Organization of the enantiomeric and racemic forms of an amphiphilic resorcinol derivative at the air-water and graphite-1-phenyloctane interfaces

Article abstract of DOI:10.1002/chir.21976

This article describes a study of the outcome of racemate condensation in different types of monolayers. The study was performed on a resorcinol surfactant bearing an octadecyl chain and a lactate group which formed a monolayer at the interface of graphit

Full text of DOI:10.1002/chir.21976

CHIRALITY 24:155–166 (2012)
Organization of the Enantiomeric and Racemic Forms of an
Amphiphilic Resorcinol Derivative at the Air–Water and
Graphite–1-Phenyloctane Interfaces
1
2
3
3
4
5
´
PATRIZIA IAVICOLI, HONG XU, TAMAS KESZTHELYI, JUDIT TELEGDI, KLAUS WURST, BERNARD VAN AVERBEKE,
WOJCIECH J. SALETRA,1 ANDREA MINOIA,5 DAVID BELJONNE,5 ROBERTO LAZZARONI,5
STEVEN DE FEYTER,2 AND DAVID B. AMABILINO
1
*
1Institut de Ci`encia de Materials de Barcelona (CSIC), Campus Universitari, 08193-Bellaterra, Spain
2Division of Molecular Imaging and Photonics, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, B-3001,
Leuven, Belgium
3Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri’ut 59-67, Budapest 1025, Hungary
4Institut fu¨r Allgemeine Anorganische und Theoretische Chemie, Universita¨t Innsbruck, A-6020, Innrain 52a, Austria
5Service de Chimie des Mate´riaux Nouveaux, Universite´ de Mons/Materia Nova, B-7000 Mons, Belgium
ABSTRACT
This article describes a study of the outcome of racemate condensation in dif-
ferent types of monolayers. The study was performed on a resorcinol surfactant bearing an octa-
decyl chain and a lactate group which formed a monolayer at the interface of graphite and 1-
phenyloctane as well as a Langmuir film at the air–water interface. Control experiments with
the enantiopure materials provided the characteristics of the chiral organizations. The results
obtained on the racemate show that on graphite the molecule forms chiral domains, indicating
that spontaneous resolution takes place at the surface, a phenomenon that has been rationalized
using molecular modeling. The X-ray crystal structure of the DMSO solvate of one of the enan-
tiomers shows a similar type of packing to this monolayer. On the other hand, in the Langmuir
layer it appears that the formation of a racemic compound is favoured, as it is in the solid state
in three dimensions. The work shows how the symmetry restrictions in different environments
can have a critical influence on the outcome of racemate organization, and underline the tend-
ency of graphite to favour symmetry breaking in monolayers formed at its surface. Chirality
C
VV
24:155–166, 2012.
2011 Wiley Periodicals, Inc.
KEY WORDS: spontaneous resolution; conglomerate; STM; lactate; langmuir; SAM; SFG;
molecular modeling
INTRODUCTION
impose restrictions on the symmetry operations open to cer-
tain molecules which pack on top of it. The most frequently
encountered structural feature of adsorbents to graphite is a
long alkyl chain.6 The reason for this trait is that saturated n-
alkyl chains form a series of [CꢀꢀHꢁꢁꢁp] interactions between
the methylene groups of the hydrocarbon and the aromatic
rings of the graphite, which encourages these chains to lie
parallel to the carbon surface.7 However, the moieties which
are attached to the alkyl chains in physisorbed systems play
a definite role in the packing of the monolayers: the symme-
try and number of alkyl chains can lead to different packing
The factors that determine whether racemic mixtures of
amphiphiles self-assemble into either homo- or heterochiral
monolayer domains on a surface in the form of two-dimen-
sional crystals are mainly based on symmetry arguments.1,2
The most common symmetry elements in 3D crystals are
the centre of inversion, glide plane, 2-fold screw axis and
translation operators.3,4 The first two of these—which are
the most common—are associated with racemic crystals. In
a 2D system a surface cannot possess a centre of inversion
and can only maintain translation, rotation and reflection mir-
ror symmetry planes normal to the surface.
In addition to the reduced symmetry possibilities in
the presence of a substrate, the balance between adsorb-
ate-substrate interactions and adsorbate–adsorbate inter-
actions plays a role.5 Should the adsorbates have any
specific interaction with the surface then symmetry of
the surface plays a fundamental role in the possible
packing modes of a molecule upon it. So the ability to
form a racemic conglomerate will depend on the nature
of the substrate because of symmetry, molecular orienta-
tion, and the strength of the molecule–surface and mole-
cule–molecule interactions.
Contract grant sponsor: European Community’s Seventh Framework Pro-
gramme; Contract grant number: NMP4-SL-2008-214340.
Contract grant sponsor: Marie Curie Research Training Network CHEXTAN;
Contract grant number: MRTN-CT-2004-512161.
Contract grant sponsor: Generalitat de Catalunya; Contract grant number:
2009 SGR 158.
Contract grant sponsor: MICINN; Contract grant number: CTQ2010-16339.
Contract grant sponsor: Belgian Federal Science Policy Office;
Contract grant number: IAP-6/27.
Contract grant sponsors: Project RESOLVE, K. U. Leuven (GOA), Fund for
Scientific Research - Flanders (F.W.O.), Belgian National Fund for Scientific
Research (FNRS-FRFC), OPTI2MAT ‘Poˆle d’Excellence’ Programme of
Re´gion Wallonne.
*Correspondence to: David B. Amabilino, Institut de Cie`ncia de Materials de
Barcelona (CSIC), Campus Universitari, 08193-Bellaterra, Spain.
E-mail: amabilino@icmab.es
Received for publication 5 February 2011; Accepted 27 September 2011
DOI: 10.1002/chir.21976
The symmetry elements open to molecules which pack co-
planar with a surface are quite different to that of amphi-
philes at a surface. The basal plane of graphite, comprised of
the familiar hexagonal net of carbon–carbon bonds, does
Published online 19 December 2011 in Wiley Online Library
(wileyonlinelibrary.com).
C
VV
2011 Wiley Periodicals, Inc.

156
IAVICOLI ET AL.
3-(Octadecyloxy)phenol (3). To a solution of 3-(octadecyloxy)-
phenyl benzoate (2) (4.00 g, 8.57 mmol) in methanol (50 mL) so-
dium hydroxide (2N, 1.2 mL, 25.71 mmol) was added and the mix-
ture stirred overnight at room temperature. The reaction mixture
was concentrated, diluted with 30 mL of water and was acidified to
pH 2 with hydrochloric acid (2N). The mixture was extracted with
dichloromethane and the organic layers were combined and dried
over sodium sulfate. The organic solution was filtered and concen-
trated to give a white solid. The residue was purified by flash chro-
matography (SiO2, EtOAc/n-Hexane 1:3) to give 3 as white solid
(2.50 g, 80% yield).
forms and, if the cores interact strongly, their orientation rel-
ative to the substrate can be influenced.8 So when the whole
molecule does not follow the graphite axes, but orients with
a certain angle with respect to them, the glide plane symme-
try is eliminated.
We chose molecule 1 as an example to probe spontaneous
resolution because the 1,3 substitution pattern at the ben-
zene ring should break strict linearity and therefore pack in
a nontrivial way. The long alkyl chain ensures absorption to
graphite while also helping in the formation of Langmuir
layers. The source of chirality is the lactate group. At the
graphite–1-phenyloctane interface, we will show that the sys-
tem displays spontaneous resolution, while in Langmuir
layers it appears that a racemic compound is formed. These
data are put into the context of the X-ray crystal structure of
the compound.
m.p.: 698C; MALDI-TOF/MS (aCN) m/z (%): 362.45 (100) [M]1. Calc
for C24H42O2; Mw: 362.59; 1H NMR (250 MHz, CDCl3): 7.19-7.08 (m, 1H,
ArH), 6.51-6.40 (m, 3H, ArH), 5.02 (s, 1H, ArOH), 3.92 (t, J 5 6.5,
2H,OCH2(CH2)16CH3), 1.79-1.71 (m, 2H, ꢀꢀOCH2CH2(CH2)15CH3), 1.81-
1.71 (m, 2H, ꢀꢀOCH2CH2(CH2)15CH3), 1.37-1.42 (m, 2H, ꢀꢀO(CH2)2CH2
(CH2)14CH3), 1.27 (m, 28H, ꢀꢀO(CH2)3(CH2)14CH3), 0.89 (t, J 5 13.1,
3H, ꢀꢀO(CH2)17CH3) ppm. FTIR (KBr): 3346 (m, OH), 2917 (s, CH2),
2849 (s, CH2), 1599 (m, phenyl), 1504 (m, phenyl), 1463 (m), 1382 (w),
1285 (m), 1226 (w), 1186 (m), 1154 (m), 683 (w) cm-1.
(R)-Methyl 2-(3-(octadecyloxy)phenoxy)propanoate ((R)-4). 3-
(Octadecyloxy)phenol (3) (3.11 g, 8.58 mmol), (S)-methyl lactate (983
lL, 10.29 mmol) and triphenyl phosphine (2.70 g, 10.29 mmol) were
dissolved in dry THF (70 mL) with stirring under an atmosphere of
nitrogen and the mixture was cooled in an ice–salt bath. A solution of
diisopropylazodicarboxylate (DIAD, 2.03 mL,10.29 mmol) in THF (5
mL) was added drop wise over a period of 30 minutes and the mixture
was stirred overnight. After addition of water 20 ml, THF was removed
in vacuum and the residue was partitioned between CH2Cl2 and water.
The aqueous phase was extracted once more with CH2Cl2. The com-
bined organic phases were dried over sodium sulfate, filtered and
stripped of solvent. The residue was purified by column chromatogra-
phy (SiO2, CH2Cl2/n-Hexane 1:1) to give (R)-4 as a white solid (2.44 g,
62% yield).
MATERIALS AND METHODS
General Methods
Reagents and starting materials were used as obtained from SDS,
Fluka, Aldrich Chemical and Merck. Acetonitrile (MeCN) was distilled
over phosphorous pentoxide under a nitrogen atmosphere. Tetrahydro-
furan was distilled over metallic sodium under an inert atmosphere.
Silica gel 60 (35–70 mesh, SDS) was used for column chromatography.
NMR measurements were performed using a Bruker Avance 250-MHz
spectrometer. The reference used in the spectra was either tetramethyl-
silane or solvent residue. IR measurements were done using a Perkin
Elmer Spectrum One Fourier transform spectrometer.
m.p.: 498C; MALDI-TOF/MS m/z (%) (aCN): 448.67 (100) [M]1, Calc.
for C28H48O4; Mw: 448.36; [a]546 5 15.9 deg cm2 g21 (c 5 50 mM,
CH2Cl2); 1H NMR (250 MHz, CDCl3): 7.14 (t, J 5 8.2, 1H, ArH), 6.54-6-
40 (m, 3H, ArH), 4.76 (q, J 5 6.8, 1H, ꢀꢀOCHCH3COOMe), 3.91 (t, J 5
6.7, ꢀꢀOCH2(CH2)16CH3) 3.76 (s, 3H, ꢀꢀOCHCH3COOMe), 1.79-1.73 (m,
2H, ꢀꢀOCH2CH2(CH2)15CH3), 1.61 (d, J 5 6.6, 3H, ꢀꢀOCHCH3COOMe),
1.49-1.39 (m, 2H, ꢀꢀO(CH2)2CH2 (CH2)14CH3), 1.26 (m, 28H,
ꢀꢀO(CH2)3(CH2)14CH3), 0.88 (t, J 5 0.03, 3H, ꢀꢀO(CH2)17CH3) ppm.
FTIR (KBr): 2917 (s, CH2), 2849 (s, CH2), 1731 (m, CO), 1606 (m, phe-
nyl), 1588 (m, phenyl), 1492 (m), 1471 m), 1334 (w), 1284 (m), 1263
(m), 1181 (m), 1155 (m), 1052 (m), 835 (w), 718 (w), 685 (w), 770 (w),
3-(Octadecyloxy)phenyl benzoate (2). Predried and ground
K2CO3 (8.30 g, 25 mmol) was suspended in MeCN (170 mL) in a two
neck round bottom flask and the mixture was degassed with a flow of
nitrogen for 30 minutes. Resorcinol monobenzoate (5.00 g, 23 mmol)
was added as a solid and the resulting mixture was heated to reflux tem-
perature all under nitrogen. After thirty minutes, the reaction mixture
was cooled, and 1-bromooctadecane (93 mmol) was introduced as a liq-
uid. The temperature of the mixture was then raised to gentle reflux,
and these conditions were maintained for 48 hrs. The reaction mixture
was filtered at the pump while it was still warm, and the residual solid
was washed with MeCN. After twenty minutes at the pump crystals had
started to form in the liquid. After a further thirty minutes at the pump
the solution was filtered and the crystals were collected under gravity fil-
tration and then dried on a vacuum pump after being washed with cold
MeCN. A further crop of crystals was obtained by evaporating some of
the acetonitrile in a rotary evaporator and then allowing the resulting so-
lution to stand for a while in air. The total amount of crystal of (2)
obtained was 6.50 g (56% yield).
718 (w), 685 (w) cm21
.
(S)-Methyl 2-(3-(octadecyloxy)phenoxy)propanoate ((S)-4). This
compound was prepared in an identical manner to its enantiomer, but
employing (R)-methyl lactate as the starting material. All analytical data
was identical to that of the enantiomer except [a]546 5 25.7 deg cm2
g21 (c 5 0.075 M, CH2Cl2).
(R)-2-(3-(Octadecyloxy)phenoxy)propanoic acid ((R)-1). To a
solution of (R)-methyl 2-(3-(octadecyloxy)phenoxy)propanoate (R)-4
(1.20 g, 2.68 mmol) in methanol (30 mL) was added sodium hydroxide
(aq, 2N, 4 mL) and the mixture stirred overnight at room temperature.
The reaction mixture was concentrated, diluted with water and acidified
to pH2 with 2N hydrochloric acid. The precipitate was filtered and
washed with water. (R)-1 was obtained as a white solid (1.13 g, 95%
yield).
m.p.: 688C; MALDI-TOF/MS m/z (%) (aCN matrix): 466.54 (100)
[M]1 Calc. for C31H46O3; 466.70; 1H NMR (250 MHz, CDCl3): 8.22-8.18
(m, 2H, ArH), 7.64-7.61 (m, 1H, ArH), 7.54-7.48 (m, 2H, ArH), 7.31-7.26
m.p.: 808C; MALDI-TOF/MS m/z (%) (aCN): 434.73 (100) [M]1. Calc.
for C27H46O4; 434.65; [a]546 5 16.5 deg cm2 g21 (c 5 0.046M, MeOH);
1H NMR (250 MHz, d6 DMSO): 7.16 (t, J 5 8.0, 1H, ArH), 6.53–6.41 (m,
3H, ArH), 4.80 (q, J 5 6.7, 1H, ꢀꢀOCHCH3COOMe), 3.92 (t, J 5 6.1, 2H,
ꢀꢀOCH2(CH2)16CH3), 1.70-1.66 (m, 2H, ꢀꢀOCH2CH2(CH2)15CH3), 1.49
(d, J 5 6.7, 3H, ꢀꢀOCHCH3COOMe), 1.48–1.39 (m, 2H, ꢀꢀO(CH2)2CH2
(CH2)14CH3), 1.25 (m, 28H, ꢀꢀO(CH2)3(CH2)14CH3), 0.87-0.84 (m, 3H,
ꢀꢀO(CH2)17CH3); FTIR (KBr): 3137 (w, OH), 2918 (s, CH2), 2849 (s,
(m, 1H, ArH), 6.84-6.76 (m, 3H, ArH), 3.96 (t,
J 5 6.5, 2H,
ꢀꢀOCH2(CH2)16CH3), 1.82-1.74 (m, 2H, ꢀꢀOCH2CH2(CH2)15CH3, 1.44-
1.40 (m, 2H, ꢀꢀO(CH2)2CH2 (CH2)14CH3), 1.29 (m, 28H,
ꢀꢀO(CH2)3(CH2)14CH3), 0.88 (t, J 5 13.2, 3H, ꢀꢀO(CH2)17CH3 ppm.
FTIR (KBr): 2917 (s, CH2), 2849 (s, CH2), 1735 (s, CO), 1600 (m, phe-
nyl), 1491 (m, Phenyl), 1470 (m), 1268 (m), 1242 (s), 1143 (s), 1171
(m),1063 (m), 889 (w), 775 (w), 709 (m), 681 (w) cm-1.
Chirality DOI 10.1002/chir

ENANTIOMERIC AND RACEMIC FORMS
157
TABLE 1. Crystallographic data of (S)-1 crystals
Empirical formula
Formula weight
Color
C
29 H52 O5 S
512.77
Colorless
b (8)
g (8)
Volume (A )
91.970(3)
90.704(3)
1532.74(14)
2
1.111
564
2.55–22.99
4870
3
˚
Crystal size (mm3)
Temperature (K)
0.4 3 0.3 3 0.02
233(2)
MoKa 0.71073
Triclinic
Z
Calculated density (g/cm3)
F (000)
˚
Radiation wavelength (A)
Crystal system
Theta range for data collection (8)
Reflections collected
Reflections observed, I > 2r
Parameters
Space group
P 1 (no. 1)
5.4669(3)
˚
a (A)
4216
644
0
˚
b (A)
7.9950(4)
35.095(2)
˚
c (A)
Restriction
a (8)
90.813(3)
R1 for I > 2r (I)
0.0439
CH2), 1708 (m, CO), 1666 (w, CO), 1613 (m, phenyl), 1582 (m, phenyl),
1494 (m), 1470 (m), 1332 (m), 1287 (m), 1233 (m), 1186 (m), 1173 (m),
1039 (m), 853 (w), 756 (w), 720 (w), 683 (w) cm21; UV–vis (CHCl3)
Langmuir-Blodgett Films
By the correct choice of the subphase pH value (pH at 5.6) and sur-
face pressure (18 mN m21) Langmuir films were deposited by vertical
lifting at a speed of 10 mm min21. The glass substrates (10 mm by 14
mm) were cleaned in chromic acid, and soaked in piranha solution
(H2O2/concentarted H2SO4 1:2) before monolayer preparation. They
were washed several times with copious amounts of deionized water and
finally sonicated in ethanol for 15 minutes.
k
max/nm (e/mol L21 cm21): 201 (18,040), 274 (1240); Calculated for
C27H46O4: C 74.61%, H 10.67%, found: C 74.60%, H 10.74%.
(S)-2-(3-(Octadecyloxy)phenoxy)propanoic acid ((S)-1). This
compound was prepared in an identical manner to its enantiomer, but
employing (S)-4 as the starting material. All analytical data was identical
to that of the enantiomer except [a]546 5 26.9 deg cm2 g21 (c 5 75
mM, CH2Cl2). The compound was crystallized by slow evaporation from
DMSO obtaining nice needle-like crystals. Crystallographic data are
given in Table 1.
Sum-Frequency Spectroscopy
Sum-frequency vibrational spectra were collected by an EKSPLA (Vil-
nius, Lithuania) sum-frequency spectrometer, described in detail in our
earlier publications.9–11 The visible beam (532 nm) is generated by dou-
bling the fundamental output of a Nd:YAG laser (1064 nm wavelength,
20 ps pulse width, 20 Hz repetition rate). The tuneable IR beam is
obtained from an optical parametric generation/difference frequency
generation system, pumped by the third harmonic and the fundamental
of the Nd:YAG laser. The IR and visible beams are temporally and spa-
tially overlapped on the sample surface with incident angles of 558 and
608, respectively. Beam energies were kept below 200 lJ. Sum frequency
light is collected in the reflected direction through a holographic notch
filter and monochromator, and detected by a photomultiplier tube. Spec-
tral resolution is determined by the <6 cm21 line width of the IR pulse.
Spectra presented were measured using the ssp (s-polarized sum fre-
quency, s-polarized visible, and p-polarized IR radiation), polarization
combination. Spectra were collected in the 2800–3000 cm21 spectral
region using 3 cm21 increments and 50 laser pulses at each step. Five
spectra were collected and averaged.
Scanning Tunnelling Microscopy
STM images presented here were obtained at the liquid–solid inter-
face using a PicoSPM (Agilent). STM tips were mechanically cut from
Pt/Ir wire (80%/20%, diameter 0.25 mm). Highly oriented pyrolytic
graphite (HOPG, grade ZYB, Advanced Ceramics, Cleveland, OH) was
used as a substrate. A 7.6 3 1023 M solution of the pure enantiomers
was prepared in 1-phenyloctane (Aldrich, 99%) and heated for about 10
minutes at 708C before applying a drop of one of these enantiomer solu-
tions to the basal plane of freshly cleaved HOPG. In case of the 1:1 mix-
ture, the concentration of each of the enantiomers is 3.8 3 1023 M. After
about half an hour, the STM tip was immersed into the solution and
scanned in the variable height mode. The setpoint current (Iset) is typi-
cally smaller than 0.3 nA. The bias voltage was (Vbias) applied to the sam-
ple in such a way that at negative bias voltage electrons tunnel from the
sample to the tip. For analysis purposes, the imaging of a molecular
layer was immediately followed by recording at a lower bias voltage and
higher setpoint current the graphite lattice, under otherwise identical ex-
perimental conditions. Drift effects were corrected for using scanning
probe image processor (SPIP) software (Image Metrology ApS). Note
that only images containing a small drift were used for analysis.
Modeling
The modeling of the physisorbed layers was carried out using a molecu-
lar mechanics/molecular dynamics (MM/MD) approach. The DREID-
ING12 force field, as implemented in the FORCITE tool pack of Materials
Studio, was used, since it is particularly adapted for an accurate descrip-
tion of the hydrogen bonding that promotes the self-assembly of the mole-
cules. This joint MM/MD approach has been used to model the structure
and energetics of self-assembled monolayers of (S)-resorcinol molecules
on graphite. The initial geometries come from assembly models proposed
on the basis of STM images; the calculations consisted in two steps of
energy minimization, to relax possible geometrical constraints, followed
by two subsequent MD simulation steps, first in the NVE ensemble (con-
stant number of particles, volume and energy) for 500 ps, then in the NVT
ensemble at 298K for 3 ns. The long-range nonbonded interactions were
Langmuir Layer Preparation
Langmuir films were obtained spreading an aliquot of the materials
(30 ll, 2.3 mM) dissolved in CHCl3 (Chemolab) on the surface of ultra-
pure water held in a Teflon trough (NIMA Technology, 611D). The
water used for the subphase was obtained from a Millipore system (18.2
MX cm resistivity). The subphase temperature was controlled within
60.58C by a thermostat. After complete evaporation of the organic sol-
˚
turned off with a cubic spline cutoff set at 18 A.
vent (10 min) the compression was initiated at a speed of 50 cm2 min21
.
Molecular area isotherms were obtained under continuous compression
in the trough. Surface pressures were measured with a Wilhelmy plate
balance. The isotherms reported are the average of at least three experi-
ments. Measurements on water-subphase were performed at different
pH values in the range 2–10. A Brewster Angle Microscope (NIMA, Min-
iBAM) with a video mounted on the Langmuir film balance was used for
the visualization of monolayer.
RESULTS AND DISCUSSION
Synthesis and X-Ray Structure
The synthetic route used for the preparation of the chiral
amphiphilic derivative (S)-1 is shown in Scheme 1 (the syn-
thesis of the S isomer only is shown), and its enantiomer
was obtained using the same route but employing (S)-methyl
Chirality DOI 10.1002/chir

158
IAVICOLI ET AL.
bonyl adsorption at 1714 cm21 together with the O-H out of
plane at 928 cm21. For a carbonyl group to appear at such
low wavenumber is clearly consistent with a strong hydrogen
bond. The relative broadness of the 928 cm21 band is charac-
teristic of the centrosymmetric carboxyl dimer. The IR spec-
tra of the enantiopure sample shows two differences with the
racemate; first the carbonyl band is split into two bands at
lower wavenumber (1706 and 1666 cm21), and second the
OH wag is shifted to the lower wavenumber of 897 cm21
.
The less rigid OH group in the catemeric structure of the
enantiopure derivative could explain the latter. The hydroxyl
OH stretch gives a broad band at 3069 cm21
.
The differential scanning calorimetry gave an exothermic
peak at 808C for both enantiomers and a peak at 1208C for
the racemic mixture. The higher melting point for the equi-
molar mixture of the enantiomers is indicative of the forma-
tion of racemic compound in the solid.
Colorless needle-like crystals of (S)-1 were obtained by
slow evaporation of a dimethyl sulfoxide (DMSO) solution.
(S)-1/DMSO crystallizes in the trigonal P1 space group with
two molecules of (S)-1 and two molecules of DMSO in the
unit cell. An ORTEP representation of the head to tail arrange-
ment of the molecules in the tapes formed by the alkyl chain
van der Waals intersections is presented in Figure 1.
The two molecules of 1 in the unit cell adopt different con-
formations from one another. The alkyl chain is slightly
twisted as it joins the phenyl ether group in one molecule
while in the other the mean planes are virtually coplanar, as
shown by the torsion angles C12-C11-C8-O4 and C39-C38-C35-
O8 which are 270.48 and 178.88. The b-methyl group in the
lactate moiety lies approximately in the plane of the phenoxy
group with the plane of the carboxylic acid residue anticlini-
cally related. The angle between the phenyl group and the chi-
ral substituent of each molecule in the unit cell is slightly dif-
ferent from each other: the C4-O3-C2-C1 torsion angle is 73.88
whereas C31-O7-C29-C28 measures 71.18. The conformation
of the lactate groups in this crystal structure is comparable to
those of other phenyl derivatives.13–16
The acid OH group of each molecule is associated with
the SO group of the nearest solvent molecule by strong
hydrogen-bond (O2ꢁꢁꢁO9 2.567 and O6ꢁꢁꢁO10 2.549). The po-
lar layer is reinforced by weak hydrogen-bonds (CH3ꢁꢁꢁO)
between the ꢀꢀCH3 and both the SO of DMSO molecules
and the CO of the acid group of (S)-1.
Scheme 1. Synthesis of compound 1.
lactate as the source of chirality. Reaction of commercially
available resorcinol monobenzoate with 1-bromooctadecane
in the presence of K2CO3 afforded the corresponding ether
2. Deprotection of the phenol group with NaOH in MeOH
gave 3 which was then reacted with both (S)-methyl lactate
and (R)-methyl lactate giving (R)-4 and (S)-4 respectively.
The last step was a hydrolysis conducted by using NaOH in
MeOH which gave (R)-1 and (S)-1 which were characterized
by polarimetry, NMR, FTIR, UV–vis, X-ray single crystal dif-
fraction (vide infra) and MALDI TOF/MS. The racemic mix-
ture was prepared by mixing equal amounts of the two enan-
tiomers. The IR of the racemic mixture shows the single car-
The molecular packing (Fig. 2), can be described as an
alternation of polar and apolar layers along the b direction of
Fig. 1. ORTEP representation of the asymmetric unit of (S)-1/DMSO crystals.
Chirality DOI 10.1002/chir

ENANTIOMERIC AND RACEMIC FORMS
159
Fig. 2. Molecular packing of (S)-1/DMSO in its crystals. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
the crystal. The apolar layer is composed of the alkyl chains
interacting with each other by van der Waals forces and
which are interdigitated. One noncovalent bond more is
found in this layer: the last methyl group of each alkyl chain
interacts with the nearest phenyl ring via a CꢀꢀHꢁꢁꢁp interac-
tion. The polar part is comprised of the acid groups of the
chiral substituents and the DMSO molecules interacting
with each other by hydrogen-bonds.
scanning tunnelling microscopy (STM) at the liquid-graphite
interface, which is a particularly sensitive way to study chiral
structure at submolecular resolution.17–19 The molecules
were dissolved in 1-phenyloctane (7.6 3 1023 M) and a drop
of this solution was cast onto highly oriented pyrolytic graph-
ite (HOPG). The monolayer formation and structure was fol-
lowed using STM in the constant current mode at the liquid–
solid interface. After a while, the spontaneous formation of
highly ordered adlayers was observed.
Monolayers of 1 at the Graphite–1-Phenyloctane Interface
Typical STM images of (R)-1 and (S)-1 physisorbed inde-
pendently at the 1-phenyloctane-graphite interface are shown
in Figure 3. A bright (dark) contrast reflects an increased
The self-assembly of solutions of (R)-1, (S)-1 and the equi-
molar (R)-1/(S)-1 mixture on graphite were investigated by
Fig. 3. STM images of 1 physisorbed at the 1-phenyloctane-HOPG interface. (A) (R)-1, large area with multiple domains (Iset 5 0.05 nA, Vset 5 1.04 V). (B) (R)-1,
high-resolution image (Iset 5 0.03 nA, Vset 5 1.00 V; IHOPG 5 1.09 nA, VHOPG 5 0.02 V). (C) (S)-1 (Iset 5 0.02 nA, Vset 5 1.04 V; IHOPG 5 0.79 nA, VHOPG 5 0.2 V).
The periodicity perpendicular to the double rows D is indicated by white arrows. The inset shows the corresponding graphite image. The white solid lines in the insets
indicate the direction of the major symmetry axes of HOPG (<-1 2 21 0>, <1 1 22 0>, and <2 21 21 0>). The black dashed line in the insets and main images, run-
ning perpendicular to one of the main symmetry axes, is a reference axis selected to evaluate the orientation of the monolayer with respect to the substrate. Yellow
solid lines indicate the propagation direction of the double rows.[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Chirality DOI 10.1002/chir

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IAVICOLI ET AL.
Fig. 4. STM images of (R)- and (S)-1 mixture (ratio 1:1) physisorbed at the 1-phenyloctane-HOPG interface. (A) Large area with multiple domains (Iset 5 0.06
nA, Vset 5 1.04 V). (B) Smaller-scale image containing both types of domains (Iset 5 0.05 nA, Vset 5 1.02 V; IHOPG 5 1.09 nA, VHOPG 5 0.02 V). The inset shows
the corresponded graphite image. The white solid lines in the inset indicate the direction of the major symmetry axes of HOPG (<-1 2 21 0>, <1 1 22 0> and
<2 21 21 0>). The black dashed lines in the inset and main image, running perpendicular to main symmetry axes, are reference axes selected to evaluate the
orientation of the monolayer with respect to the substrate. Yellow solid lines are the propagation direction of molecular double rows.[Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
(decreased) tip-sample distance, due to higher (lower) topo-
graphical features and/or a higher (lower) tunnelling effi-
ciency.
When an equimolar solution of the (R)-1 and (S)-1 in 1-
phenyloctane was deposited onto HOPG domains similar to
the ones observed to the single enantiomers were registered
(Fig. 4).
The STM images are composed of double rows of bright
spots separated by often less bright lines running perpendic-
ular to these double rows. The bright spots in the double
rows are attributed to the phenoxy propionic acid head
groups which potentially dimerize through hydrogen bonds.
Note though that the contrast along the double rows is not
always constant, indicating a variation or flexibility in the ori-
entation and the interaction of the phenoxy propionic groups.
The lines running almost perpendicular to the double rows
correspond to the alkyl chains. They are adsorbed adopting
an all-trans conformation, and those of adjacent rows are
interdigitated. Due to the variation in image contrast in many
of the images, it is not possible to determine the distance
between two adjacent phenoxy propionic units in a straight-
forward manner. Alternatively, the distance between next-
nearest-neighbour interdigitating alkyl chains was deter-
mined to be 0.98 6 0.03 nm. The periodicity perpendicular to
the double rows (D) is easier to determine and measures 3.7
6 0.2 nm. The insets in Figure 3 illustrate the graphite sub-
strate (not to scale with the STM image of the monolayer)
where the main symmetry axes are indicated by white lines.
The alkyl chains run parallel to one of the main symmetry
axis of graphite.
The chirality of the enantiopure monolayers was analyzed
evaluating the orientation of a molecular row with respect to
graphite. Therefore, for each domain a reference axis was
selected, running perpendicular to one of the major symme-
try axes of graphite, to evaluate the orientation of the mono-
layer with respect to the substrate. The reference axis
selected is the one which forms the smallest angle with the
lamellae. The angle between the graphite reference axis and
the tape axis of the monolayer is on average 23.78 6 2.18 for
(R)-1 (Fig. 5A) and 14.58 6 2.98 for (S)-1 (Fig. 5B). A simi-
lar relationship is observed in all domains probed for both
enantiomers. This fact indicates clearly that the stereogenic
centres influence strongly the molecule-surface interaction.
To investigate the possibility of spontaneous resolution20,21
of the racemic mixture (R)-1/(S)-1, the chirality in each do-
main formed was analyzed. Both domains with positive and
negative y angles were found, i.e., 23.08 6 2.38 and 13.48 6
2.58 (Table 2). The results of the analysis of chirality in differ-
ent domains are presented in the histograms in Figure 5 for
both enantiomers and the racemic mixture. These domains
are not evident in large scale images, because the difference
in angle of the domains is only ꢂ68 and the resolution and
correction of the images in large areas is not possible, their
appearance is similar to the large areas of the enantiopure
compound shown in Figure 3. Close-up images of domains
with reliable graphite reference axes must be taken to iden-
tify the domains with different chirality, as shown in Figure
4B. These data indicate the spontaneous resolution of the ra-
cemic mixture at the surface.
In the 2D adsorbed structure 1 is arranged in a slightly
different way from that in the 3D crystal while the gross
supramolecular structure – head to head carboxylic groups
and interdigitated alkyl chains – is similar. The main differ-
ence is the presence of solvent molecules in the crystal
which are between two (S)-1 molecules forming hydrogen-
bonds, whereas in the 2D structure dimers of (S)-1 presum-
TABLE 2. Monolayer descriptors: D is the repetitive row to
row distance (see Fig. 3)
D (nm)
y (8)
N
(R)-1
(S)-1
(R)-11(S)-1 (1:1)
3.7 6 0.2
3.7 6 0.1
3.8 6 0.1
3.8 6 0.1
23.7 6 2.1
14.5 6 2.9
23.0 6 2.3
13.4 6 2.5
13
21
28
21
y is the angle between the graphite reference axis and the row propagation
axis. N is the number of domains analyzed.
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ENANTIOMERIC AND RACEMIC FORMS
161
Modeling has been used to unravel the role played by
supramolecular interactions and chirality on the formation of
monolayers at HOPG surfaces, examining the specific case
of (S) enantiomer (the (R) enantiomer is expected to give
similar results of opposite handedness, only the orientation
of the layer inverted with respect to the HOPG symmetry
axis). The initial structure corresponds to the model pro-
posed from the STM data, i.e., a monolayer composed of
double rows of phenoxy groups forming hydrogen bonds,
separated by interdigitated alkyl chains.
The modeling of the (S)-1 monolayer has been performed
in two stages: in the first step, we investigate the internal or-
ganization of the assembly, as well as its orientation with
respect to graphite symmetry axes. To do so, we analyse the
stability of starting models upon comparison of the geometri-
cal parameters obtained by STM with the predicted MD val-
ues. The simulations are carried out on a graphite layer (19.5
3 19.5 nm2), with 40 molecules of (S)-1 arranged as indi-
cated in Figure 6 on top. The second step consists in assess-
ing the relative stability of monolayers featuring different ori-
Fig. 5. Histograms of the angle § observed for physisorbed monolayers
formed of solutions of (A) (R)-1; (B) (S)-1; (C) racemic (R)-1/(S)-1.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
ably interacts with itself by forming hydrogen-bonded
dimers. The distance between two alkyl chains in the 3D
˚
crystals is ꢂ4.1 A, in the 2D crystal on graphite this distance
˚
is 5 6 1 A. These results demonstrate that both the 2D clos-
est packing principles, related to the minimization of the sur-
face free energy, and hydrogen bond play a role in the
adsorption process. The final adsorption structure is deter-
mined by the balance of these two forces.
Fig. 6. Modeling result for (S)-1 assembly at the HOPG surface: internal
organization of the monolayer (top) and orientation of the layer with respect
to graphite (bottom). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
Chirality DOI 10.1002/chir

162
IAVICOLI ET AL.
Scheme 2. Representation of the angle between rows of molecules and
the <-1 1 0 0> symmetry axis of graphite upon molecular displacement.
Fig. 7. Potential energy profile for the rotation of the layer of (S)-1 mole-
cules with respect to the <-1 1 0 0> reference graphite axis. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
entations with respect to the symmetry axes of the graphite
surface. To do so, we consider a smaller system (16 mole-
cules of (S)-1 on top of a 12.1 3 12.1 nm2 graphite sheet), as
represented at the bottom of Figure 6. The analysis of the
energy-minimized geometries extracted from these MD sim-
ulations, and the comparison to the experimental data allows
pinpointing those interactions that are responsible for the
specific orientation of the monolayers on graphite, i.e. the
chiral signature of the assembly.
A closer look at the supramolecular structure within the
monolayers of (S)-1 shows that the formation of hydrogen
bonds between facing resorcinol ‘‘heads’’ leads to a rather
large separation between adjacent molecules in a given row.
As a result, the empty space between the alkyl chains along
one row can be filled through interdigitation with the alkyl
tails of the molecules belonging to the neighbouring rows.
The second set of MD simulations aimed at investigating
the energy of (S)-1 monolayer as a function of rotation over
graphite revealed that the alignment of the alkyl chains along
one of the main symmetry axes of graphite also determines
the global orientation of the stack. This is shown in Figure 7,
where we see that the equilibrium angle of the (S)-1 mono-
layer (that amounts to ꢂ128 in our simulations) allows for
optimal Van der Waals interactions between the (S)-1 tails
and the graphite surface. This equilibrium rotation angle for
the chiral domains is in reasonable agreement with the ex-
perimental value of ꢂ14.58.
The starting models of the monolayers were built on the
basis of STM parameters. The construction process per se al-
ready gave us some hint on the origin for the rotation of the
rows formed by the molecular cores with respect to graphite:
it has been observed that a small displacement between
repeating units in one row promotes more favourable (Van
der Waals) interactions among molecules belonging to the
same row. However, due to the presence of the chiral group,
the repeating units in one row are displaced with respect to
each other in a specific direction, which depends on the mo-
˚
lecular chirality: by ꢂ10.5A along the alkyl chain axis for
˚
(S)-1 and by ꢂ20.5 A along the alkyl chain axis for (R)-1, as
sketched in Scheme 2.
The geometrical parameters obtained from the modeling
are found to be overall in good agreement with experiment,
see Table 3. Thus, the model proposed for the organization
of the molecules within the monolayers accounts well with
most of the salient features extracted from the STM images.
Slight differences between theory and experiment probably
arise from not accounting for solvent effects in the calcula-
tions. Namely, the MD simulations tend to overestimate the
interactions between alkyl chains yielding lower intermolecu-
lar distances compared to the measured values.
Monolayers of 1 at the Air–Water Interface
Langmuir monolayers of the enantiomers and racemic
forms of amphiphile 1 were prepared by placing drops of a
chloroform solution of the samples on deionised water and
then compacting them with the barrier in the Langmuir
trough. To produce the most compact Langmuir layers the
influence of the pH on the Langmuir monolayers of the three
samples were studied by surface pressure-area isotherms
(Fig. 8). Surface pressure-molecular area isotherms have
been proven to be a sensitive way of studying the chiral dis-
TABLE 3. Geometrical parameters typical of the (S)-1 monolayer
d/nm
a/nm
b/nm
c/nm
D/nm
Theoretical (DRIEDING–FORCITE)
Theoretical (MM3 Tinker)
Experimental
0.54 6 0.07
0.50 6 0.04
0.49 6 0.02
1.1 6 0.08
0.90 6 0.07
0.98 6 0.03
1.2 6 0.02
1.22 6 0.11
1.23 6 0.10
0.85 6 0.22
0.92 6 0.07
1.11 6 0.13 nm
3.78 6 0.17
3.98 6 0.22
3.68 6 0.21
d refers to the alkyl–alkyl separation; a, b, and c refer to the distances between the cores of the (S)-1 molecules; D is the distance between two (S)-1 stacks.
Chirality DOI 10.1002/chir

ENANTIOMERIC AND RACEMIC FORMS
163
Fig. 8. Pressure-area isotherms for monolayers of (R)-1, (S)-1, and (R)-1/(S)-1 1:1 mixture at subphase pH value of (A) 2, (B) 5.6, (C) 7.8. [Color figure can
be viewed in the online issue, which is available at wileyonlinelibrary.com.]
crimination effects of enantiomeric and racemic mono-
layers.22–24
identical and practically no hysteresis was observed even
during subsequent isotherms. For the racemic compound
the isotherms exhibited hysteresis and a decrease in limiting
The shape of the isotherms at pH 2 and 5.6 – which show
2
˚
no appreciable surface pressure below 40 A per molecule
mean molecular area on successive compression was
2
˚
and then a very steep rise—are typical of an amphiphile that
forms a tightly packed monolayer with the alkyl chain of
neighbouring molecules interacting strongly. The increase of
the pH value to 7.8 results in a shallower isotherm and a shift
to a bigger area per molecule value, resulting in a Langmuir
monolayer with a bigger molecular area requirement. The
isotherm relative to the racemic mixture is further shifted
with respect to that of the enantiopure samples. It is clear
how the subphase pH conditions influence the ionic state of
the head group and so the intermolecular interactions. At pH
7.8 the carboxylic groups are in the anionic state and repul-
sive electrostatic forces between the head groups dominate
their packing.
There is no substantial difference between the slopes of
the isotherms at subphase pH 2 and 5.6 meaning that there
is not a big difference in the packing at the air-water interface
of the two enantiomers and of the racemic mixture in either
condition. Although there are slight differences between the
racemic modification and the enantiomers over the whole
isotherms, the only difference at both pH values was
observed around the collapse point of the isotherm, and this
effect could depend on factors other than the chirality. While
for the pure enantiomers there is a decrease of pressure after
the collapse, for the racemic the pressure stays constant indi-
cating a different behavior of the racemic monolayer at the
higher pressure.
observed. The limiting molecular area goes from 33 A per
2
˚
molecule to 27 A per molecule.
The smaller limiting area per molecule of the racemic mix-
ture could be partially attributed to the reorientation of the
molecules during the decompression and recompression
steps, forming a better packed monolayer after the cycle.
Other explanations are plausible, such as the formation of
nonmonolayer aggregate regions within a monolayer host,
the dissolution of the monolayer into the aqueous phase or
the partial overlapping of the molecules in the monolayer
films leading to a multilayer stacks on the surface of water.
On the other hand it was found that during the experiments
on the racemate when the moving barrier was stopped at 22
mN m21 the molecular area decreases about 25% after 15
min relaxation. This phenomenon suggests that a stereose-
lective kinetic process must occur upon film compression.
The Langmuir layers are not very stable and confirm the
The limiting molecular area in the condensed phase at ei-
ther pH 2 or 5.6 at zero pressure, which is obtained by
extrapolating from the slope of the solid phase to zero sur-
2
˚
face pressure, is 33 A per molecule for both the enantiopure
and racemic compounds and represents the hypothetical
area occupied by one molecule in the condensed phase at
zero pressure. This value is of the order of that expected for
the head-group of the compound.
Figure 9 shows the hysteresis of two successive compres-
sion/expansion cycles of the racemic and enantiomeric
monolayers of the amphiphilic resorcinol derivative. For the
enantiomers, the first and second compression isotherms are
Fig. 9. Surface pressure/area isotherms for the compression/expansion
cycle of (A) enantiomeric and (B) racemic 1 monolayers at 258C on a pure
2
˚
water subphase (pH 5.6). Rate of compression/expansion was 5 A /molecule
per minute.
Chirality DOI 10.1002/chir

164
IAVICOLI ET AL.
Fig. 10. BAM images of (R)-1 taken at pH 5.6, 258C: (A) after spreading; (B) at the liquid-phase; (C) at the solid phase; (D) after collapsing.
tendency of the molecules to find a more stable interaction
state. This big difference in behavior between enantiomers
and racemate is further evidence of the formation of a race-
mic compound at the interface.
The visualization of the monolayers from zero pressure to
collapse pressure was registered by a Brewster Angle Micro-
scope (BAM) to observe the homogeneity and the domain
structure of the monolayers. In Figure 10 the images of the
monolayer of (R)-1 at pH 5.6 during the course of its com-
pression are shown. The areas of different brightness arise
from different molecular layer densities.
After spreading the enantiomeric amphiphile in chloroform
on the water’s surface, a monolayer of scattered, lace-like
structure appears. During compression, homogeneous struc-
tures with smaller and larger hole are formed (liquid-con-
densed phase). Further compression results in a homogene-
ous thin film (crystal-like solid phase) without any black
spots (which represents the water subphase). Then at higher
surface pressure the monolayer undergoes a transformation,
and, because of the collapse of the film,25 a few white spots
appear. The images suggest well shaped condensed phase
domains, which mean a highly ordered molecular packing is
found for the enantiomers and the racemic mixture.
mic compounds at subphase pH 7.8 (Fig. 12). Both enantiom-
ers form three tipped star-shaped structures during compres-
sion, whereas the racemic mixture forms more acicular and
inhomogeneous structures of larger domain dimensions.
That confirms the different interactions existing in the race-
mic mixture which were found during the observation of the
isotherms and the isocycles for the deprotonated amphiphile.
After the correct choice of subphase pH (5.6) stable mono-
layers of (R)-1, (S)-1 and their racemic mixture at the air/
water interface were produced and deposited by vertical lift-
ing onto normal glass, mica and hydrophobic glass. The mo-
lecular packing orientation and arrangement within the LB
films was further investigated by sum-frequency vibrational
spectroscopy. The sum-frequency ssp spectra of (R)-1, (S)-1
and (R)-1/(S)-1 (1:1) monolayers deposited onto glass by
the Langmuir-Blodgett technique are shown in Figure 13.
The good signal-to-noise ratios of the sum-frequency spectra
clearly indicate that the transfer of ordered monolayers to
the glass substrates was successful. The sum-frequency
spectra of the monolayers of the (S)-1 and (R)-1 enantiomers
(bottom and middle traces in Fig. 13, respectively) in the
CꢀꢀH stretch region (2800–3000 cm21) are very similar. The
spectrum of the monolayer of the racemic mixture (top trace
in Fig. 13) exhibits the same peaks but with somewhat differ-
ent intensity ratios and a slightly lower signal-to-noise ratio
as compared with the enantiopure samples.
The images of (R)-1, (S)-1 and the racemic (R)/(S)-1 at
the two-dimensional gas phase (before compression) were
compared and no substantial difference was noted between
the structure of the domains (Fig. 11).
The morphology of the domains at subphase pH 2 was
found to be similar to the ones at pH 5.6, whereas clear dif-
ferences were found between the enantiopure and the race-
The assignment of the major peaks in the spectra is well
established in the literature.26 The spectra are dominated by
the symmetric methyl stretch (r1 mode) at 2878 cm21, þand
the Fermi resonance of the same mode at 2940 cm21 (rFR).
Chirality DOI 10.1002/chir

ENANTIOMERIC AND RACEMIC FORMS
165
CONCLUSIONS
The self-assembly of the racemic amphiphile 1 at the 1-
phenyloctane-graphite interface results in spontaneous reso-
lution of the enantiomers, because of the angle that the enan-
tiomers form with the graphite axis and the preference for
the lactate group to induce a certain angle. On the other
hand, p/A isotherms and BAM imply that when racemic mix-
tures of the amphiphiles are spread on water and com-
pressed then it seems that there is no spontaneous resolu-
tion. The racemic mixture domains are not as stable as the
enantiopure ones and an overlapping of the monolayers hap-
pens during the second compression in a compression/
Fig. 11. BAM images of (A) (R)-1; (B) (S)-1; (C) Racemic (R)-1/(S)-1
taken at pH 5.6, 258C after spreading.
The antisymmetric methyl stretch (r2) shows up as a
shoulder at 2956 cm21. The broad peak at 2850 cm21 arises
from the symmetric methylene stretch (d1 mode). The
I(r1)/I(d1) intensity ratio of the methyl and methylene sym-
metric stretch modes in the spectra of the monolayers pre-
pared from the pure enantiomers indicates rather high con-
formational order of the alkyl chains, with a only a small frac-
tion of gauche conformations.27 The relative intensity of the
methyl and methylene symmetric stretch peaks of the race-
mic (R)-1/(S)-1 (1:1) monolayer is lower than for the pure
enantiomers indicating a somewhat lower conformational
order of the alkyl chains. The sum-frequency spectra thus
suggest slightly different structures in the Langmuir-Blodg-
ett monolayers of the pure (R)-1 and (S)-1 enantiomers as
compared to the racemic monolayer.
Fig. 12. BAM images of (A) (R)-1; (B) (S)-1; (C) Racemic (R)-1/(S)-1
taken at pH 7.8, 258C during compression.
Chirality DOI 10.1002/chir

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IAVICOLI ET AL.
7. Yin SX, Wang C, Xu QM, Lei SB, Wan LJ, Bai CL. Studies of the effects
of hydrogen bonding on monolayer structures of C18H37X (X 5 OH, SH)
on HOPG. Chem Phys Lett 2001;348:321–328.
8. Plass KE, Grzesiak AL, Matzger AJ. Molecular packing and symmetry of
two-dimensional crystals. Acc Chem Res 2007;40:287–293.
9. Varga I, Keszthelyi T, Me´sza´ros R, Hakkel O, Gila´nyi T. Observation of a
liquid-gas phase transition in monolayers of alkyltrimethylammonium
alkyl sulfates adsorbed at the air/water interface. J Phys Chem B
2005;109:872–878.
10. Keszthelyi T, Pa´szti Z, Rigo´ T, Hakkel O, Telegdi J, Guczi L. Investiga-
tion of solid surfaces modified by Langmuir-Blodgett monolayers using
sum-frequency vibrational spectroscopy and X-ray photoelectron spec-
troscopy. J Phys Chem B 2006;110:8701–8714.
´
11. Kiss E, Keszthelyi T, Korma´ny G, Hakkel O. Adsorbed and spread layers
of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block
copolymers at the air-water interface studied by sum-frequency vibrational
spectroscopy and tensiometry. Macromolecules 2006;39:9375–9384.
12. Mayo SL, Olafson BD, Goddard WA III. DREIDING: a generic forcefield.
J Phys Chem 1990;94:8897–8909.
13. Kennard CHL, Smith G, White AH. Structural aspects of phenoxyalka-
noic acids. The structures of phenoxyacetic acid, (1-)-2-phenoxypro-
pionic acid, (1-)-2-(4-chlorophenoxy)propionic acid, 2-methyl-2-phenoxy-
propionic acid and 2-(4-chlorophenoxy)-2-methylpropionic acid. Acta
Cryst 1982;B38:868–875.
Fig. 13. Sum-frequency ssp spectra of Langmuir-Blodgett monolayers of
(R)-1, (S)-1, and (R)-1/(S)-1 1:1. Symbols are measured data points, solid
lines are guides to the eye. The spectra have been vertically displaced for
clarity. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
14. Raghunathan S, Chandrasekhar K, Pattabhi V. 2-(4-Chlorophenoxy)pro-
pionic acid. Acta Cryst 1982;B38:2536–2538.
15. Sorensen HO, Collet A, Larsen S. (R)-(1)-2-(4-chlorophenoxy)propionic
acid. Acta Cryst 1999;C55:953–956.
16. Minguet M, Amabilino DB, Wurst K, Veciana J. Chirality of a-nitronyl
nitroxide radicals in the solid state. J Solid State Chem 2001;159:440–450.
expansion cycle and when keeping a pressure for few
minutes. These results illustrate the key role that orientation
and substrate have upon chiral separation processes in low
dimensions, and are consistent with results on the curtailed
spontaneous resolution of a bis-lactate on a metal surface
reported recently.28 Together with the observation that the
bulk material forms a racemic compound, the observations
infer that more than a subtle change in head group of the
amphiphile is necessary to generate surfactants that will
resolve spontaneously under any condition, a challenge par-
ticularly relevant for studies of chirality at the nanoscale29
and one we are pursuing.
17. Raval R. Chiral expression from molecular assemblies at metal surfaces:
insights from surface science techniques. Chem Soc Rev 2009;38:707–721.
18. Elemans JAAW, De Cat I, Xu H, De Feyter S. Two-dimensional chirality
at liquid-solid interfaces. Chem Soc Rev 2009;38:722–736.
19. Katsonis N, Lacaze E, Feringa BL. Molecular chirality at fluid/solid inter-
faces: expression of asymmetry in self-organised monolayers. J Mater
Chem 2008;18:2065–2073.
20. Pe´rez-Garcı´a L, Amabilino DB. Spontaneous resolution under supramo-
lecular control. Chem Soc Rev 2002;31:342–356.
21. Pe´rez-Garcı´a L, Amabilino DB. Spontaneous resolution, whence and
whither: from enantiomorphic solids to chiral liquid crystals, monolayers
and macro- and supra-molecular polymers and assemblies. Chem Soc
Rev 2007;36:941–967.
22. Harvey NG, Mirajovsky D, Rose PL, Verbiar R, Arnett EM. Molecular
recognition in chiral monolayers of stearoylserine methyl-ester. J Am
Chem Soc 1989;111:1115–1122.
ACKNOWLEDGMENTS
D.B. is a research director of FNRS.
LITERATURE CITED
23. Parazak DP, Uang JYJ, Turner B. Stine KJ, Fluorescence microscopy
study of chiral descrimination in Langmuir monolayers of N-acylvaline
and N-acylalanine amphiphiles. Langmuir 1994;10:3787–3793.
1. Weissbuch I, Kuzmenko I, Berfeld M, Leiserowitz L, Lahav M. Stereo-
chemical studies on chiral systems in two dimensions. J Phys Org Chem
2000;13:426–434.
24. Nandi N, Vollhardt D. Chiral discrimination and recognition in Langmuir
monolayers. Curr Opin Colloid Interface Sci 2008;13:40–46.
25. Steffens S, Oldendorf J, Haufe G, Galla H-J. Organized collapse structures
in mixtures of chiral ethyl 2-azido-4-fluoro-3-hydroxystearates. Langmuir
2006;22:1428–1435.
2. Weissbuch I, Leiserowitz L, Lahav M. Stochastic ‘‘Mirror symmetry
breaking’’ via self-assembly, reactivity and amplification of chirality: rele-
vance to abiotic conditions. Top Curr Chem 2005;259:123–165.
26. MacPhail RA, Strauss HL, Snyder RG, Elliger CA. Carbon-hydrogen
stretching modes and the structure of n-alkyl chains. 2. Long, all-trans
chains. J Phys Chem 1984;88:334–341.
3. Jacques J, Collet A, Wilen SH. Enantiomers, racemates and resolutions.
Malabar: Krieger Publishing; 1994.
4. Pratt-Brock C, Dunitz JD. Towards a grammar of crystal packing. Chem
Mater 1994;6:1118–1127.
27. Bell G.R, Bain CD, Ward RN. Sum-frequency vibrational spectroscopy of
soluble surfactants at the air/water interface. J Chem Soc Faraday Trans
1996;92:515–523.
5. Amabilino DB, De Feyter S, Lazzaroni R, Gomar-Nadal E, Veciana J,
Rovira C, Abdel-Mottaleb MM, Mamdouh W, Iavicoli P, Psychogyiopou-
lou K, Linares M, Minoia A, Xu H, Puigmart´ı-Luis J. Monolayer self-as-
sembly at liquid-solid interfaces: chirality and electronic properties of
molecules at surfaces. J Phys: Condens Matter 2008;20:184003.
28. Robin A, Iavicoli P, Wurst K, Dyer MS, Haq S, Amabilino DB, Raval R. A
racemic conglomerate nipped in the bud: a molecular view of enantiomer
cross-inhibition of conglomerate nucleation at a surface. Cryst Growth
Des 2010;10:4516–4525.
6. Ilan B, Florio GM, Hybertsen MS, Berne BJ, Flynn. Scanning tunneling
microscopy images of alkane derivatives on graphite: role of electronic
effects. Nano Lett 2008;8:3160–3165.
29. Amabilino DB, editor. Chirality at the nanoscale. Weinheim: Wiley-VCH;
2009.
Chirality DOI 10.1002/chir