Contact angles and external protein adsorption onto UF membranes

Contact angles and external protein adsorption onto UF membranes

L. Palacioa,*, J.I. Calvoa, P. PraÂdanosa, A. HernaÂndeza, P. VaÈisaÈnenb, M. NystroÈmb

aDept. TermodinaÂmica y FõÂsica Aplicada, Fac. Ciencias, Univ. Valladolid, 47071 Valladolid, SpainbLaboratory of Technical Polymer Chemistry, Lappeenranta University of Technology, PO Box 20, 53851 Lappeenranta, Finland

Received 17 April 1998; received in revised form 3 June 1998; accepted 2 July 1998

Abstract

Both receding and advancing contact angles of pure water and BSA solutions against the active layers of several retentive

ultra®ltration membranes are studied here. In particular, three membranes from desalination systems, whose active layers are

made of aromatic polyamides: G5, G20 and G50 have been chosen.

It has been shown that these membranes are hydrophilic with equal advancing contact angles against pure water while the

receding ones decrease with increasing pore radii.

Finally, both the contact angles against the BSA solutions are studied at different pH and concentrations. It is shown that the

stationary advancing contact angles correspond to the early adsorption steps while receding ones correspond to the ®nal

adsorption level. When analyzed within this frame, the results obtained can easily be explained in terms of the qualitatively

known molecule±molecule and molecule±membrane surface interactions. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Contact angles; Protein adsorption; Surface roughness; Pore size; Ultra®ltration membranes

1. Introduction

In membrane technology, a problem usually

encountered is the fouling of the membranes. The

nature of the forces or interactions causing solute

deposition and eventual fouling is discussed in litera-

ture. In any case it is undoubtful that solute deposition

can be caused by chemical binding, hydrophobicity of

the membrane, electrostatic attraction or short range

forces as Van der Waals forces, etc. The extent to

which these forces are present depends on both the

particles in solution and the membrane material.

Deposition should depend, for instance, on solution

pH and ionic strength because the solute molecules

should change their charged state as a consequence of

modi®cations in their solution environment.

The aim of this work is to study adsorption of

bovine serum albumin (BSA) on UF membranes by

investigating how surface properties of membranes

and proteins affect adsorption. Electrical and chemical

properties of the membranes have also been studied

previously, as well as porosity, in order to understand

this interaction and to have as complete as possible a

characterization of the membranes.

During the adsorption of external particles on the

membrane surface, the hydrophilicity of the porous

material increases if the adsorbed molecules are more

hydrophilic than the material or decrease otherwise.

Journal of Membrane Science 152 (1999) 189±201

*Corresponding author. Tel.: +34-83-423013; fax: +34-83-

423134.

0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 3 7 6 - 7 3 8 8 ( 9 8 ) 0 0 2 0 3 - 8

These changes in hydrophilicity of membrane surfaces

due to protein adsorption can be studied using contact

angle methods, [1±4], given that hydrophobic surfaces

should lead to high contact angles.

Until now, some contact angle measurements for

®lms and material surface characterization have

been used. Gregoris et al. [5] studied polymers and

hydrogel surfaces by contact angle procedures due

to their biological interest. As indicative of surface

heterogeneity, Cain et al. [6] used dynamic contact

angles on smooth and rough surfaces. The interest of

contact angles in membrane science and technology is

noticeable; Gekas et al. [7] used contact angle experi-

ments in food technology. Hydrophilic membranes

have also been characterized by Oldani and Schock

[8].

On the other hand, different techniques have been

used to measure contact angles: captive bubble, sessile

drop, Wilhelmy method, etc. Among them, a modi®-

cation of the Wilhelmy method seems to be the most

easy and meaningful technique. Some details of the

method are being intensively studied as, for instance,

the in¯uence of the penetration velocity [9], contact

angle hysteresis in¯uence [10], and adequate data

acquisition [11,12].

Here, a new way to measure contact angles is

proposed; namely studying the advancing angle after

the receding one for a membrane immersed into a

protein solution in order to analyze both, the initial and

®nal, protein adsorption processes. This will be done

at different pH and concentrations to interpret the

results according to the expected or qualitatively

known interactions.

2. Contact angle measurements

Hydrophobicity of a solid material is usually

expressed in terms of a contact angle (�) or a surface

tension ( ), and although they are closely related it is

necessary to distinguish between them. Surface ten-

sion describes the interface forces between two

phases, and contact angle describes the edge of the

two-phase boundary where it ends at a third phase.

Hence two phases must be speci®ed to describe sur-

face tension, while three are needed to describe con-

tact angle [8]. Our aim is to study this last parameter

which is given by the Young equation in thermo-

dynamic equilibrium

cos � � SV ÿ SL

LV

; (1)

where SV and LV are the surface tensions of the solid

and the liquid with the vapor of the liquid and SL is

the tension of the solid±liquid interface.

The most widely used method (sessile drop) is a

direct measurement of � on a liquid drop deposited on

a surface, where the angle is determined by construct-

ing a tangent to the pro®le at the point of contact of the

drop with the solid surface. This can be done on a

projected image or a photograph of the drop or directly

by using a telescope ®tted with a goniometer eyepiece.

Another similar method is the captive bubble where a

small air bubble is put in contact with the membrane

immersed into the liquid and the pro®le of this bubble

is measured. The most important difference between

these methods is that when the captive bubble method

is used the membrane is in wet state and it is not

needed to worry about major rearrangements of the

surface or other damages due to drying, while with the

sessile drop method the membrane should be pre-

viously dried [13].

Here the vertical rod method is used, which is based

on another more general and indirect technique: the

so-called Wilhelmy plate method. In our case, contact

angles can be measured directly on a vertical rod

partially immersed into the liquid. Advancing and

receding angles are obtained simply by immersing

and withdrawing the rod [14]. Since the liquid pro®le

does not move within the ®eld of view during immer-

sion or withdrawal, dynamic as well as static angles

can be measured easily.

In fact, contact angles measured by all the exposed

methods include the effect of the so-called line tension

that is only eliminated when the drop or rod radius

tends to in®nity. This effect comes from the tendency

of the interface to minimize the length of its perimeter

which adds a contribution to the effective contact

angle. This does not apply to our work here, as far

as only relative values with and without protein in the

solution are relevant and line tension effects are con-

stant for not very concentrate solutions [15]. In any

case the rod used has a large enough diameter to give

very small line tension effects.

Apart from these direct methods there are also

indirect ones as, for instance, tensiometric and

190 L. Palacio et al. / Journal of Membrane Science 152 (1999) 189±201

capillary methods [14]. Other methods can also be

found in the bibliography [13,14,16±19].

With all these techniques, advancing and receding

angles can be obtained as shown in Fig. 1. The

difference in both angles is called contact angle

hysteresis. Hysteresis depends on the conformation

changes of the membrane and also on surface rough-

ness. It should be taken into account that a wet

membrane behaves differently than a dry one.

3. Other characterization techniques

Several other characterization methods have been

used to complement the contact angle technique. They

help to characterize the membrane structure as well as

to characterize the colloidal behavior of BSA solu-

tions.

Pore size distribution can be obtained from ®ltra-

tion experiments of macromolecules whose size dis-

tribution is assumed as well-known. Flux and retention

are studied in tangentially ®ltered for several trans-

membrane pressures and recirculation velocities.

Results are analyzed with the ®lm theory for the

concentration polarization phenomenon, obtaining

the mass transfer coef®cient along with the true

retention coef®cient as a function of the feed circula-

tion velocity and the molecular weight of the solute.

The resulting standard retention curves can be

expressed in terms of molecular size and treated to

obtain the effective pore size distribution of the mem-

brane, [20,21].

Porosity can be obtained from a method developed

recently [22]. In this method an inert gas penetrates the

void volume of the membrane in controlled tempera-

ture under different applied low pressures. The mass

versus pressure dependence ®ts the ideal gas one,

which after an adequate calibration allows the bulk

porosity of the sample to be obtained.

The surface structural characteristics of the mem-

branes, especially roughness, can be analyzed by

using atomic force microscopy. Atomic force micro-

scopy (AFM) sometimes called sweeping probe

microscopy (SPM) can detect topography by using

contact, non-contact or tapping modes [23±25]. The

last one which will be used here is a variation of the

contact mode where a stiff cantilever is oscillated at its

resonant frequency with high amplitudes allowing it to

touch the sample during the oscillations; highly sen-

sible photodiodes allow to detect amplitude variations

by the corresponding de¯ections of a laser beam. The

relative height of the sample allows to de®ne an

average roughness as

Ra � 1

n

Xn

i�0

jzi ÿ smj; (4)

where n is the total number of points where height, zi,

is measured and zm is the mean height.

It is well known that streaming potential follows the

sign of charges or potentials at the shear surface [17].

Thus, in order to take into account electrical properties

of the membrane surfaces, we will here analyze the

streaming potential versus pH behavior. In effect, the

streaming potential can be measured easily as

�p � �V

�p

� �I�0

; (5)

where �V is the electric potential appearing when a

transmembrane pressure �p is applied at zero electric

conduction.

Finally, the colloidal properties of macromolecular

solutes can be studied in terms of zeta potential, i.e.

the electric potential appearing at the shear surface

surrounding the macromolecules. This potential gives

a mobility in response to an externally applied elec-

trical ®eld. This electrophoretic mobility can be mea-

sured by analyzing the dispersion of a laser beam, and

it can be subsequently transformed to zeta potentials

according to the appropriate theory [26].

Fig. 1. Scheme of the contact angle measurement in a gas±liquid±

solid interface.

L. Palacio et al. / Journal of Membrane Science 152 (1999) 189±201 191

4. Experimental

4.1. Membranes and chemicals

Three different composite membranes made by

Desalination Sys. have been used. They consist of

an aromatic polyamide active layer on a polysulfone

support. The membranes are named: G5, G20 and

G50, and their nominal molecular weight cut-offs

(MWCO) are 2000, 3500 and 15 000 g/mol, corre-

sponding to diameters of 2.82, 3.46 and 7.75 nm,

respectively [27,28].

Bovine serum albumin (BSA) has been used to foul

the membranes. It has been obtained from Sigma

Chemical. Some properties of BSA relevant for our

study are:

1. Molar mass: 66 700 g/mol [29].

2. Isoelectric point: values from 4.7 to 4.9 are given in

the literature [30] while a value of 4.8 has been

obtained by us from measurements of electro-

phoretic mobilities.

3. Size: several even somehow divergent dimensions

are reported for BSA. Nevertheless, the results of

Peters [29], obtained from many independent tech-

niques, give 4.16�4.16�14.09 (nm) for a BSA

molecule in water, while Nakatsuka and Michaels

[31] give a gyration radius of 3.64 nm.

The water used was Millipore RO treated with a

conductivity less than 1 mS cmÿ1. For contact angle

experiments, the pH was controlled by adding small

amounts of HCl and KOH of pa grade. The BSA

concentrations used were 0.001, 0.005, 0.01 and

0.05 g/l while the pH values were 3, 5 and 8.

Retention experiments were done with aqueous solu-

tions (0.1%, w/w) of polyethylene glycols (PEG) whose

molar masses were 300, 600, 1000, 2000, 3000, 4000,

6000, 10 000 and 12 000 g/mol at neutral pH.

Finally the streaming potential was measured with

KCl solutions at 1�10ÿ3 mol/l, being pH set also by

small addition of HCl or KOH. While solution zeta

potentials have been measured for BSA solutions of

5 g/l with 0.01 mol/l of NaCl and controlled additions

of HCl or NaOH.

4.2. Experimental devices

The contact angles between the liquid phase, air and

the membrane surfaces were measured at the Labora-

tory of Technical Polymer Chemistry in Lappeenranta

by using a thick rod with a diameter of 2 cm [32]. The

set-up for the measurements is pictured in Fig. 2. The

movement of the rod was controlled to give a constant

speed of 1 mm/min. The equipment was attached to a

video camera provided with magnifying optics so that

Fig. 2. Scheme of the contact angle device. (1) TV monitor, (2) video recorder, (3) video camera, (4) cast rod immersed into the liquid phase,

(5) motor attached to lifting shrew, (6) control board for lifting speed regulation and (7) light source.


the contact angle change with time could be measured

dynamically. It is worth mentioning that the rod was

covered by the membrane leaving the active surface

free to interact with the liquid phase in the reservoir.

The membrane was not allowed to roll up completely

in order to avoid capillary effects, as shown in Fig. 2.

All membranes were previously cleaned with pure

water in an ultrasound bath. Afterwards they were kept

immersed in pure water until receding and advancing

processes started, ®rstly with pure water and ®nally

with each BSA solution. The measurement process

has been designed in such a way that just when the

protein solution enters the measurement vessel the rod

starts to be extracted thus obtaining receding angles. In

this way the contact time of protein molecules and

membrane surface increases until the advancing pro-

cess starts. Now, when the rod is advancing, contact

angles are measured for membrane surfaces that have

been in contact with the protein for decreasing times.

The pore size distributions were obtained from

experiments performed at the Group of Membrane

Science and Technology in Valladolid in isothermal

conditions at 298 K with pressures up to 7 bar and

recirculation velocities from 0.02 to 4.6 m/s, by using

a ¯at membrane tangential ultra®ltration device that

has been described elsewhere [33]. The membrane cell

used for retention experiments was a Minitan-S mani-

fold from Millipore made of methacrylate. On the

membrane, there are nine ducts of rectangular section

0.40 mm�7.0 mm and a length of L�55.0 mm. The

resulting channel hydraulic diameter is dh�0.76 mm,

giving an effective membrane area of 36.8 cm2 and a

channel section of 2.8 mm2.

The permeate ¯ux through the membrane is mea-

sured by timing and weighting with a high precision

balance with errors lower than �1�10ÿ7 kg. The

retentate and permeate concentrations have been mea-

sured by an adequately calibrated differential refracto-

meter.

The streaming potentials were measured at the

Laboratory of Technical Polymer Chemistry in Lap-

peenranta by using a ¯at-sheet cross-¯ow ®ltration

module equipped with a set of Ag/AgCl electrodes,

which could measure the streaming potential across

the pores of the membrane [34]. Pressure (0±1 bar) and

temperature (298 K) were measured with appropriate

sensors. Temperature was measured at four different

points along the feed circuit. Concentrate was circu-

lated through a magnetic pump. Concentration was

measured by a calibrated conductivity meter placed at

the concentrate entrance of the ¯ow cell; while pH was

measured with a pH-meter at the thermostated feed

vessel. The permeate was weighted in a balance. All

the measurements were programmed to be performed

at constant time intervals (4 min) and the measure-

ment signal was ampli®ed and converted to binary

code with an analog to digital converter and processed

by a computer.

Referring to solution zeta potentials they have been

measured at the Group of Membrane Science and

Technology in Valladolid by using a Zetamaster from

Malvern. Atomic Force Microscopy has been done at

the same laboratory by using tapping mode with a

Nanoscope IIIA from Digital Inst. The tips were

etched silicon pyramids TESP. Finally gas penetration

volumetry was conducted by using an Omnisorp

100CX from Coulter and with He as penetrating

gas by the Group of Membrane Science and Technol-

ogy in Valladolid [22].

5. Results

According to the gas penetration method, the por-

osities of the membranes studied are (in %): 55.1�0.5

for G5, 46.2�0.6 for G20 and 55.6�0.7 for G50 [35].

An AFM three-dimensional picture of the active

layer of the G50 membrane is shown in Fig. 3. As can

be seen roughness is quite signi®cative giving Ra

values of 7.67 nm for G5, 10.79 nm for G20 and

12.93 nm for G50.

The streaming potentials versus pH of the mem-

branes used are shown in Fig. 4. It can be seen that

negative streaming potentials are obtained through all

the pH range. This shows that the membranes do not

present an isoelectric point in contact with KCl, but

rather they are always negatively charged. Given that

Clÿ anions are known to adsorb easily on all poly-

meric materials; most of the charge responsible for the

streaming potential sign should come from this

adsorption process. The measured zeta potential for

BSA is shown against pH in Fig. 5.

Retention experiments with PEGs can be inter-

preted in terms of sizes by taking into account that

the mean gyration radius of a molecule of PEG is

given as a function of its molecular weight in terms of


Fig. 3. AFM picture of the active layer of the G50 membrane.

Fig. 4. Streaming potential as a function of pH for the three

membranes studied. Fig. 5. Zeta potential of BSA versus pH.


the theory of Flory [33]

rg � 16�775� 30� � 10ÿ13

��Mw

p� �; (6)

where Mw is the molecular weight in g/mol giving rg in

m. The corresponding true retentions are presented in

Table 1. The corresponding cut-off values are 4000,

6000 and 10 000 g/mol, for the membranes G5, G20

and G50, respectively. These values correspond to

95% retention. The resulting distributions of equiva-

lent pore sizes [36] are shown in Fig. 6, along with the

corresponding 95% retention pore sizes.

It can be seen that both in terms of molar masses and

equivalent pore sizes these results differ signi®catively

from the nominal data. This is not surprising as far

as effective retentions can differ depending on the

module, batch, etc.

In order to have a reference to account for the

in¯uence of protein on wettability, ®rstly both reced-

ing (�r,w) and advancing (�a,w) contact angles were

measured with RO-treated water. The so obtained

results are constant but different for receding and

advancing angles as shown in Table 2. These values

correspond to the hydrophilic character of the aro-

matic polyamide on the active layers of the mem-

branes.

Then each protein solution (different concentrations

and pH levels) was brought into contact with the

membrane and again receding (�rec) and advancing

(�adv) contact angles were measured. Both angles were

measured during 10 min. The receding and advancing

processes were switched without any time delay.

Nevertheless, sometimes advancing contact angles

were measured for longer periods letting the contact

angles stabilize close to the pure water value; so that

the solution could reach portions of the membrane not

being contacted with the solute previously.

Some representative results for contact angles are

shown in Figs. 7±11.

6. Discussion and conclusions

The measured streaming potentials show that all

membranes have a negative charge in KCl solutions.

In fact these streaming potentials refer to the pore

surfaces rather than to the external membrane surfaces

but no changes on the membrane sign should be

Table 1

True retention in percentage for PEGs and membranes studied

300 600 1000 2000 3000 4000 6000 10 000 12 000

G5 1.1 7.5 16.2 47.7 90.0 97.0

G20 9.4 40.2 65.1 77.8 97.0

G50 5.0 15.0 45.0 80.0 96.8 99.9

Fig. 6. Pore size distributions from retention experiments showing

the 95% retention equivalent radius.

Table 2

Receding and advancing contact angles for pure water

�adv �rec

G5 73�4 35�2

G20 73�5 17�4

G50 73�5 16�3


expected from inner to outer surfaces of the same

material.

It is known that protein is positively charged for pH

below the isoelectric point (IEP) while it is negatively

charged for pH over the IEP. It is worth noting that the

protein zeta potential measurements were done with

BSA solutions of 5 g/l with 0.01 mol/l of NaCl as

mentioned. In fact, the contact angles have been

Fig. 7. Receding and advancing contact angles for membrane G50

and pH 3.


and pH 5.


and pH 8.

Fig. 10. Receding and advancing contact angles for membrane

G20 and pH 5.


measured for substantially lower concentrations than

those used to measure zeta potentials which should be

affected by very high relative errors if lower concen-

trations were used. Also, solutions with almost neg-

ligible ionic strengths were used when measuring

contact angles. Nevertheless, low ionic strengths

should cause the protein to aggregate at the relatively

high BSA concentrations used in electrophoretic

experiments. In any case, lower BSA and salt con-

centrations should not change qualitatively the main

features of the results obtained, i.e. IEP value, and sign

of its charge below and over it.

From these results and on the basis of electrostatic

attraction/repulsion, it should be expected that adsorp-

tion should be greater at pH values below the BSA IEP

than over it. Nevertheless, these aspects need and will

be reconsidered after an adequate re¯ection on the

results obtained here.

In Fig. 12 the receding contact angle of pure water

is shown as a function of pore radii (as taken from

Fig. 6) and roughness. It is seen that receding contact


and pH 5.

Fig. 12. Receding contact angle of pure water as a function of pore radius and surface roughness. Both contact angles versus the contact angle

hysteresis are also shown.


angles decrease with increasing pore radius and rough-

ness. This contact angle versus pore radius behavior

has been reported for Sartorius polysulfone mem-

branes by Gekas et al. [37]. A linear dependence

was suggested by them, nevertheless this dependence

is hardly obtained in our case. A better linearity seems

to correlate contact angles and roughness. Both the

presence of pores and/or valleys should clearly lead to

decreasing receding angles as far as the liquid should

penetrate easier into a porous material. Nevertheless,

this dependence could be somehow obscured by the

different porosities that the G membranes have. This

and the fact that we only study three membranes of the

same kind should allow neither to conclude nor to

discard a linear dependence of water receding contact

angle on pore size and/or roughness.

In any case, these pure water contact angles mean

that the G membranes are fairly hydrophilic. On the

other hand, if the linear decrease of receding contact

angles with increasing pore radii and roughness were

accepted, in spite of their lack of an actual justi®ca-

tion, the ®ts could be extrapolated to zero radius,

�rec�508, or to zero roughness, �rec�628. An alter-

native approach, followed by Schultze et al. [38], is to

plot both the contact angles versus the contact angle

hysteresis (��advÿ�rec), in such a way that linear

dependencies should be expected. It is clear that

zero hysteresis should correspond to a perfectly ¯at

non-porous solid surface. When our data are repre-

sented in this way, two perfect straights are effectively

obtained leading by extrapolation to a Young's equili-

brium contact angle of 738, as shown in Fig. 12. The

differences of extrapolated contact angles should

be due to the interaction of both pore size and rough-

ness along with porosity, whose effects cannot be

separated.

Referring to both receding and advancing angles for

membrane-protein experiments, no consistent differ-

ences are noted for the different membranes, which

seems to indicate that they are determined by the

interaction of the liquid with a protein layer deposited

on the membrane as a consequence of their total

retentivity for BSA. Things are substantially different

if the concentration dependence of contact angles are

analyzed for different pH, as will be shown below.

As seen for example in Figs. 7±11, at long times

contact angles reach almost static values. These pla-

teau values correspond to the initial interface for

advancing angles and to the ®nal interface for receding

angles. In effect, as mentioned, receding contact

angles refer to increasing surface-protein contact

times, while advancing angles are measured for mem-

brane surfaces that have been in contact with protein

for decreasing times. In fact, during the receding

process the contact angle is almost constant after

nearly 5 min for all membranes, concentrations and

pH. This means that the last receding contact angles

measured refer effectively to membrane portions

where adsorption has been totally established. For

the advancing process this is not so clear, as far as

sometimes the plateau is not totally reached. This can

be due to small variations in the time gap from solution

entrance in the vessel to effective receding of the rod.

These times can be very critical considering the short

characteristic times for adsorption. On the other hand,

it is clear that the last advancing angles correspond to

membrane portions that have been outside the solution

for more than 10 min, this is why a certain drying

process can be relevant. Thus advancing contact

angles should be carefully extrapolated to long times

to be sure that they refer to the initial adsorption.

To resume we can accept that ®nal receding angles,

�rec, refer to ®nal adsorption, i.e. to these correspond-

ing to an in®nite adsorption time (t!1), while the

®nal advancing angles, �adv, should lead to the contact

angles for initial adsorption times (t!0), when only

the ®rst protein layer or layers appear on the mem-

brane.

Thus, both the �rec and �adv were averaged for all

membranes, ®tted and extrapolated for long measure-

ment times to get the contact angle for ®nal (t!1)

and initial (t!0) adsorption, respectively. These

values are shown in Figs. 13 and 14. It is clear that,

in this process the in¯uence of some long time advan-

cing contact angles affected by drying effects has been

minimized.

First of all, it is seen that all these values correspond

to very hydrophilic surfaces as it should when corre-

sponding to BSA layers.

On the other hand, referring to early adsorption, it is

seen in Fig. 13 that:

1. At pH 8, �adv is always in the pure water zone for

all concentrations. This should be due to the

repulsion between BSA molecules, negative at this

pH, and the always negatively charged membrane

surface.


2. At pH 3, �adv decreases below the pure water

values. In effect, at this pH BSA's positively

charged protein molecules are attracted by the

membrane. The so formed protein layer is thicker

for increasing concentrations leading to decreasing

contact angles which should be more and more

determined only by the protein surfaces.

3. Finally at the isoelectric point the protein is neither

repelled nor attracted by the membrane surface or

by the other protein molecules. Thus they can

approach enough the solid surface to be attracted

by short range forces. Thus forming a very thick

protein layer on the membrane surface leading to

even lower contact angles. Of course, adsorption in

these conditions should be higher for increasing

concentrations, as far as more BSA molecules are

present at the interface, thus leading to decreasing

contact angles.

As far as late or ®nal adsorption is concerned, from

Fig. 14, it can be seen that for high concentrations �rec

is almost equal for all pH which means that a thick

enough protein layer is formed at all pH values, which

gives a contact angle totally determined by the protein

surfaces. This should be due to the disposability of

enough BSA molecules in the solution to be adsorbed.

For these high concentrations, at the isoelectric point

there is a slightly lower adsorption than for the other

pH, possibly due to the high intermolecular short

range attraction that balances somehow the BSA±

membrane interaction. For low concentrations, some

signi®cant differences are detected for different pH:

1. At the isoelectric point the protein is easily

adsorbed even at low concentrations as far as it

can approach the membrane and be linked, even

forming multilayers, by short range interactions

leading to low �rec. Charged proteins form with

dif®culty more than one layer at low concentra-

tions and thus have higher �rec.

2. At pH 3 the protein is positively charged thus being

easily adsorbed in the first layer but hardly

adsorbed at long times due to repulsion for second

and subsequent layers. This layer to layer repulsion

should be relatively strong as far as the correspond-

ing zeta potential for BSA at pH 3 has the highest

absolute value for the pH range measured (see

Fig. 5).

3. At pH 8 a slightly better adsorption than at pH 3 is

indicated by the lower �rec. This can be due to a

partial displacement of Clÿ anions, which are

responsible for at least a part of the negative charge

Fig. 13. Advancing contact angle for t�0 as a function of protein

concentration for the three pH used. The contact angles shown are

averaged values for all the membranes.

Fig. 14. Receding contact angle for t�1 as a function of

concentration for the three pH used. The contact angles shown

are averaged values for all the membranes.


of the membrane, by the protein molecules that are

also negatively charged and whose affinity for the

membrane organic polymer is high. The secondary

layers should also be repelled but to a lower extent

than at pH 3 as indicated by the protein zeta

potentials of ÿ15 mV and �23 mV at pH 8 and

3, respectively (see Fig. 5).

In conclusion it has been shown that the dynamic

measurement of contact angle is an important and

useful technique to characterize adsorption and

deposition behavior of membranes in contact with

protein solutions. In particular dynamic measurements

of contact angles on membranes when immersed into

protein solutions help to distinguish between early and

®nal adsorption steps.

Acknowledgements

The Spanish authors would like to thank the ®nan-

cial support of this work through projects QUI96-0767

of the Spanish `̀ Plan Nacional de InvestigacioÂn y

Desarrollo'' (CICYT) and VA10496 of the J. Castilla

y LeoÂn, MSc Pasi VaÈisaÈnen would like to thank the

Tekes foundation for its support.

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Contact angles and external protein adsorption onto UF membranes

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