Z. MO: STUDY OF THE PROPERTIES AND INTERFACIAL ENERGY OF GRAPHENE-MODIFIED ASPHALT ...
501–507
STUDY OF THE PROPERTIES AND INTERFACIAL ENERGY OF
GRAPHENE-MODIFIED ASPHALT BASED ON MOLECULAR
DYNAMICS
[TUDIJ LASTNOSTI IN ENERGIJE NA MEJAH Z GRAFENOM
MODIFICIRANEGA ASFALTA S POMO^JO MOLEKULARNE
DINAMIKE
Zhenlong Mo
East China Jiaotong University, School of Transportation Engineering ,Nanchang 330013, China
Prejem rokopisa – received: 2023-05-12; sprejem za objavo – accepted for publication: 2023-08-05
doi:10.17222/mit.2023.878
In this paper, a molecular dynamics simulation was used to examine the physical, mechanical, and interfacial adhesion charac-
teristics of graphene-modified asphalt. The results show that the physical properties, mechanical properties and interfacial adhe-
sion work of modified graphene are higher than those of the base asphalt model, indicating that the addition of graphene can im-
prove the mechanical properties and interfacial interaction of asphalt. Asphalt and aggregate mainly interact through physical
adsorption, and the Van der Waals force plays an important role in the adhesion behavior of the asphalt-calcite interface. There
is an optimal value for the content of graphene, such that the content of graphene added to the asphalt should not be too high.
Considering the graphene price factor and modification effect, the graphene content studied is optimal at 1.79 w/%.
Keywords: asphalt, molecular dynamics, graphene, adhesion interface
Povzetek: v tem ~lanku avtor opisuje simulacije na osnovi molekularne dinamike za dolo~itev fizikalnih in mehanskih lastnosti
ter medmejne adhezije z grafenom modificiranega asfalta. Rezultati simulacij so pokazali, da ima z grafenom modificirani asfalt
bolj{e fizikalne in mehanske lastnosti ter ve~je adhezijsko delo kot osnovni asfalt. Dodatek grafena asfaltu torej lahko izbolj{a
mehanske lastnosti in medmejno interakcijo med sestavinami asfalta. Asfaltno vezivo (obi~ajno bitumen) in dodani agregati
(pesek oziroma apnenec) medsebojno v glavnem reagirajo preko fizikalne adsorpcije in Van der Waalsove sile igrajo pomembno
vlogo pri adheziji na mejah med asfaltom in kalcitom. Avtor ugotavlja, da obstaja optimalna vsebnost grafena in da dodatek
le-tega ne sme biti previsok. Upo{tevajo~ ceno grafena in u~inek modifikacije z grafenom, avtor na osnovi izveden {tudije
ugotavlja, da je 1,79 masnih % optimalni dodatek grafena.
Klju~ne besede: asfalt, molekularna dinamika, grafen, adhezija na mejah med sestavinami asfalta
1 INTRODUCTION
Asphalt is a complex mixture of organic molecules
with various molecular weights, polarities, and func-
tional groups.
1
An asphalt pavement is vulnerable to fa-
tigue cracking, rutting, and other pavement diseases due
to its material and structure, which reduces the use ca-
pacity of the asphalt mixture.
2
Among them, the struc-
tural stability of an asphalt mixture is largely determined
by the adhesion of the asphalt and aggregate in the mix-
ture. It has an important influence on the service perfor-
mance of the asphalt pavement.
3
Thus, to improve the
mechanical properties of the asphalt mixture, it is essen-
tial to study and improve the adhesion strength of the as-
phalt-aggregate interface.
Since the preparation and separation of monolayer
graphene were first reported in 2004,
4
it has been widely
used in various fields. It is considered to be a revolution-
ary nanomaterial,
5
because the addition of nanomaterials
can improve the compatibility of asphalt materials.
6
In
recent years graphene has been used in modified asphalt
by many researchers. Fernández et al.
7
studied the me-
chanical properties and thermal sensitivity of graphene-
modified asphalt. Test results showed that 0.05 %
graphene-modified asphalt had better heat transfer, a
complex modulus and resistance to plastic deformation
at high temperatures. Although there has been a signifi-
cant improvement in this ability, it does not have an ef-
fect on the self-healing ability. Yang et al.
8
found that the
addition of graphene and carbon nanotubes increased the
stiffness, high-temperature performance, and durability
of asphalt through DSR and microstructure analysis.
Nazki et al.
9
thought that the anti-rutting properties of
graphene-modified asphalt improved with the rheologi-
cal test, and the optimum content of graphene was be-
t w e e n1%a n d1 . 5% .Y a n ge ta l .
10
considered that
graphene could improve the elasticity, viscosity, and rut-
ting resistance of asphalt. Hafeez et al.
11
found GNPs to
be effective in increasing the pavement modulus, reduc-
ing the phase angle, increasing the pavement rutting re-
sistance, significantly reducing the pavement water sen-
sitivity and improving the pavement durability.
At present, in the field of road asphalt materials, mo-
lecular dynamics simulation is an effective and intuitive
Materiali in tehnologije / Materials and technology 57 (2023) 5, 501–507 501
UDK 622.337:52-334.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)501(2023)
*Corresponding author's e-mail:
mozhenlong@ecjtu.edu.cn (Zhenlong Mo)

method to characterize the molecular model and nano-
structure of the asphalt binder and quantify its physical
and mechanical properties,
12–15
which provides an effec-
tive and considerable test and evaluation method for un-
derstanding and revealing the molecular basis of the as-
phalt material’s performance evolution and attenuation
mechanism. This shows that a molecular dynamics simu-
lation could be used to investigate the interaction be-
tween asphalt and aggregate at the nanoscale, providing
vital insights into the relationship between the chemical
composition of asphalt concrete and its macroscopic be-
havior. Shishehbor et al.
16
studied the adhesion properties
of asphalt, graphene, and mineral aggregate at different
temperatures with a molecular dynamics simulation. The
results showed that the graphene improved the viscosity,
but there was no advantage in the interaction between
binder and aggregate. Yao et al.
17
used molecular dynam-
ics to simulate graphene nanosheet-modified asphalt, in
which the asphalt model used a three-component model.
The results showed that the modified asphalt’s density,
viscosity, and thermal conductivity were higher than the
asphalt model. The trend of laboratory data is consistent
with the change in the these characteristics at different
temperatures. Qu et al.
18
studied the self-healing proper-
ties of asphalt binders by molecular dynamics and found
that graphene positively affected the self-healing process
of asphalt binders. Zhou et al.
19
used a molecular simula-
tion to investigate the influence of graphene and carbon
nanotubes on the thermomechanical characteristics of as-
phalt binders. They discovered that adding graphene or
carbon nanotubes to asphalt considerably enhanced its
thermodynamic characteristics. Hu et al.
20
also studied
the mechanism of SBS-modified asphalt alteration by in-
cluding different percentages of graphene oxide (GO) in
the molecular model.
In this paper the physical properties, mechanical pro-
perties, and interfacial adhesion properties of graphene-
modified asphalt were studied with a molecular dynam-
ics simulation. Firstly, the AAA-1 asphalt and graphene
model were constructed, the content of graphene sheets
in the blended asphalt model was controlled by adjusting
the number of sheets, and the physical and mechanical
properties of the modified asphalt were calculated.
Finally, the asphalt-calcite interface model was con-
structed to calculate the interaction energy of the inter-
face under different contents. The mechanical properties
and interface effects of graphene-modified asphalt mo-
lecular dynamics simulation were studied.
2 ESTABLISHING THE MODEL
2.1 Asphalt model
This paper used the molecular dynamics simulation
software Materials Studio2019 to establish the molecular
models of base asphalt and graphene-modified asphalt.
The COMPASS II force field was used to describe the
interaction potential of the molecular system, and the im-
provement effect of different graphene contents on the
adhesion of asphalt and limestone was studied. The
four-component, 12-molecule asphalt model proposed by
Li and Greenfield
21
in 2014 on behalf of the American
Strategic Highway Research Program AAA-1 asphalt
was selected as the base asphalt, and the molecular struc-
ture is shown in Figure 1. Gray is the carbon atom;
white is the hydrogen atom; blue is the nitrogen atom;
yellow is the sulfur atom; and red is the oxygen atom.
The basic information about its various compounds is
shown in Table 1. In the simulation, the graphene and
asphalt were physically blended because the graphene
molecules did not break their chemical bonds to form a
new structure. Firstly, the 12-component monomer struc-
ture model of asphalt was established by the Monte
Carlo method using the amorphous unit module in MS,
and each molecule’s energy minimization and geometric
optimization were carried out. After the molecular sys-
tem reached stability, the initial asphalt model was con-
structed by mixing it into a box. The initial density was
set to 0.8 g/cm
3
. After geometric optimization and an-
nealing, the 200 ps NVT simulation was carried out. To
enable the system to stabilise initially, a Nose-Hoover
thermostat was used to control the constant temperature.
The NPT simulation of 300 ps was carried out, and the
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502 Materiali in tehnologije / Materials and technology 57 (2023) 5, 501–507
Table 1: Details of the AAA-1 bitumen model system.
Components Molecular Label
Molecular For-
mula
Number of
Atoms
Molar mass
Number in
Model system
Mass Ratio
(%)
Asphaltene
Pyrrole C 66
H 81
N 148 888.4 2 5.45
Phenol C 42
H 54
O 97 574.9 3 5.29
Thiophene C 51
H
62
S 114 707.1 3 6.51
Resin
Quinolinohopane C 40
H
59
N 100 553.9 4 6.80
Benzobisbenzothiophene C 18
H 10
S 2 30 290.4 15 13.36
Thioisorenieratane C 40
H 60
S 101 573.0 4 7.03
Pyridinohopane C 36
H
57
N 94 503.9 4 6.18
Trimethylbenzene-oxane C 29
H
50
O 80 414.7 5 6.36
Saturate
Squalane C 30
H 62
92 422.8 4 5.19
Hopane C 35
H
62
97 482.9 4 5.93
Aromatic
PHPN C 35
H
44
79 464.7 11 15.68
DOCHN C 30
H
46
76 406.7 13 16.22

density of the system was stabilized by using Andersen’s
constant pressure controller to control a constant pres-
sure. Finally, the 100 ps NVT simulation was carried out
to stabilize the system further, and finally, the base as-
phalt and graphene-modified asphalt model were ob-
tained.
According to the literature, graphene is modeled by
Qu et al.
18
, as shown in Figure 2. The content of
graphene in modified asphalt was calculated according to
the graphene content in modified asphalt, as shown in
Table 2.
Table 2: Content of graphene in the model.
Number of graphene
sheets/sheet
012345
Graphene content (w/%) 0 1.79 3.52 5.19 6.8 8.36
2.2 Aggregate molecular model
Limestone is widely used as aggregate for road pave-
ment materials in China. Limestone is ground into pow-
der for an XRD test to test its mineral composition. The
results are shown in Figure 3a.
According to the results of the X-ray diffraction anal-
ysis, the content of calcite in this batch of limestone is
92.2 w/%, and the content of dolomite is 7.8 w/%. There-
fore, this paper will establish a calcite crystal model to
simulate the aggregate model. For calcite surfaces, past
researchers have found that the crystal {104 } cleavage
surface is the most stable cleavage plane because it has
the largest interlayer spacing and the lowest surface en-
ergy.
21
Therefore, the {104 } cleavage surface is used to
construct a supercell for building the calcite molecular
model, and the aggregate model is shown in Figure 3b.
2.3 Asphalt-calcite interface model
The asphalt-calcite interface model is constructed by
combining the asphalt and aggregate mineral models
with the interface construction tool. To eliminate the in-
fluence of the three-dimensional periodic boundary con-
ditions, a 5-nm vacuum layer is placed at the top of each
asphalt-calcite interface model. When the model size is
Z. MO: STUDY OF THE PROPERTIES AND INTERFACIAL ENERGY OF GRAPHENE-MODIFIED ASPHALT ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 501–507 503
Figure 3: Aggregate composition and model: a) aggregate composition, b) aggregate molecular model
Figure 2: Typical molecular structure of graphene
Figure 1: Asphalt 4-component 12-molecule system

greater than 3.5 nm × 3.5 nm, the system energy con-
verges and meets the accuracy requirements. In the final
modeling, the size is roughly 4 nm × 4 nm. The con-
structed asphalt-calcite interface system was first sub-
jected to 5000 iterations of energy and geometry optimi-
zation, followed by a 300 ps dynamic equilibrium
operation with an NVT ensemble at 298.15 K. The last
20 frames of the trajectory are exported for subsequent
energy calculations.
2.4 Verification of the equilibrium of the model
Density is an important physical and mechanical
property of asphalt, which is normally used to verify the
reliability of the molecular model in a MD simulation.
To determine whether the system reaches equilibrium in
the last 1 ns dynamics, the density-time curve of all as-
phalt models is calculated. It can be seen from Table 3
that the density of the model is stable in the last 1 ns.
Table 3: Density Fluctuation Data
Graphene con-
tent (%)
Average den-
sity (g/cm
3
)
Final density
(g/cm
3
)
Standard devi-
ation
0 0.979 1.003 0.034
1.79 0.996 1.009 0.027
3.52 0.998 1.011 0.031
5.19 1.005 1.022 0.032
6.80 1.012 1.042 0.026
8.36 1.034 1.055 0.030
That is, it fluctuates within a specific range. The stan-
dard deviation of the density fluctuation is
0.026–0.034 g/cm
3
, which is very small and far less than
the average density, indicating that the simulated density
fluctuation range is weak and tends to be stable. There-
fore, it is determined that all the models reach the equi-
librium state in the last 1ns, and the following analysis
can be carried out. The density calculated by the AAA-1
asphalts model system at 298.15K is 1.003g/cm
3
, which
is close to the experimental density of 1.023g cm
3
. This
result verifies the correctness of the simulation process.
Furthermore, the density increases with the increase of
graphene content, which is consistent with the actual
conditions.
3 PERFORMANCE SIMULATION
3.1 Simulation of Physical Properties of Modified As-
phalt
The cohesive energy density (CED) is the energy re-
quired to overcome intermolecular forces per unit vol-
ume of aggregates, which is an index to evaluate the
strength of intermolecular interactions. Figure 4a shows
the cohesive energy-density simulation results of the
base asphalt and the graphene-modified asphalt models
at different temperatures. The results show that the cohe-
sive energy density of the modified asphalt model is
higher than that of the base asphalt model. This result is
reasonable because the introduction of graphene in-
creases the intermolecular non-bond interaction, espe-
cially the van der Waals force. Solubility is frequently
utilized to assess the validity of the asphalt molecular
model, as it serves as a metric for the intermolecular
forces among the asphalt molecules and one of the char-
acteristics of the asphalt itself. The simulation results of
the solubility parameters of the original asphalt and the
graphene-modified asphalt model at different tempera-
tures are shown in Figure 4b. The solubility parameter
of the base asphalt simulated in this paper is 18.548
(J/cm
3
) ^ 0.5, and the solubility parameter range after
modification is 18.708–19.368, which is within the solu-
bility parameter range of asphalt 13.3–22.5.
22
This indi-
cates that the MD simulation parameter setting and opti-
mization process used in this paper are reliable for
analyzing and evaluating the physical properties of the
Z. MO: STUDY OF THE PROPERTIES AND INTERFACIAL ENERGY OF GRAPHENE-MODIFIED ASPHALT ...
504 Materiali in tehnologije / Materials and technology 57 (2023) 5, 501–507
Figure 4: Change of physical properties of modified asphalt with graphene content: a) cohesive energy density, b) solubility parameters

asphalt model, the base asphalt model, and the
graphene-modified asphalt model constructed in this pa-
per are also reasonable. No matter the cohesive energy
density or solubility parameter, the modified asphalt is
larger than the base asphalt model. The physical proper-
ties under the van der Waals force also show the same re-
sults. It can also be seen from Figure 4 that the cohesive
energy density and solubility parameter are mainly af-
fected by the van der Waals force, and the electrostatic
force is relatively weak. With the increase of graphene
content, the cohesive energy density and solubility are as
in the figure.
3.2 Simulation of Mechanical Properties of Modified
Asphalt
After the structural optimization and dynamic relax-
ation, the mechanical properties of the modified asphalt
were studied with a molecular dynamics simulation of
different moduli of graphene-modified asphalt, including
bulk modulus, shear modulus, and Young’s modulus.
The constant- strain method is used to solve the mechan-
ical parameters, and the bulk modulus B and shear
modulus S of the graphene-modified asphalt are calcu-
lated. Then Young’s modulus is calculated by the isotro-
pic material’s standard Equation (3), and multiple tests
take the average value. The results are shown in Fig-
ure 5. The V oigt method is used to approximate the max-
imum values of the bulk modulus and shear modulus,
and the minimum values are approximately calculated by
the Reuss method. Hill
23
found that the actual bulk and
shear modulus are usually between the estimated values
of V oigt and Reuss, so the VRH (V oigt-Reuss-Hill) ap-
proximation method was proposed to correct them, as
shown in Equations (1) and (2).
BB
BB
==
+
VRH
VR
2
(1)
SS
SS
==
+
VRH
VR
2
(2)
E
S
SB
=
+
9
3/
(3)
It can be seen from Figure 5 that for the bulk modu-
lus, the modulus of graphene-modified asphalt with
1.79 w/% content is the largest and then this gradually
decreases but gradually increases with the increase of the
content. The bulk modulus at 8.36 w/% is 2.369 GPa.
The shear modulus generally increases first and then de-
creases with the increase of the content, and the maxi-
mum modulus of graphene at 5.19 w/% is 0.975 GPa.
Young’s modulus shows a wave trend with the increase
of graphene content, and the maximum modulus of
graphene at 5.19 w/% is 2.554 GPa. In general, when the
graphene content is in the range of 8.36 %, the bulk
modulus, shear modulus, and Young’s modulus of modi-
fied asphalt are greater than those of the base asphalt. It
shows that the increase of graphene can improve the me-
chanical properties of the asphalt. However, when the
content is too high above 5.19 %, it can be seen that the
shear modulus and Young’s modulus decrease, so the
content of graphene in asphalt should not be too high.
3.3 Interface Adhesion of Modified Asphalt
Adhesion energy is defined as the energy required to
desorb the asphalt-mineral interface model into two inde-
pendent asphalt and mineral models. Many researchers
have proved this calculation method to be effective for
evaluating the interface model’s bonding strength
24
.A s -
phalt-aggregate interface’s interaction energy and adhe-
sion work are calculated as Equations (4) and (5). In this
paper, the interface interaction is evaluated by the adhe-
sion work calculated by the interaction energy. Interfa-
cial interaction increases with adhesion work.
EEEE
interactions asphalt aggregate total
=+− (4)
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Materiali in tehnologije / Materials and technology 57 (2023) 5, 501–507 505
Figure 6: Adhesion work of modified asphalt-calcite interface Figure 5: Change of modified asphalt modulus with graphene content

W
EEE
A
ass -agg
asphalt aggregate total
=
+−
(5)
In the formula, W
as-agg
is the adhesion energy between
asphalt and aggregate interface (mJ/m
2
); E
interaction
is the
interaction energy of interface (mJ); E
total
is the total po-
tential energy of the interface model after NVT equilib-
rium (mJ); E
asphalt
and E
aggregate
are the potential energy
(mJ) of the single asphalt model and the aggregate sur-
face model, respectively; A is the contact area between
the asphalt and the aggregate interface (m
2
).
The bond strength of the interface between the
graphene-modified asphalt and the aggregate is shown in
Figure 6. In the calcite-asphalt interface model, the in-
teraction between the asphalt and the aggregate is mainly
physical adsorption behaviour, as the total adhesion work
is mainly composed of the non-bonding energy. The Van
der Waals interaction energy and electrostatic interaction
energy are significant contributors to the adhesion en-
ergy. It can also be seen from the diagram that the van
der Waals force plays a significant role in the adhesion
behavior of the base asphalt-calcite interface. The main
reason for this phenomenon is that the parallel arrange-
ment structure of the base asphalt can provide a more
considerable intermolecular interaction, which helps to
improve the van der Waals interaction. Therefore, the in-
terface between asphalt and aggregate has a higher van
der Waals energy. In general, the adhesion work in-
creases first and then decreases, and then increases with
the increase of dosage and is higher than that of unmodi-
fied base asphalt. Although the graphene content reached
the maximum value of 317.03 mJ/m
2
at 8.36 w/%, it was
not much different from the 303.75 mJ/m
2
at 1.79 w/%
and did not bring a great improvement. With comprehen-
sive graphene price factors and modified interface effect,
a graphene content of 1.79 w/% is the optimal value.
4 CONCLUSIONS
In this paper we conducted a molecular dynamics
simulation to investigate the interfacial mechanical prop-
erties of graphene-modified asphalt. Initially, the density
of AAA-1 asphalt was determined at room temperature
and pressure (p = 1 atm, T = 298.15 K), resulting in a
value of 0.979 g/cm
3
. Moreover, the solubility parameter
fell within the range of the asphalt test and closely re-
sembled the experimental value, confirming the accuracy
of our asphalt model. Subsequently, we conducted exper-
iments to explore the mechanical properties and interface
interactions of the graphene-modified asphalt through a
molecular dynamics simulation, leading to the following
conclusions.
(1) Upon analysing the cohesive energy density and
solubility parameter of the modified asphalt in compari-
son to the base asphalt model, higher values for both pa-
rameters were observed in the modified asphalt. The
physical properties of both the base and modified asphalt
were primarily influenced by the van der Waals force,
with the electrostatic force playing a relatively minor
role. Interestingly, as the graphene content increased, the
cohesive energy density and solubility exhibited a pattern
of initial increase, followed by a decrease, and finally a
subsequent increase again, reaching a peak at 1.79 w/%.
Moreover, the bulk modulus, shear modulus, and
Young’s modulus of the modified asphalt were signifi-
cantly higher than those of the base asphalt, indicating
that the addition of graphene enhanced the mechanical
properties of the asphalt. However, it is worth noting that
when the graphene content exceeded 5.19 w/%, both the
shear modulus and Young’s modulus showed a decline.
(2) Graphene-modified asphalt significantly enhances
the adhesion performance with weakly alkaline aggre-
gate calcite compared to the base asphalt. The main in-
teraction between the asphalt and aggregate is the physi-
cal adsorption behavior. Both the van der Waals and
electrostatic interaction energies make a substantial con-
tribution to the adhesion energy. The adhesion behavior
of the asphalt-calcite interface is mainly governed by the
Van der Waals force.
(3) From a comprehensive perspective, including
physical properties, mechanical properties, and adhesion
with the aggregate interface, it was found that the perfor-
mance of the modified asphalt does not improve indefi-
nitely with an increasing graphene content. Instead, there
exists an optimal value for the graphene content, and the
addition of graphene to the asphalt content should not
exceed this value. Considering both the graphene price
and its modification effect, the optimal graphene content
studied in this paper is 1.79 w/%.
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