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Molecular dynamics simulations on the orientation of n-alkanes with different lengths on graphene
Surface Science 690 (2019) 121468
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Molecular dynamics simulations on the orientation of n-alkanes with different lengths on graphene
T
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Liu Yan Fanga, Yang Huaa, , Zhang Zhi Menga, Zhang Huib a Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of Chemistry, Tianjin Normal University, Tianjin 300387, People's Republic of China b School of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molecular dynamics simulation Graphene n-alkanes Orientation
The orientation of molecules on surface is very important in the design of nanomaterials. Molecular dynamics simulation was used to study the mechanism of n-alkane molecules orientation on graphene surface. The chain length effects the orientation of n-alkanes on graphene surface. Short n-alkanes are perpendicular to the graphene surface and the long n-alkanes are laid parallel to the surface. The two kinds of orientation processes are shown directly and are considered as a three-step process (adsorption, orientation, and growth). The critical temperatures of the orientation of n-alkanes on graphene surface increases with chain length increasing. In addition, the simulation results show the interactions of n-alkane-n-alkane and n-alkane-graphene playing a key role in the orientation of n-alkanes on graphene surface.
1. Introduction Liquid crystals, are a state of matter which has properties (fluidity, anisotropy, etc) between those of conventional liquid and those of solid crystals. Liquid crystal materials consist most of organic molecules such as aliphatic, aromatic and stearic acids, etc. The study of liquid crystal has become popular in the past few years because it plays an important role in the materials science [1–3]. For example, the liquid crystal display technology has promoted the technological development of microelectronics and optoelectronic information. The orientation of molecules in liquid crystals is an important field of studying liquid crystals. The surface has a great influence on the orientation of molecules on it [4–6]. Graphene plays a key role in the study of molecules adsorbed on a surface because it has good mechanical properties and good symmetry. Due to the complexity of studying the orientation of aliphatic, aromatic and stearic acids, we can focus first on the orientation of much simpler physical systems: n-alkanes on graphene. This can help us better analyze and understand the orientation process of molecules in liquid crystals deeply. In recent years, the study of the thermodynamic properties of molecules physically adsorbed on substrates has become a very active field. The structures and behaviors of molecules on a surface have been investigated both experimentally and computationally [7–36]. Masnadi and Urquhart [7] studied the effect of substrate temperature on the epitaxial growth of oriented n-alkane thin films on graphite, confirming ⁎
that molecular orientation depends on the evaporation conditions (substrate temperature, substrate identity, and evaporation rate) and chain length. They found that the coexisting lateral and normal orientation emerges at higher temperature. The morphology and molecular orientation of linear alkanes on NaCl (001) surface were investigated by NEXAFS spectroscopy and microscopy [9]. The strong interaction between the longer chain molecules and surface is responsible for the lateral orientation in the investigated substrate temperature range. When the interaction between shorter molecules and the surface is weaker, the substrate temperature plays an important role in defining the normal molecular orientation. Pint et al. [16] referred that the importance of the competition between molecule-molecule and molecule-substrate interactions leading to the molecular rolling and tilting. Firlej and his coworkers [17] found that the electrostatic force plays an important role in the process of melting by analyzing the dynamic behavior of a tetracosane (C24H50) monolayer adsorbed on graphite. Molecular simulations were performed to study the melting of hextane monolayers adsorbed on graphite by Wexler et al. [18] They found that the monolayer melts directly from the herringbone solid to a fluid with residual local order reflecting the graphite substrate's symmetry. And the size of reorienting domains decreases with increasing temperature. Roth et al. [23] found that the neighboring layers and confinement effects determine the amount of gauche defects during the melting of alkane C24H50 bilayer and trilayer systems adsorbed on graphite. Whether in the bilayer or in the trilayer, the lower layer
Corresponding author. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.susc.2019.121468 Received 27 March 2019; Received in revised form 2 July 2019; Accepted 22 July 2019 Available online 22 July 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved.
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simulation system is about 1500. Periodic boundary conditions were utilized. The parameter z was enlarged to 120 Å to ensure that the interactions between the adsorbed n-alkanes and the periodic images of graphene in the top plan could be ignored. The three-dimensional periodicity inherent in the model was transformed into actual two-dimensional periodicity thus simulating an infinity extended surface. The COMPASS [43,44] force field has been successfully used in simulating the interaction between alkane/polyethylene and the carbon nanotube/graphite/graphene [45–48]. The five n-alkanes/graphene systems were described by the COMPASS force field. The models relaxed by 50,000 steps firstly. The resultant structures were subjected to subsequent 10,000 ps NVT MD simulations in the temperature range of 300–550 K. The integration time step of 1 fs was used. Nose–Hoover [49,50] method was applied to control the temperature. The cut off distance for the nonbonded interaction was taken to be 12.5 Å.
exhibits fewer defects than the monolayer at the same temperature, and the upper layer exhibits more. These mentioned studies are of great significance for the study of molecules orientation on surface. However, none of them studied the influences of molecular length on the orientation of n-alkanes on a solid surface directly, which is also important in the application of liquid crystals materials. It is necessary to understand the formation mechanism of n-alkanes thin film in order to control the film structure precisely. Number of researches [37–42] has been done on the orientation of organic molecules on surface by vapor deposition method. The orientation processes were observed by a quartz crystal microbalance technique. Kinetic and thermodynamics models have been proposed to explain the mechanism of molecules orientation on surface by vapor deposition. Both models can describe the temperature dependence of linear organic compounds orientation in vapor-deposited film. In our previous study, we presented the results of the isothermal orientation of n-decanes melt confined between graphene sheets by molecular dynamics simulations [24]. The n-decanes are normally oriented to the basal plane of graphene at low temperatures. With increase in temperature, the n-decanes are laterally oriented to the surface. When temperature is high enough, the n-decanes form a disordered structure. In this study, we continue to study the orientation of n-alkanes melt on graphene surface by means of molecular dynamics (MD) simulation. The effect of chain length on the orientation of n-alkane molecules will be shown in detail. The united atom model [16,21] was adopted to simplify the calculations in most of previous works. But they neglected the effect of hydrogen and electrostatic interaction in the simulations [17,21]. To compensate for the deficiencies, the all-atom description of n-alkane molecules is used in our simulations. This paper is organized as follows: First, the last conformations of n-alkane molecules at different temperatures will be described; second, the orientation processes of n-alkanes on the graphene surface will be shown; Third, the effect of chain length on the orientation of n-alkane molecules will be explored; Finally, the effect of alkane-alkane interaction and alkane-surface interaction on the orientation of molecules will be discussed.
3. Results and discussion 3.1. Last conformations of n-alkanes on graphene surface In order to explore the effect of the chain length on the orientation of n-alkane molecules on graphene surface, 10,000 ps NVT MD simulations were performed at different temperatures for the five systems. Fig. 2 displays the last conformations of C6, C10, C12, C20, C30 molecules on graphene surface at 400, 450, 500 K. They can form three kinds of structures: parallel multilayer structure, perpendicular ordered structure, disordered structure. At 400 K, C6, C10, C12 molecules are all perpendicular to the surface. And C6 molecules form two ordered regions. Some C6 molecules are desorbed from the surface and appear below the surface due to periodic boundary conditions. Most of C20, C30 molecules are parallel orientation on the surface and the molecules in the ordered region are extended and not always parallel to each other. At 450 K, the C6 molecules are disordered on the surface and more C6 molecules are desorbed. Most of C10, C12 molecules are perpendicular to the surface. Some of C10 molecules are also desorbed. More C20, C30 molecules arrange parallel to the surface and they are parallel to each other. They form a multi-layer structures. At 500 K, the short n-alkanes (C6, C10, C12 molecules) form disordered structures on the surface. The number of C6 molecules desorbed is bigger than that of C10 molecules. C20, C30 molecules are still parallel to the surface and almost all arrange in the ordered structures. Thus, it is found that the length of n-alkane molecules effects the orientation of them on graphene. At lower temperature, the short n-alkane molecules are perpendicular to the surface. But long n-alkanes are parallel to the surface and form multi-layer structures. At relatively higher temperature, the nalkanes become disordered on the surface.
2. Simulation details To obtain the relation between the length of n-alkanes and the orientation of them on the graphene surface, five kinds of n-alkane molecules n-hextane (C6), n-decane (C10), n-dodecane (C12), n-eicosane (C20), n-triacontane (C30) were chosen in our study. We placed 250C6, 150C10, 130C12, 75C20, 50C30 randomly in a simulation box, respectively. Two parallel graphene surfaces with the lattice parameters x = 51.4 Å, y = 49.2 Å, z = 120 Å, α = β = γ = 90° were built. After that we put the five kinds of n-alkane molecules between the surfaces, respectively. Then NVT MD simulations were performed at relatively high temperature. We deleted one of the graphene surface and got five systems 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA. The initial configurations of the five systems are shown in Fig. 1. The number of carbon atoms in the alkane molecules of the
3.2. The isothermal orientation processes of 150C10/GRA and 75C20/ GRA Our simulations show that short n-alkane molecules prefer to be perpendicular to the surface and long ones prefer to be parallel to the surface. Fig. 3a and b shows the orientation processes of 150C10/GRA (short) and 75C20/GRA (long) at 450 K, respectively. When the simulation begins, both C10 molecules and C20 molecules are disordered on graphene surface. For 150C10/GRA, some C10 molecules are absorbed near the surface and are parallel to the surface and others are still disordered at 400 ps. A C10 molecule is desorbed and appears below the graphene surface due to periodic boundary conditions. At 600 ps, most of C10 molecules form a perpendicular ordered region on the surface, and several C10 molecules appear on the top of the perpendicular ordered region. As the simulation proceeding, more C10 molecules are desorbed and appear below the surface. At the end of the simulation, most of the C10 molecules are still perpendicular to the surface and they are extended and parallel to each other. In addition, the C10 molecules have a hexagonal close-packed structure on the surface,
Fig. 1. The initial configurations of the five systems. (a. 250C6/GRA, b.150C10/GRA, c.130C12/GRA, d.75C20/GRA, e.50C30/GRA). 2
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layer structure and they are extended and parallel to each other. The six-layer structure becomes more regular at the end of the simulation. Almost all C20 molecules in six layers are extended and parallel to each other. The orientation starts in the first layer near the graphene surface and then in the next layer. This reflects the effect of graphene on the orientation of C20 molecules. C20 molecules require about 1500 ps MD simulations to orient laterally to the surface and C10 molecules require about 600 ps MD simulations to orient normally to the surface at 450 K. The parallel orientation of C20 molecules appears later than the perpendicular orientation of C10 molecules at 450 K resulting from the C20 molecules relaxing more slowly than C10 molecules. To further describe the orientation processes of C10 and C20 molecules on the surface at 450 K, we also calculated the bond-orientation order parameter OPb, average end-to-end distance, local mass density ρ(z) during the two orientation processes. OPb is defined by
OPb =
3 cos2 ϕ − 1 2
(1)
where φ is the angle between the subbond vector and the z axis, and the subbond vector is formed by connecting the centers of two adjacent bonds. The parameter OPb would assume a value of 1.0 for the n-alkanes whose subbonds parallel to z axis. OPb = 0.0 indicates the molecules are laid randomly on the surface and OPb = -0.5 indicates the subbonds of molecules are perfectly perpendicular to the z axis. Fig. 4 shows the evolutions of the bond-orientation order parameter OPb for 150 C10 and 75 C20 molecules on graphene at 450 K. At the beginning of the simulations, both the OPb of C10 and C20 molecules are about −0.1, which means that the molecules are laid randomly on the surface. The OPb of C10 molecules increases to 0.1 before 450 ps and increases to 0.7 quickly from 450 to 700 ps. After that, the OPb of C10 molecules fluctuates around 0.7. This indicates that most of C10 molecules are perpendicular to the graphene plane after 700 ps. The OPb of C20 molecules fluctuates around −0.1 from 1 to 1000 ps and decreases to −0.4 gradually from 1000 to 7000 ps. C20 molecules are disordered from 1 to 1000 ps and gradually arrange into a lateral orientation ordered structure after that. From 7001 to 10,000 ps, the OPb of C20 molecules fluctuates around −0.4, which means that most of C20 molecules are parallel to the surface from 7001 ps to the end of the simulation. This shows two kinds of orientation processes quantitatively. The average end-to-end distance (Red) can also be used to describe the conformation changes of C10 and C20 molecules. When the molecules adopt extended conformation, their average Red is close to the length of extended molecules. But when the molecules adopt gauche conformation, their average Red is much smaller than the length of extended molecules. The Red of extended C6, C10, C12, C20, C30 molecules are 6.3, 11.3, 13.8, 23.8 and 36.4 Å, respectively. Fig. 5 shows the evolutions of the average Red of 150 C10 and 75 C20 molecules at 450 K. It can be seen that the average Red of 150 C10 and 75 C20 molecules are about 9 Å and 15.8 Å when the simulation begins. They are shorter than the extended C10 and C20 molecules, respectively. This means that both C10 and C20 molecules adopt gauche conformation at the beginning of the simulation. The average Red of 150C10 increases slowly to 9.5 Å before 450 ps and increases to 11.5 Å quickly from 450 to 700 ps. Then the average Red of C10 molecules fluctuates around 11.5 Å after 700 ps, which is close to 12.8 Å. This shows that the C10 molecules change from gauche conformation to extended conformation during the orientation process and keep extended conformation in the perpendicular ordered structure after that. For 75C20/GRA, the average Red of C20 molecules fluctuates around 16 Å from 1 to 1000 ps and increases to 19 Å gradually from 1000 to 2000 ps. They are shorter than the extended C20 molecules. This indicates that C20 molecules are curved in the disordered structure. After that, the average Red fluctuates around 19 Å before 5000 ps and increases to 20.5 gradually from 5000 to 9000 ps. This shows that many
Fig. 2. The last conformations of C6, C10, C12, C20, C30 molecules on graphene surface at 400, 450 and 500 K. (a. 250C6/GRA b. 150C10/GRA c. 130C12/GRA d. 75C20/GRA e. 50C30/GRA).
which is similar to the crystal of polyethylene [20,51]. And the C10 molecules between the ordered region and the graphene are extended and parallel to the surface after 400 ps. For 75C20/GRA, some of C20 molecules are also absorbed near the surface and parallel to the surface at 500 ps, which are curved. Others are disordered on the surface. Local orientation region appears at about 1200 ps. The C20 molecules in the first layer and in the second layer are curved at this time. With the simulation proceeding, more C20 molecules are orientated laterally to the surface. At 1500 ps, most of C20 molecules are laid parallel to the surface and form a five-layer structure. More C20 molecules in the first layer are extended and some of them are parallel to each other. Some of the molecules in the second and third layers are extended but not parallel to each other. Few molecules in the fourth and fifth layer are extended. A six-layer structure appears at about 2000 ps. Most of the C20 molecules in the first and second layer are extended and parallel to each other. Some of the C20 molecules in the third and fourth layer are extended and parallel to each other. The amount of the molecules in the top layer is least but all of them are oriented. (We can see them in the figure S1 in the Supporting Information.) With the simulation going, more molecules enter the six3
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Fig. 3. Isothermal relaxation processes of 150C10/GRA (a) and 75C20/GRA (b) at 450 K. The C10 and C20 molecules are denoted in different colors. Left: side view and right: top view. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
Fig. 4. The evolution of bond-orientation order parameter OPb for 150C10/ GRA and 75C20/GRA at 450 K.
Fig. 5. The evolution of the average end-to-end distance (Red) of 150C10/GRA and 75C20/GRA at 450 K. The inset is an enlarged part of the last conformation (1–1000 ps).
C20 molecules become extended before 5000 ps and more molecules adopt extended conformation in the ordered structure from 5000 to 9000 ps. Compared with the evolution of the average Red of C10 molecules, the fluctuate amplitude of the average Red of the C20 molecules is violent because C20 molecule is longer than C10 molecule. From 9001 ps to 10,000 ps, the average Red of C20 molecules fluctuates
around 20 Å. Almost all of C20 molecules are extended after the lateral orientation ordered structure forms on the surface. The orientation processes of C10 and C20 molecules can also be described by local mass density ρ(z). The local mass density ρ(z) shows the mass distribution of molecules at different positions (z). Fig. 6a and 4
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adsorption layers. This is consistent with the results of OPb and Red. There are six peaks appearing at 4.02, 7.52, 11.52, 15.52, 19.52, 24.52 Å at the end of the simulation. This shows that C20 molecules on the surface are an ordered layer-like structure. The distance (D1) between the first peak and second peak is 3.5 and the distance (D2) between the second peak and third peak is 4, which is the same as the distance (D3) between the third peak and fourth peak. The distance (D4) between the fourth peak and fifth peak is 4. The distance (D5) between the fifth peak and sixth peak is 5. It is found that D1 < D2 = D3 = D4 < D5. This is due to the interactions between the C20 molecules and graphene increasing with z decreasing. By analyzing OPb, Red and ρ(z) of C10, C20 molecules, the orientation process of alkane molecules on graphene surface can be divided into three steps: adsorption, orientation and growth. When the simulation begins, the molecules are adsorbed to the graphene surface and n-alkane molecules are not oriented and curved. The Red of them are shorter than the length of the extended molecules and the average OPb of them are close to 0, and the local mass density distributions of them are wide. With the simulation proceeding, the average Red of n-alkane molecules increases quickly, and the average OPb of molecules increases or decreases quickly. The local mass density distributions narrow and the heights of peaks increase. The orientations of n-alkane molecules happen. The average Red of them are close to extended molecules and the average OPb of molecules are about 1 or −0.5. After the orientation structure is formed, the regularity of the orientation structure increases with the time increasing. The Red, OPb and local mass density distribution of molecules and the heights of peaks change little. 3.3. Effect of chain length on the orientation of n-alkanes The n-alkanes molecules are ordered orientation on graphene at lower temperature and form a disordered structure at relative high temperature. The average bond-orientation order parameter OPb and end-to-end distance (Red) of C6, C10, C12, C20, C30 molecules at different temperatures are also calculated and shown in Figs. 7 and 8, respectively. The Red of extended C6, C10, C12, C20, C30 molecules are given in the previous section. For 250C6/GRA, the average OPb is 0.7–0.9 in the temperature range of 300–441 K and the average Red is about 6.3 Å, which is equal to the length of extended C6 molecule. This means that most of C6 molecules are extended and perpendicular to the graphene surface. When the temperature is 442 K, the average OPb decreases to −0.1 suddenly
Fig. 6. Local mass density distributions of 150C10/GRA (a) and 75C20/GRA (b) at different simulation time at 450 K. The graphene is at z = 0. The desorbed C10 molecules below the graphene are ignored.
b gives the local mass density of 150C10/GRA and 75C20/GRA at different time at 450 K, respectively. The graphene sheet is at z = 0. For 150C10/GRA and 75C20/GRA, there is a wide distribution of CHx at different z from the surface and two high peaks at the ends of the distribution when the relaxation begins. This is due to the initial configurations were gotten under NVT MD simulations at high temperature between two graphene sheets. It can be seen that there is a high peak near the graphene surface during the orientation simulation, which means that more molecules or segments are adsorbed near the graphene surface. With the simulation proceeding, the local mass density distributions of C10 molecules narrow and the heights of peaks close to the surface increase from 200 to 600 ps. A continuous density distribution appears after 600 ps for C10 molecules. This shows that C10 molecules form a perpendicular ordered structure. For 75C20/GRA, there is a wide continuous distribution of CHx and the local mass density narrows and the heights of the peaks increase from 200 to 1000 ps. A five-peak distribution appears at 1500 ps. C20 molecules form a five-layer structure. At 2000 ps, a six-peak distribution appears and the height of peak decreases with increasing z, indicating that there is a six-layer structure of C20 molecules on the surface and the adsorbed molecules or segments on the top layer are the least, and more molecules or segments are near the surface. After that, the heights of six peaks increase and the heights of five valleys decrease due to more molecules or segments enter the
Fig. 7. The average OPb for the last 1000 configurations of 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA at different temperatures. The error bars are the standard deviation calculated from the average (9001–10,000 ps). 5
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Fig. 8. The average end-to-end distance (Red) of the last 1000 configurations of 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA at different temperatures. The error bars are the standard deviation calculated from the average (9001–10,000 ps).
Fig. 9. The critical temperature of orientation changes as a function of chain length.
temperature of orientation and chain length. Fig. 9 shows the critical temperature of orientation changes as a function of chain length. The longer n-alkanes are, the higher the critical temperature of molecular orientation on graphene surface is. The relaxation time of C6 and C10 molecules is short. They are easy to orient and form an ordered structure at low temperatures. The longer n-alkane is; the longer relaxation time is. For C12, C20 and C30 molecules, they require more time to relax. So they are still disordered at 300 and 350 K. Why the orientations are different for long and short n-alkanes? These could be understood by considering intermolecular or molecule-surface interactions. [7,9,24]
and the average Red also decreases. They both change little from 442 to 500 K. The C6 molecules are curved and disordered on the graphene surface when the temperature is above 442 K. Thus the critical temperature of orientation for 250C6/GRA is approximately 441 K in the simulations. The variation of the average OPb of 150C10/GRA and 130C12/GRA is similar to that of 250C6/GRA. The average OPb of 150C10 molecules is 0.6–0.8 from 300 to 483 K and OPb of 130C12 molecules is about 0.8 from 400 to 489 K. And the average Red of 150C10 and 130C12 molecules is close to the length of extended C10 molecule (11.3 Å) and C12 molecule (13.8 Å), respectively. That manifests that most of C10/C12 molecules are perpendicular to the graphene surface and adopt extended conformation in the ordered structure. Differently, the average OPb of 130C12/GRA increases from −0.02 to 0.1 in the temperature range of 300–350 K. C12 molecules are disordered on the surface at these temperatures. This is due to C12 molecules are longer than C10 molecules and require more time to relax. The average OPb of C10 molecules is about −0.05 from 483 to 550 K and the average OPb of C12 molecules is about −0.1 from 489 to 550 K. At the same time, the average Red of C10 and C12 molecules decreases to 9.0 Å and 10 Å, indicating that the disordered structures appear and C10 and C12 molecules adopt gauche conformations at these temperatures. It is found that the critical temperature of orientation for 150C10/GRA and 130C12/GRA is approximately 483 K and 489 K. The critical temperature of orientation of n-alkane increases with chain length increasing. The average OPb of C20 molecules are about −0.15 and 0.05 at 300 and 350 K, respectively. And the average Red of C20 molecules is about 17 Å which is smaller than the length of extended C20 molecules (23.8 Å). This means that C20 molecules are curved and randomly arranged on the surface at low temperatures. The average OPb decreases from 350 to 450 K and is about −0.35 at 450 K and −0.45 at 503 K. And the average Red of C20 molecules increases from 350 to 503 K. This shows that more C20 molecules arrange parallel to the surface and are extended conformations when the temperature is 503 K. At 504 K, OPb is about −0.05 and the average Red decreases suddenly. Both OPb and Red change little after 504 K. This means that C20 molecules are curved and disordered. Hence, the critical temperature of orientation of 75C20/GRA is about 503 K. Similar to 75C20/GRA, the critical temperature of orientation for 50C30/GRA is approximately 514 K. The critical temperature of orientation of 75C20/GRA is lower than that of 50C30/GRA. Obviously, there is a certain relation between the critical
3.4. The effect of the interaction energy on the orientation To understand the influence of the interaction energy on the orientation of n-alkanes, the interaction energies of molecule-surface (Eint) and molecule-molecule (EM−M) were calculated. Here, Eint was defined as Eint = Etot – (Echain +Eplane), where Etot is the potential energy of the n-alkanes and the surface combined after orientation process, and Echain and Eplane are the potential energies of the n-alkane molecules and the surface, respectively. Fig. 10 shows the variation of the interaction energies Eint and EM−M in 250C6/GRA and 50C30/GRA with increasing temperature. For 250C6/GRA, as the temperature increases, EM−M increases and Eint decreases slowly. EM−M are much lower than Eint from 300 to 441 K. The C6 molecules are perpendicular to the surface in this temperature range. At 442 K, the EM−M increases and Eint decreases dramatically. EM−M is as much as Eint from 442 to 500 K when C6 molecules form a disordered structure on the surface. For 50C30/GRA, EM−M changes little from 300 to 400 K and decreases from 400 to 514 K; Eint increases from 300 to 514 K. EM−M is also lower than Eint when the temperature is lower than 514 K. The C30 molecules are parallel to the surface. When C30 molecules form a disordered structure on the surface at 514 K, EM−M and Eint increases suddenly. EM−M and Eint of 150C10/GRA, 130C12/GRA and 75C20/ GRA changes similarly (Figure S2 in the Supporting Information). In addition, the Eint of 250C6 molecules is about −500 kcal/mol and the EM−M of 250C6 molecules is −4500–4000 kcal/mol when C6 molecules are perpendicular to the surface. The Eint of 50C30 molecules is about −600 kcal/mol and the EM−M of 50C30 molecules is −2500 kcal/mol when the C30 molecules are parallel to the surface. The difference between Eint and EM−M for 250C6 molecules is about 3500–4000 kcal/mol and the Eint–EM−M of 50C30 molecules is about 1900 kcal/mol. Hence, it can be speculated that the orientation of nalkanes is related to the Eint–EM−M. Fig. 11 shows the differences 6
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that of n-alkanes parallel to the surface such as C20 and C30 molecules. Note that, the C6 molecules are disordered on the surface at 450 K, so Eint–EM−M of C6 molecules is about −84 kcal/mol. In summary, the orientation of n-alkanes on the surface is effected by Eint and EM−M. EM−M plays an important role to make molecules close and parallel to each other. When EM−M are much lower than Eint, the n-alkanes can orient on the surface. In our simulations, n-alkanes are perpendicular to the surface when Eint–EM−M is about 4000 kcal/ mol and are parallel to the surface when Eint–EM−M is about 2000 kcal/ mol. When EM−M are close to Eint, the n-alkanes form a disordered structure. This was consistent with the results of previous studies of orientation of amorphous molecules on surface. [24,36] While the ‘experiment’ of n-alkanes melt orientation is different to the organic chain molecules orientation during vapor deposition, the driving forces here is different to the thermodynamics and kinetic perspectives of organic molecules orientation on surface by vapor deposition. [37,39] 4. Conclusions We performed all-atom molecular dynamics simulations of five kinds of n-alkanes on graphene surface. Our results showed that the short molecules are perpendicular to the surface, and the long molecules are parallel to the surface and form a layer-like structure. The orientation process of n-alkanes on the surface were also discussed by analyzing OPb, Red and ρ(z) of C10, C20 molecules and can be divided into three steps: adsorption, orientation and growth. In the first step, the molecules are adsorbed to the graphene surface and they are a disordered structure. In the second step, local orientations of n-alkane molecules appear and more molecules enter the ordered region quickly. In the third step, the orientation structure form and the regularity of the orientation structure increases with the time increasing. There is a certain relation between chain length and the critical temperature of the orientation of n-alkane molecules. The longer the chain is, the higher the critical temperature of the orientation is. In addition, the interaction energies of EM−M and Eint control the orientation of n-alkane on graphene. When Eint.-EM−M is big, the n-alkanes are perpendicular to the graphene surface; when Eint.-EM−M is relatively small, the n-alkanes are parallel to the graphene surface; when EM−M is close to Eint, the molecules are disordered on the surface. Our simulations give more valuable insights into the mechanism of the lateral and normal orientation formations of n-alkanes on graphene surface. The alkane-alkane and alkane-graphene interactions are important in understanding the orientation of n-alkane molecules confined on graphene. More research is needed to further clarify the influence of the kinds of interactions on the orientation in the future.
Fig. 10. The variation of the molecule-surface interaction energies (Eint) and intermolecular interaction energy (EM−M) for two systems (a. 250C6/GRA b. 50C30/GRA) with increasing temperature.
Acknowledgments This work is supported by the Program for Innovative Research Team in University of Tianjin [grant number TD13-5074] and the young and middle-aged teacher of Tianjin Normal University [52XC1201]. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2019.121468. References Fig. 11. The difference between Eint and EM−M of five kinds of n-alkanes on graphene surface at 400 and 450 K.
[1] Y. Shao, T.W. Zerda, Phase transitions of liquid crystal PAA in confined geometries, J. Phys. Chem. B 102 (1998) 3387–3394. [2] Y. Sagara, T. Kato, Stimuli-responsive luminescent liquid crystals: change of photoluminescent colors triggered by a shear-induced phase transition, Angew. Chem. Int. Edit. 47 (2008) 5175–5178. [3] S. Singh, H. Singh, T. Karthick, P. Tandon, V. Prasad, Phase transition analysis of Vshaped liquid crystal: combined temperature-dependent FTIR and density
between Eint and EM−M for the five systems at 400 and 450 K. It is found that for n-alkanes perpendicular to the surface such as C6, C10 and C12 molecules, Eint–EM−M is about 4000 kcal/mol, which much bigger than 7
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Surface Science journal homepage: www.elsevier.com/locate/susc
Molecular dynamics simulations on the orientation of n-alkanes with different lengths on graphene
T
⁎
Liu Yan Fanga, Yang Huaa, , Zhang Zhi Menga, Zhang Huib a Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of Chemistry, Tianjin Normal University, Tianjin 300387, People's Republic of China b School of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molecular dynamics simulation Graphene n-alkanes Orientation
The orientation of molecules on surface is very important in the design of nanomaterials. Molecular dynamics simulation was used to study the mechanism of n-alkane molecules orientation on graphene surface. The chain length effects the orientation of n-alkanes on graphene surface. Short n-alkanes are perpendicular to the graphene surface and the long n-alkanes are laid parallel to the surface. The two kinds of orientation processes are shown directly and are considered as a three-step process (adsorption, orientation, and growth). The critical temperatures of the orientation of n-alkanes on graphene surface increases with chain length increasing. In addition, the simulation results show the interactions of n-alkane-n-alkane and n-alkane-graphene playing a key role in the orientation of n-alkanes on graphene surface.
1. Introduction Liquid crystals, are a state of matter which has properties (fluidity, anisotropy, etc) between those of conventional liquid and those of solid crystals. Liquid crystal materials consist most of organic molecules such as aliphatic, aromatic and stearic acids, etc. The study of liquid crystal has become popular in the past few years because it plays an important role in the materials science [1–3]. For example, the liquid crystal display technology has promoted the technological development of microelectronics and optoelectronic information. The orientation of molecules in liquid crystals is an important field of studying liquid crystals. The surface has a great influence on the orientation of molecules on it [4–6]. Graphene plays a key role in the study of molecules adsorbed on a surface because it has good mechanical properties and good symmetry. Due to the complexity of studying the orientation of aliphatic, aromatic and stearic acids, we can focus first on the orientation of much simpler physical systems: n-alkanes on graphene. This can help us better analyze and understand the orientation process of molecules in liquid crystals deeply. In recent years, the study of the thermodynamic properties of molecules physically adsorbed on substrates has become a very active field. The structures and behaviors of molecules on a surface have been investigated both experimentally and computationally [7–36]. Masnadi and Urquhart [7] studied the effect of substrate temperature on the epitaxial growth of oriented n-alkane thin films on graphite, confirming ⁎
that molecular orientation depends on the evaporation conditions (substrate temperature, substrate identity, and evaporation rate) and chain length. They found that the coexisting lateral and normal orientation emerges at higher temperature. The morphology and molecular orientation of linear alkanes on NaCl (001) surface were investigated by NEXAFS spectroscopy and microscopy [9]. The strong interaction between the longer chain molecules and surface is responsible for the lateral orientation in the investigated substrate temperature range. When the interaction between shorter molecules and the surface is weaker, the substrate temperature plays an important role in defining the normal molecular orientation. Pint et al. [16] referred that the importance of the competition between molecule-molecule and molecule-substrate interactions leading to the molecular rolling and tilting. Firlej and his coworkers [17] found that the electrostatic force plays an important role in the process of melting by analyzing the dynamic behavior of a tetracosane (C24H50) monolayer adsorbed on graphite. Molecular simulations were performed to study the melting of hextane monolayers adsorbed on graphite by Wexler et al. [18] They found that the monolayer melts directly from the herringbone solid to a fluid with residual local order reflecting the graphite substrate's symmetry. And the size of reorienting domains decreases with increasing temperature. Roth et al. [23] found that the neighboring layers and confinement effects determine the amount of gauche defects during the melting of alkane C24H50 bilayer and trilayer systems adsorbed on graphite. Whether in the bilayer or in the trilayer, the lower layer
Corresponding author. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.susc.2019.121468 Received 27 March 2019; Received in revised form 2 July 2019; Accepted 22 July 2019 Available online 22 July 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved.
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simulation system is about 1500. Periodic boundary conditions were utilized. The parameter z was enlarged to 120 Å to ensure that the interactions between the adsorbed n-alkanes and the periodic images of graphene in the top plan could be ignored. The three-dimensional periodicity inherent in the model was transformed into actual two-dimensional periodicity thus simulating an infinity extended surface. The COMPASS [43,44] force field has been successfully used in simulating the interaction between alkane/polyethylene and the carbon nanotube/graphite/graphene [45–48]. The five n-alkanes/graphene systems were described by the COMPASS force field. The models relaxed by 50,000 steps firstly. The resultant structures were subjected to subsequent 10,000 ps NVT MD simulations in the temperature range of 300–550 K. The integration time step of 1 fs was used. Nose–Hoover [49,50] method was applied to control the temperature. The cut off distance for the nonbonded interaction was taken to be 12.5 Å.
exhibits fewer defects than the monolayer at the same temperature, and the upper layer exhibits more. These mentioned studies are of great significance for the study of molecules orientation on surface. However, none of them studied the influences of molecular length on the orientation of n-alkanes on a solid surface directly, which is also important in the application of liquid crystals materials. It is necessary to understand the formation mechanism of n-alkanes thin film in order to control the film structure precisely. Number of researches [37–42] has been done on the orientation of organic molecules on surface by vapor deposition method. The orientation processes were observed by a quartz crystal microbalance technique. Kinetic and thermodynamics models have been proposed to explain the mechanism of molecules orientation on surface by vapor deposition. Both models can describe the temperature dependence of linear organic compounds orientation in vapor-deposited film. In our previous study, we presented the results of the isothermal orientation of n-decanes melt confined between graphene sheets by molecular dynamics simulations [24]. The n-decanes are normally oriented to the basal plane of graphene at low temperatures. With increase in temperature, the n-decanes are laterally oriented to the surface. When temperature is high enough, the n-decanes form a disordered structure. In this study, we continue to study the orientation of n-alkanes melt on graphene surface by means of molecular dynamics (MD) simulation. The effect of chain length on the orientation of n-alkane molecules will be shown in detail. The united atom model [16,21] was adopted to simplify the calculations in most of previous works. But they neglected the effect of hydrogen and electrostatic interaction in the simulations [17,21]. To compensate for the deficiencies, the all-atom description of n-alkane molecules is used in our simulations. This paper is organized as follows: First, the last conformations of n-alkane molecules at different temperatures will be described; second, the orientation processes of n-alkanes on the graphene surface will be shown; Third, the effect of chain length on the orientation of n-alkane molecules will be explored; Finally, the effect of alkane-alkane interaction and alkane-surface interaction on the orientation of molecules will be discussed.
3. Results and discussion 3.1. Last conformations of n-alkanes on graphene surface In order to explore the effect of the chain length on the orientation of n-alkane molecules on graphene surface, 10,000 ps NVT MD simulations were performed at different temperatures for the five systems. Fig. 2 displays the last conformations of C6, C10, C12, C20, C30 molecules on graphene surface at 400, 450, 500 K. They can form three kinds of structures: parallel multilayer structure, perpendicular ordered structure, disordered structure. At 400 K, C6, C10, C12 molecules are all perpendicular to the surface. And C6 molecules form two ordered regions. Some C6 molecules are desorbed from the surface and appear below the surface due to periodic boundary conditions. Most of C20, C30 molecules are parallel orientation on the surface and the molecules in the ordered region are extended and not always parallel to each other. At 450 K, the C6 molecules are disordered on the surface and more C6 molecules are desorbed. Most of C10, C12 molecules are perpendicular to the surface. Some of C10 molecules are also desorbed. More C20, C30 molecules arrange parallel to the surface and they are parallel to each other. They form a multi-layer structures. At 500 K, the short n-alkanes (C6, C10, C12 molecules) form disordered structures on the surface. The number of C6 molecules desorbed is bigger than that of C10 molecules. C20, C30 molecules are still parallel to the surface and almost all arrange in the ordered structures. Thus, it is found that the length of n-alkane molecules effects the orientation of them on graphene. At lower temperature, the short n-alkane molecules are perpendicular to the surface. But long n-alkanes are parallel to the surface and form multi-layer structures. At relatively higher temperature, the nalkanes become disordered on the surface.
2. Simulation details To obtain the relation between the length of n-alkanes and the orientation of them on the graphene surface, five kinds of n-alkane molecules n-hextane (C6), n-decane (C10), n-dodecane (C12), n-eicosane (C20), n-triacontane (C30) were chosen in our study. We placed 250C6, 150C10, 130C12, 75C20, 50C30 randomly in a simulation box, respectively. Two parallel graphene surfaces with the lattice parameters x = 51.4 Å, y = 49.2 Å, z = 120 Å, α = β = γ = 90° were built. After that we put the five kinds of n-alkane molecules between the surfaces, respectively. Then NVT MD simulations were performed at relatively high temperature. We deleted one of the graphene surface and got five systems 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA. The initial configurations of the five systems are shown in Fig. 1. The number of carbon atoms in the alkane molecules of the
3.2. The isothermal orientation processes of 150C10/GRA and 75C20/ GRA Our simulations show that short n-alkane molecules prefer to be perpendicular to the surface and long ones prefer to be parallel to the surface. Fig. 3a and b shows the orientation processes of 150C10/GRA (short) and 75C20/GRA (long) at 450 K, respectively. When the simulation begins, both C10 molecules and C20 molecules are disordered on graphene surface. For 150C10/GRA, some C10 molecules are absorbed near the surface and are parallel to the surface and others are still disordered at 400 ps. A C10 molecule is desorbed and appears below the graphene surface due to periodic boundary conditions. At 600 ps, most of C10 molecules form a perpendicular ordered region on the surface, and several C10 molecules appear on the top of the perpendicular ordered region. As the simulation proceeding, more C10 molecules are desorbed and appear below the surface. At the end of the simulation, most of the C10 molecules are still perpendicular to the surface and they are extended and parallel to each other. In addition, the C10 molecules have a hexagonal close-packed structure on the surface,
Fig. 1. The initial configurations of the five systems. (a. 250C6/GRA, b.150C10/GRA, c.130C12/GRA, d.75C20/GRA, e.50C30/GRA). 2
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layer structure and they are extended and parallel to each other. The six-layer structure becomes more regular at the end of the simulation. Almost all C20 molecules in six layers are extended and parallel to each other. The orientation starts in the first layer near the graphene surface and then in the next layer. This reflects the effect of graphene on the orientation of C20 molecules. C20 molecules require about 1500 ps MD simulations to orient laterally to the surface and C10 molecules require about 600 ps MD simulations to orient normally to the surface at 450 K. The parallel orientation of C20 molecules appears later than the perpendicular orientation of C10 molecules at 450 K resulting from the C20 molecules relaxing more slowly than C10 molecules. To further describe the orientation processes of C10 and C20 molecules on the surface at 450 K, we also calculated the bond-orientation order parameter OPb, average end-to-end distance, local mass density ρ(z) during the two orientation processes. OPb is defined by
OPb =
3 cos2 ϕ − 1 2
(1)
where φ is the angle between the subbond vector and the z axis, and the subbond vector is formed by connecting the centers of two adjacent bonds. The parameter OPb would assume a value of 1.0 for the n-alkanes whose subbonds parallel to z axis. OPb = 0.0 indicates the molecules are laid randomly on the surface and OPb = -0.5 indicates the subbonds of molecules are perfectly perpendicular to the z axis. Fig. 4 shows the evolutions of the bond-orientation order parameter OPb for 150 C10 and 75 C20 molecules on graphene at 450 K. At the beginning of the simulations, both the OPb of C10 and C20 molecules are about −0.1, which means that the molecules are laid randomly on the surface. The OPb of C10 molecules increases to 0.1 before 450 ps and increases to 0.7 quickly from 450 to 700 ps. After that, the OPb of C10 molecules fluctuates around 0.7. This indicates that most of C10 molecules are perpendicular to the graphene plane after 700 ps. The OPb of C20 molecules fluctuates around −0.1 from 1 to 1000 ps and decreases to −0.4 gradually from 1000 to 7000 ps. C20 molecules are disordered from 1 to 1000 ps and gradually arrange into a lateral orientation ordered structure after that. From 7001 to 10,000 ps, the OPb of C20 molecules fluctuates around −0.4, which means that most of C20 molecules are parallel to the surface from 7001 ps to the end of the simulation. This shows two kinds of orientation processes quantitatively. The average end-to-end distance (Red) can also be used to describe the conformation changes of C10 and C20 molecules. When the molecules adopt extended conformation, their average Red is close to the length of extended molecules. But when the molecules adopt gauche conformation, their average Red is much smaller than the length of extended molecules. The Red of extended C6, C10, C12, C20, C30 molecules are 6.3, 11.3, 13.8, 23.8 and 36.4 Å, respectively. Fig. 5 shows the evolutions of the average Red of 150 C10 and 75 C20 molecules at 450 K. It can be seen that the average Red of 150 C10 and 75 C20 molecules are about 9 Å and 15.8 Å when the simulation begins. They are shorter than the extended C10 and C20 molecules, respectively. This means that both C10 and C20 molecules adopt gauche conformation at the beginning of the simulation. The average Red of 150C10 increases slowly to 9.5 Å before 450 ps and increases to 11.5 Å quickly from 450 to 700 ps. Then the average Red of C10 molecules fluctuates around 11.5 Å after 700 ps, which is close to 12.8 Å. This shows that the C10 molecules change from gauche conformation to extended conformation during the orientation process and keep extended conformation in the perpendicular ordered structure after that. For 75C20/GRA, the average Red of C20 molecules fluctuates around 16 Å from 1 to 1000 ps and increases to 19 Å gradually from 1000 to 2000 ps. They are shorter than the extended C20 molecules. This indicates that C20 molecules are curved in the disordered structure. After that, the average Red fluctuates around 19 Å before 5000 ps and increases to 20.5 gradually from 5000 to 9000 ps. This shows that many
Fig. 2. The last conformations of C6, C10, C12, C20, C30 molecules on graphene surface at 400, 450 and 500 K. (a. 250C6/GRA b. 150C10/GRA c. 130C12/GRA d. 75C20/GRA e. 50C30/GRA).
which is similar to the crystal of polyethylene [20,51]. And the C10 molecules between the ordered region and the graphene are extended and parallel to the surface after 400 ps. For 75C20/GRA, some of C20 molecules are also absorbed near the surface and parallel to the surface at 500 ps, which are curved. Others are disordered on the surface. Local orientation region appears at about 1200 ps. The C20 molecules in the first layer and in the second layer are curved at this time. With the simulation proceeding, more C20 molecules are orientated laterally to the surface. At 1500 ps, most of C20 molecules are laid parallel to the surface and form a five-layer structure. More C20 molecules in the first layer are extended and some of them are parallel to each other. Some of the molecules in the second and third layers are extended but not parallel to each other. Few molecules in the fourth and fifth layer are extended. A six-layer structure appears at about 2000 ps. Most of the C20 molecules in the first and second layer are extended and parallel to each other. Some of the C20 molecules in the third and fourth layer are extended and parallel to each other. The amount of the molecules in the top layer is least but all of them are oriented. (We can see them in the figure S1 in the Supporting Information.) With the simulation going, more molecules enter the six3
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Fig. 3. Isothermal relaxation processes of 150C10/GRA (a) and 75C20/GRA (b) at 450 K. The C10 and C20 molecules are denoted in different colors. Left: side view and right: top view. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
Fig. 4. The evolution of bond-orientation order parameter OPb for 150C10/ GRA and 75C20/GRA at 450 K.
Fig. 5. The evolution of the average end-to-end distance (Red) of 150C10/GRA and 75C20/GRA at 450 K. The inset is an enlarged part of the last conformation (1–1000 ps).
C20 molecules become extended before 5000 ps and more molecules adopt extended conformation in the ordered structure from 5000 to 9000 ps. Compared with the evolution of the average Red of C10 molecules, the fluctuate amplitude of the average Red of the C20 molecules is violent because C20 molecule is longer than C10 molecule. From 9001 ps to 10,000 ps, the average Red of C20 molecules fluctuates
around 20 Å. Almost all of C20 molecules are extended after the lateral orientation ordered structure forms on the surface. The orientation processes of C10 and C20 molecules can also be described by local mass density ρ(z). The local mass density ρ(z) shows the mass distribution of molecules at different positions (z). Fig. 6a and 4
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adsorption layers. This is consistent with the results of OPb and Red. There are six peaks appearing at 4.02, 7.52, 11.52, 15.52, 19.52, 24.52 Å at the end of the simulation. This shows that C20 molecules on the surface are an ordered layer-like structure. The distance (D1) between the first peak and second peak is 3.5 and the distance (D2) between the second peak and third peak is 4, which is the same as the distance (D3) between the third peak and fourth peak. The distance (D4) between the fourth peak and fifth peak is 4. The distance (D5) between the fifth peak and sixth peak is 5. It is found that D1 < D2 = D3 = D4 < D5. This is due to the interactions between the C20 molecules and graphene increasing with z decreasing. By analyzing OPb, Red and ρ(z) of C10, C20 molecules, the orientation process of alkane molecules on graphene surface can be divided into three steps: adsorption, orientation and growth. When the simulation begins, the molecules are adsorbed to the graphene surface and n-alkane molecules are not oriented and curved. The Red of them are shorter than the length of the extended molecules and the average OPb of them are close to 0, and the local mass density distributions of them are wide. With the simulation proceeding, the average Red of n-alkane molecules increases quickly, and the average OPb of molecules increases or decreases quickly. The local mass density distributions narrow and the heights of peaks increase. The orientations of n-alkane molecules happen. The average Red of them are close to extended molecules and the average OPb of molecules are about 1 or −0.5. After the orientation structure is formed, the regularity of the orientation structure increases with the time increasing. The Red, OPb and local mass density distribution of molecules and the heights of peaks change little. 3.3. Effect of chain length on the orientation of n-alkanes The n-alkanes molecules are ordered orientation on graphene at lower temperature and form a disordered structure at relative high temperature. The average bond-orientation order parameter OPb and end-to-end distance (Red) of C6, C10, C12, C20, C30 molecules at different temperatures are also calculated and shown in Figs. 7 and 8, respectively. The Red of extended C6, C10, C12, C20, C30 molecules are given in the previous section. For 250C6/GRA, the average OPb is 0.7–0.9 in the temperature range of 300–441 K and the average Red is about 6.3 Å, which is equal to the length of extended C6 molecule. This means that most of C6 molecules are extended and perpendicular to the graphene surface. When the temperature is 442 K, the average OPb decreases to −0.1 suddenly
Fig. 6. Local mass density distributions of 150C10/GRA (a) and 75C20/GRA (b) at different simulation time at 450 K. The graphene is at z = 0. The desorbed C10 molecules below the graphene are ignored.
b gives the local mass density of 150C10/GRA and 75C20/GRA at different time at 450 K, respectively. The graphene sheet is at z = 0. For 150C10/GRA and 75C20/GRA, there is a wide distribution of CHx at different z from the surface and two high peaks at the ends of the distribution when the relaxation begins. This is due to the initial configurations were gotten under NVT MD simulations at high temperature between two graphene sheets. It can be seen that there is a high peak near the graphene surface during the orientation simulation, which means that more molecules or segments are adsorbed near the graphene surface. With the simulation proceeding, the local mass density distributions of C10 molecules narrow and the heights of peaks close to the surface increase from 200 to 600 ps. A continuous density distribution appears after 600 ps for C10 molecules. This shows that C10 molecules form a perpendicular ordered structure. For 75C20/GRA, there is a wide continuous distribution of CHx and the local mass density narrows and the heights of the peaks increase from 200 to 1000 ps. A five-peak distribution appears at 1500 ps. C20 molecules form a five-layer structure. At 2000 ps, a six-peak distribution appears and the height of peak decreases with increasing z, indicating that there is a six-layer structure of C20 molecules on the surface and the adsorbed molecules or segments on the top layer are the least, and more molecules or segments are near the surface. After that, the heights of six peaks increase and the heights of five valleys decrease due to more molecules or segments enter the
Fig. 7. The average OPb for the last 1000 configurations of 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA at different temperatures. The error bars are the standard deviation calculated from the average (9001–10,000 ps). 5
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Fig. 8. The average end-to-end distance (Red) of the last 1000 configurations of 250C6/GRA, 150C10/GRA, 130C12/GRA, 75C20/GRA, 50C30/GRA at different temperatures. The error bars are the standard deviation calculated from the average (9001–10,000 ps).
Fig. 9. The critical temperature of orientation changes as a function of chain length.
temperature of orientation and chain length. Fig. 9 shows the critical temperature of orientation changes as a function of chain length. The longer n-alkanes are, the higher the critical temperature of molecular orientation on graphene surface is. The relaxation time of C6 and C10 molecules is short. They are easy to orient and form an ordered structure at low temperatures. The longer n-alkane is; the longer relaxation time is. For C12, C20 and C30 molecules, they require more time to relax. So they are still disordered at 300 and 350 K. Why the orientations are different for long and short n-alkanes? These could be understood by considering intermolecular or molecule-surface interactions. [7,9,24]
and the average Red also decreases. They both change little from 442 to 500 K. The C6 molecules are curved and disordered on the graphene surface when the temperature is above 442 K. Thus the critical temperature of orientation for 250C6/GRA is approximately 441 K in the simulations. The variation of the average OPb of 150C10/GRA and 130C12/GRA is similar to that of 250C6/GRA. The average OPb of 150C10 molecules is 0.6–0.8 from 300 to 483 K and OPb of 130C12 molecules is about 0.8 from 400 to 489 K. And the average Red of 150C10 and 130C12 molecules is close to the length of extended C10 molecule (11.3 Å) and C12 molecule (13.8 Å), respectively. That manifests that most of C10/C12 molecules are perpendicular to the graphene surface and adopt extended conformation in the ordered structure. Differently, the average OPb of 130C12/GRA increases from −0.02 to 0.1 in the temperature range of 300–350 K. C12 molecules are disordered on the surface at these temperatures. This is due to C12 molecules are longer than C10 molecules and require more time to relax. The average OPb of C10 molecules is about −0.05 from 483 to 550 K and the average OPb of C12 molecules is about −0.1 from 489 to 550 K. At the same time, the average Red of C10 and C12 molecules decreases to 9.0 Å and 10 Å, indicating that the disordered structures appear and C10 and C12 molecules adopt gauche conformations at these temperatures. It is found that the critical temperature of orientation for 150C10/GRA and 130C12/GRA is approximately 483 K and 489 K. The critical temperature of orientation of n-alkane increases with chain length increasing. The average OPb of C20 molecules are about −0.15 and 0.05 at 300 and 350 K, respectively. And the average Red of C20 molecules is about 17 Å which is smaller than the length of extended C20 molecules (23.8 Å). This means that C20 molecules are curved and randomly arranged on the surface at low temperatures. The average OPb decreases from 350 to 450 K and is about −0.35 at 450 K and −0.45 at 503 K. And the average Red of C20 molecules increases from 350 to 503 K. This shows that more C20 molecules arrange parallel to the surface and are extended conformations when the temperature is 503 K. At 504 K, OPb is about −0.05 and the average Red decreases suddenly. Both OPb and Red change little after 504 K. This means that C20 molecules are curved and disordered. Hence, the critical temperature of orientation of 75C20/GRA is about 503 K. Similar to 75C20/GRA, the critical temperature of orientation for 50C30/GRA is approximately 514 K. The critical temperature of orientation of 75C20/GRA is lower than that of 50C30/GRA. Obviously, there is a certain relation between the critical
3.4. The effect of the interaction energy on the orientation To understand the influence of the interaction energy on the orientation of n-alkanes, the interaction energies of molecule-surface (Eint) and molecule-molecule (EM−M) were calculated. Here, Eint was defined as Eint = Etot – (Echain +Eplane), where Etot is the potential energy of the n-alkanes and the surface combined after orientation process, and Echain and Eplane are the potential energies of the n-alkane molecules and the surface, respectively. Fig. 10 shows the variation of the interaction energies Eint and EM−M in 250C6/GRA and 50C30/GRA with increasing temperature. For 250C6/GRA, as the temperature increases, EM−M increases and Eint decreases slowly. EM−M are much lower than Eint from 300 to 441 K. The C6 molecules are perpendicular to the surface in this temperature range. At 442 K, the EM−M increases and Eint decreases dramatically. EM−M is as much as Eint from 442 to 500 K when C6 molecules form a disordered structure on the surface. For 50C30/GRA, EM−M changes little from 300 to 400 K and decreases from 400 to 514 K; Eint increases from 300 to 514 K. EM−M is also lower than Eint when the temperature is lower than 514 K. The C30 molecules are parallel to the surface. When C30 molecules form a disordered structure on the surface at 514 K, EM−M and Eint increases suddenly. EM−M and Eint of 150C10/GRA, 130C12/GRA and 75C20/ GRA changes similarly (Figure S2 in the Supporting Information). In addition, the Eint of 250C6 molecules is about −500 kcal/mol and the EM−M of 250C6 molecules is −4500–4000 kcal/mol when C6 molecules are perpendicular to the surface. The Eint of 50C30 molecules is about −600 kcal/mol and the EM−M of 50C30 molecules is −2500 kcal/mol when the C30 molecules are parallel to the surface. The difference between Eint and EM−M for 250C6 molecules is about 3500–4000 kcal/mol and the Eint–EM−M of 50C30 molecules is about 1900 kcal/mol. Hence, it can be speculated that the orientation of nalkanes is related to the Eint–EM−M. Fig. 11 shows the differences 6
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that of n-alkanes parallel to the surface such as C20 and C30 molecules. Note that, the C6 molecules are disordered on the surface at 450 K, so Eint–EM−M of C6 molecules is about −84 kcal/mol. In summary, the orientation of n-alkanes on the surface is effected by Eint and EM−M. EM−M plays an important role to make molecules close and parallel to each other. When EM−M are much lower than Eint, the n-alkanes can orient on the surface. In our simulations, n-alkanes are perpendicular to the surface when Eint–EM−M is about 4000 kcal/ mol and are parallel to the surface when Eint–EM−M is about 2000 kcal/ mol. When EM−M are close to Eint, the n-alkanes form a disordered structure. This was consistent with the results of previous studies of orientation of amorphous molecules on surface. [24,36] While the ‘experiment’ of n-alkanes melt orientation is different to the organic chain molecules orientation during vapor deposition, the driving forces here is different to the thermodynamics and kinetic perspectives of organic molecules orientation on surface by vapor deposition. [37,39] 4. Conclusions We performed all-atom molecular dynamics simulations of five kinds of n-alkanes on graphene surface. Our results showed that the short molecules are perpendicular to the surface, and the long molecules are parallel to the surface and form a layer-like structure. The orientation process of n-alkanes on the surface were also discussed by analyzing OPb, Red and ρ(z) of C10, C20 molecules and can be divided into three steps: adsorption, orientation and growth. In the first step, the molecules are adsorbed to the graphene surface and they are a disordered structure. In the second step, local orientations of n-alkane molecules appear and more molecules enter the ordered region quickly. In the third step, the orientation structure form and the regularity of the orientation structure increases with the time increasing. There is a certain relation between chain length and the critical temperature of the orientation of n-alkane molecules. The longer the chain is, the higher the critical temperature of the orientation is. In addition, the interaction energies of EM−M and Eint control the orientation of n-alkane on graphene. When Eint.-EM−M is big, the n-alkanes are perpendicular to the graphene surface; when Eint.-EM−M is relatively small, the n-alkanes are parallel to the graphene surface; when EM−M is close to Eint, the molecules are disordered on the surface. Our simulations give more valuable insights into the mechanism of the lateral and normal orientation formations of n-alkanes on graphene surface. The alkane-alkane and alkane-graphene interactions are important in understanding the orientation of n-alkane molecules confined on graphene. More research is needed to further clarify the influence of the kinds of interactions on the orientation in the future.
Fig. 10. The variation of the molecule-surface interaction energies (Eint) and intermolecular interaction energy (EM−M) for two systems (a. 250C6/GRA b. 50C30/GRA) with increasing temperature.
Acknowledgments This work is supported by the Program for Innovative Research Team in University of Tianjin [grant number TD13-5074] and the young and middle-aged teacher of Tianjin Normal University [52XC1201]. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2019.121468. References Fig. 11. The difference between Eint and EM−M of five kinds of n-alkanes on graphene surface at 400 and 450 K.
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