Molecular simulation method for low-temperature thermodynamic behaviors of wax crystals in wax-containing asphalt and medium

Abstract

The invention belongs to the technical field of molecular simulation, and particularly relates to a molecular simulation method for low-temperature thermodynamic behaviors of wax crystals in wax-containing asphalt and a medium, and the molecular simulation method comprises the following steps: constructing model wax molecules, unaged asphalt molecules and aged asphalt molecules; selecting a molecular force field; geometric modeling is carried out on model wax molecules, unaged asphalt molecules and aged asphalt molecules, and a molecular dynamics model is established; simulating Van der Waals interaction and electrostatic interaction; optimization is carried out based on the minimization of model energy; carrying out balance simulation under an isothermal-isobaric ensemble and a regular ensemble; and calculating thermodynamic parameters of the wax-containing asphalt. Compared with the prior art, the invention overcomes the defects that the interaction mechanism of wax and asphalt is not explained from the molecular level and the influence of oxidation aging on interaction is not explained in the prior art. According to the scheme, deep mechanism interpretation is provided for low-temperature macroscopic rheology and microscopic experiment results of existing wax-bearing asphalt on the molecular scale.

Description

Technical Field

[0001] The invention belongs to the technical field of molecular simulation, and in particular relates to a molecular simulation method and a medium for low-temperature thermodynamic behavior of wax crystals in waxy asphalt. Background Art

[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 a variety of 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 extensive research on the performance of waxy asphalt at low temperatures. In cold regions, wax crystallization can lead to physical hardening, which can reduce the low-temperature cracking resistance of waxy asphalt, but the degree of physical hardening is not always correlated with 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 considered the effect of oxidative aging on physical hardening at four typical low temperatures and concluded that long-term aging can reduce the physical hardening of waxy asphalt and wax-blended asphalt at most temperatures. On the other hand, the honeycomb structure on the asphalt surface was observed at room temperature and low temperature using atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM), and wax provided evidence of 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, destruction and regeneration characteristics, which have certain reference significance for asphalt performance evaluation. However, these macroscopic evaluation methods have limitations in deeply understanding the internal mechanisms of asphalt materials, including the oxidation reaction mechanism of asphalt molecules during oxidative aging, the intermolecular interactions between different modifiers and asphalt molecules, the effects of aging and modification on asphalt thermodynamic parameters, the atomic scale mechanism of adhesive failure, the diffusion behavior of oxygen molecules in asphalt and asphalt mixtures, and the self-repair mechanism of asphalt at the molecular scale. These problems are difficult to reveal only through macroscopic experiments.

[0005] In order to overcome the shortcomings of traditional macroscopic tests, molecular dynamics (MD) simulation can reduce the need for unnecessary experimental studies 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 underlying mechanism of experimental results from a nanoscale perspective. At present, MD simulation methods have been widely used in asphalt materials to understand the interaction between asphalt molecules and additives, and further predict the key thermodynamic properties of additives. Existing methods have determined the macroscopic rheology of waxy asphalt at low temperatures, such as the patent document with patent number CN114591583A, which discloses a heat-resistant reversible aging asphalt wax inhibitor and asphalt and a preparation method thereof, wherein the heat-resistant reversible aging asphalt wax inhibitor is a nano-silica hybridized vinyl acetate copolymer. The patent document with patent number CN115368747 A discloses a dispersant for improving the low-temperature performance of waxy asphalt, asphalt and a preparation method thereof, wherein the dispersant is a vinyl graphene oxide modified ionic liquid nanocomposite. In addition, 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, AZU, 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 properties of waxy asphalt, especially the physical hardening behavior, there is currently no systematic explanation of the interaction mechanism between wax and asphalt and the effect of oxidative aging on the interaction from the molecular level. Summary of the invention

[0006] The purpose of the present invention is to provide a molecular simulation method and medium for the low-temperature thermodynamic behavior of wax crystals in waxy asphalt in order to solve at least one of the above problems, so as to solve the defect of the prior art that there is no explanation of the interaction mechanism between wax and asphalt and the influence of oxidation and aging on the interaction at the molecular level. This scheme provides an in-depth mechanism explanation of the existing low-temperature macro-rheology and micro-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 purpose of the present invention is achieved through the following technical solutions:

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

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

[0010] T2: Select molecular force field;

[0011] T3-1: Geometric modeling of model wax molecules, unaged asphalt molecules and aged asphalt molecules, and establishment of molecular dynamics models;

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

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

[0014] T3-4: Equilibrium simulations in isothermal-isobaric and canonical ensembles;

[0015] T4: Calculation of thermodynamic parameters of waxy asphalt.

[0016] Preferably, in step T1, the model wax molecule is a pure wax molecule with a straight-chain saturated carbon chain structure; the unaged asphalt molecule is a twelve-component model, including saturated components, aromatic components, resins and asphaltene; and the aged asphalt molecule is formed by adding carbonyl and sulfoxide functional groups to the unaged asphalt molecule.

[0017] Preferably, the model wax molecules include pure wax molecules of C18, C24 and C40; the saturated components include fatty chain structures with branches and cycloalkanes, the aromatic components include hydrocarbon structures with light molecular weight and minimum polarity in the asphalt system, the resin includes asphaltene stabilizers, and the asphaltene includes the components with the largest particle size and the strongest polarity in the asphalt system.

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

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

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

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

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

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

[0024] A second aspect of the present invention discloses a computer storage medium, wherein the computer storage medium includes a stored program, wherein when the program is running, the device where the computer storage medium is located is controlled to execute any of the above methods.

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

[0026] The present invention provides a molecular simulation method for the low-temperature thermodynamic behavior of wax crystals for low-temperature crack resistance and toughening of wax-containing asphalt in 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 parameters such as solubility parameters, free volume fraction, diffusion coefficient, and radial distribution function, an in-depth mechanism explanation is provided for the existing low-temperature macro-rheology and micro-experimental results of wax-containing asphalt at the molecular scale, and an 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 THE DRAWINGS

[0027] Figure 1 Schematic diagram of the molecular simulation method of the present invention;

[0028] Figure 2 The schematic diagram of the structure of the model wax molecule, in which gray: carbon atoms, white: hydrogen atoms;

[0029] Figure 3 This is a schematic diagram of the structure of an unaged asphalt molecule, where gray: carbon atoms, white: hydrogen atoms, red: oxygen atoms, blue: nitrogen atoms, and yellow: sulfur atoms;

[0030] Figure 4 This is a schematic diagram of the structure of aged asphalt molecules, where gray: carbon atoms, white: hydrogen atoms, red: oxygen atoms, blue: nitrogen atoms, and yellow: sulfur atoms;

[0031] Figure 5 The molecular models of three types of asphalt molecules and model wax molecules;

[0032] Figure 6 is the difference in 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 detection radii (free volume: blue, occupied volume: gray, gap: white);

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

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

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

[0037] Fig. 9 are the radial distribution functions of three model wax molecules in the asphalt system under different temperature conditions. DETAILED DESCRIPTION

[0038] To make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the present invention is described in detail below in conjunction with the drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the detailed description of the embodiments of the present application provided below in conjunction with the drawings is not intended to limit the scope of protection of the application claimed for protection, but merely represents the 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 making creative work belong to the scope of protection of the present application. The present invention is further described below in conjunction with the drawings.

[0039] For matters not covered in the following description, conventional methods or existing technologies in the art may be used.

[0040] The purpose of the present invention is to establish a molecular model of unaged, short-term and long-term aged asphalt mixed with C18, C24 and C40 model waxes taking into account the temperature effect in view of the above-mentioned deficiencies in the prior art. On this basis, by calculating parameters such as solubility parameters, free volume fraction, diffusion coefficient and radial distribution function, an in-depth mechanism explanation of the existing low-temperature macro-rheological and micro-experimental results of waxy asphalt is provided at the molecular scale, and an important reference value and research means are provided for inhibiting the low-temperature physical hardening of waxy asphalt to avoid premature cracking of asphalt pavement.

[0041] In order to achieve the above object, the present invention adopts the following technical solutions:

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

[0043] S1. Establishment of molecular model

[0044] S11, pure wax model

[0045] Pure wax molecules are selected as molecular models of model waxes with different chain lengths, and the model wax molecules all have a straight-chain saturated carbon chain structure.

[0046] S12, Unaged Asphalt Model

[0047] The unaged asphalt molecules consider the composition system of the twelve-component molecular model, which is composed of four components: saturated components, aromatic components, resins and asphaltenes.

[0048] S13, Aged Asphalt Model

[0049] Carbonyl and sulfoxide functional groups are added to the molecular structure of unaged asphalt to form the corresponding aged asphalt molecular model. Among them, the molecular structure of the saturated group in the asphalt does not change with oxidation aging.

[0050] S14, waxy asphalt model

[0051] The model wax molecules refer to step S11, and the asphalt model refers to steps S12 and S13. The model wax molecules are used to establish molecular models of asphalts with different wax types in different aging states according to a 3% weight ratio of unaged and aged asphalt.

[0052] S2. Force field selection

[0053] COMPASS II force field is used to fully consider the molecular models of pure wax and asphalt as well as the intramolecular and intermolecular interactions of the pure wax and asphalt mixed system.

[0054] S3. Simulation details

[0055] S31, Model Geometry Optimization

[0056] Pure wax and asphalt were modeled using the Visualizer module, and molecular dynamics models of the model wax and unaged, short-term, and long-term aged asphalt were established using the Amorphous Cell module.

[0057] S32, Model parameters

[0058] With the force field determined in step S2, for the simulation of van der Waals interactions, a cubic spline method based on atomic cutoff distance was used, and a long-range correction was performed beyond this distance. For electrostatic interactions, an Ewald-based summation method was used. A Nose thermostat and an Andersen barostat were used to control the temperature and pressure of the pure wax and asphalt systems, respectively.

[0059] S33, Model Energy Minimization

[0060] The geometry of the molecular dynamics models of model wax and unaged, short-term and long-term aged asphalt were optimized in the Forcite module, and the energy of the model was preliminarily minimized through iteration.

[0061] S34, Annealing simulation

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

[0063] S35. Run molecular dynamics simulation

[0064] The equilibrium kinetics of the annealing model was carried out under isothermal-isobaric and canonical ensemble conditions to achieve energy convergence and rationalization of molecular structure.

[0065] S4. Analysis of simulation results of wax crystal thermodynamic behavior

[0066] S41. Compatibility analysis

[0067] The solubility parameters of unaged and aged asphalt and model wax were calculated respectively, and the compatibility of wax with asphalt in different aging states was analyzed.

[0068] S42, free volume analysis

[0069] The total volume, occupied volume and free volume of the model were calculated respectively, and the free volume fraction was used to characterize the effects of different temperatures and model wax on the free volume of asphalt.

[0070] S43. Diffusion behavior analysis

[0071] The root mean square displacement parameters of wax molecules are calculated to determine the diffusion behavior of wax molecules in the waxy 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 waxy asphalt model.

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

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

[0076] Furthermore, in step S12, the aromatic component is a hydrocarbon structure with a light molecular weight and minimum polarity in the asphalt system, and is composed of two molecules, perhydrophenanthrene-naphthalene and dioctyl-cyclohexane-naphthalene.

[0077] Furthermore, in step S12, the resin acts as a stabilizer for asphaltene and plays an important role in the stability of asphalt. The resin is composed of five molecules, namely: quinoline hopane, pyridine hopane, thioisopinene, trimethylphenoxyethane and benzodibenzothiophene.

[0078] Furthermore, in step S12, asphaltene is the component with the largest particle size and the strongest polarity in the asphalt system, and is composed of three molecules, respectively named asphaltene-phenol, asphaltene-pyrrole and asphaltene-thiophene.

[0079] Further, in step S13, the asphalt is subjected to short-term and long-term aging respectively by a rotating thin film oven (RTFOT) and a pressure aging test apparatus (PAV), wherein the short-term aging is carried out according to the method specified in the 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 intercept distance is The accuracy is 0.001 kcal / mol.

[0081] Furthermore, in step S33, the number of iterations is 20,000.

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

[0083] Furthermore, in step S41, the solubility parameter calculation formula of the waxy asphalt and the wax molecules 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, respectively; δ vdw and δ elect are the van der Waals and electrostatic solubility parameters, respectively, and are obtained when the model is in a steady state.

[0086] Furthermore, in step S42, the free volume fraction of the waxy asphalt and wax molecules is calculated as follows:

[0087]

[0088] 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. These volume parameters are obtained when the model is in a stable state.

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

[0090]

[0091] 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 of mass at time t and the initial time, respectively.

[0092] Furthermore, in step S44, a 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 from r to r+dr; ρ 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 spacing is All the results were obtained when the model was 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), the mass of normal alkanes distributed between C18 and C38 in the asphalt wax composition accounts for 32% of the total asphalt wax mass, and the mass of normal and iso-alkanes distributed between C21 and C47 accounts for 95%. At the same time, referring to the model wax constructed by Blom et al., three normal (straight-chain) alkanes C18, C24 and C40 were selected as typical representatives of pure wax molecules, and the molecular model of unaged, short-term and long-term aged asphalt mixed with C18, C24 and C40 model waxes was established. On this basis, by calculating parameters such as solubility parameters, free volume, diffusion coefficient and radial distribution function, the interaction between model wax and unaged, short-term and long-term aged asphalt is considered at the molecular level.

[0097] This embodiment specifically provides a molecular simulation method for the low-temperature thermodynamic behavior of wax crystals in waxy asphalt, referring to Figure 1 , the specific process is as follows:

[0098] (1) Screening of research objects: Pure wax molecules such as C18, C24 and C40 were selected as molecular models of model waxes with different chain lengths. The three model wax molecules all have a straight chain saturated carbon chain structure. The molecular model structure established in Materials Studio software is as follows: Figure 2 As shown, the grey atoms represent carbon atoms and the white atoms represent hydrogen atoms.

[0099] The molecular model of unaged asphalt refers to the composition system of the commonly used twelve-component molecular model proposed by Li, Greenfield and Ren. Asphalt is composed of four components: saturated components, aromatic components, resins and asphaltene (SARA); in the construction of the molecular model, the saturated components are usually a fatty chain structure with branches and cycloalkanes, and squalane and hopane molecules are selected to represent the saturated components, while the aromatic components are hydrocarbon structures with light molecular weight and minimum polarity in the asphalt system, consisting of two molecules: perhydrophenanthrene-naphthalene (PHPN) and dioctyl-cyclohexane-naphthalene (DOCHN). As a stabilizer of asphaltene, resin plays an important role in the stability of asphalt. It is composed of 5 molecules, namely: Quinolinohopane, Pyridinohopane, Thioisorenieratane, Trimethylbenzeneoxane and Benzobisbenzothiophene. Asphaltene is the component with the largest particle size and the strongest polarity in the asphalt system. According to the asphaltene structure proposed by Mullins, asphaltene is determined to be 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 as follows: Figure 3 shown.

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

[0101] Table 1 Information on the twelve components of unaged asphalt

[0102]

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

[0104] Carbonyl and sulfoxide functional groups are added to the potential reaction positions of the molecular structure of the unaged asphalt components to form the corresponding aged asphalt molecular model. Among them, the saturated components in the asphalt are the least sensitive to oxidative aging. Due to the lack of polar atoms or aromatic rings, the molecular structure does not change with oxidative aging. The molecular models of all components of aromatic components, resins and asphaltene after aging are as follows: Figure 4 As shown, at the same time, in order to characterize the differences in the asphalt molecular model caused by different aging degrees, the molecular numbers of the twelve components after short-term and long-term aging are also different, as shown in Table 2 and Table 3, respectively, which are used to construct the molecular model of oxidative aging asphalt and its mixed molecular system with wax.

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

[0106]

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

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

[0109]

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

[0111] (2) Force field selection: The condensed matter optimized molecular force field (COMPASS force field) for atomic level simulation studies was originally developed by Rappe et al. to describe the interactions 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 molecular and chemical systems. It contains more parameters and correction factors and can be used to simulate the behaviors between different organic molecules, biomolecules and polymer atoms, including forces and energies. It has strong universality and accuracy. At present, the COMPASS II force field is a commonly used force field in asphalt materials. Therefore, the COMPASS II force field will be used to fully consider the intramolecular and intermolecular interactions of the pure wax and asphalt molecular models and the pure wax and asphalt mixed systems.

[0112] (3) Simulation details: The molecular dynamics models of model wax and asphalt were established using Materials Studio 2020 software. For the molecular model of a single model wax, it is composed of 200 corresponding normal alkane molecules, and the number of molecules of the unaged, short-term and long-term aged asphalt components is referred to Tables 1 to 3, respectively. The Visualizer module was used to model pure wax and asphalt, and the molecular dynamics models of model wax and unaged, short-term and long-term aged asphalt were established using the Amorphous Cell module. The COMPASS II force field was selected, and for the simulation of van der Waals interactions, the Atom based cutoff distance was used. The cubic spline method was used, and a long-range correction was performed beyond this distance. 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 used to control the temperature and pressure of the pure wax and asphalt systems, respectively.

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

[0114] Annealing simulations were performed on these geometrically optimized structures to further achieve global energy minimization, 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 falling into a local potential well.

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

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

[0117] (4) Calculation of thermodynamic parameters: The interaction mechanism between asphalt and wax is understood at the molecular level through the difference in solubility parameters between model wax and asphalt, the change in free volume of asphalt molecular model before and after wax addition, and the aggregation and diffusion behavior of wax molecules in asphalt. Among them, the molecular models of asphalt doped with different wax types in different aging states are established according to the weight ratio of 3% of unaged, short-term and long-term aged asphalt by model wax molecules. At the same time, considering the influence of temperature, two extreme temperatures are selected, 253.15K and 283.15K.

[0118] Indicator 1: Cohesive energy density (CED) can be understood as the strength of mutual attraction between molecules. The relationship between the 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, respectively; δ vdw and δ elect are the van der Waals and electrostatic solubility parameters, respectively, and are obtained when the model is in a steady state.

[0121] In addition, based on the difference in 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] refer to Figure 6 , is the solubility parameter difference between the model wax and asphalt system, including unaged, short-term and long-term aged asphalt. The solubility parameter differences between asphalt and C18, C24 and C40 model waxes at 253.15K after short-term and long-term aging increased by 22.26% and 193.02%, 12.62% and 109.42% and 12.07% and 104.68% respectively compared with unaged asphalt. At 283.15K, the solubility parameter differences between asphalt and C18, C24 and C40 model waxes 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 inferred that oxidative aging can increase the solubility parameter difference between model wax and asphalt, thereby making the compatibility of model wax and asphalt worse. This can also explain from the micromolecular perspective that the size of the "honeycomb structure" on the asphalt surface increases after short-term and long-term aging in Chapter 3. At the same time, according to the solubility parameter change rate, short-term aging and long-term aging have a greater impact on the solubility parameter difference between model wax and asphalt, thereby further increasing the risk of phase separation between model wax and asphalt.

[0123] Indicator 2: The free volume fraction is used to characterize the effect of different temperatures and model wax on the free volume of asphalt, and is calculated using the following formula.

[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. These volume parameters are obtained when the model is in a stable state.

[0126] refer to Figure 7a , Figure 7a The free volume distribution of unaged asphalt at different temperatures under different detection radii is shown in Figure 2. Increase to The amount of free volume (blue) of unaged asphalt decreases significantly at 253.15 K and 283.15 K, and the distribution of free volume changes from island distribution to sporadic distribution. When the temperature increases from 253.15 K to 283.15 K, it can be seen that the free volume increases slightly at different probe radii.

[0127] refer to Figure 7b , the figure shows the effect of model wax on the free volume fraction of asphalt at different temperatures. Short-term and long-term aging reduce the free volume fraction of wax-added asphalt, and long-term aging has a greater effect on the free volume fraction. When the temperature increases from 253.15K to 283.15K, the free volume fraction of wax-added asphalt increases. These characteristics are consistent with the unwaxed asphalt and the wax molecular model. At the same time, by comparing the free volume fractions of unwaxed and wax-added 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 both temperatures. Therefore, model wax further reduces the free volume fraction of asphalt at lower temperatures, thereby further aggravating the physical hardening degree of asphalt, which can also serve as a molecular explanation for the experimental phenomenon that the addition of model wax increases the physical hardening index of asphalt.

[0128] Indicator 3: The mean square displacement (MSD) is introduced to determine the diffusion dynamics of wax molecules in the mixed system. 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 of mass at time t and the initial time, respectively.

[0131] In order to further quantify the diffusion coefficient of wax molecules in asphalt, a relatively stable interval in the MSD curve is selected for fitting, and the diffusion coefficient of the molecule (D) is introduced. The calculation formula is as follows:

[0132]

[0133] refer to 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 at 283.15K of each wax-doped asphalt molecular model are almost all above 253.15K, which indicates that the increase in temperature accelerates the diffusion rate and the probability of molecular collision in asphalt by increasing the mean square displacement of wax molecules, which is consistent with the conclusions in previously published literature. Therefore, the dynamic mobility of wax molecules in asphalt has a certain temperature dependence, which can also provide an explanation for the decrease in cohesive energy density and the increase in free volume caused by the increase in temperature. At the same time, it was also found that at the same temperature, the mean square displacement values ​​of wax molecules in short-term and long-term aged asphalt were smaller than those in unaged asphalt, and as the degree of aging deepened, the mean square displacement values ​​further decreased, which indicates that the mobility of wax molecules in aged asphalt is lower than that in unaged asphalt, which is manifested as a relatively slow diffusion rate. This result is consistent with the simulation results of free volume. Therefore, the reduction in free volume in the aged asphalt model can be used to explain the reduced mobility of wax molecules in asphalt.

[0134] refer to Figure 8b , the figure shows the diffusion coefficient values ​​of the three model waxes in asphalt at different temperatures. The oxidative aging of asphalt reduces the diffusion coefficients of the three model wax molecules. When the temperature is 253.15K, short-term and long-term aging reduce 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 283.15K, short-term and long-term aging reduce 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, with the increase of asphalt aging, the diffusion coefficients of the three model waxes in asphalt further decrease.

[0135] Indicator 4: Use the g(r) curve to characterize the aggregation behavior of pure wax molecules in asphalt in the wax-doped asphalt system. The calculation formula is as follows:

[0136]

[0137] 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 from r to r+dr; ρ 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 spacing is All the results were obtained when the model was in a stable state.

[0138] refer to Fig. 9 The figure shows the radial distribution functions of three model wax molecules in the asphalt system under different temperature conditions. As can be seen from the figure, when the temperature of the model system is reduced from 283.15K to 253.15K, the radial distribution functions of each model wax are The peak at the position increases, thereby enhancing the aggregation of the model wax in asphalt. The above rule can also be drawn in the short-term and long-term aged asphalt. As for the effect of asphalt aging on the aggregation of model wax, at the same temperature, compared with unaged asphalt, the peak of the radial distribution function of C18 and C24 wax molecules gradually decreases as the asphalt aging deepens, that is, the aggregation degree decreases. This may be due to the fact that short-term and long-term aging play a leading role in reducing the mobility of C18 and C24 wax molecules. Interestingly, the peak of the radial distribution function of C40 wax molecules decreases first and then increases as the asphalt aging deepens, that is, the aggregation degree decreases first and then increases. The increase may be caused by the reduced compatibility of C40 wax molecules with asphalt after long-term aging. Therefore, oxidative aging has an effect on the aggregation degree of model wax in asphalt, and the degree of influence varies with the type of model wax. This effect is caused by the compatibility of model wax with asphalt and the motion state of model wax in asphalt.

[0139] The analysis results of the low temperature thermodynamic behavior of wax crystals in the waxy asphalt in this example are as follows:

[0140] Oxidative aging can increase the cohesive energy density and solubility parameters of asphalt, and increase the solubility parameter difference between model wax and asphalt, thereby making the compatibility between model wax and asphalt worse. This can provide an explanation for the experimental results that oxidative aging increases the degree of physical hardening of asphalt.

[0141] The decrease in free volume fraction of unaged, short-term and long-term aged asphalts and model waxes caused by the decrease in temperature, and the free volume collapse can be another explanation for the physical hardening of asphalt at low temperatures.

[0142] Temperature increase can enhance the diffusion coefficient of model wax in asphalt, while oxidative aging can reduce the mobility of wax molecules in asphalt, which provides an explanation for the experimental conclusion that aging at most temperatures reduces the degree of physical hardening of asphalt.

[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 it is ordered in the short distance and disordered in the long distance, 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 site, and the degree of aggregation was positively correlated with the relative molecular mass.

[0144] The lowering of system temperature can enhance the aggregation of model wax in asphalt, which can explain the experimental phenomenon that short-chain wax C18 asphalt hardens significantly at lower temperatures. Oxidative aging can change the aggregation of model wax in asphalt, which is determined by the compatibility of model wax with asphalt and the migration behavior of wax in 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 a 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 to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and 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 molecular simulation method for the low-temperature thermodynamic behavior of wax crystals in waxy asphalt, characterized in that: The steps include: T1: construct model wax molecules, unaged asphalt molecules and aged asphalt molecules; T2: Select molecular force field; T3-1: Geometric modeling of model wax molecules, unaged asphalt molecules and aged asphalt molecules, and establishment of molecular dynamics models; T3-2: Simulate van der Waals interactions and electrostatic interactions; T3-3: Optimization based on minimization of model energy; T3-4: Equilibrium simulations in isothermal-isobaric and canonical ensembles; T4: Calculation of thermodynamic parameters of waxy asphalt.

2. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T1, the model wax molecule is a pure wax molecule with a straight saturated carbon chain structure; the unaged asphalt molecule is a twelve-component model, including saturated components, aromatic components, resins and asphaltene; the aged asphalt molecule is formed by adding carbonyl and sulfoxide functional groups to the unaged asphalt molecule.

3. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 2, characterized in that: The model wax molecules include pure wax molecules of C18, C24 and C40; the saturated components include aliphatic chain structures with branches and cycloalkanes, the aromatic components include hydrocarbon structures with light molecular weight and minimum polarity in the asphalt system, the resin includes a stabilizer for asphaltene, and the asphaltene includes the component with the largest particle size and the strongest polarity in the asphalt system.

4. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T1, the aged asphalt molecules are obtained by subjecting unaged asphalt molecules to short-term and / or long-term aging through a rotating thin film oven aging and / or pressure aging test.

5. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T2, the molecular force field includes the COMPASS II force field.

6. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T3-2, the van der Waals interaction is simulated by a cubic spline method based on atomic intercept distances, and the electrostatic interaction is simulated by a summation method based on Ewald.

7. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T3-3, the optimization includes geometry optimization and annealing simulation.

8. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T4, the waxy asphalt is a molecular model established by model wax molecules according to a 3% weight ratio of unaged asphalt molecules to aged asphalt molecules.

9. The molecular simulation method for low-temperature thermodynamic behavior of wax crystals in waxy asphalt according to claim 1, characterized in that: In step T4, the thermodynamic parameters include: solubility parameters and free volume fractions of model wax molecules, unaged asphalt molecules and aged asphalt molecules, and the diffusion coefficient and radial distribution function of the calculated model wax molecules.

10. A computer storage medium, characterized in that: The computer storage medium includes a stored program, wherein when the program is running, the device where the computer storage medium is located is controlled to execute any method according to claims 1 to 9.

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