Langmuir 2004, 20, 11821-11823

Adsorption of Disk-Shaped Janus Beads at
Liquid-Liquid Interfaces
Yoshimune Nonomura,*,† Shigeyuki Komura,‡ andKaoru Tsujii§
Tokyo Research Laboratories, Kao Corporation,
2-1-3 Bunka, Sumida-ku, Tokyo 131-8501, Japan,
Department of Chemistry, Faculty of Science, Tokyo
Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi,
Tokyo 192-0397, Japan, and Nanotechnology ResearchCenter, Research Institute for Electronic Science, Hokkaido
University, N21, W10 Kita-ku, Sapporo 001-0021, Japan
Received August 2, 2004. In Final Form: October 1, 2004

Surface-active powders have recently attracted muchattention in the field of colloid and surface science,
particularly in emulsion science.1 When certain wettability
conditions are satisfied, solid particles exhibit surface
activity.1-4 Hydrophobic silica, iron oxide, clay mineral,carbon black, and polymer latex are some examples of theparticles. The adsorbed surface-active powders align at
liquid-liquid interfaces and form self-assembled structures.2,3 The so-called “Janus beads” (J-beads) having two
surface regions of different wettability are unique particles
which can be compared with ordinary surfactant molecules.5 As a result of their amphiphilic nature, the J-beads
exhibit stronger surface activity than the homogeneous
surface-active powders. It was shown that sphericalJ-beads are anchored along the boundary line of two
surface regions.
A mechanism for the adsorption of homogeneous
surface-active powders has been described by Levine etal. based on the adsorption energy argument.6 When aspherical particle is adsorbed at a planar liquid-liquidinterface, the adsorption energy is governed by the various
interfacial tensions between the particle and the liquids
as well as the particle radius. The particle is adsorbed at
the interface when the interfacial tensions among the threephases are balanced according to the Young’s equation.
For the J-beads, on the other hand, Clint and Binks
obtained the corresponding adsorption energy to see theeffect of the particle amphiphilicity.7 They showed thatthe amphiphilic J-beads have a typically larger adsorptionenergy (hence, stronger adsorption) than the homogeneousparticles.
For traditional surfactant molecules, effects of themolecular shape on the interfacial properties have been
extensively studied by using the models proposed byIsraelachvili or Helfrich.8,9 However, there is little precedent literature as to the shape of the surface-active* To whom correspondence should be addressed. Tel.: 81-3-56309419. Fax: 81-3-5630-9339. E-mail: nonomura.yoshimune@kao.co.jp.
† Kao Corp.
‡ Tokyo Metropolitan University.
§ Hokkaido University.
(1) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.
(2) Nonomura, Y.; Sugawara, T.; Kashimoto, A.; Fukuda, K.; Hotta,
H.; Tsujii, K. Langmuir 2002, 18, 10163.
(3) Nonomura, Y.; Fukuda, K.; Komura, S.; Tsujii, K. Langmuir 2003,
19, 10152.
(4) Nonomura, Y.; Komura, S.; Tsujii, K. J. Oleo Sci., in press.
(5) Casagrande, C.; Fabre, P.; Raphael, E.; Veyssie, M. Europhys.
Lett. 1989, 9, 251.
(6) Levine, S.; Bowen, B. D.; Paetridge, S. K. Colloids Surf. 1989, 38,
325.
(7) Clint, J. H.; Binks, B. P. Langmuir 2002, 18, 1270.
(8) Israelachvili, J. N. Intermolecular and Surface Forces; Academic
Press: London, 1992; Chapter 17.

11821

particle. In our previous experimental studies, we showedthat the shape of the particles indeed affects their selfassembled structures.2,3 Rossmy also reported that themorphology of the emulsions stabilized by J-beads depends
on the geometrical property of the beads.10 Recently, Brown
et al. have shown that amphiphilic curved disks distort
an air-water interface, giving rise to anisotoropic forces
between the disks.11 These observations imply that the
self-assembled structures of the surface-active particles
are governed not only by the wettabilities but also by theshapes of the particle.
In the present work, we obtain the adsorption energyof the disk-shaped J-beads as illustrated in Figure 1a. In
contrast to the spherical J-beads, the adsorption energychanges discontinuously for the disk-shaped J-beads. We
also show the effect of the adsorption on the geometry of
the liquid-liquid interface.
We discuss the adsorption of a disk-shaped J-bead at
a liquid-liquid interface. As shown in Figure 1a, a diskshaped J-bead consists of part P1 with thickness l1 andpart P2 with thickness l2. Let F be the adsorption energy
per bead when it is adsorbed at the interface between theliquid A and the liquid B (see Figure 1b). When the whole
bead is immersed either in liquid A or in liquid B, F isgiven by eq 1 or 2:

F ) πRp2(ΓAP1 + ΓAP2) + 2πRp(l1ΓAP1 + l2ΓAP2)
in liquid A (1)F ) πRp2(ΓBP1 + ΓBP2) + 2πRp(l1ΓBP1 + l2ΓBP2)
in liquid B (2)where Γij is the interfacial tension between phases i andj and Rp is the radius of the disk-shaped particle. Whenthe bead is adsorbed at the interface between the liquid
A and the liquie B, F is now given by eqs 3 and 4:

F ) πRp2(ΓAP1 + ΓBP2 - ΓAB) + 2πRp(l1ΓBP1 +l2ΓBP2) + 2πRp(ΓAP1 - ΓBP1)zfor 0 e z e l1 (3)F ) πRp2(ΓAP1 + ΓBP2 - ΓAB) + 2πRp{l1ΓAP1 - l1ΓAP2 +(l1 + l2)ΓBP2} + 2πRp(ΓAP2 - ΓBP2)zfor l1 e z e l1+ l2 (4)where z is the depth of immersion into the liquid A. In
Figure 2 we plot F/(πRp2ΓAB) as a function of z/Rp whenΓAP1/ΓAB ) 0.25, ΓBP1/ΓAB ) 1, ΓAP2/ΓAB ) 0.9, ΓBP2/ΓAB ) 0.3,
and l1/Rp ) l2/Rp ) 0.1. It is remarkable that the adsorptionenergy changes discontinuously at z ) 0 and l1 + l2, thatis, when the plane surface of the bead contacts with theliquid-liquid interface. The energy gap at z ) 0 is ∆F1 )
πRp2(-ΓAP1 + ΓBP1 + ΓAB), while that at z ) l1 + l2 is ∆F2) πRp2(ΓAP2 - ΓBP2 + ΓAB). The adsorption energy changes
linearly with z as a result of the contact of the side surface(9) Safran, S. A. Statistical Thermodynamics of Surfaces, Interfaces,and Membrane; Addison-Wesley: New York, 1994; Chapter 6.
(10) Rossmy, D. Prog. Colloid Polym. Sci. 1998, 111, 17.
(11) Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Phys. Rev. E 2000,
62, 951.

10.1021/la0480540 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/18/2004

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Langmuir, Vol. 20, No. 26, 2004

Notes

Figure 1. Schematic illustrations of (a) a disk-shaped J-bead and (b) an interface adsorbing the bead. The bead consisting of the P1
part and P2 part is adsorbed at the interface between liquid A and liquid B. In the illustrations, Rp is a radius of the disk-shaped bead,l1 and l2 are the thicknesses of the P1 and P2 parts, and θ is a contact angle. The interfacial tensions of ΓAP1, ΓBP2, and ΓAB are balanced
at the contact line.

with liquid A and attains a minimum at z ) l1. The
corresponding minimum energy is given by

Fmin ) πRp2(ΓAP1 + ΓBP2 - ΓAB) + 2πRp(l1ΓAP1 +l2ΓBP2) (5)It is characteristic of disk-shaped J-beads that they areanchored at the liquid-liquid interface along the boundaryline of two surface regions. Similar to the present case,the position of the liquid-liquid interface does not changewith the wettabilities between the bead and the liquids
for spherical J-beads.5
Suppose that we can control the depth of immersion z
in Figure 1, and then the contact angle θ of the contactline would depend on z. If the side surface of the particle
is completely smooth and homogeneous, θ should bedetermined by the Young’s equation. As discussed above,
the adsorption energy is minimized when the contact line
is attached to the boundary line between P1 and P2. In
this case, however, θ can take any value which satisfies
θ1 < θ < θ2, where θ1 and θ2 are the contact angles between
P1/A/B phases and between P2/A/B phases, respectively(Figure 3).12,13 Namely, θ1 and θ2 are related to theinterfacial tensions Γij by the Young’s equation:cos θ1 )-ΓAP1 + ΓBP1
ΓAB

and cos θ2 )

ΓAP2 + ΓBP2
ΓAB

(6)

From a simple geometrical consideration, the curvature
1/Ri of the liquid-liquid interface is given by eq 7 whenΓAP1 < ΓBP1 and ΓAP2 > ΓBP2,-ΓAP2 + ΓBP2 1
ΓAB

Rp

e

-ΓAP1 + ΓBP1 1
1
e
Ri
ΓAB
Rp

(7)

because the relation cos θ ≈ Rp/Ri holds. The curvaturechanges from a positive value to negative one at theboundary line. It indicates that the outer phase of theemulsion could be inverted. However, the actual situation(12) Kralchevsky, P. A.; Nagayama, K. Particles at Fluid Inrterfaces
and Membranes; Elsevier: Amsterdam, 2001; Chapter 2, 6.
(13) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity andWetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer-Verlag:
New York, 2003; Chapter 3, 9.

Figure 2. Dimensionless adsorption free energy F/(πRp2ΓAB)against immersion depth z/Rp for (a) a disk-shaped J-bead and(b) a homogeneous surface-active powder at constant radius of
the disk-shaped powder Rp and interfacial tensions Γij. The
parameters are as follows: ΓAP1/ΓAB ) 0.25, ΓBP1/ΓAB ) 1, ΓAP2/ΓAB) 0.9, ΓBP2/ΓAB ) 0.3, and l1/Rp ) l2/Rp ) 0.1 (l1/Rp ) 0.2 and l2/Rp) 0 for a homogeneous surface-active powder).

NotesFigure 3. Contact angle θ against immersion depth z for a
disk-shaped J-bead. The contact angle θ is calculated when thebead moves from phase B to phase A.

is more complicated than the prediction. The curved
surface of the emulsion droplet leads to a pressure
difference across the interface. If the particle is attachedat the droplet surface, the pressure difference pushes it
outward. The curvature of the interface may be determinedfrom the condition that the surface tension force counterbalances the Laplace pressure force acting on theparticle.
A contact angle hysteresis is expected for a disk-shaped
J-bead. We consider how θ varies as the bead is moved
from the phase B to the phase A through the liquidliquid interface. The contact angle θ is a constant value
θ1 when z ) 0 e z e l1, while it is θ2 when l1 e z e l1 +l2. In the process, three discontinuous changes are observed
at z ) 0, l1, and l1 + l2. On these special positions, θ changes
discontinuously as a result of the pinning effect at theboundary line between the P1 and the P2 parts and at theedge of the particle. This unique behavior is characteristicof the disk-shaped surface-active particles and is notobserved for spherical beads.
A homogeneous particle consisting of the P1 part onlyis the special case of the J-bead. In such a case, theadsorption energy is minimized at z ) l1 as shown in Figure
2b. This occurs when the plane surface of the particle
contacts with the liquid-liquid interface. The adsorptionof such a homogeneous particle affects the shape of theliquid-liquid interface similar to the disk-shaped J-bead.
The discontinuous change of θ is also expected when z )0, l1.
In general, the adsorption of the spherical or the diskshaped surface-active particles at liquid-liquid interfaces
is much stronger than that of ordinary surfactant mol-

Langmuir, Vol. 20, No. 26, 2004 11823

ecules. The adsorption energy for the surface-active
particle of 1-10 µm size is approximately 108-109 kBT,
while that for the ordinary surfactant molecule is 1-10
kBT. For example, Fmin is 4 × 108 kBT when ΓAB ) 36 mN
m-1, ΓAP1 ) 6 mN m-1, ΓBP2 ) 32 mN m-1, Rp ) 10 µm, l1) l2 ) 1 µm, and the temperature ) 298 K. An extremely
large gain in the adsorption energy results in an irreversible adsorption of the particle at the liquid-liquidinterface.1 Detachment of the disk-shaped particle fromthe interface is almost impossible because of the large
energy gap either at z ) 0 or at z ) l1 + l2 in Figure 2a.
The irreversible adsorption of the powder leads to thestabilization of emulsions, because the coalescence ofemulsion droplets depends on the surface area covered
with the particles. These predictions are confirmed byour experimental investigations where we showed thatsolid-stabilized emulsions are stable for more than 2
weeks.2,3 Arditty et al. reported that the growth of solidstabilized droplets is rather slow because their coalescenceis supressed by the adsorbed powders.14 The stability ofthe emulsion is a great advantage in the production ofindustrial materials.
The geometrical change of the liquid-liquid interface
caused by the adsorption of the disk-shaped J-bead is an
important result of the present study. As shown in eq 7,
the range of Ri is determined by the interfacial tensions
between the liquid and the solid phases. The hysteresisof the contact angle is one of the important properties ofthe disk-shaped particles. For ordinary surfactants, themolecular structure rules the curvature of the interface.8,9However, the curvature of the interface adsorbing the diskshaped particles is not uniquely determined. Therefore,
the common sense for the surfactants does not apply tothe surface-active particles.
In summary, the adsorption energy of the surface-active
disk-shaped J-beads at the liquid-liquid interfaces has
been obtained. The estimated adsorption energy is remarkably larger than that of the ordinary surfactant
molecules. We also predict the geometrical change of theliquid-liquid interfaces as a result of the adsorption of
the beads and the contact angle hysteresis. Some predictions in this work have not yet been proved by theexperiments. Further experimental and theoretical studies
are necessary to understand the behavior of the surfaceactive powders.
LA0480540(14) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; Leal-Calderon,F. Eur. Phys. J. E 2003, 11, 273.