Sacrificial polyelectrolyte multilayer coatings as an ...tarabara/docs/pubs...IPC-N 4). Four...

© 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

1

Sacrificial polyelectrolyte multilayer coatings as an approach to

membrane fouling control: Disassembly and regeneration

mechanisms

Accepted for publication in the Journal of Membrane Science on April 18, 2015.

Published article DOI: 10.1016/j.memsci.2015.04.041

Pejman Ahmadiannamini†, Merlin L. Bruening‡, Volodymyr V. Tarabara†*

† Department of Civil and Environmental Engineering, Michigan State University, East

Lansing, MI 48824, USA

‡ Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA

* Corresponding author:

[email protected]; phone: (517) 432-1755; fax: (517) 355-0250

http://dx.doi.org/10.1016/j.memsci.2015.04.041


2

Abstract

This study evaluates polyelectrolyte multilayers (PEMs) as sacrificial separation layers

on polysulfone ultrafiltration (UF) membranes. Exposure to surfactants and a swing in

pH can disassemble PEMs by disrupting non-ionic and electrostatic bonds within the

PEM and between the PEM and the support. Trends in frequency and dissipation in

quartz crystal microbalance studies confirm layer-by-layer (LbL) adsorption of PEMs on

the quartz crystal and subsequent PEM removal in response to acid/base treatment.

After disassembly of PEMs on UF membranes, water permeability increases and

methylene blue rejection decreases, in some cases reaching values close to those for

pristine ultrafilters. PEM removal occurs even with fouled membranes. After fouling by

aqueous solutions of bovine serum albumin and alginate, the PEM disassembly via

exposure to acid, base and surfactant results in a 99% decrease in the hydraulic

resistance of the PEM (compared to the unfouled PEM) and, on average, more than

80% recovery of pure water permeability (compared to an uncoated UF membrane).

Repeated cycles of PEM disassembly and readsorption give stable water permeabilities

and methylene blue rejections for the coated membranes.

Keywords:

polyelectrolyte multilayers; fouling; sacrificial films; disassembly; regeneration


3

1. Introduction

Membrane-based separations have made remarkable inroads against competing

technologies in applications such as water desalination and reuse, and gas separations

[1, 2]. Significant advantages of membrane processes may include relatively low energy

consumption and a small environmental footprint [3, 4]. Nevertheless, membrane fouling

remains a major challenge that increases the energy costs for separations and limits

membrane lifetime [5]. Managing membrane fouling requires substantial knowledge

and experience, especially because fouling is often feed-specific. In most applications,

periodic hydraulic cleaning limits short-term fouling, whereas more expensive chemical

cleaning partly removes the hydraulically-irreversible fraction of the added resistance

due to foulants [5-7]. With the increasing importance of water reuse and reliance on

low-quality water sources of high fouling propensity, development of new cleaning

approaches and anti-fouling membrane materials becomes even more important [8-12].

Unfortunately, the impact of surface morphology control and chemical modifications that

improve fouling resistance will not endure after formation of the first fouling layer on the

membrane surface. Ultimately, once chemically irreversible fouling exceeds a certain

level, membrane replacement is required.

This work investigates the possible use of PEMs as sacrificial coatings that a membrane

may shed along with foulants that cannot be removed by hydraulic cleaning. LbL

assembly of PEMs is attractive for coating membranes due to its conceptual simplicity

and control over film thickness, composition and surface charge [13, 14]. Thus, a


4

number of studies investigated PEMs as membrane skins for nanofiltration [15-21],

reverse osmosis [22, 23], forward osmosis [24] and pervaporation [25]. As with other

specially designed antifouling surfaces, however, PEMs may temporarily resist

adhesion of feed components, but eventually a fouling layer will coat the surface and

negate antifouling properties. Thus, we desire to develop conditions for removing PEMs

from porous substrates prior to adsorption of a new PEM to create a fresh membrane

surface. In this regard, previous studies employed surfactants [26-28] as well as

changes in pH [21, 22], temperature [29] and ionic strength [30, 31] to disassemble

PEMs. Hydrolysis [32], solvent exchange [33], electrochemical methods [34] and

enzymes [35] may also alter PEMs, but such methods may be difficult to apply in

membrane modules.

Previously, we studied the feasibility of regeneration of (PSS/PAH)4 and (PSS/PAH)4.5

PEMs fouled by SiO2 colloids. To regenerate a PEM with a layer of colloids adsorbed

on its surface, we employed a three-step procedure: (i) backflushing with deionized (DI)

water, (ii) soaking in a buffer solution at pH 10, and (iii) PEM re-deposition by the LbL

method [16]. Although the regenerated membranes showed permeabilities similar to

those of the initial PEM-modified membranes, the rejection was 50% lower than that

with the initial PEM. Apparently, residual polyelectrolytes or foulants decreased

rejection, perhaps by creating film defects, but they did not add significant hydraulic

resistance.


5

This study aims to develop PEM design and disassembly approaches that afford

membrane regeneration for nearly complete recovery of flux and rejection after

hydraulically-irreversible fouling. We assembled PEMs using poly(vinylsulfonic acid,

sodium salt) (PVS), poly(sodium 4-styrenesulfonate) (PSS), deprotonated poly(acrylic

acid) (PAA) and deprotonated poly(methacrylic acid) (PMAA) as polyanions; and

poly(diallyldimethylammonium chloride) (PDADMAC), poly(allylamine hydrochloride)

(PAH) and protonated chitosan (Chi) as polycations. Quartz crystal microbalance with

dissipation monitoring (QCM-D) was employed to examine PEM assembly and

disassembly kinetics on gold or silicon oxide surfaces, whereas reflectance FTIR

spectroscopy demonstrated film removal from model spin-coated polyethersulfone

substrates. Filtration studies confirmed that PEM-removal approaches restore

membrane flux and rejection to values close to those of uncoated UF support

membranes. Sequential acid/base and surfactant treatments effectively removed PEMs

to give a membrane ready for adsorption of a new polyelectrolyte (PE) coating. Finally,

we investigated whether removal methods are effective in disassembling PEMs fouled

with bovine serum albumin (BSA) and alginate.


6

2. Materials and Methods

2.1. Reagents

PVS (25 wt.% in H2O); PSS (Mw∼70,000) ; PAA (Mw∼1,800); PMAA (Mw∼9,500,

sodium salt solution, 30 wt.% in H2O); PDADMAC (Mw∼100,000-200,000, 20 wt.% in

H2O), PAH (Mw∼15,000), Chi (medium Mw), BSA, alginate, calcium chloride dihydrate

(CaCl2 · 2H2O) and sodium chloride (NaCl) were purchased from Sigma-Aldrich. HCl

solution (36.5% HCl in water, EMD, Millipore), NaOH (Macron) and Triton X-100

(Sigma-Aldrich) were used to prepare PEM disassembly solutions.

Ethylenediaminetetraacetate (EDTA, Sigma-Aldrich) and sodium dodecyl sulfate (SDS,

Roche) served as components of aqueous cleaning solutions for NF 270 membranes

[36]. DI water used in all experiments was supplied by a commercial ultrapure water

system (Lab Five, USFilter) equipped with a terminal 0.2 µm microfilter (PolyCap,

Whatman Plc); the water resistivity was greater than 16 MΩcm.

2.2. Quartz crystal microbalance with dissipation (QCM-D) measurements

A four-channel quartz crystal microbalance with dissipation (QCM-D, Q-Sense E4) was

employed for real-time studies of PEM adsorption and removal processes. Silicon

oxide- or gold-coated quartz crystals with a resonance frequency of 4.95 MHz ± 50 kHz

served as substrates for PEM deposition. Crystals were cleaned according to standard

protocols provided by the manufacturer (see Supplementary Content (SC) file, section


7

S1). PEMs were prepared via the LbL technique by flowing alternating polyanion and

polycation solutions through the QCM-D chamber using a peristaltic pump (Ismatec

IPC-N 4). Four bilayers were deposited for each PE pair by pumping 1 g/L PE solutions

in DI water through the QCM-D chamber and subsequently rinsing with DI water after

each PE deposition step at a flow rate of 0.15 mL/min. PE deposition and rinsing steps

were allowed to proceed until QCM frequency and dissipation reached constant values.

PE solutions contained no added salt, and the pH of the PEM solutions and the pKa

values (when known) of the corresponding PEs were as follows: PAA (pH=3.2, pKa=4.2-

6.5 [37, 38]), PMAA (pH=9.2, pKa=5.5 [39]), PSS (pH=5.4, pKa≈1.0 [40]), PVS (pH=6.5),

Chi (pH=6.0, pKa=6.5 [41]), PAH (pH=4.0, pKa=8.5-9.5 [38, 40]), PDADMAC (pH=4.7).

PEMs were then removed from the substrate by exposing them to HCl (1 M) or NaOH

(1 M) solutions or both. The solutions were pumped through the chambers until the

QCM frequency and dissipation reached constant values. All frequency shifts and

dissipation factors are based on data recorded for the 5th overtone.

2.3 PEM adsorption on membranes and filtration tests

PEMs were adsorbed on PES UF membranes (Pall) with a molecular weight cutoff

(MWCO) of 30 kDa. A small subset of tests was also done using PES membranes with

a MWCO of 10 kDa. A PES membrane was placed in a stirred dead-end filtration cell

(Sterlitech, membrane area of 14.6 cm2), and DI water was filtered through the

membrane at 5 bar until a steady-state flux was achieved. A total of 15 mL of PE


8

solution (1 g/L) was then poured into the cell and stirred for 5 min. The solution was

discarded and the membrane was rinsed thoroughly with DI water. The PE-adsorption

and rinsing steps were repeated to form four PE bilayers on the PES substrate surface.

PE solutions contained no added salt and the pH of the solutions was not adjusted.

In filtration experiments, membrane coupons were first compacted by filtering DI water

at a transmembrane pressure of 5 bar until a steady permeate flux was attained. The

permeate flux was calculated based on the time-derivative of the permeate mass, which

was measured by collecting the filtered water on a digital balance (AV8101C, Ohaus)

interfaced with a computer. Methylene blue (MB) served as a rejection probe. To

evaluate rejection, the filtration cell was filled with 300 mL of 6 mg/L aqueous MB, and

the feed solution was filtered under fast-stirring conditions (~1300 rpm) to minimize

concentration polarization. In total, 30 mL of solution were filtered in each experiment

and the first 20 mL was discarded to eliminate the effect of MB adsorption on rejection

data. The MB concentration in the permeate was determined using UV-Vis

spectrophotomery (MultiSpec 1501-Shimadzu). To account for an increase in the feed

concentration during the filtration of the first 30 mL of solution, the rejection was

calculated based on a simple differential equation (see SC, section S4).

The membrane in the cell was then exposed to 1 M HCl or 1 M NaOH or both

sequentially, for 15 min each, to remove the PEM coating. Subsequent exposure to

Triton X-100 solutions (2 wt% in water) for 15 min was also employed in some cases to

disrupt non-ionic interactions. During the backflush treatment, the membrane was


9

flipped over in the filtration cell and a surfactant solution was directed from the permeate

side of the membrane to PES-PEM interface at the transmembrane pressure of 5 psi. DI

water flux and MB rejection measurements were repeated for the cleaned membranes.

The efficiency of PEM removal was determined by comparing the DI water permeate

flux and MB rejection values of the support PES membrane, the same membrane

coated with a PEM film, and the same membrane after PEM removal.

To study the effectiveness of the removal methods applied to fouled membranes, dead-

end filtration was performed using a feed solution that contained BSA (0.3 g/L), alginate

(0.3 g/L), NaCl (20 mM) and CaCl2 (10 mM). Disassembly of fouled PEM-coated

membranes was attempted as described above, and DI water flux was measured to

quantify the efficiency of the disassembly methods.

2.4 Reflectance Fourier transform infrared (FTIR) spectroscopy

PES (17 mg) was dissolved in 10 mL of CH2Cl2 and 0.1 mL of this solution was placed

onto Au-coated Si wafers (1.1 × 2.4 cm2) for spin coating at 500 rpm [42]. Using the

same PE solutions and conditions described above for membrane modification, PEMs

were deposited by the LbL technique on the PES-coated wafers. Reflectance FTIR

spectra of PEMs on PES were acquired using a Thermo Nicolet 6700 FTIR

spectrometer equipped with a PIKE grazing angle (80°) attachment. A wafer cleaned

with a combination of UV and ozone served as the IR background, and absorbance

from PES (see SC, section S2) was subtracted from all spectra.


10

3. Results and Discussion

This study aims to exploit PEMs as sacrificial coatings and enable detachment of fouling

layers that accumulate during filtration and cannot be fully removed by hydraulic

cleaning. The subsections below describe our strategy for PEM removal, QCM-D

studies that demonstrate PEM loss, MB nanofiltration before and after PEM

disassembly to validate the PEM removal on membranes, and finally detachment of

PEMs after filtration of protein and polysaccharide solutions to show that the strategy

applies to fouled membranes.

3.1 Strategy for removal of PEMs

Layer-by-layer adsorption of polyelectrolytes typically occurs through multiple

electrostatic interactions, although non-ionic interactions sometimes help stabilize PEMs

[43-45]. However, when the first PE layer and the substrate possess charges of the

same sign, the initial adsorption occurs only due to non-ionic bonding. Strategies for

removal of PEMs from substrates must take into account the forces that stabilize films

and bind them to the substrate (Fig. 1).

For PEMs containing a weak-base polycation or a weak-acid polyanion, exposure to

high or low pH solutions promotes disassembly by decreasing the degree of PE

ionization and limiting the extent of electrostatic cross-linking in the film [19, 46, 47].

Nevertheless, pH changes will not remove the initial layer from the substrate when


11

Figure 1: Mechanisms operative in the assembly of PEMs and their attachment to a

supporting substrate in two cases: a) The first PE layer has the same

charge as the substrate; and b) The first PE layer has a charge opposite

to that of the substrate. For clarity, the diagram does not show the

overlap that occurs over several layers or the release of charge-

compensating counterions that helps to drive adsorption [48-50].

adsorption occurs primarily through non-ionic interactions. In this case desorption will

require disruption of non-ionic bonds, for example through exposure of the PE-substrate

interface to a surfactant. With membranes, this can occur during crossflow filtration of a

surfactant solution through the PEM-coated membrane or, when the PEM is

impermeable to the surfactant, by a backflush with the same solution. Figure 2

summarizes the disassembly strategy employed in this study when the first PE layer

and support have charges of the same sign. Note that surfactant alone may be

insufficient to remove a highly ionically cross-linked film from the surface without

acid/base treatment.


12

Figure 2: Proposed PEM disassembly algorithm for the case when the first PE layer

and support have charges of the same sign.

3.2. Examination of film formation and disassembly on a quartz crystal microbalance

Figure 3 shows QCM-D data for the assembly and disassembly of (Chi/PAA)4 on a

QCM-D crystal coated with silicon oxide. After introduction of PE solutions into the

QCM-D chamber, the crystal oscillation frequency decreased and then plateaued,

indicating successful PE adsorption. Frequency drops after both Chi and PAA

adsorption increased exponentially (𝑟2 = 0.9973) with the number of adsorbed layers

(-28, -97, -284 and -765 Hz for 1, 2, 3 and 4 bilayers, respectively), consistent with prior

studies that showed exponential growth for low charge-density PEs such as Chi [51].


13

Figure 3: QCM-D data for adsorption and disassembly of a (Chi/PAA)4 multilayer on a SiO2-coated crystal.

Chi, PAA, HCl, and NaOH labels denote times when solutions of the corresponding compounds

were introduced into the QCM-D chamber. The rinse label points to times when the QCM-D

chamber was rinsed with DI water. Note: The data set presented in Fig. 3 shows a PEM removal of ~97%, which is lower than the 100% we observed in most

other experiments. This specific data set was selected to facilitate graphical illustration of ∆𝜈2.


14

After depositing each PE layer, the QCM-D crystal was rinsed with DI water to remove

loosely bound PEs from the surface. As expected, the oscillation frequency increased

during rinsing, especially in the case of PEMs that included PAA.

We attempted to disassemble PEMs containing weak polyanions or weak polycations

through treatments with dilute HCl and NaOH solutions, respectively. We did not

employ surfactants in QCM-D tests because the interaction with the silica surface of the

QCM-D crystal should be primarily electrostatic. Because Chi is a weak polybase and

PAA is a weak polyacid, electrostatic interactions in (Chi/PAA)4 should decline when the

PEM is exposed to either HCl or NaOH. Figure 3 shows a dramatic increase in

frequency for the (Chi/PAA)4-modified QCM-D crystal after exposure to HCl, clearly

demonstrating removal of most of the film from the substrate. Subsequent exposure to

NaOH followed by rinsing decreases the frequency to nearly the same level as for a film

with only a single PAA layer. We note that the SiO2 coating on the crystal might be

unstable in 1 M NaOH. However, the oscillation frequency returned to nearly its original

value, suggesting minimal SiO2 dissolution.

We carried out the same experiments with PEMs built from several different PE pairs

(see the SC file for QCM-D frequency shifts for these PEMs (Figures S-1 – S-12) as

well as dissipation data (section S2)). Table 1 summarizes QCM-D data on the

efficiency of PEM removal, :

=∆𝜈1−∆𝜈2

∆𝜈1100% ,

(1)


15

where ∆𝜈1 and ∆𝜈2 are QCM frequency shifts (relative to DI water prior to PEM

deposition) before and after PEM disassembly, respectively (see Figure 3 for an

example).

Table 1: Efficiency of PEM disassembly, (wt% removed) in response to sequential

treatment by acid/base solutions as measured in QCM-D experiments with

4-bilayer PEMs.

Notes: * weak PE

** strong PE

# PEMs assembled on Au-coated QCM-D crystals; all other PEMs were

assembled on SiO2-coated crystals

↕ Based on one QCM-D test

Polyanion

Polycation

Chi * PAH * PDADMAC **

PAA * 99.4 ± 1.3 92.3 ± 3.0 94.3 ± 4.4

PMAA * 95.4 # ↕ 86.0 ± 4.9 82.4 ± 9.0

PSS ** 84.3 ± 13.9 # 86.5 ± 5.9 7.5 ± 3.2

PVS ** 94.5 ± 7.8 72.1 ± 6.1 17.8 ± 11.3

Among the PEMs, removal was highest for (PAA/Chi)4, which contains two weak PEs.

The high water solubilities of Chi and PAA and the very low MW of PAA may also lead to

the essentially 100% disassembly of films containing these polymers. Moreover, all the

films that contained at least one weak PE had removal efficiencies higher than 72%. As

expected, disassembly was least effective with PEMs composed of two strong PEs:


16

(PSS/PDADMAC)4 and (PVS/PDADMAC)4. In these cases, a change in pH had little

effect on ionization and removed on average 20% or less of the mass of the PEMs.

We note that, ideally, the surface for QCM-D tests should be the same as the surface of

the UF supports for PEM adsorption. Unfortunately, such an ideal model is not feasible.

Spin-coating a QCM-D crystal with polysulfone would produce a non-porous polymer

layer that would interact with the 1st PE layer differently than a porous layer of the UF

support’s skin. In principle, one can attempt to replicate the phase inversion process to

prepare a UF polysulfone membrane on the surface of a QCM-D crystal. While the

desired surface morphology might form, the membrane would be too thick (300 μm)

and, therefore, too heavy to serve as a coating for a QCM-D crystal. In addition, the

small size of the crystal would lead to boundary effects, and insufficient post-processing

(i.e. no possibility to pre-compact the membrane or to flow water through the membrane

to flush out the porogen) would make true replication impossible.

3.3. Filtration before and after PEM disassembly

To initially assess disassembly of a range of PEMs on membranes, we measured DI

water flux and MB rejection before and after PEM disassembly by the acid/base

treatment. Figure 4 shows results of these tests with the data plotted in the order of

decreasing DI water permeate flux values for membranes with as-assembled PEMs.

Adsorption of PEMs on the UF substrates decreased permeability to DI water and

increased MB rejection.


17

Figure 4: DI water flux (top) and MB rejection (bottom) of PEM-coated 30 kDa PES

membranes before (dark gray) and after (light gray) PEM removal. For

comparison the figure shows data for pristine PES membranes at the far

left (hashed gray). All PEMs were disassembled by sequential treatment

with acid and base solutions.


18

Upon PEM disassembly, flux and rejection did not return to the levels typical of the

unmodified UF support suggesting incomplete removal of the PEM. In the few cases

where disassembly conditions decreased flux, e.g. (PAH/PMAA)4, or increased

rejection, e.g. (PDADMAC/PMAA)4, the acid and base likely induce conformational

changes in the film without removing sufficient PE to enhance permeability [52].

Flux recovery after PEM disassembly (flux after disassembly divided by the flux through

the pristine membrane) does not correlate directly with the extent of polyelectrolyte

removal because both the PEM and the underlying PES membrane provide resistance

to flow. Thus, we employed a resistance-in-series model to determine the resistances

of the UF substrate, the PEM and residual material left on the membrane after PEM

removal. These data allow definition of the resistance removal efficiency (𝜀𝑅), defined

as follows:

𝜀𝑅 = 1 −𝑅𝑟𝑒𝑠

𝑅𝑃𝐸𝑀 ,

(2)

where 𝑅𝑃𝐸𝑀 and 𝑅𝑟𝑒𝑠 are hydraulic resistances of the PEM and residual film left after

PEM removal, respectively. The resistance removal efficiency is indicative of the extent

of PEM removal, so we also term it the PEM removal efficiency.

Table 2 provides values for both flux recovery after PEM removal and 𝜀𝑅. Note that for

(Chi/PAA)4 (top entry in the table), even though flux recovery is only 54%, the PEM

removal efficiency (i.e. resistance removal efficiency) is 98%.


19

Table 2: Recovery and regeneration of PEM properties (DI water permeate

flux and rejection) in response to four methods of PEM disassembly.

PEM

MWCO of PES support,

kDa

PEM removal method Flux recovery,

%

PEM removal

efficiency 𝜀𝑅, % M1 M2 M3 M4

1

PE

Ms w

ith a

poly

catio

n a

s t

he 1

st l

ayer

(Chi/PAA)4

30

x x 54 98

2 (Chi/PMAA)4 x x 32 0

3 (Chi/PSS)4 x x 54 78

4 (Chi/PVS)4 x x 22 25

5 (PAH/PAA)4 x x 82 100

6 (PAH/PMAA)4 x x 39 14

7 (PAH/PSS)4 x x 52 -8

8 (PAH/PVS)4 x x 90 86

9 (PDADMAC/PAA)4 x x 18 90

10 (PDADMAC/PMAA)4 x x 17 -28

11

PE

Ms w

ith a

poly

anio

n a

s t

he 1

st l

ayer

(PAA/Chi)4

30 x x 42 ± 7* 96.5 ± 0.3*

12

10

x x 59 ± 9* 98.7 ± 0.5*

13 x x x 94 99.8

14 (PSS/Chi)4 x x 86 96.5

15 (PSS/PDADMAC)4

x 56 75.5

16 x 95.9 98.2

17 (PAA/Chi)4/fouled† x x x 110 ± 16* 100.3 ± 0.5*,‡

18 (PSS/Chi)4/fouled† x x 80 ± 15* 97.8 ±2.2*,‡

19 (PSS/PDADMAC)4/fouled† x 93 ± 12* 99.6 ± 0.5*,‡

Notes: M1 = exposure to high pH; M2 = exposure to low pH; M3 = exposure to Triton X-100;

M4 = backflush with Triton X-100.

* Uncertainties represents standard deviations based on sets of 3 replicate measurements

performed with different membranes. Other data in the table were obtained in experiments

with only one membrane for each PE pair.


20

† Prior to PEM removal, these membranes were fouled by filtering solutions containing BSA,

alginate, and Ca2+. ‡ Note that in the calculations of εR

(i) for fouled membranes, RPEM represents the hydraulic

resistance of fouled PEM membranes.

This occurs because 𝑅𝑃𝐸𝑀 is very high compared to the resistance of the untreated

membrane (see the SC, section S3, for a discussion of the series resistance model).

Similarly, for (PAH/PAA)4, the flux recovery is only 82%, but the resistance recovery

rounds to 100%. All other PE pairs show lower removal efficiencies. Thus, of all the PE

pairs, (Chi/PAA)4 and (PAH/PAA)4 are the best choices for disassembly by acid/base

treatment and regeneration. However, the DI water flux through membranes coated

with (PAH/PAA)4 is very low, so the work that follows focuses on membranes coated

with (Chi/PAA)4. The (Chi/PAA)4 system also showed the highest removal efficiency in

QCM-D studies (Table 1), and these films on membranes show relatively high MB

rejections. We note that zero or negative 𝜀𝑅 values in Table 1 represent membranes

whose flux does not increase after a disassembly treatment. A pH swing or exposure to

surfactant may induce PEM conformational changes that are not accompanied by

sufficient removal of PEs to decrease PEM resistance, so 𝜀𝑅 is not positive.

3.4. Effects of substrate MWCO on PEM disassembly and regeneration

To understand how the pore size of the substrate affects PEM removal, we examined

assembly and disassembly of (Chi/PAA)4 films on PES substrates with MWCOs of 10

kDa and 30 kDa. Further, to study the durability of membranes under chemical

treatments and whether the PEM slowly builds up on the membrane, we repeated the


21

assembly-disassembly cycle three times (Fig. 5). Flux and rejection data show similar

trends for filtration through PEMs prepared on both 10 kDa and 30 kDa PES supports,

although flux is lower and rejection higher with the 10 kDa support. However,

disassembly does not lead to complete flux recovery with either membrane, suggesting

partial blocking of pores by residual PEs. The incomplete PEM removal even on the 10

kDa membrane suggests that a mechanism other than physical entrapment retains

some PE on the membrane. In particular, non-ionic interactions may prevent

detachment of the first PE layer from the PES substrate. Nevertheless, the 𝜀𝑅 values in

Table 2 (see entries 11 and 12) suggest that a higher membrane MWCO may result in

greater polyelectrolyte entrapment and lower removal efficiency. Most importantly,

fluxes and rejections with the PEM-coated membranes are similar for the initial film and

films formed after one or more regeneration cycles (i.e. disassembly-buildup), although

both flux and rejection decline after the second disassembly and third film-formation

step.


22

(a)


23

(b)

Figure 5: DI water permeate flux and MB rejection data during filtration of a 17.5 μM

MB solution through (Chi/PAA)4 films on the surface of (a) 30 kDa PES

membranes and (b) 10 kDa PES membranes. (Each error bar represents


24

a standard deviation for a set of 3 replicate measurements with different

membranes.)

3.5. Disassembly with surfactants

Surfactants may disrupt non-ionic interactions between the PES substrate and PEs to

enable more complete PEM removal [26-28]. However, if both electrostatic and non-

ionic interactions immobilize the initial PE, removal of this layer may require

simultaneous treatment with base/acid and surfactant rather than sequential treatments.

Thus, to prevent electrostatic interactions between the PES membrane, which carries a

negative surface charge, and the first PE layer, we employed a polyanion as the initial

layer in PEM-coated membranes on a PES substrate with a 10 kDa MWCO. After

treating the (PAA/Chi)4-modified membrane with low and high pH solutions, we filtered a

surfactant solution through the membrane for 15 min and reexamined its filtration

properties. Although the pH treatment results in only 59 ± 9% recovery of the DI water

permeate flux through the bare membrane (Table 2, entry 12), subsequent treatment

with surfactant leads to 94% flux recovery (Table 2, entry 13). The MB rejection

decreased from 99% to 55% after PEM removal by pH swing and to 36% after PEM

removal by pH swing and surfactant. Note that MB rejection by the PEM-free UF

membrane (10 kDa MWCO) is 31%, or nearly the same as by the membrane after PEM

disassembly.

We also examined the effectiveness of surfactant treatment for the disassembly of

(PSS/Chi)4, a PEM that contains one strong PE. For these films, pH affects only the


25

ionization of the weakly basic Chi, so the disassembly procedure included sequential

treatment with NaOH (no HCl) and surfactant to break electrostatic and non-ionic

bonds, respectively. DI water filtration through (PSS/Chi)4-coated PES membranes

showed flux recoveries (relative to the bare PES membrane) of 28% after treatment with

NaOH and 86% after subsequent treatment with surfactant (Table 2, entry 14). MB

rejection decreased from 73% for (PSS/Chi)4-coated membranes, to 54% after

treatment with NaOH and 34% after subsequent surfactant filtration. Clearly, surfactant

treatment enhanced the PEM removal.

Finally, we studied surfactant-induced disassembly of membranes composed of two

strong PEs (PSS and PDADMAC). Because pH does not affect the charge density of

strong PEs, the disassembly procedure only included exposure to the surfactant.

Remarkably, after surfactant treatment, flux recovery was 96% (Table 2, entry 16) while

MB rejection decreased from 48% to 18%, a rejection slightly lower than that of even

the pristine membrane. We speculate that in this case, the PEM film remains integral

and is “peeled off” the substrate.

3.4. Investigation of PEM disassembly on PES-coated wafers using FTIR spectroscopy

(PAA/Chi)4, (PSS/Chi)4 and (PSS/PDADMAC)4 films were also prepared on PES-

coated wafers to enable corroborating FTIR spectroscopy studies of PEM removal from

a surface similar to that of PES membranes (although without pores). Figures 6 (a-c)

present reflectance FTIR spectra for the three PEMs before and after different


26

disassembly treatment steps. In the spectrum of (PAA/Chi)4 (Fig. 6a), peaks at ~1550

and ~1400 cm−1 most likely stem from the PAA asymmetrical and symmetrical –COO-

stretches, respectively.


27

Figure 6: FTIR spectra of PEMs prepared on Au-coated silicon wafers

modified with spin-coated PES and treated with various steps in the

PEM buildup/disassembly sequence: after PEM adsorption, after pH

treatment, and after surfactant treatment. The PEMs were a)

(Chi/PAA)4; b) (PSS/PAA)4; c) (PSS/PDADMAC)4.

The peak at 1720 cm−1 results from the acid carbonyl stretch, and the absorbance at

1457 cm−1 is a -CH2 band. Most importantly, these peaks disappear after either HCl or

NaOH treatment, and the spectrum is featureless after the NaOH treatment, implying

complete film removal.

The spectra of PEMs containing PSS show absorbance peaks at 1127, 1036 and 1009

cm-1, which correspond to sulfonic acid group (SO3-) stretches (Fig. 6b and Fig. 6c).

With PSS/Chi films, these peaks disappear after treatment with NaOH. (We did not

treat PSS/Chi films with HCl because they do not contain a weak acid.) However, the

surfactant treatment alone did not appear to completely remove the films from PES-

coated wafers. In contrast to filtration results, the spectra of (PSS/PDADMAC)4 coatings

suggest no removal after surfactant treatment. This could be due to the high density of

the (PSS/PDADMAC)4 films, which does not allow the surfactant molecules to diffuse

through the PEM and reach the substrate-PEM interface to break the non-ionic bonds

between substrate and first PE layer. Note that surfactant treatment for the

(PSS/PDADMAC)4 membrane was performed via back flushing or filtration to bring the

surfactant to the PEM-substrate interface.


28

Figure 7: DI water flux through (1) 10 kDa PES membranes, (2) NF 270 or PEM-

modified PES membranes, (3) NF270 or PEM-coated membranes fouled by

a solution of BSA and alginate, (4) hydraulically cleaned NF270 or PEM-

coated membranes, (5) PES membranes after PEM removal, (6) PES

membranes with re-deposited PEMs or chemically cleaned NF 270. (a)


29

(Chi/PAA)4, (b) (PSS/PAA)4, (c) (PSS/PDADMAC)4 and (d) NF 270. PEM

removal protocols are described in Table 2, entries 17-19. Notes:

1) Each error bar represents a standard deviation for a set of 3 replicate measurements with

different membranes.

2) DI water fluxes for PAA/Chi, PSS/Chi, PSS/PDADMAC and NF 270 membranes are 1.67·10-

11, 4.44·10-11, 4.67·10-11, 3.17·10-11 m/s/Pa (6.0, 16.0, 26.8 and 11.4 LMH/bar), respectively.

3.5. Removal of fouled PEMs

We also examined removal of fouled PEMs and whether a second PEM deposition

gives the same separation properties as the originally deposited coating. These

experiments included filtering mixtures of BSA and alginate through (PAA/Chi)4-,

(PSS/Chi)4- and (PSS/PDADMAC)4-coated PES membranes, application of PEM

removal protocols, coating the cleaned substrate with a new PEM, and reexamining

filtration performance. Figure 7 shows that in all cases, removal of the fouled PEM

(chemical treatment column) led to DI water fluxes within ±10% of those through the

bare membrane. Adsorption of a fresh PEM gave fluxes within ± 20% of those through

the initial PEM-coated membrane. However, we should note that fouling is partly

hydraulically reversible as indicated by the significant (73 ± 19 %) flux recovery

observed for the NF 270 membrane. Treatment of fouled NF 270 with a cleaning

solution containing 1 mM EDTA and 10 mM SDS at pH 10 resulted in complete flux

recovery (115 ± 15%).


30

3.6. Limitations and possible applications of the proposed approach

The proposed PEM removal procedures may not be compatible with some membrane

materials. For example, cellulose acetate and, to a lesser extent, its di- and tri-acetate

derivatives hydrolyze at high and low pH. However, PEMs can adsorb to almost any

material and serve as sacrificial coatings on polymeric or ceramic membranes that are

stable over a wide range of pH. This work focused on surface fouling, but for

membranes with very large pores PEMs may serve as sacrificial coating to mitigate

intrapore fouling. When pores are too large to be bridged by PEs, their internal pore

surface can be coated. Perhaps the biggest limitation of PEMs is the multistep removal

and film formation process. Simplification of the deposition procedure will likely be

necessary to make the regeneration process economical. Although polyelectrolyte

redeposition may require more than one hour, replacement of only the top layers of a

film [27, 28] could shorten the process.

The removal strategy has several potential advantages. First, regenerating a

membrane’s selective layer in situ can allow for on-demand assembly of a membrane

with desired surface and separation properties. Second, regeneration of the selective

layer is an alternative method of managing irreversible fouling; this method may remove

hydraulically-irreversible fouling as a substitute for or in addition to chemical cleaning.

Third, the membrane can be “replaced” without changing the support layer (UF

membrane) or the membrane module, potentially leading to cost savings.


31

4. Conclusions This work examined controlled disassembly of PEMs on several substrates and

explored the use of sacrificial PEMs as an approach to managing membrane fouling.

On the surface of QCM crystals, exposure to 1 M HCl or 1 M NaOH or both effectively

removed most (70-100%) of the mass of PEMs composed of weak PEs. In contrast,

PEMs made of only strong PEs were stable under these conditions. For the same

PEMs assembled on the surface of PES UF membranes (MWCO 10 kDa and 30 kDa),

exposure to 1 M NaOH and 1 M HCl did not completely restore permeate flux to the

value of a PEM-free support. A subsequent treatment with a surfactant successfully

removed PEMs consisting of weak/weak, weak/strong and strong/strong PE pair

combinations and restored >86% of the flux, presumably by disrupting non-ionic

interactions between the PES support and PEs that remain adsorbed after exposure to

low or high pH or both. If a PEM contains strong PEs only, the PEM can be removed

from the substrate by disrupting non-ionic bonding between the substrate and the first

PE layer by a surfactant treatment. The proposed method is also efficient in removing

PEMs that have been fouled by a mixture of BSA and alginate.


32

Acknowledgements

This work was supported in part by the NSF Partnerships for International Education

and Research Grant IIA-1243433.


33

List of Figures

Figure 1: Mechanisms operative in the assembly of PEMs and their attachment to a

supporting substrate in two cases: a) The first PE layer has the same

charge as the substrate; and b) The first PE layer has a charge opposite

to that of the substrate. For clarity, the diagram does not show the

overlap that occurs over several layers or the release of charge-

compensating counterions that helps to drive adsorption [48-50].

Figure 2: Proposed PEM disassembly algorithm for the case when the first PE layer

and support have charges of the same sign.

Figure 3: QCM-D data for adsorption and disassembly of a (Chi/PAA)4 multilayer on

SiO2-coated crystal. Chi, PAA, HCl, and NaOH labels denote times where

solutions of the corresponding compounds were introduced into the QCM-

D chamber. The rinse label points to times when the QCM-D chamber

was rinsed with DI water.

Note:

The data set presented in Fig. 3 shows a PEM removal of ~97%, which is lower than the

100% we observed in most other experiments. This specific data set was selected to

facilitate graphical illustration of ∆𝜈2.

Figure 4: DI water flux (top) and MB rejection (bottom) of PEM-coated 30 kDa PES

membranes before (dark gray) and after (light gray) PEM removal. For

comparison the Figure shows data for pristine PES membranes at the far

left (hashed gray). All PEMs were disassembled by the sequential

treatment with acid/base solutions.

Figure 5: DI water permeability and MB rejection data during filtration of a 17.5 μM

MB solution through (Chi/PAA)4 films on the surface of (a) 30 kDa PES

membranes and (b) 10 kDa PES membranes. (Each error bar represents

a standard deviation for a set of 3 replicate measurements with different

membranes.)

Figure 6: FTIR spectra of PEMs prepared on Au-coated silicon wafers modified with

spin-coated PES and treated with various steps in the PEM

buildup/disassembly sequence: after PEM adsorption, after pH treatment,

and after surfactant treatment. The PEMs were a) (Chi/PAA)4; b)

(PSS/PAA)4; c) (PSS/PDADMAC)4.


34

Figure 7: DI water permeate flux through (1) 10 kDa PES membrane, (2) NF 270 or

PEM-modified PES membrane, (3) NF270 or PEM-coated membranes

fouled by a solution of BSA and alginate, (4) hydraulically cleaned NF270

or PEM-coated membranes, (5) PES membranes after PEM removal, (6)

PES membranes with re-deposited PEMs or chemically cleaned NF 270.

a) (Chi/PAA)4, b) (PSS/PAA)4, c) (PSS/PDADMAC)4 and NF 270. PEM

removal protocols are described in Table 2.

Notes:

1) Each error bar represents a standard deviation for a set of 3 replicate

measurements with different membranes.

2) DI water fluxes for PAA/Chi, PSS/Chi, PSS/PDADMAC and NF 270

membranes are 1.67·10-11, 4.44·10-11, 4.67·10-11, 3.17·10-11 m/s/Pa (6.0, 16.0,

26.8 and 11.4 LMH/bar), respectively.


35

List of Tables

Table 1: Efficiency of PEM disassembly, (wt% removed) in response to sequential

treatment by acid/base solutions as measured in, based on QCM-D

experiments with measurements for films 4-bilayer PEMs composed of 4

bilayers.

Notes: * weak PE

** strong PE

# PEMs assembled on Au-coated QCM-D crystals; all other PEMs were

assembled on SiO2-coated crystals

↕ Based on one QCM-D test

Table 2: Recovery and regeneration of PEM properties (DI water permeate flux and

rejection) in response to the four methods of PEM disassembly.

Notes: M1 = exposure to high pH; M2 = exposure to low pH; M3 = exposure to Triton

X-100;

M4 = backflush with Triton X-100.

* Uncertainties represents standard deviations based on sets of 3 replicate

measurements performed with different membranes. Other data in the table

were obtained in experiments with only one membrane for each PE pair. † Prior to PEM removal, these membranes were fouled by filtering solutions

containing BSA, alginate, and Ca2+. ‡ Note that in the calculations of εR

(i) for fouled membranes, RPEM represents

the hydraulic resistance of fouled PEM membranes.


36

References

[1] V. Abetz, T. Brinkmann, M. Dijkstra, K. Ebert, D. Fritsch, K. Ohlrogge, D. Paul, K.-V. Peinemann, S. Pereira-Nunes, N. Scharnagl, Developments in membrane research: from material via process design to industrial application, Adv. Eng. Mater., 8 (2006) 328-358.

[2] E.M.V. Hoek, V.V. Tarabara, Encyclopedia of Membrane Science and Technology, Volume 3: Applications, John Wiley & Sons, 2013.

[3] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science, 333 (2011) 712-717.

[4] S. Judd, The MBR book: principles and applications of membrane bioreactors for water and wastewater treatment, 2nd ed., Elsevier, 2010.

[5] AWWA, Committee Report: Recent Advances and Research Needs in Membrane Fouling, J. Am. Water Works Ass., 97 (2005) 79-89.

[6] P. Le-Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci., 284 (2006) 17-53.

[7] G.L. Amy, M.M. Clark, J. Pellegrino. NOM rejection by, and fouling of, NF and UF membranes, Project report, American Water Works Association Research Foundation, 2001.

[8] M. Elimelech, Z. Xiaohua, A.E. Childress, S. Hong, Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes, J. Membr. Sci., 127 (1997) 101-109.

[9] I. Pinnau, B.D. Freeman, Formation and modification of polymeric membranes: overview, in: Membrane Formation and Modification, American Chemical Society, 1999, pp. 1-22.

[10] V. Kochkodan, D.J. Johnson, N. Hilal, Polymeric membranes: surface modification for minimizing (bio)colloidal fouling, in: Adv. Colloid Interface Sci., 2014, pp. 116-140.

[11] C.-C. Ho, A.L. Zydney, Overview of fouling phenomena and modeling approaches for membrane bioreactors, Sep. Purif. Technol., 41 (2006) 1231-1251.

[12] Y. Ye, P.L. Clech, V. Chen, A.G. Fane, Evolution of fouling during crossflow filtration of model EPS solutions, J. Membr. Sci., 264 (2005) 190-199.


37

[13] N. Joseph, P. Ahmadiannamini, R. Hoogenboom, I.F.J. Vankelecom, Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation, Polym. Chem., 5 (2014) 1817-1831.

[14] Q. Zhao, Q.F. An, Y. Ji, J. Qian, C. Gao, Polyelectrolyte complex membranes for pervaporation, nanofiltration and fuel cell applications, J. Membr. Sci., 379 (2011) 19-45.

[15] P. Ahmadiannamini, X. Li, W. Goyens, B. Meesschaert, I.F.J. Vankelecom, Multilayered PEC nanofiltration membranes based on SPEEK/PDDA for anion separation, J. Membr. Sci., 360 (2010) 250-258.

[16] W. Shan, P. Bacchin, P. Aimar, M.L. Bruening, V.V. Tarabara, Polyelectrolyte multilayer films as backflushable nanofiltration membranes with tunable hydrophilicity and surface charge, J. Membr. Sci., 349 (2010) 268-278.

[17] R. Malaisamy, A. Talla-Nwafo, K.L. Jones, Polyelectrolyte modification of nanofiltration membrane for selective removal of monovalent anions, Sep. Purif. Technol., 77 (2011) 367-374.

[18] B.W. Stanton, J.J. Harris, M.D. Miller, M.L. Bruening, Ultrathin, multilayered polyelectrolyte films as nanofiltration membranes, Langmuir, 19 (2003) 7038-7042.

[19] P. Ahmadiannamini, X. Li, W. Goyens, N. Joseph, B. Meesschaert, I.F.J. Vankelecom, Multilayered polyelectrolyte complex based solvent resistant nanofiltration membranes prepared from weak polyacids, J. Membr. Sci., 394-395 (2012) 98-106.

[20] P. Ahmadiannamini, X. Li, W. Goyens, B. Meesschaert, W. Vanderlinden, S. De Feyter, I.F.J. Vankelecom, Influence of polyanion type and cationic counter ion on the SRNF performance of polyelectrolyte membranes, J. Membr. Sci., 403-404 (2012) 216-226.

[21] L. Ouyang, R. Malaisamy, M.L. Bruening, Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations, J. Membr. Sci., 310 (2008) 76-84.

[22] W. Jin, A. Toutianoush, B. Tieke, Use of polyelectrolyte layer-by-layer assemblies as nanofiltration and reverse osmosis membranes, Langmuir, 19 (2003) 2550-2553.

[23] J. Xu, X. Feng, C. Gao, Surface modification of thin-film-composite polyamide membranes for improved reverse osmosis performance, J. Membr. Sci., 370 (2011) 116-123.

[24] P. Pardeshi, A.A. Mungray, Synthesis, characterization and application of novel high flux FO membrane by layer-by-layer self-assembled polyelectrolyte, J. Membr. Sci., 453 (2014) 202-211.


38

[25] G. Zhang, W. Gu, S. Ji, Z. Liu, Y. Peng, Z. Wang, Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures, J. Membr. Sci., 280 (2006) 727-733.

[26] X. Huang, A.B. Schubert, J.D. Chrisman, N.S. Zacharia, Formation and tunable disassembly of polyelectrolyte-Cu2+ layer-by-layer complex film, Langmuir, 29 (2013) 12959-12968.

[27] J. Irigoyen, N. Politakos, R. Murray, S.E. Moya, Design and fabrication of regenerable polyelectrolyte multilayers for applications in foulant removal, Macromol. Chem. Phys., 215 (2014) 1543–1550.

[28] J.I. Ramos, I. Llarena, S. Moya, Selective removal of hybrid di-block polyelectrolyte multilayers by means of quaternary ammonium surfactants, J. Membr. Sci., 45 (2010) 4970-4976.

[29] R. Steitz, V. Leiner, K. Tauer, V. Khrenov, R.v. Klitzing, Temperature-induced changes in polyelectrolyte films at the solid-liquid interface, Appl. Phys. A Mater. Sci. Process, 74 (2002) s519-s521.

[30] N. Wang, G. Zhang, S. Ji, Z. Qin, Z. Liu, The salt-, pH- and oxidant-responsive pervaporation behaviors of weak polyelectrolyte multilayer membranes, J. Membr. Sci., 354 (2010) 14-22.

[31] A.A. Antipov, G.B. Sukhorukov, H. Möhwald, Influence of the ionic strength on the polyelectrolyte multilayers' permeability, Langmuir, 19 (2003) 2444-2448.

[32] Y. Zhu, J. Shi, W. Shen, X. Dong, J. Feng, M. Ruan, Y. Li, Stimuli-responsive controlled drug release from a hollow mesoporous silica sphere/polyelectrolyte multilayer core-shell structure, Angew. Chem., 117 (2005) 5213-5217.

[33] Y. Lvov, A.A. Antipov, A. Mamedov, H. Möhwald, G.B. Sukhorukov, Urease encapsulation in nanoorganized microshells, Nano Lett., 1 (2001) 125-128.

[34] R. Zahn, J. Vörös, T. Zambelli, Swelling of electrochemically active polyelectrolyte multilayers, Curr. Opin. Colloid Interface Sci., 15 (2010) 427-434.

[35] V.H. Orozco, V. Kozlovskaya, E. Kharlampieva, B.L. López, V.V. Tsukruk, Biodegradable self-reporting nanocomposite films of poly(lactic acid) nanoparticles engineered by layer-by-layer assembly, Polymer, 51 (2010) 4127-4139.

[36] Q. Li, M. Elimelech, Organic fouling and chemical cleaning of nanofiltration membranes: measurements and mechanisms, Environ. Sci. Technol., 38 (2004) 4683-4693.

[37] C. Echeverria, N.A. Peppas, C. Mijangos, Novel strategy for the determination of UCST-like microgels network structure: effect on swelling behavior and rheology, Soft Matter, 8 (2012) 337-346.


39

[38] S.W. Cranford, C. Ortiz, M.J. Buehler, Mechanomutable properties of a PAA/PAH polyelectrolyte complex: rate dependence and ionization effects on tunable adhesion strength, Soft Matter, 6 (2010) 4175-4188.

[39] J. Zhang, N.A. Peppas, Synthesis and characterization of pH- and temperature-sensitive poly(methacrylic acid)/poly(N-isopropylacrylamide) interpenetrating polymeric networks, Macromolecules, 33 (1999) 102-107.

[40] S.R. Lewis, S. Datta, M. Gui, E.L. Coker, F.E. Huggins, S. Daunert, L. Bachas, D. Bhattacharyya, Reactive nanostructured membranes for water purification, PNAS, 108 (2011) 8577-8582.

[41] J. Mao, S. Kondu, H.-F. Ji, M.J. McShane, Study of the near-neutral pH-sensitivity of chitosan/gelatin hydrogels by turbidimetry and microcantilever deflection, Biotechnol. Bioeng., 95 (2006) 333-341.

[42] P. Jain, J. Dai, S. Grajales, S. Saha, G.L. Baker, M.L. Bruening, Completely Aqueous Procedure for the Growth of Polymer Brushes on Polymeric Substrates, Langmuir, 23 (2007) 11360-11365.

[43] F. Caruso, H. Lichtenfeld, E. Donath, H. Möhwald, Investigation of electrostatic interactions in polyelectrolyte multilayer films: binding of anionic fluorescent probes to layers assembled onto colloids, Macromolecules, 32 (1999) 2317-2328.

[44] J.B. Schlenoff, S.T. Dubas, Mechanism of polyelectrolyte multilayer growth: charge overcompensation and distribution, Macromolecules, 34 (2001) 592-598.

[45] P.T. Hammond, Recent explorations in electrostatic multilayer thin film assembly, Curr. Opin. Colloid Interface Sci., 4 (1999) 430-442.

[46] J.B. Schlenoff, H. Ly, M. Li, Charge and mass balance in polyelectrolyte multilayers, J. Am. Chem. Soc., 120 (1998) 7626-7634.

[47] S.S. Shiratori, M.F. Rubner, pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes, Macromolecules, 33 (2000) 4213-4219.

[48] G. Ladam, P. Schaad, J. Voegel, P. Schaaf, G. Decher, F. Cuisinier, In situ determination of the structural properties of initially deposited polyelectrolyte multilayers, Langmuir, 16 (2000) 1249-1255.

[49] R.v. Klitzing, Internal structure of polyelectrolyte multilayer assemblies, Phys. Chem. Chem. Phys., 8 (2006) 5012-5033.

[50] P.A. Neff, A. Naji, C. Ecker, B. Nickel, R. v. Klitzing, A.R. Bausch, Electrical detection of self-assembled polyelectrolyte multilayers by a thin film resistor, Macromolecules, 39 (2006) 463-466.


40

[51] C. Picart, J. Mutterer, L. Richert, Y. Luo, G. Prestwich, P. Schaaf, J.-C. Voegel, P. Lavalle, Molecular basis for the explanation of the exponential growth of polyelectrolyte multilayers, PNAS, 99 (2002) 12531-12535.

[52] R.A. Ghostine, R.M. Jisr, A. Lehaf, J.B. Schlenoff, Roughness and salt annealing in a polyelectrolyte multilayer, Langmuir, 29 (2013) 11742-11750.

Sacrificial polyelectrolyte multilayer coatings as an ...tarabara/docs/pubs...IPC-N 4). Four...

Documents

Transcript of Sacrificial polyelectrolyte multilayer coatings as an ...tarabara/docs/pubs...IPC-N 4). Four...