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Competitive adsorption of n-alkanes onto exposed and corrosion-protected
hematite surfaces.
Miguel A. San-Miguel* and P. Mark Rodger
Department of Chemistry, University of Warwick
Coventry CV4 7AL, UK
ABSTRACT
Molecular dynamics simulations have been used to study the behaviour of a liquid mixture of
octacosane and heptane between two planar hematite surfaces; one of the surfaces was coated
by a monolayer of an oleic imidazoline (OI). It was found that the that octacosane could be
inserted into the OI monolayer when it was aligned with the alkene tails of the OIs, but that the
reaction rate for such an insertion was slow. A much more rapid process was the adsorption of
the octacosane onto the exposed hematite surface, forming at least two layers and with a
packing that was strongly reminiscent of the octacosane crystal structure.
Keywords: Molecular Dynamics; Iron Oxide Surface; Corrosion Inhibitor Films; Wax; Alkane;
Deposition; Solvent
E-mail: smiguel@us.es and P.M.Rodger@warwick.ac.uk
*
Current address: ???
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1
INTRODUCTION
There are a number of processes that affect the viability of oil and gas pipelines, including
corrosion, scale deposition, wax deposition and clathrate hydrate formation.1,2 The industrial cost
of preventing such problems is high, and so there is a considerable drive from the oil and gas
industry to find cheap and effective additives to control these problems. Protecting pipelines from
all these effects simultaneously, however, is an extremely complex problem.3 Most research has
therefore been targeted at studying these processes independently, with separate research
programmes to develop scale inhibitors, wax inhibitors, corrosion inhibitors or hydrate inhibitors.
In each case there has been a focus on developing surface-active materials, usually in the form of
polymeric or self-assembled monolayer coatings. The target for these coatings may be the pipe
itself, as in the case of corrosion and scale inhibition; or the surface of the growing crystals, as in
the case of hydrate and wax deposition.
There has been considerable success in developing the separate inhibitors, and well tested lead
compounds exist for corrosion,4,5scale,6 wax7 and hydrate inhibition.8,9 However, there is little or
no understanding of the interplay between the different inhibition and deposition processes. It is
clear that such interplay does occur. It has been found, for example, that threshold inhibitors for
hydrate formation do not work effectively unless a corrosion inhibitor is also present.10 The
interaction is not always synergistic and the balance between competitive and synergistic effects
can be difficult to predict. A good example of this is the interdependence of corrosion and wax
inhibition. Wax formation has been found to enhance the effectiveness of corrosion inhibitors
(CIs), with indications that the wax can form an additional protective coating.10 Thus the use of
wax inhibitors might be expected to reduce the efficacy of Langmuir film corrosion inhibitors. On
the other hand, the paraffinic nature of common CIs11,12 may actively promote deposition of
hydrocarbons onto the surface of the CI film. Without an understanding of the molecular
mechanisms underlying this interplay, it will not be possible to design wax or corrosion inhibitors
that will not exacerbate the complementary problem.
Several recent molecular modelling studies have begun to shed light on the molecular processes
associated with wax deposition. Simulations of liquid films of small alkanes on surfaces13,14 have
shown that the hydrocarbon liquids form ordered layers at the solid surface, and with an
enrichment of the longer alkanes at the solid surface 13. Simulations have also been performed
with longer alkanes on iron oxide surfaces,15 and these have shown that the some iron oxide can
provide a weak templating effect that is compatible with alkane crystal structure. Separate
simulations have also been performed on CI films at oxide surfaces 16 However, to date, no efforts
have been made to simulate the effect CI films may have on the structure of paraffin oils, or the
consequent effects this may have for wax deposition.
In this paper we present the results of a direct simulation wax deposition in the presence of a CI
film. A slit pore model was used in which a hydrocarbon liquid was confined between two planar
iron oxide surfaces. The liquid was a mixture of heptane (C7) and octacosane (C28), and one of the
iron oxide surfaces was coated by a monolayer of an oleic imidazoline.
2
THE SYSTEM
Molecular dynamics (MD) simulations were used to model an infinite film of liquid oil
confined between two [10 11] hematite surfaces. One of the surfaces (referred to as the lower
surface hereafter) was coated with a monolayer of oleic imidazoline (OI) molecules (1) (surface
2
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coverage, 62.6 Å2 / molecule), while the second (upper) surface was the clean, anion terminated
oxide surface. The oil was modelled as a mixture of n-octacosane (C28) and n-heptane (C7)
Put a structural diagram of the OI here
The design of the initial configuration for the simulations is shown in Figure 1. All simulations
were conducted using an orthorhombic simulation cell, with z taken to be the direction normal
to the surfaces; z will be described as the vertical dimension in this paper. Both iron oxide
surfaces were the anionic termination of the [10 11] surface, with surface dimensions 43.5 × 40.3
Å (x × y) and were ??? Å thick. Ionic positions for these were taken from a previous study15
and were fully relaxed surface configurations. As the vibrational amplitudes for ions within the
oxide are small under conditions pertinent to wax formation, the oxide surfaces were held rigid
in the present study.
Figure 1: initial simulation cell ...
The OI coating was initially formed as a regular array of 28 molecules on the lower [10 11]
surface (Figure 2). This has been shown to be a stable arrangement of the monolayer17 with
surface coverage close to that observed experimentally. The octacosane molecules were
inserted as an ordered layer taken from the crystal structure, and inserted more than 15 Å away
from both the exposed oxide surface and the nearest point of the OI coating. The remaining
vacant space was then filled with n-heptane molecules, with configuration taken from an
equilibrated simulation of pure liquid heptane at 300 K and 1 bar.
Simulations were performed at two different compositions. The first, system A, used 8 C28 and
288 C7 molecules, with the C28 molecules placed so that they were initially equidistant from
both the exposed oxide surface and the OI coating (measured to the nearest point, i.e. the
terminal CH3 group on the C12H24 chain). For the second, system B, a further ??? heptane
molecules were inserted between the C28 layer and the exposed oxide surface to ensure that the
octacosane started much closer to the OI coating than to the oxide surface. The resultant z
dimensions (defined as the distance between the two surface layers of oxygen ions) was ???
and ??? Å for systems A and B, respectively.
Figure 2: initial arrangement of OIs
The intermolecular potentials were the same as those used in our earlier studies 15,18. Alkane
chains (including the tail group of the OI) were modelled with a united atom potential, based on
CHARMM22 but with parameters adjusted to give a better description of the crystal structure
of n-alkanes.15. The CHARMM22 explicit atom potentials were used for the OI head group.
Interactions between the iron oxide and the organic compounds were described by LennardJones potentials. All non-bonded interactions were truncated at 10 Å (> 2.5 σ).
The electrostatic contribution to the forces was omitted from the calculations. In adopting this
strategy we note that—within the united atom model for the solvent, wax molecules and CI
tails—the most essential components of the system involve only neutral interaction sites.
Furthermore, both the structure and dynamics of the CI film was found to be insensitive to the
electrostatic interactions for the coverage, monolayer geometry, oxide surface and temperature
used in this study.17 Thus the inclusion of coulombic forces would increase enormously the
computer time required, but would not add further information about the influence of the
corrosion inhibitor coating on wax deposition.
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Simulations were performed with DL_POLY,19 using the NVT ensemble with Nose-Hoover
thermostat. The temperature was 310 K [???], the thermal relaxation time was 0.1 ps and the
time step was 1 fs. Two-dimensional periodic boundaries were used in the x-y plane. A typical
set of input files is provided as supplementary material.
The overall protocol followed in the simulations was:
3
(1)
an initial configuration for the iron oxide surfaces, OI coating and C28 layer was
constructed;
(2)
the requisite number of heptane molecules was added, avoiding any close overlap
between any of the molecules;
(3)
a short (??? ps) simulation was conducted, during which the OI and C28 were kept
immobile (frozen) while the liquid heptane was allowed to equilibrate;
(4)
a 4 ns NVT MD simulation, with all organic molecules fully mobile, was performed
at 300 K for equilibration purposes;
(5)
extended MD simulations of up to 20 ns, again with all organic molecules fully
mobile, were performed and the trajectory saved ever ??? ps for subsequent
analysis.
RESULTS AND DISCUSSION
3.1
SYSTEM A (55??? Å LIQUID LAYER)
A series of configurations, taken from different times during the simulation of system A is
presented in Figure 3; these snapshots provide a convenient “executive summary” of the overall
results. Initially the long chain alkanes are located centrally between the two surfaces. The
initial movement carries the molecules in all directions so that by 6 ns octacosane molecules
can be found at both the OI coating and the exposed oxide surface. There appears to be little
penetration of the alkanes into the OI monolayer, and over time these molecules diffuse back
away from the OI layer. In contrast, the C28 alkanes that diffuse towards the exposed oxide
surface adsorb onto this surface and begin to aggregate so that by 8 ns there is clear evidence of
the beginnings of two layers of an embryonic wax crystal.
Figure 3: snapshots of system A at 4 different times
A more quantitative measure of the C28 behaviour can be obtained by calculating the average
density of C28 molecules as a function of their height (i.e. the z coordinate). This has been
calculated, based on the location of the centre of mass for each molecule, and the results are
shown in Figure 4. To show the time evolution of the system more clearly, these z-densities
have been averaged over successive 1 ns blocks of the trajectory. By 5 ns the distribution of C28
molecules is bimodal, with a very sharp peak at the exposed oxide surface (z > 50 Å), and a
very broad peak spanning 10–40 Å. The OI coating extends about 15 Å out from the lower
surface (i.e. 0 < z < 15 Å), so there is some penetration of the C28 into this layer, but the effect
is small. By 8 ns, there are two sharp peaks in the C28 distribution at the exposed oxide surface,
with a hint of a third peak developing. The gap between these peaks is 4.3 Å, which is very
close the interlayer separation found in alkane crystals 15. In contrast, the broad peak above the
OI film has disappeared by 8 ns, being replaced by a homogeneous background distribution
spanning most of the liquid region (8 Å < z < 40 Å). There is a suggestion of small peaks
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Competitive adsorption of n-alkanes …
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appearing within the OI layer, but these disappear when averaged over a longer time interval
(the final 3 ns; see Figure 4), indicating that they are a transient feature.
Figure 4: density profile of octacosane molecules across the z dimension
It is interesting to note that the adsorption site and aggregate structure adopted by those C28
molecules that do adsorb onto the exposed oxide surface are precisely the site and geometry
predicted from our earlier, more constrained, simulations of wax growth on this surface 15.
In order to check the repeatability of these results, a second and analogous simulation was
performed. This used the same initial positions for the C28 and OI molecules, but used a
different arrangement of C7 molecules, and assigned different initial velocities to all molecules.
The two sets of simulations were found to be very similar. In particular, we again saw no
appreciable incorporation of the octacosane into the OI film, but strong adsorption onto the
exposed oxide surface leading to the formation of two octacosane layers with a geometry that
was strongly reminiscent of the octacosane crystal structure.
3.2
SYSTEM B (70??? Å LIQUID LAYER)
In order to analyze more comprehensively what are the driving forces acting on the molecules
towards one direction or the other, we devised a further experiment. An additional heptane
solvent slab of 15 Å was introduced at height of 45 Å after 2 ns equilibration stage from
starting the simulation in System A, in such a way that the wax molecules around the central
region would see the CI tail film at shorter distance that the bare surface. The distance between
the two oxide surfaces was then adjusted until the density of heptane at the centre of the liquid
matched the final density observed in system A.
The configuration of system B after 20 ns is shown in Figure 5. Once again, the adsorption of
C28 onto the exposed hematite surface is clearly evident. Two layers are seen to form, with a
structure similar to that observed for system A, although in this case a vacancy in the first C28
layer is apparent in Figure 5.
For system B, one molecule is seen to be retained in the OI monolayer. This molecule has
aligned with the OI tails and penetrated essentially to the oxide surface—in exactly the
geometry required if the OI coating was going to promote wax deposition. As will be shown
below, this is a long-lived incursion, being retained over the final 7 ns of the simulation. We
note that for system A, all encounters between C28 molecules and the OI monolayer involved
the wrong alignment, with the octacosane lying across the top of the OI layer rather than
aligning with it. Thus, it may be that the alignment and penetration of long chain alkanes into
the OI layer is favourable, but that the alignment stage represents and entropic bottle-neck, and
so makes such adsorption events rare at the concentrations and temperatures considered here.
Figure 5: final configuration of system B
The dynamics of the C28 deposition processes can be see from the C28 z-density profiles (Figure
6) and the way the height of the centre of mass of each C28 molecule varies during the trajectory
(Figure 7). The formation of the two layers at the exposed hematite surface takes longer than in
system A, due to the larger distances over which they have to diffuse, but is almost complete by
about 15 ns. The details of the trajectories (Figure 7) show that many of the C28 molecules
diffuse initially towards the OI film (which was designed as the closer surface in system B) but
subsequently diffuse away again. In contrast, encounters with the exposed oxide surface led
resulted in adsorption of the octacosane with almost 100% efficiency.
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Figure 6: z-density profile for C28, averaged over various time intervals.
Figure 7: height of the centre of mass of each C28 molecule as a function of time.
The conformation of the octacosane molecules has been monitored by calculating both the
average end-to-end intramolecular distances and the percentage of gauche torsion angles. The
results are shown, as a function of height, in Figure 8 and Figure 9. For reference we note that
the OI head group occupies the region 0 Å < z < 5 Å, while the OI tails extend out to about 15
Å from the lower surface. Both figures show that the location of the longer alkane molecule
makes a substantial difference to its conformation. End-to-end distances are smallest within the
region of the OI monolayer, increasing to about 10 Å within the liquid heptane, but then
increasing to about 30 Å when adsorbed to the upper surface (i.e. the exposed hematite
surface). For comparison, the end-to-end distance of an all-trans C28 molecule is ???. These
distances correlate well with the existence of gauche torsion angles, which was about 25% near
the OI film and in liquid heptane, but decreased to les than 5% in the layer that deposited on the
hematite surface. It is the existence of these gauche defects that is probably responsible for the
slightly larger interlayer-distances found in the adsorbed octacosane compared with the crystal.
This percentage decreases with time in the two adsorbed layers, suggesting that crystalline
distances would be regained if much longer simulation times were accessible.
Figure 8: ene-to-end distance of octacosane molecules as a function of height (z)
Figure 9: percentage of gauche torsion angles in C28 as a function of height (z)
Structuring effects have also been examined for the solvent (heptane); density profiles at
various times are depicted in Figure 10. Up to four equally spaced layers, separated by 4.3 Å,
are evident on the hematite. Three peaks, with similar spacing but much smaller amplitudes, are
also evident within the tail group region of the OI film. Penetration of the heptane into the
head-group region was observed, but was extremely rare. The solvent distribution is seen to
equilibrate very rapidly and be stable in time. Some decrease is seen in the height of the two
peaks adjacent to the hematite surface as the heptane is displaced by the adsorption of
octacosane, but no other time dependence is evident in these profiles.
Figure 10: density profiles for the heptane solvent
These simulations also provide an opportunity to examine the structure of the the OI film, and
in particular, to determine the alignment and tilt of the OI tails. The tilt angle, ,ϑ has been
calculated as the angle between the surface normal, and the principal axis associated with the
smallest eigenvalue of the moment of inertial tensor for the alkene tail of each molecule; ϑ = 0
(cos ϑ = 1) corresponds to the long axis of the tails aligning perpendicular to the surface.
The distribution of tilt angles has been averaged over all OI molecules and over all
configurations saved within various 1 ns windows of the trajectory, and the results are
presented in Figure 11.
Figure 11: tilt angles of OI tail groups at different times.
AT all times there is a strong preponderance of tails oriented perpendicular to the lower
surface:, with???% of the molecules having titl angles greater than .At 6 ns the distribution of
tilt angles is br(cos ϑ > 0.8). At all times, there is also a small percentage (how many
molecules?) that lie along the surface. At early times there is a significant number of tails that
generate a broad distribution of tilt angles in the range [?°,?°], but this distribution steadily
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declines over time, with a corresponding increase in the near-perpendicular region. It is
interesting to note that this relaxation preceded the penetration of the one C28 molecule
observed to remain within the OI film by the end of the 20 ns simulation. It is possible that this
is merely coincidental, but it is also possible that the degree of alignment within the OI film is
important in determining whether the OI film will act as a nucleation site for wax formation.
More extensive simulations to investigate this possibility are in progress.17
4
CONCLUSIONS
In this paper we have reported the results of a molecular dynamics simulation study into the
interplay between wax precipitation and corrosion inhibition in oil pipelines. Multi-nanosecond
simulations have been performed on a liquid mixture of short (C7) and long (C28) chain alkanes
in the presence of both an exposed iron oxide surface (hematite, [10 11] ) and a monolayer film
of an oleic imidazoline on iron oxide. The simulations revealed a strong tendency for the C28
molecules to adsorb onto the exposed hematite surface, spontaneously adopting an extended,
all-trans geometry and aggregating into a layered structure reminiscent of the corresponding C28
crystal. The enthalpic driving force to such adsorption has been shown to be relatively weak15
and so there must be a significant entropic advantage to this behaviour. Preferential adsorption
of larger molecules or particles at surfaces has certainly been found in other systems.
Very little mixing was observed between the long chain alkanes and the OI film. This may
suggest that OI films do not act as nucleation sites for the formation of wax deposits, but
several other explanations are also possible, and further investigation is required. In particular,
a single long-lived insertion of a C28 molecule into the OI film was observed, and was
preserved for the final 1/3 of the total simulation time. This is suggestive that an aligned
mixture of C28 and the OI tails within the OI monolayer is favourable, but that the rate of
mixing is too slow to occur readily on the time- and length-scales currently accessible to
molecular simulation. Indeed, the single event observed in this study did not occur until there
was substantial alignment of the OI tails — an alignment that arose very slowly on the
simulation timescale. More extensive simulations, probably at a higher concentration of waxforming molecules, are needed to establish whether this possibility is of more general
significance.
ACKNOWLEDGEMENTS
This work was supported through EPSRC grant GR/L73739.
7
Competitive adsorption of n-alkanes …
29 July 2004
TABLES
Table 1.
CAPTION FIGURE
Figure 1. Height of centre of mass for each wax molecule along the
simulation.
Figure 2. Snapshots at different time. Hexane molecules have been
removed.
Figure 3. Height of centre of mass of each wax molecule along time.
Figure 4. Snapshot of final configuration after 20 ns.
Figure 5. C28 density profiles at different times.
Figure 6. End to end distance of wax molecules.
Figure 7. Percentage of gauche defects against time.
Figure 8. Percentage of gauche defects against height.
Figure 9. Solvent density profiles.
Figure 10. CI tilt angles at different times.
Reference List
1 Misra, S.; Baruah, S.; Singh, K. Spe Production & Facilities 1995, 10, 50-54.
8
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3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
29 July 2004
Mayer, M.; Crowe, T. Chemical Engineering Research & Design 1996, 74, 149-157.
Tabatabaei, A. R.; Danesh, A.; Tohidi, B.; Todd, A. C. Ann.N.Y.Acad.Sci. 2000, 912, 392-402.
Quraishi, M. A.; Jamal, D. Materials Chemistry and Physics 2001, 68, 283-287.
Getmanskiy, M. D.; Zaharov, L. G.; Nam, O. S. Neftyanoe Khozyaistvo 1998, 65-66.
Webb, P. J. C.; Nistad, T. A.; Knapstad, B.; Ravenscroff, P. D.; Collins, I. R. Spe Production &
Facilities 1999, 14, 210-218.
Hennessy, A. J.; Neville, A.; Roberts, K. J. Journal of Crystal Growth 1999, 199, 830-837.
Argo, C. B.; Blain, R. A.; Osborne, C. G.; Priestley, I. D. Spe Production & Facilities 2000, 15,
130-134.
Palermo, T.; Goodwin, S. P. Gas Hydrates: Challenges for the Future 2000, 912, 339-349.
Harrob, D., BP Exploration, private communciations
Chen, Y.; Jepson, W. P. Electrochimica Acta 1999, 44, 4453-4464.
Klenerman, D.; Hodge, J.; Joseph, M. Corrosion Science 1994, 36, 301-313.
Smith, P.; Lynden-Bell, R. M.; Smith, W. Molecular Physics 2000, 98, 255-260.
Duffy, D. M.; Rodger, P. M. Physical Chemistry Chemical Physics 2001, 3, 3580-3585.
San Miguel, M. A.; Rodger, P. M. Physical Chemistry Chemical Physics 2003, 5, 575-581.
San Miguel, M. A.; Rodger, P. M. Journal of Molecular Structure-Theochem 2000, 506, 263-272.
San Miguel, M. A. and Rodger, P. M. manuscript in preparation
San Miguel, M. A.; Rodger, P. M. Molecular Simulation 2001, 26, 193-+.
Smith, W.; Yong, C. W.; Rodger, P. M. Molecular Simulation 2002, 28, 385-471.
9
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System A
0.30
Molecule density
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
35
40
45
50
Height
Figure 1. Molecule density based on the position of the molecule centre of mass
position during the last 3 ns (7 to 10 ns).
8 ns
7 ns
6 ns
5 ns
CI
Fe2O3
55
0 ns
6 ns
7 ns
8 ns
Figure 2. Snapshots of side views of the system at four different times.
Center of Mass displacements
70
60
Heigth (A)
50
40
30
20
10
0
1
0.
6
0.
1
1.
6
1.
1
2.
6
2.
1
3.
6
3.
1
4.
6
4.
5.
1
5.
6
1
6.
6.
6
1
7.
6
7.
1
8.
6
8.
1
9.
6
9.
.1
10
.6
10
.1
11
.6
11
.1
12
.6
12
Time (ns)
Figure 3. Height of centre of mass of each wax molecule along time.
.1
13
.6
13
.1
14
.6
14
Figure 4. Snapshot of final configuration after 20 ns.
Wax density
5
4.5
4
Density
3.5
3
2.5
2
1.5
1
0.5
0.
38
2
4.
04
8
7.
71
4
11
.3
8
15
.0
47
18
.7
13
22
.3
79
26
.0
45
29
.7
11
33
.3
78
37
.0
44
40
.7
1
44
.3
76
48
.0
42
51
.7
09
55
.3
75
59
.0
41
62
.7
07
66
.3
74
0
Height
5 ns
10 ns
Figure 5. C28 density profiles at different times.
15 ns
20 ns
35
End_to_end distance
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Height
Figure 6. End to end distance of wax molecules.
Gauche defects
35
30
% gauche
25
20
15
10
5
Time (ns)
Figure 7. Percentage of gauche defects against time.
12
12
.7
13
.4
14
.1
14
.8
9.
9
10
.6
11
.3
9.
2
8.
5
7.
8
7.
1
6.
4
5
5.
7
4.
3
3.
6
2.
9
2.
2
1.
5
0.
8
0.
1
0
35
30
% gauche
25
20
15
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
Height
Figure 8. Percentage of gauche defects against height.
Density
4.5
4
3.5
Density
3
2.5
2
1.5
1
0.5
0.
38
2
4.
04
8
7.
71
4
11
.3
8
15
.0
47
18
.7
13
22
.3
79
26
.0
45
29
.7
11
33
.3
78
37
.0
44
40
.7
1
44
.3
76
48
.0
42
51
.7
09
55
.3
75
59
.0
41
62
.7
07
66
.3
74
0
Height (A)
Figure 9. Solvent density profiles.
70
0.025
6 ns
0.020
10 ns
Probability
15 ns
20 ns
0.015
0.010
0.005
0.000
0.0
0.2
0.4
0.6
cos(tilt angle)
Figure 10. CI tilt angles at different times.
0.8
1.0