Molecular simulation method of low temperature thermal dynamics behavior of wax crystals in waxy asphalt and medium

Abstract

The present application belongs to the technical field of molecular simulation, and particularly relates to a molecular simulation method and medium for low-temperature thermal dynamics behavior of wax crystals in waxy asphalt, comprising the following steps: constructing model wax molecules, unaged asphalt molecules and aged asphalt molecules; selecting a molecular force field; performing geometric modeling on the model wax molecules, unaged asphalt molecules and aged asphalt molecules, and establishing a molecular dynamics model; simulating van der Waals interaction and electrostatic interaction; optimizing based on minimization of model energy; performing equilibrium simulation under isothermal-isobaric ensemble and canonical ensemble; and calculating thermal dynamics parameters of the waxy asphalt. Compared with the prior art, the present application solves the defect in the prior art that there is a lack of explanation of the interaction mechanism of wax and asphalt from the molecular level and the influence of oxidative aging on the interaction. The present scheme realizes in-depth molecular explanation of the low-temperature macroscopic rheology and microscopic experimental results of the existing waxy asphalt.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of molecular simulation, and particularly relates to a molecular simulation method and medium for low-temperature thermal dynamics behavior of wax crystals in waxy asphalt. BACKGROUND

[0002] Asphalt pavement is widely used due to its high performance and driving comfort, and the performance of asphalt pavement is highly related to the performance of asphalt. Asphalt is obtained by distillation of crude oil and usually contains various wax components, which is called "waxy asphalt". According to the Strategic Highway Research Program (SHRP) of the United States and Canada, the wax content of waxy asphalt is between 1% and 5%. Therefore, waxy asphalt is a commonly used binder in pavement engineering.

[0003] Most researchers have conducted a large number of studies on the performance of waxy asphalt at low temperatures. In cold regions, wax crystallization can cause physical hardening, thereby reducing the low-temperature anti-cracking performance of waxy asphalt, but the degree of physical hardening is not always related to the wax content. Therefore, it is necessary to consider physical hardening when determining the low-temperature performance grade (PG) of waxy asphalt. Interestingly, some researchers have considered the effect of oxidative aging on the physical hardening of four typical low temperatures, and concluded that long-term aging can reduce the physical hardening of waxy asphalt and waxy asphalt at most temperatures. On the other hand, the honeycomb structure on the surface of asphalt was observed at room temperature and low temperature by atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM), and wax provided evidence for the honeycomb structure. In addition, oxidative aging can increase the size of wax crystals and reduce the number of crystals.

[0004] In addition, various chemical, mechanical and morphological methods have been used to reveal the potential mechanisms of asphalt aging, modification, damage and regeneration characteristics, which have certain reference significance for the evaluation of asphalt performance. However, these macroscopic evaluation methods have limitations in understanding the internal mechanism of asphalt materials, including the oxidation reaction mechanism of asphalt molecules during oxidative aging, the intermolecular interaction between different modifiers and asphalt molecules, the influence of aging and modification on the thermodynamic parameters of asphalt, the atomic scale mechanism of adhesive failure, the diffusion behavior of oxygen molecules in asphalt and asphalt mixture, and the self-repairing mechanism of asphalt at the molecular scale. These problems are difficult to reveal only by macroscopic tests.

[0005] To overcome the shortcomings of traditional macroscopic tests, molecular dynamics (MD) simulation can reduce the need for unnecessary experimental research and expensive microscopic tests to predict results. In particular, the simulated environment such as temperature and pressure is more controllable than laboratory tests. Based on this, MD simulation can ultimately explain the potential mechanism of experimental results from the perspective of nanoscale. At present, the MD simulation method has been widely applied to asphalt materials to understand the interaction between asphalt molecules and additives and further predict the key thermodynamic properties of additives. The existing method has determined the macroscopic rheology of waxy asphalt at low temperature, such as the patent with the patent number CN114591583A, which discloses an anti-thermal reversible aging asphalt wax inhibitor and asphalt and a preparation method thereof, wherein the anti-thermal reversible aging asphalt wax inhibitor is a nano-silicon dioxide hybrid vinyl acetate copolymer. The patent with the patent number CN115368747A discloses a dispersant for improving the low-temperature performance of waxy asphalt, asphalt thereof, and a preparation method thereof, wherein the dispersant is a vinyl graphene oxide modified ionic liquid nanocomposite. In addition, the existing literature has determined the low-temperature physical hardening behavior of waxy asphalt and the effect of temperature on physical hardening (Zhang, H., Soenen, H., Pipintakos, G., Blom, J., Abadeen, A. Z. U., Qiu, Y., & Van den Bergh, W. (2024). Exploring physical hardening in bitumen based on 4mm DSR measurements. Materials and Structures, 57(7), 149). Although the above test method shows that the wax component has a significant effect on the low-temperature performance of waxy asphalt, especially the physical hardening behavior, there is still no systematic explanation of the interaction mechanism between wax and asphalt from the molecular level and the effect of oxidative aging on the interaction. SUMMARY

[0006] The present application aims to solve at least one of the above problems by providing a molecular simulation method for the low-temperature thermal dynamics behavior of wax crystals in waxy asphalt and a medium, to solve the defect that the existing technology lacks an explanation of the interaction mechanism between wax and asphalt from the molecular level and the effect of oxidative aging on the interaction. The present scheme realizes in-depth mechanistic explanation of the existing low-temperature macroscopic rheology and microscopic experimental results of waxy asphalt at the molecular scale, and provides important reference value and research means for inhibiting the low-temperature physical hardening of waxy asphalt to avoid premature cracking of asphalt pavement.

[0007] The object of the present application is achieved by the following technical solutions:

[0008] The first aspect of the present application discloses a molecular simulation method for the low-temperature thermal dynamics behavior of wax crystals in waxy asphalt, comprising the following steps:

[0009] T1: constructing model wax molecules, unaged asphalt molecules, and aged asphalt molecules;

[0010] T2: selecting a molecular force field;

[0011] T3-1: geometrically modeling the model wax molecules, the unaged asphalt molecules, and the aged asphalt molecules, and establishing a molecular dynamics model;

[0012] T3-2: simulating van der Waals interactions and electrostatic interactions;

[0013] T3-3: optimizing based on minimization of model energy;

[0014] T3-4: performing equilibrium simulations under isothermal-isobaric ensembles and canonical ensembles;

[0015] T4: calculating thermodynamic parameters of waxy asphalts.

[0016] Preferably, in step T1, the model wax molecules are pure wax molecules with straight-chain saturated carbon chain structures; the unaged asphalt molecules are twelve-component models including saturates, aromatics, resins, and asphaltenes; and the aged asphalt molecules are formed by adding carbonyl and sulfoxide functional groups to the unaged asphalt molecules.

[0017] Preferably, the model wax molecules include C18, C24, and C40 pure wax molecules; the saturates include fatty chain structures with branches and naphthenes; the aromatics include carbon-hydrogen compound structures with light molecular weights and minimum polarity in asphalt systems; the resins include stabilizers of asphaltenes; and the asphaltenes include components with the largest particle sizes and the strongest polarity in asphalt systems.

[0018] Preferably, in step T1, the aged asphalt molecules are obtained by short-term and / or long-term aging of the unaged asphalt molecules through a rotary thin-film oven aging test and / or a pressure aging test.

[0019] Preferably, in step T2, the molecular force field includes a COMPASS II force field.

[0020] Preferably, in step T3-2, the van der Waals interactions are simulated by a cubic spline method based on atomic segment distances, and the electrostatic interactions are simulated by an Ewald-based summation method.

[0021] Preferably, in step T3-3, the optimization includes geometric optimization and annealing simulation.

[0022] Preferably, in step T4, the waxy asphalt is a molecular model established according to a 3% weight ratio of the model wax molecules to the unaged asphalt molecules and the aged asphalt molecules.

[0023] Preferably, in step T4, the thermodynamic parameters include solubility parameters, free volume fractions of the model wax molecules, unaged asphalt molecules and aged asphalt molecules, and diffusion coefficients and radial distribution functions of the model wax molecules.

[0024] The second aspect of the present application discloses a computer storage medium comprising a stored program, wherein the program, when executed, controls a device in which the computer storage medium is located to perform the method of any one of the above.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] The present application provides a molecular simulation method of low-temperature anti-cracking and toughening wax crystal low-temperature thermodynamic behavior of wax-containing asphalt based on asphalt pavement, and establishes a molecular model of unaged and aged asphalt mixed with model wax considering the influence of temperature. On this basis, by calculating the solubility parameters, free volume fractions, diffusion coefficients and radial distribution functions and other parameters, the existing low-temperature macro-rheological and micro-experimental results of wax-containing asphalt are provided with in-depth machine interpretation at the molecular scale, and important reference value and research means are provided for inhibiting the low-temperature physical hardening of wax-containing asphalt to avoid premature cracking of asphalt pavement. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a flowchart of the molecular simulation method of the present application;

[0028] Figure 2 It is a structural schematic diagram of the model wax molecule, wherein gray: carbon atom, white: hydrogen atom;

[0029] Figure 3 It is a structural schematic diagram of the unaged asphalt molecule, wherein gray: carbon atom, white: hydrogen atom, red: oxygen atom, blue: nitrogen atom, yellow: sulfur atom;

[0030] Figure 4 It is a structural schematic diagram of the aged asphalt molecule, wherein gray: carbon atom, white: hydrogen atom, red: oxygen atom, blue: nitrogen atom, yellow: sulfur atom;

[0031] Figure 5 It is a molecular model of three kinds of molecular asphalt molecules and model wax molecules;

[0032] Figure 6 It is the difference of solubility parameters between the model wax molecules and the asphalt system;

[0033] Figure 7aThe free volume distribution of unaged asphalt molecules at different temperatures under different probe radii (free volume: blue, occupied volume: gray, gap: white);

[0034] Figure 7b The influence of model wax molecules on the free volume fraction of the asphalt system at different temperatures;

[0035] Figure 8a The MSD value of three model wax molecules in the asphalt system changes with the simulation time;

[0036] Figure 8b The diffusion coefficient value of three model wax molecules in the asphalt system at different temperatures;

[0037] Figure 9 The radial distribution function of three model wax molecules in the asphalt system under different temperature conditions. DETAILED DESCRIPTION

[0038] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the present application will be described in detail below in combination with the drawings and specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, not all. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in combination with the drawings is not intended to limit the protection scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The present application will be further described below in combination with the drawings.

[0039] The unfinished matters in the following description can be solved by using conventional methods or prior art in the art.

[0040] The present application aims to establish a molecular model of C18, C24 and C40 model wax unaged, short-term and long-term aged asphalt considering the temperature influence, aiming at the above-mentioned deficiencies of the prior art. On this basis, by calculating the solubility parameter, free volume fraction, diffusion coefficient and radial distribution function and other parameters, the low-temperature macroscopic rheology and microscopic experimental results of the existing wax-containing asphalt are provided with in-depth machine interpretation at the molecular scale, and important reference value and research means are provided for inhibiting the low-temperature physical hardening of wax-containing asphalt to avoid premature cracking of asphalt pavement.

[0041] In order to achieve the above-mentioned purposes, the present application adopts the following technical solutions:

[0042] The present application provides a molecular simulation method for low-temperature thermodynamic behavior of wax crystals in wax-containing asphalt, comprising the following specific steps:

[0043] S1, Establishment of molecular model

[0044] S11, Pure wax model

[0045] Pure wax molecules were selected as the molecular model representatives of different chain length model waxes. The model wax molecules were all straight-chain saturated carbon chain structures.

[0046] S12, Unaged asphalt model

[0047] The unaged asphalt molecule was considered to be a system composed of twelve components, consisting of saturated components, aromatic components, resins, and asphaltene components.

[0048] S13, Aged asphalt model

[0049] The carbonyl and sulfoxide functional groups were added to the unaged asphalt molecular structure to form the corresponding aged asphalt molecular model. Among them, the molecular structure of the saturated component of the asphalt does not change with oxidation aging.

[0050] S14, Waxy asphalt model

[0051] Referring to step S11, the asphalt model refers to steps S12 and S13, and the model wax molecule is established according to the 3% weight ratio of unaged and aged asphalt to establish the molecular model of asphalt mixed with different types of wax in different aging states.

[0052] S2, Force field selection

[0053] The COMPASS II force field was used to fully consider the intramolecular and intermolecular interactions of pure wax and asphalt molecular models and pure wax and asphalt mixed systems.

[0054] S3, Simulation details

[0055] S31, Model geometry optimization

[0056] The Visualizer module was used to model pure wax and asphalt, and the Amorphous Cell module was used to establish the molecular dynamics model of model wax and unaged, short-term and long-term aged asphalt.

[0057] S32, Model parameters

[0058] For the simulation of van der Waals interactions, a cubic spline method based on atomic segment distance was used, and a long-range correction was performed beyond this distance. For electrostatic interactions, the Ewald-based summation method was used. The Nose thermostat and Andersen pressure controller were selected to control the temperature and pressure of the pure wax and asphalt system, respectively.

[0059] S33, Model energy minimization

[0060] Geometric optimization of the molecular dynamics model of model wax and unaged, short-term and long-term aged asphalt in the Forcite module, and preliminary energy minimization in the model is achieved by iteration.

[0061] S34, annealing simulation

[0062] Annealing simulation of the model in step S33 is to further achieve energy minimization in the global range to avoid the model from falling into a local potential well.

[0063] S35, running molecular dynamics simulation

[0064] Equilibrium dynamics process is carried out on the annealed model under isothermal-isobaric, canonical ensemble conditions to achieve energy convergence and rationalization of molecular structure.

[0065] S4, analysis of the simulation results of wax crystal thermodynamic behavior

[0066] S41, compatibility analysis

[0067] The solubility parameters of unaged and aged asphalt and model wax are calculated respectively to analyze the compatibility of wax with asphalt in different aging states.

[0068] S42, free volume analysis

[0069] The total volume, occupied volume and free volume of the model are calculated respectively, and the influence of model wax on the free volume of asphalt at different temperatures is characterized by the free volume fraction.

[0070] S43, diffusion behavior analysis

[0071] The root mean square displacement parameter of wax molecules is calculated to determine the diffusion behavior of wax molecules in the wax-containing asphalt molecular model.

[0072] S44, aggregation behavior analysis

[0073] The radial distribution function curve of wax molecules is calculated to characterize the aggregation behavior of pure wax molecules in the wax-containing asphalt model.

[0074] Further, in step S11, the model wax is C18, C24 and C40 pure wax molecules.

[0075] Further, in step S12, squalane and hopane molecules are selected to represent saturated molecules, which are usually aliphatic chain structures with branches and naphthenes.

[0076] Further, in step S12, aromatic molecules are light molecular weight and minimum polarity carbon-hydrogen compound structures in asphalt systems, composed of two molecules of perhydrophenanthrene-naphthalene and dioctyl-cyclohexane-naphthalene.

[0077] Further, in step S12, the resin plays an important role in the stability of asphalt as a stabilizer of asphaltene, which is composed of 5 molecules, namely quinoline hopane, pyridine hopane, thioisopimane, trimethylphenoxane and benzodithiophene.

[0078] Further, in step S12, asphaltene is the largest particle size and the most polar component in the asphalt system, which is composed of three molecules, namely asphaltene-phenol, asphaltene-pyrrole and asphaltene-thiophene.

[0079] Further, in step S13, the asphalt is subjected to short-term and long-term aging by rotary thin film oven (RTFOT) and pressure aging test device (PAV), respectively. Among them, the short-term aging is carried out according to the method specified in European standard EN 12607–1:2014, and the long-term aging is carried out according to the method specified in EN 14769:2012.

[0080] Further, in step S32, the segment distance is The precision is 0.001 kcal / mol.

[0081] Further, in step S33, the iteration number is 20000 times.

[0082] Further, in step S34, the initial temperature is 300K, the intermediate cycle temperature is 1000K, and a total of 5 annealing cycles are set.

[0083] Further, in step S41, the solubility parameter calculation formula of the wax-containing asphalt and the wax molecule is:

[0084]

[0085] In the formula, E coh , E vdw , E elect and V are the total cohesive energy, van der Waals cohesive energy, electrostatic cohesive energy and volume of a single simulation system; δ vdw and δ elect are the van der Waals and electrostatic solubility parameters, which are obtained when the model is in a stable state.

[0086] Further, in step S42, the free volume fraction calculation formula of the wax-containing asphalt and the wax molecule is:

[0087]

[0088] In the formula, V f (%) is the free volume fraction; V and V O are the total volume and occupied volume in the molecular dynamics model, and these volume parameters are obtained when the model is in a stable state.

[0089] Further, in step S43, the mean square displacement (MSD) and the diffusion coefficient (D) are introduced to determine the diffusion kinetics of the wax molecules in the mixed system, and the calculation method is as follows:

[0090]

[0091] In the formula, MSD is the mean square displacement of the calculated particle; N is the number of the calculated particles; t is the simulation time; r j (t) and r j (0) are the positions of the particle mass center at time t and the initial time, respectively.

[0092] Further, in step S44, the radial distribution function (RDF) is introduced, and the calculation formula is as follows:

[0093]

[0094] In the formula, r is the distance between the calculated molecule and the reference molecule; dN is the number of molecules counted in the range of r to r+dr; and p represents the average density of the molecular system. When calculating the radial distribution function of the model wax molecule, the cutoff radius is set to The distance is All are obtained when the model is in a stable state.

[0095] Example 1

[0096] According to the latest research results of gas chromatography-mass spectrometry (GC-MS) and field ionization mass spectrometry (FIMS) tests, it can be known that the mass of n-alkanes in the asphalt wax composition distributed between C18 and C38 accounts for 32% of the total asphalt wax mass, and the mass of n-alkanes and isoalkanes distributed between C21 and C47 accounts for 95%. Meanwhile, referring to the model wax constructed by Blom et al., C18, C24 and C40 n-alkanes are selected as typical representatives of pure wax molecules, and the molecular models of unaged, short-term and long-term aged asphalts mixed with C18, C24 and C40 model waxes are established. On this basis, by calculating the solubility parameter, free volume, diffusion coefficient and radial distribution function, the interaction between the model wax and the unaged, short-term and long-term aged asphalts is considered from the molecular level.

[0097] The embodiment specifically provides a molecular simulation method for low-temperature thermal dynamic behavior of wax crystals in a waxy asphalt, which refers toFigure 1 The specific process is as follows:

[0098] (1) Screening of research subjects: C18, C24 and C40 pure wax molecules are selected as molecular model representatives of different chain length model waxes. The three model wax molecules are all straight chain saturated carbon chain structures, and the molecular model structures established in the Materials Studio software are shown in Figure 2 The gray atoms in the figure represent carbon atoms, and the white atoms are hydrogen atoms.

[0099] The molecular model of unaged asphalt is referred to the commonly used twelve-component molecular model proposed by Li and Greenfield and Ren. Asphalt is composed of four components of saturates, aromatics, resins and asphaltenes (SARA); in the construction of the molecular model, saturates are usually fatty chain structures with branches and naphthenes, and squalane (Squalane) and hopane (Hopane) molecules are selected to represent saturates, while aromatics are light molecular weight and minimum polarity carbon-hydrogen compound structures in the asphalt system, consisting of two molecules of perhydrophenanthrene-naphthalene (PHPN) and dioctyl-cyclohexane-naphthalene (DOCHN). Resin, as a stabilizer of asphaltenes, plays an important role in the stability of asphalt, which is composed of 5 molecules, namely: quinolinohopane, pyridinohopane, thioisorenieratane, trimethylbenzeneoxane and benzobisbenzothiophene. Asphaltenes are the largest particle size and the most polar component in the asphalt system, according to the structure of asphaltenes proposed by Mullins, asphaltenes are composed of three molecules, named asphaltenes-phenol, asphaltenes-pyrrole and asphaltenes-thiophene. Based on the above components, the molecular structure model of unaged asphalt is shown in Figure 3 .

[0100] Based on the above component molecular model, Table 1 shows the molecular information of the twelve components of unaged asphalt, including chemical formula, molecular mass and quantity, which is used for the construction of unaged asphalt molecular model and its mixed molecular system with wax.

[0101] Table 1 Twelve-component information of unaged asphalt

[0102]

[0103] Note: a Indicates the components of unaged asphalt.

[0104] The potential reaction sites of adding carbonyl and sulfoxide functional groups to the molecular structure of unaged asphalt components are formed into corresponding aged asphalt molecular models. Among them, the saturated component group in asphalt is the most insensitive to oxidative aging, and the molecular structure does not change with oxidative aging due to the lack of polar atoms or aromatic rings. The molecular models of all components of aromatic, resin and asphaltene after aging are shown in Figure 4 In order to characterize the differences in asphalt molecular models caused by different aging degrees, the number of molecules of the twelve components after short-term and long-term aging is also different, respectively shown in Table 2 and Table 3, which are used to construct the oxidative aging asphalt molecular model and the mixed molecular system of wax.

[0105] Table 2 Information of twelve components of short-term aged asphalt

[0106]

[0107] Note: a indicates the components of unaged asphalt, b indicates the components of short-term aged asphalt.

[0108] Table 3 Information of twelve components of long-term aged asphalt

[0109]

[0110] Note: a indicates the components of unaged asphalt, b、c、e indicates the components of long-term aged asphalt.

[0111] (2) Force field selection: The condensed matter optimized molecular potential force field (COMPASS force field) used for atomic level simulation research was originally developed by Rappe et al. for describing the interaction of organic molecules. On this basis, the COMPASS II force field is an improvement and extension of the original COMPASS force field, aiming to improve the accuracy and applicability to various molecules and chemical systems, which contains more parameters and correction factors, and can be used to simulate the behavior between different organic molecules, biological molecules and polymer atoms, including force and energy, with strong universality and accuracy. At present, the COMPASS II force field is commonly used in asphalt materials, therefore, the COMPASS II force field will be adopted to fully consider the intramolecular and intermolecular interactions of pure wax and asphalt molecular models and pure wax and asphalt mixed systems.

[0112] (3) Simulation details: Molecular dynamics models of model wax and bitumen were built using Materials Studio 2020 software. For the single model wax molecular model, it was composed of 200 corresponding n-alkane molecules, while the number of molecules for unaged, short-term and long-term aged bitumen components were referred to Tables 1-3, respectively. Pure wax and bitumen were modeled using the Visualizer module, and molecular dynamics models of model wax and unaged, short-term and long-term aged bitumen were built through the Amorphous Cell module. The COMPASS II force field was selected, and for the simulation of van der Waals interactions, the three-spline method based on an atom-based cutoff distance of 9.5 A was used, beyond which a long-range correction was performed. For electrostatic interactions, the Ewald-based summation method with an accuracy of 0.001 kcal / mol was used. In addition, the Nose thermostat and Andersen barostat were selected to control the temperature and pressure of the pure wax and bitumen system, respectively.

[0113] The molecular dynamics models of model wax and unaged, short-term and long-term aged bitumen were geometrically optimized in the Forcite module, and the minimization of model energy was initially achieved through 20,000 iterations.

[0114] Annealing simulation of these geometrically optimized structures was further implemented to minimize the energy in the global range, with an initial temperature of 300 K, an intermediate cycle temperature of 1000 K, and a total of five annealing cycles to avoid the model structure from falling into a local potential well.

[0115] The annealed model was subjected to an equilibrium dynamics process under isothermal-isobaric ensemble (NPT, Constant atom number N, pressure P, and temperature T) conditions to further achieve energy convergence and rationalization of the molecular structure, with a total time of 1000 ps, a time step of 1 fs, a total number of iteration steps of 1000000 steps, a temperature control of 298.15 K, and a pressure of 0.0001 GPa.

[0116] Based on the above NPT equilibrium model, further rebalancing of the molecular model was achieved by applying the equilibrium dynamics process of the canonical ensemble (NVT, Constant atom number N, cell volume V, and temperature T), with the temperature value, thermostat type, time step, and total simulation time in the NVT process being consistent with the previous NPT step. After obtaining the final equilibrium state of the molecular model Figure 5 ), it was used for further calculation of thermodynamic parameters.

[0117] ​(4) Thermodynamic parameters calculation: The interaction mechanism between asphalt and wax was understood from the molecular level by the difference of solubility parameters between model wax and asphalt, the change of free volume of asphalt molecular model before and after waxing, and the aggregation and diffusion behavior of wax molecules in asphalt. Among them, the model wax molecules were established in different aging states according to the 3% weight ratio of unaged, short-term and long-term aged asphalt to establish the molecular model of asphalt with different wax types, and the influence of temperature was considered, two extreme temperatures were selected, 253.15 K and 283.15 K.

[0118] Index 1: Cohesive energy density (CED) can be understood as the strength of intermolecular attraction, the relationship between total solubility parameter δ and CED is as follows:

[0119]

[0120] In the formula, E coh , E vdw , E elect and V are the total cohesive energy, van der Waals cohesive energy, electrostatic cohesive energy and volume of a single simulation system; δ vdw and δ elect are the van der Waals and electrostatic solubility parameters, which are obtained when the model is in a stable state.

[0121] In addition, based on the difference of solubility parameters between pure wax and asphalt molecules, the compatibility potential of pure wax and asphalt molecules can be evaluated from the perspective of molecular blending:

[0122] Reference Figure 6 , the solubility parameter difference between model wax and asphalt system, including unaged, short-term and long-term aged asphalt, the solubility parameter difference between short-term and long-term aged asphalt and C18, C24 and C40 model wax at 253.15 K increased by 22.26% and 193.02%, 12.62% and 109.42%, and 12.07% and 104.68% respectively compared with unaged asphalt, while at 283.15 K, the solubility parameter difference between asphalt and C18, C24 and C40 model wax caused by short-term and long-term aging increased by 1.91% and 32.03%, 2.34% and 39.31%, and 2.67% and 44.85% respectively. Therefore, it can be concluded that oxidative aging can increase the solubility parameter difference between model wax and asphalt, thereby making the compatibility of model wax and asphalt worse, which can also be explained from the microscopic molecular point of view. In the third chapter, the size of the "bee structure" on the surface of the asphalt after short-term and long-term aging becomes larger, at the same time, according to the solubility parameter change rate, the influence of short-term aging and long-term aging on the solubility parameter difference between model wax and asphalt is greater, thereby further increasing the risk of phase separation between model wax and asphalt.

[0123] Index 2: The effect of temperature and model wax on the free volume of asphalt was characterized by the free volume fraction, which was calculated using the following equation.

[0124]

[0125] where V f is the free volume fraction; V and V O are the total volume and occupied volume in the molecular dynamics model, respectively, which were obtained when the model was in a stable state.

[0126] Reference Figure 7a , Figure 7a is the free volume distribution of unaged asphalt at different temperatures under different probe radii. It can be seen that as the probe radius increases from to the number of free volumes of unaged asphalt (blue) decreases significantly at both 253.15 K and 283.15 K, and the distribution of free volumes changes from island distribution to sporadic distribution. For the temperature increasing from 253.15 K to 283.15 K, it is intuitively seen that the free volume under different probe radii increases slightly.

[0127] Reference Figure 7b , the effect of model wax on the free volume fraction of asphalt at different temperatures is shown in the figure. Short-term and long-term aging reduces the free volume fraction of waxed asphalt, and long-term aging has a greater effect on the free volume fraction. When the temperature increases from 253.15 K to 283.15 K, the free volume fraction of waxed asphalt increases, which is consistent with the characteristics of unaged asphalt and wax molecular model. At the same time, by comparing the free volume fractions of unaged and waxed asphalt at the same temperature, the addition of model wax reduces the free volume fraction of unaged, short-term and long-term aged asphalt at two temperatures. Therefore, model wax further reduces the free volume fraction of asphalt at lower temperatures, thereby further increasing the physical hardening degree of asphalt, which can also be used as a molecular explanation for the experimental phenomenon of increasing the physical hardening index of asphalt by adding model wax.

[0128] Index 3: The mean square displacement (MSD) was introduced to determine the diffusion kinetics of wax molecules in the mixed system, and the calculation method is as follows:

[0129]

[0130] where MSD is the mean square displacement of the calculated particles; N is the number of calculated particles; t is the simulation time; r j (t) and r j (0) are the positions of the particle center at time t and the initial time, respectively.

[0131] To further quantify the diffusion coefficient of wax molecules in asphalt, the relatively stable section in the MSD curve was selected for fitting, and the diffusion coefficient (D) of the molecule was introduced, and the calculation formula is as follows:

[0132]

[0133] Reference Figure 8a , Figure 8a The MSD values of three model waxes in asphalt change with simulation time. As can be seen from the figure, the mean square displacement curves of wax molecules in each wax-doped asphalt molecular model at 283.15 K are all above 253.15 K, which shows that the increase in temperature increases the mean square displacement of wax molecules and thus accelerates the diffusion rate of wax molecules in asphalt and the probability of molecular collision, which is consistent with the conclusion in the previously published literature. Therefore, the dynamic migration of wax molecules in asphalt has a certain temperature dependence, which can also explain the decrease in cohesive energy density and the increase in free volume caused by the increase in temperature. At the same time, it is also found that at the same temperature, the mean square displacement values of wax molecules in short-term and long-term aged asphalt are less than those in unaged asphalt, and the mean square displacement values further decrease with the increase in aging degree, which shows that the migration rate of wax molecules in aged asphalt is lower than that in unaged asphalt, and the diffusion speed is relatively slow. This result is consistent with the simulation results of free volume, so the decrease in free volume in the aged asphalt model can be used to explain the decrease in flowability of wax molecules in asphalt.

[0134] Reference Figure 8b The diffusion coefficient values of three model waxes in asphalt at different temperatures are shown in the figure. Oxidative aging of asphalt reduces the diffusion coefficient of three model waxes. When the temperature is 253.15 K, short-term and long-term aging reduces the diffusion coefficients of C18, C24 and C40 wax molecules by 42.98% and 73.55%, 37.51% and 67.05%, and 26.32% and 78.95%, respectively, while when the temperature is 283.15 K, short-term and long-term aging reduces the diffusion coefficients of C18, C24 and C40 wax molecules by 57.93% and 68.27%, 50.01% and 70.67%, and 9.88% and 56.79%, respectively. Based on the above change rates, the diffusion coefficients of three model waxes in asphalt further decrease with the increase in the aging degree of asphalt.

[0135] Index 4: g(r) curve is used to characterize the aggregation behavior of pure wax molecules in wax-doped asphalt system, and the calculation formula is as follows:

[0136]

[0137] where r is the distance between the calculation molecule and the reference molecule; dN is the number of molecules counted in the range of r to r+dr; and p represents the average density of the molecular system. In the calculation of the radial distribution function of the model wax molecules, the cutoff radius is set to The distance is All are obtained when the model is in a stable state.

[0138] Referring to Figure 9 , the radial distribution functions of the three model wax molecules in the asphalt system under different temperature conditions are shown in the figure. As can be seen from the figure, when the temperature of the model system decreases from 283.15 K to 253.15 K, the peak value of the radial distribution function of each model wax at increases, thereby enhancing the aggregation degree of the model wax in the asphalt. The above rule can also be consistent in the short-term and long-term aged asphalt. For the influence of the aging degree of the asphalt on the aggregation of the model wax, at the same temperature, compared with the unaged asphalt, the peak value of the radial distribution function of the C18 and C24 wax molecules gradually decreases with the deepening of the aging degree of the asphalt, that is, the aggregation degree decreases, which may be due to the short-term and long-term aging reducing the migration rate of the C18 and C24 wax molecules. Interestingly, the peak value of the radial distribution function of the C40 wax molecule first decreases and then increases with the deepening of the aging degree of the asphalt, that is, the aggregation degree first decreases and then increases, and the reason for the increase may be due to the reduced compatibility of the C40 wax molecule with the asphalt after long-term aging. Therefore, the oxidative aging has an influence on the aggregation degree of the model wax in the asphalt, and the influence degree has certain difference with the type of the model wax, and the influence is jointly acted by the compatibility of the model wax with the asphalt and the motion state of the model wax in the asphalt.

[0139] The analysis results of the low-temperature thermal dynamic behavior of the wax crystals in the wax-containing asphalt in the embodiment are as follows:

[0140] The oxidative aging can increase the cohesive energy density and the solubility parameter of the asphalt, and increase the difference between the solubility parameters of the model wax and the asphalt, so that the compatibility of the model wax with the asphalt becomes poor, which can provide an explanation for the test results that the oxidative aging increases the physical hardening degree of the asphalt.

[0141] The decrease of the free volume fraction of the unaged, short-term and long-term aged asphalt and the model wax caused by the decrease of the temperature can further be used as another explanation for the physical hardening of the asphalt at low temperature.

[0142] The increase of the diffusion coefficient of the model wax in the asphalt caused by the increase of the temperature, and the decrease of the migration rate of the wax molecules in the asphalt caused by the oxidative aging can provide an explanation for the test conclusion that the aging reduces the physical hardening degree of the asphalt at most temperatures.

[0143] The radial distribution function peaks of the three model waxes appear at a distance of The model wax is within the range of , and presents a short-range order and a long-range disorder state, which can be used to explain that the ordered state of alkane molecules promotes the crystallization of wax molecules. Significant agglomeration and crystallization occurred at the pores, and the degree of aggregation was positively correlated with the relative molecular mass.

[0144] Lowering the system temperature can enhance the aggregation of model wax in asphalt, which explains the significant hardening of C18 asphalt blended with short-chain wax at lower temperatures. Oxidative aging can also alter the aggregation of model wax in asphalt, which is determined by the compatibility of the model wax with asphalt and the wax's migration behavior in the asphalt.

[0145] In summary, the present invention establishes a molecular model of unaged and aged asphalt mixed with model wax taking into account the influence of temperature. On this basis, by calculating parameters such as solubility parameters, free volume fraction, diffusion coefficient and radial distribution function, the interaction between model wax and unaged and aged asphalt under low temperature conditions is considered from the molecular level, thereby providing an in-depth mechanism explanation of the existing low-temperature macro-rheological and micro-experimental results of wax-containing asphalt at the molecular scale, and making reasonable suggestions for inhibiting the low-temperature physical hardening of wax-containing asphalt to avoid premature cracking of asphalt pavement.

[0146] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method of molecular simulation of low temperature thermodynamic behavior of wax crystals in a waxy bitumen, characterized in that, The method comprises the following steps: T1: constructing model wax molecules, unaged asphalt molecules and aged asphalt molecules; T2: selecting a molecular force field; T3-1: geometrically modeling the model wax molecules, the unaged asphalt molecules and the aged asphalt molecules, and establishing a molecular dynamics model; T3-2: simulating van der Waals interaction and electrostatic interaction; T3-3: optimizing based on minimization of model energy; T3-4: performing equilibrium simulation under isothermal-isobaric ensemble and canonical ensemble; T4: calculating thermodynamic parameters of the waxy asphalt, wherein the waxy asphalt is a molecular model of the model wax molecules in a 3% weight ratio of the unaged asphalt molecules and the aged asphalt molecules.

2. The method of molecular simulation of low temperature thermodynamic behavior of wax crystals in a wax-containing bitumen according to claim 1, characterized in that, In step T1, the model wax molecules are pure wax molecules with a straight-chain saturated carbon chain structure; the unaged asphalt molecules are twelve-component models including saturates, aromatics, resins and asphaltenes; and the aged asphalt molecules are formed by adding carbonyl and sulfoxide functional groups to the unaged asphalt molecules.

3. The method of molecular simulation of low temperature thermodynamic behavior of wax crystals in a wax-containing bitumen according to claim 2, characterized in that, The model wax molecules include C18, C24 and C40 pure wax molecules; the saturates include a fatty chain structure with branches and naphthenes; the aromatics include a structure of hydrocarbons with a light molecular weight and a minimum polarity in the asphalt system; the resins include a stabilizer of asphaltenes; and the asphaltenes include a component with the largest particle size and the strongest polarity in the asphalt system.

4. The method of claim 1, wherein the method is characterized by, In step T1, the aged asphalt molecules are obtained by short-term and / or long-term aging of the unaged asphalt molecules through a rotary thin-film oven aging test and / or a pressure aging test.

5. The method of claim 1, wherein the method is characterized by, In step T2, the molecular force field includes a COMPASS II force field.

6. The method of claim 1, wherein the method is characterized by, In step T3-2, the van der Waals interaction is simulated by a cubic spline method based on an atomic segment distance, and the electrostatic interaction is simulated by an Ewald-based summation method.

7. The method of claim 1, wherein the method is characterized by, In step T3-3, the optimization includes geometric optimization and annealing simulation.

8. The method of claim 1, wherein the method is characterized by, In step T4, the thermodynamic parameters include solubility parameters, free volume fractions of the model wax molecules, the unaged asphalt molecules and the aged asphalt molecules, and diffusion coefficients and radial distribution functions of the model wax molecules.

9. A computer storage medium, characterized in that The computer storage medium comprises a stored program, wherein the program controls a device in which the computer storage medium is located to perform the method according to any one of claims 1-8 when the program is executed.

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