Influence of Asphaltenes in the Properties of Liquid−Liquid Interfacebetween Water and Linear Saturated HydrocarbonsDheiver Santos,*,† Walisson Souza,‡ Cesar Santana,§ Everton Lourenco,§ Alexandre Santos,∥

and Marcio Nele⊥

†Centro Universitario Tiradentes, Av. Comendador Gustavo Paiva, 5017Cruz das Almas, Maceio, Alagoas 57038-00, Brazil‡Faculdade Pio Decimo, Campus III. Av. Pres. Tancredo Neves, 5655Jabutiana, Aracaju, Sergipe 49075-010, Brazil§Nucleo de Estudos em Sistemas Coloidais, Instituto de Tecnologia e Pesquisa, PEP/UNIT, Aracaju, Sergipe 49032-490, Brazil∥Universidade Federal do Parana, Rua Coronel Francisco Heraclito dos Santos, 210Jardim das Americas, Curitiba, Parana 82590-300, Brazil⊥Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitaria, Rio de Janeiro, 49032-490 Sergipe, Brazil

*S Supporting Information

ABSTRACT: Molecular dynamics simulations have been performed on the interface between linear saturated hydrocarbons andwater in the presence of an asphaltene molecule by measuring the properties such as mean square displacement, radialdistribution function, density profile using ave/spatial command, and interfacial tension (IFT) by OPLS and TIP3P FF (forcefields). The box of simulation contained one particle of asphaltene, 100 linear saturated hydrocarbons molecules, and 300 watermolecules in mixture with interfacial appropriate positioning. The main results show that a small amount of asphaltene in theinterface does not significantly alter the data of IFT and that the aliphatic and aromatic groups have preferred orientation.

1. INTRODUCTION

Molecular modeling studies of asphaltene1 are importantbecause with them it is possible to relate important molecularproperties such as structure factors and physicochemicalproperties such as molecular electronic structure, density,viscosity, and principally the friccohesity2 property to be relatedwith dipole momentum values. With this tool, it becomespossible to develop algorithms capable of calculating a specificmolecular structure and predict the properties of thesemolecules in different solvents and different temperature andpressure conditions.Headen et al.3 worked with asphaltene in combination with

toluene or heptane using the molecular dynamics (MD)technique. At large, it was detected that asphaltene forms bothdimers and trimers in toluene as in heptane, that is, aggregates.The authors observed that the aggregates hold longer inheptane than in toluene.The stability study of concentrated emulsions of water in oil

is imperative to provide a scientific approach to a majorproblem in the oil industry, helping to eliminate the empiricismpresent in the scientific community. Some studies have

proposed the use of MD tools for understanding the conditionsof the asphaltene in petroleum system models. It is importantto note that these studies were useful for understanding theconditions of stability of oil/water emulsion systems.Mikami et al.4 observed the interfacial rheology5−7 perform-

ance of asphaltene molecules in the oil/water interface usingMD simulations. It was found that asphaltene is preferentiallytransmitted in the oily phase (toluene), whereas theyaccumulate in oil/water interface to pure heptane. The authorsalso found that the interfacial tension (IFT)8,9 systemcontaining few asphaltene molecules is next to a system ofheptane/pure water.Ruiz-Morales and Mullins10 verified that the asphaltene

molecules remain in oil/water interface with the preferentialorientation, where the aromatic region lies in the hydrocarbon−water interface plane, although the aliphatic chains areperpendicular to the hydrocarbon−water interface. The authors

Received: January 28, 2018Accepted: March 23, 2018Published: April 5, 2018

Article

Cite This: ACS Omega 2018, 3, 3851−3856

© 2018 American Chemical Society 3851 DOI: 10.1021/acsomega.8b00102ACS Omega 2018, 3, 3851−3856

This is an open access article published under an ACS AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.

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also clarify that the rigidity interfacial film must have a directcorrelation with guidance as to the oil/water interface plans.The authors quoted above have contributed so far to the

understanding of the accommodation process of asphaltenemolecules at the interface. The literature needs to advance inother aspects to answer a series of problems, mainly, which arethe substances that manage to destabilize the grouping ofasphaltenes1 in the interface, favoring the rupture of theinterfacial film.In this work, we have performed MD simulations to obtain

information on the asphaltene influence in the interface, withthe aid of computer tools to measure properties such as radialdistribution function (RDF), mean square displacement(MSD), IFT, and density profile.

2. RESULTS AND DISCUSSIONSThis work provides new data from the molecular simulation ofasphaltene, linear saturated hydrocarbons, and water. Theresults are related to the positioning of water molecules andhydrocarbon molecules, forming an interface with asphaltene,similar to systems of water-in-oil emulsions for rigidity andstability study of interfacial film. The initial configuration wasproperly positioned in such a way as to form an interface; thedensity profile data from the ternary mixture reporting thepositioning of molecules along a few time steps, IFT for rigidityanalysis of film with added asphaltene and without addition ofasphaltene, RDF data for the analysis of preferential orientationof aromatic and aliphatic groups present in asphaltenemolecules, and MSD data for diffusive process analysis arethe main results these work (diffusion coefficients wereobtained from an unweighted least-squares fit to the MSD).Figure 1 presents the initial system configuration of linear-

saturated hydrocarbons−water−asphaltene. The data simula-

tion of the positions of atoms can be found in the SupportingInformation. The interface formation is one of the main factorsfor the analysis of configurational energy; in this work, theconfigurational energy is approximately equal to 5617.753(kcal/mol), and these values have been obtained with a cutoffboundary of long-range interactions equal to 12 Å. These datashow that simulations will be able to start properly withoutorbital overlap occurring.11 The system density data were closeto 0.35 g/cm3. The results of density are obtained consideringthe simulation box dimensions, in this work, lx, ly, and lz were36.25, 36.25, and 72.50 Å, respectively, and atom number 2025.It becomes a most interesting science that the configurationenergy share is contributed by both the solvents and the thirdchemical substance, and hence experimental verifications aredone with a survismeter device, which gives authentic data,where in real systems, asphaltene occupy freedom to approach

anywhere as per its interactive compatibility with either solvent,as shown in Table 1.

Figure 2 shows profile density data in the system withwater−hydrocarbon−asphaltene. The data in black, blue, and

red are, respectively, of the decane molecules, asphaltene, andwater. At the initial simulations and at the finished state, thepermanence of the interface between water and linear saturatedhydrocarbons is normally realized; substances with lowchemical affinity and the asphaltene molecules are mostlypositioned closest to the decane molecules. It is possible thatthe carbon chains of the asphaltene molecule must be with aspecial guidance for linear saturated hydrocarbon molecules.The van der Waals interactions of hydrocarbons (HC−decane)are dominated, and the polar groups such as nitrogen, oxygen,and sulfur are oriented toward the bulk water by dipole−dipoleforces.Figure 3 shows a typical behavior analysis of IFT as a

function of time, obtained by calculations of IFT obtainedthrough MD simulations and experimental data. Therefore, thedecrease in IFT systems cannot be explained by the diffusion ofasphaltene molecules to the interface because there is possibly

Figure 1. Initial configuration for decane−water−asphaltene interface.

Table 1. Values of the Averages and Uncertainty of theProperties of Molecular Simulations Obtained withUncorrelated Data

property average uncertainty

temperature (K) 293.114 0.041pressure (atm) −59.762 159.760potential energy (kcal/mol) 5617.753 581.032kinetic energy (kcal/mol) 1768.401 0.246density (g/cm3) 0.356enthalpy (kcal/mol per atom) 7303.114 552.048molecular energy (kcal/mol) 9235.338 553.523pairwise energy (kcal/mol) −3617.585 89.708volume (Å3) 95 277.111pressure tensor pxx (atm) −106.827 141.011pressure tensor pyy (atm) −78.289 124.627pressure tensor pzz (atm) 5.829 214.334box lengths in lx (Å) 36.251box lengths in ly (Å) 36.251box lengths in lz (Å) 72.502

Figure 2. Density profiles for decane−water−asphaltene interfacesystems (black; decane, red; water, and blue; asphaltene).

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an asphaltene barrier that is fully connected to its self-aggregation. The figure presents the data obtained from amodel system of water, decane, and asphaltene at 293 K. Afterthe data reached the mechanical equilibrium, the IFT value,indicating that in these conditions, would tend to stabilize forlong periods of time; this value would be for asphaltenemolecules that migrated to the HC−water interface. Thecomputational experiment duration was 107 ps correspondingaccording to the computer configuration used for 2 days; thistime is considered the time of balance needed to obtain a set ofIFTs. The values obtained during the simulations are on thesame order of magnitude (∼45−60 mN/m) of the experimentsusing model system (HC + water) with IFT using the pendantdrop method, as shown in Table 2, which gives the tests high-

reliability computing in proceedings of interaction force fields(TIP3P (water), CHARMM, and OPLS (asphaltene) (dec-ane)); another result that enhances the quality of the forcefields used is the pure-component density values for watermolecules (1.002 and 0.75 g/cm3) to decane,;these values wereobtained at the simulations’ end through the mean of resultsobtained for different computational experiments.In Figure 4, the central atom CH2 of the HC interaction data

was formed with sulfur of asphaltene (SM) atoms in eachasphaltene molecule in the first hydration layer up to ∼7 A, thesecond layer of the pair CH2/SM is present in ∼11 A; this

information can be obtained also for CH3/SM pair that has thefirst layer with ∼6 A, the second layer of the pair CH3/SM ispresent in ∼10 A and the third layer of the pair CH3/SM in∼11 A. These large distances can relate the geometry of lineardecane molecules and the amount of asphaltene molecules(when the algorithm traverses the whole phase space, therewould be a trouble in finding the grouping SM for possessing asingle molecule in the system, unlike the CH3 and CH2 groupsthat have a reasonable amount; the RDF is defined as a localdensity). The results also show that the pair CH3/SM has agreater interaction because of the greater number of hydrationlayer of molecular solvation. Interaction data and RDF can helpelucidate the interface position and asphaltene molecules polargroups orientation before the hydrocarbon−water pair, withthese new information related to adsorption isotherms12

modeled using this information. Therefore, it can be concludedthat the aromatic rings belonging to the asphaltene groups wereoriented to the oil phase because they have multiple solvationlayers in the CH3/SM−CH2/SM groups and small interactiondistances. Further studies are necessary to support thisaffirmation.Figure 5 and Table 3 are, respectively, the mean-squared

deviation of decane molecules, water, and asphaltene at 293 K

Figure 3. Results of MD simulations and experimental data ofinterfacial tension (mN/m).

Table 2. Interfacial Tension Experimental Values at 296.65K and 1600 sa

average (1) u (1) average (2) u (2)

Hexane + Waterγ (mN/m) 49.21 0.12 49.23 0.12A/mm2 50.14 0.02 50.14 0.02V/μL 34.41 0.02 34.42 0.01Heptane + Waterγ (mN/m) 49.94 0.08 49.93 0.09A/mm2 48.66 0.03 48.66 0.01V/μL 32.67 0.03 32.68 0.01Decane + Waterγ (mN/m) 43.76 0.74 43.87 0.62A/mm2 45.5 0.05 42.89 0.02V/μL 30.14 0.04 27.52 0.02

aInterfacial tension γ (mN/m). Uncertainty (u).

Figure 4. RDF (g(r)) for decane−water−asphaltene. CH2 (decane)−SM (sulfur of asphaltene) in black and CH3 (decane)−SM (sulfur ofthe asphaltene) in red.

Figure 5. MSD for decane (red)−water (black)−asphaltene (blue).

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and diffusion coefficient values of asphaltene molecules in theinterface model of oil/water at various temperatures. For aliquid−liquid interface composed of asphaltene molecules,hydrocarbons, and water, it is expected that a decreased IFTwith a rapid diffusion and a rapid process of asphaltenemolecule adsorption in the formed interface is observed. Thisorganization is independent of time and is increasingly used formolecules’ relaxation processes in the formed interface. Anemulsified system and its stabilizing mechanism is attributed tothe formation of mechanically strong and hard film or skin atthe interface (O/W) which prevents a droplets union. The skinis the asphaltene molecules association resulting in the liquid−liquid interface and the multilayer formed (a kind of packagingprocess).13

Asphaltene molecules may not have a classical aggregationstate in solution (CMC). Instead, they form nanoaggregatesand clusters that are prone to reorganization and relaxationprocesses. However, the colloidal stability studies of asphaltenemolecules in crude oil, when the system is not precipitated,prove that stability is related to the asphaltene molecules’aggregation state; these asphaltene precipitations can beinterpreted by physical forces, balance, mass, and the strengthof cross-association interactions between asphaltene moleculesand other substances in the system or other mixtures. In actualfact, consolidated asphaltene films around the droplets ofemulsion do not coalesce after certain aging times. Obviously,adsorption kinetics and MD studies play a key role in themechanical properties of films and an asphaltene adsorptionwith diffusion controlled at short times with individualmolecules, transitioning possibly to a barrier-controlled regimein which larger aggregates are involved,14 all of which helps inunderstanding the interfacial phenomena system assisting in thearea development.The asphaltene molecules have small diffusion coefficient

values (D ≈ 10−10 m2/s) because adsorption mechanisms arenot a diffusion-controlled type because of the high molecularweight of these compounds. This phenomenon has beendescribed before and seems to be linked to the Gibbs−Marangoni surface effects, in which the molecular diffusion ofsurfactants between the bulk phase and interface plays a roleduring droplet oscillations.6,15 By analyzing the effect oftemperature on the diffusion coefficient, the increasingtemperature increases the diffusion coefficient; thus, moregaps can be observed (the concentration of vacancies isthermally activated) and more thermal energy will be available;therefore, the rate of atomic diffusion increases with temper-ature.16

By analyzing saturated hydrocarbon effect on diffusionprocesses analysis, it has not been possible to conclude thatthe number increase of vacancies, that is, the use of smallhydrocarbons would entail increased diffusion processes byhaving greater space contact atoms (vacancies). The MSDshows that decane molecules sweep areas larger than watermolecules, and asphaltene molecules seem to prey on theinterface forming a rigid film with the adsorption process(showing the chart a constant area).

3. CONCLUSIONS

The results generated with the Playmol package initial setup arevery important for the studies of liquid−liquid interface.Density profile presents effectively the road traveled by theasphaltene molecules; it was expected that the asphaltenemolecules have preference for decane molecules instead ofwater molecules, but it is important to note that during thestimulation study of time steps, the asphaltene molecules donot spread fully into the bulk-phase linear saturated hydro-carbons (oil phase); therefore, we can interpret this situation asthe rigidity of the interfacial film is reinforced by the results ofMSD that presents constant values for asphaltene molecules.Asphaltene molecules cannot sweep large areas that reinforcethe theory of interfacial film stiffness another important resultand that is precedent in the literature with Mikami et al.,4 whichconcerns the influence of IFT with asphaltene molecules andno asphaltene molecules; it seems that few asphaltenemolecules affect very little, IFT that is, the IFT withasphaltene−linear saturated hydrocarbons−water molecules isvery similar to IFT of linear saturated hydrocarbons−watermixture. The asphaltene molecules have small diffusioncoefficient values (D ≈ 10−10 m2/s) because the adsorptionmechanisms are not a diffusion-controlled type because of thehigh molecular weight of these compounds. These results aremeant for studies of water/oil emulsion breaking.

4. METHODOLOGY

4.1. Molecular Simulation Protocol. The simulationswere conducted with the assistance LAMMPS package.17 Theinitial structure for the mixture of linear saturated hydrocarbons+ water + asphaltene was performed using the Playmolpackage18 that generates initial settings of any atoms andmolecules in space, creating file system structures. Thesesystems have assumed an interfacial initial structure betweenwater and hydrocarbons molecules.19 The box of simulationcontained one particle of asphaltene, 100 molecules of linearsaturated hydrocarbons, and 300 molecules of water in mixturewith appropriate interfacial positioning. These settings wereused in MD simulations. Previously, the asphaltene molecules’force field were obtained online using the SwissParam program;the water molecules used parameters TIP3P20 and, for linear-saturated hydrocarbons, OPLS.21

Playmol was used to define 3D space by optimizing thepairwise distances between atoms to avoid large van der Waalsrepulsive forces. The PACKMOL allows the user to define thedistance constraint between atom pairs that are not to bedisregarded. The package is written in FORTRAN and wasdeveloped by researchers at the ATOMS laboratory at UFRJ(Federal University of Rio de Janeiro). The package wasmodified to read the asphaltene molecules CHARMM,22 watermolecules TIP3P,20 and linear saturated hydrocarbons OPLS.21

The file format, including the contained atom types, generates a

Table 3. Self-Diffusion Coefficients (10−10 m2/s) forAsphaltene Using Different Parts of the Slope of MSD(t) vs t(0−2000 ps) at Different Temperaturesa

saturated hydrocarbons 283 K Fobj 293 K Fobj 303 K Fobj

hexane 4.66 1.04 4.95 1.72 0.80 6.33heptane 1.45 2.07 1.49 3.04 1.52 2.01octane 5.14 1.51 5.10 2.58 5.20 1.56nonane 0.56 7.29 0.59 8.27 0.61 7.23decane 0.99 6.08 1.02 6.02 1.04 5.96

aDiffusion coefficients can be calculated directly in MD simulations bytracking the mean square displacement of the center of mass ofasphaltene molecules as a function of time. The literature reports arelatively wide variation in the simulation results from 0.01 × 10−10 to8 × 10−10 m2/s.30−32 Fobjobjective function values.

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final system of molecules that meets the pairwise distanceconstraint and guarantees all molecules fit inside the user-defined MD cell. Playmol works with containing hundreds ofatoms, but it does require equally vast hardware to accomplishsuch work. A random seed value can be used to generatemultiple MD cells for MD study. The Playmol package inputfiles were described in the Supporting Information.In short, LAMMPS (http://lammps.sandia.gov) integrates

the motion equations of Newton for groups of atoms,molecules, or macroscopic particles that interact through forcesof short or long range; for the integration of the equation ofmotion, LAMMPS also uses the Verlet algorithm,23 which arerecomputed every few time steps (new velocities and newpositions were obtained by integrating the motion equations).LAMMPS uses neighboring list atoms to give a better efficiencyin their simulations and nearby particle monitoring. The readyfor systems with particles that are repulsive to form shortdistances that the density is not affected in a significant way.LAMMPS was suggested as an MD simulation tool because ofits robust collection capabilities. LAMMPS has integrated thesupport for MSD and RDF to study the movement of smallatoms in interface systems. LAMMPS contains a deformationfeature that allows the researcher to apply normal and shearstrains to a system to calculate stresses in interface for theproposed study. The LAMMPS requires an ensembleassociated with the boundary conditions imposed on thesystem under study. The commonly studied simulation ofliquids in systems is under the canonical ensemble conditions,with temperature T, volume V, and N number of molecules(NVT). This work used the NVT ensemble to simulatemixtures of linear-saturated hydrocarbons + water + asphaltene,1 ns integration time, temperature of 293 K, cutoff radius = 12Å, and long-range interactions by means of Ewald algorithm.24

The Playmol input files were described in the SupportingInformation.4.2. Density Profile. The densities were calculated using

the space command “fix/spatial” LAMMPS package, whichsummarizes the densities by atom and the averages for slices ofa specified thickness along a specified axis. Along the z-axis, thestructural change that the asphaltene molecule that turns awayfrom the other axes x and y was chosen to observe. Figure 2shows the general profile of mass density of linear saturatedhydrocarbons + water + asphaltene.4.3. Interfacial Tension. The interfacial rheology tension

can be determined from the molecular dynamics.25−27 Theinterfacial rheology tension γ(t) is defined as

∫γ = −t p n t p n t n( ) [ ( , ) ( , )]dnn tt (1)

where n is the axis normal to the interface, and hence pnn and pttare normal and tangential constituents, respectively, of thepressure tensor, p. The integral must be carried out from thebulk location of one phase to that of another phase. The IFT γcan be considered by averaging γ(t) over the simulation time

γ γ= ⟨ ⟩t( ) (2)

In our situation, because the z-axis is perpendicular to theinterfaces, eq 3 becomes

∫γ = −+⎡

⎣⎢⎢

⎤⎦⎥⎥p z t

p z t p z tz( , )

( , ) ( , )

2dzz

xx yy

(3)

4.4. Radial Distribution Function. The RDF28 is aprobability distribution of definition an atom at a separation, r,from other atoms (this methodology has the utility to analyzethe probability of separation between the atoms of interest).RDFs exist for both equilibrium and nonequilibrium systems.LAMMPS has a well-organized functionality to compute theRDF for a given cutoff distance radius. The RDF command isappealed with a compute command. The following examplecomputes four RDFs, for 1-1, 1-2, 2-1, and 2-2 pairs. The 1-2and 2-1 RDF should be identical (atom type).Example: compute RDF all RDF 100 1 1 1 2 2 1 2 2.4.5. Mean Square Displacement. MSD29 is a quantity of

the mean distance squared of a particle or an atom. It is definedas

= ⟨ ⟩ = ⟨ − ⟩t x t x t xMSD( ) ( ) ( ( ) (0))i i i2 2

(4)

where (xi(t) − xi(0))2 is the distance of particle i traveling over

about time interval. ⟨...⟩ ensemble average is defined as theaverage of a quantity that is a function of the individual states ofthe system. If we consider all of the particles in the system, thenwe could write

∑= ⟨ ⟩=

tN

x tMSD( )1

( )i

N

i1

2

(5)

where N is the number of particles in the system.4.6. Experimental Data of IFT. The IFTs (γ) were

measured at 296.65 K using the automated Teclis Tracker (ITConcept, France) by axisymmetric interface profile analysistechniques. This work was made the measurement of interfacetension such that pendant drops were in line with the opticsand charge-coupled device camera as the instrument to obtainvisualization on the computer screen (this equipment uses asoftware which has the resolution of equations of Young−Laplace). The linear saturated hydrocarbon samples were mixedwith water in bulk. Syringe (10 μL) with an attached u-shapedneedle was prefilled with the linear saturated hydrocarbonssample. The syringe was mounted on the syringe holderpositioned above the cuvette that contained 25 mL of waterphase. The syringe was dropped such that the needle wassubmerged in the cuvette’s water phase, and a 25 μL drop(interfacial area, A = 41 mm2) expelled the sample in the needletip. The IFTs were instantaneously measured as a function ofthe drop aging time until near-equilibrium values reachedlikewise; dynamic IFT γ(t) versus time t for linear saturatedhydrocarbons−water was measured. Currently, the new devicewhich is most accurate and streamlines several variables,especially the pressure and temperature fluctuation, has beenevolved, which produces most accurate results withoutinterference of the experiential forces which induce transitionsas the asphaltene has a larger number of pi conjugations.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsomega.8b00102.

Force field for all molecules of the studies and script forLAMMPS simulations (PDF)

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■ AUTHOR INFORMATIONCorresponding Author*E-mail: dheiver.francisco@souunit.com.br, dheiver.santos@gmail.com (D.S.).ORCIDDheiver Santos: 0000-0002-8599-9436NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to the CNPQ [grant number 400309/2017-3], ATOMS laboratory at UFRJ (Federal University ofRio de Janeiro), and Sandia National Laboratories for theirgreat service.

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ACS Omega Article

DOI: 10.1021/acsomega.8b00102ACS Omega 2018, 3, 3851−3856

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