Article
Cite This: J. Phys. Chem. C 2019, 123, 16681−16689

pubs.acs.org/JPCC

Inﬂuence of Coulombic Interaction on the Interfacial Self-Assemblyof Discotic Liquid Crystal Amphiphiles: A Combined Experimental
and Computer Simulation Study
Prakhar Verma,† Shakkira Erimban,‡ Nishant Kumar,† Snehasis Daschakraborty,*,‡ Alpana Nayak,*,†and Sandeep Kumar§
Department of Physics and ‡Department of Chemistry, Indian Institute of Technology Patna, Patna 801106, India§
Raman Research Institute, Sadashivanagar, Bengaluru 560080, India
Downloaded via INDIAN INST OF TECH PATNA on August 31, 2019 at 13:42:14 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information*

ABSTRACT: Self-assembly of amphiphilic molecules largely
depends on the structure and electronic properties of the polar
head groups. An important class of amphiphiles with technologicalapplications comprises the discotic liquid crystal (DLC) amphiphiles.
Here, we report remarkable diﬀerences in the self-assembly
properties of two similar discotic amphiphiles with dissimilar polar
head groups, viz., imidazole-tethered with hexaalkoxytriphenylene(neutral-ImTp) and imidazolium-tethered with hexaalkoxytriphenylene (ionic-ImTp). Surface manometry reveals that the ionic-ImTp
exhibits a larger limiting area, higher collapse pressure, and smaller
compressional elastic modulus at the air−water interface ascompared to the neutral-ImTp system. At the air−solid interface,ionic-ImTp can be transferred only up to a bilayer structure with
undulated morphology, whereas the neutral-ImTp exhibits smooth morphology and higher transfer eﬃciency. These results are
explained by density functional theory (DFT) calculations and molecular dynamics (MD) simulations, which elucidated that
the Coulombic interaction is the dominant factor that controls the organization of these molecules. DFT calculations predicteda T-shaped π-stacking geometry for the ionic-ImTp and a parallel-displaced stacking geometry for the neutral-ImTp. MD
simulation predicted the orientation of molecules and their strength of hydrogen bonding. Understanding the intermolecularinteractions governing self-assembly is important to engineer molecular packing that controls the charge transport in DLCbased organic electronics.

■

been the basis for the creation of all living organisms.8 This
simple process is yet very complex, as evident from theastonishing structures of lipid membranes, folded proteins, andDNA to name a few. Although mimicking such structures is
extremely challenging, these have been inspiring researchers todevelop nanostructures for applications in emerging nanoscale
devices. Interestingly, DLCs with an interlayer spacing of 0.34
nm have been regarded as a chemically synthesized version of
DNA-mimic.9 Moreover, hybrid structures of DLC−DNA
complexes have been reported in the bulk10 as well as atinterfaces.11,12 Currently, the self-assembly properties of DLCs
are a hot topic of research because of the unique charge- andenergy-transport properties that oﬀer huge possibilities fordeveloping quantum conductive devices.13
Among various noncovalent interactions such as van der
Waals, entropic, steric, electrostatic, hydrophobic/hydrophilic,and hydrogen bonding, the π-stacking interaction is thought to

INTRODUCTION
Discotic liquid crystals (DLCs) have emerged as one of thepotential components for organic semiconductor devices suchas light-emitting diodes, ﬁeld eﬀect transistors, and solar
cells.1,2 A DLC molecule generally consists of a rigid central
core made up of fused aromatic rings and peripheral alkyl
chains attached to the core.3 The anisotropic charge-carrying
properties of these molecules are greatly enhanced by the πstacking interactions between the central aromatic cores.4 Suchinteractions are also responsible for self-assembly underlying
stable mesophases not only in the bulk but also at interfaces.5
Unlike bulk, the asymmetric forces at an interface enabledistinctly diﬀerent material properties that shape the dynamic
reorganization and response of self-assembled molecules.6
Nevertheless, interfaces are ideal platforms for the evolution ofunique structure/property relations. This provides route to
tunable physicochemical architectures for technologicallyapplicable thin ﬁlms.7
Self-assembly is ubiquitous in nature. The fundamental
biological process of self-assembly, in which an organizedstructure builds itself from a collection of smaller units, has© 2019 American Chemical Society

Received: March 22, 2019
Revised: May 31, 2019
Published: June 14, 2019
16681

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C
be predominant particularly for the DLC systems.14 Incorporation of an amphiphilic character by attaching suitable polar
head groups oﬀers further tunability of self-assembly properties
at interfaces. Moreover, if the polar head group is ionic, theelectrostatic interaction starts to compete with other driving
forces. Since the π-stacking interaction is known to be
dominant among DLCs, any factor that alters the strength of
π-stacking between neighboring molecules should have adramatic impact on their self-assembly properties.
Here, a systematic study of the self-assembly of two similar
discotic amphiphiles with dissimilar polar head groups, viz.,imidazole-tethered with hexaalkoxytriphenylene (neutralImTp) and imidazolium-tethered with hexaalkoxytriphenylene(ionic-ImTp), has been investigated at air−water and air−solidinterfaces. While both formed stable monolayers at the air−water interface, there were remarkable diﬀerences in themolecular packing and their transfer eﬃciencies on solid
surfaces. These experimental observations are explained bydensity functional theory (DFT) calculations and molecular
dynamic (MD) simulations. It has been revealed that the
Coulombic interaction is the dominant factor that gave rise to
several interesting aspects such as T-shaped π-stacking
geometry, distinct orientations of the molecules, and diﬀerences in the strength of hydrogen bonding. It should be
mentioned here that a series of experimental studies on similarmolecular systems were reported before.12,15−17 However,computer simulation studies have been carried out, for the ﬁrst
time, on these molecular systems to understand the detailed
self-assembly properties. Since thin ﬁlms of imidazolium-based
DLCs have potential for device applications such as solar
thermal energy generators and electrolytes for batteries/
capacitors,2 understanding their self-assembly properties is
crucial.

synthesized, and their thermotropic liquid crystalline properties were reported earlier.18 The neutral-ImTp material did notshow any liquid crystalline phase in the bulk. On heating, theneutral-ImTp material directly went from a crystalline phase to
an isotropic liquid phase at 50 °C. On the other hand,incorporation of an ionic group induced liquid crystal
properties. The ionic-ImTp exhibited the following mesophasesequence: solid−columnar, 67 °C; columnar−isotropic, 101°C. On cooling, the columnar mesophase appeared at 98 °C
with the mesophase solidifying at 38 °C.
Measurements. The surface manometry experiments werecarried out using a NIMA 611M Langmuir−Blodgett (LB)trough. The subphase used was ultrapure deionized water (pH5.8) obtained from a Millipore Milli-Q system. The stock
solution of 0.236 mM concentration was prepared using
chloroform (HPLC-grade). After spreading the stock solution
on the subphase using a Hamilton μL syringe, the ﬁlm was left
for 20 min, allowing the solvent to evaporate. The surfacepressure versus area per molecule isotherms were obtained bysymmetric compression of the barriers with a constantcompression rate of 0.103 nm2/molecule/min. The surfacepressure was measured using the standard Wilhelmy platetechnique. The Langmuir−Blodgett (LB) technique was
employed to transfer various layers of the ﬁlms ontohydrophilic and hydrophobic substrates at diﬀerent target
surface pressures with a dipping speed of 2 mm/min. To
obtain hydrophilic surfaces, we treated polished silicon wafers
for about 5 min with hot piranha solution (mixture ofconcentrated H2SO4 and H2O2 in 3:1 ratio), which were thenrinsed with ultrapure deionized water and later dried. For
hydrophobic surfaces, freshly prepared hydrophilic silicon
substrates were dipped in hexamethyldisilazane (HMDS) for
12 h and then rinsed with HPLC-grade chloroform. All ﬁlms
with odd number of layers were transferred onto hydrophilic
substrates, and those with even number of layers weretransferred onto hydrophobic substrates. The atomic force
microscope (AFM) studies on these LB ﬁlms were performed
using a PicoPlus (Molecular Imaging 5500) system. Silicon tips
with a spring constant of 21 N/m and resonance frequency of
250 kHz were used. The AFM images were procured using the
AC mode in ambient conditions. All of the experiments werecarried out at room temperature (∼300 K).
Theoretical Methods. Gas Phase DFT Calculations. The
initial structures of the neutral- and ionic-ImTp molecules
were built using Avogadro software.19 To decrease thecomputational cost, the carbon chains were reduced to methylgroups only in the DFT calculation. Pairs of neutral- and ionicImTp molecules were placed respectively at an interplanar
separation of 0.3 nm using Gaussview20,21 software. The ﬁrstprinciples calculations were carried out for these two systems
based on DFT at the B3LYP/6-311G**22−25 level and basis
set. To obtain the energy proﬁle as a function of the interplanar
separation between the geometry-optimized structures, singlepoint energy calculations were performed for each separation
between the two molecules.
MD Simulations. MD simulation studies were performed
using GROMACS software26 to obtain a supramolecularassembly of the neutral- and ionic-ImTp molecules in thewater−vapor interface. Using packmol software,27 the initial
conﬁguration was built up on the surface of a liquid water slab
for four diﬀerent systems with 25, 34, 50, and 70 molecules ofneutral-ImTp and with 25, 37, 73, and 98 molecules of ionicImTp (snapshots are shown in the Supporting Information■

METHODS
Experimental Methods. Materials. ImTp is a discotic
molecule, which consists of a triphenylene core attached to a
imidazole (or imidazolium) group by an aliphatic carbon chain
as shown in (Figure 1). The imidazole (or imidazolium) part,being hydrophilic in nature, oﬀers a ﬁlm-forming ability on
water surface. These two molecules are similar except for theirpolar head groups; imidazole is neutral and imidazolium isionic. The materials neutral-ImTp and ionic-ImTp were

Figure 1. Energy-minimized molecular structures and chemical
structures of (a) neutral-ImTp and (b) ionic-ImTp molecules.
16682

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C

molecules on the ultrapure deionized water subphase areshown in Figure 3. The surface pressure is deﬁned as the

Figure S1). These numbers were chosen based on theexperimental surface area obtained from the isotherm plots(Figure 3). A typical conﬁguration for the system with 70
molecules of neutral-ImTp has been shown in Figure 2. In the

Figure 2. Initial structure of the system generated from Packmol forthe surface area of 0.7 nm2/molecule with 70 neutral-ImTp moleculesand 4700 water molecules at 300 K.

Figure 3. (a) Surface pressure (π) versus area per molecule (Am)
isotherms and compressional elastic modulus (|E|) plots for neutralImTp (dashed line) and ionic-ImTp (solid line), respectively. (b)Close-packed edge-on arrangement and (c) loosely packed edge-on
arrangement of ImTp molecules at the air−water interface.

case of the ionic system, an equal number of bromide ionswere added to maintain the electroneutrality of the system. To
create suﬃcient vacuum along the Z-axis, the length of the box
was set at 100 Å. Periodic boundary conditions were employed
along X-, Y-, and Z-axis. Each simulation trajectory was
initiated using a slab of water with thickness ∼30 Å (4700
water molecules). The ImTp molecules/ions and the bromideions were modeled using the OPLS-AA force ﬁeld.28−30 The
SPC/E force ﬁeld31 was chosen for water. All of the systems
were ﬁrst equilibrated at 300 K temperature for 10 ns using an
NVT ensemble. All of the bonds were held constraint using the
LINCS algorithm.32 The long-range electrostatic interactions
were handled using the particle mesh Ewald summation
technique.33,34 The cutoﬀ radius was set to 0.9 nm. The
equilibrated systems were then run for 20 ns during which the
conﬁgurations were saved every 2 ps. These production
trajectories were utilized for various analyses.
In addition, surface pressure (π)−area per molecule (Am)
isotherms were predicted qualitatively using Gromacs.35,36 The
surface pressure is given by π = γ0 − γ, where γ0 is the surface
tension of pure water and γ is the surface tension of water withmonolayer. The surface tension γ is given by
ÉÑ
Ä
1 Å
1
γ = 2 LzÅÅÅÅPzz − 2 (Pxx + Pyy)ÑÑÑÑ, where Lz is the length of theÖÇ
box in the z-direction; Pzz, Pxx, and Pyy are the pressure tensor
terms; and the fractional term 1 accounts for the presence of2
two surfaces in the system. A previously reported value35 of γ0
of 63 mN/m for the SPC/E water model at 300 K was used in
the calculations. Various cutoﬀ values of Lennard-Jones
interactions were considered in the calculations.

diﬀerence between the surface tensions of water with andwithout the monolayer. The surface pressure of neutral-ImTp(black dashed line) starts increasing at around an Am of 2.0
nm2/molecule, which is followed by a steep rise, until itreaches collapse at ∼0.7 nm2/molecule. The isotherm shows alimiting area (Ao) of 1 nm2/molecule and a collapse pressure ofabout 31.4 mN/m. The collapse pressure is deﬁned as the
maximum surface pressure that the monolayer can withstand
before it collapses and escapes out of the surface in the form of
three-dimensional aggregates or crystallites. For the ionicImTp isotherm (red solid line), the Ao value and the collapsepressure are signiﬁcantly high with values of 1.4 nm2/moleculeand 44 mN/m, respectively. The variation of compressionalelastic modulus (|E|) with Am for these monolayers is shown in
the inset of Figure 3. Here, |E| is given by Am(dπ/dAm), which
can be calculated from the π−Am isotherms. A maximum |E|
value of 76 mN/m for the neutral-ImTp monolayer was
obtained, which is about 29% higher than the (|E|) value
attained by the ionic-ImTp monolayer (53.9 mN/m).
On the basis of the molecular dimensions, the obtained Aovalues suggest an edge-on arrangement in both the systems.
The smaller Ao and larger |E| values suggest a close-packededge-on arrangement in the neutral-ImTp system compared to
the loosely packed arrangement in the ionic-ImTp system, as
shown in Figure 3b,c. These observations are attributed to thefact that, in the case of an ionic discotic monolayer, there is acompetition between two types of interactions: (i) the π−πstacking interaction between the discotic cores, which favors
close packing of molecules in the monolayer, and (ii) the
electrostatic repulsion between the molecules, which opposes
this close packing.4■

RESULTS AND DISCUSSION
Experimental Section. The surface pressure (π)−area permolecule (Am) isotherms of the neutral- and ionic-ImTp
16683

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C
Next, monolayer ﬁlms were transferred from the air−waterinterface to silicon substrates by the LB technique. Figure 4

Figure 4. AFM topography images of LB ﬁlms with two layers of (a)neutral-ImTp and (b) ionic-ImTp on hydrophobic silicon substrates.
The respective height proﬁles corresponding to the lines drawn on theimages are shown below. The surface of ionic-ImTp shows
undulations (∼1 nm), while that of neutral-ImTp is smooth.

Figure 5. Transfer ratio as a function of number of layers of LB ﬁlms
deposited for (a) neutral-ImTp and (b) ionic-ImTp on hydrophilic
silicon substrates. (c) Schematic diagram of diﬀuse double layer
overlapping in the meniscus region during the upstroke deposition ofa charged Langmuir monolayer. θ is the dynamic contact angle. (d)Schematic diagram of convective ﬂow in the vicinity of a three-phase
contact line. v is the velocity of the surface motion.

shows AFM topography images of LB ﬁlms with two layers of
the neutral-ImTp transferred at 25 mN/m and ionic-ImTptransferred at 35 mN/m onto hydrophobic silicon surfaces. For
both the systems, the ﬁlm height of about 4 nm was obtained,
which corresponds to the estimated height of the moleculesarranged in an edge-on conﬁguration.15 Qualitatively, the top
surface of the neutral-ImTp ﬁlm appeared signiﬁcantly
smoother than that of the ionic-ImTp ﬁlm. The latter showed
clear undulations on the surface. The peak-to-peak roughness
value was about 1 nm for the ionic-ImTp ﬁlm and about 0.2
nm for the neutral-ImTp ﬁlm.
Interestingly, while neutral-ImTp formed multilayers
eﬃciently on silicon surfaces, the ionic-ImTp could not form
more than two layers of ﬁlms. This emphasizes the eﬀect of theionic polar group on the LB deposition process. To quantify
the transfer eﬃciency, the transfer ratio (τ) was obtained forboth the ﬁlm systems. The τ is deﬁned as the ratio of thedecrease in the monolayer surface area to the total surface areaof the substrate to be coated. A τ of unity is indicative ofperfect deposition. The τ’s as a function of number of layers of
LB ﬁlms deposited for the neutral- and ionic-ImTp monolayers
are shown in Figure 5 for hydrophilic substrates. It should be
mentioned here that, for a hydrophilic substrate, the ﬁrst
downstroke does not coat a monolayer because the hydrophilic
surface faces the hydrophobic parts of the amphiphiles.
Therefore, the next subsequent upstroke should be consideredas the ﬁrst deposition stroke. The value of τ was close to unity
for the ﬁrst upstroke and the next downstroke of thehydrophilic substrate, leading to the formation of a bilayer
structure. However, for the subsequent strokes of ﬁlm
deposition, the τ value was found to decrease. Notably, theeﬃciency of transfer was observed to be more than 50% evenup to 12 layers for the neutral-ImTp system. On the contrary,
for the ionic-ImTp system, except for the ﬁrst two layers, the τvalues for the successive cycles of deposition showeddesorption in every upstroke and adsorption in every
downstroke, thus leading to no net transfer of ﬁlms.
The dynamics of ﬁlm deposition can be explained
qualitatively based on the complicated physical and chemical

processes between the monolayer and substrate. In theliterature, it is known that a charged monolayer at the air−water interface and the adjacent diﬀuse layer of counterions inthe subphase form a totally electroneutral electric double
layer.37 The interfacial charge of the ionized surface groups iscompletely compensated by the opposite charge that is spreadout within the diﬀuse layer. During deposition, close to thethree-phase contact line, the electric double layer, which isformed at the air−water interface, overlaps with the double
layer at the solid−water interface, as shown in Figure 5c. The
overlapping causes deﬁciency of counterions in the meniscus
region.38 Nevertheless, the deposited LB ﬁlm from a charged
monolayer should be actually electroneutral. Therefore, during
the deposition process, the monolayer must bind counterions
from the subphase to compensate the surface charge. However,for a complete compensation, all counterions within the diﬀuse
layers should move with the same velocity as the charged
surfaces. Generally, the convective ﬂux of the counterions
produced by the substrate motion is not suﬃcient tocompensate the ﬂux of the surface charges. This takes placesince only those parts of the diﬀuse layers that are adjacent tothe surfaces can move with the same velocity as that of thesurfaces in the direction of the contact line, as shown in Figure
5d. The backﬂow of the subphase expelled from the threephase contact line hinders the transfer of the counterions to
the region of strong overlap of the diﬀuse layers, thereby
hindering the ﬁlm deposition. Although these explanations are
qualitative in nature, they guide us to develop a morequantitative theory, which will be a part of the future work.
These are complex dynamics, which are typical of electrodes
and membrane systems39,40 and are also reported for thedeposition of arachidic acid monolayer and arachidic saltmonolayer under various concentrations of counterions in thesubphase.41,42
Theoretical Methods. Gas Phase DFT Calculation. The
geometry-optimized structures obtained from the ﬁrst16684

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C

Figure 6. (a) Optimized structures for two interacting neutral-ImTp molecules (top) and ionic-ImTp molecules (bottom) in the gas phase. (b)Interaction energy as a function of separation between the two molecules for neutral- and ionic-ImTp systems.

Figure 7. (a) Vector considered for Z-orientation along with various labeled atoms. (b) & (c) Percentage of orientation of neutral-ImTp and ionicImTp moleculesas a function of the angle with the Z-axis for diﬀerent surface concentrations.

principles DFT calculations showed that π-stacking is adominant phenomenon among these molecules. The πstacking of these molecules can be explained by the presenceof triphenylene group at the core. While T-shaped stacking
geometry was observed for the ionic-ImTp molecules, theneutral-ImTp molecules showed parallel-displaced π-stacking.
The optimized structures for both the molecules are shown in
Figure 6a. The interplanar separation between the neutral
molecules and the ionic molecules are 7.72 and 9.77 Å,
respectively, as shown in Figure 6b. The longer interplanar
distance between the ImTp ions stems from the electrostatic
repulsion between the two imidazolium functional groups.
These results are consistent with the experimental observationsof larger limiting area per molecule at the air−water interface
for the ionic-ImTp as compared to that for the neutral-ImTp(Figure 3). Further, these results explain the diﬀerences
observed in the morphology of LB ﬁlms of neutral- and ionicImTp systems (Figure 4). While a relatively smooth ﬁlm
surface may stem from the parallel-displaced π-stacking of theneutral-ImTp molecules, the rough and wavy nature of thesurface can be a result of T-shaped stacking of the ionic-ImTp

molecules. Further details can be obtained by performing
simulations on solid surfaces.
Next, the relative stability between the two optimized
structures was investigated. The DFT calculation clearly
indicated that the T-shaped stacking of ionic-ImTp molecules
is more stable than that of the parallel-displaced neutral
molecules. Similar stabilization of T-shaped stacking of abenzene dimer is also reported in the literature.43 Due to thelack of electrostatic repulsion in T-shaped geometry, it has
better stability than that of parallel or parallel-displaced
stacking. The DFT calculation predicted that the ionic-ImTp
molecule is electronically stable by about 4.49 kcal/mol overthe neutral-ImTp molecule. The interaction energy as afunction of separation between the two ImTp molecules isshown in Figure 6b. This result explains the observation ofmuch higher collapse pressure (∼44 mN/m) for the ionicImTp monolayer than that for the neutral-ImTp monolayer(∼31 mN/m). Notably, the calculation predicted correctly thefact that, despite the stacking of neutral-ImTp molecules being
more compact (given by smaller value of Ao), the stability in
16685

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C

Z-Density of a Head-Group and a Tail-Group Atom. The
probability densities of ﬁnding a head-group atom and a tailgroup atom along the Z-axis have been calculated. The
nitrogen atoms of the imidazolium group were considered as
the head-group atoms and the O6 atom as the tail-group atom.
Nitrogen and oxygen atoms are labeled as shown in Figure 7a.
The Z-density plots for the neutral- and ionic-ImTp molecules
are shown in Figure 8a,b, respectively. A major diﬀerence

the ionic-ImTp system is much higher (given by a higher valueof the collapse pressure).
MD Simulations. Orientational Structure. To probe further
into the system, the angular distribution of the ImTp
molecules on water surface along the vector, as shown in
Figure 7a, was obtained. Initial conﬁgurations for diﬀerent
concentrations of molecules used for the neutral- and ionicImTp systems are shown in Table 1. Lennard-Jones parameters
Table 1. Initial Conﬁguration for Diﬀerent Concentrationsof Molecules Used for the Neutral-ImTp and the IonicImTp Systems As Generated from Packmol for MD
Simulationsa
surface conc.

no. of neutral-ImTp

no. of water molecules

0.7
1.0
1.5
2.0
surface conc.

70
50
34
25
no. of ionic-ImTp

4700
4700
4700
4700
no. of water molecules

0.75
1.0
2.0
3.0

98
73
37
25

4700
4700
4700
4700

a

Surface concentration is in the unit of nm2/molecule.

for water and bromide ions used in the simulations are shownin Table 2. The probability densities of the angles between the
Table 2. Lennard-Jones Parameters for Water and Bromide
Ions Used in MD Simulationsa
atom/ion

ϵ (kJ/mol)

σ (nm)

H
O
Br

0.00000.3166
0.42800.00000.65002.9706a

The cross-parameters are calculated using the Lorentz−Berthelot
combination rules.

Z-axis and molecular vector are shown in Figure 7b. For boththe systems, as the surface area per molecule decreases, thepeak of the angular distribution shifts toward a lower angle.
This indicates that the molecules gradually align parallel to theZ-axis to attain the edge-on conformation. For the smallest
surface area, the molecules become oriented more vertically.
However, Figure 7b also shows signiﬁcant diﬀerences in thealignment of molecules in the two systems. For the neutralImTp system, while the peak is observed at 60° for the larger
surface area of 2.0 nm2/molecule, the peak shifts to 30° for thesmallest surface area of 0.7 nm2/molecule. However, for theionic-ImTp system, the peak shifts from 50 to 29° on
decreasing the surface area from 3.0 to 0.75 nm2/molecule.
Thus, the surface area inﬂuences the alignment of the neutralImTp molecules more strongly than that of the ionic-ImTp
molecules. In other words, neutral-ImTp molecules exhibit amore closely packed and upright edge-on conformation ascompared to the ionic-ImTp molecules. This is consistent withthe experimental observation that the surface pressure
increases more rapidly for the neutral-ImTp than that for theionic-ImTp system. Consequently, the compressional elastic
modulus of the neutral-ImTp was observed to be ∼29% higher(76 mN/m) than that of the ionic-ImTp monolayer (54 mN/m).

Figure 8. Z-Density probabilities for (a) neutral-ImTp and (b) ionicImTp molecules as surface concentration varies with respect to the
distance along the Z-axis.

observed between the two systems is that the separation of thepeaks was higher for neutral molecules than for ionicmolecules. The higher the separation, the closer the packingof these molecules. This provides further explanation for theexperimental results of higher compressional elastic modulus
and lower limiting area for the neutral-ImTp as compared tothat for the ionic-ImTp molecules.
Hydrogen Bonding. The role of hydrogen-bonding
interactions in the stability of ﬁlms of the neutral and ionicImTp molecules at the air−water interface was investigated.
The average number of hydrogen bonds between diﬀerent
atoms of the ImTp molecules and water were computed. Here,the widely used geometric deﬁnition of the hydrogen bondbased on the donor···acceptor heavy atom distance (<3.5 Å)and the donor−hydrogen−acceptor angle (<30°) has been
16686

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C

Table 3. Average Number of Hydrogen Bonds Associated with Diﬀerent Atoms of Neutral- and Ionic-ImTp Molecules,
Obtained from MD Simulationsa
neutral
conc.

N1

N2

O1

O2

O3

O4

O5

O6

0.7
1.0
1.5
2.0

1.008
1.002
1.046
1.098

0.492
0.531
0.549
0.596

0.129
0.208
0.316
0.251

0.023
0.063
0.111
0.102
ionic

0.010
0.054
0.071
0.103

0.011
0.035
0.059
0.097

0.113
0.231
0.277
0.391

0.206
0.312
0.371
0.432

conc.

N1

N2

O1

O2

O3

O4

O5

O6

0.75
1.00
2.00
3.00

0.004
0.004
0.004
0.004

0.003
0.003
0.003
0.003

0.109
0.164
0.210
0.286

0.041
0.080
0.140
0.196

0.006
0.007
0.045
0.022

0.009
0.017
0.052
0.025

0.189
0.204
0.249
0.222

0.206
0.210
0.233
0.219

a

Surface concentration is in the unit of nm2/molecule.

Figure 9. Radial distribution (g(r)) as a function of the distance (r) between head-group atoms and water oxygen atoms for (a) neutral and (b)ionic systems.

between N1 and N1, the peak intensity was the highest for thesystem with the maximum surface area per molecule (i.e., 2
nm2 for the neutral system and 3 nm2 for the ionic system).
This may be due to some sort of structure formation between
the molecules. While the ﬁrst peak arises at about 0.46 nm forthe neutral system, for the ionic system, the ﬁrst peak shifts toaround 1 nm. Also, the intensity is higher for the neutral
system compared to the ionic system. This indicates a
substantial repulsion between the head groups in the ionicsystem. The ﬁrst maximum for radial distribution between C2and C2 is situated at about 0.5 nm for both the neutral and theionic molecules. The above similarity of the ﬁrst peak position
suggests that the stacking interaction between the fusedaromatic regions is unchanged on converting the head groupfrom neutral to ionic. The g(r) curves between the N1 atom
and water Ow for the neutral and the ionic systems provide
insight into the interaction between the head groups and water.
The intensities are observed to be the same for all of thesurface areas. This corroborates the fact that the head-groupatoms remain strongly hydrogen-bonded with water at theinterface. A higher intensity of the ﬁrst peak is observed for theionic system compared to that for the neutral system. This isdue to the fact that the ionic molecules interact with watermuch more strongly than the neutral molecules. This isconsistent with the H-bond interaction, Table 3.

adopted. The respective numbers of hydrogen bonds for theneutral- and ionic-ImTp molecules are shown in Table 3. For
neutral molecules, N1 and N2 atoms actively participated informing hydrogen bonds with water, whereas for ionicmolecules, because of the presence of a positive charge, thehydrogen bonding was rare. Notably, the strength of hydrogenbonding was as high as 2 orders of magnitude for the neutralImTp as compared to the ionic-ImTp system. However,hydrogen bonding between the peripheral oxygen atoms andwater molecules remained almost the same for both thesystems. Further, as the surface area per molecule is decreased,
the average number of hydrogen bonds decreases because themolecules tend to orient more toward the Z-axis. Therefore,
for smaller surface areas, the distribution of these molecules ismore uniform.
Radial Distribution Function. To understand the hydration
structure of the head groups and the alignment of head-groupatoms among themselves, the radial distribution function g(r)between the head-group atoms and water was calculated.
Figure 9 presents g(r)’s between N1/C2 atoms of the molecule(see Figure 7 for the atom labels) and water Ow for diﬀerent
surface areas of the neutral and ionic systems. For both thesystems, a similar qualitative trend is observed in the g(r)curves. The position of peaks remained the same with varying
surface areas. However, the intensities varied. For the g(r)16687

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C

determining the molecular packing and their transfer
eﬃciencies on solid surfaces. The neutral-ImTp formed stable
multilayers with as many as 20 layers on silicon surfaces,
whereas the ionic-ImTp formed only a bilayer. DFT
calculations predicted a T-shaped π-stacking geometry ofionic-ImTp molecules arising due to Coulombic repulsion,
which is electronically stable by about 4.49 kcal/mol over theparallel-displaced π-stacking geometry of neutral-ImTp molecules. MD simulations predicted that, due to Coulombic
repulsion, the z-orientation of the ionic-ImTp moleculeschanged gradually upon compression as compared to therapid change observed for the neutral system. Further,comparative analysis of the H-bonding between the neutral
and the ionic-ImTp with water molecules revealed a signiﬁcantreduction of H-bonding strength by about 2 orders ofmagnitude for the imidazolium group. Finally, a qualitative
trend of the isotherms of the neutral and ionic molecules has
been predicted based on the simulation results. DLCs aretechnologically important materials; therefore, understanding
intermolecular interactions governing self-assembly is aprerequisite for any successful device application.

Further, to investigate the eﬀect of chain length on selfassembly, the simulations were repeated for both the neutral
and ionic molecules with reduced carbon chains to methylgroups. The simulation results are presented in the Supporting
Information, Figures S2 and S3. Interestingly, the new
simulation results are similar and give the same trend of
calculated proﬁles as that obtained for the complete molecules.
The only diﬀerence is that all of the calculated proﬁles aremuch clearer when the simulations are performed with thecomplete molecules. For instance, in the Z-orientational proﬁle(Figure S2), a vertical alignment of the vector with a decreasein surface area per molecule is seen as expected, but a clearer
picture can be seen in Figure 7. Similarly, the Z-density proﬁlesgive a much clearer picture on the vertical alignment of themolecules (as shown in Figure 8) when the simulations were
performed for the complete molecule. Therefore, it appears
that, unlike fatty acids where alkyl chains are known to be thegame changer in determining the phases of monolayer,44 theself-assembly of discotic molecules are primarily ruled by thediscotic core responsible for the π-stacking interaction. Thus,
the eﬀect of alkyl chains on the self-assembly of discotic
systems seems to be secondary.
Finally, the surface pressure−area isotherms of the neutral
and ionic systems were predicted qualitatively (Supporting
Information Figure S4). The surface pressure values were
calculated for the four diﬀerent surface concentrations ofmolecules, as mentioned in Table 1. The calculated values of
surface tensions and surface pressures are tabulated in the
Supporting Information. As can be seen, although the trend ofisotherm can be qualitatively predicted, the calculated values
do not match with the experimental ones. Further, the neutral
system showed a dip in the isotherm at the point 1 nm2/
molecule followed by an increase as the surface area was
reduced. The ionic system showed the expected trend ofincreasing surface pressure with decreasing surface area.
Following the recent report by Javanainen et al.,36 various
cutoﬀ values of Lennard-Jones interactions were used ranging
from 0.9 to 2 nm; however, it was found to have a minor
inﬂuence on the accurate calculation of surface tension. As
mentioned by Javanainen et al., most of the simulation models
reported so far cannot reproduce experimental pressure−area
isotherms with quantitative accuracy. There is a need forproper choice of force-ﬁeld parameters and an appropriate
combination of water models that could provide the correct
surface tension for a more realistic MD simulation of Langmuirmonolayers. Since the simulation studies of Langmuirmonolayers of discotic amphiphiles are not available in theliterature, it is a subject of future investigation on how thesimulation models for the discotic molecules as well as the
combination with diﬀerent water models inﬂuence thesimulated isotherm. Thin ﬁlms of discotic molecules areinteresting both scientiﬁcally and technologically. The present
work predicts the behavior of these molecules at the air−waterinterface only; however, similar studies at the air−solidinterface are in progress.

■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.9b02713.

■

Calculated values of surface tensions and surfacepressures at diﬀerent surface concentrations; snapshotsof the monolayers at 300 K for 0.7, 1.0, 1.5, and 2 nm2for the neutral-ImTp molecules and 0.75, 1.0, 2.0, and
3.0 nm2 for the ionic-ImTp molecules after a 30 ns
simulation run; the vector considered for Z-orientation
along with various labeled atoms for the simpliﬁed
simulation of systems with carbon chains reduced tomethyl groups; the percentage of orientation of ImTp
molecules as a function of the angle with the Z-axis; forthe simpliﬁed systems with carbon chains reduced tomethyl groups, Z-density probabilities as the surface areavaries with respect to the distance along the Z-axis; andqualitative estimation of the trend of surface pressure−area isotherms of both the neutral and ionic systems(PDF)

AUTHOR INFORMATION

Corresponding Authors*E-mail: snehasis@iitp.ac.in (S.D.).
*E-mail: anayak@iitp.ac.in (A.N.).
ORCID

Snehasis Daschakraborty: 0000-0002-2694-7142
Alpana Nayak: 0000-0002-3924-0875
Notes

The authors declare no competing ﬁnancial interest.

■■

ACKNOWLEDGMENTS
Authors thank Dr S. K. Pal at IISER-Mohali for the synthesis ofmolecules during his Ph.D. under the guidance of Prof. S.
Kumar at RRI, Bangalore, India. A.N. thanks the Science andEngineering Research Board, DST, Govt. of India, for the
research grant under sanction ECR/2017/002239. Authors
thank the reviewers for the constructive suggestions.

CONCLUSIONS
Here, the eﬀect of electrostatic interaction on the self-assemblyof two similar discotic amphiphiles with dissimilar polar headgroups, viz., neutral-ImTp and ionic-ImTp, has been
investigated at air−water and air−solid interfaces. While bothformed stable monolayers at the air−water interface, thecharge on the polar head group played a decisive role in
16688

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689

Article

The Journal of Physical Chemistry C■(25) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
Ab initio calculation of vibrational absorption and circular dichroism
spectra using density functional force field. J. Phys. Chem. B 1994, 98,
11623−11627.
(26) Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark,A. E.; Berendsen, H. J. C. GROMACS: fast, flexible, and free. J.
Comput. Chem. 2005, 26, 1701−1718.
(27) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M.
PACKMOL: A package for building initial configurations formolecular dynamics simulations. J. Comput. Chem. 2009, 30, 2157−
2164.
(28) Jorgensen, W. L.; Tirado-Rives, J. Potential energy functions foratomic-level simulations of water and organic and biomolecularsystems. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 6665−6670.
(29) Dodda, L. S.; Vilseck, J. Z.; Tirado-Rives, J.; Jorgensen, W. L.
1.14*CM1A-LBCC: Localized bond-charge corrected CM1A charges
for condensed-phase simulations. J. Phys. Chem. B 2017, 121, 3864−
3870.
(30) Dodda, L. S.; Cabeza de Vaca, I.; Tirado-Rives, J.; Jorgensen,
W. L. LigParGen web server: an automatic OPLS-AA parameter
generator for organic ligands. Nucleic Acids Res. 2017, 45, W331−
W336.
(31) Mark, P.; Nilsson, L. Structure and dynamics of the TIP3P,
SPC, and SPC/E water models at 298 K. J. Phys. Chem. A 2001, 105,9954−9960.
(32) Hess, B.; Bekker, H.; Berendsen, H. J.; Fraaije, J. G. LINCS: A
linear constraint solver for molecular simulations. J. Comput. Chem.
1997, 18, 1463−1472.
(33) Darden, T.; York, D.; Pedersen, L. Particle mesh Ewald: An
Nlog(N) method for Ewald sums in large systems. J. Chem. Phys.
1993, 98, 10089−10092.
(34) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.;
Pedersen, L. G. A smooth particle mesh Ewald method. J. Chem. Phys.
1995, 103, 8577−8593.
(35) Vega, C.; de Miguel, E. Surface tension of the most popularmodels of water by using the test-area simulation method. J. Chem.
Phys. 2007, 126, No. 154707.
(36) Javanainen, M.; Lamberg, A.; Cwiklik, L.; Vattulainen, I.;
Samuli Ollila, O. H. AtomisticModel for Nearly Quantitative
Simulations of Langmuir Monolayers. Langmuir 2018, 34, 2565−
2572.
(37) Kovalchuk, V. I.; Vollhardt, D. Ion redistribution and meniscus
stability at Langmuir monolayer deposition. Adv. Colloid Interface Sci.
2005, 114-115, 267−279.
(38) Kovalchuk, V. I.; Bondarenko, M. P.; Zholkovskiy, E. K.;
Vollhardt, D. Influence of ion transfer kinetics on the composition ofLangmuir-Blodgett films. J. Phys. Chem. B 2008, 112, 11333−11340.
(39) Dukhin, S. S. Non-equilibrium electric surface phenomena. Adv.
Colloid Interface Sci. 1993, 44, 1−134.
(40) Lakshminarayanaiah, N. Transport Phenomena in Membranes;
Academic Press: New York, 1969.
(41) Mahnke, J.; Vollhardt, D.; Stockelhuber, K. W.; Meine, K.;
Schulze, H. J. Regular stripe patterns in skeletonized LangmuirBlodgett films of arachidic acid. Langmuir 1999, 15, 8220−8224.
(42) Kovalchuk, V. I.; Bondarenko, M. P.; Zholkovskiy, E. K.;
Vollhardt, D. Mechanism of meniscus oscillations and stripe patternformation in Langmuir-Blodgett films. J. Phys. Chem. B 2003, 107,
3486−3495.
(43) Tauer, T. P.; Sherrill, C. D. Beyond the benzene dimer: aninvestigation of the additivity of pi-pi interactions. J. Phys. Chem. A
2005, 109, 10475−10478.
(44) Bell, G. R.; Manning-Benson, S.; Bain, C. D. Effect of chain
length on the structure of monolayers of alkyltrimethylammoniumBromides at the air-water interface. J. Phys. Chem. B 1998, 102, 218−
222.

REFERENCES(1) Kato, T.; Yoshio, M.; Ichikawa, T.; Soberats, B.; Ohno, H.;
Funahashi, M. Transport of ions and electrons in nanostructured
liquid crystals. Nat. Rev. Mater. 2017, 2, 17001.
(2) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Discotic liquid crystals: a
new generation of organic semiconductors. Chem. Soc. Rev. 2007, 36,
1902−1929.
(3) Chandrasekhar, S.; Ranganath, G. S. Discotic liquid crystals. Rep.
Prog. Phys. 1990, 53, 57−84.
(4) Goossens, K.; Lava, K.; Bielawski, C. W.; Binnemans, K. Ionic
liquid crystals: Versatile materials. Chem. Rev. 2016, 116, 4643−4807.
(5) Kaafarani, B. R. Discotic liquid crystals for opto-electronic
applications. Chem. Mater. 2011, 23, 378−396.
(6) Perova, T. S.; Vij, J. K.; Kocot, A. Observation of an anchoringtransition in a discotic liquid crystal. Europhys. Lett. 1998, 44, 198−
204.
(7) Bellier-Castella, L.; Caprion, D.; Ryckaert, J.-P. Surface orderingof diskotic liquid crystals. J. Chem. Phys. 2004, 121, 4874−4883.
(8) Whitesides, G. M.; Boncheva, M. Beyond molecules: Selfassembly of mesoscopic and macroscopic components. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 4769−4774.
(9) Materials Science of DNA; Grote, J., Jin, J., Eds.; CRC Press:
Taylor and Francis, 2012.
(10) Cui, L.; Miao, J.; Zhu, L. Spacer length controlled obliquecolumnar to lamello-columnar mesophase transition in liquid
crystalline DNA-discotic cationic lipid complexes. Macromolecules
2006, 39, 2536−2545.
(11) Mallik, S.; Nayak, A.; Daschakraborty, S.; Kumar, S.; Suresh, K.
A. Supramolecular self-assembly of ionic discotic liquid crystalline
dimer with DNA at interfaces. ChemistrySelect 2018, 3, 7318−7326.
(12) Nayak, A.; Suresh, K. A. Discogen-DNA complex films at airwater and air-solid interfaces. J. Phys. Chem. B 2008, 112, 2930−2936.
(13) Zou, C.; Sun, J.; Wang, M.; Wang, J.; Wu, Y.; Zhang, L.; Zhu,
Z.; Xiong, G.; Jiang, L.; Ikeda, T.; Yang, H. A UV-responsive
multifunctional photoelectric device based on discotic columnar
nanostructures and molecular motors. Adv. Mater. 2019, 31,
No. 1806016.
(14) Foster, E. J.; Jones, R. B.; Lavigueur, C.; Williams, V. E.
Structural factors controlling the self-assembly of columnar liquid
crystals. J. Am. Chem. Soc. 2006, 128, 8569−8574.
(15) Nayak, A.; Suresh, K. A.; et al. Films of novel mesogenic
molecules at air-water and air-solid interfaces. J. Phys. Chem. B 2007,
111, 11157−11161.
(16) Nayak, A.; Suresh, K. A. Mechanical properties of LangmuirBlodgett films of a discogen-DNA complex by atomic force
microscopy. J. Phys. Chem. B 2009, 113, 3669−3675.
(17) Nayak, A.; Suresh, K. A. Conductivity of Langmuir-Blodgett
films of a disk-shaped liquid-crystalline molecule−DNAcomplex
studied by current-sensing atomic force microscopy. Phys. Rev. E
2008, 78, No. 021606.
(18) Kumar, S.; Pal, S. K. Synthesis and characterization of novel
imidazolium-based ionic discotic liquid crystals with a triphenylene
moiety. Tetrahedron Lett. 2005, 46, 2607−2610.
(19) Hanwell, M. D.; Curtis, D. E.; Lonie, D. C.; Vandermeersch, T.;
Zurek, E.; Hutchison, G. R. Avogadro: an advanced semantic chemical
editor, visualization, and analysis platform. J. Cheminf. 2012, 4, 17.
(20) Dennington, R.; Keith, T. A.; Millam, J. M. Gauss View, version
6, 2016.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H. et al. Gaussian 16, revision B.01, 2016.
(22) Becke, A. D. Density-functional thermochemistry. III. The roleof exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(23) Lee, C.; Yang, W.; Parr, R. G. Development of the ColicSalvetti correlation-energy formula into a functional of the electron
density. Phys. Rev. B 1988, 37, 785.
(24) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent
electron liquid correlation energies for local spin density calculations:a critical analysis. Can. J. Phys. 1980, 58, 1200−1211.
16689

DOI: 10.1021/acs.jpcc.9b02713
J. Phys. Chem. C 2019, 123, 16681−16689