Langmuir 2005, 21, 9409-9411

9409

Surface-Active Particles with Microstructured Surfaces
Yoshimune Nonomura,*,† Shigeyuki Komura,‡ and Kaoru 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, Nanotechnology Research Center,Research Institute for Electronic Science, Hokkaido University, N21, W10 Kita-ku,
Sapporo 001-0021, Japan
Received July 6, 2005. In Final Form: August 23, 2005
Interfaces adsorbing solid particles have recently attracted great attentions in the field of materials
science, because they are useful as templates of well-ordered particle arrays or of microstructured hollow
spheres. The solid particles are adsorbed at the interfaces and form self-assembled structures when theparticles have suitable wettability to both fluids. Here, we show theoretically how the microstructure on
the particle surface affects their adsorption properties. The physical properties of the interface adsorbinga particle will be described with consideration for surface area magnification due to the microstructure.
The microstructure on the surface changes drastically the wettability and the equilibrium position of theadsorbed particle and prevents the particle from adsorption at the interface. The range of the interfacialtensions at which the particle is adsorbed becomes narrower with the increase of the magnification. On
the other hand, the particle which is partially covered with the microstrucutred surface is adsorbed firmly
at the interface in an oriented state. We should consider not only the interfacial tensions but also thesurface structure to control the adsorption behavior of the particle.

Introduction
The surface-active adsorbed particles align at fluid-fluid
interfaces1,2 and stabilize emulsions and foams.3,4 These
emulsions and foams have been applied to food or cosmetic
products, because they are stable due to the adsorptionof the particles on the interfaces.5,6 In recent years, somescientists have focused on the emulsions as a template offunctional composite materials. A hollow sphere and aribosome-like nanocapsule were obtained through theemulsion state.7,8
The mechanism for the adsorption of the surface-activeparticle has been discussed based on the interfacial energy
argument.9 The equilibrium position of the adsorbed
particle can be quantitatively predicted from the interfacial energy. When spherical particles are adsorbed at aplanar interface, the energy depends on the three kindsof interfacial tensions, i.e., between the particle surface
and the two fluids. The particle is adsorbed at the interface
when the interfacial tensions among the three phases arebalanced according to Young’s equation. The effect of theparticle shape on the adsorption state was also describedelsewhere.10,11* To whom correspondence should be addressed. E-mail:nonomura.yoshimune@kao.co.jp. Tel: 81-3-5630-9419. Fax:81-3-5630-9339.
† Kao Corporation.
‡ Tokyo Metropolitan University.
§ Hokkaido University.
(1) Binks, Curr. Opinion Colloid Interface Sci. 2002, 7, 21.
(2) Kralchevsky, P. A.; Nagayama, K. Particles at Fluid Inrterfaces
and Membranes; Elsevier: Amsterdam, 2001.
(3) Nonomura, Y.; Sugawara, T.; Kashimoto, A.; Fukuda, K.; Hotta,H.; Tsujii, K. Langmuir 2002, 18, 10163.
(4) Nonomura, Y.; Fukuda, K.; Komura, S.; Tsujii, K. Langmuir 2003,19, 10152.
(5) Dickinson, E. An Introduction to Food Colloids; Oxford University
Press: Oxford, 1992.
(6) Kubo, S.; Hirano, I.; Yamada, J. IFSCC VIIIth International
Congress 1974, A8.
(7) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12,2374.
(8) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. D.; Marquez, M.;
Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006.
(9) Levine, S.; Bowen, B. D.; Paetridge, S. K. Colloids Surf. 1989, 38,325.

The interfacial properties of the particles with themicrostructured surface are interesting due to their great
potential as industrial materials, such as electronicmaterials, medical materials, and supported catalysts.12
Complicated rough structures which exist on the solid
surfaces change their wettability drastically.13 Wenzel,Cassie, and Baxter supposed the theoretical models which
describe the effect of the roughness on the wettability.14,15
For example, Tsujii et al. showed that fractal surfaces ofa wax, alkylketene dimer, is super water-repellent.16-19
Then, the microstructure must affect the behavior of thesurface-active particle.
In the present work, we have calculated the interfacial
energy of the fluid-fluid interface adsorbing sphericalparticles with a microstructured surface as illustrated in
Figure 1a. We study the effect of the microstructured roughsurface on the interfacial energy. The surface area of thespherical particle is r times larger than that of a sphericalparticle having a smooth surface. The quantity of r isdesignated as the surface area magnification factor. We
can obtain r by measuring specific surface area or by
analyzing the surface geometry based on the concept offractal.17 The interfacial energy can be calculated taking
the increase of the particle surface area into account. We
can also predict the behaviors of the spherical particles
partially covered with microstructured surface. In general,
partial modification of the surface causes the particles tobe adsorbed with a fixed orientation at the fluid-fluid(10) Nonomura, Y.; Komura, S.; Tsujii, K. Langmuir 2004, 20, 11821.
(11) Nonomura, Y.; Komura, S.; Tsujii, K. J. Oleo Sci. 2004, 53, 607.
(12) Yu, S.; Colfen, H. J. Mater. Chem. 2004, 14, 2124.
(13) Johnson, R. E.; Dettre, R. H. Contact angle, Wettability andAdhesion; Wiley: New York, 1964; p 112.
(14) Wenzel, R. N. Ind. Eng. Chem. 1949, 28, 988.
(15) Cassie, A. B. D., Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
(16) Onda, T.; Shibuishi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12,2125.
(17) Shibuishi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem.
1996, 100, 19512.
(18) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem.,
Int. Ed. 1997, 36, 1011.
(19) Shibuishi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid
Interface Sci. 1998, 208, 287.

10.1021/la051816m CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/10/2005

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Langmuir, Vol. 21, No. 21, 2005

Letterszmin ){}ΓAB - r(ΓAP - ΓBP)
RΓAB

(2)

Then the corresponding minimized energy is given by{ΓAB - r(ΓAP - ΓBP)}2Fmin ) 4πrR2ΓBP - πR2(3)ΓABFigure 1. Effect of microstructured surface on the adsorbing
state of the particle. (a) Schematic illustration of an interface
adsorbing a particle P. The particle P is adsorbed at the interface
between fluid A and B. z is an immersion depth of the particle.
(b) The dimensionless interfacial energy F/(πR2ΓAB) against
immersion depth z/R. The parameters are as follows, ΓAP/ΓAB) 0.4, ΓBP/ΓAB ) 0.5. The surface area magnification factor r isthe ratio of the surface area of the particle P to that of theparticle of smooth surface whose diameter is R. (c) The
adsorption-dispersion state diagram of the particle as functionsof (ΓAP - ΓBP)/ΓAB and r.

interface.20 Some methods for the partial modification havebeen proposed recently.21,22 We demonstrate here that thepartial modification of the surface is one of the mosteffective methods to control the surface activity of theparticles.
Results and Discussion
We now discuss the adsorption of particle P at the fluidfluid interface as shown in Figure 1a. Let F be theinterfacial energy per particle when it is adsorbed at theinterface between fluid A and B. Then F is given by F )4πrR2ΓAP or F ) 4πrR2ΓBP when the whole particle isimmersed either in the fluid A or B, respectively. Here R
is the radius of the particle, and Γij is the interfacial tension
between the phases i and j. When the particle is adsorbed
at the interface between the fluid A and B as shown in
Figure 1a, F is given by eq 1F ) 4πrR2ΓBP - 2πR{ΓAB - r(ΓAP - ΓBP)}z + πΓABz2(1)where z is the immersion depth into the fluid A. In thisinterfacial energy, we have taken into account both thecontact of the particle surface with the fluid A and theelimination of the interface between the fluid A and B.
The above energy F attains a minimum at zmin(20) Casagrande, C.; Fabre, P.; Raphael, E.; Veyssie, M. Europhys.
Lett. 1989, 9, 251.
(21) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H.
Langmuir 1999, 15, 4630.
(22) Hugonnot, E.; Carles, A.; Delville, M. H.; Panizza, P.; Delville,J. P. Langmuir 2003, 19, 226.

where the first term is the energy when the particle isimmersed in the fluid B, and the second term is the energychange due to the adsorption of the particle at the fluidfluid interface.
The surface area magnification factor r highly changes
the adsorbing state of the particle. Figure 1b shows theprofile of F in the range of 1 e r e 5 when ΓAP/ΓAB ) 0.4
and ΓBP/ΓAB ) 0.5. Both zmin and Fmin become larger withincreasing the factor r. The equilibrium position of theparticle at the fluid-fluid interface is determined not onlyby the interfacial tension of the particle surface but also
the surface area of the microstructured particle. This canbe understood by noting that the interfacial energy F
consists of two parts. The first one is the surface energy
between the particle P and the liquids. The second one isthe energy change due to the elimination of the interface
between fluids A and B. The effective increment of theparticle surface area influences the first factor directly,
although it does not affect the second one. Therefore, theequilibrium position shifts to the fluid A side when theparticle has a relatively higher affinity to the fluid A.
The particle cannot be anchored at the fluid-fluid
interface and is dispersed in the fluid phases when its
surface area becomes large. Thus, wettability of theparticle determines if the particle exhibits the surface
activity or not. The particle is adsorbed at the interface
when the equilibrium position satisfies 0 e zmin e 2R and,hence, eq 4

ΓAP - ΓBPe r-1
ΓAB-r-1 e(4)The above condition is drawn in Figure 1c which indicatesthat the particle adsorbs at the fluid-fluid interface whenthe values of r and Γij are suitable. The particle is dispersedin the fluid A [B] if (ΓAP - ΓBP)/ΓAB is smaller than r-1[larger than r-1]. The range of the interfacial tensions at
which the particle is adsorbed becomes narrower with theincrease of r. The allowed range for adsorption isΓAP - ΓBPe1
ΓAB-1 ewhen r ) 1, whereas it is-0.2 e

ΓAP - ΓBPe 0.2
ΓABwhen r ) 5. We should consider not only the interfacial
tensions but also the surface structure to control theadsorption behavior of the particle.
Let us consider next a spherical particle partiallycovered with the microstructured surface as shown inFigure 2a,b. We assume here that (i) the hemisphere Pm
is covered by microstructured surface, whereas the otherhemisphere Ps is a smooth one, and (ii) the surface areaof the Pm region is r times larger than that of Ps. Theinterfacial energy F decreases drastically when the particle
in fluid B approaches to the interface from the Pm side

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Langmuir, Vol. 21, No. 21, 2005 9411

interfacial energy is seen as multiplied by the factor r,
does not work when the roughness is above a critical value
rc.23 If r > rc, the contact surface of the particle with thefluid consists of the solid surface and the fluid which exist
in the ditch on the particle surface. The quantity rc isfound to be (φs - 1)/cos θ + φs which is close to -1/cos θ.24,25
Here, θ and φs are the Young contact angle at the threephase contact line and the actual ratio of the solid-fluid
interface on the surface. For example, the quantity of rc
is 10 when ΓAP ) 16 mN m-1, ΓBP ) 20 mN m-1 and ΓAB) 40 mN m-1.

Figure 2. Adsorption of partially microstructured particle.
(a) The particle consisting of the hemisphere of Pm and Ps part
approaches from the Pm side (type 1). (b) The particle of above
approaches from the Ps side (type 2). (c) The dimensionlessinterfacial energy F/(πR2ΓAB) against immersion depth z/R. The
parameters are as follows, ΓAP/ΓAB ) 0.4, ΓBP/ΓAB ) 0.5, r ) 5.

(Figure 2a,c type 1). In this case, the energy F decreases
with the particle adsorption and the cusp is located at theboundary between the Pm and the Ps part, i.e., z ) R. On
the other hand, when the particle contacts with theinterface from the Ps side, the energy change is smaller
than the previous case (Figure 2b,c type 2). The cusp
appears also at z ) R. These results indicate that thepartial modification of the surface increases the surface
activity and fixes the particle direction at the fluid-fluid
interface, even if the bulk material is completely the same.
The present theory can be applied only with the particle
having fine microstructures. The assumption, that theConclusions
In this paper, we have shown that the surface activityof the solid particle can be controlled by the microstructureon the particle surface. The strength of the adsorption
and the direction of the particle at the fluid-fluid interface
are governed by the interfacial tensions and the microstructure of the particle surface. We can inscribe themicrostructure on the particle surface. For example, oneof the present authors (K.T.) has succeeded in preparing
microstructured particles by the spray dry method.26 Some
predictions in this work have not yet been proven by theexperiments. Further experimental and theoretical studies
are necessary to understand the behavior of the surfaceactive particles.
Acknowledgment. The authors thank Drs. T. Onda,
N. Sato, S. Shibuichi, Y. Murase, and H. Mayama for theirkind and useful discussions.
LA051816M(23) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity andWetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer-Verlag:
New York, 2003; Chapter 9.
(24) Bico, J.; Thiele, U.; Quere, D. Colloids Surf. A 2002, 206, 41.
(25) Patankar, N. Langmuir 2003, 19, 1249.
(26) Mayama, H.; Tsujii, K. Creation of a Fractal Body, 11th
International Conference on Organized Molecular Films, O55, June 2630, Sapporo, Japan 2005.