JOURNAL OF OLEO SCIENCE
Copyright ©2004 by Japan Oil Chemists’ Society
J. Oleo Sci., Vol. 53, No. 12, 607-610 (2004)

JOS

Adsorption of Rod-Shaped Surface-Active Particles
at Liquid-Liquid Interfaces
Yoshimune NONOMURA1＊, Shigeyuki KOMURA2 and Kaoru TSUJII3
1Kao Corporation(Bunka, Sumida-ku, Tokyo, JAPAN)
2
Tokyo Metropolitan University(Minami-Osawa, Hachioji-shi, Tokyo, JAPAN)
3
Hokkaido University(Kita-ku, Sapporo, JAPAN)
Edited by M. Iwahashi, Kitasato Univ., and accepted July 8, 2004 (received for review June 10, 2004)

Abstract: The adsorption of a rod-shaped (rectangular parallelpiped) surface-active
particle at a liquid-liquid interface has been theoretically studied. The adsorption energy, whichis the free energy of the interface adsorbing the particle, is governed by the interfacial tensions
between the particle and the liquids and the particle shape. The aspect ratio of the rod-shaped
particle determines its adsorbing direction at the interface. The rod-shaped particle having aspectratio larger than unity adsorbs in parallel direction with the liquid-liquid interface, while theplate-shaped one having the aspect ratio less than unity does perpendicularly. These theoreticalresults are important to understand the effects of the particle shape on the behavior of thesurface-active particles in the self-assembled structures.
Key words: surface activity, adsorption energy, rod-shaped particle

1

Introduction

Surface-active particles have recently attracted muchattention in the field of colloid and surface science, particularly in emulsion science (1, 2). The particles act
like surfactant molecules if certain wettability conditions are satisfied. In such case, the particles areadsorbed at liquid-liquid interfaces and exhibit surface
activity. Hydrophobic silica, iron oxide, clay mineral,carbon black and polymer latex are some examples ofthe particles. The particles form self-assembled structures and act as emulsifiers in the preparation of emulsions (3-5).
Thermodynamics is powerful discipline to understand the behavior of the surface-active agents (6).
Interfacial tension, a free energy per unit area or a force
per unit length, is one of the factors for the geometry

and physical properties of the interfaces. Therefore, themechanism for the adsorption of the surface-active particles has been proposed based on the adsorption energy
argument (7). In the argument, the adsorption energy is
the free energy of the interface with the surface-activeparticle. When a spherical particle is adsorbed at a planar liquid-liquid interface, the energy depends on thevarious interfacial tensions between the particle surface
and the two liquids. The particle is adsorbed at theinterface when the interfacial tensions among the threephases are balanced according to the Young’s equation.
For traditional surfactant molecules, effects of themolecular shape on the interfacial properties have been
extensively studied by the models of Israelachvili orHelfrich (8, 9). However, there is few precedent literature as to the shape of the surface-active particle. In ourprevious studies, we showed that the shape of the parti-＊Correspondence to: Yoshimune NONOMURA, Kao Corporation, 2-1-3, Bunka Sumida-ku 131-8501, Tokyo
E-mail: nonomura.yoshimune@kao.co.jp
Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online
http://jos.jstage.jst.go.jp/en/607

Y. Nonomura, S. Komura and K. Tsujii

cle actually affects their self-assembled structures (3-5).
Rossmy also reported that the morphology of solid-stabilized emulsion depends on the geometry of the particles (10). These observations imply that the self-assembled structures of the surface-active particles are governed not only by the wettabilities but also the shape of
the particle.
In the present work, we have calculated the adsorption energy of rod-shaped surface-active particles asillustrated in Fig. 1. Several situations of adsorption
seem possible due to their strong anisotropic shape, andthe adsorption energy is important to understand thestability of each condition. The direction of the particle
at the liquid-liquid interface is estimated by the calculated adsorption energy. It is the first theoretical report
to discuss the influence of the anisotropic shape on thebehavior of the surface-active particle.

2

Results and Discussion

We now make discussions for the adsorption of arod-shaped surface-active particle at a liquid-liquidinterface. In the model, l is the length of one side of thesquare, h is the height of the rectangular parallelepiped
particle, and Gij is the interfacial tension between thephases i and j. Let F1 [F2] be the adsorption energy perparticle when the rod-shaped particle P adsorbs at theinterface between liquids A and B, and the main axis ofthe particle is parallel [perpendicular] to the interface(Fig. 1). Then, the energies are represented by F1 = F2 =2l(2h + l)GAP or F1 = F2 = 2l(2h + l)GBP when the wholeparticle is immersed either in liquid A or B. When theparticle is adsorbed at the interface between the liquids
A and B, the adsorption energy changes with the formation of the new interfaces between the particle P and thetwo liquids and the disappearance of the interface
between liquids A and B. The adsorption energies F1and F2 are given by eqs (1) and (2), respectively:

Fig. 1 Schematic Illustrations of a Rod-Shaped
Surface-Active Particle P Adsorbing (a)Parallel or (b) Perpendicular to the Interfaceof Liquid A and B. The interfacial tensionsof G AP , G BP and G AB are balanced at thecontact line. In the illustrations, l is thelength of one side of the square, h is theheight of the particle, and z is theimmersion depth into liquid A.

plot the dimensionless F 1 /(l 2 G AB ) against z/l when
GAP/GAB =0.2, GBP/GAB =0.6, and h/l=10 in Fig. 2(a). It isremarkable that the adsorption energy changes discontinuously at z=0 and l, that is, when the plane surfaceof the particle comes into contact with the liquid-liquidinterface. The energy gap at z=0 is ∆F10=lh(–GAP +GBP +GAB),
while that at z=l is ∆F1l =lh(GAP+GBP–GAB). The adsorption energy changes linearly with z due to the contact ofthe side surface with liquid A, and attains a minimum atz= l. The corresponding minimum energy is given by
F1min =lh(3GAP +GBP – GAB)+2l2GAP

F1 =lh(GAP +3GBP–GAB)+2l2GBP +2(l+h)(GAP–GBP)z(1)F2 =l2(GAP +GBP–GAB)+4lhGBP +4l(GAP–GBP)z(2)where z is the depth of immersion into the liquid A.
These equations are complicated, because they containmany interfacial or geometrical parameters. Then, we(3)On the other hand, we plot dimensionless F2/(l2GAB)
against z/l in Fig. 2(b) with the same parameter numbers. In such a case, the adsorption energy is minimized
at z= h. The energy gap at z=0 is ∆F20 =l2(–GAS +GBS +GAB), while that at z = h is F2h = l2(GAS – GBS + GAB). The
corresponding minimum energy is given by
F2min =l2(GAP +GBP – GAB)+4lhGAP

608

J. Oleo Sci., Vol. 53, No. 12, 607-610 (2004)(4)

Adsorption of Rod-Shaped Surface-Active Particles

adsorbed condition for the particle. In Fig. 3, we plot
F1min/(l2GAB) and F2min/(l2GAB) against the aspect ratio
h/l. Both energies of F1min and F2min increase with h/l
and coincides when h/l= 1. The minimum energy F1min
is smaller than F2min when the aspect ratio is smaller
than unity, and vice versa when the aspect ratio is morethan unity. Thus, the rod-shaped particle adsorbs in parallel direction to the liquid-liquid interface, while theplate-shaped one does in perpendicular direction. In ourprevious study, we actually observed solid-stabilized
emulsions in which plate-shaped surface-active powders adsorbed perpendicularly at the liquid-liquid interface (4).

3Fig. 2 The Dimensionless Adsorption Energies
against Immersion Depth z/l when theParticle Adsorbs (a) Parallel [F1/(l2GAB)] or(b) Perpendicular [F 2 /(l 2 G A B )] to theInterface. The parameters are as follows;
GAP/GAB=0.2, GBP/GAB= 0.6, and h/l=10.

The minimum energies F1min and F2min and the energygaps ∆F1 and ∆F2 are useful parameters to determine if
the particle adsorbs at the liquid-liquid interface. Whenthese parameters are positive, the particle adsorbs at theliquid-liquid interface spontaneously. These parametersindicate the stability of the interface adsorbing the particle, because they are the energies required to move theparticle from the interface to the liquids. The larger theenergy gap is, the more difficult it is to move the particle from the interface. If the energy gap is large enough,the adsorption is substantially irreversible.
The calculated adsorption energy indicates that theadsorbing condition depends on the anisotropy reflectedby the aspect ratio h/l. The energy show the most stable

Conclusions

In the present study, we study the physical propertiesof the interface adsorbing the rod-shaped surface-active
particle based on the adsorption energy argument. Theenergy profiles indicate that the particle adsorbs in parallel direction to the liquid-liquid interface, while theplate-shaped one does in perpendicular direction. The
conversion of the adsorbing direction is a unique behavior of the surface-active particle. Our investigations willcontribute to controlling the behavior of the surfaceactive particles and their applications.

Fig. 3 The Dimensionless Minimum Adsorption
Energies F1min/(l2GAB) (–––) and F2min/(l2GAB)() against the Aspect Ratio h/l. The
parameters are as follows; G AP /G AB = 0.2,
GBP/GAB= 0.6, and h/l=10.
609

J. Oleo Sci., Vol. 53, No. 12, 607-610 (2004)

Y. Nonomura, S. Komura and K. Tsujii

References
1. B.P. BINKS, Particles as Surfactants-Similarities and Differences, Curr. Opinion Coll. Int. Sci., Vol. 7, 21-41 (2002).
2. Y. NONOMURA, Development of Cosmetics with Solid-stabilized Emulsions, J. Jp. Soc. Colour Mat., Vol. 77, 23-25 (2004).
3. Y. NONOMURA, T. SUGAWARA, A. KASHIMOTO, K.
FUKUDA, H. HOTTA and K. TSUJII, Self-Assembly of Surface-Active Powder at the Interfaces of Selective Liquids. 1:
Multiple Structural Polymorphism, Langmuir, Vol. 18, 1016310167 (2002).
4. Y. NONOMURA, K. FUKUDA, S. KOMURA and K. TSUJII,
Self-Assembly of Surface-Active Powder at the Interfaces ofSelective Liquids. 2: Behavior of an Organic-Crystalline Powder, Langmuir, Vol. 19, 10152-10156 (2003).
5. Y. NONOMURA, S. KOMURA and K. TSUJII, Adsorption of
Disk-Shaped Janus-Beads at Liquid-Liquid Interfaces,Langmuir, submitted.
6. A.W. ADAMSON, A.P. GAST, Physical Chemistry of Surfaces,
Wiley Interscience: New York, Chapter II (1997).
7. S. LEVINE, B.D. BOWEN, S.K. PAETRIDGE, Stabilization ofEmulsions by Fine Particles I. Partitioning of Particles Between
Continuous Phase and Oil/Water Interface, Colloids Surf, Vol.
38, 325-343 (1989).
8. J.N. ISRAELACHVILI, Intermolecular and Surface Forces,Academic Press, London, Chapter 17 (1992).
9. S.A. SAFRAN, Statistical Thermodynamics of Surfaces, Interfaces, and Membrane, Addison-Wesley, New York, Chapter 6(1994).
10. G. ROSSMY, From Polyurethane Foam Stabilization toAmphiphilic Particles. An Example of Industrial R&D in Colloid Chemistry, Progr. Colloid Polym. Sci., Vol. 111, 17-26(1998).

610

J. Oleo Sci., Vol. 53, No. 12, 607-610 (2004)