In situ surface structure study of polyelectrolyte multilayers byliquid-cell AFM

J.-Luis Menchaca a, Barbara Jachimska b,c, Frederic Cuisinier c, Elıas Perez a,*a Instituto de Fısica, Universidad Autonoma de San Luis Potosı, Alvaro Obregon 64, 78000 San Luis Potosı, SLP, Mexicob Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 1, 30-239 Cracow, Poland

c INSERM U 424, Federation de Recherche Odontologiques, Universite Louis Pasteur, 11 rue Humann, 67085 Strasbourg Cedex, France

Abstract

Liquid cell Atomic Force Microscopy (AFM) is used to image in situ self-assembly polyelectrolyte films (SAPFs). We

show that the technique is appropriate to study surface structure of these systems. In this work, we report images of

layer-by-layer deposition for negative poly(sodium 4-styrenesulfonate) (PSS) and positive poly(allylamine hydrochlo-

ride) (PAH), when an initial layer of positive poly(ethylenimine) (PEI) starts the multilayer polymer film on a glass

surface. This work is addressed to study first layers of SAPFs. Height AFM images are obtained using contact mode at

different pH buffer where polyelectrolytes are initially dissolved. The pH values were: 3.5, 6.8 and 10.5. In all the cases,

the polyelectrolyte film surfaces are not flat, they show rough surfaces with average grain domains ranging from 50 to

90 nm in diameter. The roughness and grain domains slightly grow as a function of deposited layer. In order to

understand the origin of grain structure observed by AFM, size of positive�/negative polyelectrolyte complex was

determined by Dynamic Light Scattering. The results suggest that grains of granulate surface are formed by

polyelectrolyte complexes. In the present work, we also discuss effects of kinetics and preparation on the surface

structure.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Multilayers; Surface structure; Polyelectrolytes; AFM liquid-cell; Polyelectrolyte complexes

1. Introduction

Self-assembly polyelectrolyte films (SAPFs)

formed by alternating adsorption of polycations

and polyanions in aqueous solution on a charged

surface have been introduced as a new method to

modify surfaces [1]. They have some advantages

over self-assembled monolayer (SAM) or

Langmuir�/Blodget monolayer because their for-

mation is limited to the presence of charged

macromolecules and surfaces. It is not the case

for SAM where surfaces are modified by molecules

with terminal groups like alkysilanes [2] or for

Langmuir�/Blodget monolayer, which are made

from amphiphilic molecules at the air�/water inter-

face and then translated to solid surface [3].

Indeed, multilayer polyelectrolyte films have been

used to incorporate organic, inorganic and biolo-

gical compounds in their structure with the condi-

* Corresponding author. Tel.: �/52-444-826-2363x134; fax:

�/52-444-826-1338.

E-mail address: elias@ifisica.uaslp.mx (E. Perez).

Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194

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tion that they should be electrically charged insolution. The incorporation is done introducing

the charged compounds instead the corresponding

polyelectrolyte during the process of SAPFs for-

mation. Examples of these compounds have been

DNA [4], proteins [5], nanoparticles [6] or full-

erenes [7]. These possibilities have evidently shown

SAPFs as a very attractive method to modify

surfaces for several applications [8].In these applications, it is important to know the

structure of SAPFs in order to understand and

control how compounds are incorporated into the

film. However, it is especially important to know

how they are attached on the surface during the

SAPFs buildup. Much effort has been done to

elucidate the internal structure and the growing

rate of these films. Some of the techniquesemployed have been: Scanning Angle Reflectome-

try [9], Optical Waveguide Lightmode Spectro-

scopy (OWLS) [10], Neutron [11] and X-ray [12]

diffraction, Quartz Microbalance [13], and Atte-

nuated Total Reflection Fourier Transform Infra-

red (ATR-FTIR) [14]. The studied polyelectrolytes

have been: poly(ethylenimine) (PEI), poly(allyla-

mine hydrochloride) (PAH), polydiallyldimethyl-ammonium chloride (PDDA), poly(L-lysine)

(PLL), as polycations, and poly(sodium 4-styrene-

sulfonate) (PSS), poly(acrylic acid) (PAA), and

poly(L-glutamic acid) (PLA), as polyions. Never-

theless, there are few techniques to study surface

structure of SAPFs, for example: Scanning Elec-

tron Microscopy (SEM) and Atomic Force Micro-

scopy (AFM). However, they have theinconvenience of requiring pre-treatment of sam-

ples that does not allow to observe the surface

structure in solution, where the process of deposi-

tion occurs. Liquid cell AFM may be an appro-

priated technique to study of surface structure of

SAPFs. AFM has already been used to investigate

different issues of SAPFs: fabrication of micro-

porous films for PAA/PAH by a pH-inducedphase separation [15], salt effect on polyelectrolyte

multilayer film morphology [16], kinetic of film

formation of PAA/PSS on a positive charged SAM

[17], thickness measurements [18], and comparison

of surface structure of polyelectrolyte films for a

linear (PSS/PAH) and exponential (PGA/PLL)

growth regime [19]. However, all the reported

images have been obtained in air after a naturaldrying process or in water solution but after a

transfer from the film preparation devise to the

liquid cell AFM [19]. Therefore, the observed

surfaces are only an approximation of wet sur-

faces, where the real deposition takes place.

In the present paper, we use a liquid cell AFM

where the SAPFs are buildup and are imaged in

situ the polyelectrolyte surface film. For simpli-city, without loss of generality, we have addressed

on this study only the first layers of SAPFs. We

observed a granulate structure on the surface of

SAPFs. We have investigated the dependence of

buffer pH, where polyelectrolytes are initially

dissolved, on the surface structure of SAPFs.

Three pH values were chosen: 3.5, 6.8 and 10.5.

pH 6.8 represents a neutral solution while theother two represent two extreme pH solutions.

Film thickness is an important parameter but it is

very difficult to measure in the AFM liquid cell, as

it is done for dried systems [18], therefore no

attempt was done to determinate it.

We have observed randomly domains similar to

granulate structure on the surface of SAPFs.

Granulate structure has been already reportedwith multilayer films on solid macroscopic sub-

strates [20�/23], on melamin formaldehyde and on

biological cell [24]. However, all these studies were

also done in air where drying process is involved.

There are evidences that granulate structure is

related to conformation of polyelectrolytes in

solution that are employed during film construc-

tion. Grain size, roughness and film thicknessgrow as the salt concentration increases in a

PDDA/PSS system [16]. These results were inter-

preted as a conformation transition of polyelec-

trolytes from extended rod to globular coil in

polymer solution as function of salt concentration

[16]. There are also evidences that polyelectrolyte

complexes are formed inside the multilayer [25].

Neutron reflectometry has shown that two con-secutive layer in the SAPFS are strongly inter-

digitated [11]. The precise form of these

polyelectrolyte complexes inside SAPFs has not

been elucidated.

Granulate structure observed by AFM may be

related to complex size that should be formed if

positive and negative polyelectrolyte were mixed in

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194186

solution. In order to test this idea, we havemeasured the characteristic size of polyelectrolyte

complexes by Dynamic Light Scattering (DLS) at

different concentrations in a 50�/50% wt. of

positive�/negative polyelectrolyte mixing. Evolu-

tion of surface roughness, grain size and grain

number with time was also inspected. Grain size

and roughness decrease with the time and oppo-

sitely grain number increases.

2. Materials and methods

2.1. Material

Polyelectrolyte solutions were prepared in a

buffer solution of Tris(hydroximethyl)amino-methane (TRIS) and 2-(N -morpholino)ethanesul-

fonic (MES) and NaCl were purchased from

Sigma. The buffer was prepared with; 25 mM

TRIS, 25 mM MES and 100 mM NaCl. The pH

was adjusted at 3.5, 6.8 and 10.5 with a HCl and

NaOH solutions. Polymer concentration was 1 mg

ml�1 in all the cases. We used the following

polyelectrolytes: PEI (Mw:/7�/104 Da, Poly-sciences, Warrington, USA), PSS (Mw:/7�/104

Da, Sigma, USA) and PAH (Mw:/7�/104 Da,

Sigma, USA). All products were used as received.

Solutions were prepared with ultra-pure water

(Millipore, USA) with resistivity of 18.2 MVcm�1.

2.2. Atomic Force Microscopy (AFM)

Glass slides of 14 mm of diameter (Micro Cover

Glass, France) were cleaned just before the ima-

ging process. They were put in a sample holder in

vertical position and cleaned using glass-detergent

(Cole-Parmer Instrument Company, USA) and

H2SO4 in water at 1% vol. both at 608 for 15

min in this order. Slides were carefully rinsed with

ultra-pure water at the end of each cleaning step.They were finally dried with nitrogen flux (high

purity). The glass slides were mounted directly on

the AFM piezoelectric.

We drop 50 ml of buffer solution on the glass

slide before to close the cell. It is formed by the

Nanoscope liquid cell accessory, the silicon o-ring

and the glass slide where the multilayer will be

formed. This is schematically shown in Fig. 1. The

liquid cell volume is approximately 28 ml. We

tested the liquid cell AFM taking the first image of

bare glass slide. Buffer solution is then flowed

from an open syringe, with a height of 10 cm (see

Fig. 1). The flux was of 0.25 ml s�1 in this

geometry. PEI is first and once injected, after

rinsing PSS is injected and next PAH, in this order.

Polymers are injected and let 15 min in the liquid

cell AFM. Between each polyelectrolyte solution,

rinsing is done injecting five times 1 ml of solution

of buffer. We can calculate the buffer volume used

to rinsing the SAPFs formed in our liquid cell

AFM. The relationship is: 1 ml versus 0.028 ml, we

rinse with 36 times the volume of cell for 4 s in five

occasions, which gives a total of 180 times the

volume of liquid cell AFM. This is an efficient way

to rinse multilayer buildup. Images are taken 15

min after last buffer injection. We have imaged

each layer from the first PSS layer until the second

PAH or third PSS layer. Nanoscope III (Digital

Instrument Santa Barbara, CA, USA) was used

and silicon nitride tips MLCT-AUHW were pur-

chased from Park Scientific (USA). Cantilevers

with spring constant of 0.01 N m�1 are used. Few

days before to image surfaces, AFM tips were

silanized with octadecyltrichlorosilane (OTS) to

transform the hydrophilic AFM tip into hydro-

phobic one. The AFM tips were placed close to

Fig. 1. Schematic representation of the AFM liquid cell set up

utilized in this work. Buffer solution was always present during

multilayer building. Injection container was 10 cm high. If it

was less than 10 cm it does not allow buffer circulation while

higher that 10 cm cause buffer draining through the joining of

the o-ring.

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194 187

OTS solution contained in a beaker and both werekept in a glass closed-container overnight. OTS

solution was prepared with 5 ml of hexadecane,

five drops of carbon tetrachloride (CCl4), and five

drops of OTS. Height images were captured in

AFM contact mode. All images reported here were

taken at 1 Hz with resolution of 512�/512 pixels.

Images at 10�/10, 5�/5 and 4�/4 mm2 were

obtained. We will mainly discuss the 5�/5 and4�/4 mm2, where surface structure is better ob-

served. Surface roughness of the film was obtained

from the RMS value given by Nanoscope software

that is defined by RMS2�(1=N)aNj (z(j)�z0)2

where z(j) is the height of point j , z0 is the average

height and N is the number of z values used

(512�/512). Grain size and grain number were

also obtained using Grain Size program fromNanoscope software. The threshold limit was

half of correlation depth. The program gives grain

size in area units. It is transformed in linear

dimension assuming equivalent circular areas.

2.3. Dynamic Light Scattering (DLS)

DLS apparatus is formed by: a multiple tau

digital real time correlator ALV 6010/16, a singlephoton detector ALV/SO-SIPD, a goniometer

Brookhaven BI-200SM and an argon-ion laser of

800 mW, emitting at 488 nm. Detection angle was

fixed at 908. Light scattering by polyelectrolyte

complexes was in general low and the maximum

laser intensity was used. We prepared a first

complex solution from solutions of PSS and

PAH polyelectrolytes at 1 mg ml�1 in a 50�/50%wt. proportion. A turbid solution was immediately

observed. Six subsequent complex solutions were

obtained from the original one in the following

way: 2 ml from the first solution were diluted in 4

ml of buffer to obtain the second solution and

same process was followed to obtain the third

solution from the second one and so on. The last

solution was of 0.02 mg ml�1. The solutions were

prepared 15 days before the analysis to let themreach the equilibrium. DLS requires of refraction

index and viscosity of solution to determinate

complex size. We have assumed that these values

correspond to the buffer. Data where analyzed

using a second cumulant method assuming a

spherical form of polyelectrolyte complexes.

3. Results

We have decided to investigate pH buffer

influence on SAPFs surface structure. Three pH

values were chosen: 3.5, 6.8 and 10.5. pH 6.8

represents a neutral solution while the other two

represent extreme pH solutions. Buffer at pH 3.5

has preferentially dissociated hydronium cations

H� while that pH 10.5 has preferentially disso-ciated hydroxyde anions OH�. These extreme pH

solutions do not represent real buffer solutions,

because the TRIS�/MES range of pH is limited.

However, the SAPFs are very sensitive to these

H� and OH� ion excess, as we will show. Layer-

by-layer polyelectrolyte deposition at pH 10.5 is

shown in Fig. 2. SAPFs have been denoted by PEI,

PEI�/PSS, PEI�/(PSS/PAH) depending on deposi-tion sequence. The images start with the bare glass

surface, next the first layer of PSS. PEI layer is not

shown here because image was not systematically

captured due to AFM tips-film sticking problems.

Height and friction AFM images were obtained,

however, we report only the height images because

friction images have similar information. We can

observe more clearly the randomly domains orgranulate structure at the end of the series but

there are already present at beginning of process.

Fig. 3 shows only the first and the last layer of

multilayer process at pH 3.5 (left side) and 6.8

(right side). Granular structure is again presented

but it is less defined in comparison with such one

at pH 10.5. At pH 3.5 the first PEI polyelectrolyte

was not able to be adsorbed onto the glass surface,

Fig. 2. 4�/4 mm2 height AFM images during multilayer buildup at pH 10.5. First image corresponds to flat glass surface.

Polyelectrolyte films are shown from (PEI/PPS) layer to PEI�/(PSS/PAH)2�/PSS layer. In the last image the granulate structure is more

compact and well defined. Images are presented at the same z scale of 20 nm to better appreciate the evolution.

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194188

Fig. 2

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194 189

probably due to neutralization of negative glass

surface by hydronium cations H�. PEI was

injected at pH 6.8 to induce PEI adsorption, rinsed

at pH 3.5, and then the multilayer was built up at

pH 3.5. This SAPF was successfully built-up with

this starting layer. Two parameters are obtained

from these images: roughness and lateral dimen-

sion of the grains. Roughness does not have a

systematic variation and the RMS values are very

close to each other. The SAPF deposited at pH 6.8

show big agglomerates, which did not allow us to

calculate grain size. We observe a slight increase of

grain size during deposition. The average grain

size, for pH 3.5 and 6.8, is ranged from 50 to 90

nm.

In order to compare grain size obtained by

AFM we determinate complex sizes at different

concentrations of a 50�/50% wt. PSS�/PAH com-

plex by DLS. We have not measured PEI com-

plexes because this polymer is used only once at

the beginning of the SAPFs buildup and the

molecular size is very similar to positive PAH

polyelectrolye. The diameter of PSS/PAH poly-

electrolyte complex obtained by DLS observes a

constant value independent of polyelectrolyte

complex concentration, this for the three pH

Fig. 3. 4�/4 mm2 height AFM images corresponding at beginning and end of multilayer buildup at pH 3.5 (right) and 6.8 (left).

Granulate structure is present in both cases but it is less defined than at pH 10.5 case. Images are presented at the same z scale (20 nm).

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194190

values. In order to compare AFM and DLS

results, we used the average grain size for eachpH and the averaged diameter obtained by DLS.

They are reported in Table 1. The polydispersity

meausred by DLS is not an artifact of the

technique. Proper solutions and enough correla-

tion time were employed but large polydispersity

was obtained systematically for all our measures.

4. Discussion

To our knowledge, this is the first time that

liquid cell AFM is employed to analyze in situ

multilayer buildup. The observed granulate sur-face structure was previously reported in the

literature [20�/25] but was imaged in air after a

drying process. It is easy to show that observed

grain domains are too larger to correspond to one

polyelectrolyte complex. Assuming a gaussian

chain, the complex square diameter is given by

d2�/nl2, where n is the number of unities compos-

ing the complex chain and l is the length of theseunities. We assume that a positive and a negative

chain form complex chain and then, in our case, n

is approximately equal to 140,000/200�/700 and l

to 0.25 nm, this gives d :/7 nm. We can assume

other kind of complex configuration but the

calculate complex size is of the same the order of

magnitude as calculated here [24]. It is thus

legitimate to research the origin of the granulatedomain surface structure.

Our results indicate that roughness is not very

sensible to the number of multilayer, at least at the

beginning of SAPFs. Roughness had a little

variation (less than 3 nm) and without clear

correlation with pH. This is different for SAPFs

observed in air where it has been observed thatfilm roughness grow (see for example Fig. 7 in Ref.

[16]). Our results are closer to such obtained by

Lavalle et al. [19]. They have observed a slightly

increasing of roughness until a plateau at around 4

nm after PEI�/(PSS/PAH)5. In our case, grain size

shows a slightly growth tendency for pH 3.5 and

10.5. This low grain size increasing is surprising

because after one looking we have the impressionthat grain size is growing faster. This result is

consequence of the multilayer formation. We

realize it by counting the number of grains used

to determinate the grain size: it decreases as the

multilayer is built-up: 619 grains for first PSS layer

and 216 for the third PSS layer at pH 10.5; 718

grains for first PAH layer and 552 for the third

PAH layer at pH 3.5. This decreasing allowsupposing initial multiple grains that coalesce to

form flat large grains, as function of the SAPF

formation. The above difference in grain number

is not correlated with a grain size increasing. This

show that grains not only coalesce but also could

disappear.

DLS results reveal that grain size observed by

AFM are in the same order of magnitude ofpolyelectrolyte complexes obtained by mixing

50�/50% in weight of negative PSS and positive

PAH polyelectrolyte (see Table 1). The size

depends on pH values but it is independent of

polyelectrolyte concentration. It is again far to

represent only one polyelectrolyte complex. These

results indicate that electrical forces are bigger

than thermic and dispersive ones and that theinitial complex formed at the beginning of complex

preparation is stable in the dilution process. These

results suggest that granulate structure observed

by AFM is related to polyelectrolyte complexes.

The film structure is very difficult to image at

pH 6.8 (Fig. 3, right side) and the grain size

analysis of images is not able to calculate indivi-

dual grain size due to agglomerates. At pH 10.5individual grains are more visible and grain size is

measurable. To understand such observations, we

have followed the evolution of surface structure

with time at pH 6.8. Fig. 4(A) shows a multilayer

PEI�/(PSS/PAH)2 at pH 6.8 after 15 min. We let

the film for 4 h in the AFM cell and then we

capture a new image. We observe an appreciable

Table 1

Polyelectrolyte 50�/50% PSS�/PAH complex diameters ob-

tained by AFM (in the height mode using grain size program,

threshold limit was half of image depth) and DLS

pH\diameter (nm) Height AFM DLS

3.5 629/09 3769/120

6.8 �/ 2589/58

10.5 809/10 2999/70

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194 191

change on the surface structure, shown in Fig.

4(B). Roughness and grain size analysis gives not

to far values for both images (Fig. 4(A): 3.7 and

133 nm; Fig. 4(B): 3.26, 93 nm), however, grain

number change from 166 (Fig. 4(A)) to 350 (Fig.

4(B)). The surface structure at pH 6.8, shown in

Fig. 3, is not well defined due the relative short

time of the imaging process. After rinsing we wait

15 min before imaging and we spend until 60 min

before to inject the next polymer. Apparently this

15 min is not enough to have the complete

formation of the new SAPFs surface. Fig. 4

suggests that in the first 15 min complexes are

formed, during the next 4 h new complexes are

formed on the surface due to diffusive process and

grain diameter is decreasing. The opposite evolu-

tion of grain size (decreasing with the time) and

grain number (increasing with the time) indicates

that the number of polymer chains involved on

surface complexes is constant. This result indicates

that films at pH 6.8 after 15 min is not totally

mature. They progress to more compact films and

images shown in Fig. 3 (right side) represent a state

of development. At pH 10.5 polymer diffusion is

faster than at pH 6.8 or 3.5 and the film acquire

quickly its definitive granular surface structure.

Kinetic effects have been already studied for a

system (PSS/PAH) deposited on a positive-

charged surface [17], it is observed that a very

short time (1�/2 min) PSS polyelectrolyte is in-

homogeneously adsorbed on select sites but a

longer deposition time (10�/30 min) polyelectrolyte

forms a more homogenous film. In our case, the

kinetics is slower and we need 4 h to observe a

homogenous distribution of grain on the surface.

This is not however the situation for pH 10.5, in

fact, this is the opposite situation of pH 3.5 where

PEI was not able to adsorb on the glass negative

surface. At pH 10.5 there are favorable conditions

to occur a rapid PEI adsorption.

The influence of relaxation time and rinsing is

critical on film surface in our in situ system. We

have made a comparative study with an alternative

SAPFs preparation method proposed in Ref. [19],

where SAPFs are prepared outside the liquid cell

but imaged inside liquid cell AFM. Fig. 5(A)

shows the film of PEI�/(PSS/PAH) prepared in

this form at pH 6.8. On this figure, we observe

some agglomerates whose average size is 60 nm in

diameter, which is comparable to average size

reported in Ref. [19] and also to grain size

observed at pH 3.5 and 10.5. The film perhaps

acquires this surface structure because of a non-

efficient buffer rinsing between each polyelectro-

lyte deposition and of a partial drying film during

transfer to the liquid cell AFM. In order to induce

Fig. 4. Kinetic effects on multilayer formation. (A) corresponds to PEI�/(PSS/PAH)2 film at pH 6.8. (B) represents the same film but it

was captured after 4 h. There is evidently a contribution due to film formation kinetics. Roughness, grain size and grain number are:

(A) 3.7 nm, 133 nm and 166; (B) 3.26 nm, 93 nm and 350 (Images of 5�/5 mm2).

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194192

an adsorption of these agglomerates onto the

surface we practice a drying�/wetting process for

this film. Fig. 5(B) shows the same film of Fig.

5(A) after 10 h in air and then hydrated before

imaging. We observe smaller agglomerates that

have been integrated onto surface given an image

with granulate structure. Comparison of rough-

ness, grain size and number before and after

drying (respectively, Fig. 5(A): 6.38 nm, 60 nm

and 1617; Fig. 5(B): 3.30 nm, 51 nm and 1978)

show clearly a strong kinetic effect induced by the

drying�/wetting process. First, the film after wet-

ting is flatter, represented by the low roughness,

grain size goes down while the number of grain

grows. It is important to note that the grain

structure observed in Fig. 5(A) is similar to Fig.

4(B), after 4 h of relaxation. We can observe that

drying�/wetting process induce grain size reduction

and increasing in grain number, in very similar

way as kinetic effect observed from Fig. 4(A) to

Fig. 4(B). However, experiments made by OWLS

on same polyelectrolyte films have shown a

stability of film thickness and optical refraction

index after 15 min of last rising at pH 7.4 [9]. This

indicates that the strong effects observed for film

surface are not related to a modification of film

thickness and structure density. Kinetics of poly-

electrolyte films is evidently an interesting subject

of study in this field.

5. Conclusions

We have presented a new methodology to image

in situ SAPFs buildup. Set up consists in liquid cell

AFM with one entry and one exit tubing to injectpolyelectrolyte solutions and buffer, it allows

rinsing in a very efficient way. Layer-by-layer

polyelectrolyte multilayer formation is imaged

with this methodology. We have studied the first

layers for a PSS/PAH system when a PEI layer

covers a negative glass surface. We have observed

a granulate structure on these polyelectrolyte films

at different pH: 3.5, 6.8 and 10.5. Roughness andgrain size have been measured using Nanoscope

software. We have observed a small roughness

variation with apparent no correlation to deposed

layer, for all the three series, and the grain

domains slightly grow as the multilayer is formed

at pH 3.5 and 10.5. Size grain analysis was not

Fig. 5. (A) shows a film PEI�/(PSS/PAH) prepared as Ref. [19]. Apparently, the agglomerates on the surface are consequence of a non-

efficient rinsing process. (B) shows the same film but under a drying�/wetting process the structure is very similar to reported in our

work. Drying was of 10 h. Roughness, grain size and grain number are: (A) 6.38 nm, 60 nm and 1617; (B) 3.30 nm, 51 nm and 1978

(Images of 5�/5 mm2).

J.-L. Menchaca et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 185�/194 193

possible at pH 6.8 because films were not wellformed. It is due to short time (between 15 and 60

min) used to image these surfaces, we have

observed that for pH 6.8, 4 h are enough to

observe a well defined granulate structure indicat-

ing a clear kinetic effect in the SAPFs formation.

Buffer at pH 10.5 is a good condition for the

SAPF formation on negative surfaces. We have

measured size complex by DLS and observed thatthis is dependent of pH value but independent of

the concentration of polyelectrolyte complex. It is

also in the same order of magnitude of grain size

observed by AFM. These results suggest that

grains on the surfaces are formed by polyelec-

trolyte complex. Finally, we have observed that

surface structure is very sensitive to the prepara-

tion methods. The film surface is continuouslyrearranging, the kinetics of this rearranging is

related to pH, time and drying process. The main

outcome of this work is the great variability of the

SAPFs surface. Any preparation method, even in

situ method [19] modifies the surface structure.

Further studies about SAPFs have to take into

account film roughness and rearranging kinetics.

Acknowledgements

This work was supported by the ‘Biomolecular

Material Project’ CONACYT-Mexico (ER026)

and by Mexico�/France program: SEP-CONA-

CYT-ANUIES-ECOS France (M01-S01). J.-L.

Menchaca and E. Perez thank Dr Jaime Ruiz-Garcıa, UASLP-Mexico, for the AFM facilities.

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