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Self-Accumulation of Uncharged Polyaromatic Surfactants (PASs) at Crude Oil-Water Interface; A Mesoscopic DPD Study Hossein Rezaei, Sepideh Amjad-Iranagh, and Hamid Modarress Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00254 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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Self-Accumulation of Uncharged Polyaromatic Surfactants (PASs) at Crude Oil-Water Interface; A Mesoscopic DPD Study

1 2 3 4 5 6

Energy & Fuels

Hossein Rezaei, Sepideh Amjad-Iranagh, Hamid Modarress* Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), 424 Hafez Ave., Tehran, Iran

Abstract

7

The dissipative particle dynamics (DPD) technique was applied to study the behavior of several

8

uncharged perylene bisimide-based polyaromatic surfactant (PAS) molecules, with the same

9

polyaromatic core but with different terminal functional types (TP, C5Pe, PAP and PCH) at the

10

crude oil−water interface. We considered the SARA crude oil model with Persian Gulf oil field

11

composition, which includes saturates, 59%; aromatics, 28.5%; resins, 9.7% and asphaltenes, 2.8%

12

at two temperatures 298K and 363K. The DPD interaction parameters for the bead pairs needed in

13

the DPD simulations were evaluated by using the well-known correlation equation, where the

14

required Flory-Huggins interaction parameter in this equation, was calculated by the blend

15

methodology model. The results indicated that the C5Pe terminal functional type of PAS is

16

absorbed more effectively on the water droplet interface in the crude oil system and can reduce the

17

interfacial tension (IFT) to facilitate the oil-water separation. The results of this simulation can be

18

used to choose proper demulsifier surfactant for application in various processes in the oil industry

19

as well as enhanced oil recovery (EOR).

20 21

Keywords: oil-water interface, interfacial tension (IFT), dissipative particle dynamics (DPD),

22

enhanced oil recovery (EOR), surfactant, PAS

23 24 25

*

Corresponding author. Tel.:+98 21 64543176, Fax: +98 21 66405847. Email addresses: [email protected] (H. Modarress)

1

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1 2

1. Introduction

3

The augmentation of crude oil production from oil reservoirs by using the enhanced oil recovery

4

(EOR) methods can be considered as an energy resource, since the oil reservoirs are limited in

5

number and in capacity. Molecular additives, such as surfactants, are used to reduce IFT of

6

oil/water interface in two main stages of crude oil production: (i) in oil recovery from reservoir for

7

EOR, and (ii) in oil/water emulsion separation after crude oil recovery at desalting plants. Using an

8

effective additive for reducing the IFT is very important, since at very low IFTs, the water/oil

9

emulsion will stabilize and the separation of water from crude oil (in the second stage), will be

10

more difficult 1, 2. However, it can be stated that IFT reduction is of extreme interests in all stages

11

of oil production including oil extraction from reservoir and oil/water separation at desalting

12

plants. Also microemulsion formation is considered as an effective tool in EOR techniques

13

because of its high level of extraction efficiency by reducing oil–water IFT

14

established relationship between the microemulsion formulations and IFT reduction, it is common

15

in the industry to screen surfactants according to their efficiency in IFT reduction2-5. Therefore, it

16

is important, to understand the behavior of existing interfaces between crude oil and reservoir

17

fluids which are usually salty aqueous mixtures. Only very limited information is available about

18

the noncentrosymmetric environment around the interface between two immiscible liquids. One of

19

the reasons are that; the experimental measurements on such interfaces is challenging due to their

20

buried nature with the width of a few molecular diameter and relatively small size

21

Computational methods such as molecular mechanics (MM), Monte Carlo (MC), molecular

22

dynamics (MD) and dissipative particle dynamics (DPD), are useful complements to the

23

experimental measurements 7 to describe the molecular behaviors at water/oil interfaces. Computer

24

simulations have been used in numerous studies on aggregation behavior of various molecules at

25

molecular level in the past decade. Most of these studies were limited to the structure of static 2


2, 3

. Due to the well-

6

.

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Energy & Fuels

6, 8-11

1

interfaces formed between water and pure hydrocarbons

or to the interfaces containing

2

surfactant additives

3

hydrocarbon molecules is a highly complex medium

4

hydrocarbon phase. Generally, group-type analysis of petroleum as a crude oil after refinery, can

5

be known by the acronyms:

6

paraffins, naphthenes, and aromatics), PONA (paraffins, olefins, naphthenes, and aromatics),

7

PIONA (paraffins, iso-paraffins, olefins, naphthenes, and aromatics), or SARA (saturates,

8

aromatics, resins, and asphaltenes). For the crude oil itself, SARA is the most popular method of

9

classification, which is based on polarity and solubility differences of its components 19. The main

10

crude oil component is asphaltene which consists of the three subcomponents: (1) polycyclic

11

aromatic rings, known as the aromatic core which constrains the molecular behavior of the

12

asphaltenes (2) the aliphatic chain, and (3) the heteroatoms, such as oxygen, sulfur or nitrogen.

13

Therefore, the size, number and kind of these subcomponents define the chemistry of the

14

asphaltene structure

15

form a monolayer (film) by absorbing at the oil/water interface21 and affects the interface

16

properties. As polyaromatic association in the crude oil is one of the important and interesting

17

phenomena in industrial processes such as oil recovery, transportation and oil-contaminated

18

wastewater treatment21-24, we focus in this work on the behavior of polyaromatic surfactant (PAS)

19

molecules as the surface active materials to be used for reducing the interfacial tension of the crude

20

oil/water interface. Polyaromatic associations strongly influence physical properties of the crude

21

oil such as density, viscosity, solubility. In some cases, for example, during transportation and

22

production, the condensed polyaromatic fractions of the crude oil precipitates which can lead to

23

blocking of reservoir porous rocks as well as oil transportation pipes 23, 25-28.

24

In application of simulation methods to study physicochemical properties, such as oil/water

25

interfacial behavior, the number of employed molecules and the time and length scales of the

12-15

. However, crude oil which consists of tens of thousands of various

20

18

16, 17

and cannot be considered as a single

PNA (paraffins, naphthenes, and aromatics), PINA (paraffins, iso-

. The surfactants due to their hydrophobic and hydrophilic characters can

3


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1

simulation are very important to obtain the results with high statistical accuracy. To shorten the

2

time scale by using small number of molecules, the obtained density profile resolution will be

3

inadequate and thereby the structural feature becomes obscured. Water droplets’ size in the crude

4

oil emulsions might be up to 100 µm in diameter, which is large

5

droplets begins on the nanometer scale, where the detailed experimental information on the

6

mechanism of water molecules aggregation to form droplets in a crude oil environment is not

7

adequately available. DPD as a mesoscopic large-scale molecular dynamics method can overcome

8

the above mentioned difficulties to be used for investigating the behavior of water droplets in the

9

crude oil. Therefore, the purpose of this work is to utilize DPD mesoscopic method to study the

10

molecular organization and the interfacial properties of PAS molecules in the complex crude oil-

11

water systems.

29

, but aggregation of water

12

2. The Models

13

2.1.

14

DPD

15

DPD simulation is a coarse grain mesoscopic method in which instead of atom-atom interactions,

16

the interactions between soft particles called “beads” which contain only several single groups in a

17

molecule are used in the simulation to evaluate the properties of the studied systems. In the case of

18

spring-bead model for macromolecules like polymers, the polymer chain is considered as several

19

beads that are joined together by the springs, which representing the bonds, in-between the beads

20

30

21

of the beads 9. The acting force on the ith bead, fi , is expressed as the conservative force ( FijC ),

22

dissipative force ( FijD ), random force ( FijR ) and bonding force ( FijBond ) which are presented by the

23

following equations 31-33:

. Similar to the conventional MD, the Newton’s laws of motion is used to follow the movements

fi =

24

∑ (F

C ij

+ FijD + FijR + Fijbond )

j ≠i

(1) 4


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1

FijC = aij ω C (rij )rîj

2

FijD = −ζω D (rij )( v ij .rîj )rîj

,

(3)

3

FijR = σω R (rij )ξ ij ∆t − (1 / 2) rîj

,

(4)

4

FijBond = − k S ( rij − r0 )rˆ ij

5

In Eqs. (2) to (5); the repulsive force parameter, aij , which is also called DPD interaction

6

parameter, shows the interaction strength of i and j beads in the simulation system. rîj = rij / rij is

7

the unit vector in the direction of rij . The vector v ij = v i − v j is the relative velocity of i and j beads

8

and

9

is the dissipation coefficient, where the noise coefficient, σ , controls the random force amplitude.

10

The following fluctuation-dissipation relation should be satisfied to sample the canonical ensemble

11

distribution34-36:

12

ω D ( rij ) = [ω R ( rij ) ]

13

Eq. (6) guarantees the Gibbs equilibrium condition and the ω D (rij ) , ω C (rij ) and ω R ( rij ) which are

14

the weight factors are related in the following form 35:

,

(2)

(5)

ξ is a randomly fluctuating variable with the stochastic properties.

2

σ 2 = 2k BTζ

and

ω C ( rij ) = ω R ( rij ) = [ω D ( rij ) ]

1/ 2

15

∆ t is the time step and

ζ

(6)

1 − rij rC , = 0,

rij ≤ rC rij > rC

(7)

16

where, rC is the usual effective interaction range or the cutoff distance and is defined as the length

17

scale in DPD simulation. In Eq. (5), the spring constant (or the rigidity parameter), k S controls the

18

stiffness of the molecule and r0 is the spring equilibrium distance. There are various suggestions in

19

the literature for values of k S and r0

: ( kS = 4 and r0 = 0 )

31, 37

5


9, 12, 19, 32, 38-40

, ( k S = 10 and

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1

r0 = 0.86 )

2

and reliable values among them is ( kS = 4 and r0 = 0 ) that were used in our DPD simulations.

3

Also, in the DPD simulations, there are two main factors which usually determine the results; (i)

4

the procedure of coarse-graining of the molecules into beads and (ii) the DPD interaction

5

parameter. These two factors will be explained in the following sections.

41

and ( kS = 100 and r0 = 0.7 )

Page 6 of 42

33, 42

. As it is seen by the references, the most common

6 7

2.2. Crude oil model

8

In this work, a SARA model of Khark Island (Persian Gulf) crude oil

9

composition of the main components: saturate, aromatics, resins and asphaltenes in weight percent,

10

as presented in Table 1. It should be noted that the crude oil characteristics such as physico-

11

chemical properties, composition and specially the amount of asphaltene depends on the oil-field

12

and even on the oil-well 44. As the asphaltenes are fractionated to a high variety of samples, due to

13

their apparent similarities, it is not proper to use the analytical results of one sample for another

14

asphaltene sample. Therefore, to investigate the effect of asphaltene structure and also to have a

15

better control over the concentrations and interfacial behavior in a water-oil emulsion, it is useful

16

to design and apply a single, well-defined model compound. Generally, asphaltene molecules

17

consist of an aromatic core, one or more aliphatic chains and a heteroatom, such as nitrogen,

18

oxygen or sulfur. The aromatic core which characterizes the molecular behavior contains many

19

polycyclic aromatic rings. Therefore, the number, size, and kind of these components define the

20

chemistry of the asphaltene molecules

21

chosen: “island type” (C53SH58; MW = 726), “archipelago type” (C90SOH120; MW = 1248) and

22

“resin type” (C26NS2H41; MW = 431) as are shown in Figure 1a, b and c, respectively. For low-

23

molecular-weight asphaltenes (500−2000 amu), these models can be generated from an improved

24

quantitative molecular representation (QMR) method by applying an optimizing algorithm to

20

43

was investigated with the

. In this work, at first, three types of asphaltene were

6


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select the best structure among a group of ~1000 asphaltene molecules according to the available

2

experimental data

3

aliphatic chains connected to an aromatic core. The “island type” asphaltene, compared to the

4

other two types, has an intermediate aromatic core size, since it has an intermediate number of

5

aromatic rings between two other types. Also the structure of the “island type” asphaltene has

6

been confirmed by theoretical studies and the experimental

7

fluorescence emission measurements

8

structure. Furthermore, we compared the selected asphaltene structure with the asphaltene structure

9

in studied crude oil by using the H/C (hydrogen to carbon) ratio. A low H/C ratio is a good

10

indication of the presence of polynuclear aromatic systems, whereas, a high H/C ratio indicates the

11

presence of straight-chain compounds

12

samples from three oil samples from three Iranian south west oilfields

13

asphaltenes were: 1.11, 1.09 and 1.12. We calculated the H/C ratio for the proposed three types of

14

asphaltenes (island-type = 1.09, archipelago-type = 1.33 and resin-type = 1.65) and we found that

15

the H/C ratio of island-type is closer to the reported experimental values. Therefore we selected the

16

island-type model for asphaltene in our DPD simulation. The experimental H/C ratio for the

17

studied crude oil reveal that the asphaltene fractions are not very aromatic which confirmed that

18

the island-type asphaltene with an intermediate core size is an appropriate selection47. Also, the

19

ratios of (saturate/aromatic) and (asphaltene/resin) are respectively 2.07 and 0.28, and therefore the

20

studied systems in this work are in the limit between stable and unstable water-in-oil (w/o)

21

emulsions

48, 49

20, 45

. The “island type” asphaltene structure contains one sulfur atom and three

20, 46

47

investigations, such as UV

. Therefore, we select the “island type” asphaltene

. According to the experimental results of crude oil 47

, the H/C ratio for

.

22 23

2.3. Polyaromatic Surfactants (PASs) models

24

Polyaromatic surfactants (PASs), which can be considered as nanomaterials are constructed from

25

stacked polyaromatic building blocks and are of interest in designing novel functional compounds 7 ACS Paragon Plus Environment

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1

21, 23, 44, 50

2

were used as the building blocks to investigate the structural effect of these surfactants, on the

3

crude oil-water interfaces. Considering the structure of PAS, their molecular cores consists of a

4

perylene bisimide (PBI) group connected on one side to a pronged aliphatic double chain. The core

5

and the double chains are the same for all the four studied PAS macromolecules, as are shown in

6

Figure 2a. The other side of the PBI core is connected to various head groups of different structural

7

types of aliphatic alkyl groups or amino acids such as phenylalanine, tryptophan, β-alanine and

8

branched n-alkanes, as are shown in Figure 2b 21, 23, 44. The head groups are the active part of PAS

9

and control the interfacial behavior of PAS at the two-phase liquid interface. The connected side

10

groups in the studied PAS macromolecules include: N-(1-hexylheptyl)-N′-(2-indol-3-yl-propanoic

11

acid)-perylene-3,4,9,10-tetracarboxylicbisimide,

12

carboxylic

13

hexylheptyl)-N′-(2-phenylpropanoic acid)-perylene-3,4,9,10-tetracarboxylicbisimide, C46H46N2O6

14

(PAP),

15

tetracarboxilicbisimide and N-(1-hexylheptyl)-N′-(methyl 2-phenylpropanpate)-perylene-3,4,9,10-

16

tetracarboxylicbisimide , C47H48N2O5 (PCH) (Figure 2b). The molecular weights of the four PAS

17

macromolecules (in g/mol) are: TP (762), C5Pe (689), PAP (723) and PCH (721). The structures

18

TP, C5Pe and PAP are specially designed and synthesized with definite compoundings whereas

19

the PCH is a hypothetical compound. In PCH, the hydrogen atoms of the hydroxyl groups (-OH)

20

of PAP are substituted by a group of alkyl (-CH3) 21, 23, 44, 50.

. In this study, four well-defined perylene bisimide based polyaromatic surfactants (PAS)

C48H47N3O6

(TP),

pentyl)-perylene-3,4,9,10-tetracarboxylicbisimide,

N-(1-hexylheptyl)-N′-(5-

C47H48N2O6 (C5Pe),

N-(1-

N-(1-hexylheptyl)-N′-(methyl2-phenylpropanoate)-perylene-3,4,9,10-

21 22

2.4. Coarse grained model

23

Coarse graining (CG) is the first and essential step in performing the DPD simulations which is

24

done to obtain the physical properties of macromolecules at mesoscopic scale by simulation

25

procedure in a significantly reduced computational time 9. In a DPD CG model the bead volume is 8


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defined as: bead volume = Nm × (volume of a water molecule) where Nm which is the number of

2

water molecules in a bead, is known as the degree of coarse-graining

3

the bead volume according to a given value for Nm., the macromolecules in DPD CG model are

4

divided into optimum number of beads. In this work, we have selected a set of different prototype

5

molecules as the building blocks of the different molecular components in the systems which are

6

shown in Figures 2 and 3. It is assumed that, the interfacial characteristics of the macromolecules

7

are represented by theprototype molecules. The criteria for selection of prototype molecules is

8

their molecular structure similarity and chemical properties to the studied macromolecules 19. The

9

number Nm ,which is used to convert

the DPD units to the

51-54

. Therefore, by defining

meaningful physical units as

10

represented by the following equations 32, 52:

11

m = N m m water

(8)

12

rC = 3 . 107 ( ρ N m ) 1 / 3

(9)

13

τ = (14 . 1 ± 0 . 1) N m5 / 3

(10)

14

Eqs. (8 to 9) indicate how Nm is used to define mass m (in mass units), length rC (in Å) and time τ

15

(in ps). In the present case study, to simulate SARA crude oil model, 18 water molecules as well as

16

saturates and aromatics were considered as a single bead (i.e. Nm). The two molecular types which

17

were used as saturates (Sat1 and Sat2) and two aromatic types (Ar1 and Ar2) are shown in Figure

18

3a and 3b respectively. Also, the core models basis

19

SARA crude oil systems, was utilized in this work for selecting resin and asphaltene cores. The

20

core used for the resin was alicyclic group (Cr) connected to an alkyl chain (A1) where the

21

structure of the resin consist of these two connected beads (Figure 3c). The island-type asphaltene

22

was selected as a mesomolecular structure consisting of a core region bead (CAi), two aliphatic

23

chain beads (A1)s and a sulfur-contained fragment (S) bead as are shown in Figure 3d. The

24

structure of four PAS mesomolecules were selected as one core bead (PBI), two aliphatic-like 9

19, 55

as previously used for simulating of


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1

beads (A4) and a head group bead (TP, C5Pe, PAP or PCH) as are shown in Figure 2a and 2b (the

2

coarse grained beads structure of studied PAS molecules are presented in Figure S1 of the

3

Electronic Supplementary Information (ESI) of this manuscript). The beads are interconnected by

4

harmonic springs where the connection rigidity is represented by the same spring constant (ks) for

5

all beads used in DPD simulation. It should be noted that for the sake of simplification, as it has

6

been used in the similar simulation procedure

7

considered as a single bead.

19, 55

the cores of asphaltene, resin and PAS were

8 9

2.5. DPD parameters in CG model

10

DPD interaction parameter, a ij (as it was introduced in Eq. (2) ) can be related to the χ-parameter

11

in Flory-Huggins (FH) theory of polymers solutions based on the Groot and Warren relationship in

12

1997 35. Many other researchers have approximated a ij in terms of χ ij parameter in the following

13

form 31, 33, 35, 56:

14

a ij = a ii + bχ ij

15

where, aii indicates the DPD interaction parameter for two similar beads and b is fixed constant

16

(b=3.27).

17

In this work, aii of pure water was used for all types of beads and for the interaction of the similar

18

beads, we used the standard value of aii =25, as it is reported by other researchers15, 57, 58 that, using

19

this value gives more accurate results for the IFT of oil/water systems. For pure components or the

20

identified light components of an oil model (which can be considered as a pure pseudo-

21

component), χij can be calculated from the solubility parameter by using

22

mesurements or all-atom molecular dynamics (MD) simulation where the all-atom MD simulation

23

procedure has been described in the previous publications

24

oil systems, due to their complex structures, a blend methodology model, was used to evaluate χij

(11)

10

9, 12

experimental

. But in this work, for SARA crude


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Energy & Fuels

19, 55

1

without recourse to solubility parameter calculation of a pure component

2

estimates the miscibility behavior of binary mixtures and requires only the molecular structures

3

and a force field as the inputs data to predict the χij parameter, directly for beads i and j

4

this work, we used the Blends module of the Materials Studio, Accelrys Inc.

5

Huggins parameters (χij) with the understanding that this prediction would be qualitative and not

6

quantitative, since this parameter depends on the choice of initial conditions and force fields used

7

as well as molecular configuration and the charge equilibration. The computational details will be

8

given in the next section. The calculated aij values for the SARA crude oil systems at 298 K and

9

363 K, converted to DPD units, are presented as a symmetric matrix in Table 2.

60

. This method

55, 59

. In

to predict Flory-

10 11

3. Simulation details 9, 12

12

The DPD simulations were performed in NVT ensemble using the DPD module

. The

13

simulations were carried out in a cubic box with a size of (20× 20× 20)rC by using periodic

14

boundary conditions (pbc) in three dimensions at temperatures 298 K (kBT=1 in DPD unit) and 363

15

K (kBT=1.218 in DPD unit) where on the energy scale, they are respectively 2.4765 kJ/mol and

16

3.0213 kJ/mol. In the DPD simulation, rC is the unit length and all DPD particles (beads) have the

17

same mass (m=1). The DPD units of size and time can be converted to physical units, by using

18

Eqs. (9) and (10). The dependence of simulation time step on temperature can be ignored up to

19

∆t = 0.06 35,

20

position and velocities of the particles, the modified velocity-Verlet algorithm35 was used for the

21

equation of motion. To have an acceptable (N,V,T) ensemble in DPD method, the fluctuation−

22

dissipation relation (Eq. 6) should be satisfied 34, 35. Since for SARA heavy crude oil systems, the

23

resin and asphaltene cores were each considered as a single bead and the rigidity of the core is not

24

a determining factor and it is not an issue to concern with 19. However, for all the connected beads

3

52

, therefore, in this work the time step was chosen as ∆t = 0.05 . To obtain the

11


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Page 12 of 42

1

of the studied DPD systems, the rigidity parameter, k S was kept fixed constant in the DPD unit (

2

k BT / rC2 ) as k S = 4

3

mk B T / rC2 )

61

32, 38-40

. The dissipation strength was assumed as

ζ =4.5 in DPD units (

and the noise strength as σ = 3, to control the temperature via fluctuation

4

dissipation theorem (Eq. 6). It is reported that for a typical DPD simulation of such oil/water

5

systems, only about 5 × 10 4 steps are required to follow the beads motion to study the formation

6

and decomposition of aggregates

7

2 × 10 4 time steps (13,433 ps and 12,171 ps at 298 K and 363 K respectively) for equilibration

8

which was followed by 8 × 10 4 steps for completion of the DPD simulation. Therefore, each system

9

was simulated up to 10 5 time steps (67,163 ps and 60,853 ps, respectively at 298K and 363 K). The

10

configurations were saved at every 100 steps to be used for further analysis of the simulation

11

results that is, the length of each frame of snapshots was set to 100 time steps. All the DPD

12

properties can be derived from rC, m, and, kBT respectively for length, mass and energy, where in

13

physical units they are respectively 11.74 Å, 324 amu and 2.4765 kJ/mol at 298 K and 3.0213

14

kJ/mol at 363 K. Also, the time scales for DPD simulations, as obtained from Eq. (10), were 13.43

15

at 298 K and 12.17 at 363K. Before running the DPD simulations, the interaction parameter (aij)

16

was calculated by using the Blends module of Materials Studio.

55

. In the simulations of this work, at first we used at least

17 18 19 20

4. Results and discussion 4.1. The spatial structure of crude oil/water/polyaromatic surfactant (PAS) systems 4.1.1. Crude oil/Water Interface without PAS

21

DPD simulations were performed by using periodic boundary condition for the simulation boxes of

22

volumes 20 × 20 × 20 in units of ( rC ) which were separated into two regions: the crude oil (region

23

I), and three water droplets (region II). The radii of water droplets were 1.5, 3 and 5 in DPD units

3

12


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Energy & Fuels

1

(1.76, 3.52 and 5.87 nm) and their centers were located at DPD fractional coordinates: (0.1665,

2

0.1665, and 0.1665), (0.8335, 0.8335 and 0.8335) and (0.5, 0.5, and 0.5), respectively in (x,y and

3

z). Figure 4 shows the initial and final configurations of water droplets in SARA crude oil systems

4

at 298K, where the oil beads, in this figure, were concealed in order to have a better visibility of

5

the water droplets. The full description of the aggregation of the water droplets can be seen in

6

Figure S2 of the Electronic Supplementary Information (ESI) of this manuscript. Simulation

7

results in Figure S2 show that the aggregation of water droplets begins after 10 frames of DPD

8

simulation or after 671 ps, when the two larger droplets which have the lowest distances from each

9

other, begin to join together. Then, after 40 frames (or 2686 ps), there is only one water droplet left

10

in the system, and at a later time, at equilibrium state, the shape of this water droplet fluctuates

11

between spherical and non-spherical shapes (as it is seen in Figure 4b and c), due to changes in the

12

configuration of other molecules in the oil phase which have tendency to reach their equilibrium

13

configuration with their minimum energy and then fluctuate around their equilibrium configuration

14

(as it is seen in Figure 5).

15

By considering the results of simulation as represented in Figures 5 to 7 and S2 to S6, the

16

following qualitative discussion on aggregate formation can be presented, which may be beneficial

17

in better understanding of the crude oil/water interface in the absence and presence of PAS.

18

Figure 5a, shows the configuration of the crude oil components: saturates (Sat1 and Sat2 with the

19

ratio of 50:50), aromatics (Ar1and Ar2 with the ratio of 50:50), resin and asphaltene, during

20

different time steps of DPD simulations at 298 K. As it is shown in this figure 5 a, at the initial

21

steps of simulation (before the 100th frame), a single spherical aromatic aggregate is formed, where

22

the Ar2 beads are located at its core and the Ar1 beads are located around the aggregate core. As it

23

is seen in Figure 5a, the resin molecules are surrounding these aromatic aggregates. At the

24

beginning of the simulation, the asphaltene molecules are initially near the aromatic core, then they

25

start to move into the bulk which consists of saturate components of the crude oil to form 13


Energy & Fuels

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Page 14 of 42

1

asphaltene aggregates. At the later time, as the simulation proceeds, the asphaltene aggregates

2

approach the water droplet and change their locations, in the vicinity of the water droplet in the

3

course of the simulation. The results represented in Figure 5b, are obtained at the same simulation

4

condition as those of Figure 5a. The only difference between these two figures is that; Figure 5a

5

represents (Sat1 and Sat2 with the ratio of 50:50) and (Ar1and Ar2 with the ratio of 50:50) and

6

Figure 5b represents, only (Sat1) and (Ar1) and other components of crude oil. Figure 5b shows

7

that, at the final simulation steps, the asphaltene aggregates are not attracted to the water droplet. A

8

similar configuration is presented in Figure 5c, for simulation at 363 K, where no large aromatic

9

spherical aggregates are formed and the resin and asphaltene molecules are spread in the

10

simulation box, without forming any ordered aggregate, except at the final simulation steps, where

11

a few small aggregates are formed. Consequently, as a result of asphaltene and resin spreading in

12

the saturate bulk, and the collision of water molecules with these molecules, the water droplets

13

formation is retarded and takes longer time in comparison with the previous case, as was explained

14

for Figure 5b. From the results presented in Figure 5c, it is seen that at the 20th frame, two small

15

water droplets have been formed compared with Figure 5b, where at the 20th frame all the droplets

16

have joined together and formed a large droplet. On comparing the results, it can be concluded

17

that, the type of aromatics and saturates affect the behavior of the beads involved in the

18

aggregations and as a result, they determine the final configuration. Also, it was found that, the

19

temperature increase causes the water droplets’ separation from the oil phase, to occur in a longer

20

time. In the next sections, it will be shown that, the crude oil components (the type of aromatics

21

and saturates), as well as the surfactant presence, will play a significant role in the interfacial

22

properties of oil/water emulsion.

23 24

4.1.2. Crude oil-water-PAS (after adding PAS):

14


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Energy & Fuels

1

Figure 6, shows the aggregation configuration of different PAS molecules (TP, C5Pe, PAP and

2

PCH) around the water droplet in the crude oil/water system after 105 steps of DPD simulation at

3

298K and 363 K. Also in Figure S3, the positions of PAS molecules in the simulation box are

4

presented for the equilibration steps of 2 × 10 4 (13,433 ps). The comparison of the results at two

5

temperatures indicates that at 298 K, the PAS molecules aggregate around the water droplet to

6

form a compact structure, whereas, at 363 K, they disintegrate from each other and take distance

7

from the water droplets. Figure 6 indicates the behavior of different PAS types, when added to the

8

oil/water system. In this figure, it is observed that at 298K, the PAS types including C5Pe (Figure

9

6b) and PCH (Figure 6d) are aggregating around the water droplet, whereas the PAS type TP

10

molecules (Figure 6a) prefer to aggregate by themselves, rather than being attracted to the water

11

droplet. However, the PAS type PAP molecules (Figure 6c) have an intermediate behavior, that is,

12

they want to surround the water droplet as well as to aggregate by themselves. Figures 6a to 6d

13

represent the behavior of PAS molecules as the reverse-micelle. Their hydrophilic head groups

14

(TP, C5Pe, PAP and PCH) are partially shielded from the oil phase whereas their hydrophobic

15

chains are arrayed toward the oil phase. By definition, a micelle is an aggregate formed by

16

surfactants, where their hydrophobic sections are locating in the interior of the aggregate and their

17

hydrophilic sections are facing toward the aqueous medium 62. The reverse-micelle, by definition,

18

is an aggregate formed by surfactants where their hydrophilic sections are in the interior and their

19

hydrophobic sections are extending from the aggregate centre toward the oil medium 62, 63.

20

As Figures 6a to 6d indicate, all the studied PAS molecules at 298 K exist in the form of the

21

reverse-micelles and their concentrations are most probably above the CMC. However, the CMC

22

of a surfactant (PAS) in a complicated system such as crude oil, due to difficulties in experimental

23

measurements are not reported, but on the basis of research reports on similar simple systems

15


64-

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1

67

2

surfactant is above CMC .

3

At 363 K, this behavior becomes somehow different, where no trace of reverse-micelles is

4

observed (Figures 6 (e, f and h)), except those represented in Figure 6g for PAP. However, as

5

Figure 6e to 6h show, all PAS molecules are not attracted to the water droplets and they prefer to

6

gather around the aromatic aggregates. These phenomena are seen in Figure S4, where the

7

aromatic molecules have been presented, whereas their presence was concealed in Figure 6e to 6h.

8

Page 16 of 42

, it can be stated that when the micelles (direct or inverse) are formed, the concentration of the 7

4.1.3. PAS concentration effects

9

An increase in the concentration of PASs in the oil-water systems leads to an easier separation of

10

water and oil. To show this, the DPD simulations were done for 5% and 10% of PAS

11

concentrations. The results of simulation on 5% PAS concentration are represented in Figures 6a to

12

6h , Figures S3 and S4. In Figures 7a to 7h, the configuration of four PAS molecules at 298K, with

13

a concentration of 10%, around the water droplets are shown at frames 200 and 1000, respectively,

14

for 1,343 and 6,7173 ps of simulation time. Comparing these figures (Figures 7a to 7h) with the

15

snapshots, taken for 5% of PAS concentration (Figures 6a to 6h and Figure S3 and S4), it is

16

observed that, the PAS molecules behavior in aggregate formation around the water droplets is

17

similar at these two concentrations. At 10% concentration and for the frame 200, it seems that the

18

PAP is not able to force all the small water droplets to join each other, and still two water droplets

19

are left, separated from each other (Figure 7e). Similar behavior is seen for PAP at 5% (Figure

20

S4c). But in the case of PCH in the equilibrium stage and at the surfactant concentration Cs=10%,

21

one water droplet is present, whereas at Cs=5% (Figure S4d) there is one main water droplet and

22

several small water aggregates. Also at 298K and 363 K, the effect of the studied PAS molecules

23

on the interfacial tension (IFT) of the crude oil/water system, was studied by simulating the

24

systems with different PAS concentrations. The results are discussed in the following section.

25 16


Page 17 of 42

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Energy & Fuels

4.2. Interfacial behavior

1 2

There are two main factors which can define the overall efficiency of a surfactant: the tendency for

3

absorption at the interface and its ability in IFT reduction. For this purpose, in this work, we

4

studied various concentrations of surfactant (Cs) to calculate the IFT of different oil/water systems

5

containing

6

Lx × Ly × Lz = (40× 20× 20)rC3 , but the other conditions were the same as those used in the previous

7

mentioned simulations. The water molecules were put in the middle part (with length of L x = 10 rC )

8

and the oil phase was located at the two sides of the simulation box (with length of Lx = 2 × 15 rC ).

9

This arrangement of the simulation box was used in all the simulations.

various

PAS

types.

The

simulation

box

dimensions

were

chosen

as

10

The interfacial tension (IFT) is obtained based on the well-known Irving-Kirkwood equation

11

29,30,31,24,25] as derived by Lyklema 68, 69:

12

γ =

13

where , Pn and Pt are the normal and tangential pressures respectively, and x is the thickness of

14

the interface. The DPD IFT ( γ DPD) was calculated by the following equation 57, 70:

15

γ DPD =

16

where, the first factor of 1/2 is to account for two existing interfaces in the simulation box. Lx is

17

the box length in the x-direction and x is converted to a dimensionless value in the DPD

18

simulations, defined as x = x / rC . The angle brackets show the ensemble average of the local

19

dimensionless pressure tensors components ( p xx , pyy and pzz ) at x, y and z directions.

∫

x

0

( Pn − Pt ) dx

1 1  p xx − 2 2

(12)

(p

yy

)

 + p zz  L x 

(13)

20 21

4.2.1. Interfacial density

17


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Page 18 of 42

1

Figure S5 represents the density profiles and simulation snapshots of the studied systems, obtained

2

over 105 time steps for the studied crude-oil systems, in the presence and absence of PAS

3

molecules at 298K and 363 K. The density distribution of the system was computed by dividing

4

the simulation box into 40 equal slabs, along the x-direction (perpendicular to the oil−water

5

interface) and then the density of components was calculated for each slab. In all the studied

6

systems, the density curves of SARA crude oil components and water intersect each other. This

7

means that some of the mesomolecules have penetrated into the water phase or vice versa. In

8

Figure S5, it is seen that, the saturate as SARA component has higher tendency to aggregate, at the

9

oil-water interface, than the other SARA components. For the systems in the absence of PAS

10

molecules, at 298 K (Figure S5 a1), some of the aromatic molecules are attracted to the interface,

11

whereas at 363 K (Figure S5 a2), they tend to avoid the interface. In these systems, by adding PAS

12

to the systems, the asphaltene and resin molecules stay in the oil phase, close to the interface

13

(Figures S5 b1-e2), these results are interpreted as follows: At 298 K, all the uncharged PAS

14

molecules prefer to stay in the water-oil interface, in a configuration that, their head groups be

15

located at the interface and their lipophilic tails prefer to be in the oil phase. In Figure S5, the

16

absorption of PAS molecules into the oil-water interface is shown by red curves, where the highest

17

peak of this curves crosses the water density curves (blue curves) at the water-crude oil interface.

18

As it is shown in this snapshot (Figure S5), the TP molecules are aggregated in the oil phase,

19

whereas a large number of other PAS molecules (C5Pe, PAP and PCH) are distributed along the

20

interface. However, at 363 K, most of the PAS molecules have tendency to form small aggregates

21

in the oil phase and mostly around the aromatics, rather than to be absorbed at the water-oil

22

interface .The C5Pe molecules are more crowded at the oil/water interface ( Figure S5 c2) than

23

the TP, PAP and PCH molecules and as Figures S5 b2, d2 and e2 indicate their density distribution

24

curves, at the oil/water interface, while exhibiting smaller peaks in comparison with C5Pe, they are

25

extending to the oil bulk phase. These results are in accord with those presented in Figure 6, which 18


Page 19 of 42

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Energy & Fuels

1

indicate that the adsorption of the PAS molecules at the crude oil-water interface is lowered by

2

increase in temperature. At 363 K, the chain molecules such as various types of PASs, resins and

3

asphaltenes, spread in the oil phase rather than aggregating or being absorbed on the aromatic

4

aggregates or water droplet surfaces. This behavior is in contrast with the behavior of these chain

5

molecules at 298 K. The final configurations of TP and C5Pe molecules with various

6

concentrations (Cs = 0, 5%, 10%, 15% and 20%), obtained at the end of the DPD simulations at

7

298K and 363 K, are represented in Figure S6.

8

An important property for a PAS molecule is its concentration at the interface (Ci). To calculate

9

Ci, first of all, the thickness of the interfacial region should be defined. There are two criteria for 71-74

10

defining the interfacial thickness: (i) “10-90” criterion

11

as the distance between two positions in the studied system where the components’ density varies

12

from 10 to 90% of their bulk densities. (ii) “90-90” criterion 75 in which the interfacial thickness is

13

defined as the distance between two positions where the densities of two phase (here oil and water)

14

are 90% of their own bulk densities. In this work, we used the criterion (ii) which is more

15

appropriate for the complicated interfaces

16

variation of the components at 298 K, in the system of {oil phase [saturate + aromatic + resin +

17

asphaltene + PAS (PCH)] /water phase}, as an example, is presented in Figure S7 where the

18

calculated interfacial thickness based on criterion (ii) is indicated. Also, the calculated interfacial

19

concentrations (Ci) of the PAS molecules for variation of global concentration of PAS molecules

20

(Cs) are reported in Figure 8 and Table S1 which indicates on increasing Cs the interfacial

21

concentration (Ci) increases for all surfactant molecules. The results in this figure are in agreement

22

with those presented previously in Figures 6, 7, S4, S5 and S6 which show that the C5Pe PAS

23

molecules has the highest tendency to locate themselves at the interface.

75

which defines the interfacial thickness

such as those studied in this work. The density

24 25

4.2.2. Interfacial tension (IFT) 19


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Page 20 of 42

1

The interfacial tension (IFT) is an influential property of crude oil in the reservoirs which affects

2

the oil production 20. The calculated IFT of the studied systems by using Eq. (13) in the presence

3

and absence of PAS molecules at 298K and 363 K, are reported in Table 3 and Figure 9a. The

4

obtained IFT for water-crude oil systems (in the absence of PAS) are 4.32 and 3.78, in DPD units

5

at 298K and 363 K respectively, which are equal to 13.75 and 12.89 mN/m in the real physical

6

units. Table 3 and Figure 9a show that, as expected the IFT values at 363 K are lower than 298 K

7

and the IFTs decrease with the PAS concentration. Also, as it is seen in Figure 9a, the IFT values

8

are lower than those of TP, PAP and PCH, as a result of C5Pe addition to the oil/water systems.

9

Upon comparing the results presented in Table 3, with the related snapshots for the configurations

10

of TP and C5Pe as illustrated in Figure S6, it is seen that at 298K, the C5Pe molecules are attracted

11

to the water-oil interface and are distributed all through the interface in a well-ordered form. The

12

main reason for this behavior can be attributed to the strong attraction between the water beads and

13

C5Pe head groups which is manifested by the lower DPD interaction parameter ( a ij = 30.46 ),

14

compared with the value of this parameter for the other PAS molecules. As a result of strong

15

interaction of C5Pe head groups with the water beads at the interface, as the Table 3 indicates, the

16

IFT of the water/oil interface due to the C5Pe absorption, in all concentrations, is lower than those

17

of other PAS molecules. Unfortunately, there is no experimental result on IFT of water/crude oil

18

systems in presence of the studied PAS molecules, Therefore, on the basis of the obtained results

19

of this DPD simulation, it can be proposed that C5Pe is the best surfactant to be used for the IFT

20

reduction of the crude oil-water interface, that is, the addition of a fixed amount of this surfactant

21

is more effective and economical, since it lowers the IFT more than the other surfactants. As it is

22

seen in Figure 9 a, at 363K, the observed reduction of IFT for various PASs does not differ

23

significantly from each other. However, as this figure represents, at low concentrations (Cs=5% for

24

TP) and at high concentrations (Cs=20% for C5Pe), the IFT values are respectively 2.88 (10.45

25

mN/m) and 2.35 (8.54 mN/m) which are the lowest IFT values due to addition of PAS to the crude 20


Page 21 of 42

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Energy & Fuels

1

oil-water systems. But, at medium concentrations (Cs=10%), the lowest value of IFT (2.35 (8.54

2

mN/m)) belongs to PCH. Also the IFT variation as a function of PAS molecules concentration in

3

the interface (Ci) was calculated and presented in Figure 9 b. As expected, this Figure indicates

4

that IFT decreases with PAS interfacial concentration (Ci), where the effect of temperature and

5

type of PAS on decreasing the interfacial tension, as discussed earlier, are clearly observed in this

6

figure. It should be noted that, Figure 9 b suggests some kind of similarity between the studied

7

PASs when they are placed at the interface. However, there are some clear differences between

8

them which can be due to their different “affinity” or partitioning between interface and bulk. This

9

point, as explained earlier, can affect the PASs tendency to approach the interface and manifest

10

their characteristics. This behavior is also seen in Figure 8, where C5Pe tends to be preferentially

11

placed at the interface when compared with PCH, for instance.

12

However, Figure 9 (and also Figure 8) are quantitative and provide a clear idea of the distribution

13

of each PAS between the bulk and interface and the fact that they have the same “surfactant”

14

potential once placed at the interface. This information is precisely the one that cannot be easily

15

obtained in experiments.

16

4.2.3. Diffusion coefficient

17

In dissipative particle dynamics (DPD), a DPD-particle is a virtual construction not existing in

18

nature and it is essential to relate any bead diffusion to real molecular diffusion by using proper

19

scaling law

20

considered in calculating the mean square-displacement (MSD) in the well-known Einstein

21

relation

22

particular PAS molecule has higher mobility than another one and estimate the tendency of their

23

aggregation, in the studied system, the diffusion coefficient of each PAS molecule was evaluated

24

by using the following equation 23, 60:

23, 60

32, 35

where the center of mass of group molecules (COM) in a bead should be

. As a first approximation and to present a qualitative descriptor to show whether a

21


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1 d ( MSD ) 1 d 2 lim = lim r (t ) − r ( 0 ) t → ∞ t → ∞ 6 dt 6 dt

Page 22 of 42

1

D=

2

Where, the diffusion coefficient (D) was calculated by linear fitting of the MSD. The results are

3

presented in Table 4 which indicate that the diffusion coefficient (D) of PAS molecules at 363 K is

4

higher than 298 K. However, as the oil phase is not a pure fluid, there are other effective factors

5

that should be considered, for a precise interpretation of diffusion behavior of PAS molecules,

6

which ultimately may facilitate or prevent their aggregations. As it is seen in this table, the order of

7

the diffusion coefficient (D) of PAS molecules at 298 and 363 K, after the equilibrium stage are

8

PCH>C5Pe>PAP>TP and C5Pe>PCH>PAP>TP

9

interaction of PCH with the water molecules, as it is manifested by the DPD interaction parameters

10

reported in Table 2, they are attracted to the water droplet interface. This is also seen in Figure 6d,

11

where PCH molecules are surrounding the water droplets. As the diffusion coefficient of C5Pe is

12

higher than TP and PAP, and also the C5Pe-W interaction in stronger (aij=30.46), the C5Pe

13

molecules are attracted to the water molecules, and as a result, they surround the water droplet,

14

whereas as illustrated in Figure 6c, the PAP small aggregates are not similar to PCH aggregates

15

and also they are not surrounding the water droplets like C5Pe and PCH. This behavior can be

16

attributed to the lower diffusion of PAP molecules in comparison with PCH and C5Pe and the

17

lower attraction of PAP to the water beads (aij= 46.13). Due to the lower attraction force of TP

18

molecules with water, which is manifested by higher DPD interaction parameter (aij=72.63), TP

19

molecules in their aggregate forms are not attracted to the oil-water interface, and they remain in

20

the oil phase as the dispersed aggregates.

21

At 363 K, the order of diffusion coefficient, as mentioned above (C5Pe>PCH>PAP>TP), indicates

22

a higher rate of diffusion and tendency of C5Pe molecules to disperse into the oil phase. Also, the

23

stronger attraction ( a ij = 38 .05 , in Table 2) of C5Pe core (PBI bead) with the aromatic (Ar1 bead),

24

the C5Pe molecules are attracted to the aromatic aggregate (as it is seen in Figure 6 f). Initially,

(14)

22

respectively. At 298 K, due to stronger


Page 23 of 42

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Energy & Fuels

1

PAP molecules diffuse with higher rate to form small aggregates, but the small aggregates cannot

2

move as fast as PCH and C5Pe to approach the (oil/water) interface and (aromatic aggregate) in

3

the oil phase to form interfacial aggregates. Therefore, it can be stated that the interaction between

4

all kinds of beads including PAS (head group, PBI core, tail group), water and Ar1, can be

5

considered as the dominant effect, which determine the diffusivity of molecules and their

6

aggregates in the oil/water system.

7

It should be noted that in calculating the transport properties such as diffusion coefficient by DPD

8

method an optimization of the dissipation strength ( ζ ) to modify the equations of motion is

9

required 76, 77. However, since the application of standard dissipation strength of

ζ =4.5 have been

10

used efficiently in studying the thermodynamic behavior (such as interfacial density and IFT) of

11

various systems, as well as oil-water-surfactant 9, 48, 52, 56, 66, we used this “standard” value ( ζ =4.5)

12

of dissipation strength for calculating the diffusion coefficient (D) as a first approximation, to

13

assess the ability of DPD method to estimate the diffusivity of PAS molecules in such complex

14

system.

15

The significance of surfactant diffusion in the oil phase toward the oil-water interface is justified

16

by considering the order of magnitude of diffusion coefficient (~10-9 m2s-1) which qualitatively

17

indicates that, the hydrophilic heads of the surfactant molecules are not instantainously attracted

18

into the oil-water interface. That is, the adsorption of surfactant into the oil-water interface is a

19

diffusion controled process which can influence the expected behavior of surfactant as a result of

20

its application to the oil-water systems.

21 22

4.2.4. End-to-end distance of surfactant

23

The structure and orientation of PAS molecules at the oil/water interface can be analyzed by

24

considering the degree of their chain curliness which is indicated by calculating the root mean 23


Energy & Fuels

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1/ 2

39, 78

Page 24 of 42

1

square (RMS) end-to-end distance ( R 2

2

molecules in the crude oil/water systems were evaluated at 298K and 363 K. As it is shown in

3

Table 3, an increase in R 2

4

perpendicularly to the oil/water interface. As a result of PAS absorption on the interface, the

5

interfacial number density (defined as the number of PAS molecules absorbed on the unit area of

6

the oil/water interface) increases which leads to a decrease of the conformational mobility of the

7

PAS molecular chain which means that the PAS molecules are oriented in an ordered form at the

8

interface and as they have less available space for their movement (as it is seen in Figure S5 and

9

9). However, the variations of R 2

1/ 2

)

. In this work, RMS values for the studied PAS

of the PAS molecules suggests that the molecules are oriented

1/ 2

as reported in Table 3 are not so significant, since the

10

molecules studied in this work (PAS, asphaltene and resins) were considered as the mesomolecular

11

chains, consisting of only two beads connected by a spring. In spite of this fact, by inspecting the

12

snapshots of TP and C5Pe at different concentrations and at two temperatures (298K and 363 K) as

13

represented in Figure S6 provide a meaningful insight into the interfacial orientation of the PAS

14

molecules (TP and C5Pe as examples) at the oil/water interface. This figure indicates that at 298

15

K, almost all the C5Pe molecules have been attracted and packed together at the oil/water interface

16

in a well ordered manner. The strong attraction between water beads and C5Pe head groups causes

17

the C5Pe molecules to be oriented as stretched chains perpendicular to the water/oil interface. This

18

would restrict the chain motions and results in an increase in the R 2

19

the results in Table 3, a decrease in the IFT values can be related to the orientation of PAS

20

molecules at the oil/water interface, which is manifested by the increase of the R 2

21

obvious that the PAS concentration does not affect the

22

significantly.

23

R2

5. Conclusion 24


1/ 2

1/ 2

values. As it is seen from

1/ 2

values. It is

values of asphaltene chain

Page 25 of 42

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Energy & Fuels

1

Three dimensional DPD simulations method was employed to study the aggregation of water

2

droplets in a crude oil-water system in the absence and presence of the polyaromatic surfactants

3

(PAS). For the crude oil in this study, a SARA crude oil model of Persian Gulf was used. The

4

molecules were divided into appropriate beads and the interaction parameters of the beads as the

5

initial DPD parameters, were obtained by applying the Blend module of Material studio. The

6

results of DPD simulation were analyzed by considering the evaluated spatial configurations,

7

diffusivity, end to end distance, interfacial density and interfacial tension (IFT). The efficiency of

8

the four PAS molecules in IFT reduction at different concentration was investigated at tow

9

temperatures 298K and 363K. The obtained results are summarized as follows:

10

-

11

In the SARA crude oil-water systems, upon the temperature increase the joining of water droplets and as a result the oil/water separation was delayed.

12

-

13

The aromatics in SARA crude oil aggregated as spherical droplets at 298 K, and forced the resins to be absorbed around them. This was in contrast with the behavior at 363 K.

14

-

It was observed that, the TP molecules prefer to form self-aggregates, as reverse micelles,

15

where C5Pe molecules prefer to surround the water droplets at 298K, whereas at 363 K,

16

they prefer to surround aromatic aggregates.

17

-

The spreading of PAS molecules increased by increasing the temperature. As the

18

diffusivity of PCH was higher than the other surfactants, dispersion of PCH in the oil phase

19

was higher than the other PASs.

20

-

21

Among the studied surfactants, C5Pe was the most efficient surfactant in the reduction of the IFT of the oil/water interface.

22 23

.

24

■ ASSOCIATED CONTENT

25

Supporting Information 25


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Page 26 of 42

1

Figure S1 shows the coarse grained structure of PAS molecules. Figures S2 to S4 represent the

2

morphology of water droplets during the simulation and the snapshot of PAS molecules located

3

around the water droplets at equilibrium steps at 298K and 363 K. Figure S5 presents the density

4

profiles and simulation snapshots of SARA crude oil/water systems in absence and presence of

5

PASs at 298 and 363 K. Figure S6 represents snapshots obtained for simulation of (crude

6

oil/water/ TP and C5Pe PAS) system for different surfactant concentrations (Cs): 0, 5%, 10%, 15%

7

and 20% at 298K and 363 K and Figure S7 shows 90-90 criterion for obtaining interfacial

8

thickness. Table S1 shows the interfacial concentration (Ci) versus global concentration (Cs) for

9

different studied PAS molecules. The supporting information is available free of charge via the

10

Internet at http://pubs.acs.org.

11

■AUTHOR INFORMATION

12

Corresponding Author

13

*E-mail: [email protected]

14

Notes

15

The authors declare no competing financial interest.

16 17

References

18 19 20 21 22 23 24 25 26 27 28 29 30

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79. Boek, E. S.; Headen, T. F.; Padding, J. T., Multi-scale simulation of asphaltene aggregation and deposition in capillary flow. Faraday discussions 2010, 144, 271-284.

Figure captions: Figure 1: Chemical structures of asphaltene (a) “island” type asphaltene structure with one large aromatic core, (b) an “archipelago” type asphaltene with two condensed aromatic cores connected by an aliphatic chain (c) a structure at the resin end of the asphaltene spectrum 79.

Figure 2. 3-dimensional schematic representation of the perylene bisimide-based polyaromatic (PA) molecules used in this study. (a) a polyaromatic perylene core with two alkyl chains attached to aliphatic chain end and (b) a different functional groups (PCH, TP, PAP and C5Pe) attached to the other end of the molecules ( (R) in part (a) is substituted by these functional groups) 21, 23, 44. Figure 3. Schematic representation of coarse-grained crude oil SARA models. (a) Saturated, namely, Sat1: 3-methy-l-octane, Sat2: 3,5-dimethyl-4-ethyl octane (b) aromatic, condensed aromatic rings in its structure, (c) resins, one alicyclic groups and one alkyl chains in its structure, (d) asphaltenes, “island-type” core consisting of one alicyclic group, two alkyl chains and a sulfurcontain chain which is connected to alkyl chain. Figure 4. The oil-water emulsions. (a) 3 spherical water droplets the initial configuration. (b) aggregation of water small droplets to form an individual spherical droplet after 40 frames and (c) deformation of water droplets spherical shape after 200 frames. All the crude oil fractions are concealed. Figure 5. The configurations of water and SARA crude oil components at different time steps ( the time frames are 0, 100, 200, 400, 700 and 1000) of the temperatures: (a) T=298 K where crude oil components are : saturates (Sat1 and Sat2 which are concealed to have a better view of the other beads), aromatics (Ar1:light violet, Ar2:dark violet), resin (core: dark green; tail: light green) and asphaltene (core (CAi): blue; sulfur chain (S):red and aliphatic chain (A1): light green). (b) T=298 K, where for crude oil components, only Sat1 and Ar1 are considered as saturate and aromatics. The others conditions are similar to (a). (c) T=363 K, the other conditions are similar to (b). Water beads are shown in cyan for all. (For better vision of the figures and the color used in, the reader is referred to the web version of this article.)

Figure 6. The location of PAS molecules around the water droplet at 298 K: (a) TP, (b) C5Pe, (c) PAP and (d) PCH. At 363 K: (e) TP, (f) C5Pe, (g) PAP and (h) PCH. The concentration of PAS in oil phase is 5%. The participant particles are water (cyan), PAS head (red), PAS core (PBI) (orange), PAS tail (double A4) (yellow). The SARA crude oil components (saturates, aromatics, resins and asphaltenes) are concealed in order to have a clear view of the other molecules.

Figure 7. The configuration of various PAS molecules with a concentration of 10% around the water droplet at 298 K: (a and b): TP, (c and d): C5Pe, (e and f): PAP and (g and h): PCH during the DPD simulation equilibration time steps (frame 200:1,343 ps) and the last step ( frame 1000:67,173 ps) 31


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1 2 3 4 5 6 7 8 9

10

Figure 8. Interfacial concentration (Ci) versus global concentration (Cs) for different studied PAS molecules at 298 K and 363. Figure 9. Interfacial tension (IFT) versus (a) PAS global concentration (Cs) and (b) PAS interfacial concentration (Ci) at 298 and 363 K.

Tables Table 1. Compositions’ of studied SARA crude oil model of Khark Island (Persian Gulf) 43 component

symbol

Saturations Aromatics Resins Asphaltenes

Sat Ar R As

Composition (wt%) 59 28.5 9.7 2.8

11 12

32


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Table 2 . DPD interaction Parameters (in DPD units) used in the DPD simulations for crude oil at 298K and 363K. Bead type

Symbol

Sat1

Sat2

Ar1

Ar2

CAi

A1

S

Saturates

Sat1 Sat2 Ar1 Ar2 CAi A1 S W Cr C5Pe PAP TP PCH PBI

25.00 25.20 107.59 95.76 125.05 25.93 26.71 63.83 58.49 43.74 57.50 73.32 28.48 196.71

25.00 103.82 96.02 133.67 25.26 26.33 61.31 63.08 43.20 57.34 74.26 29.06 195.18

25.00 35.97 84.16 124.04 120.46 192.92 47.18 133.49 105.50 126.59 100.11 54.72

25.00 109.78 101.82 102.84 156.20 67.94 102.79 104.00 109.11 93.87 10.26

25.00 144.85 142.45 188.68 46.96 131.77 98.34 127.43 100.18 33.06

25.00 26.66 58.68 68.98 43.60 58.75 76.43 29.91 202.14

25.00 41.68 67.25 37.47 46.13 63.92 26.47 201.37

26.40

25.67

140.57

116.87

130.31

25.34

25.00 25.03 100.23 87.33 89.82 25.48 25.47 53.89 41.06 34.00 41.54 48.91 27.24 183.53

25.00 105.32 88.40 96.95 25.17 25.96 52.16 44.45 34.09 42.05 49.51 27.43 181.55

25.00 41.35 62.19 113.37 112.97 168.48 37.66 107.46 103.83 95.97 99.93 38.05

25.00 77.43 92.42 92.76 130.89 43.23 87.77 85.47 86.07 86.39 36.15

25.00 103.92 101.96 148.86 56.10 105.13 92.33 95.90 89.28 93.38

25.00 25.94 50.21 48.27 34.44 43.16 50.81 28.08 187.85

25.00 38.98 47.89 30.96 35.41 43.00 25.92 186.66

25.83

25.76

124.71

101.65

109.14

25.22

26.49

Aromatics Core of Asphaltene Aliphatic chain Sulfur contained Water Core of resin PAS heads

Core of PAS Aliphatic chain of PAS Saturates Aromatics Core of Asphaltene Aliphatic chain Sulfur contained Water Core of resin PAS heads

Core of PAS Aliphatic chain of PAS

A4 Sat1 Sat2 Ar1 Ar2 CAi A1 S W2 Cr C5Pe PAP TP PCH PBI A4

W 298 K

Cr

C5Pe

PAP

TP

PCH

25.00 67.71 65.12 72.12 58.13 94.83

25.00 22.06 32.32 28.83 200.79

25.00 48.78 35.53 211.40

25.00 52.81 200.12

25.00 191.03

25.00

74.58

45.55

62.56

73.28

31.40

225.01

25.00 86.68 31.31 37.79 50.82 38.06 262.60

25.00 49.30 48.87 47.22 40.53 76.60

25.00 25.65 31.74 27.58 184.80

25.00 31.11 28.98 192.14

25.00 31.76 160.59

25.00 179.37

25.00

48.85

51.91

34.82

43.70

52.84

28.61

202.77

25.00 122.49 30.46 45.58 72.63 39.19 306.55

26.92 56.64 363 K

33 ACS Paragon Plus Environment

PBI

A4

25.00

25.00

Energy & Fuels

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Page 34 of 42

Table 3. Interfacial tension (IFT) and Root mean square end-to-end distance ( R 2

1/ 2

) of PAS,

resins and asphaltenes mesomolecules for different PAS (TP, C5Pe, PAP, PCH) concentrations at 298K and 363 K. 298 K PAS Conc. (%)

R2

IFT (DPD unit)

363 K 1/ 2

R2

IFT

1/ 2

(mN/m)

Asph.

Resin

PAS

(DPD unit)

(mN/m)

Asph.

Resin

PAS

Absence of PAS 0.00

4.32

12.89

1.66

1.10

-

3.78

13.75

1.98

1.17

-

5.00 10.00 15.00 20.00

3.79 3.74 3.53 3.18

11.32 11.17 10.52 9.48

1.77 1.78 1.76 1.77

1.14 1.14 1.13 1.14

1.38 1.40 1.35 1.36

2.88 2.67 2.39 2.49

10.45 9.69 8.70 9.05

1.96 2.01 1.99 2.04

1.17 1.17 1.16 1.16

1.41 1.42 1.48 1.55

3.38 2.86 2.62 2.78

10.08 8.54 7.81 8.30

1.81 1.79 1.75 1.79

1.13 1.14 1.14 1.14

1.81 1.51 1.88 1.91

3.07 2.59 2.35 2.35

11.16 9.40 8.53 8.54

2.03 2.04 2.05 1.98

1.16 1.17 1.18 1.18

1.56 1.56 1.63 1.64

3.66 2.95 3.06 2.92

10.93 8.81 9.13 8.70

1.81 1.78 1.73 1.74

1.14 1.14 1.15 1.14

1.46 1.40 1.41 1.40

3.22 2.50 2.46 2.52

11.70 9.09 8.93 9.16

2.03 1.96 1.96 1.98

1.16 1.17 1.17 1.18

1.64 1.60 1.64 1.66

3.71 3.09 2.80 2.96

11.07 9.21 8.34 8.84

1.85 1.81 1.71 1.84

1.14 1.14 1.12 1.15

1.64 1.64 1.55 1.50

3.11 2.35 2.57 2.45

11.29 8.54 9.32 8.91

1.96 2.01 2.03 1.99

1.17 1.17 1.17 1.17

1.49 1.57 1.59 1.61

TP

C5Pe 5.00 10.00 15.00 20.00 PAP 5.00 10.00 15.00 20.00 PCH 5.00 10.00 15.00 20.00

Table 4. Diffusion coefficient, D of PA surfactant molecules in DPD and real units. Component TP C5Pe PAP PCH

D (DPD unit)

D ( m 2 s −1 × 10 9 )

298 K 0.0331 0.0444 0.0370 0.0483

298 K 1.4467 1.9401 1.6180 2.1104

363 K 0.0474 0.0673 0.0533 0.0560


363 K 2.0716 2.9404 2.3291 2.4469

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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

Energy & Fuels

Figure 1


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

Figure 2.


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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

Energy & Fuels

Figure 3.


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

Figure 4.

Figure 5.


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Page 39 of 42

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

Energy & Fuels

Figure 6.


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

Figure 7.


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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

Energy & Fuels

Fig 8.


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

Fig. 9.


Page 42 of 42

Self-Accumulation of Uncharged Polyaromatic ... - ACS Publications

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