Article
Sakai et al.
of surfactants in such products. One is the development of gemini
surfactants under the reduced synthesis and purification cost strat-
egies, and in our recent works we have provided such materials
A fresh piece of silica was used for each experiment and then
discarded.
The waterusedinthecurrent study wasfiltered with a Millipore
membrane filter (0.1 μm in pore size) after deionization with a
Barnstead NANO pure diamond UV system.
1
7,18
synthesized from oleic acid derivatives.
The other approach is
the use of mixtures of gemini surfactants with conventional mono-
meric ones. The aim of our current study is, therefore, to demon-
strate the adsorption of monomeric/gemini surfactant mixtures at
a solid/aqueous solution interface and to compare the resultant
data with the corresponding data obtained from the gemini single
system.
2.2. Measurements. Static surface tension measurements
were performed by using a Kr u€ ss K100C Wilhelmy auto surface
tensiometer with a platinum plate. Continuous measurements
were carried out until a change in the surface tension becomes less
-1
than 0.01 mN m per 90 s.
A Q-Sense QCM-D E1 was used to assess both the mass of the
surfactants adsorbed at the silica/aqueous solution interface and
the viscoelastic nature (energy dissipation) of the adsorbed layer.
The adsorbed mass was calculated from a change in the third
overtone of the resonance frequency by applying Sauerbrey’s
Herein, we present the adsorption characteristics of monomeric/
gemini surfactant mixtures at the silica/aqueous solution interface
as a function of their mixing ratio. We note that the adsorbed
layer morphology of monomeric surfactant mixtures has been
previously studied by using the soft-contact atomic force micros-
2
4
relationship. A single sensor crystal with a silica coating was
used for all QCM-D experiments. The cleaning of the sensor
crystal and an O-ring that seals the cell/sensor assembly has been
made according to the procedure mentioned in the previous
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9-23
19
copy (AFM).
For example, Ducker and Wanless have
demonstrated a change in the adsorbed layer morphology of the
cationic surfactant (dodecyltrimethylammonium bromide, DTAB)
and the zwitterionic one ((dodecyldimethylammonio)propane-
sulfonate, DDAPS) on mica and found that the increased mole
fraction of DTAB results in the gradual shape transition of the
adsorbed aggregates from spheres to rods. To the best of our knowl-
edge, however, no systematic studies focusing on the adsorption
of monomeric/gemini surfactant mixtures have been reported yet.
In our current study, we have mainly performed two measure-
ments: one is the soft-contact AFM to study the adsorbed layer
morphology at the silica/aqueous solution interface, and the other
is the quartz crystal microbalance with dissipation monitoring
25
paper. After assembly in the QCM-D instrument, an electrolyte
solution (10 mmol dm NaBr) was injected into the cell, and the
-3
system was allowed to equilibrate. The flow rate of the electrolyte
solution was always fixed at 0.1 cm /min. Then, a surfactant solution
3
at a fixed mixing ratio was injected after a stable baseline in the
electrolyte solution was achieved. Again the system was allowed
to equilibrate in the surfactant solution under the continuous flow,
and then the solution was replaced by a new surfactant solution.
Here the surfactant concentration was increased step by step to
obtain the QCM-D adsorption isotherm and dissipation data at
the fixed mixing ratio.
In-situ imaging of the surfactant layers adsorbed on a flat silica
plate was performed with a Seiko SPI3800 AFM. Cantilevers with
an integral silicon nitride tip (Olympus OMCL-TR800PSA, the
(
QCM-D) to estimate both the mass and viscoelastic nature of the
surfactant mixtures adsorbed at the interface. The gemini surfactant
used in this study was cationic 1,2-bis(dodecyldimethylammonio)-
ethane dibromide (12-2-12). The following three surfactant mixtures
have been studied: DTAB/12-2-12, HTAB/12-2-12, and C EO /
-1
nominal spring constant = 0.15 N m ) were used for all AFM
experiments. The chemically oxidizedsilicon wafer was assembled
in the AFM instrument, and the surfactant solution prepared at
1
2
8
3
the desirable concentration (∼1 cm ) was injected into the AFM
26
12-2-12, where DTAB and HTAB (hexadecyltrimethylammonium
fluid cell. After equilibration for 1 h, images were collected using
bromide) are cationic and C EO (octaoxyethylenedodecyl ether)
12
8
the soft-contact method with a scan rate of 3-4 Hz; this uses the
is nonionic.
minimum force necessary to obtain an image, thereby minimizing
scanning-induced deformations of the adsorbed layer. All images
presented herein are deflection images. Interaction forces between
the cantilever tip and the surfactant layer adsorbed at the silica/
aqueous solution interface were also measured.
2
. Experimental Section
.1. Materials. The cationic gemini surfactant (12-2-12) was
synthesized in our laboratory according to the procedure reported
2
4
0
0
previously. Briefly, N,N,N ,N -tetramethylethylenediamine was
refluxed with 1-bromododecane in ethanol for 48 h, and then
the product obtained here was recrystallized several times from
methanol/acetone mixtures. The cationic monomeric surfactants
All measurements reported here were performed at a constant
-3
temperature of 25 °C in the presence of 10 mmol dm NaBr as
a background electrolyte.
(
DTAB and HTAB) were purchased from the following suppliers
3. Results and Discussion
and used after recrystallization several times from methanol/
acetone mixtures (DTAB: Tokyo Chemical Industry (TCI); HTAB:
Aldrich). The nonionic surfactant (C EO ) was kindly supplied
from Nikko Chemicals and used without further purification.
The other chemicals used in this study were of analytical grade
(Wako Pure Chemical Industries) and again used without further
purification.
Flat silica plates were prepared from silicon wafers (Nilaco): a
3
.1. Surface Tension. Before presenting the adsorption
characteristics of the monomeric/gemini surfactant mixtures at
the silica/aqueous solution interface, it is fundamentally impor-
tant to see their cmc(s) as a function of mixing ratio. Figure 1
shows the mixed cmc data of the three surfactant systems (DTAB/
12-2-12, HTAB/12-2-12, and C EO /12-2-12) asa functionofthe
mole fraction of 12-2-12 (R). We note that each cmc was deter-
mined by the static surface tensiometry (see Supporting Informa-
tion Figure S1): for all the surfactant mixtures, their surface tensions
decrease sharply with increasing concentration (in the region of
low surfactant concentrations) and attain a plateau level. The break
point observed here was assumed to be the cmc of the surfac-
tant mixtures at each mixing ratio. Also shown in Figure 1 are the
cmc curves of each mixture, computed using Clint’s equation
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8
1
2
8
silicon wafer was immersed in a mixed solution of H
2 3
O:NH :
H O =5:1:1 (in volume) for 15 min at 80 °C, followed by a copious
2
2
rinsing with deionized water to give a hydroxylated silica surface.
(
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(
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7120 DOI: 10.1021/la1028367
Langmuir 2010, 26(22), 17119–17125