, changes between that incurred on approach of two surfaces and that which occurs at contact. This indicates that a reexamination of current interaction formulations is required. Additionally, even when surface roughness results in a weaker adhesion, γ + and γ groups may play some role in determining the magnitude of the resulting adhesion. The results presented here have direct implications for evaluating the long-term colloidal fouling propensity of water treatment membranes and may also provide a new means for evaluating hydrodynamic and/or chemical cleaning techniques to remove surface adsorbed solutes. References [1] A. Batra, S. Paria, C. Manohar, K.C. Khilar, Removal of surface adhered particles by surfactants and fluid motions, AIChE J. 47 (2001)