Additive perturbed molecular assembly in two-dimensional crystals: Differentiating kinetic and thermodynamic pathways

Article abstract of DOI:10.1021/ja210933h

During attempts to produce novel two-dimensional cocrystals by coadsorbing components in a binary mixture, the formation of a metastable form was observed in analogy to the phenomenon of additive-induced polymorph formation reported in three-dimensional c

Full text of DOI:10.1021/ja210933h

Article
pubs.acs.org/JACS
Additive Perturbed Molecular Assembly in Two-Dimensional
Crystals: Differentiating Kinetic and Thermodynamic Pathways
Seokhoon Ahn and Adam J. Matzger*
Department of Chemistry and the Macromolecular Science and Engineering Program, University of Michigan, Ann Arbor,
Michigan 48109-1055, United States
S
* Supporting Information
ABSTRACT: During attempts to produce novel two-dimensional
cocrystals by coadsorbing components in a binary mixture, the
formation of a metastable form was observed in analogy
to the phenomenon of additive-induced polymorph formation
reported in three-dimensional crystallization. Mechanistic in-
sights into this phenomenon were gained through the use of
scanning tunneling microscopy and several adsorbate/additive combinations. One additive plays a critical role in forming a disordered
assembly through a process that is primarily kinetic whereas another additive thermodynamically stabilized an intermediate form,
resulting in interrupting a phase transformation to a more stable form. These additive effects elucidate one of the potential pathways
to kinetically isolate a metastable polymorph formed during cocrystallization in three-dimensional crystallization.
fashion,20−28 and the results are interpreted in the context of
kinetics and thermodynamics.
INTRODUCTION
■
Crystal polymorphism, because of its importance across a wide
range of fields in solid-state chemistry such as pharmaceuticals,1,2
RESULTS AND DISCUSSION
explosives,3,4 and nonlinear optical materials,5,6 is of considerable
■
To explore the effect of additives on phase selection in 2D
crystals, an amide amphiphile with 18 carbons in the alkyl chain
(18-amide) was selected for study because it has been shown
to form at least six phases.29 The chemical structure of 18-
amide and the additive molecules investigated are shown in
Figure 1. A concentration of 100 μM total adsorbate was used
for each binary mixture with the molar ratio varied as specified.
All STM imaging was performed at the phenyloctane/highly
oriented pyrolytic graphite (HOPG) interface. During this
investigation, new crystallization behaviors, distinguished from
those occurring in the homogeneous solutions of 18-amide,
were observed and in some cases the results are kinetic in
nature whereas for others the presence of an additive exerts a
thermodynamic influence. The implications of these results for
understanding additive effects on three-dimensional (3D)
polymorph selection are discussed in this context.
economic interest; furthermore, because of the unresolved
challenges associated with predicting the occurrence of this
phenomenon and experimentally producing forms, it is one of the
most significant unresolved issues in solid state chemistry. In
general, polymorphs are discovered through crystallization by
screening methods varying solvent and temperature7 although
more modern discovery methods such as tailor-made addi-
tives,8−10 epitaxial crystal growth,11−13 and polymer-induced
heteronucleation14 are entering the toolbox of the solid-state
researcher. Recently, the discovery of new single component
crystalline polymorphs during attempts to grow cocrystals has
been reported,15−19 and this phenomenon may be distinguished
from previous methods of polymorph discovery. For example, an
impressive four polymorphs of pure benzidine were obtained
from attempted cocrystallizations with benzophenone and
diphenyl sulfoxide.16 The generation of metastable polymorphs
during cocrystallization trials may prove to be a useful crystal form
discovery technique contributing to the important goal of access-
ing all energetically viable polymorphs of a given compound;
however, at the present time, the mechanism of form production
is unclear, making the path toward further developments
uncertain. Because reduced dimensionality dramatically simplifies
crystallization phenomena, studying phase selection in two-
dimensional (2D) crystallization at solution/solid interfaces can
provide mechanistic insights at the molecular level. Here we
demonstrate that phase selection arises in attempted coadsorption
of several additives with a simple amide amphiphile capable of
forming multiple phases. These results were obtained by scanning
tunneling microscopy (STM), offering submolecular resolution of
both periodic and nonperiodic packing in a time dependent
18-Amide with 17-m-Diester. As a baseline for under-
standing additive effects, the case of 18-amide with 17-m-
diester is discussed. At high concentrations of the amide, a pure
close packed phase (Figure 2, phase I) is observed. A rhombic
nanoporous network of 18-amide (Figure 2, phase II) was
observed from 1:3 to 1:11 (18-amide:17-m-diester) whereas
coadsorption occurred from 1:11 to 1:20 to form 1D-cocry-
stals.28 These molecular assemblies showed a marked depend-
ence on the relative mole fraction of two components in
solution. The structural characteristics of these phases have
been discussed previously.28−30 In particular, coexistence of
Received: November 21, 2011
Published: February 6, 2012
© 2012 American Chemical Society
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Figure 1. Chemical structures of the molecules investigated.
Figure 2. Assemblies obtained from the competition study of 18-amide and 17-m-diester. Blue insets identify phases. (a) STM images (100 × 100 nm2)
showing phase transformation from the close packed phase (phase I) to the rhombic nanoporous network (phase II) observed at 1:6
(18-amide/17-m-diester). (b) STM image (50 × 50 nm2) showing coadsorption of phase II of 18-amide and 1D-cocrystal at a 1:11 ratio. (c) STM
image (200 × 200 nm2) obtained from 1:9 ratio with 52 μM 18-amide. (d) STM image (200 × 200 nm2) obtained from 1:9 ratio with 10 μM
18-amide.
phase II of 18-amide and the 1D-cocrystal without phase
transformation was observed at 1:11 (Figure 2b), indicating
that competitive adsorption occurs near this molar ratio so that
the formation of phase II of 18-amide may be influenced by
molecular interaction with 17-m-diester. However, the above
data are also consistent with a simple dilution effect in a single
or multicomponent solution,24,29,31−34 which has been shown
to stabilize phase II in a homogeneous 18-amide solution.29 To
test this hypothesis, the concentration of 18-amide in the
mixture was varied while keeping the molar ratio to the additive
constant. Using 53 μM 18-amide and 460 μM 17-m-diester
(1:9), the concentration of 18-amide is above the stability
crossing point between phase I and II observed in
homogeneous solution (33 μM).29 Under these conditions,
phase II was not observed and some disorder, which may be
kinetically induced by competition, was observed (Figure 2c).
However, at 10 μM 18-amide and 90 μM 17-m-diester (1:9),
phase II was observed as the thermodynamically stable form
(Figure 2d). This competition study with 17-m-diester does
not illuminate new thermodynamic behavior based on the
presence of an additive although competitive adsorption exerts
some influence on kinetics of nucleation and aggregate
formation as evidenced by the formation of disorder. This
kinetic effect exerted by an additive was also observed in the
competition with 17-triester as discussed below.
18-Amide with 17-Triester. A highly disordered assembly
of 18-amide domains was observed from 2:1 (18-amide/
17-triester) without discernible 17-triester coadsorption
(Figure 3a) whereas coadsorption with phase segregation
between the disordered assembly of 18-amide and pure 17-
triester was observed at a 1:1 ratio (Figure 3b). Because a
disordered structure has not been observed in dilute solutions
of 18-amide,29 amide amphiphile analogues with shorter alkyl
chains,30 nor in the mixture with other additives in the present
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Figure 3. Assemblies obtained from the competition study of 18-amide and 17-triester. (a) STM image (50 × 50 nm2) of the disordered structure
of 18-amide initially observed at 2:1 (18-amide/17-triester). (b) STM image (50 × 50 nm2) showing coadsorption of 18-amide and 17-triester at a
1:1 ratio.
Figure 4. Time course observation of assemblies indicating that the disordered phase of 18-amide is a kinetic product induced by the competition
with 18-amide and 17-triester. (a) STM images (25 × 25 nm2) obtained at 2:1 (18-amide/17-triester) showing conversion to the more ordered
structure. (b) STM images (50 × 50 nm2) showing the transformation from a disordered 18-amide phase to the ordered 17-triester phase at a 1:1
ratio.
study, the disordered assembly is believed to be induced from
molecular interaction between 18-amide and 17-triester and
furthermore is a kinetic product. This latter claim is supported
by the observation of the transformation from disordered to
ordered structures during sequential STM images (Figure 4).
The disordered structure observed at a 1:1 ratio reorganizes to
form more ordered structures indicated by white arrows in Figure
4a, a type of Ostwald ripening (see Supporting Information).35−39
With further increase of the 17-triester fraction, the disordered stru-
ctures of 18-amide are replaced by 17-triester phase (Figure 4b).
In this case, the dilution is unlikely to significantly affect crystallization
because the concentration of 18-amide in the present case is greater
than the 33 μM stability crossover point between phase I and II in
homogeneous solution; therefore the displacement reflects a
competition for limited adsorption sites with the most strongly
adsorbed molecule covering the surface. In a previous study it was
demonstrated that 18-amide has the ability to form and stabilize
various aggregates through noncovalent interactions and when
two different aggregates are incorporated in a unit cell, highly
complex features such as one-dimensional order and wave-like
patterns of voids are formed.29 The same phenomenon manifests
in the present case because the existence of various aggregates in
the disordered assembly is observed while maintaining
reasonably close packing due to compatibility of aggregates. In
other words this compatibility between various aggregates makes
disorder possible to observe because the thermodynamic penalty
is not great.
18-Amide with 12-Amide. During 2D crystal growth of
18-amide in the presence of 12-amide, the formation of the
honeycomb network (phase III) and the rhombic nanoporous
networks (phase II) of 18-amide were observed at ratios from
1:3 to 1:7 (18-amide/12-amide) (Figure 5) whereas the
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Figure 5. Assemblies obtained from the competition study of 18-amide and 12-amide. (a) STM image (200 × 200 nm2) of the honeycomb
nanoporous network (phase III) with 95% surface coverage at 1:3 (18-amide/12-amide). (b) STM images (100 × 100 nm2) showing phase
transformation from phase III to II observed at 1:3. (c) Plots regarding phase transformation from phase III to II observed from the mixture and the
diluted homogeneous solution of 25 μM of 18-amide at room temperature. N is the number of molecules counted.
rhombic nanoporous network of 12-amide was observed at a
ratio of 1:9. The formation of any bilayer was not observed
under different STM bias voltages. The present case differs
from 2D crystallization from 18-amide homogeneous solutions
in two ways: (1) The phase III of 18-amide obtained in the
presence of additive was observed as a major phase with over
95% surface coverage whereas the surface coverage by phase III
in homogeneous solutions was less than 10%,29 and (2) the
phase transformation rate from phase III to II dramatically
decreased in the two component system compared with that
observed in homogeneous solutions. The phase transformation
rate was obtained by counting the number of molecules (N)
associated with each phase in successive STM images. To
eliminate concentration effects on the rate, the concentration of
18-amide for both the mixture and the homogeneous solution
were fixed at 25 μM. Because the phase transformation
occurred most rapidly in the scanned area, the scanning param-
eters such as current, bias, and scanning rate were fixed and the
average of the rates obtained by STM imaging with different
tips was used to minimize tip artifacts. The obtained values of
ln(N)/sec were −0.031 0.007 for the diluted homogenenous
solution and −0.0034 0.0012 for the mixture (Figure 5c).
This 10-fold difference is consistent with an increase in
activation energy required to transform from phase III to II in
the presence of the additive. Using the Arrhenius equation, the
change of the energy barrier (ΔEa) associated with this trans-
form is generally not possible in 3D crystallization because in
a pure phase additive can only interact with the surface of
the crystals and this exerts a negligible effect on stability. The dif-
ferentiation between these possibilities in 2D could be achieved
by comparing STM images obtained during the phase trans-
formation in homogeneous solution and the mixture (Figure 6).
In the homogeneous solution of 25 μM 18-amide, because phase
III is kinetically formed after desorption of a part of phase I and
rapidly transformed to phase II, phase I and II exist as major
phases as shown in Figure 6a. In contrast, phase I was rarely
observed in the mixture of 18-amide and 12-amide as shown in
Figure 6c whereas phase III exists as a major phase. This
observation can not be explained by the kinetic pathway. In that
case phase I should be observed as one of the major phases
because phase I remains more stable than phase III. However, the
observed phenomenon suggests that phase III is more stable than
phase I. To verify this thermodynamic stabilization pathway, 0.5
μL of 100 μM of 18-amide was placed on HOPG. After
confirming the formation of phase I by STM, 1.5 μL of 100 μM
12-amide was added to the solution on HOPG to make a 1:3
mixture that is 25 μM in 18-amide. The phase transformation
from phase I to III occurred as shown in Figure 7. This result
indicates that phase III is more stable than phase I under these
conditions, a result of combining dilution effects and a
thermodynamic stabilization of phase III by 12-amide molecules.
Thermodynamic stabilization of a nanoporous network in 2D
crystallization of a single component solution can occur by three
general pathways:29,31,32 (1) epitaxial stabilization by due to
substrate, (2) equilibrium of adsorption−desorption (dilution),
and (3) solvent coadsorption. The constancy of the substrate and
concentration employed points to the role of 12-amide
molecules as stabilizers of phase III through a coadsorption
mechanism analogous to (3). A closely related phenomenon is
cocrystallization through host−guest chemistry. For example, the
formation can be estimated as 1.33
0.36 kcal/mol (see
Supporting Information).40,41 This increase of energy barrier
(Ea) can be achieved by either destabilization of the transition
state between phase III and II (kinetic mechanism) or
stabilization of phase III (thermodynamic mechanism).
Although kinetic factors often explain the formation of a
metastable form during crystallization with additives in 3D
crystallization, the thermodynamic stabilization of a metastable
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Figure 6. STM images ((a) 400 × 400 nm2, (c) 200 × 200 nm2) and reaction coordinate diagram of the phase transformation from (a, b) the
homogeneous solution of 25 μM and (c, d) the mixture of 1:3 (18-amide/12-amide) with 25 μM of 18-amide. In the 18-amide homogeneous
solutions, phase III is observed as a kinetic intermediate at the boundary area between phase I and II. However, in the mixture with 12-amide, phase
III is observed as a major phase and slowly transformed to phase II where phase I was rarely observed, indicating that phase III can be more stable
than phase I in the mixture.
Figure 7. STM images (50 × 50 nm2) showing the role of 12-amide molecules as stabilizers of phase III. Phase I formed from the homogeneous
solution was transformed to phase III by adding 12-amide solution to make a 1:3 mixture (18-amide/12-amide).
linear structure of dehydrobenzo[12]annulene derivatives were
transformed to the honeycomb structure by adding excess
coronene molecules that fill empty space.42 The failure to image
coadsorbed 12-amide molecules in the present case must be
related to their high mobility. Therefore, the high surface coverage
of phase III and the transformation rate decrease from phase III to
II in the mixture can only be rationalized by the thermodynamic
stabilization of phase III by 12-amide molecules.
Implications. The above three cases illustrate that an additive
can kinetically and/or thermodynamically affect the 2D crystal-
lization of 18-amide (Figure 8). From these competition studies,
three major factors in 2D phase selection from multicomponent
solutions were identified. First, the mole fraction of components
plays a critical role, in concert with the free energy of monolayer
formation, in determining which phase adsorbs at equilibrium (see
Supporting Information for the monolayer formation dependence
on mole fraction in the present cases). Second, dilution effects
serve as a key thermodynamic factor in 2D crystallization. Third,
additive structures play a critical role in determining 2D crystal
structure through a kinetic and/or thermodynamic mechanism. In
particular, assembly structure as well as stabilization of a metastable
form can be controlled through modification of additive structure.
The above 2D phenomena involving competitive molecular
adsorption offers a mechanism to explain changes in the course
of phase selection from multicomponent solutions in 3D
crystallization. Heterogeneous nucleation is a ubiquitous
phenomenon in 3D crystallization and therefore the structure
of adlayers, such as physisorbed monolayers examined here,
may kinetically drive the outcome of crystallization. The above
examples illustrate that monolayer structure can be altered, in a
dynamic fashion, both through kinetic and thermodynamic
pathways; inasmuch as such structures can exert kinetic
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Figure 8. Overview of the competition study with an additive. The molar ratio, concentration effect, and an additive structure play a critical role in
determining the assembly of 18-amide.
influence over phase selection in 3D crystallization through a
heteronucleation mechanism this offers one pathway for
additive-induced polymorph selection/discovery in 3D crystal-
lization.
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
CONCLUSION
*matzger@umich.edu
■
The phenomenon of additive-induced phase selection reported in
3D crystallization has close analogy in 2D crystallization. The
reduced dimensionality and ability to probe the 2D system by
STM allows mechanistic insights into this phenomenon at the
molecular level. The molecular interaction between two
components plays a critical role in determining assembly structure
through kinetic and thermodynamic mechanisms and each plays a
role that is dependent on the details of monolayer structure.
Because these observations in 2D crystallization can be applied to
heteronucleation occurring during attempted cocrystallization,
the influence of an additive on adlayer formation may be one
of the operative pathways to kinetically produce a metastable
polymorph. In other words, although thermodynamic stabilization
of a metastable form by an additive is without analogy in 3D
crystallization of polymorphs, the thermodynamic selection of 2D
crystal forms does offer a potential explanation for kinetic control
of polymorphism in cases where heteronucleation prevails.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-0957591).
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ASSOCIATED CONTENT
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* Supporting Information
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