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Direct observation of surfactant aggregate behavior on a mica surface
using high-speed atomic force microscopy

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Published on 25 March 2011 on http://pubs.rsc.org | doi:10.1039/C0CC05762B

Shigeto Inoue,*a Takayuki Uchihashi,bc Daisuke Yamamotob and Toshio Andobc
Received 24th December 2010, Accepted 1st March 2011
DOI: 10.1039/c0cc05762b
The processes of dynamic aggregate transformations of micelles
at a mica surface were observed using high-speed atomic force
microscopy. We showed that salt concentration aﬀects not only
the equilibrium state adsorbed structure but also the timerequired to reach the equilibrium state on the mica surface.
Surfactant molecules containing long hydrophobic alkyl
chains and hydrophilic groups can organize into various
self-assemblies, both in bulk solution and at a solid interface.
Atomic force microscopy (AFM) has been employed to demonstrate a strong correlation between the bulk self-assembly
structure and the morphology of adsorbed surfactant layers.1
Adsorbed layers comprised of globular and cylindrical aggregateson hydrophilic substrates, of which mica is the most widely
studied, have been well-characterized.1c–e Micellar structuresof common cationic surfactants, such as hexadecyltrimethylammonium bromide (CTAB) and hexadecyltrimethylammonium
chloride (CTAC), adsorbed at the mica/liquid interface havebeen directly observed by AFM.1c–e These ordered interfacial
aggregates have well-deﬁned sizes and shapes that can becontrolled by the addition of metal halide salts.1 When ahigher concentration of competing cations is introduced, thedensity of available lattice anions on the mica surface isreduced, so that the spacings between surfactant headgroups
increase. In other words, the inﬂuence of the mica interface isreduced by the introduction of the salt, and hence the surface
aggregated structure becomes more similar to the micellar
structure found in bulk solution.
Lamont and Ducker have shown that CsCl tends to increase
the curvature of CTAC aggregates on a mica surface due tothe transformation from cylinders to globules with variationof the salt concentration.1c However, these measurements wereperformed at equilibrium conditions and no information wasgained regarding the initial aggregation dynamics, due to thelack of available techniques for observing the dynamic behaviorof surfactant aggregates. Rapid acquisition of successive highresolution images of surfactant aggregates is required. The
scanning speeds of commercially-available AFMs are usuallyaKao Corporation, Wakayama 640-8580, Japan.
E-mail: inoue.shigeto@kao.co.jp; Fax: +81-73-426-8626;
Tel: +81-73-426-8514
bKanazawa University, Kanazawa 920-1192, JapancCREST, Japan Science and Technology Agency (JST), Japan4974

Chem. Commun., 2011, 47, 4974–4976

several minutes per frame, and are therefore too slow tomonitor many physicochemical processes in liquids that occur
at much faster rates. Recently, this limitation has been challenged
by the use of high-speed AFM,2 which can capture images at aframe rate of more than 30 frames s 1. Various imaging
studies have attempted to capture biological processes onvideo at high imaging rates.2 In this study, high-speed AFM
was used to observe the initial aggregate behavior of CTAC at
the mica–water interface, and provide new insights regarding
surfactant aggregation dynamics at a mica surface.
The muscovite mica used in this study was cleaved immediately
prior to use. Water was prepared using a Milli-Q system andthe resulting water had a conductivity of 18 M O cm 1. CTAC
(495%, TCI, Tokyo, Japan) was recrystallized three times
from ethanol. Analytical grade CsCl (99.9%, Wako, Tokyo,Japan) was used as received. Observations of the CTAC
aggregates were performed in soft tapping mode using alaboratory-built high-speed AFM. The cantilever (Olympus,Tokyo, Japan) is 6–7 mm long, 2 mm wide, and 90 nm
thick with a spring constant of 0.1–0.2 N m 1. Its resonant
frequency in an aqueous solution is B1 MHz. To achieve asmall tip–sample loading force, the free-oscillation peak-topeak amplitude of the cantilever (A0) was set to B1 nm andthe amplitude set point was set at more than 0.9 A0. The probe
tip was grown on the tip of a cantilever by electron beam
deposition and was further sharpened by argon plasma
etching. The tapping force estimated was less than 30 pN.
All observations were performed at room temperature (25 1C)
and at a surfactant concentration equal to twice the critical
micelle concentration (CMC) in the absence of salt. The
scan area was 200  200 nm2 with 200  200 pixels. The
imaging rate was 1–2 frames s 1. The high-speed AFM usedin this study can be used to observe at a video rate of ca.
15–30 frames s 1 for a scan range of ca. 250 nm under
feedback operation that is capable of preventing weak interactions from being disturbed by the scanning tip. However,high-resolution observations were necessary in this study, so
we measured at 1–2 frames s 1. High-speed AFM measurements were recorded immediately after injection of CsClsolutions including the CTAC. The diameters of the observed
globular or cylindrical micelles are approximately 9–11 nm.
Taking the tip radius into account, the true sizes of the micelles
are actually smaller than the observed diameters.3 Their
heights are in fact approximately 5–6 nm.
This journal is c The Royal Society of Chemistry 2011

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Fig. 1 Continuous high-speed AFM images of CTAC aggregates at
the interface between the mica surface. 0.0027 mol L 1 CTAC. Images
are 200  200 nm2. (a) At 10 s worm-like micelle shapes are evident,
while at (b) 20 s and at (c) 30 s cylindrical micelle shapes are observed,
and (d) at 300 s a ﬂat bilayer is observed.

Fig. 1 shows continuous high-speed AFM images of theCTAC aggregate structure at the mica surface. Interestingly,for CTAC at twice the CMC, worm-like and cylindricalstructures were observed at 10 to 30 s, which then transformed
into a ﬂat layer after approximately 300 s. It is known that
CTAC forms a bilayer at a mica surface.1c However, thisdynamic behavior shows that CTAC is not instantaneously
stabilized by binding to the mica surface, but graduallychanges into a bilayer over a few minutes. Mica has exchangeablesurface cations, and the density of charges on the mica
substrate (1 negative lattice charge per 0.48 nm2)4 is such that
surfactants are adsorbed to the surface with their head-groups
closer together than that for equilibrium bulk separation(the head-group area for CTAC in a micelle is 0.64 nm2).5 It
is considered that this dynamic behavior leads to adsorbed
aggregates with a lower degree of curvature, such as a ﬂatbilayer, than the corresponding micelles in solution.
Fig. 2 and 3 show the structure of CTAC aggregates at themica surface for diﬀerent CsCl concentrations and with aconstant CTAC concentration of 0.0027 mol L 1. When CTAC
was coexistent with a relatively low concentration of CsCl
(0.034 mol L 1), a transformation from globular to cylindrical
micelles was observed after approximately 20 s, and an
equilibrium state with stable cylindrical structures was reachedafter about 30 s (Fig. 2). When CTAC was coexistent with ahigh concentration (0.1 mol L 1) of CsCl, the adsorbed
structure formed globular micelles and no further transformation was observed, that is, an equilibrium state was reachedafter 10 s (Fig. 3). These equilibrium states of CTAC aggregate
shapes on a mica surface with addition of CsCl have beenreported,1c and reported shapes are very similar to our results
at 300 s. Fig. 4 shows a schematic representation of the CTAC
aggregation states on the mica surface for diﬀerent exposureThis journal is c The Royal Society of Chemistry 2011

Fig. 2 Continuous high-speed AFM images of CTAC aggregatesat the interface between the mica surface. 0.0027 mol L 1 CTAC/0.034 mol L 1 CsCl. Images are 200  200 nm2. (a) At 10 s globular
micelle shapes are evident, (b) at 20 s and (c) 30 s globular tocylindrical micelle shapes are observed, and (d) at 300 s cylindrical
micelle shapes are evident.

Fig. 3 Continuous high-speed AFM images of CTAC aggregatesat the interface between the mica surface. 0.0027 mol L 1 CTAC/
0.1 mol L 1 CsCl. Images are 200  200 nm2. (a) At 10 s globular
micelle shapes are evident, while at (b) 20, (c) 30, and (d) 300 s,
no further transformation was observed.

times. It is known that CTAC forms cylindrical aggregates
at a mica surface at equilibrium state with the addition of0.034 mol L 1 CsCl;1c however, there have been no previous
reports of the change in shape from globular to cylindricalaggregates. The initial globular structure is induced by the adsorption of Cs+ to negative sites of the mica lattice, which decreases
Chem. Commun., 2011, 47, 4974–4976

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Downloaded by UNIVERSITY OF KANAZAWA on 23 April 2011
Published on 25 March 2011 on http://pubs.rsc.org | doi:10.1039/C0CC05762B

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Fig. 4 Schematic representation of CTAC aggregation states at the interface between the mica surface for diﬀerent exposure times. (a) 0.0027 mol L 1CTAC, (b) 0.0027 mol L 1 CTAC/0.034 mol L 1 CsCl, and (c) 0.0027 mol L 1 CTAC/0.1 mol L 1 CsCl.

the number of adsorption sites available for CTAC, and it isconsidered that the structural change of the aggregate shape iscaused by the slow exchange of CTA ions for potassiumions in the mica lattice. These transformations occur in theopposite order to that observed in bulk solution, indicating a
diﬀerent mechanism for surfactant aggregation at a micainterface.
We conclude that salt concentration aﬀects not only theequilibrium state adsorbed structure but also the time required
to reach the equilibrium state on the mica surface, which is a
new result that was previously unknown. In this study, we
directly observed the initial dynamics using a high-speed AFM
until an equilibrium state was reached. We observed thegradual transformation of the adsorbed structures until anequilibrium state was reached and clariﬁed for the ﬁrst timethat the addition of CsCl aﬀects the time required to reachan equilibrium state. Thus, we believe that it is important
to determine the time required for the adsorbed structure toreach an equilibrium state in order to understand the nature ofvarious phenomena that take place at the interface.
The authors are grateful to Dr Katsumi Kita and Dr Tatsushi
Wakisaka for enlightening discussions, and to the Kao
Corporation for permission to publish this study.

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Chem. Commun., 2011, 47, 4974–4976

Notes and references
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3 J. Vesenka, S. Manne, R. Giberson, T. Marsh and E. Henderson,Biophys. J., 1993, 65, 992.
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This journal is c The Royal Society of Chemistry 2011