Download pdf - Assessing short-range membrane–colloid interactions using surface energetics

Journal of Membrane Science 203 (2002) 257–273

Assessing short-range membrane–colloid interactionsusing surface energetics

Jonathan A. Brant, Amy E. Childress∗Department of Civil Engineering, University of Nevada at Reno, Mail Stop 258, Reno, NV 89557-0152, USA

Received 7 August 2001; received in revised form 14 December 2001; accepted 4 January 2002

Abstract

The contribution of acid–base (AB) (polar) interactions to the total interaction energy between membranes and colloidswas investigated. The surface energetics of several membranes and colloids were evaluated using the Lifshitz–van der Waalsacid-base approach. This provides the van der Waals (LW) and polar interaction energies between various surfaces frommeasurements of contact angles of different probe liquids on these surfaces. In addition, the surface potentials of the mem-branes and colloids were determined using electrokinetic measurements, yielding the electrostatic (EL) interaction betweenthese surfaces. The three interaction energy components (LW, EL, and AB) were combined according to the extendedDerjaguin–Landau–Verwey–Overbeek (extended DLVO or XDLVO) approach to evaluate membrane–colloid interactionenergies. Predictions of interaction energy based on the XDLVO approach were compared to the corresponding predictionsfrom the classical DLVO theory. For all the membrane–colloid combinations studied, the DLVO potentials were quite sim-ilar. However, inclusion of AB interactions resulted in a substantially different (qualitative and quantitative) prediction ofshort-range (separation distances<5 nm) interaction energies for several of the membrane–colloid combinations investigated.Finally, all of the membranes studied were found to have substantially low surface energies compared to the colloids andthe interaction energy between the membranes and colloids was primarily dictated by the surface energies of the colloids.Fouling experiments for a membrane and three colloids supported the fouling trends predicted by the XDLVO approach.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Polar interactions; XDLVO approach; Fouling; Contact angle; Reverse osmosis

1. Introduction

During the past decade, numerous investigationshave focused on elucidation of the mechanisms in-volved in colloidal membrane fouling (e.g. [1–17]).These investigations generally reveal that colloidalfouling results from the deposition of particles ontomembrane surfaces through a combination of physical(hydrodynamic) and chemical (colloidal) interactions

∗ Corresponding author. Tel.:+1-775-784-6942;fax: +1-775-784-1390.E-mail address:[email protected] (A.E. Childress).

[5–7,10,12,13]. Furthermore, these studies clearlyshow that while hydrodynamic drag forces bring theparticles close to the membrane surface, it is the col-loidal (chemical) interactions that cause binding ofthe particles to the membrane [9,12].

The chemical interactions leading to colloidalmembrane fouling are generally assessed usingthe classical Derjaguin–Landau–Verwey–Overbeek(DLVO) theory [18,19], which considers two typesof interactions, namely, van der Waals (LW) andelectrostatic (EL) double layer interactions. Quiteoften, these two interactions alone fail to explain thefouling behavior of a membrane [8,20]. Under such

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(02)00014-5

258 J.A. Brant, A.E. Childress / Journal of Membrane Science 203 (2002) 257–273

Nomenclature

ac colloid radiusA Hamaker constante electronic charge (1.6 × 10−19 C)�G interaction energy per unit area

between two infinite planar surfacesh surface to surface separation

distance between flat plate and sphereJ permeate fluxJ0 initial permeate fluxk Boltzmann’s constant

(1.38× 10−23 J/K)ni number concentration of ion

i in bulk solutionT temperatureU total interaction energy between

membrane and particley separation distance between two

infinite planar surfacesy0 minimum separation

distance (0.158 nm)zi valence of ioni

Greek lettersεrε0 permittivity of the suspending liquidγ surface tensionκ inverse Debye screening lengthλ decay length of AB interactions

in waterθ contact angleζ zeta potential

SuperscriptsAB acid–baseDLVO Derjaguin–Landau–Verwey–OverbeekEL electrostaticLW Lifshitz–van der WaalsTOT totalXDLVO extended DLVO+ electron acceptor− electron donor

Subscriptsc colloidl liquidm membranes solid

circumstances, the discrepancy between the theoreti-cal DLVO predictions and experimental observationsis typically accounted for using additional mecha-nisms and hypotheses. For example, these discrepan-cies have sometimes been attributed to chemical andmorphological heterogeneity of the membrane sur-faces [10,21–25], while in other instances, they havebeen attributed to additional types of interactions be-tween the membranes and foulants [26–29]. It is quitepossible that the presence of morphological and chem-ical heterogeneities can result in surfaces with verydifferent energy distributions than smooth or chem-ically homogeneous surfaces [10,21–25], and hence,can result in substantially different fouling behaviorcompared to theoretical DLVO predictions. Equallylikely, however, is the presence of additional interac-tions between a membrane and a foulant, particularlyat very small separations, leading to “aberrant” foul-ing propensities. For many uncharged materials, thepresence of additional interactions induced by the po-larity of the solvent has been observed [26–29], andthis has led to a school of thought that proposes anextended DLVO (XDLVO) type approach to accountfor the total interactions in such systems [3,27,29–33].

The additional interaction in the XDLVO modelis often attributed to a short-ranged acid–base (AB)(electron donor/electron acceptor) interaction (possi-bly stemming from hydrogen bonding) between twosurfaces immersed in a polar solvent (e.g. water)[3,27,30,32–35]. This interaction may be attractive(hydrophobic attraction) or repulsive (hydrophilic re-pulsion). The presence of these interactions is stronglysupported by the measurement of surface tensionsof various substances. For example, in the case ofwater, the major contribution (51 mJ/m2) to the totalsurface tension (72.8 mJ/m2) stems from these polarinteractions, while the van der Waals contribution(21.8 mJ/m2) is much lower. Interfacial tension mea-surements for many polymeric surfaces immersed inwater have also revealed a substantial AB contribution[4,20,31,36]. The fouling of polymeric membranes, istherefore, likely to be influenced by AB interactionsand the contribution of polar interactions to colloidalmembrane fouling should be examined.

Comparisons between the DLVO theory and theXDLVO approach have been made in previous inves-tigations concerning deposition and aggregation inaqueous systems (e.g. [31,33,37]). Meinders et al. [31]

J.A. Brant, A.E. Childress / Journal of Membrane Science 203 (2002) 257–273 259

investigated deposition efficiencies and reversibilityof bacterial adhesion on various substratum surfacesand compared those results to DLVO and XDLVOpredictions. It was concluded that bacterial adhesionto the surfaces studied was more accurately explainedby the XDLVO approach. Ohki and Ohshima [37]analyzed the interaction and aggregation of lipid vesi-cles using the DLVO and XDLVO models and foundthat the XDLVO approach was more accurate in de-scribing interactions occurring at a separation distance<2 nm, where hydrophobic interactions are strong.Wu et al. [33] examined interactions occurring in par-ticle suspensions and compared experimental resultswith both DLVO and XDLVO predictions. It wasconcluded that the XDLVO approach nearly alwaysdescribed the interactions of immersed or dissolvedspecies more accurately than the DLVO theory. Theabove studies represent a small cross-section of a vastbody of evidence showing the considerable influenceof polar interactions, particularly at small separationsbetween the interacting surfaces, on particle depo-sition and attachment. This leads to the question ofwhether these interactions also influence colloidalmembrane fouling and, if so, to what extent.

Several studies use the concept of hydrophilicityand hydrophobicity of membranes to assess theirpropensity toward fouling. A membrane can be classi-fied as either having a high surface energy (i.e. beinghydrophilic) or as having a low surface energy (i.e.being hydrophobic) [15,27,38]. In numerous previousinvestigations (e.g. [1,36,39–44]), the hydrophobicnature of a membrane has been estimated from itscontact angle with water and correlated with mem-brane fouling. It has generally been concluded that hy-drophobic membranes tend to foul more rapidly thanhydrophilic membranes [1,4,15,39,41–43,45–49].This is presumably because hydrophobic membranesprefer to be covered with colloids than with water[4,50]. Additionally, it has been shown that membranefouling can be reduced through the hydrophilizationof ultrafiltration (UF) [1,39,41,42,51] and reverseosmosis (RO) [46,48,52] membrane surfaces.

Many earlier studies present a somewhat incom-plete, and occasionally even misleading picture aboutthe role of polar interactions in membrane fouling.First, only the hydrophobic/hydrophilic nature ofthe membrane has been considered in many studieswhen evaluating membrane fouling. However, the

hydrophobic/hydrophilic nature of the potentialfoulant must also be considered when analyzing theinfluence of polar interactions on colloidal fouling.Second, it is not sufficient to simply relate foulingpropensity of a surface to the contact angle of wateron that surface. Polar interactions cannot be rigor-ously quantified solely on the basis of contact anglemeasurement with water, which merely provides aqualitative estimate of these interactions. To eluci-date the influence of polar interactions on colloidalmembrane fouling, a systematic characterization ofthe membrane–colloid system should be performedand a self-consistent analysis that quantifies the polarproperties should be used.

The objective of this paper is to systematicallyinvestigate the effects of polar (hydrophobic/hydro-philic) interactions on colloidal membrane fouling.To do this, the surface energetics (or more specif-ically, the polar properties) of several membranesand colloids are measured using the Lifshitz–vander Waals acid-base (or simply acid-base) approach[27,31]. These results are used in an XDLVO analysisto evaluate membrane–colloid interactions upon closeapproach. By comparing the XDLVO results with pre-dictions from the classical DLVO theory for differentmembrane–colloid combinations, the contribution ofpolar interactions relative to EL and LW interactionsis examined.

2. Theory and principles of contact angleand interaction energies

2.1. DLVO and XDLVO interaction

The DLVO theory expresses total interactionenergy, or energy of adhesion, between a colloid anda membrane as the sum of LW and EL interactionenergies according to

UDLVOmlc = ULW

mlc + UELmlc (1)

whereUDLVO is the total interaction energy betweenthe membrane and colloid immersed in water,U LW

the LW interaction term, andU EL is the EL inter-action term. The subscripts m, l, and c correspondto the membrane, bulk feed solution, and colloid,respectively.


It has been suggested by van Oss [27] that energybalances performed for aqueous systems must alsoaccount for the AB interaction energy in addition tothe LW and EL interaction energies. Inclusion of theAB interaction term results in the extended DLVO ap-proach, which may be written as

UXDLVOmlc = ULW

mlc + UELmlc + UAB

mlc (2)

whereUXDLVO is the total interaction energy betweenthe membrane and colloid immersed in water andUAB

is the AB interaction term.

2.2. Lifshitz–van der Waals acid–base approach

Application of the XDLVO approach requires thatthe surface energy parameters of the membrane andcolloid be determined experimentally. Surface energydata can be calculated for membranes and colloidsusing contact angle data and the acid-base approach[27,31]. The acid-base approach has been widely usedas a method for determining surface tension compo-nents of a solid and interfacial free energies per unitarea between two phases (e.g. [3,53,54]).

van Oss et al. [55] represented the total surface ten-sion of a pure substance as the sum of an LW and anAB component, yielding

γ TOT = γ LW + γ AB (3)

where γ TOT is the total surface tension andγ LW

andγ AB are the LW and AB components of surfacetension, respectively. The non-polar LW componentrepresents a single electrodynamic property of a givenmaterial, while the polar AB component comprisestwo non-additive electron acceptor and electron donorparameters [27,53]. The polar AB component of amaterial’s surface energy is given by [3,36,53,56]

γ AB = 2√

γ +γ − (4)

whereγ + is the electron acceptor parameter andγ −is the electron donor parameter.

The surface tension components/parameters of asolid surface (γ LW

s , γ +s , γ −

s ) can be determined byperforming contact angle measurements using threeprobe liquids with known surface tension parameters(γ LW

l , γ +l , γ −

l ) and employing the extended Youngequation [27]. The extended Young equation relatesthe contact angle of a liquid on a solid surface to the

surface tension parameters of both the solid and theliquid and is given by [4,36,56]

(1 + cosθ)γ TOTl

= 2

(√γ LW

s γ LWl +

√γ +

s γ −l +

√γ −

s γ +l

)(5)

whereθ is the contact angle. The subscripts s and lcorrespond to the solid surface and the liquid, respec-tively. The extended Young equation is best describedas an equilibrium force balance. The left-hand sideof the equation represents the free energy of cohe-sion per unit area of the liquid (l) and the right-handside represents the free energy of adhesion per unitarea between the liquid (l) and the solid (s) [27,57].According to van Oss [27], two of the probe liquidsshould be polar and one of the probe liquids shouldbe apolar. The apolar liquid is used to calculatethe non-polar component of surface tension (γ LW

s )[27,58]. Furthermore, high energy (polar and apolar)liquids are recommended to produce larger, moreeasily measured contact angles [58].

2.3. Free energy of adhesion

Surface tensions for membranes and colloids cal-culated using the acid-base approach can be used toevaluate the free energies of adhesion per unit area,�GLW

y0and �GAB

y0, between these surfaces. Expres-

sions for the LW and AB adhesion energies per unitarea are, respectively given by [27]

�GLWy0

= 2

(√γ LW

l −√

γ LWm

) (√γ LW

c −√

γ LWl

)(6)

and

�GABy0

= 2√

γ +l

(√γ −

m +√

γ −c −

√γ −

l

)

+2√

γ −l

(√γ +

m +√

γ +c −

√γ +

l

)

−2

(√γ +

m γ −c +

√γ −

m γ +c

)(7)

The free energy of adhesion per unit area signifies theinteraction energy per unit area between two planarsurfaces (bearing the properties of the membrane andthe colloid) that are brought into contact. Because it


is known that the LW interaction between two ma-terials diverges when the surfaces physically toucheach other, contact is assumed to occur at a hypothet-ical minimum equilibrium cut-off distance [31]. Theminimum equilibrium cut-off distance,y0, is usuallyassigned a value of 0.158 nm (±0.009 nm) and maybe regarded as the distance between the outer elec-tron shells (van der Waals boundaries) of adjoiningnon-covalently interacting molecules [20,59].

2.4. Interaction energy between a sphericalcolloid and an infinite planar surface

As the separation distance between two surfacesincreases, the LW and AB interaction energy com-ponents diminish from their corresponding adhesionenergy (i.e. the interaction energy at contact given byEqs. (6) and (7)) following a unique decay pattern.The LW interaction energy per unit area decays withthe inverse square of the distance between two infiniteplanar surfaces [56]:

�GLW(plate–plate) = − A

12πy2(8)

whereA is the Hamaker constant andy is the separa-tion distance between the two interacting planar sur-faces. The Hamaker constant can be calculated fromthe LW component of the free energy of adhesion byrearranging Eq. (8):

A = −12πy20 �GLW

y0(9)

Estimates of free energy of adhesion obtained fromthe surface tension components provide informationabout interaction energy per unit area between two in-finite planar surfaces. To obtain the actual interactionenergy between the membrane (assumed to be an infi-nite planar surface) and the colloidal particle (assumedto be a sphere), a technique that converts the interac-tion energy per unit area to the total interaction energyfor a given geometry is required. Derjaguin’s tech-nique [60], was therefore, used to scale the interactionenergy per unit area between two infinite flat surfacesto the corresponding interaction energy between a flatsheet (membrane) and a sphere (colloid). Applyingthis technique to Eq. (8), the LW interaction energybetween an infinite flat plate and a spherical colloid is

ULW(plate–sphere) = −Aac

6h(10)

whereac is the radius of the spherical colloid andhis the surface to surface separation distance betweenthe flat plate (membrane) and the sphere (colloid).Combining Eqs. (9) and (10) gives the expression forthe LW interaction energy between a membrane anda colloid in an aqueous environment:

ULWmlc (h) = 2π �LW

y0

y20ac

h(11)

The expression for the AB interaction energy as afunction of separation distance is derived similarly.The AB interaction energy per unit area decays expo-nentially with separation distance between two infiniteplanar surfaces [3]:

�GAB(plate–plate) = �GAB

y0exp

[y0 − y

λ

](12)

whereλ is the characteristic decay length of AB in-teractions in water, whose value is between 0.2 and1.0 nm [61]. A commonly used value ofλ for aqueoussystems is 0.6 nm [20,34]; this value was used in thecurrent investigation. Applying Derjaguin’s techniqueto Eq. (12), the decay behavior of AB interactionsbetween a membrane and a colloid in an aqueousenvironment is

UABmlc = 2πac λ �GAB

y0exp

[y0 − y

λ

](13)

The EL interaction energy per unit area between twoinfinite planar surfaces decays with separation distanceaccording to [62]

�GEL(plate–plate) = ε0εrκ

2(ζ 2

m + ζ 2c )

×(

1 − coth(κy) + 2ζmζc

(ζ 2m + ζ 2

c )csch(κy)

)(14)

where ε0εr is the dielectric permittivity of the sus-pending fluid,κ the inverse Debye screening length,andζm andζ c are the surface potentials of the mem-brane and colloid, respectively. Once again, applyingDerjaguin’s technique to Eq. (14), the decay behaviorof the EL interaction energy between a membraneand a colloid in an aqueous environment is

UELmlc(h) = πεrε0ac

(2ζcζm ln

(1 + e−κh

1 − e−κh

)

+(ζ 2c + ζ 2

m) ln(1 − e−2κh)

)(15)


In the current investigation, the surface potentialswere assumed to be the same as the measured zetapotentials of the surfaces involved. The zeta potentialsof the membranes were determined from streamingpotential measurements while the zeta potentials ofthe colloids were determined from electrophoreticmobility measurements. For both membranes andcolloids, zeta potential values at pH 5.6 were usedto determine the EL interaction energy term. The in-verse Debye screening length was determined usingthe following relationship [63]:

κ =√

e2∑

niz2i

εrε0kT(16)

where e is the electron charge,ni the number con-centration of ioni in the bulk solution,zi the valenceof ion I, k Boltzmann’s constant, andT is absolutetemperature. For this investigation, a backgroundelectrolyte of 0.01 M NaCl was assumed.

3. Materials and methods

3.1. Representative membranes

The three RO membranes selected for this investi-gation were the FT-30, CD, and CE membranes. TheFT-30 membrane (Film Tec, Minneapolis, MN) is athin-film composite polyamide membrane. The CDand CE membranes (Osmonics Desal, Vista, CA) areheat-treated cellulose triacetate/diacetate blend mem-branes. All of the membranes were stored in ultrapurewater at 5◦C.

3.2. Representative colloids

The three colloids selected for this investigationwere commercial silica colloids, aluminum oxide col-loids, and polystyrene microspheres. The commercialsilica colloids (MP-1040, Nissan Chemical AmericaCorporation, Houston, TX) were supplied dispersedin deionized water; the dispersion was stored at roomtemperature. According to the manufacturer, the silicacolloids have an average particle diameter of 100 nmand a density of 2.2 g/cm3. The aluminum oxidecolloids (Aluminum Oxide C, Degussa Corporation,Akron, OH) were supplied as a powder and stored at

room temperature. According to the manufacturer, thealumina colloids have an average particle diameterof 13 nm and a density of 3.2 g/cm3. The polystyrenemicrospheres (PSO2N, Bangs Laboratories, Inc.,Fishers, IN) were supplied dispersed in deionizedwater; the dispersion was stored in the refrigerator at5◦C. According to the manufacturer, the polystyrenemicrospheres have a mean diameter of 20 nm and adensity of 1.05 g/cm3.

Because it is known that colloids tend to bondtogether irreversibly to form aggregates [5,64–66],dynamic light scattering measurements (Zeta PALS,Brookhaven Instruments Corp., Holtsville, NY) wereused to determine aggregate size. Measurements re-vealed that the silica, alumina, and polystyrene col-loids have diameters of 97.9, 220.5, and 128.2 nm,respectively. Thus, the silica colloids appear to remainas primary particles and the alumina and polystyrenecolloids appear to form aggregates.

3.3. Electrokinetic measurements

Zeta potentials of the membranes were determinedusing a streaming potential analyzer. Measurementswere conducted using a 1 mM KCl solution at 25◦C.Streaming potential was evaluated over the pH rangeof 3–9 for each membrane. Zeta potential was cal-culated from the measured streaming potential usingthe Helmholtz–Smoluchowski equation with the Fair-brother and Mastin substitution. Detailed descriptionsof the instrument, measurement procedure, and zetapotential calculation can be found elsewhere [67–71].

Zeta potentials of the colloids were determinedusing microelectrophoresis (Zeta PALS, BrookhavenInstruments Corp., Holtsville, NY). Electrophoreticmobility was evaluated with a background electrolyteof 0.01 M NaCl at three pH values (3.0, 5.6, and8.0). Zeta potential was calculated from the measuredelectrophoretic mobility using the Smoluchowskiequation. A detailed description of the instrument,measurement procedure, and zeta potential calcula-tion can be found elsewhere [17].

3.4. Goniometric measurements

3.4.1. Automated goniometerContact angle measurements were performed us-

ing an NRL Contact Angle Goniometer (Rame Hart,


Mountain Lakes, NJ). This instrument is a standard go-niometer with image analysis attachments (i.e. videocamera, computer with monitor, and image analysissoftware). The goniometer has the capability of mea-suring contact angle using both the sessile drop andcaptive bubble methods. In the sessile drop method,a drop of probe liquid is placed on a dry surface; inthe captive bubble method, the surface is placed in aquartz cell, which contains the probe liquid, and an airbubble is released onto the surface. The sessile dropmethod was used to measure the surface energeticsof the colloids because the packed colloidal bed dis-perses once immersed in a liquid. The captive bubblemethod was used to measure the surface energeticsof the membrane surfaces because by immersing themembrane sample in solution and completely hydrat-ing it, contact angle measurement is less influencedby pores and swelling [15,42,72–75]. Additionally, thedrying process that is necessary for the sessile dropmethod can be damaging to the membrane [73].

3.4.2. Probe liquidsAs discussed earlier, to determine the surface ten-

sion components of solid surfaces, it is necessary toperform contact angle measurements using three probeliquids with well known surface tension properties.The probe liquids selected for this investigation areglycerol, ultrapure water, and diiodomethane. Theseprobe liquids were chosen on the premise that twomust be polar (glycerol and ultrapure water) and onemust be apolar (diiodomethane) [27]. The glycerol anddiiodomethane were obtained from Fisher Scientific(Pittsburgh, PA). Ultrapure water was obtained froma Millipore (Burlington, MA) water purification sys-tem. The three surface tension components (γ LW, γ +,andγ −) as well as the polar energy component (γ AB)and the total free energy component (γ TOT) of eachprobe liquid are shown in Table 1. It should be noted

Table 1Surface tension properties (mJ/m2) of probe liquids at 20◦C

γ LW γ + γ − γ AB γ TOT

Ultrapure water 21.8 25.5 25.5 51.0 72.8Glycerol 34.0 3.9 57.4 30.0 64.0Diiodomethane 50.8 0.0 0.0 0.0 50.8Formamide 39.0 2.3 39.6 19.0 58.0

Data taken from van Oss [27].

that although formamide was not used in the currentinvestigation, its surface tension parameters are shownin Table 1 because data taken from the literature [76]for the surface tension of the polystyrene colloids usedformamide as a probe liquid instead of glycerol.

3.4.3. Measurement of membrane surface energyMembrane samples were conditioned using

ultrapure water for 45 h prior to contact angle mea-surement. A membrane coupon having the approxi-mate dimensions of 1.0 in.×0.25 in. was cut from theconditioned membrane sample, mounted on the under-neath side of the stainless steel sampling plate with theactive layer facing down, secured on the goniometer’sviewing stage, and then lowered into the probe liquid.A 10�l air bubble was delivered to the membrane sur-face using a 20�l syringe with a bent U-shaped nee-dle attachment (Hamilton Instruments, Reno, NV). A10�l bubble was used in this investigation to providea bubble of significant size to make its edges clearlyvisible. Two contact angle measurements (one on eachside of the bubble) were taken at time zero and thenat 5 min intervals over the next 20 min. A minimumof three membrane coupons and three air bubbles percoupon was used for each of the three membrane types.

3.4.4. Measurement of colloid surface energyColloids were deposited onto a non-porous RO

membrane using a dead-end membrane filtrationsystem for a 24 h period. The membrane was removedfrom the membrane filtration system to allow the col-loidal deposit to dry for 1 h. Contact angle measure-ments were then performed on the colloidal depositusing the sessile drop method. A 5�l liquid dropletof the probe liquid was released from the micrometersyringe at a minimum distance from the colloidal de-posit. Two contact angle measurements (one on eachside of the droplet) were taken immediately follow-ing deposition of the liquid droplet on the colloidaldeposit.

3.5. Membrane fouling experiments

3.5.1. Cross-flow membrane test unitA flat-sheet, closed loop bench-scale membrane test

unit was used for conducting membrane fouling tests.The feed solution is housed in a 30 l temperature con-trolled reservoir (Cole Parmer Polystat, Vernon Hills,


IL). The feed solution is fed to two flat-sheet mem-brane test cells operating in parallel by a positivedisplacement pump (CAT Model 280, Minneapolis,MN) capable of providing hydraulic pressures up to1000 psi and a flowrate of 11.36 lpm. Feed flowrateis controlled using a bypass valve on the feed line.The rectangular cells contain membranes with an ac-tive area of 19.35 cm2. Permeate is removed from thetop of each membrane test cell, while the concentrateexits from the side of the test cell. Transmembranepressure is controlled using a needle valve on the con-centrate stream. The flowrate, conductivity, and pH forboth the concentrate and permeate are measured us-ing in-line digital probes (Cole Parmer, Vernon Hills,IL) connected to a computer. Transmembrane pressureis recorded using pressure transducers (Cole ParmerModel 07356-66, Vernon Hills, IL) located at both theinlet and outlet of the membrane test cells. The con-centrate and permeate are re-circulated back to the re-frigerated reservoir.

3.5.2. Membrane fouling testsMembrane coupons were cut from flat-sheet mem-

branes stored in water at 5◦C. Membranes wereequilibrated for 45 h using a background electrolytesolution of 0.01 M NaCl at which point permeate fluxand salt rejection were constant. After equilibration,the pressure was adjusted to generate a permeate fluxof 1.0 m/day, which was the initial flux used for allmembrane fouling tests. A concentrated stock solutionof colloids was added to provide a feed colloid con-centration of 1.37×1015 colloids/l. Permeate flux wasthen monitored for 5 h. Conductivity and pH were alsomeasured throughout the fouling experiment to moni-tor ion rejection and to ensure that the chemical condi-tions were constant throughout the test. Permeate fluxresults following the addition of colloids to the feedstream were recorded in terms of a normalized flux.

4. Results and discussion

4.1. Surface energy determinations

Average contact angle measurements for the probeliquids on the three membranes (FT-30, CD, andCE) and the three colloids (silica, alumina, andpolystyrene) are shown in Table 2. The values in

Table 2Average contact angles of membranes and colloids

Ultrapure water Glycerol Diiodomethane

MembranesFT-30 49◦ (±0.8◦) 47◦ (±1.0◦) 54◦ (±0.6◦)CD 49◦ (±1.2◦) 58◦ (±1.5◦) 43◦ (±2.4◦)CE 45◦ (±2.1◦) 41◦ (±1.6◦) 44◦ (±1.4◦)

ColloidsSilica 67◦ 80◦ 50◦Alumina 44◦ 23◦ 33◦

Average contact angles of the polystyrene colloid in ultrapurewater, formamide, and diiodomethane are 90, 74, and 44◦, respec-tively (data taken from Busscher et al. [76]).

Table 2 for the membranes represent the mean ofat least 18 air bubbles, and are reported with theirrespective 95% confidence limits. The membranecontact angle measurements were highly reproduciblefor all membrane–probe liquid combinations investi-gated with 95% confidence limits of<3◦. The valuesfor the silica and alumina colloids are reported as themean of three air bubbles. Values for the polystyrenecolloids were taken from Busscher et al. [76].

Table 3 shows the calculated surface tension pa-rameters and the free energy of cohesion for each ofthe membranes and colloids. The surface energy datashow that all three membranes have high electrondonor monopolarity, or high electron donor com-ponents (γ −) and relatively low electron acceptorcomponents (γ +). These results agree with previousstudies (e.g. [4,36]) that have found that polymericmembranes are typically characterized by a highelectron donor monopolarity. Specifically, the CDmembrane has an especially low electron acceptorcomponent, which translates into a very low AB

Table 3Surface energy parameters (mJ/m2) of membranes and colloids

γ LW γ + γ − γ AB γ TOT �Gsws

MembranesFT-30 32.02 1.82 28.68 14.46 46.48 2.57CD 38.07 0.01 37.43 1.40 39.47 16.58CE 37.54 1.69 28.51 13.89 51.43 0.10

ColloidsSilica 34.27 1.04 31.72 11.50 45.77 6.57Alumina 42.94 3.66 19.55 16.91 59.85−14.97Polystyrene 37.54 0.57 5.27 3.47 41.00−51.60


component. Furthermore, the CD membrane isstrongly hydrophilic while the FT-30 and CE mem-branes cannot be said to be hydrophobic or hy-drophilic with �Gsws approximately equal to zero.Similar to the membranes, the colloids are character-ized as having high electron donor monopolarity. Thesilica colloid is hydrophilic while the alumina andpolystyrene colloids are hydrophobic.

The free energy of cohesion is the interaction freeenergy (per unit area) when two surfaces of the samematerial are immersed in a solvent (in this case,water) and brought into contact. These values providea quantitative insight regarding the hydrophobicity/hydrophilicity of the membranes and colloids. Positivevalues of the cohesive energy imply hydrophilic sur-faces, while negative values indicate hydrophobic sur-faces. The free energy of cohesion may provide somequalitative insight into potential interactions betweenmembranes and colloids. For example, based on thevalues given in Table 3, the short-range interactionbetween the CD membrane and silica colloid may beconsidered to be strongly hydrophilic and therefore,repulsive. However, in many cases, cohesive energiesmay also provide misleading notions about the in-teraction between membranes and colloids, primarilydue to its inability to provide a quantitative assess-ment of adhesive energies between the membrane andcolloid. Free energies of cohesion are discussed inrelation to free energies of adhesion in Section 4.3.

4.2. Zeta potential determination

The pH dependence of the zeta potentials for thethree membranes is shown in Fig. 1. For the CDmembrane, the potential is positive in the lower pHrange, passes through an isoelectric point around pH4, and then becomes negative in the higher pH range.The CE and FT-30 membranes are negatively chargedover the entire pH range investigated. Detailed de-scriptions of the mechanisms controlling the surfacecharge of thin-film composite and cellulose acetatemembranes were presented in earlier publications(e.g. [69,71]). The membrane zeta potential curveswere determined using a background electrolyte of1 mM KCl. A notable feature regarding the charge be-havior of the membranes that becomes evident whenFig. 1 is compared to Fig. 2 is that all the membraneshave very small zeta potentials (<10 mV). Because

Fig. 1. Variation of the zeta potential of membranes with pH.Measurements were carried out with a background electrolyte of1 mM KCl.

Fig. 2. Zeta potential of colloids as a function of pH. Measurementswere carried out with a background electrolyte of 10 mM NaCl.


the zeta potentials of the membranes are expectedto vary only minimally with ionic strength [69], themeasured values of zeta potential were employed incalculations of DLVO and XDLVO interactions thatassume a background electrolyte of 10 mM NaCl.

Zeta potential curves for the three colloids as a func-tion of pH are presented in Fig. 2. With an isoelectricpoint of approximately 7.5, the alumina colloids arepositively charged over the majority of the pH rangestudied. The silica and polystyrene colloids are nega-tively charged over the entire pH range investigated.In sharp contrast with the membranes, the zeta poten-tials of the colloidal particles were found to be sub-stantially larger in magnitude over certain pH ranges.Therefore, EL double layer interactions between themembranes and colloids involve highly asymmetricsurfaces with very different potentials.

4.3. Interaction energy at contact

Table 4 shows DLVO and XDLVO predictionsof the free energy of adhesion per unit area for thevarious membrane–colloid pairs tested. These val-ues were calculated using Eqs. (6), (7) and (14).In Eq. (14), the minimum separation distance of0.158 nm was used. The DLVO theory predicts aweak attraction for all of the membrane–colloid com-binations with all interaction energies ranging fromapproximately−3 to−7 mJ/m2. Even though both themembranes and colloids (except for alumina) are neg-atively charged, the DLVO theory (based on constantpotential EL interactions) predicts mild attractionbecause the membranes have very low surface poten-tials compared to the colloids. In general, when the

Table 4DLVO and XDLVO predictions for interaction energy at contact(mJ/m2)

Silica Alumina Polystyrene

DLVOFT-30 −5.63 −5.18 −2.98CD −7.16 −6.90 −4.53CE −6.56 −7.06 −4.31

XDLVOFT-30 1.14 −7.91 −20.74CD 7.20 −6.40 −22.54CE 0.14 −9.95 −22.48

constant potential assumption is used, highlyasymmetric surfaces are mildly attractive; however, ifconstant charge is assumed, this may not be the case[77].

The XDLVO predictions show that inclusion ofAB interactions results in a substantially differentprediction for the free energy of adhesion for sev-eral of the membrane–colloid combinations. In thecase of the silica colloid, the XDLVO predictionsshow a repulsive interaction at contact with all of themembranes, which is qualitatively different from thepredictions of the DLVO theory. Specifically, a com-plete reversal of the free energy of adhesion (from−7.2 to +7.2 mJ/m2) is observed for the CD–silicacombination. In the case of the polystyrene colloid,XDLVO predictions show a much stronger attractionthan was predicted by the DLVO theory. The largenegative value of the free energy of adhesion for thepolystyrene colloid based on the XDLVO approachcorroborates the strongly hydrophobic nature of thiscolloid. The substantial differences between DLVOand XDLVO predictions imply that the fouling pre-dictions for the membranes and colloids would becompletely different if the AB component is ignored.

It is important to note the differences be-tween the free energy of adhesion results for eachmembrane–colloid combination (Table 4) and the freeenergy of cohesion results for the individual mem-branes and colloids (Table 3). For example, accordingto the cohesion results, all of the membranes wouldbe expected to have low fouling tendencies becausethe free energies of cohesion are positive. However,when the free energies of adhesion for the differentmembrane–colloid combinations are considered, amore comprehensive picture of the fouling tendencyis developed. The CD membrane, which is expectedto have the lowest fouling tendencies according tothe free energy of cohesion values, actually does notexhibit a substantially different fouling tendency thanthe other membranes when exposed to alumina orpolystyrene.

It was stated earlier that the presence of additionalinteractions (such as polar interactions) is just onepossible mechanism to explain deviations from DLVOtheory. A second plausible mechanism is surfaceroughness [10]. In this context, the results from Table 4give considerable insight into the relative importanceof morphological heterogeneity and polar interactions


on the deviation of the total membrane–colloidinteraction energy from the DLVO prediction. Be-cause the membrane–colloid interaction energy dic-tates the initial fouling behavior, careful analysis ofthe surface energies from Table 4 may be used todelineate the conditions under which roughness alonecannot explain aberrant fouling tendencies.

Earlier studies dealing with the influence of rough-ness on DLVO interactions have found that roughnessprimarily brings the surface energies closer to zero(reduces the magnitude of the interaction). Morespecifically, calculations for interaction energies be-tween rough surfaces near contact reveal that ELand LW interactions are considerably lower thanthey are for corresponding smooth surfaces [22,23].The presence of roughness alone can never inducea complete reversal of the interaction energy, ormore specifically, render the interaction repulsive.The CD–silica adhesive energies in the current in-vestigation, however, change from−7 to +7 mJ/m2

(see Table 4) in the presence of AB interactions.Such an enhanced repulsion could never be solelyattributed to surface roughness. Hence, the foulingbehavior of CD membranes in the presence of silicacould not have been explained by roughness alone.Careful analysis of the surface energetics can, thus,provide considerable insight into the relative impor-tance of polar interactions and physical heterogeneityon the membrane–colloid interactions. In order togain a comprehensive insight into membrane foul-ing, it is, therefore, advisable to simultaneously as-sess the effects of roughness and polar interactions(surface energetics).

4.4. DLVO and XDLVO energy profilesfor membrane–colloid systems

Fig. 3 shows DLVO and XDLVO interaction energyprofiles for the nine membrane–colloid systems. Foreach membrane–colloid combination, Eqs. (11), (13)and (15) were used to calculate the LW, EL, and ABcomponents of the interaction energies. These com-ponents were then added together according to eitherEq. (1) or Eq. (2) to obtain the DLVO or XDLVOenergy, respectively. The horizontal axis (separationdistance) is plotted using a logarithmic scale in orderto emphasize the substantial short-range differencesbetween the DLVO and XDLVO predictions. It can

be noted from Fig. 3 that deviations between theDLVO and XDLVO predictions disappear at largerseparations (>5 nm).

The first observation that can be made is that thebehavior of the membrane–colloid system is gener-ally controlled by the colloid surface energetics. Inother words, no matter what membrane the colloidis paired with, the DLVO and XDLVO curves arequite similar for each colloid. The primary reason forthis behavior is the considerably higher surface ener-gies of the colloids compared to the membranes. Theexception to this is the CD–alumina system, whichhas different DLVO and XDLVO interactions than theFT-30–alumina and CE–alumina systems.

For the silica colloid, the XDLVO approach predictsrepulsion while the DLVO theory predicts attraction.For each membrane–silica combination the AB inter-action component is repulsive due to the hydrophilicnature of the silica colloid (�Gsws = 6.57 mJ/m2).In the case of the polystyrene colloid, the XDLVOapproach predicts much stronger attraction than theDLVO theory. In the membrane–polystyrene systems,the AB interaction component is attractive due to thehighly hydrophobic nature of the polystyrene col-loid (�Gsws = −51.60 mJ/m2). The FT-30–aluminaand CE–alumina systems behave similar to theirpolystyrene counterparts. The attractive AB inter-action component results in the XDLVO approachpredicting stronger attraction than the DLVO theory.However, in the case of the CD–alumina system, thehigh surface energy of the CD membrane results in arepulsive AB interaction. Unlike the polystyrene sys-tem where the strong hydrophobicity of the colloidovercame the hydrophilic tendency of the CD mem-brane, the less strong hydrophobicity of the aluminacolloid does not overcome the hydrophilic tendencyof the CD membrane. Therefore, for the CD–aluminasystem, the XDLVO prediction does not substantiallydeviate from the DLVO prediction.

On the basis of the interaction energy profiles ex-plained earlier, it is fairly straightforward to assess thefouling tendencies of a given membrane in the pres-ence of different colloids. For the membranes, col-loids, and solution chemistry in this investigation, thehydrophilic silica colloid would be expected to foula membrane to a lesser degree than the strongly hy-drophobic polystyrene colloids. It should be noted thatthis difference between silica and polystyrene is not


Fig. 3. DLVO and XDLVO interaction energy profiles for all the membrane–colloid combinations tested (ionic strength= 0.01 M NaCl;pH = 5.6). The rows designate the type of membrane, while the columns correspond to the different colloids. Values for interaction energyat contact were taken from Table 4.

predicted by the DLVO potentials, which predicts sim-ilar fouling propensities for both colloids.

4.5. Implications of DLVO and XDLVO energyprofiles for membrane fouling

Initial attachment of colloids to the membranesurface is controlled by adhesive energies betweenthe colloids and the membrane surface. Over time, amembrane surface eventually becomes covered withcolloids, and consequently, the interaction energybetween approaching colloids and deposited colloidswill govern additional fouling. At this time, fouling

tendencies will be controlled by the energy of co-hesion between approaching and deposited colloidsrather than the energy of adhesion between approach-ing colloids and the membrane surface. Fig. 4 showsDLVO and XDLVO interaction energy profiles foreach colloid–colloid system. Eqs. (11), (13) and (15)were again used to calculate the LW, EL, and ABcomponents of the interaction energies. However,the surface tension characteristics of the membranesurface were replaced with colloid surface tensioncharacteristics. The resulting components were thenadded together according to either Eq. (1) or Eq. (2)to obtain the DLVO or XDLVO energy, respectively.


Fig. 4. DLVO and XDLVO interaction energy profiles forcolloid–colloid combinations (ionic strength= 0.01 M NaCl;pH = 5.6). The cohesive energy represents the interaction be-tween a depositing colloidal particle and a previously depositedcake layer.

It is important to note that EL interactions becomevery important in cohesive interactions because theinteraction is occurring between two surfaces withthe same potential. The result, which can be seen bycomparing Figs. 3 and 4, is strong EL repulsion thatcauses a more repulsive DLVO interaction.

The DLVO and XDLVO predictions of polystyrene–polystyrene interaction energies are similar to thepolystyrene–membrane DLVO and XDLVO pre-dictions. This provides further evidence that thestrong hydrophobicity of the polystyrene colloid (seeTable 3) dominates interactions involving this colloidand results in strong attractive energies. The DLVOand XDLVO predictions of alumina–alumina interac-tion energies show that approaching colloids will beattracted to deposited colloids in generally the samemanner as the approaching colloids are attracted tothe membrane surface. The most significant exceptionto this is the case of the CD–alumina system wherethe hydrophilicity of the CD membrane causes theXDLVO approach to predict a weaker attraction thanthe DLVO theory. Whereas in the alumina–aluminasystem, because the membrane surface energetics nolonger play a role, the XDLVO approach predicts astronger attraction than the DLVO theory. Unlike thepolystyrene and alumina profiles, DLVO and XDLVOpredictions of silica–silica interaction energies showthat approaching silica colloids experience repulsionto the deposited silica colloids. Whereas in all of themembrane–silica cases where DLVO predictions areattractive and XDLVO predictions are repulsive, inthe silica–silica case, the DLVO prediction is weak re-pulsion and the XDLVO prediction is a much strongerrepulsion. The DLVO prediction of weak repulsion isdue to strong EL repulsion caused by the high neg-ative potential of the silica colloid (see Fig. 2). TheXDLVO prediction of strong repulsion in silica–silicainteractions is due to the hydrophilicity of the silicacolloid (see Table 3).

Based on the XDLVO predictions of repulsionbetween the silica colloid and the membrane surfaceand between approaching silica colloids and depositedsilica colloids, the silica colloids would be expectedto have less propensity towards forming a thick foul-ing layer than either the alumina or polystyrene col-loids. Based on the strong adhesion forces predictedbetween the membranes and the polystyrene colloidsand the strong cohesion forces predicted among the


Fig. 5. Normalized permeate flux as a function of time followingcolloid addition to the feed stream for the FT-30 mem-brane and the three colloid solutions (colloid concentration= 1.37×1015 colloids/l; ionic strength= 0.01 M NaCl; pH= 5.6).

polystyrene colloids themselves, the polystyrene col-loids would be expected to form a very thick foulinglayer. However, it is important to note that how theenergetics change with time will also depend on a va-riety of physicochemical parameters such as particleconcentration, applied pressure (which governs thepermeation drag), cross-flow velocity, and electrolyteconcentration.

4.6. Fouling results

Fig. 5 shows the flux decline behavior for the FT-30membrane in the presence of the silica, alumina, andpolystyrene colloids. The normalized permeate fluxfollowing the addition of colloids to the feed stream isplotted as a function of time. Substantial differencesexist in the lag time before losses of permeate fluxwere observed for each of the three colloids. For thepolystyrene colloid, there was a lag time of approx-imately 1.2 h before loss of permeate flux was ob-served; for the alumina colloid there was a lag timeof approximately 1.7 h; and for the silica colloid therewas a lag time of approximately 3 h.

As RO membranes intrinsically have large resis-tance to water flux, a loss of permeate flux will onlybe observed once a sufficiently thick cake layer hasformed on the membrane surface. In the earliest stages,interactions between the colloids and membrane willoccur. However, it is likely that the interactions be-tween the approaching and deposited colloids will ulti-mately control the rate of cake layer growth, andhence, the lag time before permeate flux loss is seen.

Based on the XDLVO predictions for bothmembrane–colloid and colloid–colloid interactions,the polystyrene colloid was expected to have thegreatest fouling propensity and the silica colloid wasexpected to have the least. Indeed, this fouling trendis observed in Fig. 5, where the lag time for the silicacolloid is much higher than that for the polystyrenecolloid and although these results are not a quantita-tive assessment, they provide promising evidence asto the significance of polar interactions on colloidalmembrane fouling.

5. Concluding remarks

A comprehensive analysis of membrane and colloidsurface energetics is a prerequisite for understand-ing the influence of polar (hydrophobic/hydrophilic)interactions on colloidal membrane fouling. It isdemonstrated that specifying the surface energiesof a membrane alone does not necessarily indicatewhether a membrane will be fouling resistant. Rather,it is necessary to assess the adhesive energy betweenthe membrane and the colloid to predict the foulingbehavior. With this understanding, it seems pertinentto focus on means for minimizing colloid–membraneand colloid–colloid interaction energies during mem-brane filtration processes instead of focusing attentiononly on manufacturing “low-energy membranes” anddisregarding the interfacial properties of the foulants.

The XDLVO approach (which considers AB in-teractions) predicts substantially different interactionenergies for several of the membrane–colloid com-binations investigated compared to DLVO predic-tions. Strongly hydrophilic systems (e.g. the silica–membrane combinations) demonstrate that the mono-tonic attraction predicted by the DLVO theory maybecome repulsion when analyzed using the XDLVOapproach. On the other hand, in strongly hydrophobic


systems (e.g. any of the polystyrene systems), theweak DLVO attraction may be significantly magnifieddue to the additional hydrophobic attraction.

On the basis of the DLVO theory, all of themembrane–colloid combinations were predicted tohave a weak attraction, and hence, similar foul-ing tendencies. However, when the AB interactionsare considered in the XDLVO approach, there is amarked difference in the total interaction energies.This suggests, for example, that the fouling behaviorof silica will be substantially different from the foul-ing behavior of polystyrene. Furthermore, based oncolloid–colloid surface energetics, the rate of foul-ing due to silica–silica interactions was predicted tobe substantially lower than the rate of fouling dueto polystyrene–polystyrene interactions. The higherpropensity of fouling for the polystyrene colloidsversus that for the silica colloids was supported bypreliminary fouling results for the FT-30 membraneand three colloidal solutions.

Acknowledgements

The authors acknowledge the support of the U.S.Bureau of Reclamation, Water Treatment Engineer-ing and Research Group under research grant 98-FC-81-0057. The authors thank MenachemElimelech at Yale University for his assistance withelectrophoretic mobility and dynamic light scatteringmeasurements and Marianne Nystrom at Lappeen-ranta University of Technology for her assistance withstreaming potential measurements. The authors wouldalso like to thank Subir Bhattacharjee at the Universityof Alberta for his careful review of this manuscript.

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