Aggregation and layering transitions in thin films of X-, T-, and anchor-shaped bolaamphiphiles at the air-water interface

Article abstract of DOI:10.1002/chem.201003671

Aggregation in Langmuir films is usually understood as being a disorderly grouping of molecules turning into chaotic three-dimensional aggregates and is considered an unwanted phenomenon causing irreversible changes. In this work we present the studies of 11 compounds from the group of specific surfactants, known as bolaamphiphiles, that exhibit reversible aggregation and, in many cases, transition to well-defined multilayers, which can be considered as a layering transition. These bolaamphiphiles incorporate rigid π-conjugated aromatics as hydrophobic cores, glycerol-based polar groups and hydrophobic lateral chains. Molecules of different shapes (X-, T-, and anchor) were studied and compared. The key property of these compounds is the partial fluorination of the lateral chains linked to the rigid cores of the molecules. The most interesting feature of the compounds is that, depending on their shape and degree of fluorination, they are able to resist aggregation and preserve a monolayer structure up to relatively high surface pressures (T-shaped and some X-shaped molecules), or create well-defined trilayers (X- and anchor-shaped molecules). Experimental studies were performed using Langmuir balance, surface potential and X-ray reflectivity measurements. Copyright

Full text of DOI:10.1002/chem.201003671

FULL PAPER
DOI: 10.1002/chem.201003671
Aggregation and Layering Transitions in Thin Films of X-, T-, and Anchor-
Shaped Bolaamphiphiles at the Air–Water Interface
[a]
[a]
Patrycja Niton´, Andrzej Zywocin´ski, Jan Paczesny,[a] Marcin Fiałkowski,[a]
Robert Hołyst,*[a, c] Benjamin Glettner,[b] Robert Kieffer,[b] Carsten Tschierske,*[b]
Damian Pociecha,[d] and Ewa Gꢀrecka[d]
˙
Abstract: Aggregation in Langmuir
films is usually understood as being a
disorderly grouping of molecules turn-
ing into chaotic three-dimensional ag-
gregates and is considered an unwant-
ed phenomenon causing irreversible
changes. In this work we present the
studies of 11 compounds from the
group of specific surfactants, known as
bolaamphiphiles, that exhibit reversible
aggregation and, in many cases, transi-
tion to well-defined multilayers, which
can be considered as a layering transi-
tion. These bolaamphiphiles incorpo-
rate rigid p-conjugated aromatics as
hydrophobic cores, glycerol-based
esting feature of the compounds is
that, depending on their shape and
degree of fluorination, they are able to
polar groups and hydrophobic lateral
chains. Molecules of different shapes
(X-, T-, and anchor) were studied and
compared. The key property of these
compounds is the partial fluorination
of the lateral chains linked to the rigid
cores of the molecules. The most inter-
resist aggregation and preserve
a
monolayer structure up to relatively
AHCTUNGTRENNUNG
high surface pressures (T-shaped and
some X-shaped molecules), or create
well-defined trilayers (X- and anchor-
shaped molecules). Experimental stud-
ies were performed using Langmuir
balance, surface potential and X-ray re-
flectivity measurements.
Keywords: aggregation
·
amphi-
philes · interfaces · layered com-
pounds · thin films
Introduction
molecules.[3] Hence, it appears that directed self-assembly at
surfaces is presently one of the major strategies to achieve
functional nanoscale structures. Besides self-assembly on
solid surfaces,[4] self-assembly of amphiphilic molecules on
liquid surfaces (Langmuir films) and transfer of these pre-
formed layers onto solid substrates represents another one
of the major strategies.[1,5] However, well-defined mono-
Development of new technologies, especially nanotechnolo-
gy, would not be possible without understanding the self-as-
sembly of molecules into materials with well-defined struc-
tures in a bottom-up approach.[1] For example, organic tran-
sistors based on a self-assembled monolayer were recently
reported.[2] Another example from the field of organic elec-
tronics is provided by molecular junctions with p-conjugated
AHCTUNGTREGmNNNU olecular layers could be achieved with the Langmuir tech-
nique by compressing thin films of amphiphiles. In attempts
to produce multilayers by further compression, a random,
disordered aggregation usually takes place instead of self-or-
ganization into well-defined multilayers.[1,5,6] This unwanted
process is frequently observed in Langmuir films because
the molecules tend to detach from the interface (escape to
the third dimension) and disorderly group into random
(semi)crystalline 3D aggregates. Hence, avoiding aggrega-
tion of the monolayer, and achieving reversibility of film for-
mation became an important challenge.[7–11] To the best of
our knowledge, perfect reversibility of compression/decom-
pression isotherms[12] was first reported by Tournilhac
et al.[10] in 1994 and recently, in our previous communication
paper.[11] The common feature of the amphiphilic com-
pounds used in both reports is the combination of a rodlike
mesogenic unit, favouring liquid crystalline self-assembly,
with partly fluorinated alkyl chains. Besides their distinct ef-
fects on self-assembly, the rodlike p-conjugated moieties in-
troduce functionality, as fluorescent and semiconducting
properties, which are of interest with respect to potential ap-
plications of the formed thin films.[13,14]
˙
´
´
[a] P. Niton, Dr. A. Zywocinski, J. Paczesny, Prof. M. Fiałkowski,
Prof. R. Hołyst
Institute of Physical Chemistry PAS
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Fax : (+48)223433333
E-mail: rholyst@ichf.edu.pl
[b] B. Glettner, R. Kieffer, Prof. C. Tschierske
Martin-Luther-University Halle-Wittenberg
Institute of Chemistry, Organic Chemistry
Kurt-Mothes Strasse 2, 06120 Halle (Germany)
Fax : (+49)3455527346
E-mail: carsten.tschierske@chemie.uni-halle.de
[c] Prof. R. Hołyst
´
Cardinal Stefan Wyszynski University
WMP-SNS, Dewajtis 5, 01-815 Warsaw (Poland)
´
[d] Dr. D. Pociecha, Prof. E. Gꢀrecka
Faculty of Chemistry, University of Warsaw
Pasteura 1, 02-093 Warsaw (Poland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003671.
Chem. Eur. J. 2011, 17, 5861 – 5873
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5861

The observation that partially fluorinated alkanes are able
to create stable monolayers[15] on water surfaces and behave
as “primitive surfactants” initiated intense studies of fluori-
nated alkanes[16–20] as well as fluorinated polar amphi-
philes.[21,22] Systematic studies of some fluorinated alkanes in
Langmuir films have shown reversibility of isotherms[21]
when decompressed before collapse. Hence, it seems that an
important way to increase stability of the monolayer and to
prevent an irreversible 3D aggregation is a fluorination of
particular parts of the amphiphilic molecules.[8,17,21]
of Langmuir isotherms even for very high compression/de-
compression rates and ratios. Herein we report the effects of
the number and position of the lateral chain(s) (X- versus T-
shaped molecules), the core length (terphenyls versus phen-
ylene ethynylene-type cores), the shape of the rigid core
(linear versus bent) and the degree of fluorination on the
self-assembly of rigid bolaamphiphiles in Langmuir mono-
and multilayers. Brewster angle microscopy, surface poten-
tial measurements and X-ray reflectivity measurements of
transferred films were used as the major tools to investigate
the thin films. During film formation, reversibility of the iso-
therms was observed in all classes of compounds with semi-
perfluorinated alkyl chains. Generally, the monolayers of
gaseous and expanded liquid phases with the aromatic cores
lying flat on the water surface were found for all compounds
at relatively low surface pressure (see Figure 2c). Upon fur-
ther increase of the surface pressure, depending on the mo-
lecular structure, either triple-layer formation with retention
of the orientation of the aromatic cores parallel to the sur-
face (Figure 2a) was observed as a first-order layering transi-
tion, or the monolayer structure was retained while the mol-
ecules changed their orientation to a tilted or vertical orien-
tation (Figure 2b) in a second-order transition to a con-
densed-liquid film.
Bolaamphiphiles[23] represent a special type of amphi-
philes possessing two hydrophilic groups attached to both
ends of an elongated hydrophobic segment. These special
amphiphiles, which could likewise be regarded as dimeric
end-to-end connected amphiphiles, are known to significant-
ly stabilize membranes (e.g., monolayer lipid membranes of
archae bacteria[24]) and to form other interesting self-assem-
bled structures, such as liquid crystalline phases in the bulk
state,[25] and gels[23] or helical fibres[26] in aqueous systems.
Formation of nanowells and pores in lipid membranes repre-
sents an additional example of their unique self-assembly
capability.[23] In their self-assembled structures flexible bo-
laamphiphiles can adopt different conformations: a linear
one, and if the hydrophobic moiety is sufficiently long and
flexible, also a reversed U-shape.[27,28] Introduction of a rod-
like rigid segment as a hydrophobic moiety between the
polar groups inhibits deformation to a U-shaped conforma-
tion, and hence, should modify the self-assembly behaviour
significantly.
Transformations shown schematically in Figure 2 provide
a tool for obtaining well-defined Langmuir films with dis-
tinct film thickness (mono- versus trilayers) and distinct di-
rections of the p-conjugated aromatic cores in a reversible
One of the most promising classes of rodlike bolaamphi-
philes capable of self-assembly into distinct superstructures
are those composed of a rigid p-conjugated aromatic core
terminated at both ends with hydrophilic hydrogen-bonding
glycerol moieties.[29–31] This basic structure can be modified
through the shape of the core (linear[29,30] or bent[31]) or by
additional lateral chains attached at different positions
giving T-[29] (T), X-[30] (X), or anchor-shaped[31] (A) mole-
cules, as shown schematically in Figure 1. The shape of the
molecule, combined with the segregation of the lateral
chains into distinct compartments, determines the bulk
properties of the compound. This concept of polyphilic bo-
laamphiphiles led to a wide variety of different highly com-
plex liquid crystalline (LC) soft-matter structures as de-
scribed by Tschierske et al.[29–33] In a previous communica-
tion[11] we reported three examples of partially fluorinated
X-shaped bolaamphiphiles that showed perfect reversibility
Figure 2. Different types of self-assembled layers at the air–water inter-
face as formed by X-, T- and anchor-shaped bolaamphiphiles and their
reorganization by changing the surface pressure; dark grey=rigid aro-
matic cores (simplified; bent structure of the A compounds is not consid-
ered); white circles=polar glycerol groups; light grey=regions of semi-
perfluorinated chains and alkyl chains (segregation of RF and RH seg-
ments is not considered). a) Lam-like[33] trilayer with the aromatic cores
organized parallel to the air–water interface, b) smectic-like monolayer
with an organization of the aromatic cores, on average, tilted or perpen-
dicular to the air–water interface, c) Lam-like monolayer with a single
layer of aromatic cores covering the water surface. A, B and C describe
different types of transformations leading from the Lam-like monolayers
to trilayers (A=lifting, B=reversible rollover collapse) and to smectic-
like monolayers (C=tilting).
Figure 1. Schematic sketches of the bolaamphiphilic molecules under dis-
cussion and their abbreviations; molecules assigned as T-shaped are in
fact more similar to a l-shape as the lateral chain is attached at an angle
of 608 to the rodlike core.
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The Air–Water Interface
FULL PAPER
surface-pressure-dependent self-assembly process. More-
over, these preformed layers can be transferred onto solid
substrates without loss of ordering as revealed by X-ray re-
flectivity measurements. The combination of 1) anisometric
rigid units, favouring liquid-crystalline self-assembly thereby
stabilizing the ordering, 2) the general film-stabilizing effect
of perfluorinated chains, due to their increased amphiphilici-
ty, and 3) the fluidity of semi-perfluorinated chains (inhibit-
ing crystallization)[34] were established as the key features
for successful molecular design.
rigid cores differing in length were investigated. The lateral
chains attached to each core differ in the degree of fluorina-
tion. Compounds X1 to X4 with terphenyl cores terminated
with two glycerol moieties were recently investigated and
detailed analysis of the reversibility of their isotherms was
shown for the case of compound X2.[11] The isotherms of
compounds X1 to X4 are collated in Figure 3a. The most in-
teresting feature of the isotherms of all these X-shaped mol-
ecules are the broad plateaus, as often observed for transi-
tions from monolayers to multilayers. In all cases the pla-
teaus are reached at p=20–25 mNmꢀ1, which corresponds
to an area (A) of 1.1 to 1.3 nm2 moleculeꢀ1. This area corre-
sponds to the area required by the bolaamphiphilic moiety
(aromatic core and polar head groups) lying on the water
surface and seems to be additionally affected by the number
of fluorinated segments (compare the isotherms of com-
pounds X3 and X4). It can be assumed that before the pla-
teau is reached the polar terminal groups of compounds
X1–X4 interact strongly with the water surface causing
these molecules to lie flat at the air–water interface. As the
plateau is reached the bolaamphiphilic cores become dense-
ly packed with the lateral chains arranged on top and cover-
ing these layers. Therefore, the molecular area at this kink is
mainly determined by the size of the bolaamphiphilic cores
(aromatics, glycerols and ether oxygen atoms carrying the
lateral chains) and the effect of the flexible lateral chains is
smaller. Only in the case of two fluorinated chains (com-
pound X4) does the relatively large diameter of these
chains seem to influence the required surface area. En-
largement of the fluorinated parts of the side chains enhan-
ces the stability of these layers due to an increase of hydro-
phobicity (compare compounds X2 and X4). A second rise
of the isotherms starts at A=0.3–0.4 nm2 moleculeꢀ1. The
ratio of the area per molecule at both ends of the plateau is
very close to 3, which suggests that trilayer formation is de-
veloping at the beginning of the plateau. This was already
proven[11] in the case of compound X2 by comparing the
molecular areas at both ends of the plateau as well as by X-
ray reflectivity measurements (XRR).[11] The same conclu-
sion can be drawn for compounds X3 and X4. For example,
the film of X3 was transferred onto silicon wafers at p=17
and 27 mNmꢀ1 using the Langmuir–Blodgett technique.
XRR measurements were performed on these samples to
estimate the thickness of the
Results and Discussion
All numerical results obtained in the studies of the 11 bo-
laamphiphilic compounds are collected in Table 1 for the X-
and anchor-shaped compounds X and A, and for the T-
shaped molecules T in Table 2. A detailed description of in-
vestigations is presented in the following sections according
to the shape of the molecules.
X-shaped bolaamphiphiles: Six compounds (X1–X6) belong-
ing to the group of X-shaped molecules with two types of
Table 1. Data of the Langmuir films and transferred films of compounds X and
A.[a]
Com- A1
p1
[mNmꢀ1
d1
A2
p2
[mNmꢀ1
d2
[nm]
pound [nm2 moleculeꢀ1
]
G
]
[nm] [nm2 moleculeꢀ1
]
N
]
X1
X2
X3
X4
X5
X6
A1
A2
A3
1.20
1.05
1.06
1.23
1.48
1.57[b]
1.28
1.16
0.89
17.7
19.7
23.4
24.1
9.2
0.51
19.2
20.6
23.8
25.0
10.0
43.0[c]
13.3
21.6
26.1
1.43
1.65
0.34
0.36
0.37
0.71
4.07
4.76
9.4[b]
1.57[b] 0.53[c]
0.27
3.51[c]
3.71
12.2
20.0
26.1
1.43
0.43
0.30
[a] A1, p1 =area and surface pressure at the beginning of the plateau; A2, p2 =
area and surface pressure at the end of the plateau; d1 =thickness of the mono-
layer transferred below the plateau region; d2,=thickness of the trilayer (for
compounds X2, X3 and A2, only) transferred above the plateau region. [b] The
apparent plateau corresponds to the beginning of the transition to tilted phase
(A1, p1) and d1 =thickness of the monolayer of the molecules with horizontally
oriented cores. [c] Coordinates (A2, p2) correspond to the collapse of the mono-
layer of tilted molecules and d2 =thickness of this monolayer (see Figure 5a and
discussion in the text).
films and the results are shown
in Figure 5a. Values of the layer
thickness obtained for com-
pound X3 from XRR data
fitted to the theoretical equa-
tion based on the electron den-
sity modulation derived initially
from modelling (see Figure S2
and Table S1 in the Supporting
Information) are equal to d1 =
1.65 nm for the monolayer and
d2 =4.76 nm for the film trans-
ferred at the end of the plateau
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5863

R. Hołyst, C. Tschierske et al.
Figure 3. Isotherms and BAM images for X-shaped compounds: a) the
hysteresis loop (compression and decompression) for the dialkyl-substi-
tuted compound X1 (dashed lines) and the compression runs (only) for
~
*
^
semi-perfluorinated compounds X2 ( ), X3 ( ) and X4 ( ); b)–d) the
BAM images recorded at conditions marked with the same letters on the
isotherm of compound X2: b) first domains of the trilayer appearing at
the beginning of the plateau, c) and d) the increasing area covered by the
trilayer during compression; e) (not marked on the isotherm) the remains
of the trilayer in coexistence with the monolayer observed during decom-
pression of compound X2 (not shown) at the right end of the plateau.
The white scale bar in image (b) represents a length of 400 mm.
Figure 4. Direct comparison of two pairs of compounds a) with different
rigid cores and b) with a different degree of fluorination in one lateral
chain. a) The isotherms of the compression/decompression runs and the
curves of the vertical component of molecular dipole moment for X3
with a p-terphenyl core and for X6 with a phenylene ethynylene type
core are compared; b) two cycles of compression (black lines) and de-
compression (grey lines) of two compounds with a phenylene ethynylene
based rigid core and different side chains (hydrocarbon chains in X5 and
one partially fluorinated chain in X6) are shown for comparison; letters c
and d mark the conditions at which the BAM images (also marked as c
and d) were recorded. c) The aggregates of X6 after compression beyond
collapse and d) the coexistence of liquid and gas phases (2D foam) recov-
ered after decompression. The white scale bar in image (d) represents a
length of 400 mm.
region. This ratio confirms that a trilayer is formed in the
plateau region (d2/d1 ꢁ3).
Additional information about the self-assembly of com-
pounds X1–X4 comes from the curve of the vertical compo-
nent of the dipole moment, m (A), shown for compound X3
?
in Figure 4a. An important contribution to the molecular
dipole moment comes usually from the terminal bond of the
alkyl chain. In the case of X-shaped molecules, for which
the rigid cores with the polar groups are lying flat on the
ꢀ
ꢀ
water surface, the terminal C H or C F bonds of the lateral
chains induce changes in the vertical component of the
dipole moment. If during compression of the close-packed
monolayer the lateral chains reorganize and the contribution
of the chains adopting a vertical orientation increases, the
contribution of terminal bonds becomes clearly visible in
ꢀ
X3 upon compression of the monolayer. This shows that
during the rise of the isotherm the longer hydrogenated
chains are rising up first, causing the increase of the dipole
moment. Upon further compression the contribution of ver-
tically aligned fluorinated chains increases and causes the
decrease of the curve. In the plateau region no further
change of the dipole moment was observed, which supports
the proposed model of molecular organization whereby in
the plateau region a bilayer of antiparallel-organized mole-
the m (A) curves, because the dipole moment of the C H
bond is positive and that of the C F bond is negative. A
broad peak in the m (A) curve is observed for compound
?
ꢀ
?
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The Air–Water Interface
FULL PAPER
fluorination (two fluorinated segments) exhibits a small hys-
teresis loop. The isotherms are perfectly reproducible in re-
peated cycles for X2 and X3 with all runs overlapping and
are shown in Figure S1 in the Supporting Information.
Compounds X5 and X6 consist of linear rigid cores that
are significantly longer than those in compounds X1 to X4,
due to the two additional ethynyl groups introduced be-
tween the benzene rings. A comparison of the shorter com-
pound X3 with the longer compound X6 is shown in Fig-
ure 4a. The isotherm cycle of compound X6 exhibits strong
hysteresis (in spite of the presence of a partially fluorinated
chain), which makes it distinct from compound X3 that
shows completely reversible isotherms. In addition to a re-
duced reversibility, also the shape of the isotherms of com-
pounds X5 and X6 (compared in Figure 4b) is different
from those of compounds X1–X4. Compound X6 contains
one partially fluorinated chain that is much longer than the
hydrocarbon chain, whereas compound X5 has two relative-
ly short hydrocarbon chains of the same length. The iso-
therms of compounds X5 and X6 showing two compression/
expansion cycles for each compound are presented in Fig-
ure 4b and the Brewster angle microscopy (BAM) images
acquired for X6 at the most interesting parts of compression
and decompression cycles are shown in Figure 4d and e. The
isotherms of both compounds are rather similar, but the film
of the fluorinated compound X6 is more stable as it can be
compressed to a much higher surface pressure than the non-
fluorinated compound X5. Though both compounds exhibit
strong hysteresis (no reversibility) the reproducibility is still
very good as the second compression curve nearly exactly
follows the first one.
Figure 5. Results of X-ray reflectivity measurements and curves fitted to
a theoretical equation for compounds a) X6 and b) X3. The film thick-
nesses for mono- and trilayers was estimated from the fitted parameters;
for more details, see the Supporting Information.
Analysis of the areas per molecule (A1 and A2) at both
ends of the plateau for X6 resulted in a relation A1 ꢁ2A2 at
pꢁ10 mNmꢀ1 and this excludes trilayer formation in this
case; the ratio A1/A2 ꢁ2 suggests a bilayer, but this doubtful
conclusion will be discussed below. The film of X6 was
transferred onto silicon wafers at p=8.5 and 15.0 mNmꢀ1
using the Langmuir–Blodgett technique. XRR measure-
ments were performed on these samples to estimate the
thickness of the films and the results are shown in Figure 5a.
The XRR curve measured for the film of compound X6
transferred at the ends of the plateau turned out to be very
different from those obtained for the related compound X3
under the same conditions (see Figure 5b). The curve of
compound X6 has less peaks, which indicates that the multi-
layer in this case consists of fewer layers than the structure
formed by compound X3, or alternatively, has a different
vertical distribution of the electron density. Values of the
layer thickness obtained for compound X6 from XRR data
fitted to theoretical equations are equal to d1 =1.57 nm for
the monolayer and d2 =3.51 nm for the layers transferred in
the plateau region. The single layer thickness d1 is very simi-
lar to that measured for compound X3 (d1 =1.65 nm),
whereas d2 is much smaller than the related value of X3
(d2 =4.76 nm). The very different ratios d2/d1 measured for
X3 and X6 confirm that whereas compound X3 with a short-
er terphenyl-based aromatic core creates a trilayer the X-
cules (compensated dipole moments) is formed on top of
the monolayer, leading to trilayer formation. For trilayer
formation a reversible “rollover” mechanism (Figure 2, pro-
cess B) was proposed firstly for rodlike amphiphilic liquid
crystals (aromatics perpendicular to or tilted relative to the
surface)[35–39] and then for T-shaped facial amphiphiles[40]
that have an arrangement of rodlike aromatics parallel to
the surface as also found for the bolaamphiphiles X1–X4.
More recently, trilayer formation was also reported for
semi-perfluorinated hydrocarbons,[17] amphiphiles with bulky
aromatic end groups[41] and for mixed monolayers formed
by co-assembly of cationic lipids with anionic porphyrins.[42]
For these co-assembled monolayers it was suggested that tri-
layer formation takes place in a kind of “lifting” process
(Figure 2, process A) and the reversibility of the isotherms
was explained by line tension of the trilayer domains coex-
isting with residues of monolayer.[43] Though these two
modes (rollover or lifting) of reorganization at the monolay-
er to trilayer transition are difficult to distinguish, a direct
transition from trilayer to 9-layer, as observed at high com-
pression ratio for compound A2 (see below), can only be ex-
plained by assuming a rollover process.[44]
Among the four compounds X1–X4 only those with fluo-
rinated chains (X2–X4) show perfectly reversible p(A) iso-
therms, although compound X4 with the highest degree of
Chem. Eur. J. 2011, 17, 5861 – 5873
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5865

R. Hołyst, C. Tschierske et al.
Table 2. Data of the Langmuir films and transferred films of compounds
shaped compound X6 with a longer p-conjugated core
forms a different structure in the plateau region. Another
difference between compounds X2–X4 and X5, X6 was
found for the surface pressure at the first break in the p(A)
isotherms, which is much lower for compound X6 than ob-
served for compounds X1–X4, although X6 has the longest
fluorinated tail. Apparently, the surface pressure required
for the trilayer formation is not reached for the Langmuir
films of X6 and a different process, requiring less energy,
takes place. This is also manifested by the different shape of
T.[a]
Compound
A1
A2
p2
[mNmꢀ1
d
[nm]
[nm2 moleculeꢀ1
]
[nm2 moleculeꢀ1
]
G
]
T1
T2
1.16
1.27
0.38
0.39
43.4
39.5
2.70
2.76
[a] A1 =area at the lift-off of the isotherm; A2, p2 =area and surface pres-
sure at the collapse; d=thickness of the monolayer.
the m (A) isotherm (Figure 4a) in the plateau region and
?
this will be discussed below in more detail.
Differences can also be observed for the isotherms ob-
tained for the non-fluorinated compounds X1 and X5, and
those of the fluorinated compounds X2–X4 and X6. In gen-
eral, the reversibility is reduced for all non-fluorinated com-
pounds. For example, the alkyl-substituted terphenyl-derived
compound X1 shows a strong hysteresis with consecutive
cycles systematically shifted towards smaller values of area
per molecule being the result of irreversible aggregation.
Though this is not observed for compound X5, the plateaus
in the p(A) isotherms of compounds X1 and X5 are strongly
rounded-off at the ends, so the molecular areas at these
points are hard to estimate and their ratios cannot be calcu-
lated. Also, the XRR measurements for these two com-
pounds did not give curves that could be used for fitting and
estimation of the thickness of the films. These observations
allow us to conclude that non-fluorinated X-shaped bolaam-
phiphiles collapse at higher surface pressure without creat-
ing well-defined multilayers and aggregate instead.
T-shaped bolaamphiphiles: The p(A) isotherms of the T-
shaped bolaamphiphiles T1 and T2 are completely different
from those recorded for the X-shaped molecules. These
Figure 6. The p(A) isotherms of compounds T1 and T2: a) two cycles of
compression (black lines) and decompression (grey lines) as well as the
dipole-moment changes for compound T1. The BAM images for com-
pound T2 are given as examples: letters b–e mark the conditions at
which the BAM images, also marked as b)–e), were recorded: b) the uni-
form monoACHTUNTRGNEUlGN ayer during compression, c) the aggregates formed after com-
pression beyond collapse, d) aggregates disappearing during decompres-
sion, e) coexistence of liquid and gas phases (2D foam) recovered after
decompression to p=0. The white scale bar in image (e) represents a
length of 400 mm.
compounds, which have only one semi-perfluorinated lateral
chain, give isotherms without plateaus and only one kink
close to the end of the isotherms close to A=
0.4 nm2 moleculeꢀ1 (Table 2). Moreover, their isotherms
show strong hysteresis during compression/decompression
cycles, whereas the compression runs show perfect reprodu-
cibility because in second and subsequent cycles identical
compression curves were obtained (for clarity only two
cycles for each compound are shown in Figure 6). Examples
of BAM images, recorded during compression/decompres-
sion runs of compound T2 confirm reversible aggregation of
the molecules. It should be noted that reversible aggregation
does not refer to reversibility of the isotherm in terms of the
meaning presented in the discussion of compounds X1–X4.
Here, 3D aggregates appear only after the kink (Figure 6c)
and disintegrate during decompression (Figure 6d) to disap-
pear completely after decompression to p=0 mNmꢀ1. Disin-
tegration occurs during decompression at surface pressures
of approximately 15 and 25 mNmꢀ1 for T1 and T2, respec-
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tively, but this very slow process is visible as inflection
points on the curves of decompression. Reversible aggrega-
tion manifests itself as reproducibility of the isotherms (re-
peatable subsequent cycles). The reappearing liquid phase
forms a two-dimensional foam at p=0 mNmꢀ1, as shown in
Figure 6e.
Interesting results come from a comparison of the iso-
therms of compounds T1 and T2 that differ exclusively in
the length of the rigid aromatic cores. Compound T1 has a
p-terphenyl core (L=2.48 nm as measured between the
ends of the primary OH groups in a most extended confor-
mation[45]), the same as the X-shaped compounds X1–X4,
whereas T2 has a longer core (L=2.95 nm) with two addi-
tional ethynyl groups, the same as incorporated in com-
pounds X5 and X6. Surprisingly, the areas occupied by these
two molecules at higher surface pressures, as estimated from
the p(A) isotherms, are nearly identical for both compounds
(A=0.39 nm2 moleculeꢀ1), though they have a very different
length.
on top of the monolayer should not significantly contribute
to the dipole moment (compare Figure 2a and b). The com-
pressibility modulus, Csꢀ1 =ꢀA
ACHTUNGERTN(NUNG dp/dA), corresponding to
the very small slope of p(A) at the beginning of compres-
sion, is equal to 19 mNmꢀ1 at p=3 mNmꢀ1 and is typical
for expanded liquids.[46] During compression the modulus
continuously increases to approximately 50 mNmꢀ1, which is
a typical value for condensed 2D liquid phases.[46]
Based on this experimental data the following model of
the organization of the T-shaped compounds T1 and T2 in
thin films is proposed. At the beginning of compression the
molecules are organized with rodlike cores and lateral
chains parallel to the air–water interface (see Figure 2c).
Upon further compression the molecules remain in a liquid-
like state and reorganization takes place by detachment of
one of the polar groups from the water surface and continu-
ous rising of the molecules by adopting a randomly tilted,
on average, vertical configuration of the rodlike cores. One
of the glycerol groups retains contact with the water surface,
whereas the other one at the opposite end of the bolaamphi-
philic molecule is detached from the surface and is raised to
a tilted orientation (see Figure 2, process C). The cross-sec-
tion of one molecule estimated from the isotherm in Fig-
ure 6a is equal to approximately 0.39 nm2, which seems to
be consistent with the proposed organization of the mole-
cules, on average, perpendicular to the air–water interface
(see Figure 2b). The cross-section of the fluorinated part of
the chain[47] is 0.285 nm2 and the cross-section of the ter-
phenyl moiety is less than 0.20 nm2, hence, there is enough
space for an, on average, vertical or slightly tilted arrange-
ment of the rigid cores with the partially fluorinated lateral
chains organized in the space between the rodlike cores.
It appears that the topology of the connection of the later-
al chain to the rigid aromatic core is responsible for this spe-
cial kind of behaviour. In fact, in the “T-shaped” molecules
the lateral chain is actually attached at an angle of 608 to
the aromatic core, which provides a l-like shape rather than
a true T-shape for these molecules (Figure 1b). As a conse-
quence, the lateral chain is closer to one of the polar glycer-
ol groups and restricts the interaction of this polar group
with the water surface. The other glycerol unit at the oppo-
site end is less distorted, which makes both polar groups
non-equivalent. This non-equivalence seems to allow an
easy detachment of the more distorted polar group from the
water surface. As a result, upon further compression, a
XRR measurements performed on films transferred onto
silicon wafers at p=33 mNmꢀ1 (see XRR curves of com-
pound T2 in Figure 7 as an example) gave layer thicknesses
Figure 7. X-ray reflectivity data and curve fitted to a theoretical equation
obtained for a monolayer of compound T2 transferred onto a silicon sub-
strate at a surface pressure of 33.0 mNmꢀ1. The film was transferred
using the Langmuir–Blodgett technique.
equal to d=2.70 and 2.76 nm for the transferred films of
compounds T1 and T2, respectively. These values are signifi-
cantly smaller than that expected for trilayers (e.g., d=
4.76 nm for the trilayer of X3), but very close to the length
of the rigid bolaamphiphilic moiety (T1: L=2.48 nm; T2:
L=2.95 nm; as estimated from CPK models[45]).
monoACTHUNRTGNEUNGlayer structure is retained and in this monolayer the
aromatic cores adopt a tilted or even vertical orientation
with respect to the water surface (see Figure 2b).
Surface potential (DV) measurements, presented as dipole
moment changes, m (A), shown in Figure 6a for compound
The isotherms of both T-shaped molecules T1 and T2 are
very similar with respect to the molecular area adopted at
the transition around p=40 mNmꢀ1 as the cross-section of
the aromatic cores and lateral chains is nearly identical for
both compounds. The effect of the distinct molecular length
on the layer thickness is relatively small because the longer
aromatic cores provide more space to accommodate the lat-
eral chains and a different degree of average tilt is possible.
This model of organization also provides clues for under-
?
T1 as an example, indicate a sudden decrease of the dipole
moment when the surface pressure exceeds p=10 mNmꢀ1.
The decrease of m (A) is most probably caused by the fluo-
?
rinated lateral chains rising up, which provides a negative
dipole moment, that is, upon compression above p=
10 mNmꢀ1 the fluorinated chains should adopt an orienta-
tion more or less perpendicular to the surface. This is not in
line with a trilayer formation in which the symmetric bilayer
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R. Hołyst, C. Tschierske et al.
standing the isotherms observed for the X-shaped com-
pounds X5 and X6 as explained in the following section.
monolayer-to-bilayer transition takes place in the plateau
region the dipole moments of the molecules in the lower
and upper layers should perfectly compensate and the m
?
X-shaped bolaamphiphiles X5 and X6 with extended rodlike
cores: Though the p(A) isotherms of the X-shaped mole-
cules X5 and X6 are similar to those of compounds X1–X4
forming a broad plateau, the areas per molecule (A1 and A2)
at both ends of the plateau give a relation A1 ꢁ2A2 at p
ꢁ10 mNmꢀ1 for compound X6, which is not compatible
with trilayer formation and would suggest formation of a bi-
layer. However, in light of the results obtained for the T-
shaped compounds T1 and T2, indicating that reorganiza-
tion of the molecules within the monolayer is an alternative
option for reducing the area per molecule at the air–water
interface, there is a second possible explanation as follows.
At low surface pressure, compound X6 behaves like the X-
shaped amphiphiles X1–X4, adopting an arrangement paral-
lel to the surface with both glycerol units attached to the
water surface (Figure 2c). However, for compound X6 this
arrangement seems to be less stable at increased surface
pressure, so that no trilayer is formed. Instead, a reorganiza-
tion of the molecules by tilting, as observed for the T-
shaped molecules, takes place (Figure 2, process C). The sur-
face area at the collapse of the monolayer is A2 =
0.53 nm2 molꢀ1, which is only a bit larger than that observed
for compounds T1/T2. The small difference in required area
could be explained by the presence of an additional lateral
chain in compound X6.
To distinguish these two possibilities of molecular reor-
ganization the experimentally measured XRR curves of the
films transferred at the highest possible surface pressure
were compared with the simulated XRR curves for both dis-
tinct cases: 1) a bilayer with aromatic cores arranged paral-
lel to the air–water interface created by a lifting mechanism
(see Figure 2, process A); and 2) a tilted organization of the
aromatics. In the latter case the aliphatic segments are ar-
ranged between the aromatics (see Figure 2, process C) and
the fluorinated chains form a distinct sublayer on top of the
monolayer (see Figure S2 in the Supporting Information).
This model provides a distinct electron-density profile that
can be compared to the case of bilayer formation in which
the highest concentration of fluorinated chains is expected
between the sublayers of the aromatic cores situated at the
bottom and on top of the hypothetic bilayer. Although the
fitting of the measured XRR curves to the vertical (Fig-
ure S2, image a) and horizontal (Figure S2, image b) models
was almost equally good (the vertical was slightly better),
for vertical orientation the fitted parameters describing the
electron-density distribution are only slightly different from
those estimated from the model, whereas in the case of hori-
zontal alignment the fitted parameters are significantly dif-
ferent from the ones estimated from the model, which
shows that this model is less likely (for details, see Table S1
in the Supporting Information).
(A) curve in this part should be a straight line extrapolated
to the coordinate m?(A=0)=0. In contrast, for compound
AHCTUNGTRENNUNG
X6 with a longer rodlike core, the dipole-moment curve
continuously decreases after the first kink in the p(A) iso-
therm and a decrease continues until the second kink is
reached (see Figure 4a). Since m (A) becomes negative at
?
the first kink it indicates that there is a significant contribu-
tion of fluorinated chains organized, on average, perpendic-
ularly to the surface. This behaviour of the dipole moment
upon compression is the same as that observed for the T-
shaped molecules (e.g., T2, see Figure 3a). It supports the
idea that for compounds X5 and X6 (compared in Fig-
ure 4b) the reorganization of the molecules is similar to that
observed for the T-shaped compounds, that is, no triple or
double layer is formed in the plateau region. We conclude
that the monolayers are retained and a reorganization of the
molecules to a randomly tilted or vertical orientation of the
aromatic cores takes place (see process C in Figure 2).
The distinct behaviour of compounds X1–X4 on the one
hand and X5/X6 on the other hand can be explained by the
effect of the longer aromatic cores on the self-assembly be-
haviour. As the distance between lateral chains and polar
groups is larger in compounds X5 and X6 with longer aro-
matic cores the distorting effect of the lateral chains on the
interaction of the polar groups with the water surface is re-
duced. Chain folding can additionally reduce the effective
length of these chains and this further decreases the distor-
tion of the polar groups. Moreover, in compound X6 the dis-
turbing influence of the shorter non-fluorinated alkyl chains
is smaller and the glycerol group adjacent to this chain is
less distorted than the glycerol group adjacent to the longer
fluorinated chain. Therefore, upon film compression the
shorter alkyl chains can be easily removed from the water
surface and one of the polar groups (the one besides the
shorter chain) becomes less distorted than the other one.
Hence, the amphiphilic character of the molecules changes:
at low surface pressure they behave as symmetric bolaam-
phiphiles (like compounds X1–X4) retaining an orientation
parallel to the surface, which is stable up to p=10 mNmꢀ1
(Figure 2, arrangement c). However, at higher surface pres-
sure after the first kink in the isotherm, they behave as non-
symmetric bolaamphiphiles, that is, like compounds T1/T2.
Starting from this point a rearrangement takes place in the
monolayers to an orientation tilted or perpendicular to the
air–water interface (see Figure 2, process C). Therefore, the
isotherm of X6 can be considered as a combination of the
isotherms of compounds X1–X4 at low surface pressure
(until plateau) and the isotherms of T-shaped compounds
T1/T2 at high surface pressure. The long semi-perfluorinated
chain in compound X6 allows the formation of a separate
sublayer of segregated perfluoralkyl chains, which signifi-
cantly stabilizes these monolayers. However, the presence of
a long fluorinated chain seems not to be required, because
the symmetric alkyl-substituted compound X5 shows a very
Additional information was gained from the comparison
of the shapes of dipole-moment curves recorded for com-
pounds X3 and X6 (Figure 4a), and T2 (Figure 6a). If a
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similar isotherm and it is very likely that the same kind of
molecular reorganization as proposed for X6 takes place for
X5, too.
Anchor-shaped bolaamphiphiles: Compounds A1–A3 from
this group resemble the shape of an anchor because a single
aliphatic chain is attached to a bent rigid core at the central
phenyl ring inside an angle created by the arms of the core
(bay position).
All compounds A1, A2 and A3 exhibit isotherms with
broad plateaus similar to compounds X1–X6. The isotherms
of compound A1 and A2 are shown in Figure 8. Both com-
pounds have the same rigid cores and the same length of ali-
phatic chains, the only difference between them is that the
chain in compound A1 is fully hydrogenated whereas com-
pound A2 contains a fluorinated C6F13 segment at the end
of the chain. The surface pressure at which the monolayers
lose their stability is higher for the fluorinated compound
A2, which shows that partial fluorination makes the interac-
tion with the water surface stronger. The area of
1.16 nm2 moleculeꢀ1 at the beginning of the plateau is small-
er than the area required by the X-shaped amphiphile X6
comprising a similar, but linear core. The difference between
the areas of A2 and X6, measured at the same surface pres-
sure p=9.4 mNmꢀ1, is about 0.2 nm2 moleculeꢀ1, which
agrees well with the additional space required by the second
OCH2 group of compound X6. Hence, it can be concluded
that the bent aromatic cores lay flat on the water surface
with the bend parallel to the surface and fixed on the sur-
face by the two glycerol groups and the ether oxygen atom
carrying the lateral chain. The second increase of the surface
pressure at the end of the plateau of compound A2 starts at
an area of 0.43 nm2 moleculeꢀ1 and ends at A=
0.4 nm2 moleculeꢀ1. Hence, a comparison of the areas per
molecule at both ends of the plateau leads to a ratio that is
equal to 2.7. In experiments similar to those described for
X- and T-shaped molecules the films of compound A2 were
transferred onto silicon wafers at p=20 mNmꢀ1 (before
Figure 8. Isotherms and BAM images for compounds A1 and A2: two
cycles of compression (solid symbols) and decompression (open symbols)
are shown in (a); letters b–e mark the conditions at which the BAM
images (also marked as b–e) were recorded. b) The uniform monolayer
during compression of the homogeneous liquid phase; c) the aggregates
formed after compression beyond collapse; d) aggregates disappearing
during decompression; e) coexistence of liquid and gas phases (2D foam)
recovered after decompression to p=0. The white scale bar in image (e)
represents a length of 400 mm.
reaching the plateau) and at p=30 mNmꢀ1 (before the
second transition). The X-ray reflectivity measurements per-
formed on these samples gave the results shown in Figure 9.
Values of the layer thickness obtained from the XRR data
fitted to theoretical equations are equal to d1 =1.43 nm and
d2 =3.71 nm, respectively. This value of d1 is only a little bit
smaller than the values found for the monolayers of com-
pounds X3 and X6, and it is also in good agreement with
the thickness of a model monolayer formed by the aromatic
cores lying flat on water with the disordered fluid semi-per-
fluorinated chains covering these aromatics. The ratio d2/
d1 =2.6 could be either due to a reorganization of the mole-
cules in the monolayers by adopting, on average, a perpen-
dicular organization similar to compounds X5 and X6 or to
trilayer formation. Though the compressibility at the end of
the plateau region is similar to that observed for the same
region of X6, indicating a liquid condensed film, the thick-
ness d2 =3.71 nm is much larger than can be explained by a
reorganization of the molecules inside the monolayer, even
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R. Hołyst, C. Tschierske et al.
decompression runs is much stronger here than in the case
of A2, but the second compression perfectly overlaps with
the first one showing perfect reproducibility, too. It means
that the aggregation is reversible after decompression of the
film to “zero” pressure and the aggregates need time for dis-
integration. Comparison of the isotherms of compounds A1
and A2 (Figure 8a) shows that the partial fluorination of the
lateral chain in the case of anchor-shaped molecules increas-
es the stability of the film and its tendency for self organiza-
tion into trilayers, but does not affect the reproducibility in
this case.
Compound A3 has the same fluorinated lateral chain as
compound A2, but its rigid core consists of a m-terphenyl
moiety, so this part of the molecule is smaller than the cores
of compounds A1 and A2. This compound has a compres-
sion isotherm very similar to the one of compound A2, but
with reduced molecular areas at the transition (Figure S5 in
the Supporting Information). It shows that this compound
also creates a trilayer during compression.
Figure 9. X-ray reflectivity data and curves fitted to a theoretical equa-
tion obtained for a monolayer and a trilayer of compound A2 transferred
onto the silicon substrates at surface pressures of 20 and 30 mNmꢀ1, re-
spectively; the films were transferred using the Langmuir–Blodgett tech-
nique.
if the molecules would adopt a perfectly vertical alignment
(which only would give d=2.7 nm). Therefore, we assume a
trilayer formation in the plateau region of the isotherm of
compound A2. The fact that the ratio d2/d1 is a bit smaller
than exactly three could be explained by some intercalation
of the lateral chains in the formed trilayer structure. The
proposed trilayer formation is also in line with the surface
potential measurements (see Figure S4 in the Supporting In-
formation), which indicates an increase of dipole moment in
the plateau region. In the case of a transition to a monolayer
with molecules organized, on average, perpendicularly to
the surface a decrease of the dipole moment, as observed
for compounds T1/T2 and X5/X6 would be expected due to
The fact that anchor-shaped compounds form Langmuir
films similar to the X-shaped molecules, but distinct from
those of T-shaped molecules, could partly be due to the
equal distance of the lateral chain from both polar groups in
the anchor-shaped bolaamphiphiles. In addition, the combi-
nation of the bent shape and substitution inside the bay area
between the rodlike wings distorts a parallel alignment of
these cores and hence the formation of smectic layers (cores
perpendicular to the layers) becomes more difficult. For
these reasons a Lam-type organization[33] with the cores par-
allel to the water surface becomes more favourable in this
case. Hence, bending the aromatic core of the T-shaped bo-
laamphiphiles T, leading to compounds A, has a similar
effect as grafting a second lateral chain to the opposite side
of the rodlike core leading to compounds X.
ꢀ
the negative contribution of the terminal C F bond of verti-
cally aligned RF segments.
The BAM image in Figure 8c shows a rough surface of
the film after breakdown of the trilayer at the second kink
in the isotherm curve. During decompression the trilayer
cannot be recovered, so in the image in Figure 8d the coex-
istence of the trilayer with aggregates and the monolayer
can be observed. The shape of the domains and the fact that
their size diminishes during expansion allows us to conclude
that the trilayer is still in the liquid state. It is one of the rea-
sons why the isotherm, although irreversible (because of a
small hysteresis), is perfectly reproducible in the subsequent
compression.
For compound A1 with a non-fluorinated n-alkyl chain
the second increase of surface pressure occurs at an area per
molecule much smaller than that for A2 (Figure 8a) and
smaller than the value estimated from the CPK model. The
surface pressure increase at a non-realistic value of molecu-
lar area is typically observed for completely disordered
films, which suggests that disorganized aggregates are creat-
ed also in this case. In this region the curve of the dipole
moment is a nearly straight line going through zero, which
means that all of the dipole moments cancel out because of
the completely random aggregation (Figure S3 in the Sup-
porting Information). Hysteresis between compression and
Conclusion
Molecular design leads to new types of bolaamphiphiles ca-
pable of self-assembly in distinct modes at the air–water in-
terface. For all compounds here, the densely packed mono-
layers were formed with the aromatic cores laying flat at the
air–water interface. Further compression either leads to a
first-order transition with formation of well-defined trilayers
(layering transition) by retaining the orientation of the ani-
sometric cores, or to a second-order transition retaining the
monolayers by reorganization of the cores into, on average,
vertical orientation. It seems that the degree of distortion of
the polar groups attached to the ends of the rigid p-conju-
gated cores by the laterally attached hydrophobic chains is
the key factor deciding which type of reorganization is ob-
served in the monolayer. For X-shaped bolaamphiphiles
with two relatively long lateral chains of approximately
equal length, both exceeding the polar groups at the ends,
triple-layer formation is dominating. If one chain is removed
(T-shaped bolaamphiphiles) the two polar groups at the op-
posite ends become distinct and these bolaamphiphiles pref-
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erably organize with their anisometric cores perpendicular
to the surface with the less-hindered polar group acting as
an anchor group. Even slight modification of the molecular
structure of the two fundamental types of bolaamphiphiles
(T-shaped and X-shaped) can change the self-assembly be-
haviour dramatically. For example, an elongation of the
rigid core of X-shaped compounds reduces the degree of
distortion of the polar groups by the lateral chains and it
gives rise to reorganization of the monolayers by changing
the molecular orientation (compounds X5 and X6). On the
other hand, bending the rodlike core of T-shaped molecules
(leading to the anchor-shaped compounds) makes the two
polar groups equivalent, which favours formation of a trilay-
er without changing the orientation of the molecules.
Besides the formation of well-defined mono- and trilayers,
which can be transferred onto solid supports, a second
unique feature observed for these Langmuir films is repro-
ducibility and reversibility of the compression/decompres-
sion cycles. Reproducibility of the isotherms, that is, overlap-
ping of subsequent compression runs with (or without) hys-
teresis loops during decompression, was observed for all
compounds. Reversibility of the compression/decompression
isotherms, that is, perfect overlapping of compression and
decompression runs, was only achieved with short fluorinat-
ed segments incorporated into the lateral chain(s). Non-flu-
orinated compounds do not exhibit reversibility and reversi-
bility is also reduced by elongation of the fluorinated seg-
ments. The main reason for reproducibility/reversibility of
the Langmuir films seems to be their fluidity provided by
the partly fluorinated chains, combined with the preorgani-
zation of the amphiphiles due to short-range order induced
by alignment of the rigid anisometric units.[48]
The thin films resemble the molecular organizations simi-
lar to lamellar liquid-crystalline bulk phases of the smectic
type if the molecules are organized vertically (e.g., T-shaped
molecules), whereas mono- or multilayers with molecules
organized parallel to the layer planes are related to Lam-
type LC phases[32a,49] in which rodlike units are arranged par-
allel to the smectic layers.[33] Remarkably, distinct types of
Lam phases were also observed for numerous T- and X-
shaped amphiphiles as bulk LC phases.[29,30a,32a] Though the
investigated bolaamphiphiles as bulk phases are in the crys-
talline state (see Table S2 in the Supporting Information), at
the temperature of investigation of their self-assembly in
Langmuir films (238C) the interaction of the amphiphiles
with the water molecules at the interface leads to a phenom-
enon called “surface melting”,[50] which gives the possibility
to form the fluid thin films. Moreover, hydration increases
the effective size of the polar glycerol groups, which is likely
to modify the mode of self-assembly. Therefore, the tenden-
cy of these molecules to self-assemble in these lamellar
structures is considered as a driving force for formation of
stable mono- and trilayers, though at the same temperature
in the bulk state the pure (non-hydrated) compounds are in
the crystalline state or form other non-lamellar LC phases
(see Table S2 in the Supporting Information). As there is
presently no clear evidence of long-range orientational or
positional order in the Langmuir films it is assumed that any
orientational/positional order occurring in the Lam-like
structures is of short range, only.
The function of the semi-perfluorinated chains is twofold:
1) they increase the film stability by increasing the amphi-
philicity and 2) the fluorinated chains of medium size in-
crease film fluidity due to the mismatch of chain diameter
(RH versus RF), and probably, due to the dipole moment in-
troduced at the RH–RF junctions.[17,20] Hence, beside the
presence of an anisometric rigid unit, also the partial fluori-
nation of the lateral chains turned out to be a key factor for
reversible formation of stable and well-defined mono- and
trilayers.
The knowledge gained in these studies might be useful for
the design of self-organizing p-conjugated molecules and
macromolecules, which are of importance for their applica-
tions as organic electronic materials. On the other hand,
these molecules being bolaamphiphiles have the potential to
interact with lipid membranes either by formation of inter-
nal structures, thus modifying membrane flexibility and per-
meability, or by formation of external structures at the
membrane surfaces.[51,52] There is also the possibility of stabi-
lization of membrane fragments and membrane proteins,[53]
which would be useful for applications in biochemistry and
biophysics.
Experimental Section
Materials: Compounds X1, X4, T1, A2 and A3 have been reported previ-
ously. The synthesis of the new compounds and their analytical data are
given in the Supporting Information. Chloroform (Sigma–Aldrich, HPLC
grade) was used to prepare the solutions. Ethanol 95% (Merck) used for
cleaning the trough and the barriers, as well as the other solvents were of
analytical grade. Ultra-pure water characterized by a surface tension of
72.75 mNmꢀ1 at 208C and resistivity 18.3 MWmꢀ1, used as a subphase in
the Langmuir trough was obtained from the Milli-Q water purification
system. All solutions for the monolayer spreading were prepared by dis-
solving the compounds in CHCl3 to obtain concentrations of approxi-
mately 1 mgmLꢀ1
.
Compression isotherms: Experiments were carried out using the equip-
ment from Nima Technology: a Teflon trough of size 50 mmꢂ750 mmꢂ
10 mm equipped with the two hydrophilic barriers for symmetric com-
pression and a film balance of resolution 0.01 mNmꢀ1 for surface pres-
sure (p) measurements. A rectangular piece of analytical filtering paper
(20 mmꢂ10 mmꢂ0.1 mm) was used as a surface pressure sensor. The
whole system was placed on an active anti-vibration table and closed in a
Plexiglas box to prevent the films from dust and air currents. The time
delay after spreading of the film and before its compression, necessary
for solvent evaporation and film equilibration, was about 20 min. Usually,
the films were compressed/decompressed at a rate of 5 cm2 minꢀ1, which
corresponds to approximately 0.05–0.15 nm2 moleculeꢀ1 minꢀ1 and de-
pends on the amount of substance spread on the film. However, when
the reversibility of the isotherms was checked, a compression rate as high
as 1.5 nm2 moleculeꢀ1 minꢀ1 was applied. For all the compounds studied
the isotherms of compression and decompression (hysteresis loops) were
recorded at 238C controlled with accuracy ꢂ0.28C using a cooling/heat-
ing circulating bath (Thermo Scientific, USA). Temperature was mea-
sured with two Pt100W resistance thermometers immersed at both ends
of the trough and connected to a Keithley multimeter.
Brewster angle microscopy (BAM): The trough was equipped with a
Brewster angle microscope (MiniBAM) of a resolution of 8.3 mm per
Chem. Eur. J. 2011, 17, 5861 – 5873
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Hołyst, C. Tschierske et al.
pixel from Nanofilm Technology. This technique allows for direct obser-
vations of the monolayer during compression and detection of film thick-
ness to distinguish areas of the water surface covered with a monolayer
or multilayers. It is also possible to distinguish liquid and solid phases
from the shape of the domains and the patterns observed on the inter-
face. A magnification of MiniBAM is not high, so the field of observation
is relatively big, that is, 4.8 mmꢂ6.4 mm.
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the molecular dipole moment using the Helmholtz equation [Eq. (1)]:
˙
´
´
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m
¼ DVAee0
ð1Þ
?
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in which m =mcosq (q is the angle between the surface normal and the
?
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e0 are dielectric permittivity constants of the monolayer (which is as-
sumed to be 1) and vacuum, respectively. The surface potential signal is
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Acknowledgements
This research, as a part of the European Science Foundation EURO-
CORES programme SONS, project SCALES, was supported by funds
from the Polish Ministry of Science and Higher Education (project 2007-
2010), the DFG and the Sixth Framework Programme under contract
ERAS-CT-2003-989409. Support by the DFG was also provided through
the FG 1145. The research was also partially supported by the European
Union within European Regional Development Fund, through grant In-
novative Economy (POIG.01.01.02-00-008/08). P.N. acknowledges a sup-
port from the Polish Ministry of Science and Higher Education under
programme “Iuventus Plus”, project 2010-2011. J.P. is a scholar within
Sub-measure 8.2.2 Regional Innovation Strategies, Measure 8.2 Transfer
of knowledge, Priority VIII Regional human resources for the economy
Human Capital Operational Programmeco-financed by European Social
Fund and state budget.
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Received: December 20, 2010
Published online: April 8, 2011
Chem. Eur. J. 2011, 17, 5861 – 5873
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5873