10152

Langmuir 2003, 19, 10152-10156

Self-Assembly of Surface-Active Powder at the Interfacesof Selective Liquids. 2: Behavior of an Organic-Crystalline
PowderYoshimune Nonomura,*,† Keiichi Fukuda,† 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,
N12, W6 Kita-ku, Sapporo 060-0812, Japan
Received August 18, 2003. In Final Form: September 17, 2003
A ternary system consisting of a surface-active organic-crystalline powder (fluorinated calcium N-lauroyl
taurate powder), fluorinated oil (perfluoropolymethylisopropyl ether, PFPE), and silicone oil (dimethylpolysiloxane, DMS) has been studied to examine the self-assembling behavior of the powder. Mixedstates of the ternary system were classified into eight regions: (i) network-structured state, (ii) O/F emulsion
state (O/F emulsion means DMS-in-PFPE type emulsion), (iii) O/F emulsion state with excess PFPE andDMS phases, (iv) separated state, (v) powdery state, (vi) granular state, (vii) powder dispersion state withexcess PFPE and DMS phases, and (viii) dispersion state of powder and PFPE. The separated state, inwhich a transparent DMS phase was separated from a milky white phase containing the powder and
PFPE, was observed first in this system. The O/F emulsion was able to contain a larger amount of DMS
oil than the ternary system having a spherical surface-active resin powder reported in our previous paper[Langmuir 2002, 18, 10163]. Shear-stress curve and contact angle measurements as well as theoreticalanalysis indicated that these interesting observations originated from the plate-type shape of the organiccrystalline powder and its characteristic network in the fluorinated oil.

Introduction
Amphiphilic molecules and polymers form stable supramolecular assemblies, such as micelles, microemulsions, and liquid crystals, in which their structure isdependent on thermodynamic or kinetic parameters.1,2
However, there are a few reports on surface-active powders
producing self-assembling states. Pickering reported anemulsion stabilized with powder particles in 1907.3 Since
then, some other solid-stabilized emulsion systems havebeen found.4-6 Some surface-active powders adsorb, alignat a liquid-liquid interface, and form self-assembled
structures in multicomponent systems. Because this typeof emulsion is stable due to the adsorption of the particles
on the interface, it has been applied to food or cosmetic
products.5,7 In recent years, some scientists have focusedon this type of emulsion as a template of functional
composite materials. A microstructured hollow sphere anda ribosome-like nanocapsule were obtained through the* To whom correspondence should be addressed. E-mail:nonomura.yoshimune@kao.co.jp.
† Kao Corp.
‡ Tokyo Metropolitan University.
§ Hokkaido University.
(1) Solvent Properties of Surfactant Solution; Shinoda, K., Ed.; Marcel
Dekker: New York, 1967.
(2) Surfactants and Polymers in Aqueous Solution; Lindman, J.,
Kronberg, H., Ed.; John Wiley & Sons Ltd.: West Sussex, U.K., 1998.
(3) Pickering, S. U. J. Chem. Soc. 1907, 9, 2001.
(4) Breen, P. J.; Wasan, D. T.; Kim, Y. H.; Nikolov, A. D.; Shetty, C.
S. In Emulsion and Emulsion Stability; Sjoblom, J., Ed.; Marcel
Dekker: New York, 1996; p 237.
(5) Dickinson, E. An Introduction to Food Colloids; Oxford University
Press: Oxford, 1992.
(6) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.
(7) Kubo, S.; Hirano, I.; Yamada, J. IFSCC VIIIth International
Congress 1974, A8.

emulsion state.8-11 A “liquid marble”, being an aqueous
liquid droplet covered with a hydrophobic powder, wasprepared and behaved like a soft solid in air.12
Some organic-crystalline powders behave as a surfaceactive powder. The crystalline powders of triglyceride andfatty acid affect the taste and the quality of emulsiontype foods, that is, whipped cream and margarine, throughthe dispersed state of the inner phases of the emulsions.5
On the other hand, the powders are useful to stabilize
emulsion-type cosmetics and to control the viscosity andthe lubricity.13 However, the governing factors to control
the behavior of these products are not well-known.
In our previous work, we studied the behavior of aspherical surface-active resin powder.14 In a mixture of
the powder and two liquids, multiple structures such as
O/F emulsion state (where O/F denotes an emulsion ofsilicone oil in fluorinated oil), network-structured state,
and granular state were observed. The wettability of thepowders with the fluorinated and silicone oils is thegoverning factor in the structure formation. However,these results for the resin powder should not be applied
to the system having the organic-crystalline powder,because the shape and the surface composition of thepowder are quite different from those of the spherical resin
powder. Many organic-crystalline powders, such as zinc(8) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12,2374.
(9) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12,2385.
(10) Velev, O. D.; Nagayama, K. Langmuir 1997, 13, 1856.
(11) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. D.; Marquez, M.;
Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006.
(12) Aussillous, P.; Quere, D. Nature 2001, 411, 924.
(13) JP55-81735, JP55-83707, JP56-39905.
(14) Nonomura, Y.; Sugawara, T.; Kashimoto, A.; Fukuda, K.; Hotta,
H.; Tsujii, K. Langmuir 2002, 18, 10163.

10.1021/la035511o CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/29/2003

Self-Assembly of Surface-Active Powder

stearate and N-lauroyl-L-lysine, are plate-shaped sincethese molecules contain alkyl chains and assemble to form
lamellar crystals.15-18 Moreover, they change the rheology
of some nonaqueous liquids. The powders increase theviscosity of the suspensions, since the crystalline powders
form network aggregates in the nonaqueous liquids.13
In the present study, we have focused on the fluorinatedcalcium N-lauroyl taurate powder, PF-5 CaLT, to characterize the self-assembled structure of the organiccrystalline powder (the nomenclature PF-5 denotes a
powder treated with a 5 wt % solution of the fluorinatedagent, diethanolamine salt of perfluoroalkyl phosphate).
CaLT is a lamellar molecular-crystal with laminatedlayers of linear molecules.17,18 This powder is suitable forour purpose, because such a layered structure is typicalfor the organic-crystalline powders. This powder is
expected to be useful as an ingredient of cosmetics or
lubricants because of its high lubricity and high stability
for thermolysis. The mixed state and the physicochemical
properties of the ternary system consisting of PF-5 CaLT,
fluorinated oil (perfluoropolymethylisopropyl ether, PFPE),
and silicone oil (dimethylpolysiloxane, DMS) have been
investigated. The characteristic behaviors and theirmechanisms are studied by a shear-stress curve, contact
angle measurements, and theoretical analysis.

Langmuir, Vol. 19, No. 24, 2003 10153

Figure 1. A mixed state diagram of the PF-5 CaLT powder/
PFPE/DMS mixtures: region I, network-structured state; region
II, O/F emulsion 1 state; region III, O/F emulsion 2 state; regionIV, separated state; region V, powdery state; region VI, granularstate; region VII, O + FS + F; region VIII, OF,S.

Experimental Section
Materials. CaLT was synthesized by a method described in
the literature.17 CaLT has a molecular structure of C11H23CONH(CH2)2SO3‚0.5Ca, a specific gravity of 1.25 g cm-3, and an average
particle diameter of 8 µm.17 PFPE and DMS were commerciallyavailable and were used as received. PFPE (FOMBLIN HC/04,
Ausimont Co.) has a molecular structure of CF3-[(OCF(CF3)CF2)n-(OCF2)m]-OCF3 (n/m ) 20/40), a molecular weight of 1500,
and a specific gravity of 1.79 g cm-3. DMS (KF96A(6CS), Shinestu Chemical Co., Ltd.) has a molecular structure of (CH3)3SiO[Si(CH3)2O]nSi(CH3)3, a molecular weight of 900, and a specificgravity of 0.93 g cm-3. A fluorinated powder treated with afluorinated agent, diethanolamine salt of perfluoroalkyl phosphate, was purchased from Daito Kasei Co. The degree of surface
modification was analyzed by contact angle measurements, anelectron probe microanalyzer, and an X-ray photoelectron
spectroscopy technique. The contact angle was measured for
powder tablets with water, PFPE, and DMS.
Preparation and Characterization of the Ternary Mixtures. The powder and PFPE were mixed using a conditioning
mixer (Thinky MX-201) at 2000 rpm for 3 min. After adding
DMS, the mixture was restirred under the same conditions. The
ternary mixtures thus obtained were transferred into screwcapped test tubes and placed in a thermostatic water bath at 298( 1 K. The mixed states of the samples were observed with a
Nikon OPTIPHOTO-2 optical microscope. Determination of thecontinuous and discrete phases of the mixtures was accomplishedby observing the dispersibility of the mixture drop in either PFPEor DMS. PFPE-continuous (DMS-continuous) mixtures are easily
dispersed in PFPE (DMS) and remain as drops in DMS (PFPE).
Separations of PFPE and/or DMS were observed within a few
hours after preparation of the mixtures. The mixed states wererechecked 12 h after the preparation to produce the state diagramof the PF-5 CaLT/PFPE/DMS ternary system.
Measurements. The contact angle was measured by a CA-X
contact angle measurement apparatus (Kyowa Interface ScienceCo.) 30 s after putting a DMS droplet on the powder/PFPE
mixture. Viscosity measurements were carried out by an A & D
vibro viscometer CVJ5000 at 298 ( 1 K. The shear-stress curve,that is, strain-stress relations under constant shear rates, wasmeasured with an RFS-II viscoelastic measuring apparatus(15) Vold, R. D.; Hattinangdi, G. S. Ind. Eng. Chem. 1949, 41, 2311.
(16) Takehara, M. Colloids Surf. 1989, 38, 149.
(17) Nonomura, Y.; Kurita, K.; Kashimoto, A.; Hotta, H.; Kaneko, Y.;
Iitaka, K. Chem. Lett. 2002, 2, 216.
(18) Nonomura, Y.; Kurita, K.; Kashimoto, A.; Hotta, H.; Suzuki, T.;
Kaneko, Y.; Iitaka, K. Bull. Chem. Soc. Jpn. 2002, 75, 2305.

(Rheometrics Co.) using a cone-plate rotator (diameter, 25 mm;angle, 0.04 rad). The shear rate was in the range of 0.01-20 s-1.
Samples were allowed to stand for 30 min before each run of themeasurements. The shear stress of Bingham plastics can be
written as τ ) τB + ηBγ; where τB is the yield stress, ηB is theplastic viscosity, and γ is the shear rate. The yield stress τB isadopted as an index of strength of the network aggregate andis obtained by the extrapolation of the shear-stress curves.
Interpretation of Symbolism. In the diagram, S, F, and O
denote the powder, PFPE, and DMS, respectively. FS (OS)indicates a state in which the powder particles were dispersed
in PFPE (DMS). FS,O (OF,S) means that the powder and DMS(PFPE) were dispersed in PFPE (DMS). SO,F means that thepowder component absorbs PFPE and DMS oils in the spaces
between the particles. O/F indicates a DMS-in-PFPE typeemulsion. O/FS + F indicates an emulsion with a separated phaseof PFPE, and O + O/FS + F an emulsion with two separated
phases of DMS and PFPE. O + FS indicates a dispersion witha separated phase of DMS, and O + FS + F a dispersion with two
separated phases of DMS and PFPE. O + OF,S indicates adispersion with a separated phase of DMS. The average volume
fraction of DMS in the system is denoted as R (R ) DMS/(DMS+ PFPE + powder)). The volume fraction of the powder in the
PFPE/powder mixture is denoted as β (β ) powder/(PFPE +powder)).

Results
The mixed states were mainly governed by the composition ratio of the powder and PFPE, β. The continuous
phase was PFPE when β < 0.4 (Figure 1, I-III and VII),
while that was DMS when β > 0.65 (Figure 1, VI and
VIII). In the powdery state, the continuous phase was
unclear, because the powder component contained both
PFPE and DMS oils in the inside spaces among theparticles (Figure 1, V). The separated state consisting ofa milky PFPE phase (containing PF-5 CaLT particles)
and a transparent DMS phase was observed for β ) 0.40.65 (Figure 1, IV). The details of the mixed states are as
follows:(i) Network-Structured State. When R < 0.07 and β <0.4, the mixture was in the “network-structured state”(FS,O) (Figure 1, I; Figure 2a). The effect of DMS content
on the viscosity of the PF-5 CaLT/PFPE/DMS ternary
system at β ) 0.26 is shown in Figure 3. When β was

10154Langmuir, Vol. 19, No. 24, 2003

Nonomura et al.

Figure 2. Optical microscopic photographs of the PF-5 CaLT powder/PFPE/DMS mixtures: (a) 26/72/2 (v/v/v), network-structured
state (FS,O, region I in Figure 1); (b) 19/54/27 (v/v/v), O/F emulsion 1 state (O/FS + F, region II); (c) 10/27/63 (v/v/v), O/F emulsion
2 state (O + O/FS + F, region III); (d) 42/44/14 (v/v/v), separated state (Fs phase in O + Fs, region IV); (e) 9/2/89, granular state(O + OF,S, region VI).

Figure 3. Viscosity of the PF-5 CaLT powder/PFPE/DMS
mixtures plotted against DMS composition R when β ) 0.26.

0-0.07, the viscosity increased with the DMS content
because the powder particles were bound to each other by
the addition of DMS to form a network structure.
(ii) O/F Emulsion 1 State. When R was 0.07-0.40, themixture was in the “O/F emulsion 1 state” (O/FS + F),
which was a milky DMS-in-PFPE type emulsion (Figure
1, II; Figure 2b). The emulsion was fluid, and the size of
DMS droplets with the adsorbed powder particles was
about several tens of micrometers. Flocculation andcreaming of the DMS droplets and separation of the excess
PFPE phase at the bottom were observed. When β wasconstant, the viscosity remained almost constant andindependent of the DMS composition. For example, theviscosity was 400-600 (mPa s) when β was 0.26 (Figure3). The maximum amount of DMS contained as stableemulsion droplets was about 40 vol %. This amount ofupper limit was about 1.3 times larger than that for the
spherical surface-active resin powder reported in ourprevious paper.14 The reason will be described in thediscussion section.
(iii) O/F Emulsion 2 State. When the DMS content
exceeds the limit value of the O/F emulsion 1 state (R >0.4), the mixture becomes the “O/F emulsion 2 state” (O+ O/FS + F) (Figure 1, III; Figure 2c). The DMS oil phaseseparated out by coalescence, and the PFPE phase by
creaming. The size of DMS droplets was larger and more
widely distributed compared with that in the O/F emulsion
1 state. Some emulsion droplets were distorted from thespherical shape.
(iv) Separated State. When β ) 0.4-0.65, self-assembled
structures such as solid-stabilized emulsions or granules
were not observed. The system consisted of two separated
phases: the white PFPE phase in which the powder was
dispersed and a transparent DMS phase (“separated state”(O + FS), Figure 1, IV; Figure 2d). These two phases wereseparated regardless of the composition ratio of DMS. Thisregion was not observed in the system of the sphericalsurface-active resin powder.14 To our knowledge, this is
the first finding of such an unusual region in the ternary
system consisting of the powder and two immiscible
liquids.
(v) Powdery State. When the PFPE content was lessthan the limiting amount of oil absorption to the powderSelf-Assembly of Surface-Active Powder

Langmuir, Vol. 19, No. 24, 2003 10155(β > 0.65) and R was also below a certain volume, themixture was in the “powdery state” (SO,F; Figure 1, V). In
this state, the powder component contained both PFPE
and DMS oils in the inside spaces among the particles.
(vi) Granular State. When more DMS oil was added tothe powdery state, the mixture entered into the “granularstate” (O + OF,S) (Figure 1, VI; Figure 2e). The powder
particles were granulated in the DMS medium. PFPE wasabsorbed among the particles and acted as a binding
material. A clear DMS phase was separated from the
mixture by creaming after standing for a couple of hours
and attained a stationary state after approximately 10days.
(vii) Powder Dispersion State with Excess PFPE andDMS Phases. When the powder content was small (<0.5
vol %), neither emulsion nor granules were obtained(Figure 1, VII; O + FS + F). The powder particles weredispersed in one PFPE phase, while another clear PFPE
phase and a DMS phase were separated.
(viii) Powder/PFPE Dispersion State. When the PFPE
content was small (<0.5 vol %), the powder particles and
PFPE were dispersed in DMS (Figure 1, VIII; OF,S).
Discussion
The surface-active organic-crystalline powder, PF-5
CaLT, shows some characteristics of the self-assembling
behavior resulting from its shape, the chemical composition, and the aggregation of the particles. These characteristics become clear by comparison with the behavior of
the spherical surface-active resin powder, that is, PF-5
silicone resin powder.14(a) Higher Capacity of the O/F Emulsion with theOrganic-Crystalline Powder for the DMS DropletsThan with the Spherical Resin Powder. A simple
theoretical analysis can be given, taking into account theeffect of the platelike shape of PF-5 CaLT on the maximum
amount of DMS contained as stable emulsion droplets. A
geometrical consideration has been made on the adsorptionof the plate-shaped and the spherical particles at theliquid-liquid interfaces. The plate-shaped powders covera larger interface than the spherical ones, if the volume
and the number of powder particles are the same. The
volume of emulsion droplets per unit volume φ is describedby eq 1:4φ ) πR3N
3(1)

where R and N are the radius and the number of emulsion
droplets per unit volume (number density), respectively.
If all the powder particles are adsorbed on the liquidliquid interface in a hexagonal close-packed arrangement,
N can be written as eqs 2:21
2

N)

x3 rp
n
2π R2 pplate-shaped powder

(2)

2

N)

x3 rs
n
2π R2 sspherical powder

where the plate-shaped powder is assumed to be acylindrical shape whose radius is rp, while the radius ofa spherical powder is rs (Figure 4). In eq 2, np and ns are
the number of plate-shaped powder particles and spherical(19) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17, 4540.
(20) Levine, S.; Bowen, B. D.; Paetridge, S. J. Colloids Surf. 1989,
38, 325.
(21) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622.

Figure 4. The effect of β on the shear-stress curve (a) and theyield stress τB of the powder/PFPE mixture (b). Panel b: (9)platelike PF-5 CaLT powder; (0) spherical PF-5 silicone resin
powder.

powder particles per unit volume, respectively. Equations
3 can be obtained by substituting eqs 2 into eq 1:

φ)2
Rrp2 npx3

φ)2
Rrs2 nsx3

plate-shaped powder(3)

spherical powder

Equations 3 imply that the upper-limit amount of emulsion
droplets is proportional to the quadratic power of rp or rs,
when R is constant and np ) ns. According to this result,the plate-shaped powder can stabilize more volume of theemulsion droplets than the spherical powder because rp
is usually larger than rs, if the volumes of both the powder
particles are the same, as shown in eq 4:

rp )

()

2 rs 1/2
rs
x3 d

(4)

where d is the thickness of the plate-shaped powder andis much less than rp and rs. For example, the plate-shaped
powder whose rp and d are 4.0 and 0.9 µm, respectively,
can stabilize 3 times more volume of emulsion droplets
than can the spherical powder whose rs is 2.2 µm. The
predicted difference of the plate-shaped powder from thespherical powder is much larger than our experimental
result.
The reason for the discrepancy between the theoreticaland experimental values is as follows. Although both PF-5
CaLT and the PF-5 spherical silicone resin powder are
surface-active, their chemical compositions are quite
different. The different wettability of these two powders
with PFPE and DMS might account for the above
discrepancy. The adsorption energy Es for the spherical10156

Langmuir, Vol. 19, No. 24, 2003

Nonomura et al.

powder particle adsorbed on an interface between liquid
A and liquid B can be written as 21,22(γAB - γAS + γBS)2
Es ) πrs2
γAB(5)where γAB and γAS (γBS) are interfacial tensions between
liquid A and B and between the powder surface and liquid
A (liquid B). Alternatively, the adsorption energy for theplate-shaped powder, Ep, can be described by eq 6, if theeffect of the contact between the edge of the powder andthe two liquids is neglected.

Ep ) πrp2(γAB - γAS + γBS)(6)Equations 5 and 6 indicate that not only the size of thepowder particle but also the interfacial tensions areimportant factors to determine the adsorption energy.
When these interfacial tensions are balanced, most of thepowder particles adsorb on the liquid-liquid interface.
But if the affinity of PF-5 CaLT with PFPE is higher thanthe balanced value, some powder particles might bedispersed in the PFPE phase and not contribute to theemulsification. This may be the reason the experimental
upper limit of the DMS content is less than the theoreticalvalue.
(b) Mechanism of Formation of the Separated
State. The effects of increasing β on the shear-stress curveand yield stress of the powder/PFPE mixture, τB, are shownin Figure 4. The shear stress of these dispersions increased
with increasing shear rate and β (Figure 4a). The profileof the shear-stress curves indicates that these dispersions
are thixotropic. The yield stress τB as a function of β isshown in Figure 4b. The τB of the PF-5 CaLT/PFPE mixture
increased abruptly when β > 0.4 and was higher thanthat of the spherical surface-active resin powder dispersions. These higher τB values indicate a stronger interaction between powder particles in the network aggregates.22
The contact angle of the PF-5 CaLT/PFPE mixture withDMS droplets increased sharply with the fraction of thepowder when β > 0.4, while it decreased continuously for
the PF-5 spherical silicone resin powder/PFPE mixture(Figure 5). These results indicate that the addition of PF-5
CaLT reduces the affinity of the powder/PFPE mixture
with DMS. This reduction of the affinity should prevent
the DMS oil from being emulsified into the powder/PFPE
mixture. Although the reason for these unusual behaviors
is not clear at present, the β value at the onset of increasingτB and contact angle in the PF-5 CaLT/PFPE mixture(22) Barnes, H. A.; Hutton, J. F.; Walters, F. R. S. K. In An
Introduction to Rheology; Elsevier: New York, 1989; p 115.

Figure 5. The effect of β on the contact angle of the powder/
PFPE mixtures with the DMS oil: (9) platelike PF-5 CaLT
powder; (0) spherical PF-5 silicone resin powder.

interestingly coincides with the composition at which theseparated state starts to appear (Figures 4 and 5). The
effect of the dispersed state and the affinity between twophases of the powder on the emulsion stability will be
studied in the future.
Conclusions
We have studied the ternary system consisting of theplate-shaped fluorinated CaLT powder, PFPE, and DMS
to characterize the self-assembled structure of the organiccrystalline powder. The maximum content of DMS wasapproximately 40 vol % in the O/F emulsion 1 state andwas larger than the value observed for spherical surfaceactive powder. According to the simple theoretical analysis,
the difference is mainly due to the geometrical and/or
thermodynamic characteristics of the plate-shaped powder. The separated state, in which the transparent DMS
phase was separated from the milky white phase consisting of the powder and PFPE, was observed. The resultsof the shear-stress curve and the contact angle measurements suggest that this characteristic state seems toappear due to the network aggregate of the powder
particles and the low affinity of the powder/PFPE mixture
with DMS.
In industrial applications, organic-crystalline powders
with various shapes and sizes are applied as functional
materials. Our results are useful in the efforts to improve
the properties of many industrial products, such as
foodstuffs, pharmaceuticals, cosmetics, or chemical products. The results are important not only for their potential
industrial applications but also from a fundamental
scientific viewpoint. The appearance of the separated state
is not a common phenomenon in colloid and surface scienceand may be characteristic of the plate-shaped powder
particles.
LA035511O