Li et al.
Article
Table 5. Adsorption of Partially Fluorinated Gemini and Single Chain Surfactants at the Hydrophobic (OTS)/Water Interface
2
Γ ((0.1 mg/m2)
˚
˚
˚
˚
surfactant
c (mM)
τ1 ((1 A)
τ2 ((1 A)
θ ( 0.03
σ ((0.5 A)
A (A ( 5%)
(fC4C11)2-C6
(fC5C10)2-C6
(fC6C8)2-C6
(fC8C6)2-C6
fC5C10-TAB
fC6C8-TAB
fC8C6-TAB
1.05
0.86
0.075
0.08
0.83
1.07
0.37
12.0
12.5
11.0
9.0
12.0
19.0
16.0
2.75
1.75
2.0
1.25
1.75
2.75
2.5
0.87
0.86
0.96
0.94
0.95
0.88
0.87
6.0
6.5
6.5
5.0
6.5
7.0
6.5
89
89
90
120
42
28
36
2.1
2.2
2.2
1.9
2.2
3.4
3.0
converge to a common value becuase they are largely determined
by thermal fluctuation and local roughness. This means that the
good accuracy in the values of the fragment separations means
that they are relatively very accurate and can usefully be com-
pared.
fluorocarbon and hydrocarbon chain fragments and a narrow
hydrophilic region on the outside of the layer (layer 2), which
contained the head groups and the spacer (for the geminis). The
parameters of the silica layer and the OTS were constrained to the
values obtained in the absence of surfactant. The data were fitted
using the optical matrix method with the usual constraints on the
total volume fraction at any point in the layer. To allow for dis-
order in the layer, the fractions of heads and spacers in the chain
region were used as additional fitting parameters. The need to
include these mixing parameters confirms that the layers are not
well ordered, but no useful quantitative conclusions can be drawn
from them. In addition, it was necessary to include the roughness
(σ) between the surfactant layers and between the last surfactant
layer and water. The best fits of the main geometrical parameters
plus the coverage (θ), surface excess and area per molecule, are
given in Table 5 (we have omitted the mixing parameters). The
reflectivity profiles for the corresponding gemini and single chain
cationics for the fC6C8 chain just above their respective CMCs
are shown in Figure 4. Sometimes a shift to lower Q of the sharp
dip in the reflectivity of dOTS and protonated surfactant systems
can be attributed to a thickening of the overall layer and hence of
the surfactant layer. Here, the situation is complicated by other
factors, namely the different contrast of the partially fluorinated
systems, the very different coverage of the two layers, and a slight
difference in the thickness of the OTS layer used in the two cases.
Table 5 shows that the single chain surfactant forms a thicker
layer at a much higher coverage. Figure 4 also shows that there is
some penetration of water into the OTS (and underlying silica) at
a level of about 15%. Neutron reflectometry is particularly sensi-
tive to this effect, and we have usually observed it at a level in the
range 10-20%.
In general, it is expected that adsorption of an amphiphile on a
hydrophobic surface of this type should be stronger than at the
hydrophobic air-water interface. This is the case for most of the
surfactants here but not for the gemini containing the fC8C6 chain,
for which adsorption is actually weaker, and not for the single
chain fC5C10-TAB, for which adsorption at the two surfaces is
aboutthesame. Although there is a very strong hydrophobiceffect
driving this adsorption, it requires direct contact between the fluo-
rocarbon fragment and the hydrocarbon OTS surface, and this
will become progressively less favorable as the extent of fluorina-
tion increases. Adsorption will therefore be driven by a competing
mix of the increasing hydrophobicity with extent of fluorination
and an increasing repulsion between fluorocarbon and the OTS
hydrocarbon.
Table 4 shows that the overlap of hydrocarbon and fluorocar-
bon is smallest at the two extremes of fluorination. This suggests
that there is competition between a greater tendency for larger
fluorocarbon groups to segregate from the hydrocarbon and the
restriction imposed by shortening the length of the hydrocarbon
group. Thus, the long hydrocarbon chain in the fC4C11 system
allows the fluorocarbon more freedom than in the other com-
pounds and it can segregate efficiently, but in the fC8C6 system the
greater extent of fluorination drives a stronger segregation.
However, the really striking difference is between the single chain
compounds and the gemini. In the former there is clearly much
more efficient segregation of fluoro- and hydrocarbon fragments,
as would be expected from the closer packing of the chains. The
differences in ΔhC-W within each system and between single and
gemini surfactants show no clear trends.
Finally, we note that the sublayer that was invoked to interpret
the data from the hydrocarbon geminis obtained by Li et al. was
not found to be necessary here. The data are fitted satisfactorily
with only a single molecular layer.
Adsorption at the Hydrophobic Solid/Aqueous Interface.
The d-OTS surface was first characterized by measuring reflec-
tivity profiles in three different water contrasts, D2O, and water of
˚
scattering length density 3.4 ꢁ 10-6 and 4.0 ꢁ 10-6
A
-2. As
always occurs with these layers in contact with water, there is a
very thin layer which some attribute to contamination and others
to an “air” layer.24-27 This disappears in the presence of a sur-
face active adsorbate. It is therefore necessary to include this layer
in the basic characterization but not when the adsorbed surfactant
layer is present. The surfactant layers were all studied at concen-
trations either equal to or slightly larger than their CMC.
The deuterated OTS has a similar scattering length density to
D2O. Adsorption of a layer containing hydrocarbon therefore
gives a very strongsignal. If a partially fluorinated surfactant were
to adsorb with its fluorocarbon fragment attached to the OTS
surface, the initial part of the surfactant layer would contrast only
weakly with the OTS. The main contribution to the reflectivity
would then come from the hydrocarbon part of the layer and the
protonated TAB group. In principle, the reflectivity profile is
therefore quite sensitive to the relative distribution of these two
groups within the layer. However, this was not found to be the
case and the adsorbed layer could be modeled in terms of a
hydrophobic region next to the OTS (layer 1) containing both
Given that the layer is disordered, there is the possibility of
some contact of the hydrocarbon fragment (and spacer) with the
OTS and the system should adjust to optimize this contact. The
optimization of hydrocarbon OTS contact would result in a thin-
ning of the layer, which would be expected to be greater for the
gemini because of the hydrocarbon spacer. Because only one con-
trast was used for the OTS experiments, it is not possible to make
a clear separation of the layer into its components. However, a
comparison of the overall thickness of the layers at OTS/water
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H.; Palms, D.; Ralston, J.; Honkimaki, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 101,
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Langmuir 2011, 27(2), 656–664
DOI: 10.1021/la104291w 661