Inhomogeneity of Organically Modified Montmorillonite Revealed by Molecular Dynamics Simulation.

The modification of an interlayer of layered materials by intercalation with an organoammonium ion has been a promising method to control the polarity of the two-dimensional nanospace. Montmorillonite is one of the best-known examples, and the modification with octadecyltrimethylammonium ion (Mont-C18) results in adsorption of anthracene and pyrene together with specific excimer emission, while the nanostructure is yet to be uncovered at the molecular level because the gallery height is only ca. 27 Å. We, herein, investigated the nanostructure of this nanocomposite by molecular dynamics (MD) simulation, combined with analysis of molecular orientations against the Mont layer. The gallery height of Mont-C18 was well consistent with the experimental value, which was linearly increased along with the intercalation of anthracene. Anthracene was segregated on the Mont layer with its short and long molecular axes vertical in the early and late stages, respectively. In contrast, C18 was initially rather horizontal, forming the so-called pseudotrimolecular layer. Pushed out by anthracene, distribution and orientation of C18 were gradually changed: the third molecular layer was distinctly observed in the center of the interlayer in the early stage, and the orientation was changed to vertical in the late stage. Thus, the continuous increase in the gallery height is ascribed to soft response of C18 to the intercalation. Summarizing the abovementioned results, it was concluded that Å-order inhomogeneity is introduced in the interlayer by the intercalation of anthracene, which is significant in ideal design of the two-dimensional nanospace.

Introduction

Intercalation property of layered materials has been gaining attention in a variety of fields due to their two-dimensional interlayer space. While recovery of environmental pollutants has classically been investigated,[1−3] much effort has also been devoted to applications of catalysts, sensors, and drug delivery systems.[4−7] Some clay minerals are representative layered materials due to their natural abundance. Montmorillonite (Mont) is one of the best-known smectite-type clay minerals, whose single layer basically consists of two-dimensional Si–O tetrahedral and Al–O octahedral frameworks. Because the layers are negatively charged due to isomorphous substitution of Mg2+ and Fe2+, exchangeable cations, typically Na+ and K+, are present in the interlayer for charge compensation. The intercalation of Mont is generally ascribed to the exchange of these cations and confirmed by changes in the basal spacing (sum of the height of the interlayer and thickness of the single layer) measured by powder X-ray diffraction.

The interlayer of Mont has been utilized by the intercalation of not only inorganic metal complexes,[8,9] metal cations,[10] and ionic liquids[11] but also organic molecules such as polymers,[12,13] cationic dyes,[14] and organoammonium ions,[15] the last of which endows Mont with hydrophobicity. This organically modified Mont further adsorbs nonpolar compounds such as aromatic hydrocarbon. This functionalization may seem to be derived from the hydrophobicity of the organoammonium ion, whereas the significance of interaction between the siloxane surfaces and the nonpolar compounds has been pointed out on the basis of the adsorption isotherm. In other words, the adsorbed species are assumed to be distributed inhomogeneously in the interlayer.

Spectroscopy is one of the most powerful techniques to investigate the molecular structure and its property in the interlayer. A typical example is excimer emission of anthracene (Ph3, hereafter) and pyrene intercalated in Mont modified with octadecyltrimethylammonium ion (C18).[16] Because the emission ratio of the excimer to the monomer is dependent on the preintercalated ammonium ion species, morphological control in the interlayer is significant with the aim of on-demand material design. Comprehensive understanding, however, is yet to be achieved probably because direct observation of the interlayer with a resolution of Å order is extremely difficult.

Computational chemistry is another approach to investigate the structure and the property. From a viewpoint of computational cost, molecular dynamics (MD) simulation has been extensively used rather than quantum chemical calculation. Since the late 20th century, computational methods of clays have been developed including forcefields and crystal structures.[17−23] In most cases, however, only one kind of guest species has been the subject such as polymers, organoammonium ions, ionic liquids, cationic dyes, and metal complexes.[9,24−32] In other words, the nanocomposites containing more than one guest species have hitherto been limited to relatively simple systems.[33−36] Considering the recent trend of integration with experimental studies, the development of the computational method applicable to two guest species and following understanding of the interlayer structure with the Å-order resolution will uncover fundamental strategies for the on-demand design of novel layered materials, which has ever been attempted experimentally for decades.

In the present study, we investigated the nanostructure of Ph3-intercalated Mont modified preliminarily with C18 by the MD simulation. The interlayer structure of Mont modified with C18 (Mont-C18) was well consistent with that proposed experimentally,[16] and an increase in the basal spacing of Ph3-intercalated Mont-C18 (Mont-(C18·Ph3)) was also simulated, along which inhomogeneous distribution of Ph3 in the interlayer was clearly observed. The morphologies of C18 and Ph3 in the interlayer were analyzed based on atom distributions and orientation angles against the clay layer. As a result, the relation between the swelling of Mont-(C18·Ph3) and the changes in the interlayer nanostructure was revealed, which are essential to fabricate ideal two-dimensional nanoarchitecture.

Theoretical Calculations

Outline

Molecular dynamics simulation was performed by Forcite implemented in the Materials Studio 7.0 supplied by BIOVIA Inc. All MD simulations were conducted under a three-dimensional periodic boundary condition in a canonical ensemble (NVT) or an isothermal–isobaric ensemble (NPT). Temperature and pressure were set at 300 K and 1.0 × 10–4 GPa and controlled using the Nosé–Hoover thermostat and the Parrinello–Rahman method, respectively.[37] Dependency of the initial orientation of the Ph3 molecule was fully removed by annealing MD with the NVT ensemble to obtain an equilibrium structure. The initial and midcycle temperatures were set at 300 and 2000 K. The heating ramps were set at 50, and the MD simulation was conducted at each step. The number of the MD simulations was 1000 per step with the time step of 1.0 fs. This annealing MD simulation was repeated 30 times. A pcff-interface developed by Heinz and co-workers was used to represent the atomic interaction,[38] which is suitable for the clay minerals to simulate the structure of the guest molecules.[23] The Lennard-Jones potential representing nonbonding atomic interactions was cut off at 10 Å with truncation correction of a cubic spline. For a summation of electrostatic interactions, the Ewald summation technique was used.[39]

Modeling of Mont-C18 and Mont-(C18·Ph3)

A unit cell of montmorillonite composed of two pairs of a layer and an interlayer was referred to that reported previously,[40] whose parameters are a = 25.959 Å, b = 27.0459 Å, c = 20.0457 Å, α = 90.000°, β = 95.735°, and γ = 90.000°, respectively. The model formula is Na0.4[Si4O8][Al1.6Mg0.4O2(OH)2] with 108 mequiv/100 g, which is sufficiently close to that investigated experimentally.[16] The charges of the trimethylammonium (NMe3+) head group in C18 were set to −0.1 for a N atom and +0.275 for each CH3 group, and the CH2 group directly bonded to the N atom.[41] In the modeling of Mont-C18, the unit cell was expanded to 80 Å along the c axis so as to place C18. Each Na+ ion was substituted with the cationic C18 with the carbon chain vertical against the clay layers. The N atoms were positioned where the deleted Na+ ions were. After the optimization of atomic positions by molecular mechanics with 500 steps, the MD simulation was conducted with the NPT ensemble for 30 ns, which is long enough for the basal spacing to become constant, as shown in Figure a.

Figure 1

(a) Changes in the basal spacing of Mont-C18 during the MD simulation with the NPT ensemble, (b, c) snapshots of the unit cell at 25 and 30 ns, and (d) atom distributions. All of the H atoms are not displayed for clarity. The purple, green, orange, red, blue, and gray spheres in (b, c) represent Al, Mg, Si, Osurf, N, and C in C18 atoms, respectively. The colors are also corresponding to those in (d) except the CH2 atom, which is drawn in black.

For modeling of Mont-(C18·Ph3), the obtained Mont-C18 was expanded by 10 Å along the c axis. Then, the expanded structure was energetically equilibrated by performing the annealing MD followed by the MD simulation with the NVT ensemble for 5 ns. After the equilibration, five Ph3 molecules were placed in each interlayer, whose position was further equilibrated by the same method. Finally, the expanded interlayer was shrunk by the MD simulation with the NPT ensemble for 5 ns to obtain Mont-(C18·Ph3), similar to the modeling of Mont-C18. The simulation time was long enough for the basal spacing to become constant, as shown in Figure . Inserting the Ph3 molecules by the abovementioned procedure repeatedly, Mont-(C18·Ph3) containing different numbers of the Ph3 molecules was modeled.

Figure 2

(a) Changes in the basal spacing of Mont-(C18·Ph3) during the MD simulation with the NPT ensemble containing (a) 5, (b) 10, (c) 15, (d) 20, (e) 25, and (f) 30 Ph3 molecules in each interlayer, respectively.

Structural Analysis

After modeling Mont-C18 and Mont-(C18·Ph3), the obtained nanostructures were investigated by the MD simulation with the NVT ensemble for 5 ns. The time step was set to 1.0 fs. In the following analyses, multiple snapshots in every 100 ps were used for ensemble average.

Atom distributions of Mont-C18 and Mont-(C18·Ph3) were analyzed along the z axis with an interval of 0.02 Å. We note that the clay layer was set on the xy plane. Due to the two dimensionality and the swelling property, the atom distributions along the vertical z axis are significant, while those along the lateral xy directions are not. Discussing the nanostructure in the interlayer simply and definitely, we focused on the following atoms and named as denoted in the parentheses: Al atom (Al), Si atom (Si), O atom on the surface of the clay layer (Osurf), N atom (N), C atom in the CH2 group of C18 (CH2), C atom in the CH3 tail group of C18 (Ctail), C atom in Ph3 (CP), and H atom in Ph3 (HP), respectively. The basal spacings of Mont-C18 and Mont-(C18·Ph3) were calculated by dividing the length of the z axis of the unit cell by two because the unit cell consists of two pairs of the layer and the interlayer.

Orientations of Ph3 and C18 against the clay layer were investigated using the Cartesian coordinates of four atoms. In the Ph3 molecule, the furthest four C atoms from the centroid were chosen and two molecular axes were defined, as shown in Figure a: the shorter and the longer axes are hereafter denoted as S and L, respectively. Orientation angles against the Mont layer, θ and θ, were calculated in the multiple snapshots and averaged. Here, molecular orientations are roughly divided into three groups, as shown in Figure b: V, V, and H forms. In the V form, the S and L axes are vertical and horizontal, respectively. In other words, θ and θ equal to 90 and 0°, respectively, while the opposite holds in the V form. In contrast, both θ and θ equal to 0° in the H form. Thus, the orientation of Ph3 is able to be discussed by plotting θ against θ. Similarly, the orientation angle of C18, θC18, was also defined using four atoms: the N atom and its nearest three CH2 atoms.

Figure 3

(a) Definition of S and L axes of Ph3 and (b) three forms of Ph3 with different orientations. The H atoms are not shown for clarity.

Results and Discussion

Model Validation of Mont-C18

Figure a shows changes in the basal spacing of Mont-C18 during the MD simulation with the NPT ensemble. The value is sufficiently constant after 20 ns, whose average is 20.8 Å and well consistent with the reported value, 22 Å.[16]Figure b,c shows the snapshots of the unit cell at 25 and 30 ns, respectively. In both snapshots, the NMe3+ head groups are located near the clay layers probably due to electrostatic interaction. The carbon chain of C18 is tangled in the interlayer, and the CH2 atoms are located not only horizontally near the clay layers but also in the center of the interlayer. The corresponding atom distributions are shown in Figure d. In each interlayer, two distinct peaks are observed in the CH2 atoms, which are also distributed broadly in the center of the interlayer, indicating the formation of a pseudotrimolecular layer of C18, as proposed experimentally.[16] Thus, the CH2 atoms are found to be distributed inhomogeneously in the interlayer. Figure a shows a histogram of θC18 in Mont-C18. Based on the definition of θC18, it is found that the three CH2 atoms near the N atom are rather located horizontally in Mont-C18 because θC18 is mainly distributed <15°. It is noted that similar horizontal but partially disordered nanostructures are observed theoretically in C14H29N(CH3)3+ and C22H45N(CH3)3+ intercalated in Mont with CEC of 145 mequiv/100 g.[32] Thus, it was concluded that Mont-C18 was well modeled by our MD calculations under the three-dimensional periodic boundary condition.

Figure 4

Histograms of θC18 in (a) Mont-C18 and Mont-(C18·Ph3) containing (b) 10, (c) 20, and (d) 30 Ph3 molecules in each interlayer, respectively.

Nanoscopic Structure of C18 and Ph3 in the Interlayer

Figure shows changes in the basal spacing of Mont-(C18·Ph3) during the MD simulation with the NPT ensemble, where the Ph3 molecules are intercalated five by five. The basal spacing is sufficiently constant during the late stage of the simulation in all of the models and is 24.4 Å in the case of 10 Ph3 molecules, which is longer than that of Mont-C18, 20.8 Å. The corresponding snapshots of the unit cell at 4 and 5 ns are shown in Figure a,b, respectively. Ph3 is located on the clay layers with its S axis vertical, while the center of the interlayer seems slightly dense compared to that in Mont-C18. These results are clearly confirmed by the atom distributions shown in Figure c. Ph3 is found to be distributed mainly on the clay layer. In addition, the distribution of the CH2 atoms is not similar to that in Mont-C18 shown in Figure d: the third peak is definitely observed in the center of the interlayer, which is not in Mont-C18. However, the histogram of θC18 shown in Figure b does not change significantly compared to that in Mont-C18, indicating that the orientation of the three CH2 atoms near the NMe3+ group does not change in the early stage of the intercalation of Ph3, and the CH2 atoms in the middle or on the tail side of the carbon chain get partly vertically oriented. Our simulation results that C18 and Ph3 are located inhomogeneously in the interlayer are actually noticeable. It is true that many experimental studies previously suggested the inhomogeneity,[15,16,42] but visualization by spectroscopy and/or microscopy is technically impossible. As far as we know, this is the first report on the visualization of the Å-order inhomogeneous interlayer induced by the intercalation of Ph3.

Figure 5

Snapshots of the unit cell of Mont-(C18·Ph3) containing 10 Ph3 molecules in each interlayer at (a) 4 and (b) 5 ns, (c) atom distributions, and (d) orientation plots of Ph3 in Mont-(C18·Ph3). The purple, green, orange, red, blue, gray, and brown spheres in (a, b) represent Al, Mg, Si, Osurf, N, C in C18, and CP atoms, respectively. The colors are also corresponding to those in (c) except the CH2 atom, which is drawn in black.

As mentioned above, the CP atoms are distributed only on the clay layer roughly with two peaks, for example around 8.5 and 10.6 Å. Hence, the orientation of Ph3 is mainly assumed to be in the V form, as expected from the snapshots shown in Figure a,b. Figure d shows the corresponding orientation plots of the Ph3 molecules. We note that the plots are on the broken line when the molecular plane is completely vertical to the clay layer. About 80% of the plots are located on the upper left assignable to the V form. The H form appearing in the lower left is not present at all. This result is rationalized by the distribution of the CH2 atoms shown in Figure c: the orientation of the CH2 atoms is partly vertical, resulting in the localization in the center. While the V and V forms are parallel to this locally vertical orientation of the CH2 atoms, the H form is not, leading to the conclusion that the H form is energetically unfavorable rather than the V and V forms.

By subtracting the thickness of the Mont layer (9.8 Å) from the basal spacing, the gallery height is calculated to be 14.6 Å, which is longer than the L axis, 7.3 Å. However, almost all of the Ph3 molecules are present in the V form. This result originates from the fact that two molecular layers of Ph3 are formed in each interlayer. It is noted that the presence of C18 and the HP atoms is also not negligible. In contrast, the length of the S axis is 1.4 Å. Even considering the HP atom, it is reasonable that the Ph3 molecules are mainly present in the V form in the early stage of the intercalation of Ph3, as shown in Figure a,b. In other words, it is suggested that the population ratio of the V form is possibly increased by further swelling.

Swelling-Induced Population Changes in the Orientation of Ph3

Figure a,b shows the 4 and 5 ns snapshots of the unit cell of Mont-(C18·Ph3) containing 20 Ph3 molecules in each interlayer. While Ph3 is still localized on the clay layers in the V or V form, C18 seems located entirely in the interlayer. By this intercalation of Ph3, the basal spacing is expanded to 28.0 Å. In contrast to the case of the 10 molecules, the histogram of θC18 shown in Figure c is also changed along with the swelling: the frequency around 70° is comparable to that of around 10°, indicating that the orientation of the three CH2 atoms near the N atom is getting vertical.

Figure 6

Snapshots of the unit cell of Mont-(C18·Ph3) containing 20 Ph3 molecules in each interlayer at (a) 4 and (b) 5 ns, (c) atom distributions, and (d) orientation plots of Ph3 in Mont-(C18·Ph3). The purple, green, orange, red, blue, gray, and brown spheres in (a, b) represent Al, Mg, Si, Osurf, N, C in C18, and CP atoms, respectively. The colors are also corresponding to those in (c) except the CH2 atom, which is drawn in black.

Figure c shows the corresponding atom distributions. Similar to the result in the case of the 10 molecules shown in Figure c, the three peaks of the CH2 atoms are observed near the clay layers and in the center. However, the third peak in the center is fairly broadened. This broadness is corresponding to the swelling of the interlayer, which delocalizes the carbon chain of C18 due to an increase in the volume of the interlayer. In addition, the first peak near the clay layer is split into two at 9.04 and 9.60 Å beside the peak of the N atom. Thus, it is indicated that the nearest two CH2 atoms from the N atom are somewhat located vertically as supported by the histogram of θC18 shown in Figure c.

The distribution of the CP atoms is complementary to that of the CH2 atoms, and two peaks are observed near the Osurf atoms individually, indicating that Ph3 is still localized on the clay layer. Figure d shows the orientation plots of Ph3. In the case of 20 Ph3 molecules, it is found that about 65% are assigned to the V form, while about 10% are assigned to the V form appearing in the lower right. Based on the above discussion in the case of 10 molecules, it is assumed that the gallery height is enough for Ph3 to be in the V form. Indeed, the gallery height is calculated to be 18.2 Å, which is more than twice the length of the L axis. Due to the presence of CH2 and Hp atoms in the interlayer, Ph3 can yet be present in the V form completely, leading to a rather tilted orientation plotted in the range of around 45° both in θ and θ. In addition, the first 10 Ph3 molecules are already localized in the V form on the clay layers covered with the NMe3+ head groups. Thus, it is inferred that the V form becomes more energetically favorable than the V form for the additional 10 Ph3 molecules in the middle stage of the intercalation.

Changes in the Orientation of C18 by Further Intercalation of Ph3

Figure a,b shows the 4 and 5 ns snapshots of the unit cell of Mont-(C18·Ph3) containing 30 Ph3 molecules in each interlayer. Ph3 is located on the clay layers in the V or V form, while C18 is stretched vertically to the clay layer. The latter result is supported by the distribution of θC18, as shown in Figure d. The value is mostly around 70°, and the frequency less than 30° is zero. As shown in Figure a,b, however, the carbon chains near the Ctail atoms do not seem stretched but still tangled in the center of the interlayer. This observation is supported by the distribution of the Ctail atoms, as shown in Figure a: the Ctail atoms are localized in the center of the interlayer in the presence of Ph3 on the clay layers, which is in contrast to those in Mont-C18 shown in Figure b. The basal spacing is expanded to 31.4 Å, and the gallery height is calculated to be 21.6 Å, which is sufficient for the V form as discussed above. The orientation analysis of Ph3 supports this assumption, as shown in Figure d. The ratio of the V form gets higher and comparable to that of the V form. This change is in harmony with the changes in θC18 shown in Figure d: the vertically long V form is preferable when C18 gets vertical.

Figure 7

Snapshots of the unit cell of Mont-(C18·Ph3) containing 30 Ph3 molecules in each interlayer at (a) 4 and (b) 5 ns, (c) atom distributions, and (d) orientation plots of Ph3 in Mont-(C18·Ph3). The purple, green, orange, red, blue, gray, and brown spheres in (a, b) represent Al, Mg, Si, Osurf, N, C in C18, and CP atoms, respectively. The colors are also corresponding to those in (c) except the CH2 atom, which is drawn in black.

Figure 8

Ctail atom distributions of (a) Mont-(C18·Ph3) containing 30 Ph3 molecules in each interlayer and (b) Mont-C18, respectively. The purple, orange, red, blue, and gray lines represent Al, Si, Osurf, N, and Ctail atoms, respectively.

Figure c shows the corresponding atom distributions of Mont-(C18·Ph3). Similar to the result in the case of 20 molecules, the distribution of the CH2 atoms is complementary to that of the CP atoms. In the CP atoms, shoulder bands are observed close to the center of the interlayer due to the V form: the CP atoms are roughly divided into two groups in the V form (upper and lower halves), while located rather homogeneously in the V form. Contrary to the case of 20 molecules, the broad peak in the center of the interlayer is split into two. This split originates from the continuous expansion of the interlayer by the intercalation of Ph3. Moreover, not two but three sharp peaks of the CH2 atoms are observed beside each N atom, assignable to the three CH2 atoms near the N atom used for θC18. Distances between two adjacent peaks are 1.32, 1.24, and 1.21 Å for N–C, C–C, and C–C respectively. When the molecular axis of C18 is completely vertical to the clay layer, the distances are 1.37 and 1.26 Å for N–C and C–C, respectively. Therefore, it was concluded that the three CH2 atoms near the N atom are oriented almost vertically to the clay layer. This result is consistent with the snapshots and θC18 shown in Figures a,b, and 4d, respectively.

Dynamics of Swelling and Structural Characteristics

Figure shows the correlation between the basal spacing of the nanocomposite and the number of Ph3 in each interlayer. The swelling takes place continuously by the intercalation. It is true that the swelling seems to originate from the increase in the volume due to Ph3, but the nanostructural behavior is actually not simple. First, the CH2 atoms are distributed mainly near the clay layers to form the pseudotrimolecular layer in Mont-C18, as shown in Figure d. Second, the third layer of the CH2 atoms becomes distinctly observed in the center by the intercalation of 10 Ph3 molecules, as shown in Figure c. In this nanocomposite, the swelling is ascribed to the fact that Ph3 is localized on the clay layers and pushed out the CH2 atoms in the middle or on the tail side of the carbon chain, whereas the three CH2 atoms near the N atom are still mainly horizontal, resulting in the three peaks in the distribution of the CH2 atoms. However, the expanded gallery height is not sufficient for Ph3 to be present in the V form. Consequently, the V form is dominant. Third, further swelling takes place by the intercalation of Ph3, along which the CH2 atoms near the N atom get oriented vertically, as shown in Figures c and 6c. This swelling enables the additional Ph3 molecules on the clay layer to be present in the V form. Finally, C18 gets vertical almost completely and the ratio of the V form is increased, as shown in Figures d and 7d. Therefore, the swelling in the late stage is ascribed to the changes of θC18, whose distribution is shown in Figure . In other words, the flexibility of C18 enables the interlayer of Mont to swell smoothly and determines the basal spacing.

Figure 9

Correlation between the basal spacing and the number of the Ph3 molecules in each interlayer.

Population changes in the V and V forms against the number of the Ph3 molecules are summarized in Figure . It is clearly demonstrated that the V form is dominant in the early stage but saturated in the middle stage, while the V form is drastically increased correspondingly. We assume that the increase in the population of the V form in the middle and late stages is ascribed to the coverage of the clay layers with the NMe3+ head groups and Ph3 in the V form. In the MD simulation, the orientation changes from the V to V form did not take place, suggesting that the V form is favorably interacted with the clay layer strongly compared to the V form. Based on the number of the HP atoms in Ph3 faced to the clay layer, it is convincing that the Ph3 molecules in the V form are more stabilized than those in the V form. Our result of the localization of Ph3 on the clay layers is consistent with that reported previously.[15] It was experimentally suggested that the clay layers play a significant role in the adsorption of aromatic hydrocarbons. The localization of Ph3 also possibly rationalizes the excimer emission of Ph3 in Mont-(C18·Ph3).[16] As shown in Figure a,b, the Ph3 molecules are parallel to each other, which is necessary for the formation of the excimer.

Figure 10

Population changes of the V and V forms against the number of the Ph3 molecules. The marks × and ○ represent the V and V forms, respectively. In this analysis, the V and V forms are defined when θ > 60° and θ < 30° in V, and θ < 30° and θ > 60° in V, respectively.

Because the length of C18 is approximately consistent with the gallery height of Mont-(C18·Ph3),[16] one may think that the carbon chain is stretched thoroughly at the saturated adsorption state. However, it is highly improbable due to the coverage of the clay layers with the NMe3+ head groups and Ph3 molecules. The atom distributions of the Ctail atoms shown in Figure support our assumption. The Ctail atoms are originally located both on the clay surface and in the center of the interlayer in Mont-C18. By the intercalation of 30 Ph3 molecules, the distribution is changed and localized in the center. Combined with the changes in θC18 shown in Figure , it is found that the three CH2 atoms near the N atom are oriented vertically, while the CH2 atoms in the other part of the chain are relatively flexible, leading to the localization of the Ctail atom in the center of the interlayer, as shown in Figure a,b. Therefore, nanoscopic segregations of C18 and Ph3 were revealed by the MD simulation and following structural analyses.

Conclusions

In the present study, we investigated the nanostructure of Mont-(C18·Ph3) by MD simulation combined with the analyses of the vertical atom distributions and the orientation angles of C18 and Ph3. Stepwise intercalation of Ph3 revealed that the increase in the basal spacing is continuous, which cannot be realized easily by an experiment. The Ph3 molecules are localized in the V or V form on the Mont layers. By the intercalation of Ph3, the orientation of C18 is changed gradually from horizontal to vertical. Thus, the swelling of the interlayer is ascribed not to the volume of the Ph3 molecules but to the orientation changes in C18. Summarizing the changes in the distribution of the CH2, Ctail, and CP atoms, it was concluded that the Ph3 molecules are distributed in the interlayer inhomogeneously, whereas such segregation possibly induces the characteristic photophysical behavior. We believe that our results demonstrate the significance of the flexibility of C18 and give an insight into the on-demand material design.