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The Stabilization of Aqueous PEO‐PPO‐PEO TriblockCopolymer FoamBrita Rippner Blomqvist a , Sandra Folke a & Per M. Claesson aa Institute for Surface Chemistry, Stockholm, SwedenPublished online: 06 Feb 2007.

To cite this article: Brita Rippner Blomqvist , Sandra Folke & Per M. Claesson (2006) The Stabilization of AqueousPEO‐PPO‐PEO Triblock Copolymer Foam, Journal of Dispersion Science and Technology, 27:4, 469-479, DOI:10.1080/01932690500374201

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The Stabilization of Aqueous PEO-PPO-PEO Triblock Copolymer Foam

Brita Rippner Blomqvist, Sandra Folke, and Per M. ClaessonInstitute for Surface Chemistry, Stockholm, Sweden

Foam generated by sparging of aqueous solutions of the block copolymers P85 (PEO26-PPO39-PEO26), F88 (PEO103-PPO40-PEO103), F127 (PEO99-PPO65-PEO99), and L64 (PEO13-PPO30-PEO13), has been characterized by foam volume measurements. Uniform wet foam formed,which, after drainage of the major part of the liquid, transformed to polyhedral dry foam. Con-ductance jumps across the foam column indicated that structural changes occur at a certainliquid fraction. The dry foams of P85 were less stable than those of F88 and F127. Thelatter copolymers showed similar foam stability over a period of one hour. The L64 foamwas very unstable. It is suggested that the stability of the dry foams is determined by the resist-ance to rupture of the foam films. Foam stability is discussed in relation to earlier studies onsurface rheology and to the thickness of thin foam films. A general relationship for allPEOx-PPOy-PEOx block copolymers between the dilatational modulus and the foam stabilitycould not be found. However, the ability to form thick adsorption layers, accompanied by stericrepulsive forces across foam films, appears to be a general foam-stabilizing factor. Surface dif-fusion coefficients of a fluorescent probe in single-block copolymers foam films are alsoreported for a brief discussion on Gibbs-Marangoni stabilization.

Keywords Block copolymer, polymeric surfactant, foam stability, foamability, drainage rate,surface diffusion, surface rheology, dilatational rheology, fluorescence recovery afterphotobleaching, FRAP

INTRODUCTION

Understanding the mechanisms for foam stabilization and

destabilization is of great importance for controlling foam in

various kinds of applications (Prud’homme and Khan 1995).

All foams are subjected to destabilization processes such as

disproportionation, drainage of liquid, and thin film rupture.

Foam lifetime is influenced both by bulk liquid behavior and

interfacial properties (Pugh 1996; von Klitzing and Muller 2002).

Non ionic triblock copolymers of ethylene oxide and pro-

pylene oxide (PEOx-PPOy-PEOx) are polymeric surfactants,

which can act either as foam stabilizing agents or as defoamers.

For a given composition of the polymer, the temperature and

the concentration determine which of these opposing functions

will dominate. Above a certain temperature (cloud point)

and/or concentration, aqueous solutions of PEO-PPO-PEO

polymers phase-separate into a polymer-rich phase and a

more dilute phase. Foamability and foam stability is reduced

above the phase separation temperature by the bridging of

small drops of the polymer-rich phase between the two

air-water interfaces of the foam film (Bonfillon-Colin and

Langevin 1997; Garett 1993). This causes film rupture. For

the most effective antifoaming action, the weight percentage

of the hydrophobic PPO part should be high (Alexandridis

and Lindman 2000). On the other hand, PEO-PPO-PEO co-

polymers may form metastable polyhedral foams below the

phase-separation temperature.

Several types of forces operate between the two air-water

interfaces enclosing the thin liquid film of the foam

lamellae (Exerowa and Kruglyakov 1998; Bergeron 1999).

Repulsive force contributions, such as electrostatic double

layer, steric, and structural forces, oppose film thinning. Elec-

trostatic double layer forces have been suggested for deter-

mining the foam stability for a number of alkyl glucoside

surfactants, since the addition of electrolyte led to lower dis-

joining pressures, which correlated to shorter foam lifetimes

(Waltermo, et al. 1996; Bergeron 1996). Disjoining pressure

measurements have provided information about the type and

range of forces, acting normal to the film surfaces. PEO-

PPO-PEO block copolymers and similar nonionic block copo-

lymers form interfacial films with a thickness in the order of

nanometers to tens of nanometers, depending on the surface

concentration, as demonstrated by interferometric film thick-

ness measurements (Sedev et al. 2000; Exerowa et al. 1997;

Rippner et al. 2002). and neutron reflectivity (Sedev et al.

Received 22 July 2005; Accepted 17 August 2005.Address correspondence to Brita Rippner Blomqvist, Institute for

Surface Chemistry, Box 5607, 114 86 Stockholm, Sweden. E-mail:brita.rippner@surfchem.kth.se

Journal of Dispersion Science and Technology, 27:469–479, 2006

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print/1532-2351 online

DOI: 10.1080/01932690500374201

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2002; Vieira et al. 2002). The stability of foam has often been

ascribed to the steric forces exerted by these thick surface

layers. However, the direct correlation between thin film

and foam properties has been investigated in very few

studies (Khristov et al. 2001; Khristov, et al. 2002; Sedev

et al. 1999). A correlation between foamability and film thick-

ness has been reported for the block copolymers P85 and F108

(PEO122-PPO56-PEO122) (Sedev et al. 1999), although foam-

ability is generally considered to be determined by factors

other than the disjoining pressure, since the films of a wet

foam are thicker than the surface force range; see for

example, Malysa et al. (1991).

The strength of interfacial layers and foam films, in the

lateral direction, and their response to dilatational and shear

deformations are definitely important properties in different

stages of the lifetime of a foam. However, the exact relation-

ship between rheological parameters of an interface and foam-

ability and foam stability is not yet clear (Dickinson 1999; Bos

and van Vliet 2001). For example, the stability of aqueous

foam films stabilized by cationic alkyltrimethylammonium

bromide surfactants (CnTAB) has been correlated to the dilata-

tional elasticity (Bergeron 1997). Furthermore, the nonionic

surfactants n-dodecyl-b-D-maltoside (b-C12G2) and tetraethy-

leneglycolmonodecyl ether (C10E4) have been compared at

concentrations giving the same surface potential (i.e., equal

electrostatic repulsion across the film). A correlation between

the pressure at which single foam films ruptured and the dilata-

tional surface elasticities was found (Stubenrauch and Miller

2004). On the other hand, Martin et al. (2002) concluded that

the surface rheological properties of various proteins could

not be linked to macroscopic foam stability, although they

found that the rheological parameters had an effect on

disproportionation.

In this article, we have characterized the foaming properties

of the block copolymers PEO26-PPO39-PEO26 (P85), PEO103-

PPO40-PEO103 (F88), and PEO99-PPO65-PEO99 (F127). The

block copolymers were chosen to form two pairs; one with a

very similar size of the EO block (F88 and F127) and one

with the same PO block size (P85 and F88), to distinguish

the influence of the hydrophobic PPO chain length and the

more hydrophilic PEO chain length, respectively. In addition,

we studied the foam properties of the block copolymer L64

(PEO13-PPO30-PEO13). The stability of well-drained dry

block copolymer foams is compared to the dilatational

viscoelastic properties of polymer layers. The dilatational

rheology of interfacial films that were formed from exactly

the same polymer samples used in the present study was

descibed in detail recently (Rippner Blomqvist 2005b). The

macroscopic foam stability and the relation to the surface

rheology of PEO-PPO-PEO block copolymers have to our

knowledge not been directly investigated before. The stability

of dry foams is further discussed in view of literature data on

the foam film thickness as determined by steric repulsion.

EXPERIMENTAL SECTION

Materials

The triblock copolymers Synperonic PE/P85 (PEO26-PPO39-

PEO26, Mw(mean) ¼ 4600 g/mol, batch no. 1005BD0428), Syn-

peronic PE/F88 (PEO103-PPO40-PEO103, Mw(mean) ¼ 11400 g/mol, batch no. A3), and Synperonic PE/F127 (PEO99-PPO65-

PEO99, average molecular weight Mw(mean) ¼ 12600 g/mol,

batch no. 2804BK0518) were kindly provided by ICI Surfactants

(Uniqema, The Netherlands). Pluronic L64 (PEO13-PPO30-PEO13,

Mw(mean) ¼ 2900 g/mol, batch no. NPAQ-561B) was kindly

provided by BASF (Parsnippany, N.J., USA). The samples

were used as received. PEO represents a –CH2–CH2–O–

segment, while PPO represents a –CH2–CH(CH3)–O–

segment. The generic names of the polymers L64, P85, F88,

and F127 are Poloxamer 184, Poloxamer 235, Poloxamer 238,

and Poloxamer 407, respectively. The molecular weights,

block sizes, and critical micelle concentration (CMC) values

are given in Table 1. The CMC to which we related the

solution concentrations are the values provided by the

TABLE 1

Molecular characteristics of the PEOx-PPOy-PEOx triblock copolymers

Polymer Mw (g/mol) x-y-x

CMC

(mM)aCmc range in

literature (mM, 258C)

L64 2900 13-30-13 — 5200–8800b (308C)

P85 4600 26-39-26 10900 110–8700c

F88 11400 103-40-103 8800 460–8800d

F127 12500 99-65-99 2000 555–2000e

aAccording to the manufacturer, Uniqema, Netherlands.bLopes and Loh (1998); Alexandridis et al. (1994).cKabanov et al. (1995); Alexandridis et al. (1994).dLopes and Loh (1998); Alexandridis et al. (1994).eDesai et al. (2001); Lopes and Loh (1998); Alexandridis, et al. (1994).

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supplier, which were 50000 ppm (¼ 10900mM) for P85,

100000 ppm (¼ 8800mM) for F88, and 25000 ppm (¼

2000mM) for F127. Note that the CMC values mentioned are

to be viewed as approximate and as the concentration where

the main component of the polydisperse sample has formed

micelles. For more details on the micellization and compli-

cations of CMC determinations for polydisperse block copoly-

mers, see for example, Rippner et al. (2002) and Kabanov

et al. (1995). All solutions were investigated below the phase

separation temperatures (Desai et al. 2000; Desai et al. 2001;

Alexandridis and Holzwarth, 1997).

Block copolymer solutions were prepared by dissolving the

polymer in water by gentle stirring over night at least one day

before the measurement and were stored under nitrogen at

room temperature until use. Water from a Millipore purifi-

cation system (resistivity 18.2 MV cm) was used. Sodium

chloride, purity 99.9%, was supplied by Merck Eurolab

(Sweden). An amount of 10 mM NaCl was added to solutions

used for Foamscan measurements. All glassware were

cleaned with a surfactant-free detergent, Deconex 20 NS

from Borer Chemie (Switzerland) and carefully rinsed with

Millipore water.

The fluorescent probe for the fluorescence recovery after

photobleaching measurements was 5-(N-Octadecanoyl)amino-

fluorescein (ODAF, Mw ¼ 613.79 g/mol) from Molecular

Probes Inc. Europe BV, The Netherlands (O-332, Lot no.

2051-1). A stock solution of ODAF (3 mM in EtOH) was

prepared.

Foaming Properties

Ross-Miles Method

The Ross-Miles method (Ross and Miles 1941) is standar-

dized by the American Society for Testing and Materials. In

agreement with the standard procedure, the polymer solution

was kept at the bottom of the test column (50 mL) and in a

glass reservoir placed above the column (200 mL). The outlet

of the reservoir was adjusted to a position 90 cm above the

liquid interface. Foam was created by allowing the foaming

solution to fall into the column. The initial foam volume (or

foam height) is defined as the foamability, i.e., the capacity

of the solution to generate foam. The remaining foam

volume (or foam height) after 5, 10, and 15 minutes was used

as a measure of the foam stability. The temperature was

248C. Measurements were carried out twice on each solution.

Foam Column (Foamscan)

The foam volume and the liquid volume of the foam were

measured as a function of time during the foam generation

and during one hour after the formation by using automatic

equipment at room temperature (20–228C) (Foamscan, I. T.

Concept, Longessaigne, France). Foam was generated in a

glass column (diameter 36 mm) by sparging gas (pure com-

pressed air, AGA, Sweden, filtered through a 0.2mm particle

filter) through the solution via a porous glass frit (diameter

34 mm, pore size 16–40mm). The foam volume was calculated

from an image of the column, recorded by a CCD camera. One

pair of electrode plates (area 3.5 cm2) at the bottom of the

column where the solution is placed was used to determine

the total amount of liquid present in the foam in the following

way. With each new solution, aliquots of five times 5 mL of

solution were added subsequently and the conductance was

measured across the liquid after each addition. The linear

relationship between liquid volume and conductance was

then used to automatically compute the quantity of liquid in

the foam (the difference between the initial solution volume

(25 mL) and the solution volume remaining at a certain

time). Three pairs of electrodes (cross-sectional area ¼

1.8 cm2) at different heights (5.5, 10.5, and 15.5 cm) along

the column determined the conductance across the foam.

At the start of the experiment 25 mL of solution was intro-

duced at the bottom of the column. The airflow rate was set

to 30 mL/min and was turned off when the foam volume had

reached 150 mL. The time to produce 150 mL of foam and

the rate of decrease of the foam volume were used to

estimate the foamability and foam stability, respectively.

Five complete sets of measurements were performed with

each solution. The last four were used for analysis, and the

first measurement was used to equilibrate the equipment.

FRAP (Fluorescence Recovery after Photobleaching)

The surface lateral diffusion coefficient in block copoly-

mer films of the surface-active fluorescent probe ODAF

was determined by using an experimental setup described

previously (Clark et al. 1990). The method involves irrevers-

ible photobleaching of fluorescent molecules within a limited

surface area of a thin foam film. The repopulation with (non-

bleached) fluorescent molecules by lateral diffusion into the

bleached area is monitored as a function of time. The rate

of fluorescence recovery is used to calculate the surface

lateral diffusion coefficient of the fluorophore. The wave-

length of the laser used was 488 nm (Arþ 10 W, Coherent

Innova 100-10) and the bleached spot size diameter was

2.85mm. Thin foam films were formed in a ground glass

ring (internal diameter 3 mm) mounted horizontally in a

chamber. Thin films were formed from aqueous solutions

of the block copolymer, at concentrations of 2, 20, 200,

and 1600mM. The solutions contained 0.014 mole ODAF

per mole polymer, except at the lowest concentration

(2mM), where the probe concentration had to be raised to

0.14 moles per mole polymer to achieve a detectable

signal of fluorescence. Adsorption was allowed for five

minutes before the films were formed. The measurements

were performed at 248C on films that had drained to their

equilibrium thickness at an applied capillary pressure of

25–36 Pa, as calculated according to Pc ¼ 2gRc/(Rc2 2 rf

2)

(see chapter 2.1.4 of Exarowa et al.), for the capillary

radius Rc ¼ 3 mm, the film radius rf ¼ 375mm, and surface

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tensions ranging from 37 to 54 mN/m for different polymers

and solution concentrations (20–1600mM) at t ¼ 5 minutes

(surface tension data not shown). The data was analyzed by

fittings to an expression (Axelrod 1976), defining the time

dependence of fluorescence recovery as described by Clark

et al. (1990). At least 10–15 fluorescence recovery curves

for each film were collected, and the mean values are

presented.

Surface Dilatational Rheology

The rheological data presented here were discussed in more

detail in a previous article (Rippner Blomqvist et al. 2005b).

Data were obtained by using an oscillating ring trough

method (Kokelaar et al. 1991), measuring the surface area A

and the surface tension g during sinusoidal variations of the

surface area (dA/A ¼ 5%) at room temperature (20–218C)

and a fixed frequency of 0.13 Hz. The presented values are

the complex surface dilatational moduli E ¼ dg/dlnA for

block copolymer surface films formed by adsorption during

one hour from polymer solutions of varying concentrations in

the range 0.02–1600mM (in separate experiments for each

concentration). The dilatational modulus E is presented as a

function of the surface pressure p (p ¼ g0 2 g), which

allows the construction of a master curve from data obtained

at different times and bulk solution concentrations (Rippner

Blomqvist et al. 2005b).

RESULTS

Foamability and Foam Stability

The foamability, defined as the initial foam height, and

the foam stability of P85 (PEO26-PPO39-PEO26), and F88

(PEO103-PPO40-PEO103) were first compared by the Ross-

Miles test (Figure 1), since this method is commonly used

in industry. The foamability improved with increasing

polymer concentration, until a plateau level apparently was

reached, as shown for the P85 polymer. The foamability of

P85 and F88 was similar at concentrations corresponding

to half the CMC. The decrease of foam volume with time

was, on the other hand, much faster for P85. After 15

minutes, the remaining foam volume of P85 was less than

30% of the initial volume, while it was almost 90% for

F88. Thus, an increase of the PEO chain length increases

the foam stability.

A sparging method (Foamscan) was used to further charac-

terize the block copolymer foams at two different bulk solution

concentrations: one equimolar concentration (1600mM) and

one concentration equal to half the CMC of the polymers.

The equimolar concentration was chosen to fall below the

CMC, yet be high enough to give homogeneous stable foam

for all of the polymers with the foam generation parameters

employed.

Uniform foams, as observed visually, with small spherical

bubbles (diameter ¼ 1 mm) formed initially. Similar

foamability was observed for the different polymers

(Table 2). Some solutions at lower concentrations were also

investigated. It was found that uniform small-bubble foam

was produced by F127 (PEO99-PPO65-PEO99), also at 20mM

and 200mM, while P85, at 200mM, forms foam with larger

bubbles, which decay within less than 500 s. At 20mM, the

P85 foam films collapsed immediately after formation and

the intended foam volume of 150 mL was never reached.

Thus, P85 with the smallest PEO-chains has a lower ability

to generate foam compared to the other polymers at the same

molar concentration.

The foam volume as a function of time is shown in Figure 2.

Since the process of foam collapse is partly a stochastic pro-

cess, the reproducibility between foam decay curves, in terms

of absolute foam volumes, is generally not very high. Hence,

small differences between the polymers will not be reported.

However, some clear and reproducible features were

observed, as discussed below.

TABLE 2

Foamability: Foamscan method

Concentration

P85 sa

(std dev)

F88 sa

(std dev)

F127 sa

(std dev)

1600mM 219 (2) 229 (4) 217 (1)

0.5 CMC 218 (1) 226 (7) 212 (2)

aMean value of the time to reach the predetermined foam volume of

150 mL. The number of measurements on each solution was four.

FIG. 1. Initial foam height and foam height after 5, 10, and 15 minutes.

The given block copolymer concentrations correspond to the following molar

concentrations. P85 : 0.1 wt% ¼ 220mM, 2.5 wt% ¼ 5400mM (0.5 CMC),

5 wt% ¼ 10900mM (1CMC), 7.5 wt% ¼ 16300mM (1.5CMC); F88 : 0.1

wt% ¼ 90mM, 5 wt% ¼ 4400mM (0.5CMC). Each value is a mean of

two measurements. The Ross-Miles method was used. The lines are guides

to the eye.

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After a continuous decrease with time during the first 500 s

(P85) or 1000 s (F88 and F127) after foam formation, the foam

volume decreased in a stepwise manner. This feature was par-

ticularly clear for the small polymer, P85 (see Figure 2) and

was apparently caused by collective bubble collapse, i.e., the

collapse of one bubble initiated the coalescence of a series of

adjacent bubbles. In this latter regime, the initially spherical

bubbles had transformed into a polyhedral shape. The

reduction of the P85 foam volume occurred considerably

faster compared to that of the larger F88 and F127 polymers.

The even smaller block copolymer L64 (PEO13-PPO30-

PEO13) gave rise to very unstable foam. The foamability

of L64 was also lower than that of the larger polymers. In

fact, no foam could be generated at the bubbling rate of

30 mL/min. On enhancing the flow rate to 50 mL/min, foam

was created, although it collapsed very rapidly, within 110 s.

Liquid Drainage

During foam formation by means of sparging, bubbles drag

liquid upward at the same time as liquid drains downward due

to gravitation. After the foaming has ceased, the drainage con-

tinues. In addition to gravitation, capillary forces promote

drainage and film thinning. The liquid volume in the foam as

a function of time, normalized to the liquid volume present

initially (i.e., immediately after the bubbling ceased) is displayed

in Figure 3. (Figure 3(a), 1600mM and 3(b), 0.5 CMC). The time

dependence of the foam volume and the volume of liquid in the

foam in absolute values are compared in Figure 3(c)–(d). It is

clear that the major part of the bulk liquid drained within

about 400 (P85) or 600–1000 s (F127) from the start of the

experiment, followed by significantly slower drainage at

longer times. Foams of P85 drained slightly faster than those

of F127 at the equimolar concentration. The bulk drainage at

the concentration half of the CMC was similar for the investi-

gated polymers, although the P85 foams also drained

somewhat faster at this concentration. All foams were very

well drained and dry in the latter part of the experimental period.

The foam structure changes with time by means of drainage,

bubble disproportionation, and film rupture, processes that may

occur simultaneously and result in a decrease of the foam

volume. Important information is given in Figure 3(c)–(d),

showing that the most significant decrease of the foam

volumes starts after the major part of the liquid has drained.

The foam decay curves for the different polymers are indistin-

guishable in the regime of rapid bulk drainage (up to 600–

1000 s), whereas variations of the stability are evident for

foams containing very small amounts of liquid. These obser-

vations demonstrate that differences in the drainage rate are

not responsible for the differences in stability of the dry

foams. The rate of foam volume reduction in the dry regime

is further expected to be insensitive to small variations in

bulk solution viscosity. Thus, the data strongly suggest that

the volume of dry block copolymer foam is determined by

the strength of the thin foam films. Further support for this sug-

gestion is found when considering the stepwise decrease of

foam volume, a behavior that may be due to collective

bubble collapse, which was notably clear for the P85 block

copolymer. Such behavior can be due only to variations in

foam film stability and is not consistent with a foam volume

decrease mainly determined by evaporation and dispropor-

tionation, although these processes also take place.

The corresponding stepwise decay could not be observed by

the Ross-Miles test, since the data points are too few and the

foam was studied for only 15 minutes in this case. In

summary, we conclude that P85 formed considerably less

stable foam than that formed by the larger polymers F88 and

F127 and that the foam volume of dry foam is determined by

the stability of the thin liquid films.

FIG. 2. Foam volume versus time during foam formation and decay.

Foam was created in a column by sparging air through a glass frit of pore

size 16–40mm (Foamscan method). Solution concentrations were (a)

1600mM and (b) 0.5 CMC, corresponding to 5400mM, 4400mM, and

1000mM of P85, F88, and F127, respectively. The data for L64, obtained at

a higher sparging rate, are also included in Figure 2(a).

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The foam stability of F88 and F127, having similar PEO and

differing PPO chain lengths, could hardly be separated, indicat-

ing that the length of the PEO chains play a major role, while

the size of the PPO block is less important (compare P85 and

F88). The foam stability also increases with the overall size

of the polymer (P85 being smaller than F88 and F127, which

have similar molecular weights). Further studies are required

to fully understand the influence of the hydrophilic and

hydrophobic block sizes of the copolymers.

The conductance across the foam column is shown as a

function of the liquid volume in the foam in Figure 4(a).

The jumps between ca10 and 30mS were observed in all

experiments. They occurred at different points of time for the

different polymers, but common for all three polymers was

FIG. 3. Drainage of liquid as measured by the Foamscan method; (a), (b)

volume of liquid in the foam (Lfoam) relative to the liquid volume present

immediately after foaming had ceased (Lfoam, initial) and (c), (d) foam volume

and liquid volume of the foam as a function of time. The solution concen-

trations are 1600mM, (a) and (c), and 0.5 CMC (b) and (d).

FIG. 4. Conductance measured across the foam column at the column

height 5.5 cm versus the amount of liquid in the foam during drainage. The

experiments at the concentration 1600mM are shown. (b) Volume of liquid

in the foam (Lfoam) relative to the foam volume (Vfoam) measured continuosly.

The arrows indicate when conductance jumps across the foam at the column

height 5.5 cm occured: P85 (open arrow), F88 (normal arrow), and F127 (filled

arrow).

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that the jump in conductivity was observed when the liquid

fraction of the foam was between 2.6 and 3.3%, as indicated

by the arrows in Figure 4(b). We have found that no disconti-

nuity of the conductivity is observed for foam stabilized by the

milk protein b-lactoglobulin (unpublished data), suggesting

that the jumps are due to a physical process that takes place

in the interior of the block copolymer foams (rather than an

instrumental artefact). It is suggested that the process

involves a sudden coalescence and/or rearrangement of the

bubbles occurring at the critical liquid fraction, e.g., the

transition from spherical to polyhedral bubbles. In the latter

structure, charges would be obliged to travel a longer

distance, and thus a lower conductance would be measured.

The observed feature may have the same origin as that

reported for foams of mixed sodium dodecylbenzene-

sulphonate-C10E10 solutions (Carrier and Colin 2003). In that

work, a correlation was found between a critical value of the

liquid fraction of the foam and the coalescence of bubbles.

The threshold value of the liquid fraction depended on

surfactant concentration and the type of surfactant, but was

found to be independent of the initial bubble size (Carrier

and Colin 2003).

Surface Dilatational Rheology

The complex dilatational modulus (E) of adsorbed layers of

P85, F88, and F127 at the air-water interface is a function of the

surface pressure (p) (Figure 5). In our previous report (Rippner

Blomqvist et al. 2005b) the variation of E with the surface con-

centration was discussed in terms of structural changes within

the block copolymer layers. It was concluded that a mainly flat

conformation is adopted at surface concentrations correspond-

ing to the first peak of the E ¼ f(p) curves. The second peak

was related to a stretched conformation of poly(ethylene

oxide chains). A partial distribution of propylene oxide

segments into the PEO-water sublayer at high surface pressures

was proposed (Rippner Blomqvist et al. 2005b). The surface

films were found to be predominantly elastic over the whole

range of surface pressures. Comparing the block copolymers

in Figure 5, we observe very similar properties at the low

surface pressures. At higher surface pressure (larger surface

concentration), the highest values (18–19 mN/m) of E were

reached for the block copolymers F88 and F127 (see the

second maximum). In contrast, the maximum value of the

P85 modulus was only 11 mN/m. The relation between the

measured dilatational properties and the ability to stabilize

foams will be discussed below. In order to understand the dila-

tational rheology of block copolymers, it is important to realize

the curves in Figure 5 are composed of overlapping data from

several separate experiments, obtained at different bulk

solution concentrations and equilibration times. We note, that

in contrast to rheology data of low molecular weight surfac-

tants, most data points in Figure 5 do not represent the

(adsorption) equilibrium values. However, as demonstrated in

Rippner Blomqvist et al. (2005b), the important conclusion is

that the dilatational rheological parameters of the block copo-

lymers are unique functions of the surface pressure (surface

concentration).

FIG. 5. Complex dilatational modulus of block copolymer layers adsorbed

at the airwater interface as function of the surface pressure; (a) P85 (PEO26-

PPO39PEO26) (b) F88 (PEO103PPO40PEO103), and (c) F127 (PEO99PPO65-

PEO99). The surface pressure, p, is given by p ¼ g0 2 g, where g0 is the

surface tension of water. Data redrawn from Rippner Blomqvist et al. 2005

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Surface Lateral Diffusion

The fluorescent probe ODAF is able to diffuse at the surface

of model thin foam films stabilized by the block copolymers

(Figure 6), which demonstrates a significant lateral mobility

in these layers. The lateral diffusion coefficients of the probe

are of the order of 1028 cm2/m, which is at least one order

of magnitude lower than for conventional low-molecular

weight surfactants (Clark et al. 1990; Mackie et al. 1999). In

addition, the diffusion coefficients for the block copolymers

themselves are expected to be lower than for the small probe

molecule. The diffusion coefficient falls with increasing

block copolymer bulk concentration and consequently with

the amounts adsorbed. The diffusion of the probe molecule is

also reduced when the molecular weight of the block copoly-

mer is increased, as evidenced by the decrease in surface

lateral diffusion in the order P85 . F88 . F127. Lateral

mobility of surfactants at the air-liquid interface is a prerequi-

site for foam stabilization by the Gibbs-Marangoni mechanism

(Ewers and Sutherland 1952). Our results strongly indicate that

this mechanism of stabilization, involving surfactant flow

against surface tension gradients, is possible for the block

copolymers. In contrast this mechanism is not active in foam

stabilized by proteins that form rigid gel-like interfacial skins

at the air-water interface (Mackie et al. 1999) in which the mol-

ecules are immobile. In this context we also recall that the

elastic and viscous shear moduli of adsorbed layers of P85,

F88, and F127 were small or close to zero (Rippner Blomqvist

2005a), suggesting that intermolecular interactions in the

surface layer are weak. The measurements reported above

were made in equilibrium foam films, and it should be noted

that the diffusion in equilibrium films is not necessarily identi-

cal to diffusion against surface tension gradients in a thin film.

However, the trends are expected to be the same. The largest of

the polymers, F127, is expected to have the slowest lateral dif-

fusion of the polymers in a steady foam film, yet possibly is an

effective foam stabilizer (by means of the Gibbs-Marangoni

mechanism) by being able to drag a larger amount of solvent

along the interface.

DISCUSSION

The time evolution of the block copolymer foam illustrates

the fact that foam is subjected to different destabilizing/stabi-

lizing processes at different stages of its lifetime (Figures 2 and

3(c)–(d)). If the foam had been observed for approximately

five minutes only, it would appear that the different polymers

are equally good foam stabilizers as judged from the amount

of foam present. This observation demonstrates the importance

of specifying the time scale of interest. In the following discus-

sion we focus on the relationships between the stability of the

dry foam and the surface dilatational modulus and repulsive

forces across the foam films. In this regime differences depend-

ing on block copolymer composition were observed and

the foam volume decreased with time by film rupture and

coalescence.

Foam Properties in Relation to Surface DilatationalRheology

Foamability and foam stability are generally governed by

different processes. The elasticity of surface films have been

considered most important under the dynamic conditions of

foam generation (Malysa et al. 1991), when surface tension

gradients effectively retard liquid drainage (Gibbs-Marangoni

stabilization). However, if there are attractive interactions

between the molecules in the adsorbed surface layer, as was

suggested and referred to as cohesion in the study of CnTAB

surfactants by Bergeron (1997) the dilatational elasticity will

also have an impact on foam stability. A cohesive film would

offer resistance to deformations, caused by convection due to

drainage, ambient vibrations, thermal fluctuations, and

collapse of adjacent bubbles. Bergeron (1997) furthermore

concluded that a high surface dilatational modulus (E) gener-

ally dampens spatial fluctuations. The latter implies that E is

important to foam stability whether it is a measure of the

resistance to compression or expansion. Considering the

PEO-PPO-PEO block copolymers, van der Waals and hydro-

phobic attractive interactions between the polymer segments

within the layer (due to the minimization of segment-water

contacts) are expected, which will be dominated by steric

repulsion at smaller areas per molecule.

The question of whether the measured dilatational modulus

of the block copolymer layers can be correlated to foam stab-

ility will now be addressed. One obvious complication with

the comparison of interfacial rheology results on an air-water

interface with the properties of macroscopic foam is that

neither the exact surface concentration nor the frequency of

disturbances of the foam is known. Concerning the frequency

FIG. 6. Surface lateral diffusion coefficient of the fluorophore ODAF as a

function of block copolymer solution concentration, at the surface of model

thin foam films, stabilized by the block copolymers. The fluorescence recovery

after photobleaching method was used.

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dependence of the dilatational parameters, the situation is

different for polymeric surfactants compared to low molecular

weight surfactants. The dilatational modulus of PEO-PPO-

PEO triblock copolymers depends mainly on the surface-

concentration dependent structure of the polymer chains in

the surface layer, whereas exchange of polymer molecules

between the surface and the bulk phase can be ignored

(Rippner Blomqvist et al. 2005a; 2005b). It has been shown

(Rippner Blomqvist et al. 2005b; Munoz et al. 2003; Noskov

et al. 2003) that the decrease of E observed at higher solution

concentrations cannot, as in the case of low molecular weight

surfactants, be attributed to exchange processes with the bulk

solution. Further, the influence of frequency variations in the

experimentally accessible frequency ranges appears to be

small according to several reports (Munoz et al. 2003;

Noskov et al. 2003). Hence, the fact that our study is limited

to one frequency only (0.13 Hz) does not prevent a discussion

on the relation between foam stability and surface rheology.

As mentioned already, the exact amount adsorbed at the air-

water interfaces of the foam is inaccessible, leading to the

difficulty of knowing which values of the dilatational

modulus (see Figure 5) should be related to foam stability.

Recall that the modulus is a function of the actual surface

pressure (surface concentration). However, one may conclude

that the values relevant to the situation in the foam are to be

found at the higher surface pressures. The reason is that

below the surface pressure of 10 mN/m, the surface concen-

tration is too low to stabilize foam films at all, since the corre-

sponding solution concentration in this regime is only in the

order of 0.1mM (Rippner Blomqvist et al. 2005b). As seen in

Figure 5 the modulus of P85 is lower than that of F88 and

F127 at all surface pressures above 10 mN/m. When compar-

ing the modulus corresponding to the solution concentration

of 1600mM, i.e., the same concentration as used in the

foaming experiments, a lower modulus is also found for P85.

The E values (close to equilibrium) of P85, F88, and F127 at

1600mM were 2, 4, and 7 mN/m, respectively (Rippner

Blomqvist 2005b). Thus, for these polymers a higher

modulus apparently correlates with higher foam stability.

However, as shown by Hambardzumyan et al. (2004),

similar or even higher values of the dilatational modulus at

the “second peak” of the E ¼ f(p) curve than those of F88

and F127 are attained, for example, for the block copolymers

PEO2-PPO30-PEO2, PEO13-PPO30-PEO13 (L64) and the

reversed polymer PPO25-PEO7-PPO25. We found that the

ability of L64 to stabilize foam is very small (Figure 2),

despite having a dilatational modulus similar to F88 and

F127, demonstrating that a high dilatational modulus for a

PEO-PPO-PEO copolymer is not a sufficient criterion for

high foam stability.

Foam Properties in Relation to Interlayer Interactions

It is clear that a thicker foam lamella is less vulnerable to hole

formation and rupture than a thinner one and that the thickness

of a foam film is determined by the interplay of attractive and

repulsive forces acting across foam lamellae. The repulsive

forces that determine the film thickness in foams formed by

non ionic block copolymers are electrostatic double-layer

forces and steric forces (Sedev 2000; Rippner et al. 2002).

Repulsion due to overlap of electric double layers associated

with charged air-liquid interfaces of thin liquid films have been

linked to foam film stability (Waltermo et al. 1996; Bergeron

et al. 1996). Consequently, the screening of the electric field

by the presence of added electrolyte should destabilize the

foam, provided this is the only repulsive force contribution.

Exerowa et al. (Exerowa et al. 1997) have determined how

the film thickness of PEO-PPO-PEO block copolymers, at

low pressures, depends on the electrolyte concentration

(NaCl). It was found that a transition from electrostatic stabil-

ization to a state where steric forces determined the film thick-

ness occurred at a NaCl concentration of 10 mM and 3 mM for

films of P85 and F108 (PEO122-PPO56-PEO122), respectively.

The Debye screening length k21 (i.e., the decay length of the

field) is reduced to 3 nm in the presence of 10 mM NaCl,

which was the concentration of salt added to the solutions

used in the Foamscan measurements. We note that the

adsorbed polymer layer thicknesses are larger than 3 nm (see

below). Thus, stability due to electrostatic double-layer

forces is not a dominating stabilizing mechanism in any of

the block copolymer foams investigated.

Disjoining pressure measurements across foam films (air-

solution-air) (Sedev et al. 2000; Exerowa et al. 1997;

Rippner et al. 2002) stabilized by poly(ethylene oxide)-

containing block copolymers have shown that the film thick-

ness, under conditions of screened electrostatic repulsion,

increases with the polymer molecular weight and with the

solution concentration of the polymer. Steric repulsion

develops upon approach of two polymer-covered surfaces,

and the range of the steric repulsion is rather sensitive to

the presence of extending tails. The foam film thickness

has been determined to be 17 nm for P85 (concentration

70mM) (Exerowa et al. 1997), 45 nm for F108 (concentration

range 0.7–7mM) (Exerowa et al. 1997), and 46 nm for F127

(concentration 20mM) (Rippner Blomqvist et al. 2005b), as

found by means of dynamic thin-film measurements in the

presence of 100 mM NaCl (low pressure). The film thickness

for F108 as measured by the porous plate technique at higher

pressures was 30 nm (concentration 14mM) (Sedev et al.

2000). Note that the adsorbed layer thickness for PEO-

PPO-PEO and similar block copolymers, determined by ellip-

sometry (hydrophobic solid-solution (Rippner et al. 2002) and

air-solution (Rippner Blomqvist et al. 2005a) interface) or

neutron reflectivity (air-solution interface (Sedev et al.

2002; Vieria et al. 2002)), is less than half of the reported

foam film thickness, which reflects the fact that the

adsorbed layer thickness is not a well-defined quantity

but depends on the method of measurement. The block

copolymer layer thickness found by neutron reflectivity at

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the air-aqueous solution interface was 7.7 nm for F88

(90mM) (Sedev et al. 2002), 9.8 nm for F108 (70mM)

(Sedev et al. 2002), 7 nm for PEO23PPO52PEO23 (2 and

1200mM) (Vieria et al. 2002) and 5 nm for PEO9PPO22PEO9

(480mM) (Vieria et al. 2002). The difference in thickness

values returned by the different techniques is mainly due to

differences in sensitivity to the dilute tail region.

The information in the referenced literature taken together

clearly demonstrates that the thickness of foam films and inter-

facial layers increase with the size of the PEO-PPO-PEO

polymers due to an increased range of steric repulsion. Thus,

P85 films are thinner than F88 and F127 films, and a L64

film is even thinner. It is clear that an increasing range of

steric repulsion acting normal to the film surface increases

the foam stability of the block copolymer systems investigated.

CONCLUSIONS

The volume of foam formed from aqueous solutions of

nonionic PEO-PPO-PEO block copolymers decreases signifi-

cantly after the major part of the liquid of the foam has

drained. Dry foams of F88 (PEO103-PPO39-PEO103) and

F127 (PEO99-PPO65-PEO99) are more stable than those of

P85 (PEO26-PPO39-PEO26) compared both at the solution con-

centration of 1600mM and at concentrations related to the

CMC (0.5 CMC). Foam formed by L64 (PEO13-PPO30-

PEO13) was found to be highly unstable (1600mM).

Several naturally interrelated mechanisms of film stabiliz-

ation contribute to overall foam stability. The following con-

clusions about the various mechanisms can be drawn.

Stabilization by the Gibbs-Marangoni mechanism should be

possible, since the block copolymers are mobile at the inter-

face. A quantitative relationship between the dilatational

modulus and the foam stability that is universal for all PEO-

PPO-PEO block copolymers could not be found. Regarding

stabilizing intralayer repulsion normal to the film surfaces, a

simple relationship could be inferred. For PEO-PPO-PEO

block copolymer systems, good foam stability requires

long-range steric repulsion.

ACKNOWLEDGMENTS

The work was supported by funds from The Foundation of

Strategic Research, Colloid and Interface Programme,

Sweden. We thank the Institute of Food Reasearch, Norwich,

UK for use of the FRAP equipment and Thomas Arnebrant

for discussions regarding the foaming experiments. Torbjorn

Warnheim is thanked for valuable comments on the

manuscript.

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