Langmuir-Blodgett film formation of succinic acid half-esters containing double long alkoxy chains

Article abstract of DOI:10.1039/a700716g

Two types of half-esters of succinic acid have been prepared. One contains a 1,2-dialkylated glyceryl group, while the other has a normal alkyl group as the hydrophobic group. The mono-molecular films of the half-esters developed at the air-water interface were evaluated by π-A curves. Multilayer Langmuir-Blodgett (L-B) films deposited on the substrate are discussed in terms of pinhole defects. Influences of the surface pressure on accumulation, the number of layers, and the chain length of the lipophilic group are analysed. The L-B film made from the double-chained homologues possessed fewer pinhole defects compared with that composed of the single-chained one.

Full text of DOI:10.1039/a700716g

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Langmuir–Blodgett film formation of succinic acid half-esters containing double long
alkoxy chains
Shuichi Osanai,* Makoto Higami, Yuko Ono and Eiji Ohta
Faculty of Science and T echnology, Keio University, 3-14-1 Hiyoshi, Kohhoku-ku, Yokohamashi 223, Japan
Two types of half-esters of succinic acid have been prepared. One contains a 1,2-dialkylated glyceryl group, while the other has a
normal alkyl group as the hydrophobic group. The mono-molecular films of the half-esters developed at the air–water interface
were evaluated by p–A curves. Multilayer Langmuir–Blodgett (L–B) films deposited on the substrate are discussed in terms of
pinhole defects. Influences of the surface pressure on accumulation, the number of layers, and the chain length of the lipophilic
group are analysed. The L–B film made from the double-chained homologues possessed fewer pinhole defects compared with that
composed of the single-chained one.
Considerable interest has focused on Langmuir–Blodgett (L–B)
films, which show widespread technological potential as a
novel class of materials.1–3 For the development of various
functions, there are many requirements to improve or modify
them. L–B films comprised of higher fatty acids, such as stearic
and arachidic acid and their metal soaps have been fabricated
and investigated for use as organic insulators.4 Observations
using electron microscopy and electronic measurements
revealed that there are several types of defects in the films.5–7
A large number of factors could be considered to improve the
film state. It is possible that the state and behaviour of the
arrangement of the matrix amphiphilic compound at the air–
liquid interface are the most dominant factors in this kind of
work as well as the preparation of the appropriate raw
materials for the film.
and sodium 4,4-dimethyl-4-silapentanesulfonate (DSS) in
D O]. J values are in Hz.
2
Preparation of fatty acids
Scheme 1 shows an outline of the preparation of the amphi-
philic double-chained and single-chained succinic acid half-
esters. Benzyl ether 2 was prepared according to the literature
procedures.10,11 The 1,2-di-O-isopropylidene group was then
removed by the usual method using aqueous dilute hydro-
chloric acid. Compound 3 was then converted into the
corresponding 1,2-di-O-alkylated triether 4.
1,2-Di-O-alkyl-3-O-benzylglycerol 4. Preparation of the
bis(tetradecyl) homologue 4a is described here as an example.
The diol 3 (1.91 g, 10.5 mmol) was reacted with sodium hydride
(1.0 g, 25 mmol, 60% oil emulsion) in 100 ml of dry tetrahydro-
furan (THF) solution with stirring and gentle refluxing.
Tetradecyl p-toluenesulfonate12 (9.29 g, 25.2 mmol) was added
to the alkoxide solution, and the mixture was heated for 3 h
with refluxing, then additional heating was continued for 5 h
to remove most of the THF at 140–150 °C. The residue was
It is well known that amphiphilic compounds containing
two long chain lipophilic groups have the ability to self-
organize. In addition, they provide a well-organized film
structure, and their membranes are very extensible and flexible.
Furthermore, they arrange and pack in good order at the air–
water interface. Their dominant features are compatible with
the aim of this work, that is, preparation of ‘defect-free films’.
In this study, we synthesized half-esters of succinic acid
possessing a (2,3-bis-long-chain-alkoxy)glyceryl group as the
double-chained lipophilic group. For comparison, the corre-
sponding single chain analogue was also prepared. We would
like to clarify in this work the eÂect of introducing the double-
chained structure and the relationships between the molecular
structure and the pinhole defect density in the L–B film on a
silicon substrate.8,9
Experimental procedures
General measurement
Throughout the preparation of the raw materials, Merck
precoated TLC plates (Silica gel 60F254, 0.25 mm, Art 5715)
were used for the TLC analysis. For preparative column
chromatography, Wakogel C-200 was employed. Melting
points were recorded on an MP-J3 melting point apparatus
(Yanagimoto Co) and are uncorrected. IR spectra were taken
with a JASCO spectrometer FTIR-5300 (Japan Spectroscopic
Co). The identification of the structures was confirmed from
NMR spectra. 1H and 13C NMR spectra were recorded with
JEOL FX 90A (90 MHz for 1H NMR and 22.5 MHz for 13C
NMR) and JNM-GSX 270 (270 MHz for 1H, 67.5 MHz for
13C) instruments. Chemical shifts are reported in ppm down-
field (d) from internal standard [Me Si in CDCl , (CD ) SO
Scheme 1 Reagents: i, NaH, then BnBr, THF; ii, aq. HCl; iii, NaH,
pyridine
then ROTs, THF; iv, H , Pd/C, EtOH, AcOH; v, succinic anhydride,
2
4
3
3 2
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diluted with diethyl ether. The ethereal solution of the crude
products was washed with brine and water to pH 7 of the
washed water, then dried over anhydrous sodium sulfate. The
semi-solid product 4a was purified by preparative column
chromatography. Elution using hexane–ethyl acetate (1551 v/v)
OCH CH CH ), 2.58 [s, 4H, C=(O)CH ], 3.30 (br s 1H,
2
2
2
2
OH), 4.08 (t, 2H, OCH CH ); mp of 8c: 64–65 °C.
2
2
The 1H NMR spectrum of each homologue intermediate
possessing additional alkyloxy groups was also in fair agree-
ment with their d values and integration with the theoretical
values for 4 and the subsequently prepared dialkoxy glycerol 5
and half-esters of succinic acid 6.
aÂorded 4a as a waxy product. (4.0 g, yield 67%). The R value
f
was 0.86 for each homologue, 4a–c, (hexane: AcOEt=351 v/v).
The 1H NMR spectra are summarized as follows; d
H
(90 MHz, CDCl ) 0.88 (t, 6H, CH ), 1.28 [br s, 44H,
Preparation of monolayer and measurement of p–A curves
3
3
CH (CH ) ], 1.60 (m, 4H, OCH CH CH ), 3.33–3.60 (m, 9H,
3
2 11
2
2
2
The monolayer films of the half-ester of succinic acid were
prepared on a Langmuir trough. Deionized distilled water
from a Millipore system was used to prepare the subphase for
the trough. The specific resistivity of the water used was larger
than 17 MV cm and its surface tension, as measured by the
Wilhelmy plate method, was 71 mN m−1. A chloroform solu-
tion of the amphiphilic carboxylic acid was spread on a pure
OCH CH , OCHCH ), 4.57 (s, 2H, CH Ph), 7.30 (m, 5H, Ph).
2
2
2
2
1,2-Di-O-tetradecylglycerol 5a. The 1,2-di-O-alkylglycerol 5
was obtained by the reductive debenzylation of the correspond-
ing 4 in the presence of a 10% Pd/C catalyst and hydrogen at
atmospheric pressure. The solution, which consisted of 100 ml
of ethanol and 20 ml of acetic acid containing 3.74 g (6.5 mmol)
of 4a, was allowed to hydrogenate at 50 °C overnight with
agitation. After filtration of the catalyst, the solvent was
removed by evaporation under reduced pressure. Silica gel
(Wakogel C-200) column chromatography of the product using
hexane–ethyl acetate (3051 v/v) gave pure 5a. (3.8 g, yield
water surface at a subphase temperature (T ) of 10 °C.
sp
Concentrations of the chloroform solution of the double-
chained succinic acid monoester (6a–c) and mono-chained
higher-alkyl hydrogen monosaccinate (8a–c) were 2 and 4 m,
respectively.
After 20 min of solvent evaporation, the p–A isotherm curves
were recorded with a film balance system equipped with an
Wilhelmy plate (NL-LB240S-MWA, Nippon Laser Electronics
Co, Ltd) using the moving wall method at a barrier speed of
11 mm min−1. The results for the dihexadecanoyl homologue
is shown here as an example in Fig. 1. Molecular occupied
areas and collapse pressures of the amphiphiles are summarized
in Table 1.
80%). The R values were 0.45–0.48 for each of the homologues,
f
5a–c (hexane–AcOEt, 1551 v/v). n/cm−1 (KBr) 3470, 2920,
2851, 1467, 1113, 1080. d (90 MHz, CDCl ) 0.89 (t, 6H, CH ),
H
3
3
1.28 [br s, 44H, CH (CH ) ], 1.58 (m, 4H, OCH CH CH ),
3
2 11
2
2
2
2.22 (br s, 1H, OH), 3.37–3.68 (m, 9H, OCH CH , OCHCH ).
2
2
2
Mps: 5a, 44–45 °C; 5b, 54–56 °C; 5c: 63–64 °C (lit.,9 64 °C).
Double-chained mono-ester of succinic acid 6a. To a solution
of 1.07 g (2.2 mmol) of 5a in 20 ml of pyridine in a round-
bottomed flask provided with a magnetic stirrer, a dropping
funnel, a nitrogen gas tube, and a reflux condenser fitted with
a calcium chloride tube, was slowly added pyridine solution
(15 ml) containing 1.1 g (11 mmol) of succinic anhydride. The
mixture was allowed to stand for 24 h at 40 °C with agitation.
After the reaction, the content was diluted with diethyl ether.
The ethereal phase was washed with brine and water and then
dried with anhydrous sodium sulfate. Removal of the solvent
under reduced pressure aÂorded the crude product. The crude
waxy product was chromatographed on a silica gel (Wakogel
C-200) column with hexane–ethyl acetate (751 v/v) to give the
pure compound 6a as a white powder after removal of the
Accumulating deposition procedure
Two diÂerent kinds of substrates, a silicon wafer (30×10 mm)
and a glass slide (30×10 mm), were used depending upon the
purpose of the measurement.
A single crystal silicon wafer, which has a mirror surface on
one side, was used for the observation of the deposition state
of the accumulated L–B film by its IR spectrum. It was washed
using trichloroethylene, acetone and then ethanol under sonic-
ation, and further treated with 10% hydrofluoric acid for 5 min
followed by hot concentrated nitric acid for 10 min, and then
washed and rinsed with water.
On the other hand, in order to evaluate the pinhole defects
in the L–B films, the copper decoration method was applied
and used to visualize the pinholes. For this purpose the glass
slide substrate was successively vapour-deposited with
chromium and gold.
eluent (0.67 g, yield 52%, R =0.08, hexane–ethyl acetate,
f
751 v/v). n/cm−1, (KBr), 2918, 2851, 1734, 1710, 1469, 1105;
d
(270 MHz, CDCl ) 0.88 (6H, t, J 7.5, a), 1.24 (44H, br s, b),
H
3
1.57 (4H, distorted quin, c), 2.68 (4H, distorted t, d), 3.42 (2H,
t, 7.0, one of e), 3.45 (2H, t, J 7.0, one of e), 3.57 (2H, m, f ),
During the L–B depositions, the surface pressure was kept
at 55 mN m−1. A transfer speed of 3–5 mm min−1 was applied
for the film deposition. These accumulating conditions were
applied for both of the above substrates.
3.64 (1H, quin, J 6.5, g), 4.12 (1H, dd, h1, J 13.0, Jg–h1 6.5),
gem
4.28 (1H, dd, h2, J 13.0, Jg–h2=4.5), 4.80 (1H, br s, i). Mps:
gem
6a, 38–40 °C, 6b, 43–44 °C, 6c, 48–49 °C.
Single chained mono-ester of succinic acid 8. Mono higher
alkyl hydrogen succinates 8 were prepared in a manner similar
to the synthesis of 6 just described. The normal higher alcohol
7 was used instead of 1,2-di-O-alkylglycerol 5 as the raw
material.
The crude waxy product was chromatographed on a silica
gel (Wakogel C-200) column with a mixture of hexane–ethyl
acetate (351 v/v) to give the pure compound 8b (yield 42%,
mp 59–60 °C, R =0.12); d (90 MHz, CDCl ) 0.91 (3H, t,
f
H
3
CH ), 1.34 [br s, 26H, CH (CH ) ], 1.62 (m, 2H,
Fig. 1 p–A Isotherm curves of (a) 6b and (b) 8b
3
3
2 13
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Table 1 Molecular occupied areas and collapse pressures of amphiphiles
molecular occupied
collapse pressure/
molecular occupied
collapse pressure/
˚
˚
amphiphiles
6 and 8
area/A2
mN m−1
area/A2
mN m−1
pure water
KCl aq. soln.
double-chained
amphiphiles
6a
6b
6c
40
43
44
61
65
68
40
43
43
60
61
61
CdCl aq. soln.
2
single-chained
amphiphiles
8b
8c
23
24
54
60
23
24
66
70
Detection of the pinholes on the L–B film
1H NMR and IR spectroscopy. The integral data in the 1H
NMR identified the structure of the end products.
Recently, it was confirmed that conventional fatty acid L–B
films on a solid substrate are not perfect but many defects
such as pinholes exist in the film. The evaluation of the
pinholes was carried out using the copper decoration method
through electroplating with the following conditions.6,13 If the
pinholes eventually penetrate the L–B lipid membrane, ‘the
heads of copper nails’ appeared as stains on the surface of the
p–A curves of fatty acid films
Fig. 1 shows the p–A isotherm curves of the monomolecular
films of 6b and 8b, which possess a hexadecyloxy hydrophobic
chain, at the air–water interface. Both 8 and 6 provided good
surface pressures for L–B monolayer formation under these
conditions. As summarized in Table 1, the limiting molecular
area of 8 was about half that of 6. They were estimated as
film after the decoration: electrolytic solution: 1% CuSO aq.
4
soln. (0.063 ), counter electrode: copper plate, applied voltage:
˚
40–44 A2 for the double-chained amphiphile series and
100 mV, distance between electrodes: 3.0 cm, decoration
periods: 5 min.
˚
23–25 A2 for the single chained one, respectively. The collapse
pressure of 6 was >60 mN m−1 and higher than that of 8.
But the addition of a divalent cation such as cadmium in a
sub-phase enhanced the collapse pressure of the L–B film that
consisted of a single-chained amphiphile. It has been reported
that the stability and the deposition properties of the mono-
layer on the water surface could be improved by the formation
of a poly-ion complex between the hydrophilic groups in the
ionic amphiphile and poly-ion in the subphase.14,15 Equally
good results have also been achieved in our experiment. On
the other hand, the influence of the chain length on the limiting
molecular area was slight regardless of the number of
hydrophobic groups in the molecule.
After the decoration, the small copper stains that appeared
during electroplating were counted in a unit area using an
optical microscope. This was suÃcient to permit the exact
evaluation of the defects. The influence of the surface pressure
during the deposition and the number of layers in the accumu-
lation upon the defects of the L–B films are discussed. The
results are summarized in Fig. 2.
Results and Discussion
Synthesis of the half-esters of the dibasic acids
In order to prepare the raw materials of the L–B film, double-
and single-chained fatty acids were synthesized according to
the reactions shown in Scheme 1. Synthetic studies along with
application of many methods already introduced were
accompanied with no pronounced diÃculties, although some
yields were unsatisfactory. The etherification of the hydroxy
groups of 3 was carried out according to Williamson’s method
using alkyl tosylate. Although some other O-alkylating agents,
such as the higher alkyl chlorides and bromides were used, the
corresponding alkyl toluene-p-sulfonates gave the best results.
The characterization of each compound was carried out using
Evaluation of pinhole defects in L–B films
Fig. 2 shows the correlation between the surface pressure (p)
and the number of copper spots that appeared on the single
layer film prepared under various surface pressures during the
copper decoration. The film prepared under the lowest surface
pressure at 15 mN m−1 showed an extremely large number of
copper spots, which means it had many pinhole defects. On
the other hand, the film prepared at a surface pressure between
25 and 55 mN m−1 clearly had few defects compared with the
previously mentioned film. As Fig. 2 shows, the number of
copper spots was inversely proportional to the surface pressure,
but the influence of pressure was slight at 25 mN m−1 or above.
For the influence of the number of accumulating layers
consisting of double-chained amphiphiles (6a–c), it was con-
firmed that increasing numbers of layers depressed the forma-
tion of the pinholes. As Fig. 3(a) shows, the single-layer film
had more than 120 spots of copper stain per 0.01 mm2, more
than twenty times greater than the number of spots that
appeared in the multi-cumulated films. The eÂect of the
accumulation on the improvement of pinhole defects in the
film is clear. While the diÂerence between the single and triple
layers was marked, clear diÂerences were not shown in the
number of copper stains among multi-layers accumulated over
three layers. Also there was no obvious influence of the chain
length of the two alkoxy groups over the whole accumu-
lation range.
For the films that consisted of single-chained amphiphiles,
8, on the other hand, the influence of the chain length of the
hydrophobic group was more obvious, as shown in Fig. 4. The
Fig. 2 Correlation between the density of copper spots and surface
pressure for a single layer of 6b
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The eÂect of the introduction of a double-chained lipophilic
group into an amphiphilic fatty acid was investigated in terms
of improvement of pinhole defects in the L–B film. They were
discussed based on studies of the p–A isotherm curves and
microscopic observations after the copper decoration method
by Cu electroplating. Even on the pure water surface containing
no univalent and divalent metal cations, double-chained
fatty acids were able to form a monolayer which could be
accumulated on the substrate as Y-type film by the usual
L–B method.
The number of pinhole defects in the film which consisted
of the double-chained fatty acids was decreased to one twenti-
eth by forming a multilayer. The eÂect of the accumulation on
elimination of the defects was more pronounced for the double-
chained homologues in contrast to the single-chained homol-
ogues. On the other hand, the eÂect of the lipophilic group
chain length was small.
Fig. 3 Influence of number of accumulated layers on the copper spots
for (') 6a, (p) 6b and ($) 6c
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Paper 7/00716G; Received 31st January, 1997
1408
J. Mater. Chem., 1997, 7(8), 1405–1408