Asphaltene Aggregation and Assembly Behaviors in Organic Solvents

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Asphaltene Aggregation and Assembly Behaviors in Organic Solvents with Water and Inhibitor Bin Jiang, Rongya Zhang, Na Yang, Luhong Zhang, and Yongli Sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04121 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Energy & Fuels

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Asphaltene Aggregation and Assembly Behaviors in Organic Solvents with

2

Water and Inhibitor

3

Bin Jiang,† Rongya Zhang, †,‡ Na Yang,*,† Luhong Zhang,† and Yongli Sun†

4 5

†School

of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

6 7 8

‡Department

of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 1H9

9 10 11 12 13 14 15 16 17 18 19 20 21

Corresponding author: School of Chemical Engineering and Technology, Tianjin University,

22

Tianjin 300072, People's Republic of China.

23

E-mail address: [email protected] (Na Yang) 1

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Abstract:

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Asphaltenes cause problems such as emulsion formation and deposition/precipitation

26

during crude oil production, processing and transport. A deeper understanding of the

27

behaviors of asphaltenes is needed to design remediation treatments to minimize costs

28

during production. In this study, the aggregation and assembly behaviors of

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asphaltene model compound C5PeC11 under different conditions were studied and

30

visualized through molecular dynamic simulations. C5PeC11 aggregates were formed

31

as a result of the competition between the solvation of C5PeC11 in solvents and the

32

self-association of C5PeC11 molecules. One-dimensional bent linear assembly of

33

C5PeC11 aggregates was observed upon the addition of n-heptane to toluene.

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Although the water content was low, the addition of water molecules resulted in

35

enhanced C5PeC11 aggregation and considerably different C5PeC11 aggregates

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assembly behavior. C5PeC11 aggregates formed a cluster in the way of parallel

37

horizontal connection with hydrocarbon tails located outside and polar groups located

38

inside. The network formed via the hydrogen bonding between C5PeC11 and water as

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well as between water molecules provided new bindings for C5PeC11 aggregates.

40

Water molecules were indispensable for the formation and retention of C5PeC11

41

aggregate cluster. The dodecylbenzene sulfonic acid (DBSA) and water competed

42

with each other to influence asphaltenes associations when they were both present in

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the system. Both the hydrogen bonding between C5PeC11 and water and hydrogen

44

bonding between water molecules contributing to the development of the network

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were hindered by DBSA. DBSA molecules succeeded in stabilizing C5PeC11 even in

46

the presence of water molecules. 2

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Key words: Asphaltene; Model compound; C5PeC11; Molecular dynamic

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simulation; Aggregates Structure; One dimensional assembly; Water; Cluster;

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Aggregation

inhibitor.

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1. Introduction

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Naturally occurring asphaltenes in crude-oil are known for causing considerable

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problems in the petroleum industry.1 Primary damages caused by asphaltenes include

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plugging wellbore,2 fouling transportation pipelines,3 producing a stabilized oil-water

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emulsion,4 and catalyst deactivation.5 Most of these problems can be attributed to the

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aggregation and precipitation behavior of asphaltenes, which have been widely

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observed in experimental studies.6 Given the complexity and diversity of asphaltenes,

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different model compounds have been proposed in literature to represent the

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constituents of asphaltenes, more details of asphaltene model compounds were

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reviewed by Sjöblom et al.7 Extensive studies showed that the perylene-based

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polyaromatic compounds exhibit similar solubility and interfacial properties to real

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asphaltenes, which makes them promising model compounds to mimic the properties

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of

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N-(1-undecyldodecyl)-N’-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylbisimi

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de (C5PeC11), showed similar flocculation kinetics to that of the irreversibly

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adsorbed asphaltenes.9 The irreversibly adsorbed asphaltenes containing the highest

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number of polar groups is the fraction of asphaltenes responsible for the observed

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flocculation in whole asphaltenes.9, 10 Owing to the remarkable similarity between the

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behaviors of C5PeC11 and real asphaltenes,9,11,12 C5PeC11 was used as the model

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compound in this research to study the aggregation and assembly behaviors of

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asphaltenes. Many factors can influence the aggregated structures of polyaromatic

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compounds, such as their molecular structures,13 type of solvents,14 solution

asphaltene

molecules

in

crude

oil.8

Previous

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study

found

that

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concentrations,15 and external conditions including temperature as well as pressure.16

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It is of importance to mention the effects of solvent properties because asphaltenes are

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defined as a solubility class of compounds that are toluene soluble and

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n-heptane-insoluble.17 Much work has been dedicated to the association behaviors of

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asphaltene components. However, the underlying mechanisms at molecular level

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accounting for the different size and geometry of asphlatene aggregates are still

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unanswered questions. A deeper understanding of the molecular interaction energies

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behind the aggregation and aggregates assembly is necessary for the fine tuning of the

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size and structures of asphaltene aggregates. It is also important to note that probing

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early stage dynamic process of asphaltenes association is extremely difficult if not

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impossible due to inadequate instrumentation. Computational approaches such as MD

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are particularly suitable for such dynamic investigations.

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Water, as a common inorganic solvent, is a “bad” solvent for asphaltenes.

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Despite the ubiquitous presence of water in the processing of petroleum resources, the

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effect of water effect on asphaltene aggregation and assembly behavior has not drawn

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enough attention. Asphaltenes fractionated from oil can stabilize emulsion formation

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rapidly when water is added into the system.18 However, asphaltenes’ actual

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interaction with trace amounts of water in organic solvents has received little or no

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attention. This is probably due to the low solubility of water in organic solvents. To

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study asphaltenes aggregation in the laboratory, usually the water was first removed

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from the crude oil sample, resulting in crude oil samples with very little water

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(typically around 0.5 wt %) being studied.19 However, small amounts of water were 5

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still present in crude oils. Aggregation of asphaltenes in water solution and the

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enhancement effect of water on asphaltenes aggregation in organic solvents have been

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observed experimentally, and interactions of model asphaltene molecules with water

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molecules were studied using molecular mechanics calculations.13, 16, 21-23 However,

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only one water molecule in organic solvent or pure water solution was used to probe

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its effect in the simulations.13, 16, 23 Hence, indirect evidence was given in the literature

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for the interactions between large amount of water molecules and asphaltene

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molecules in organic solvents although this bears much more similarity to the real

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case in industry. Due to the similarity between asphaltene and surfactant structure, it

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was widely accepted that asphaltenes have similar properties to surfactants. Extensive

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discussions exist on the actual role of water, even at trace levels, in the formation of

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the reversed micelle of surfactants.24-26 While the actual reason for the colloidal

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behavior of asphaltenes has not been resolved due to the complex nature of

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asphaltenic material. As trace amounts of water are present in most if not all crude

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oils, it is of interest to study the effect of water in the association process of

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asphaltenes in organic solutions.

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Due to the problematic feature of asphaltenes, considerable efforts had been

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made to disperse and stabilize asphaltene.27-31 Among the chemicals used for the

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stabilization of asphaltene, dodecylbenzene sulfonic acid (DBSA) is promising for the

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low-dosage inhibition of asphaltene precipitation.28,

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DBSA molecules in stabilizing asphaltene under harsh conditions, such as in the

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presence of water, was not studied. Furthermore, it is worth investigating how DBSA

32-34

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However, the efficiency of

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competes with water molecules at molecular level to provide guidance for future

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selection of asphaltene inhibitors under harsh conditions.

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The present paper reports findings on the interactions between asphaltene model

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molecules and organic solvent molecules as well as water molecules and asphaltene

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inhibitors at molecular level. The contribution of different interactions to the

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aggregates size and structure was probed. It needs to be mentioned that the content of

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water bearing much more similarity to the real case in industry was investigated. The

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role of water molecules in the formation of asphaltene cluster was discussed.

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Moreover, the competitive effect of water and asphaltene aggregation inhibitor on

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asphaltene behaviors in organic solvents was also investigated. These findings are

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important for the further understanding of asphaltene self-association, flocculation,

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and deposition in the oil industry.

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2. Simulation methods

129

GROMACS 5.1.2 software package with 53A6 parameter set were used for all the

130

simulations in this paper. The initial structures of C5PeC11 and DBSA (Figure S1 in

131

support information) were constructed using the Material Studio 8.0 software. Energy

132

optimizations were then conducted to achieve reasonable coordinates of those

133

molecules. The topology and GROMACS structure file for those molecules was

134

generated by supplying the coordinates of the molecule to Automated Topology

135

Builder (ATB) and Repository server (version 2.2). 35 United atoms model was used

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for CH2 and CH3 groups on the aliphatic chains, whereas the carbons in the 7

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polyaromatic region were modeled as sp2 hybridized carbons. The charges of the

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default topology obtained from ATB were then modified so that they were compatible

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with the GROMOS96 force field parameter set 53A6. The charges were changed from

140

the default values as it has been reported in literature that the default charges can lead

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to unphysical results. On the contrary, using analogous functional group existing in

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GROMOS96 has proven to be a more reliable approach.13, 14, 36 The topology for the

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toluene and n-heptane were generated from the phenylalanine amino acid and DPPC

144

fraction in the GROMOS96 force field parameter set 53A6 through the pdb 2gmx

145

routine in GROMACS.13, 37 The force field parameters for the substances are shown in

146

SI-2. The periodic boundary conditions, full electrostatics with the particle-mesh

147

Ewald method, a cutoff distance of 1.4 𝑛𝑚 for van der Waals and electrostatics

148

pairwise calculations, the LINCS algorithm to constrain all bonds for molecules, and a

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time step of 2 fs were used for all simulations.38-40 Simple-point-charge (SPC) water

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model was adopted here for modeling of water molecules. The SPC water model

151

chosen here has been widely tested in the literatures and shown to be suitable for

152

asphaltene simulations.13, 38, 40 In order to examine the sensitivity and applicability of

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force field parameters in our systems, the bulk physical properties of the solvent were

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calculated in our simulations. The results show that our simulation reproduced fairly

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well the density and self-diffusivity of solvents compared with experimental data.

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Thus the force field parameters used in this study were able to reproduce the

157

properties of the simulated systems with reasonable accuracy. (See Supporting

158

Information SI-3 for details). 8

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The simulation boxes were constructed by firstly inserting 24 C5PeC11

160

molecules in a cubic box of 12 𝑛𝑚 edges. The boxes were then solvated with toluene,

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n-heptane, water, and DBSA molecules as needed. A total of four simulation boxes

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were constructed as shown in Table 1. During each simulation, static structure

163

optimization was first performed to minimize the total potential energy. After energy

164

minimization, all simulations were carried out under the NPT ensemble at 298 K and

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1 bar pressure. For the first 3 𝑛𝑠 of NPT equilibration, the Berendsen thermostat and

166

barostat were used to quickly relax the system to a constant pressure and

167

temperature.37-40 After 3 𝑛𝑠, all of the simulations for NPT MD production were

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performed using Nosé−Hoover thermostat and Parrinello−Rahman pressure coupling

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algorithm.37-40 The pressure and temperature coupling constants of τp = 3 𝑝𝑠 and τT =

170

0.3 𝑝𝑠, respectively, were used throughout the simulations. After simulations, the

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structure and dynamic properties of the system were analyzed using the GROMACS

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built-in analytical tools. All the molecular configurations and snapshots were

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visualized and acquired with visual molecular dynamics (VMD).41

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To facilitate the discussion of the simulation results, we introduced the following

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acronyms and definitions. Each of these systems is referred by the composition of the

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simulation box. For example, the first system that contains C5PeC11 molecules and

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toluene molecules is referred to as C5PeC11_toluene system. Upon the addition of

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n-heptane, the solvent becomes heptol solution, thus system 2 is referred to as

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C5PeC11_heptol system. As to system 3 and system 4, H2O and DBSA are included

180

in

the

definition

and

represented

by

C5PeC11_heptol+H2O,

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and

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DBSA_C5PeC11_heptol+H2O respectively. According to Andersen et al,18 the

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solubilized water content in toluene without asphaltenes was around 0.04%, 400 ppm

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(w/w), and when 30 g/L asphaltene was added into toluene, the water content

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increased to around 0.12%, 1200 ppm (w/w). Thus 36 water molecules were adopted

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in C5PeC11_heptol+H2O, and DBSA_C5PeC11_heptol+H2O systems to produce

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reasonable water content in the presence of asphaltenes in organic solvent solutions.

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Demonstrations of achieving dynamic equilibrium using time evolution of radial

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distribution functions (RDFs) between polyaromatic cores of C5PeC11 molecules, as

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well as time evolution of temperature and potential energy are available in the SI-4.

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The definitions for the aromatic core group, polar group, hydrocarbon tail and plane

191

group used in the paper are given in SI-5.

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Table 1. System composition and simulation setup.

Definition

1

C5PeC11_toluene

24

80

5000+0

2

C5PeC11_heptol

24

80

2680+2670

3

C5PeC11_heptol+H2O

24

36

80

2640+2646

4

DBSA_C5PeC11_heptol+H2O

24

36

80

2432+2450

3. Results and Discussion

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3.1 Organic Solvent Effect

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DBSA number

Number of solvent molecules (Ntoluene+n-heptane)

System

193

H2O number

Time (𝑛𝑠)

C5PeC11 number

120

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Toluene is known as a “good” solvent for asphaltenes. However, n-heptane is the API

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standard-asphaltene precipitant. System 1 and system 2 were simulated to probe the

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effect of solvent properties on the asphaltenes model compound C5PeC11 aggregation

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and assembly behaviors. Although toluene is good solvent for asphaltene, aromatic

199

cores stackings indicating C5PeC11 aggregation were still found in C5PeC11_toluene

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system as indicated in Figure 1. The number of peaks as well as the positions of the

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peaks was the same for C5PeC11_toluene system and C5PeC11_heptol system.

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Therefore, adding n-heptane to toluene solution did not change the nature of the

203

interactions between C5PeC11 molecules. The first peak procured at 0.37 𝑛𝑚

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represents direct π-π stacking between aromatic cores of two C5PeC11 molecules.

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While the second and third peaks at 0.74 𝑛𝑚 and 1.11 𝑛𝑚, respectively, represent the

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extension of π-π stacking to the third and fourth molecules. It can be seen from Figure

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1 that the addition of n-heptane into toluene solution increased the possibility of π-π

208

stacking between aromatic cores of C5PeC11 molecules. Therefore, replacing toluene

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with asphaltene precipitant did increase the asphaltene model compound C5PeC11

210

aggregation.

211 11

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Figure 1. Radial distribution function g(r) for COM separation distance r (𝑛𝑚) between aromatic

213

cores of C5PeC11 molecules averaged over the last 5 𝑛𝑠 of total 80 𝑛𝑠 MD simulations for the

214

systems.

215

To better understand the n-heptane effect on the size and distribution of the π-π

216

stacked structures, the n-mers of C5PeC11 π-π stacking were further calculated and

217

shown in Table 2. As shown in Figure 1, the first RDF peak indicating the aromatic

218

core stacking between two molecules ends at 0.45 𝑛𝑚 . Therefore, 0.45 𝑛𝑚 was

219

chosen as the criteria to define molecules being aggregated with each other.15 For

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example, if the COM distance between molecules A and B was less than 0.45 and at

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the same time the distance between molecules B and C was also less than 0.45 𝑛𝑚,

222

then molecules A, B, and C were considered to form a three-molecule stacking

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(3-mers). As shown in Table 2, when n-heptane molecules were added to toluene,

224

monomers in the C5PeC11_heptol system disappeared, whereas one more 3-mers was

225

found at the cost of consuming one 2-mers. The n-mers results as well as Figure 1

226

confirmed that C5PeC11 had a higher tendency to aggregate in heptol than in toluene.

227

To quantitatively compare the role of n-heptane in enhancing aggregation, the

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average aggregation number for C5PeC11 molecules was calculated as follows.

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𝑛𝑎𝑣𝑔 = ∑𝑛𝑛𝑓(C5PeC11𝑛)

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Where the fraction of each C5PeC11 aggregate, f(C5PeC11n), was calculated as the

231

sum of the number of molecules in the aggregates at different stacking modes

232

(n-mers) multiplied by its occurrence number (𝑚), and then divided by the total

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number of C5PeC11 molecules (∑ (𝑚 × 𝑛 ― mers)) in all the aggregates (eq. 2). 𝑛

(1)

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Energy & Fuels

𝑚 × 𝑛 ― mers

𝑓(C5PeC11𝑛) = ∑ (𝑚 × 𝑛 ― mers)

(2)

𝑛

235

For C5PeC11 molecules in toluene, the 𝑛𝑎𝑣𝑔 was 3.17, while this number

236

increased to 3.33 when half of toluene molecules were replaced by n-heptane. The

237

increment was not significant considering n-heptane working as asphaltene precipitant.

238

It should be kept in mind that 𝑛𝑎𝑣𝑔 was calculated based on direct π-π stacking of

239

molecules within aggregates. This is reasonable taking into consideration the weak

240

increment for the first RDF peak in Figure 1 at 0.37 𝑛𝑚. However, it is noteworthy

241

that compared with the first peak the increment of the intensity for the RDF peaks in

242

Figure 1 is much more obvious for the second and third peak.

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Table 2. π-π stacking type, their corresponding occurrence number, and average aggregation

244

number of C5PeC11 in the systems. System C5PeC11_toluene C5PeC11_heptol C5PeC11_heptol+H2O DBSA_C5PeC11_heptol+H2O

6-mers

2 0

5-mer s

4-mer s

3-mer s

2-mer s

Monomer

𝑛𝑎𝑣𝑔

1 1 0 0

1 1 1 0

2 3 0 3

4 3 3 3

1 0 2 9

3.17 3.33 4.25 2

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First layer stacking pair (FLSP), second layer stacking pair (SLSP) and third

246

layer stacking pair (TLSP) configuration were calculated to quantitatively compare

247

the overall π-π stacking structures formed at each peak location (r = 0.37, 0.74, and

248

1.11 𝑛𝑚). The FLSP configuration was quantified by two aromatic cores with the

249

COM separation distance less than 0.45 𝑛𝑚. Pairs for aromatic cores with COM

250

separation distance between 0.45 𝑛𝑚 and 0.9 𝑛𝑚 fell into SLSP. TLSP configuration

251

corresponded to pairs for aromatic cores with COM separation distance that was 13

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larger than 0.9 𝑛𝑚 and smaller than 1.25 𝑛𝑚. By summing up FLSP, SLSP and

253

TLSP, the total stacking pair (TSP = FLSP+SLSP+TLSP) was defined as the π-π

254

stacking configuration formed by two aromatic cores with COM separation distance

255

less than 1.25 𝑛𝑚. On the basis of the above distance criteria, the number of stacking

256

pairs in each system averaged over the last 5 𝑛𝑠 of the 80 𝑛𝑠 simulation time was

257

summarized in Table 3. As shown in Table 3, the addition of n-heptane molecules

258

generally increased the number of stacking pairs, corresponding to greater peak

259

heights in C5PeC11_heptol system observed in Figure 1. The minimum increment for

260

FLSP gave explanation for the weakest enhancement in the intensity for the first RDF

261

peak at 0.37 𝑛𝑚 (Figure 1). It is important to note that introducing n-heptane into

262

toluene significantly increased the number of stacking pairs at larger COM separation

263

distance, especially for the TLSP. The extension of stacking pairs to the second and

264

third neighboring layers will have impact on the assembly behavior of C5PeC11

265

aggregates. Therefore, it is speculated that besides enhancing aggregation, n-heptane

266

molecules played an important role in changing the assembly behavior of the

267

aggregates.

268

Table 3. Number of π-π stacking pairs formed in the systems averaged over the last 5 𝑛𝑠 of the 80

269

𝑛𝑠 simulations. System

FLSP

SLSP

TLSP

TSP

C5PeC11_toluene C5PeC11_heptol C5PeC11_heptol+H2O

15 16 16

8 10 15

7 24 12

30 50 43

DBSA_C5PeC11_heptol+H2O

9

7

13

29

14

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Snapshots of C5PeC11 molecules in these two systems were obtained from the

271

trajectory to reveal the geometry differences. Table 2 shows that the largest stable

272

C5PeC11 aggregate in toluene and heptol both consisted of 5 molecules. What is

273

different is that the aggregates in toluene system were dispersed without apparent

274

order in the simulation box, while the aggregates in heptol tend to assemble in a linear

275

way (Figure 2). Considering the periodic boundary conditions used in the simulation,

276

when a molecule leaves the box by crossing a boundary, its image enters from the

277

opposite side. The 3-mers in the blue circle in Figure 2(b) was actually distributed

278

between the 5-mers (yellow circle) and the 3-mers (red circle). The snapshots vividly

279

illustrated that the addition of n-heptane led to the formation of one-dimensional

280

linearly assembled C5PeC11 aggregates. The linear one-dimensional assembly

281

phenomenon was responsible for the striking larger number of TLSP in

282

C5PeC11_heptol system (Table 3). Thus it can be concluded that the composition of

283

the solvent can affect the degrees of π-π stacking inside the aggregates. In addition,

284

attentions should be paid to its influence on the assembly behavior of aggregates. The

285

role of n-heptane in enhancing C5PeC11aggregation and changing the assembly

286

behavior of the aggregates was further confirmed by the behaviors of C5PeC11 in

287

n-heptane system (Shown in SI-6 for detail).

15

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288 289

Figure 2. Snapshot of C5PeC11 molecules at 80 𝑛𝑠 for: (a) C5PeC11_toluene system; (b)

290

C5PeC11_heptol system. Solvent molecules were removed for clarity.

291

Solvent accessible surface area (SASA) representing solvation of C5PeC11

292

molecules in the organic solvent was calculated to interpret the enhancement of

293

C5PeC11 aggregation by n-heptane. It can be seen from Figure 3 that C5PeC11

294

molecules have larger SASA in toluene than in heptol, which implies that the

295

solubility of C5PeC11 in toluene is higher. The higher solubility was caused by

296

molecular interactions between toluene and both the aromatic and aliphatic chain

297

regions of C5PeC11, while n-heptane was prone to interact with aliphatic chain

298

regions of C5PeC11 only. This inference was confirmed later in the following

299

sections. Decreased solubility in heptol is one of the contributing factors for the

300

enhancement of C5PeC11 aggregation in C5PeC11_heptol system.

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Energy & Fuels

301 302

Figure 3. Solvent accessible surface area (SASA) of the C5PeC11 molecules calculated over the

303

simulation time for the systems.

304

To elucidate the behaviors of asphaltenes in toluene and heptol at molecular level,

305

the interaction energies between molecules were calculated. The interaction energy

306

was determined by subtracting the total energy of the complex with the energies of the

307

corresponding isolated components.42,

308

interaction energy between C5PeC11 and toluene, the C5PeC11-toluene complex,

309

isolated C5PeC11 and isolated toluene were extracted, respectively. The equation

310

used for the calculation of the interaction energy was summarized as equation 3. The

311

interaction energy was resolved into two components including Coulomb electrostatic

312

interaction energy and van der Waals (vdW) interaction energy.

313

EA-Binter= EA+B − EA− EB

43

For example, in order to determine the

(3)

314

The calculated total interaction energy as well as vdW and Coulomb interaction

315

energy for these four systems are given in Tables 4, 5, 7, and 8. C5PeC11 aggregates

316

were formed as the result of competition between the solvation of C5PeC11 in solvent

317

and aggregation of C5PeC11 molecules. The solvation of C5PeC11 in the solvent was 17

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318

controlled by the interactions between C5PeC11 molecules and solvent molecules,

319

while the aggregation was resulted from C5PeC11-C5PeC11 interactions. These two

320

effects competed with each other to determine the final C5PeC11 aggregation state.

321

For C5PeC11_toluene system, the large negative interaction energy (-9589.17 kJ/mol)

322

between C5PeC11 and toluene suggests that C5PeC11 molecules have a high

323

tendency to interact with toluene, which accounts for toluene known as good solvent

324

for asphaltene. Despite this, the interaction energy between C5PeC11 molecules

325

(-4161.54 kJ/mol) was quite strong as well, thus aggregation of C5PeC11 in toluene

326

was observed in our simulation. With regards to C5PeC11_heptol system, the

327

solvation of C5PeC11 in heptol was contributed by the interactions between C5PeC11

328

and toluene, as well as the interactions between C5PeC11 and n-heptane. In total, the

329

absolute value of attractive interaction energy between C5PeC11 and solvent reduced

330

from ABS (-9589.17) kJ/mol in C5PeC11_toluene system to ABS (-7949.68) kJ/mol

331

in C5PeC11_heptol system. For the -7949.68 kJ/mol interaction energy, -4580.17

332

kJ/mol was due to the C5PeC11-toluene interaction, while -3369.51 kJ/mol was

333

attributed to the C5PeC11-(n-heptane) interaction. Since the C5PeC11-toluene

334

interaction was stronger than the C5PeC11-(n-heptane) interaction, the replacement of

335

toluene by n-heptane was destined to decrease the solubility of C5PeC11 molecules in

336

solvent. In addition, the interaction strength between C5PeC11 molecules increased

337

from -4161.54 kJ/mol in C5PeC11_toluene system to -5038.23 kJ/mol in

338

C5PeC11_heptol system. Combining the weakened C5PeC11-solvent interactions and

339

strengthened C5PeC11-C5PeC11 interactions, the enhanced C5PeC11 aggregation in 18

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Energy & Fuels

340

heptol system was not unexpected.

341

Table 4. vdW and Coulomb interaction energies between molecules in C5PeC11_toluene system.

342

Molecules

vdW (kJ/mol)

Coulomb (kJ/mol)

vdW+Coulomb (kJ/mol)

C5PeC11-toluene C5PeC11-C5PeC11

-9069.92 -3574.08

-519.25 -587.46

-9589.17 -4161.54

Table 5. vdW and Coulomb interaction energies between molecules in C5PeC11_heptol system. Molecules

vdW (kJ/mol)

Coulomb (kJ/mol)

vdW+Coulomb (kJ/mol)

C5PeC11-toluene C5PeC11-(n-heptane) C5PeC11-C5PeC11

-4311.91 -3369.51 -4328.69

-268.26 0 -709.54

-4580.17 -3369.51 -5038.23

343

The vdW and Coulomb interaction energies between C5PeC11 and solvent were

344

investigated to understand the different effects of toluene and n-heptane. -9069.92

345

kJ/mol vdW interaction energy and -519.25 kJ/mol Coulomb interaction energy were

346

found between C5PeC11 molecules and toluene molecules in C5PeC11_toluene

347

system. In C5PeC11_heptol system, the absolute value of attractive vdW interaction

348

energy was reduced to ABS (-4311.91) kJ/mol and the Coulomb interaction energy

349

was reduced to ABS (-268.26) kJ/mol for C5PeC11-toluene interaction. At the same

350

time, -3369.51 kJ/mol vdW interaction energy was found between C5PeC11

351

molecules and n-heptane molecules. By comparing the interaction energy value in

352

C5PeC11_heptol system, we can find that the absolute value of attractive vdW

353

interaction energy is larger for C5PeC11-toluene interaction (ABS (-4311.91) kJ/mol)

354

than C5PeC11-(n-heptane) interaction (ABS (-3369.51) kJ/mol). In addition, no

355

Coulomb interaction was found between C5PeC11 and n-heptane, while extra

356

attractive Coulomb interaction was found between C5PeC11 and toluene (-268.26 19

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357

kJ/mol). Toluene made up of one aromatic core and one -CH3 group showed a general

358

resemblance to the structure of C5PeC11. Both the aromatic core and the -CH3 group

359

made contributions to the pretty large negative vdW interaction energy between

360

C5PeC11 and toluene, while charges on aromatic core of toluene molecules also

361

brought attractive Coulomb interaction to C5PeC11 molecules. In the case of

362

n-heptane, no Coulomb interaction was found between C5PeC11 and n-heptane

363

molecules. In addition, the lack of aromatic core on n-heptane also resulted in smaller

364

attractive vdW interaction energy between C5PeC11 and n-heptane molecules. The

365

variation of solvent molecular structure resulted in significantly different interaction

366

energies between C5PeC11 and solvent.

367

Among these two kinds of interactions between C5PeC11 and solvent molecules,

368

vdW interaction played a leading role in causing aggregation while Coulomb

369

interaction made further contributions and intensified the strength. Coulomb

370

interaction, although weaker, was speculated to be the driving force for changing the

371

assembly behavior of C5PeC11 aggregates. In C5PeC11_toluene system, beside other

372

C5PeC11 molecules, these regions of C5PeC11 molecules carrying charges can also

373

be occupied by toluene molecules. Once n-heptane was used to replace toluene in

374

C5PeC11_heptol system, some of the charged regions of C5PeC11 molecules

375

occupied by toluene in C5PeC11_toluene system were then exposed. This result was

376

verified by the fact that the total Coulomb interactions between C5PeC11 and solvent

377

in C5PeC11_toluene system were -519.25 kJ/mol, while only -268.26 kJ/mol

378

Coulomb interactions were found between C5PeC11 and solvent in C5PeC11_heptol 20

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Energy & Fuels

379

system. To reduce the potential energy of the system, these exposed C5PeC11

380

molecules’ charged regions were promoted to interact with other C5PeC11 molecules.

381

As a result, compared with C5PeC11_toluene system, both the attractive vdW and

382

Coulomb interaction energy between C5PeC11 molecules were increased in

383

C5PeC11_heptol system. The excess interaction forces between C5PeC11 molecules

384

were the underlying reason responsible for the formation of one-dimensional linearly

385

arranged C5PeC11 aggregates in C5PeC11_heptol system as discussed above.

386

3.2 Water effect

387

Previous results showed that the aggregation of polyaromatic molecules also hinged

388

upon polar group interactions between polyaromatic molecules.9,

389

changing the structure of polyaromatic molecules through adding or deleting of polar

390

group, C5PeC11_heptol+H2O system was investigated in our study to probe the effect

391

of polar solvent on C5PeC11 aggregation. Water molecules were adopted in our study

392

considering its ubiquitous presence in the processing of petroleum resources.

393

Compared

394

C5PeC11_heptol+H2O system resulted in stronger RDF peak intensities for aromatic

395

cores of C5PeC11 molecules as shown in Figure 1. Moreover, compared with

396

C5PeC11_heptol system, new peaks at 1.48 and 1.85 𝑛𝑚 emerged. The location of

397

these two peaks was exactly four times and five times the position of the first peak.

398

Thus, the appearance of these two peaks suggested the formation of C5PeC11

399

aggregates in the form of 5-mers and 6-mers. n-mers data in Table 2 confirmed that

with

C5PeC11_heptol

system,

the

presence

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15

Instead of

water

in

the

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Page 22 of 61

400

larger aggregates, 6-mers, were formed in C5PeC11_heptol+H2O system. The peak

401

intensity for the fourth and fifth peaks in C5PeC11_heptol+H2O system in Figure 1

402

was resulted from the two 6-mers in the system. Although no individual 5-mers were

403

found, any five consecutive C5PeC11 molecules in 6-mers were responsible for the

404

fourth peak in C5PeC11_heptol+H2O system in Figure 1. The addition of water into

405

heptol increased the average aggregation number of C5PeC11 by 28% from 3.33 to

406

4.25. Taking into consideration the appearance of large aggregates and the remarkable

407

increment of 𝑛𝑎𝑣𝑔, the negligible increment of the intensity for the first peak in

408

C5PeC11_heptol+H2O system in Figure 1 was unexpected. FLSP value in Table 3

409

addressed

410

C5PeC11_heptol+H2O system was exactly the same as C5PeC11_heptol system.

411

FLSP translating to direct π-π stacking was responsible for the formation of C5PeC11

412

aggregates.

413

C5PeC11_heptol+H2O system indicated that instead of forming new stacking pairs,

414

6-mers in C5PeC11_heptol+H2O system were formed based on the connection of

415

stacking pairs available in the system. Since FLSP was the same in

416

C5PeC11_heptol+H2O system and C5PeC11_heptol system, the weak increment in

417

the intensity for the first RDF peak in Figure 1 was therefore reasonable considering

418

larger aggregates in C5PeC11_heptol+H2O system. Both the larger C5PeC11

419

aggregates and larger number of SLSP gave rise to the stronger intensity of the second

420

RDF peak at 0.74 𝑛𝑚 in C5PeC11_heptol+H2O system. It's worth noting that the

421

stronger intensity for the third peak at 1.11 𝑛𝑚 (Figure 1) for C5PeC11_heptol+H2O

the

deeper

Same

cause

number

of

for

this

FLSP

phenomenon.

together

with

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FLSP

larger

value

aggregates

in

in

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Energy & Fuels

422

system contradicted the fact that less TLSP was found in C5PeC11_heptol+H2O

423

system than that in C5PeC11_heptol system (Table 3). As mentioned above, linear

424

arrangement of C5PeC11 aggregates was responsible for the large number of TLSP in

425

C5PeC11_heptol system. Snapshots revealing C5PeC11 aggregates geometry were

426

therefore examined to understand the cause for the decrease of TLSP in

427

C5PeC11_heptol+H2O system. The snapshots in Figure 4 showed that instead of

428

extending in one direction as in C5PeC11_heptol system, C5PeC11 aggregates in

429

C5PeC11_heptol+H2O system were entangled and the final geometry formed by

430

C5PeC11 aggregates was close to a cluster. The formation of cluster instead of

431

one-dimensional linearly assembled C5PeC11 aggregates reduced the possibility of

432

finding four consecutive C5PeC11 molecules in one direction. Therefore, the number

433

of TLSP in C5PeC11_heptol+H2O system decreased (Table 3).

434

435

Figure 4. (a) Snapshot of C5PeC11 molecules in C5PeC11_heptol+H2O system at 80 𝑛𝑠. (b)

436

Snapshot of C5PeC11 molecules in the cluster illustrating the connection pattern of C5PeC11

437

aggregates.

438

Compared with C5PeC11_heptol system, the above discussion indicated boosted 23

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439

C5PeC11 aggregation in C5PeC11_heptol+H2O system. Interestingly, the SASA for

440

C5PeC11 molecules in C5PeC11_heptol+H2O system were identical to that in

441

C5PeC11_heptol system as reflected in Figure 3. Thus we can conclude that the effect

442

of trace amounts of water on solubility of asphaltene was insignificant. The

443

enhancement of C5PeC11 aggregation due to the existence of H2O molecules was

444

therefore caused by other reasons instead of solubility reduction.

445

At first glance, the snapshot in Figure 4(a) shows that C5PeC11 molecules in

446

C5PeC11_heptol+H2O system aggregated through π-π stacking and polar group

447

association without order. This is strikingly different from C5PeC11_heptol system,

448

where C5PeC11 aggregates were distributed along a line. To get a comprehensive

449

understanding of the effect of water on the assembly behavior of C5PeC11 aggregates,

450

the angle (Θ) between aromatic core plane of any two C5PeC11 molecules in

451

C5PeC11_heptol+H2O system and C5PeC11_heptol system were calculated and

452

defined as plane-plane angle. The plane was characterized using three carbon atoms

453

located in the polyaromatic core of C5PeC11 (see SI-5 for details). The angle data

454

calculated using 276 pairs of C5PeC11 molecules in each system were presented in

455

Figure 5 as the inset. We focused on the C5PeC11 molecule pairs with plane-plane

456

angle less than 25◦. The angle was chosen according to the criteria that cosΘ was

457

greater than 0.9, which means C5PeC11 molecules were almost parallel to each other.

458

It can be seen from Figure 5 that the existence of water strikingly increased the

459

number of C5PeC11 molecule pairs for plane-plane angle less than 25◦. Combining

460

with the snapshots in Figure 4(b), it can be inferred that water molecules in 24

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Energy & Fuels

461

C5PeC11_heptol+H2O system worked as a bridge connecting different C5PeC11

462

aggregates in the way that the aggregates were parallel to each other. Such different

463

aggregates geometries were caused by different aggregation mechanisms in these two

464

systems. In C5PeC11_heptol system, water was absent and π-π interaction dominated.

465

The π-π interaction and steric hindrance from long hydrocarbon tails on C5PeC11

466

worked together resulted in aggregates consisting solely of parallel stacked molecules.

467

Due to the steric hindrance between hydrocarbon tails of C5PeC11 molecules from

468

different aggregates, the connection of different aggregates was bended deviating

469

from perfect vertical line. This deviation leads to less C5PeC11 pairs parallel to each

470

other. While in C5PeC11_heptol+H2O system, besides π-π interaction between the

471

aromatic cores, additional interaction forces existed due to the presence of water. On

472

one hand, water molecules participated in interacting with polar groups of C5PeC11

473

molecules. On the other hand, water molecules connected with each other through

474

hydrogen bonding. These extra interactions provided new agent for C5PeC11

475

aggregates bindings. The network formed through water and C5PeC11, water and

476

water interactions changed the connection of C5PeC11 aggregates from bent vertical

477

linear arrangement to parallel horizontal connection. To be more specific, the steric

478

hindrance between C5PeC11 aggregates were counteracted as the hydrocarbon tails of

479

different C5PeC11 aggregates were distributed on different sides. The abruptly

480

different arrangement pattern of C5PeC11 aggregates was illustrated in the

481

association scheme in Figure 6 for visualization.

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482 483

Figure 5. Plane-Plane angles between C5PeC11-C5PeC11 pairs that are less than 25°for

484

C5PeC11_heptol system and C5PeC11_heptol+H2O system. Shown in inset are the plane-plane

485

angles for all the C5PeC11-C5PeC11 pairs.

486 487

Figure 6. Association scheme of C5PeC11 aggregates in (a) C5PeC11_heptol system; (b)

488

C5PeC11_heptol+H2O system. Aromatic core of C5PeC11 molecule was represented by black

489

solid rectangle, hydrocarbon chain was represented by a black line, and -COOH was represented

490

by red circle. A red dotted line was drawn to show the arrangement pattern in (a).

491

The number of hydrogen bonds formed between different molecules was

492

calculated and presented in Table 6 to quantify the formation of the network. The

493

existence of H2O reduced the number of hydrogen bonds between C5PeC11

494

molecules from 22.86 in C5PeC11_heptol system to 5.38 in C5PeC11_heptol+H2O 26

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Energy & Fuels

495

system. Since hydrogen bond was one of the factors contributing to C5PeC11

496

aggregation, the reduction of the number of hydrogen bonds was supposed to decrease

497

C5PeC11 aggregation. However, the loss of hydrogen bonds between C5PeC11

498

molecules was compensated by hydrogen bonds formed by H2O and C5PeC11, H2O

499

and H2O. Table 6 shows that 41.76 hydrogen bonds were formed between water

500

molecules and C5PeC11 molecules. The hydrogen bonding between C5PeC11 and

501

water brought water molecules to the vicinity of C5PeC11 molecules. Moreover, H2O

502

molecules interacted with C5PeC11 molecules can also form hydrogen bonds with

503

other H2O molecules. In concert with the 22.70 hydrogen bonds between H2O and

504

H2O, H2O molecules gathered together in close proximity to C5PeC11 aggregates. In

505

this way, H2O built a network for connecting aggregates. The connection of the

506

C5PeC11 aggregates by the network increased the C5PeC11 aggregation extent and

507

resulted in the formation of cluster as observed in Figure 4(a). The increase of RDF

508

intensity over 0.35 𝑛𝑚 in Figure 7 (a) for C5PeC11_heptol+H2O system confirmed

509

the networking of C5PeC11 aggregates. Instead of staying as individual aggregates far

510

from each other, H2O molecules in C5PeC11_heptol+H2O system brought the

511

-COOH group of different C5PeC11 aggregates close to each other and formed a

512

cluster.

27

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Page 28 of 61

513 514

Figure 7. (a) Radial distribution function g(r) for COM separation distance r (𝑛𝑚) between

515

-COOH groups of C5PeC11 molecules in C5PeC11_heptol and C5PeC11_heptol+H2O systems

516

averaged over the last 5 𝑛𝑠 of 80 𝑛𝑠 simulations; (b) Radial distribution function g(r) for COM

517

separation distance r (𝑛𝑚) between C5PeC11 molecules and water molecules, -COOH groups of

518

C5PeC11 molecules and water molecules, aromatic cores of C5PeC11 molecules and water

519

molecules for C5PeC11_heptol+H2O system averaged over the last 5 𝑛𝑠 of 80 𝑛𝑠 simulations.

520

Table 6. Number of hydrogen bonds between molecules averaged over the last 5 𝑛𝑠 for 80 𝑛𝑠

521

simulations time in different systems.

Molecules

C5PeC11_heptol

C5PeC11_heptol+H2O

DBSA_C5PeC11_heptol+H2O

C5PeC11-C5PeC11 C5PeC11-H2O H2O-H2O DBSA-H2O C5PeC11-DBSA DBSA-DBSA

22.86

5.38 41.76 22.70

3.85 22.24 6.78 36.49 48.12 45.86

522

From their limited data, Andersen et al. suggested the presence of hydrogen

523

bonding either between water and oxygen/nitrogen-containing groups in asphaltenes

524

or between water and aromatic rings.18 With regards to the 41.76 hydrogen bonds

525

between C5PeC11 and water molecules, 32.5 hydrogen bonds were caused by the

526

interactions between -COOH groups of C5PeC11 and water, 9.26 hydrogen bonds 28

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Energy & Fuels

527

were attributed to the interactions between water and O atoms on the aromatic core of

528

C5PeC11 molecules. The formation of water microdroplet at the center of the

529

C5PeC11 aggregates in C5PeC11_heptol+H2O system was revealed by the snapshots

530

in Figure 8. In addition, the exact configuration of water molecules around C5PeC11

531

aggregates was explored. It can be seen from the Figure 7(b) that the shape of the

532

RDF profile for water distribution around C5PeC11 molecules was similar to the RDF

533

profile for water distribution around -COOH groups of C5PeC11 molecules. Thus

534

Figure 7(b) confirmed that water molecules in the microdroplet were hydrated with

535

-COOH groups of C5PeC11. Considering the π-π stacking of C5PeC11 aromatic core

536

located at 0.37 𝑛𝑚, the appearance of first peak located at 0.385 𝑛𝑚 for water

537

distribution around aromatic core of C5PeC11 suggested that no water molecules

538

were found in the interlayer of any two C5PeC11 molecules.

539 29

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

540

Figure 8. Snapshots at t = 80 𝑛𝑠 of (a) C5PeC11 molecules; (b) H2O molecules; (c) C5PeC11

541

molecules and H2O molecules; (d) C5PeC11 molecules; (e) H2O molecules; (f) C5PeC11

542

molecules and H2O molecules. (a), (b), and (c) were for C5PeC11_heptol+H2O system; (d), (e),

543

and (f) were for DBSA_C5PeC11_heptol+H2O system. All other molecules were removed for

544

clarity.

545

Based on these discussions above, it was obvious that besides enhancing

546

C5PeC11 aggregation to a certain extent, water molecules played important roles in

547

building the network for aggregates bindings. To interpret how water molecules

548

affected the aggregation and assembly of C5PeC11 aggregates, the interaction

549

energies between molecules were calculated and presented in Table 7. As can be seen

550

from Table 7, the absolute value of the attractive interaction energy between water

551

and other molecules follows the order of C5PeC11-H2O > H2O-H2O > Toluene-H2O >

552

(n-heptane)-H2O. The formation of hydrogen bonds between C5PeC11 and water, as

553

well as between water and water molecules was due to the fact that toluene and

554

n-heptane are “bad” solvent for water molecules, water molecules had a high

555

tendency to interact with C5PeC11 and other water molecules. Interestingly, the

556

existence of water decreased the absolute value of attractive C5PeC11-C5PeC11

557

interaction energy from ABS (-5038.23) kJ/mol in C5PeC11_heptol system to ABS

558

(-4620.09) kJ/mol in C5PeC11_heptol+H2O system. This reduction was not caused by

559

the decreased C5PeC11 aggregation. It was actually resulted from the reduced polar

560

group association between -COOH groups as inferred from Figure 7(a). Water as

561

polar solvent has significant impacts on the electrostatic interactions between 30

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Energy & Fuels

562

molecules in the system. The vdW and Coulomb interaction energy data confirmed

563

that the addition of water decreased the absolute value of the attractive

564

C5PeC11-C5PeC11 Coulomb interaction energy from ABS (-709.54) kJ/mol in

565

C5PeC11_heptol system to ABS (-157.37) kJ/mol in C5PeC11_heptol+H2O system.

566

The absolute value of attractive C5PeC11-C5PeC11 vdW interaction energy increased

567

from ABS (-4328.69) kJ/mol to ABS (-4462.72) kJ/mol, which was caused by the

568

additional hydrocarbon associations to reduce its contact with water molecules.

569

Table 7. Interaction energies between molecules in C5PeC11_heptol+H2O system. Molecules

Interaction energy (kJ/mol)

C5PeC11-toluene C5PeC11-(n-heptane) C5PeC11-H2O Toluene-H2O (n-heptane)-H2O C5PeC11-C5PeC11 C5PeC11-C5PeC11 (vdW) C5PeC11-C5PeC11 (Coulomb) H2O-H2O

-4535.71 -3127.48 -1368.66 -199.67 -57.26 -4620.09 -4462.72 -157.37 -574.03

570

3.3 Competitive Effect of DBSA and H2O

571

The competitive effect of DBSA and water on the aggregation and assembly

572

behaviors of asphaltenes was studied through DBSA_C5PeC11_heptol+H2O system

573

to shed light on the role of inhibitor in stabilizing asphaltenes under unfavorable

574

condition. When DBSA molecules were added into the system, it can be concluded

575

from

576

DBSA_C5PeC11_heptol+H2O system, translating to the weakest C5PeC11

577

aggregation in DBSA_C5PeC11_heptol+H2O system. In addition, peaks representing

Figure

1

that

the

weakest

peak

intensity

31

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was

found

in

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 61

578

5-mers and 6-mers disappeared, indicating smaller C5PeC11 aggregates in

579

DBSA_C5PeC11_heptol+H2O

580

system, Table 2 proved that the biggest n-mers in DBSA_C5PeC11_heptol+H2O

581

system was 3-mers instead of 6-mers. Although the addition of water enhanced the

582

aggregation of C5PeC11 molecules, adding DBSA counterbalanced the observed

583

enhancement. In comparison with C5PeC11_heptol+H2O system, the addition of

584

DBSA in DBSA_C5PeC11_heptol+H2O system decreased 𝑛𝑎𝑣𝑔 by 53% from 4.25 to

585

2. The average C5PeC11 aggregation number in DBSA_C5PeC11_heptol+H2O

586

system (2) was even lower than that in C5PeC11_heptol system without water (3.33).

587

In accordance with the peak intensities in Figure 1 as well as n-mers data, the number

588

of stacking pairs in Table 3 verified that the lowest number of FLSP was found in

589

DBSA_C5PeC11_heptol+H2O system. In addition, TSP was significantly less in

590

DBSA_C5PeC11_heptol+H2O system, which confirmed the dispersion effect of

591

DBSA molecules even in the existence of water molecules. These results

592

corresponded well with the observations from snapshots at 80 𝑛𝑠 in Figure 8(d),

593

where C5PeC11 aggregates in DBSA_C5PeC11_heptol+H2O system were far from

594

each other and the largest number of monomers was found. C5PeC11 molecules in

595

DBSA_C5PeC11_heptol+H2O system had the least inclination towards aggregation.

system.

Compared

with

C5PeC11_heptol+H2O

596

When both DBSA molecules and H2O molecules were present in the system,

597

water had to compete with DBSA to associate with C5PeC11. Since the network built

598

by water molecules was the crucial reason for the high aggregation extent and

599

formation of C5PeC11 cluster in C5PeC11_heptol+H2O system, the formation of the 32

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Energy & Fuels

600

network in DBSA_120_C5PeC11_heptol+H2O system was investigated to identify

601

DBSA’s effect in the presence of water. The dynamic process shown by snapshots at

602

different times in Figure 9(a) revealed that H2O molecules in C5PeC11_heptol+H2O

603

system migrated to the polar group of C5PeC11 molecules quickly and interacted with

604

those polar groups. In addition, it shows that once H2O molecules interacted with

605

C5PeC11 molecules, they were able to cling to those C5PeC11 molecules. H2O

606

molecules then acted as “glue” for combining different aggregates. By the end of the

607

simulation, a large cluster was developed in the C5PeC11_heptol+H2O system. In

608

contrast, it can be seen from Figure 9(b) that some of H2O molecules were interacted

609

with C5PeC11 molecules in the DBSA_C5PeC11_heptol+H2O system. However, the

610

interactions were not strong enough and H2O molecules were re-dispersed. By the end

611

of the simulation, only some of the H2O molecules were interacted with C5PeC11

612

molecules and these H2O molecules were far away from each other.

613

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Page 34 of 61

614

Figure 9. Snapshots of C5PeC11 molecules and H2O molecules at different times for (a)

615

C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

616

Figure 10 proved that there were more H2O molecules distributed around

617

C5PeC11

molecules

in

C5PeC11_heptol+H2O

618

DBSA_C5PeC11_heptol+H2O

619

DBSA_C5PeC11_heptol+H2O system in Figure 10 suggested that C5PeC11-H2O

620

interaction as one of the factors resulting in the formation of the network was

621

hindered in DBSA_C5PeC11_heptol+H2O system. DBSA worked as kind of

622

protector preventing the interactions between C5PeC11 molecules and H2O

623

molecules. In addition, Figure 10(a) shows that the number of H2O molecules around

624

C5PeC11 molecules increased at the very beginning and then stayed stable over time

625

in C5PeC11_heptol+H2O system. The fast RDF increment and stability of water

626

molecules’ distribution around C5PeC11 in Figure 10(a) indicated strong interactions

627

between C5PeC11 and water molecules. However, in DBSA_C5PeC11_heptol+H2O

628

system, Figure 10(b) shows that the number of H2O molecules around C5PeC11

629

molecules fluctuated. Compared with 25-30 𝑛𝑠, the number of H2O molecules around

630

C5PeC11 molecules increased during 65-70 𝑛𝑠, then decreased during 70-75 𝑛𝑠 and

631

75-80 𝑛𝑠. It can be inferred from the fluctuation that the existence of DBSA destroyed

632

some of the interactions between C5PeC11 and H2O, dispersing those bounded H2O

633

molecules. The breaking of C5PeC11-H2O interactions by DBSA molecules

634

suggested that the interactions between DBSA and H2O were stronger than that

635

between C5PeC11 and H2O, and a more detailed discussion follows in the ensuing

system.

The

system

reduced

34

ACS Paragon Plus Environment

RDF

than

that

in

intensity

in

Page 35 of 61 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

636

Energy & Fuels

sections.

637 638

Figure 10. The radial distribution function for the COM separation distance r (𝑛𝑚) between

639

C5PeC11 molecules and H2O molecules calculated over different time windows for (a)

640

C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

641

Another factor for the formation of the network, H2O-H2O association process

642

was shown in Figure 11. The association process of H2O molecules in

643

C5PeC11_heptol+H2O system was clearly seen by comparing snapshots from

644

different time windows in Figure 11(a). In the end, these H2O molecules in

645

C5PeC11_heptol+H2O system were associated together at 80 𝑛𝑠. H2O molecules in

646

DBSA_C5PeC11_heptol+H2O system were not able to associate as reflected in

647

Figure 11(b). By the end of the simulation, water molecules were still evenly

648

distributed in the whole simulation box. The snapshots presented a vivid picture that

649

the existence of DBSA molecules hindered the association of H2O molecules in

650

DBSA_C5PeC11_heptol+H2O system. Figure 12 confirmed that there were more

651

H2O molecules close to each other in C5PeC11_heptol+H2O system than that in

652

DBSA_C5PeC11_heptol+H2O system. In addition, no increment of RDF peak was 35

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

653

seen in DBSA_C5PeC11_heptol+H2O system (Figure 12(b)), which indicated that the

654

existence of DBSA in the system impeded the interactions between different H2O

655

molecules. Since H2O-H2O interactions were disrupted due to the presence of DBSA

656

molecules, it can be inferred that the interactions between DBSA and H2O were

657

stronger than that between H2O and H2O, and a more detailed discussion follows in

658

the ensuing sections.

659 660

Figure 11. Snapshots of H2O molecules at different times for (a) C5PeC11_heptol+H2O system

661

and (b) DBSA_C5PeC11_heptol+H2O system.

662

36

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Energy & Fuels

663

Figure 12. The radial distribution function for the COM separation distance r (𝑛𝑚) between H2O

664

molecules

665

C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

and

H2O

molecules

calculated

over

different

time

windows

for

(a)

666

Considering the weakened two factors for the formation of the network, we can

667

conclude that the existence of DBSA in DBSA_C5PeC11_heptol+H2O system

668

prevented H2O molecules from building the network for C5PeC11 aggregation and

669

assembly.

670

DBSA_C5PeC11_heptol+H2O system were caused by the competitive interactions

671

between molecules promoted by functional groups associations in the system. As

672

indicated by the dash lines in Figure 13(a), two peaks were procured at 0.167 and

673

0.261 𝑛𝑚 for RDF between -COOH groups of C5PeC11. Meanwhile, two peaks at

674

exactly the same place were observed for RDF between -COOH group of C5PeC11

675

and -SOOOH group of DBSA. Those peaks occurred at the same place and both were

676

less than 0.35 𝑛𝑚 implies that -SOOOH group of DBSA competed with -COOH

677

group of C5PeC11 to interact with -COOH group of C5PeC11. The competition effect

678

resulted in the disruption of C5PeC11 aggregation through polar group association.

679

The dashed lines in Figure 13(b) indicated that two peaks were located at 0.16 and

680

0.24 𝑛𝑚 for RDF between -COOH group of C5PeC11 and H2O, as well as for RDF

681

between -SOOOH group of DBSA and H2O. This phenomenon denoted that -SOOOH

682

group of DBSA competed with -COOH group of C5PeC11 to associate with H2O.

683

This leads to H2O molecules losing its effect in linking with C5PeC11 molecules to

684

form the network for aggregation and assembly. With regards to H2O-H2O

The

different

distribution

behaviors

37

ACS Paragon Plus Environment

of

molecules

in

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

685

interaction, another contributing factor for the formation of the network, DBSA

686

molecules were able to interact with H2O molecules at closer COM separation

687

distance than that between H2O and H2O (Figure 13(c)). This fact indicated that the

688

interactions between DBSA and H2O were much easier to take place than the

689

interactions between H2O molecules. The existence of DBSA prevented H2O

690

molecules from building the network through self-associations.

691 692

Figure 13. The radial distribution function for the COM separation distance r (𝑛𝑚) between (a)

693

-COOH groups of C5PeC11, -COOH group of C5PeC11 and -SOOOH group of DBSA; (b)

694

-COOH group of C5PeC11 and H2O, -SOOOH group of DBSA and H2O; (c) -SOOOH group of

695

DBSA and H2O, H2O and H2O for DBSA_C5PeC11_heptol+H2O system calculated over the last

696

5 𝑛𝑠 for 80 𝑛𝑠 simulation.

697

Hydrogen bonds were calculated and presented in Table 6 to quantify the polar

698

group associations between molecules. Table 6 shows that the hydrogen bonds

699

number between C5PeC11 molecules was the least in DBSA_C5PeC11_heptol+H2O

700

system. Thus the polar group association between C5PeC11 molecules boosting

701

aggregation was weakened in DBSA_C5PeC11_heptol+H2O system. In addition, in

702

comparison with C5PeC11_heptol+H2O system, H2O-C5PeC11 hydrogen bonds 38

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Energy & Fuels

703

number in DBSA_C5PeC11_heptol+H2O system was decreased by 46.74% from

704

41.76 to 22.24. At the same time, H2O-H2O hydrogen bonds number was decreased

705

by

706

DBSA_C5PeC11_heptol+H2O system. The break of those hydrogen bonds resulted in

707

H2O molecules losing its effect in building the network for C5PeC11 aggregation and

708

assembly when DBSA molecules existed. Moreover, it can be seen that 48.12

709

hydrogen bonds were formed between C5PeC11 and DSBA. These hydrogen bonds

710

brought DBSA molecules to the vicinity of C5PeC11 molecules in terms of polar

711

group associations. Unlike H2O, the long straight hydrocarbon chain provided steric

712

hindrance making DBSA an inhibitor for C5PeC11 aggregation. In addition, 45.86

713

hydrogen bonds were formed by DBSA molecules. These DBSA-DBSA interactions

714

caused self-associations between DBSA molecules. The C5PeC11-DBSA interactions

715

and DBSA-DBSA interactions cooperated with each other resulting in the

716

surrounding of DBSA molecules around C5PeC11 aggregates. The distribution of

717

DBSA around C5PeC11 was in the form of polar group located in the interior and

718

hydrocarbon tails located at the outside as unveiled by the snapshots in Figure 14. The

719

snapshots were resolved into separate C5PeC11 molecules in Figure 14(a) and DBSA,

720

H2O molecules in Figure 14(b) for clarity. Those C5PeC11 molecules in red and blue

721

circles of Figure 14(a) were actually circled by DBSA molecules in red and blue

722

circles of Figure 14(b) respectively. The extension of the hydrocarbon tails of

723

surrounding DBSA molecules into the solvent increased the solubility of associated

724

C5PeC11 aggregates, which stabilized them as individual aggregates instead of

70.13%

from

22.70

in

C5PeC11_heptol+H2O

39

ACS Paragon Plus Environment

system

to

6.78

in

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 61

725

forming large cluster. As a result, despite the fact that water molecules are “bad”

726

solvent for C5PeC11, the solubility of C5PeC11 in solvents was the largest in the

727

DBSA_C5PeC11_heptol+H2O system as shown in Figure 3.

728

729

Figure 14. Snapshots of molecules distribution in the simulation box at 80 𝑛𝑠

730

DBSA_C5PeC11_heptol+H2O system. (a) C5PeC11 molecules; (b) DBSA and H2O molecules.

for

731

C5PeC11-C5PeC11, C5PeC11-H2O, and H2O-H2O interaction energies were

732

calculated and presented in Table 8 to quantify DBSA molecules’ effect in stabilizing

733

C5PeC11. Firstly, compared with C5PeC11_heptol+H2O system, the absolute value 40

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Energy & Fuels

734

of the attractive C5PeC11-C5PeC11 interaction energy decreased from ABS

735

(-4620.09) kJ/mol to ABS (-2798) kJ/mol in DBSA_C5PeC11_heptol+H2O system.

736

The reduction was attributed to the high tendency of C5PeC11 molecules to interact

737

with DBSA molecules. The C5PeC11-DBSA interaction energy was -6684.68 kJ/mol,

738

which was strikingly stronger than the C5PeC11-C5PeC11 interaction energy of

739

-2798 kJ/mol (Table 8). The stronger attractive C5PeC11-DBSA interaction than

740

C5PeC11-C5PeC11 interaction promoted DBSA molecules working as good protector

741

preventing self-aggregation between C5PeC11 molecules. Secondly, due to the

742

stronger interaction between H2O and DBSA than that between H2O and C5PeC11,

743

water molecules in the DBSA_C5PeC11_heptol+H2O system were more inclined to

744

interact with DBSA molecules. As a result, water molecules were occupied by DBSA

745

molecules instead of interacting with C5PeC11 molecules to enhance C5PeC11

746

aggregation and build the network for connecting aggregates. Moreover, the following

747

order of intensity of attractive interaction energy between molecules, DBSA-H2O >

748

C5PeC11-H2O > H2O-H2O, indicated that the self-association between water

749

molecules was also impeded. Therefore, DBSA molecules succeeded in hindering

750

both the driving forces for the formation of the network. Thirdly, the high negative

751

interaction energy between C5PeC11 and DBSA caused the attachment of DBSA

752

molecules on C5PeC11 molecules. In addition, the -16251.22 kJ/mol interaction

753

energy between DBSA molecules caused self-associations between DBSA molecules.

754

Those two facts cooperated with each other resulting in the distribution of DBSA

755

molecules around C5PeC11 molecules as shown in Figure 14. The surrounding of 41

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Page 42 of 61

756

C5PeC11 molecules by DBSA molecules in combination with the order of absolute

757

value of attractive interaction energy (DBSA-toluene > DBSA-(n-heptane) >

758

C5PeC11-toluene > C5PeC11-(n-heptane)), explained the increase of C5PeC11

759

molecules’ solubility by the associated DBSA molecules.

760

Table 8. Interaction energies between molecules in DBSA_C5PeC11_heptol+H2O system. Molecules

Interaction Energy (kJ/mol)

C5PeC11-DBSA C5PeC11-H2O C5PeC11-toluene C5PeC11-(n-heptane) DBSA-H2O DBSA-toluene DBSA-(n-heptane) toluene-H2O (n-heptane)-H2O C5PeC11-C5PeC11 DBSA-DBSA H2O-H2O

-6684.68 -839.09 -3631.70 -2644.14 -1155.95 -9156.33 -7124.28 -238.74 -56.41 -2798.0 -16251.22 -226.36

761

4. Conclusion

762

In this work, the aggregation and assembly behavior of asphaltene model compound,

763

N-(1-undecyldodecyl)-N’-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylbisimi

764

de (C5PeC11), was studied under several different conditions. Firstly, the effect of

765

organic solvent properties was investigated. Replacing toluene with n-heptane

766

increased the C5PeC11 aggregation. Besides enhancing aggregation, n-heptane

767

molecules played an important role in changing the assembly behavior of the

768

aggregates. The addition of n-heptane resulted in the formation of one-dimensional

769

linearly assembled C5PeC11 aggregates. Results showed that between the two kinds 42

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Energy & Fuels

770

of interactions, vdW interaction played a leading role in causing aggregation while

771

Coulomb interaction made further contribution and intensified the aggregation

772

tendency. Coulomb interaction, although weaker, was speculated to be the driving

773

force for changing the assembly behavior of C5PeC11 aggregates.

774

Although the content of water was low, the addition of water molecules resulted

775

in enhanced C5PeC11 aggregation and considerably different C5PeC11 aggregates

776

assembly behavior. Interactions between C5PeC11 and H2O brought H2O molecules

777

to the vicinity of C5PeC11 molecules. In addition to the interactions between H2O

778

molecules, H2O molecules built a network for connecting C5PeC11 aggregates and

779

formed a cluster whereas its effect on solubility of C5PeC11 molecules was less

780

significant. The network changed the connection of C5PeC11 aggregates from bent

781

vertical linear arrangement to parallel horizontal connection. Water molecules were

782

indispensable for the formation and retention of C5PeC11 aggregate cluster. The

783

C5PeC11-H2O hydrogen bonds were formed by the interactions between water and

784

-COOH groups of C5PeC11 molecules, water and O atoms on aromatic core of

785

C5PeC11 molecules. The formation of water microdroplet at the center of the

786

C5PeC11 aggregates was revealed. In addition, the exact configuration of water

787

molecules around C5PeC11 aggregates was explored.

788

The effect of asphaltene inhibitor, dodecylbenzene sulfonic acid (DBSA),

789

outweighed the effect of water in influencing C5PeC11 aggregation. Without

790

inhibitors, water molecules migrated to the proximity of C5PeC11 molecules rapidly

791

and stayed there linking with C5PeC11 molecules to form a network for C5PeC11 43

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aggregation and assembly. In contrast, the existence of DBSA destroyed some of the

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interactions between C5PeC11 and H2O, dispersing those bounded H2O molecules. In

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addition, the association process between different H2O molecules was also impeded

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by DBSA molecules. The network accounting for the enhanced C5PeC11 aggregation

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and formation of C5PeC11 cluster disappeared upon the addition of DBSA molecules.

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The polar group association, hydrogen bonds number as well as interaction energies

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illustrating the competitive effects were analyzed to understand how DBSA molecules

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working as protectors for preventing C5PeC11 aggregation and assembly in the

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presence of water molecules. These results are beneficial for the further understanding

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of asphaltene behaviors and providing guidance for designing asphaltene inhibitors.

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Acknowledgements

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This research was conducted under the auspices of China Scholarship Council (CSC

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NO.201606250074) and University of Alberta. The authors would like to thank Dr.

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Zhenghe Xu at the University of Alberta for providing many constructive suggestions.

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We also acknowledge computing resources and technical support from Western

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Canada Research Grid (Westgrid).

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Supporting Information

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Structures of C5PeC11 and DBSA, force field parameters for the substances,

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validation of the force field parameters, demonstrations of achieving dynamic

811

equilibrium, definitions for the functional groups, C5PeC11 aggregation and assembly 44

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behavior in C5PeC11_n-heptane system.

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References:

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(15). Liu, L.; Zhang, R.; Wang, X.; Simon, S.; Sjöblom, J.; Xu, Z.; Jiang, B., Interactions of Polyaromatic Compounds. Part 1: Nanoaggregation Probed by Electrospray Ionization Mass Spectrometry and Molecular Dynamics Simulation. Energy & Fuels 2017, 31, (4), 3465-3474. (16).Yaseen, S.; Mansoori, G. A., Molecular dynamics studies of interaction between asphaltenes and solvents. Journal of Petroleum Science and Engineering 2017, 156, 118-124. (17). Speight, J. G., The chemistry and technology of petroleum. CRC press, 2014. (18). Andersen, S. I.; del Rio, J. M.; Khvostitchenko, D.; Shakir, S.; Lira-Galeana, C., Interaction and Solubilization of Water by Petroleum Asphaltenes in Organic Solution. Langmuir 2001, 17, (2), 307-313. (19). Tharanivasan, A. K.; Yarranton, H. W.; Taylor, S. D., Asphaltene Precipitation from Crude Oils in the Presence of Emulsified Water. Energy & Fuels 2012, 26, (11), 6869–6875. (20). Tan, X.; Fenniri, H.; Gray, M. R., Water Enhances the Aggregation of Model Asphaltenes in Solution via Hydrogen Bonding. Energy & Fuels 2009, 23, (7), 3687-3693. (21). Aslan, S.; Firoozabadi, A., Effect of Water on Deposition, Aggregate Size, and Viscosity of Asphaltenes. Langmuir 2014, 30, (13), 3658-3664. (22). Tavakkoli, M.; Chen, A.; Sung, C.-A.; Kidder, K. M.; Lee, J. J.; Alhassan, S. M.; Vargas, F. M., Effect of emulsified water on asphaltene instability in crude oils. Energy & Fuels 2016, 30, (5), 3676-3686. (23). Murgich, J., Molecular Mechanics and Microcalorimetric Investigations of the Effects of Molecular Water on the Aggregation of Asphaltenes in Solutions. Langmuir 2002, 18, (23), 9080-9086. (24). Moulik, S. P.; Paul, B. K., Structure, dynamics and transport properties of microemulsions. Advances in Colloid & Interface Science 1998, 78, (2), 99-195. (25). Faeder, J.; Ladanyi, B. M., Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles. Journal of Physical Chemistry B 2000, 104, (5), 1033-1046. (26). Zana, R., Dynamics of surfactant self-assemblies : micelles, microemulsions, vesicles, and lyotropic phases. CRC Press, 2005. (27). Lowry, E.; Sedghi, M.; Goual, L., Polymers for asphaltene dispersion: Interaction mechanisms and molecular design considerations. Journal of Molecular Liquids 2017, 230, 589-599. (28). Hu, Y.-F.; Guo, T.-M., Effect of the structures of ionic liquids and alkylbenzene-derived amphiphiles on the inhibition of asphaltene precipitation from CO2-injected reservoir oils. Langmuir 2005, 21, (18), 8168-8174. (29). Goual, L.; Sedghi, M.; Wang, X.; Zhu, Z., Asphaltene aggregation and impact of alkylphenols. Langmuir 2014, 30, (19), 5394-5403. (30). Dehshibi, R. R.; Mohebbi, A.; Riazi, M.; Niakousari, M., Experimental investigation on the effect of ultrasonic waves on reducing asphaltene deposition and improving oil recovery under temperature control. Ultrasonics Sonochemistry 2018, 45, 204-212. (31). Alhreez, M.; Wen, D.; Ali, L. A novel inhibitor for controlling Iraqi asphaltene problems, In International Conference on Environmental Impacts of the Oil and Gas Industries (EIOGI), Koya , Kurdistan Region – Iraq, 2017; pp 37-41. (32). Goual, L.; Firoozabadi, A., Effect of resins and DBSA on asphaltene precipitation from petroleum fluids. AIChE journal 2004, 50, (2), 470-479. (33). Chang, C. L.; Fogler, H. S., Stabilization of Asphaltenes in Aliphatic Solvents Using 46

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Figure 1. Radial distribution function g(r) for COM separation distance r (nm) between aromatic cores of C5PeC11 molecules averaged over the last 5 ns of total 80 ns MD simulations for the systems. 84x59mm (300 x 300 DPI)

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Figure 2. Snapshot of C5PeC11 molecules at 80 ns for: (a) C5PeC11_toluene system; (b) C5PeC11_heptol system. Solvent molecules were removed for clarity.

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Figure 3. Solvent accessible surface area (SASA) of the C5PeC11 molecules calculated over the simulation time for the systems. 84x59mm (300 x 300 DPI)

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Figure 4. (a) Snapshot of C5PeC11 molecules in C5PeC11_heptol+H2O system at 80 ns. (b) Snapshot of C5PeC11 molecules in the cluster illustrating the connection pattern of C5PeC11 aggregates.

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Figure 5. Plane-Plane angles between C5PeC11-C5PeC11 pairs that are less than 25°for C5PeC11_heptol system and C5PeC11_heptol+H2O system. Shown in inset are the plane-plane angles for all the C5PeC11C5PeC11 pairs. 84x59mm (300 x 300 DPI)

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Figure 6. Association scheme of C5PeC11 aggregates in (a) C5PeC11_heptol system; (b) C5PeC11_heptol+H2O system. Aromatic core of C5PeC11 molecule was represented by black solid rectangle, hydrocarbon chain was represented by a black line, and -COOH was represented by red circle. A red dotted line was drawn to show the arrangement pattern in (a).

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Figure 7. (a) Radial distribution function g(r) for COM separation distance r (nm) between -COOH groups of C5PeC11 molecules in C5PeC11_heptol and C5PeC11_heptol+H2O systems averaged over the last 5 ns of 80 ns simulations; (b) Radial distribution function g(r) for COM separation distance r (nm) between C5PeC11 molecules and water molecules, -COOH groups of C5PeC11 molecules and water molecules, aromatic cores of C5PeC11 molecules and water molecules for C5PeC11_heptol+H2O system averaged over the last 5 ns of 80 ns simulations.

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Figure 8. Snapshots at t = 80 ns of (a) C5PeC11 molecules; (b) H2O molecules; (c) C5PeC11 molecules and H2O molecules; (d) C5PeC11 molecules; (e) H2O molecules; (f) C5PeC11 molecules and H2O molecules. (a), (b), and (c) were for C5PeC11_heptol+H2O system; (d), (e), and (f) were for DBSA_C5PeC11_heptol+H2O system. All other molecules were removed for clarity.

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Figure 9. Snapshots of C5PeC11 molecules and H2O molecules at different times for (a) C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

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Figure 10. The radial distribution function for the COM separation distance r (nm) between C5PeC11 molecules and H2O molecules calculated over different time windows for (a) C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

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Figure 11. Snapshots of H2O molecules at different times for (a) C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

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Figure 12. The radial distribution function for the COM separation distance r (nm) between H2O molecules and H2O molecules calculated over different time windows for (a) C5PeC11_heptol+H2O system and (b) DBSA_C5PeC11_heptol+H2O system.

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Figure 13. The radial distribution function for the COM separation distance r (nm) between (a) -COOH groups of C5PeC11, -COOH group of C5PeC11 and -SOOOH group of DBSA; (b) -COOH group of C5PeC11 and H2O, -SOOOH group of DBSA and H2O; (c) -SOOOH group of DBSA and H2O, H2O and H2O for DBSA_C5PeC11_heptol+H2O system calculated over the last 5 ns for 80 ns simulation.

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Figure 14. Snapshots of molecules distribution in the simulation box at 80 ns for DBSA_C5PeC11_heptol+H2O system. (a) C5PeC11 molecules; (b) DBSA and H2O molecules.

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