COMPARISON OF LINEAR AND BRANCHED MOLECULAR

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COMPARISON OF LINEAR AND BRANCHED MOLECULAR STRUCTURES OF TWO FLUOROCARBON ORGANOSILANE SURFACTANTS FOR THE ALTERATION OF SANDSTONE WETTABILITY Ivan Moncayo-Riascos, and Bibian Alonso Hoyos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02870 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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COMPARISON OF LINEAR AND BRANCHED MOLECULAR STRUCTURES OF TWO FLUOROCARBON ORGANOSILANE SURFACTANTS FOR THE ALTERATION OF SANDSTONE WETTABILITY Ivan Moncayo-Riascosa*, and Bibian A. Hoyosa a

Universidad Nacional de Colombia-Sede Medellín, Facultad de Minas, Departamento

de Procesos y Energía

ABSTRACT The aim of this work is to build a molecular dynamics model to represent the experimental evaluation of the wettability alteration of sandstone surfaces due to the action of partially fluorocarbonated organosilane surfactants with linear (perfluorodecyl) and branched chains (fluorocarbamate), using the contact angles of water and n-decane droplets as the evaluation parameter. The thickness obtained for a monolayer of coating with each surfactant was ~13 Å, in excellent agreement with the experimental measurements of thickness for a monolayer of fluorocarbonated surfactants in similar systems. The contact angles obtained with molecular dynamics simulations deviate, in most cases, by less than 3° with respect to experimental measurements. The phenomenological model reveals that for the coating with a monolayer, the

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fluorocarbonated chains of perfluorodecyl were mostly oriented away from the sandstone surface while the branches of fluorocarbamate are distributed between being adsorbed on the surface and oriented towards the fluid phase. The surface density of the perfluorodecyl monolayer is higher than that obtained for the fluorocarbamate monolayer, since the linear chain in the perfluorodecyl generates lower lateral repulsive interactions. On the perfluorodecyl coating, the droplets of n-decane and water have an appreciable amount of liquid molecules that are able to penetrate the coating, and in the bulk of these droplets, well-defined adsorption layers appear due to the effect of the solid. The configuration of the liquid droplets on the fluorocarbamate does not exhibit any stratification and the thickness of the multilayer coating eliminates the effect of the solid. Thus, the water and n-decane molecules only interact with the outer, less ordered layers of this coating, giving a droplet density profile similar to a fluid-fluid interface. Both surfactants present attractive interactions with the n-decane and water molecules. However, the liquid-liquid attraction forces in the bulk of the droplet are significantly higher than the surfactant-liquid interaction energies, which favors the liquid molecules remaining in the bulk of the droplet. The model developed represents an explanation of the experimental evidence that perfluorodecyl surfactant produces intermediate-wetting coatings, while the fluorocarbamate more efficiently alters the wettability of the surface to water.

KEYWORDS: wettability alteration, contact angle, organosilanic surfactants, EOR-chemical, molecular dynamics.

INTRODUCTION In gas fields, when the reservoir pressure drops as a consequence of gas extraction, small amounts of liquid hydrocarbons can be formed. In the early stages of the formation of these condensates, the liquid can be entrained by the gas, but as the pressure falls, the capillary forces overcome the drag forces, forming condensate banks that clog the pores available for gas extraction.1–3 The formation of these condensate banks can change the wettability state of the reservoir rock.4,5 Experimental laboratory-level evaluations show that there may be a decline in 2 ACS Paragon Plus Environment

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well productivity of up to 90% due to the formation of condensate banks, while in field studies, there have been registered productivity losses between 40 and 80% due to this type of damage.6,7

To recover the productivity lost by the formation of condensate banks, alternatives, such as hydraulic fractures and the injection of wettability modifiers, have been implemented. With the chemical alteration of the wettability of the formation around the wellbore being the most promising.3,8–10 The injection of wettability modifiers can, in addition to restoring the pore channels clogged by the condensate hydrocarbons, reduce the affinity of the surface to be in contact with the liquid, thereby facilitating the mobility of the hydrocarbons in the liquid phase.4,11–13

Fluorocarbon surfactants have been widely used to promote surfaces that generates high contact angles in both water and liquid hydrocarbons (coined with the term "gas-wet "), due to their high thermal stability, low interfacial tension and high surfactant activity.14–16 Among the various fluorocarbon surfactants, organosilanes stand out as promising structures due to their high affinity with silica surfaces, which allows them to form bonds with the surface, thereby increasing the durability of the treatment.12,13,17–20

Most studies of wettability alteration are carried out by measuring the contact angle of water or hydrocarbon droplets (usually on test cores made from outcrop sandstone rocks), as a way of determining the effectiveness of the treatment. Typically, the hydrocarbon is represented using linear alkanes, such as n-heptane13 or n-decane12, because condensate banks are associated with light hydrocarbon compounds.4,21 Previously published experimental results show that an increase in the droplet contact angle, of both water and hydrocarbons, can be generated, whereby the wettability of the surface becomes preferable to the gas phase, not the liquid phase.12–14,16,17


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Understanding the features of the wettability alteration process is important to propose treatments with the potential to promote “gas-wet” surfaces. In general, experimental works only evidence the possibility of generating this alteration of wettability, without providing details of the mechanism involved in the process. Therefore, in recent years, several studies have been developed using molecular simulation tools that complement the experimental evaluations.

By means of molecular dynamics simulations, the adsorption configurations of wettability modifiers on the reservoir rock and their impact on the alteration of the wettability can be obtained,19,22–25 as well as the relation between the structural characteristics of the chemical agents and their performance in promoting both lipophobic and hydrophobic surfaces. Previous studies show the possibility of representing, through molecular modelling, the wettability states of surfaces with and without treatments, obtaining contact angles in excellent agreement with the experimental measurements.18,26

Many research studies, both theoretical and experimental, have sought to evaluate the relation between the molecular structure of the surfactant and the alteration of wettability promoted. Studies have been published evaluating the effect, on the wettability, of the fluorocarbon chain length,27,28 the differences between fluorocarbon and hydrocarbon chains20,29–31 and the effect of the branching degree of the surfactant molecular structure30,32 and the non-fluorocarbon chain length.28,33

Based on the literature review we have selected two partially fluorocarbonated organosilane surfactants, with the aim to determine the effect of the non-fluorocarbon chain length (with and without heteroatom content) and evaluating the effect of the fluorocarbon chain structure of the surfactant (linear and branched). To do this, we present a molecular dynamics model developed to evaluate the performance of the selected surfactants in the process of altering the wettability of sandstones. As an evaluation criterion, the contact angles of n-decane and water droplets 4 ACS Paragon Plus Environment

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were used to estimate the effectiveness of the treatment. The organosilanic structures evaluated were 1H,1H,2H,2H-perfluorodecyltriethoxysilane, C8F17CH2CH2Si-[O-CH2CH3]3 (in short, perfluorodecyl), which consists of a linear fluorocarbon chain, and 1,3-bis((1,1,2,2,3,3,4,4,4nonafluoro-N-methylbutyl)sulfonamide) propan-2-yl (3-(triethoxysilyl) propyl) carbamate, [C4F9S02N(CH3)CH2]2CHOC(O)NH(CH2)3Si(OCH2CH3)3 (in short, fluorocarbamate), which has a branched structure with two fluorocarbon chains.

The organosilane surfactants selected for this study correspond to promising treatments for application at the field of production, as their ability to promote “gas-wettable” surfaces has been experimentally determined.12,13 In addition, core flooding tests show an improvement in the mobility of production fluids due to the removal of the condensate banks generated by the alteration of the wettability of the porous medium.12,13

With the aim of obtaining new insights regarding the particular mechanisms of the wettability alteration generated by these two surfactants, this work focuses on the theoretical evaluation of their efficiency to promote surfaces that generates high contact angles (in both water and liquid hydrocarbons) on the basis of the molecular characteristics of each one. To achieve this, we calculated the configuration of the wettability modifiers on the surface and the contact angles of the droplets obtained in each case. In addition, as molecular dynamics simulations allow for calculations of the energy interactions with specific parts of the molecule, we calculated the interaction energies between the droplets and sections of each molecular structure selected to study their specific contribution to the wettability alteration. This study contributes to understand, in a better way, the effect of the heteroatom content and the fluorocarbon chain structure of the surfactant (linear and branched) on the density surface coating (adsorption) and on the wettability alteration to promote gas-wetting surfaces. In worth mention that contact angles simulated (MD) represent adequately the contact angles measured experimentally, without the needed to simulate cylindrical droplets34 or to correct by contact line tension (CLT).35 5 ACS Paragon Plus Environment

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SYSTEM SIMULATED The experimental evaluation of the wettability alteration consisted of four procedures: i) clean the surface, ii) add the organosilanic surfactant and allow its reaction with the surface, iii) rinse and dry the surface coated with the surfactant and iv) add water and n-decane droplets to measure the contact angle.

For the evaluation with perfluorodecyl, a 1 x 3 cm glass surface was cleaned using sequentially ethyl alcohol and deionized water, then the surface was dried in an oven at 75 °C for 12 hours (first procedure). The surface was then suspended in the impregnation solution (2% w/w perfluorodecyl and 1% w/w triethylamine in toluene), at 112 °C for 6 hours (second procedure).13 After the reaction, the glass slide was rinsed thoroughly with ethyl alcohol and water in sequence, then it was cured overnight (> 12 hours) in an oven at 75° C (third procedure). This rinsing and drying protocol ensures a solvent-free surface. Finally, droplets of n-decane or water were added on the surface coated with the surfactant and the contact angle of the droplets were measured (fourth procedure).

The evaluation with the fluorocarbamate was performed on Berea sandstone, using an impregnation solution composed of surfactant (25% w/w), water (5% w/w), acetic acid (5% w/ w) and ethanol (65% w/w) at 140°C. The rinsing and drying protocols were similar to those reported for perfluorodecyl.12

MODEL Figure 1 shows the molecular structure of the fluorocarbonated surfactants evaluated in this work. Both surfactants have the same organosilanic portion (Si-[O-CH2CH3]3) and eight carbon atoms in the fluorocarbonated part. The most relevant differences between them are associated with the type of structure in the chain (linear for the perfluorodecyl and branched for the 6 ACS Paragon Plus Environment

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fluorocarbamate), and the functional groups that they contain: the perfluorodecyl has an ethyl group that connects the organosilanic and fluorocarbonated parts of the molecule, while the fluorocarbamate has a secondary amino group and an ester group attached to two symmetrical branches ending with four carbon atoms in the fluorocarbonated part.

Additionally, Figure 1 shows the three sections of each molecular structure, selected in order to study their specific contribution to the wettability alteration. The section labeled Tail corresponds to the group of atoms of each molecule that is not a part of the silane group and is the section responsible, in principle, for the global alteration of wettability. The other two groups of atoms correspond to a subdivision of the Tail group: the CF group, which contains the fluorocarbonated part of each structure, and the NonCF group, which contains the atoms that are not in the fluorocarbonated chain. These groups were selected to determine the specific energetic effect of the fluorocarbonated chain in the wettability alteration and the effect of the presence of heteroatoms in the non-fluorocarbonated chain.

The surfactants reported in Figure 1 were represented using the consistent valence force field potential (CVFF).36 This potential has been used to describe the properties of a large number of macromolecules and, in particular, to describe the behavior of non-ionic surfactants (polyethers, perfluoropolyether, ethylene oxides, and so on). For the specific case of perfluorodecyl, in our previous work18 we showed that the CVFF potential allows to describe in an appropriate way the adsorption energy and the surface concentration of this surfactant on silica surfaces; as well as the proper calculation of contact angles of water and n-heptane droplets on coatings of this surfactant. To the best of our knowledge, this is the first time that the fluorocarbamate molecule is modeled using molecular dynamics. From the proper results obtained for perfluorodecyl and the predictive capability of the molecular dynamics simulations, it is expected that the results obtained for the fluorocarbamate have the same degree of approximation.


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The n-decane molecules were constructed using the transferable potentials for the phase equilibria-united atom force field (TraPPE-UA)37 and the water molecules were represented by the extended simple point charge potential (SPC-E).38 All potentials used here represent van der Waals interactions with a Lennard-Jones 12-6 potential and the electrostatic interactions of the partial charges of each atom were calculated by a Coulombic potential (for the n-decane, no electrostatic interactions were considered since it has a non-polar nature). The intramolecular forces, due to bonds and angles, were modeled by harmonic interactions. The dihedral angle interactions in the surfactant molecules were also modeled by harmonic potentials, while for the n-decane, they were described by the model proposed by Watkins and Jorgensen.39 All parameters of inter- and intramolecular potentials for the surfactants, water, and n-decane are reported in Tables S1 to S4 of the Supporting Information.

The representation of the silica surface was made using the 10-4-3 wall potential40,41 (Eq. 1), which is an analytical solution of the interaction energy between a fluid and a crystalline solid that exposes (1 1 1) or (1 0 0) facets. This potential was obtained by integrating the interaction potential (Lennard-Jones) between each atom in the solid and a fluid atom as it moves from one position above a surface lattice cell into a different position above the lattice. This integration includes the interaction with infinite planes of atoms in the solid and it can be considered atomistic in the sense that it takes into account the individual contribution of each atom in the solid of three-dimensional crystals. On the other hand, we have reported18,42 that the 10-4-3 potential can adequately describe a specific wetting state of amorphous surfaces if the energy parameter of the potential is tuned.

On this basis, the potential represents the solid as an infinitely extended and infinitely thick surface that interacts with the liquid phase with the cross (solid-fluid) interaction parameters ϵsf and σsf (Eqs. 2 and 3),


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‫ۍ‬ ‫ې‬ ߪ௦௙ ସ 2 ߪ௦௙ ଵ଴ √2 ‫ۑ‬ ܷ(௥೔ೕ ) = 2ߨ߳௦௙ ‫ ێێ‬ቀ ቁ − ቀ ቁ − ଷ‫ۑ‬ 5 ‫ݖ‬ ‫ݖ‬ ‫ݖ‬ 0.61 ‫ێ‬ 3 ൬ߪ + ൰ ‫ۑ‬ ‫ۏ‬ ௦௙ √2 ‫ے‬ ߪ௦௙ =

(1)

1 ൫ߪ + ߪ௙ ൯ 2 ௦

(2)

ଵ

߳௦௙ = ൫ߝ௦ ߝ௙ ൯ଶ

(3)

where σs and ϵs are the Lennard-Jones length and energy parameters for the solid, respectively.

This type of representation allows us to properly model the wetting systems without having to describe all the heterogeneities, fractures and defects of the solid in detail.18 A value of εs = 1.02 kcal/mol has been previously reported to represent the energetic parameter for the silica surface.18 It is expected that representing the surface as a wall potential have a rather small effect on the results: the fluid density profiles43,44 and the amount of fluid adsorbed at the surface44 are practically not affected when a wall potential is used. On the other hand, the 10-4-3 potential allows a good representation of adsorption energy of water on silica surfaces.18

To determine the contact angle of the liquid droplets generated on the top of the coatings, we used the model proposed by Fan and Cagin (Eqs 4-6).45 This model, developed for nonsymmetrical drops, allows to calculate the contact angle from the geometry of the drop (height and radius).

ܿ‫ = ߠݏ݋‬1 −

ℎ ܴ

ܵ = ߨ‫ ݎ‬ଶ ܴ=

ℎ ܵ + 2 2ߨℎ


(4) (5) (6)

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where θ is the contact angle between the drop and the surface and h and r are the drop height and radius, respectively.

To obtain more accurate results, the drop geometry was measured at three different time points after equilibrium was reached (at 8, 9 and 10 ns). For each of these time points, four planes, perpendicular to the surface, were selected to measure the height and radius of the droplet, in order to obtain an adequate representation of the asymmetry of the simulated droplet (as shown in Figure S1 of the Supporting Information). Contact angles reported in this work were obtained from the average of the measurements of height and radius of each drop.

SIMULATIONS DETAILS The representation of the experimental procedure using molecular dynamics was performed by constructing a heterogeneous system consisting of one solid phase and several liquid phases (water, hydrocarbon and surfactants). In this work, the solid phase is associated with sandstone formation, represented by a silica surface. The aqueous phase was represented using water molecules (without any dissolved ions), and the hydrocarbon phase by n-decane. The air phase was not considered, since previous studies show little variation in the results with respect to simulations with an explicit atomistic representation of air.46

The comparison of the effect of the structural differences of the surfactants on the wettability alteration of the surface was made by three evaluations: i) the configuration of each surfactant on the surface at the end of the rinsing and drying protocols (prior to the addition of the liquid droplets); ii) the evaluation of the contact angles of water and n-decane droplets on the coating of each surfactant placed on the solid surface and iii) the quantification of the interaction energies between the liquid and each group of atoms presented in Figure 1.

The first procedure consisted of representing the coating of the surface with each surfactant, considering the conditions at which the experimental measurements were taken. The most 10 ACS Paragon Plus Environment

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significant difference in the experimental evaluations is the concentration used for each surfactant in the impregnation solution with which the samples were soaked. In the case of perfluorodecyl, the concentration used was 2% w/w,13 which is 12.5 times lower than the concentration at which the fluorocarbamate was employed, 25% w/w.12 Therefore, for the simulations conducted in this work, the coating with perfluorodecyl was assumed to be formed by a monolayer, and in the case of fluorocarbamate, it was considered to form a multilayer coating. Additionally, it was considered that the surfactants with organosilanic structures have a chemical reaction with the silica surface, releasing the organic CH3CH2 substituents of the silane group and thus enabling the formation of a chemical bond between the oxygen atom in the surfactant and the silica atoms in the surface.12,13,17,18

The simulations of reaction of the surfactants on the surface were not performed from the impregnation solutions, but rather the area that a certain number of molecules occupy in the formation of a monolayer of surfactant on the surface was calculated. This is justified since our aim was to reproduce the final (dry) system obtained experimentally prior to the addition of the liquid droplets. This way of simulating the reaction reduces the computational cost as the impregnation solutions have, in general, a low surfactant concentration and the simulations of these solutions would imply a very large amount of solvent molecules and the computational costs would be prohibitive.

To simulate a monolayer of perfluorodecyl, initially a single surfactant molecule was built inside a simulation box with at least 3 Å of empty space between the atoms of the molecule and each edge of the simulation box. This surfactant molecule was placed with the organosilanic part oriented towards the surface. Then, this unit box was replicated in the x and y directions to cover a 300 × 300 Å area (which required 1052 molecules). The wall potential was placed in the lower plane in the z direction and in the upper plane reflective conditions were used to guarantee a constant number of molecules throughout the simulation, with periodic conditions in the x and y directions. The relaxation for the entire coating was conducted using an NPT ensemble (1 atm 11 ACS Paragon Plus Environment

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and 298 K) for 10 ns, with a time step of 1 fs. The box length was kept constant in the z direction and variations in size were permitted in the x and y directions. The relaxation for the entire coating using an NPT ensemble seeks to obtain an equilibrium configuration of the coating, in which the surfactant molecules would have the closest possible approach between them, conditioned by the lateral interactions between the molecules. At the end of this simulation, the area occupied by the entire coating (and, thus, the equilibrium surface concentration), as well as the configuration of a monolayer of surfactant on the silica surface can be obtained.

To represent the reaction of the surfactants, the organic parts of the silane group were removed, as described elsewhere,18 and the partial charges for each atom were re-calculated as a consequence of the chemical reaction, using the equivalent charge method, Qeq.47 From this time on, the oxygen atoms were considered fixed, while the rest of the atoms in the surfactant molecules were considered mobile. In this manner, the restriction of movement generated by the formation of the Si−O bond was included in the simulations.

For the fluorocarbamate, due to the elevated concentration of treatment that was used for the experimental measurements, it was considered that the coating is composed of multiple layers of surfactant adsorbed on the surface. In this study, five layers of fluorocarbamate were built, adding four layers to the monolayer initially placed on the surface (constructed using a similar procedure as that for the perfluorodecyl). To do this, an NVT ensemble for the relaxation of the upper layers was used (maintaining the volume resulting from the simulation of the monolayer). The relaxation of the entire multilayer coating was conducted for 10 ns with a time step of 1 fs, using a total of 4615 surfactant molecules.

The configuration characteristics of the surfactants on the silica surface were evaluated using the density profiles and coordination number. The density profile allows for the determination


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of the distribution of surfactant on the direction perpendicular to the surface, while the coordination number is useful for estimating the packing degree of the coatings.

In the simulations of droplet formation on the coating with each surfactant, a total of 5832 liquid molecules were used (water or n-decane, in separate systems), using an NVT ensemble. The molecules for each liquid phase were initially placed in a cubic configuration of 18×18×18 molecules, with side lengths of 116.8 and 55.8 Å for n-decane and water, respectively. The liquid molecules were placed at a distance of 4 Å above the coating and the droplets were allowed to evolve for 10 ns with a time step of 1 fs. In addition, the contact angles of n-decane and water droplets on the uncoated surface were calculated in order to better illustrate the wettability alteration promote by the surfactants.

The number of liquid molecules used in these simulations is in agreement with a previous report that demonstrates that the contact angle calculated with molecular dynamics simulations does not change appreciably when the number of molecules increases above 5000.18

Figure S2 of the Supporting Information shows the time evolution of the energy and temperature for the water droplet formation on the surfactants evaluated in this paper. The time needed to reach the 90% value of the potential energy in equilibrium was ~0.25 ns. This result is in agreement with those obtained in other studies that involve water droplets formation (0.3026 and 0.1534 ns). Thus, a simulation time of 10 ns is large enough to ensure that the droplet is fully developed and in equilibrium.

For the calculation of the interaction energies between molecules or between the liquid droplets and sections of each molecular structure, we select entire molecules (or parts of molecules) and with them we define what is called "a group". For example, one group can be defined as “all the atoms that make up the fluorocarbon chain of the surfactant” (the CF group), and another group can be defined as “all the water molecules in the simulation box”. Once the groups have been 13 ACS Paragon Plus Environment

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defined, the energy of interaction between the elements of a group can be obtained from nonbonding interaction energies (the sum of van der Waals interaction and electrostatic energies between all atoms in the group). In the same way, the interaction energy of the elements of one group with all the elements of another group can be calculated. In this way, we calculate the liquid-liquid interactions in the bulk of the droplets, and the interactions between specific parts of the surfactant molecules and each liquid phase. The values reported correspond to the average of non-bonding interaction energy evaluated for an additional nanosecond of the simulation, with a time step of 1 fs, after the 10 ns for the droplet formation were concluded.

Simulations were conducted with parallel computing using the LAMMPS48 software. The images for determining the geometry of the droplet were obtained using the VMD49 visualization software. Lorentz-Berthelot mixing rules were used to determine the molecular interaction parameters between atoms of different natures. The Nosé-Hoover thermostat and barostat were used to maintain control over the temperature and pressure, respectively. For the long-range electrostatic interactions, the particle-particle-particle-mesh50 method was used. The cutoff radius was set at 12 Å for the van der Waals and electrostatic interactions.

RESULTS AND DISCUSSION To evaluate the wettability alteration promoted by the surfactants, the initial wetting state of the silica surface without surfactant was calculated. To do this, the simulations for droplet formation on the uncoated surface were made using a value of εs = 1.02 kcal/mol as has been previously reported to represent the energetic parameter for the silica surfaces.18 Figure 2 depicts the droplets obtained, after 10 ns of simulation time, for the n-decane and water on the uncoated silica surface.

For the n-decane, the simulation shows no droplet formation, instead a monolayer of n-decane molecules was obtained on the uncoated surface. This result is in agreement with the experimental contact angle measured on Berea Surface (0°).12 This is because the surface– n14 ACS Paragon Plus Environment

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decane adhesion energy (determined largely by the energetic parameter of the surface, εs) is much higher than the cohesion energy between the n-decane molecules (evidenced by the low interfacial tension of this substance).51 For the water droplet, it was obtained a contact angle of 24.7°, in close agreement with the experimentally reported value of 23.8°.13

Figure 3 shows the final configuration of the n-decane and water droplets on the surfactant coated surfaces. The calculated contact angle for the n-decane droplet on perfluorodecyl was 59.5°, with a difference of just 1° with respect to the reported experimental value (60.5°).13 For the fluorocarbamate coating, the obtained contact angle was 52.5°, with a difference of 2.5° with respect to the reported experimental value of 50°.12

In the case of the water droplet, the contact angle values were 94 and 109.8°, on the perfluorodecyl and fluorocarbamate coatings, respectively. In this system, a very good result was obtained with respect to the experimental value for a water droplet on a perfluorodecyl coating (93°),13 while in the case of the fluorocarbamate coating, a deviation of 50.2° with respect to the reported experimental value was encountered (160°).12 As far as we know, there is no other work that reports a water contact angle as high as 160° on fluorocarbonated coatings. The contact angle of water on the fluorocarbamate coating obtained with this simulation is consistent with that reported for water droplets on fluorocarbonated coatings (90– 130°)10,14,16,28,52. This finding leads us to recommend for future works to verify both the experimental determination and the molecular model used for this case.

Figure 4 illustrates the top view of the coatings obtained with a monolayer of each surfactant and the multilayer configuration of the fluorocarbamate. These results show that, for both surfactants, the monolayers exhibit uncovered areas of the surface. This is associated with the lateral interactions between the chains of the surfactant, which do not allow for a higher packing degree of the coating. These configurations will be analyzed in detail with the density profiles and the interaction energy results for each surfactant. For the multilayer coating of the 15 ACS Paragon Plus Environment

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fluorocarbamate surfactant, Figure 4 shows that the unoccupied spaces on the surface are hidden by the upper layers in the multilayer configuration.

The surface densities obtained for the monolayer coatings were 1.94 and 1.71 µmol/m2 for the perfluorodecyl and fluorocarbamate, respectively. Figure 5 shows the results for the coordination number (which is the number of molecules that a central atom holds as its nearest neighbors), obtained by integrating the radial distribution function between the silica atoms in the silane group of each molecule (also shown in Figure 5). The profiles show that the coating with the perfluorodecyl exhibits a higher degree of packing than that with a monolayer of fluorocarbamate, which is in agreement with the surface density results obtained for each monolayer. The higher density results for the perfluorodecyl coating are associated with this molecule having lower steric hindrance than the branched structure of the fluorocarbamate. The fluorocarbamate molecular structure increases the lateral repulsion interactions, as corroborated later with the energy results.

The spatial configuration of each surfactant on the surface was determined by calculating the density profiles in the direction perpendicular to the surface for the whole molecule and for the subgroups of atoms presented in Figure 1. In the case of perfluorodecyl, the silicon atoms, the NonCF and CF parts of the molecule were taken as subgroups. For the fluorocarbamate, the CF chains were selected in order to establish the location of the ramifications of the molecule and the atoms in the acyl group (C=O) were also analyzed in order to determine the distance between the surface and the point where the ramification begins.

Figure 6 shows the density profiles of the perfluorodecyl monolayer at the end of the simulation of relaxation for the entire coating (before the addition of water or n-decane). The thickness of the coating obtained with this surfactant was 12.5 Å, which is consistent with ellipsometry and X-ray photoelectron spectroscopy (XPS) measurements of 13 Å for the thickness of monolayers of fluorocarbonated substances on gold.28 First peak in density profile corresponds to Si atoms, 16 ACS Paragon Plus Environment

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which evidence that perfluorodecyl is bonded to the surface through the silane group (SiO3). To the other two section of the perfluorodecyl (CF and NonCF) we obtained a single peak located at ~5.5 Å. The results show that the fluorocarbon chains are mostly oriented towards the fluid phase with only a few molecules with the fluorocarbonated chains adsorbed directly on the surface (at distances less than 5 Å). The latter are precisely the ones that have lateral interactions, which prevent further packing. The density profile of the ethyl group (NonCF), which binds the silane group and the fluorocarbonated chain, exhibits a more localized peak since this group has fewer degrees of freedom for the spatial orientation because it is bonded to the silane group that reacts on the surface.

Figure 7 shows the density profiles of the perfluorodecyl monolayer upon addition of water or n-decane droplets. In both cases the coatings were densified and the thickness of the coating decreased. This behavior is due to the strong liquid-surface attraction energy, which generates a compression of the surfactant chains on the surface. The results show that the perfluorodecyl coating is more compressed in presence of n-decane compared to water, because the n-decanesurface adhesion energy is significantly higher than the cohesion energy between the n-decane molecules. While in the case of water, although the water-surface interaction energy is also high, the cohesion energy between the molecules within the droplet competes in the wetting process. These differences in the cohesion energies are physically evident in the surface tensions of each liquid, with the surface tension of the water (72.70 mN/m)52 being substantially greater than that of the n-decane (23.88 mN/m).52

Figure 8 shows the density profiles of the coatings with a monolayer and with multiple layers of the fluorocarbamate on the surface before the addition of water or n-decane. The monolayer simulation was conducted to evaluate the distribution of surfactant on the surface and the multilayer profile was made to determine the equilibrium configuration of the outer layers of surfactant that are in contact with the fluid phase (n-decane and water). The thickness obtained for a monolayer of coating with the fluorocarbamate surfactant was ~13.5 Å, in excellent 17 ACS Paragon Plus Environment

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agreement with the experimental measurements of thickness for a monolayer of fluorocarbonated surfactants in similar systems.28 For the monolayer coating (Figure 8, top), the density profile of the CF chain exhibits two distinct peaks, located at 5.5 and 9.5 Å from the surface, and a valley located at 7.5 Å, which coincides with the peak observed in the density profile of the C=O group, which in turn is the point where the ramification begins. These results indicate that in a monolayer of the fluorocarbamate coating, the branches of the molecular structure of the surfactant are distributed almost equally between being adsorbed on the surface (at distances of ~5.5 Å) and oriented towards the fluid phase. In addition, the density profile shows that the silane groups (SiO3) are oriented towards the surface, in the same way as with the perfluorodecyl coating.

For the multilayer coating with the fluorocarbamate, the density profile of the CF chain (Figure 8, bottom), shows two well-defined peaks located at 5.5 and 17.5 Å from the surface, corresponding to the adsorbed layers that are closer to the surface. Less defined peaks are also observed at distances above 25 Å from the surface, associated with the outer layers of the coating. As expected, these outer layers exhibit a lesser organization degree, since at this distance, the effect of the surface is almost completely imperceptible and the interaction is almost exclusively between surfactant molecules. The region of the first valley in the density profile of the CF group (at a distance of 13.5 Å from the surface), which corresponds to the limit between the first two layers of adsorbed surfactant on the surface, is in agreement with the thickness obtained for the monolayer coating. Upon addition of the water or n-decane droplets no significant variations of the density profiles of the multilayer coating were obtained (as shown in Figure S3 of the Supporting Information). This is mainly due to the fact that the thickness of the coating prevents the interaction between the molecules of the liquid and the surface.

The effect of the coating of each surfactant on the configuration of the n-decane droplet was determined by evaluating the density profile of the droplet in the direction perpendicular to the 18 ACS Paragon Plus Environment

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surface. Figure 9 shows the density profile for the n-decane droplet on the surface covered with a perfluorodecyl monolayer, and on a multilayer of fluorocarbamate. The results show a marked difference in the configuration of the droplet for each treatment.

The density profile of the n-decane droplet on the perfluorodecyl coating exhibits a small peak (located at ~3.5 Å from the surface) that is associated with a small amount of n-decane molecules that are able to permeate the coating and become in direct contact with the surface at places where there are no surfactant (as described in Figure 4). In the bulk of the droplet, three distinct adsorbed layers are observed (located at distances between 12 and 18 Å from the surface). This stratification of the droplet is clear evidence of the effect of the surface at these distances.

Unlike the density profile of the n-decane droplet on the perfluorodecyl coating, the configuration of the n-decane droplet on the fluorocarbamate coating does not exhibit any stratification. Instead, the density profile exhibits a dome-like shape, since the multiple layers of the coating prevents the liquid from being energetically affected by the surface and the n-decane molecules only interact with the outer layers of the coating. Thus, the density profile obtained in this case resembles that of a fluid-fluid interface, with no definite peaks of densification.53,54

Figure 10 depicts the configurations of the water droplet on the coatings with both surfactants. In this case, a similar behavior to the n-decane droplet was obtained. On the perfluorodecyl coating, the density profile shows a considerable amount of water molecules that are able to permeate the coating (at distances less than 10 Å), since the water molecules are smaller in size compared to the n-decane molecules. At a distance of ~4.5 Å from the surface, the density profile shows a first adsorption layer on the surfactant coating and the second layer is observed at a distance of ~8.5 Å, which is consistent with results reported in the literature.55


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The density profile of the water droplet on the fluorocarbamate coating has a dome-like shape, similar to that obtained for the n-decane droplet. There is no clear effect of the solid surface on the configuration of the droplet. Therefore, for both liquids on this coating, the density profile corresponds to a liquid-liquid interface. Additionally, the results are evidence that this treatment has the ability to generate both lipophobic and hydrophobic surfaces, in agreement with experimental results.12,13,17,30

The interaction energies between each surfactant and the liquid phases (n-decane and water) were evaluated from the equilibrium configuration for each droplet after 10 ns of simulation time. The results are presented in Table 1 for all systems. Positive energies are an indication of repulsive interactions, while negative energies are associated with attractive interactions. The interaction energies between the chains of each surfactant (T-T) obtained from the simulations of the coatings are also reported.

The interaction energies between the surfactant chains (T-T) are repulsive in both cases, and are higher for the fluorocarbamate compared to the perfluorodecyl. These repulsive energies generate the empty surface areas, uncovered with the surfactant, when a monolayer is applied. However, the global interaction energies (considering the whole molecule of surfactant) are attractive as a result of the contribution of all components of the molecule, and are higher for the perfluorodecyl (-52.4 kcal/mol) than those for the fluorocarbamate (-48.3 kcal/mol). These energy results explain the higher packing degree for the perfluorodecyl molecules on the surface compared to the fluorocarbamate surfactant.

Due to the non-polar nature of the n-decane molecule, the wettability alteration for this liquid is mainly governed by the van der Waals interactions. The results presented in Table 1 show that, for both surfactants, the CF and NonCF parts have attractive interactions with the n-decane molecules, with the CF-d energy higher than the NonCF-d energy in both cases. However, since the attraction forces between the liquid molecules in the bulk of the droplet (d-d energy) are 20 ACS Paragon Plus Environment

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significantly higher than the interaction energy with the chain in each surfactant, it is more favorable for the liquid molecules to remain in the bulk of the droplet, giving contact angles between 50 and 60°.

In the case of the perfluorodecyl coating, the fluorocarbonated chain (CF) is responsible for 87% of the total interaction energy with the n-decane (T-d energy), while in the fluorocarbamate it is only 77%. The results shown in Table 1 indicate that the heteroatom content in the fluorocarbamate molecular structure generates an additional contribution to the attraction energy between the NonCF parts and the n-decane molecules. Additionally, the presence of heteroatoms generates an important change in the electrostatic distribution of the molecule, which produces a greater energy of attraction between the CF parts and the n-decane molecules. Therefore, because of these two combined effects, the attractive interaction energy T-d turns out to be 25% higher for fluorocarbamate than for perfluorodecyl, which is why the contact angle of n-decane droplets obtained on fluorocarbamate is 7° smaller.

The contact angle of the droplets obtained on this coatings indicate that intermediate-wetting (i.e., lack of a strong wetting preference) was obtained with the perfluorodecyl coating and an oil-wet system was generated with the fluorocarbamate.56 These wettability states generated are due to the attraction forces in the bulk of the water droplet (w-w), which is substantially higher than the surfactant-liquid attraction energy (T-w). For the perfluorodecyl, the higher value of the surfactant-liquid attractive energy generates a smaller contact angle on this coating.

The presence of the sulfur, nitrogen and oxygen atoms in the fluorocarbamate molecular structure means that the charge distribution of this surfactant is significantly different than the charge distribution in the perfluorodecyl. This causes differences in the electrostatic contributions to the interaction energy with the water molecules. This also explains the differences obtained between the interaction energy of the liquid phase and the fluorocarbonated chains for each surfactant. 21 ACS Paragon Plus Environment

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CONCLUSIONS In this work, the wettability alteration of sandstones generated by partially fluorocarbonated organosilanic surfactants with linear (perfluorodecyl) and branched chains (fluorocarbamate) was evaluated by measuring the change in the contact angles of water and n-decane droplets.

In the coating with a monolayer of perfluorodecyl, the fluorocarbonated chains were mostly oriented toward the outside of the coating with only a few molecules with the fluorocarbonated chains adsorbed directly on the surface. In a monolayer of the fluorocarbamate coating, the branches of the molecular structure of the surfactant are distributed almost equally between being adsorbed on the surface and oriented towards the fluid phase. The surface density for the perfluorodecyl monolayer is higher than that obtained for the fluorocarbamate monolayer, since the linear chain in the perfluorodecyl generates lower lateral repulsive interactions. In the multilayer coating of the fluorocarbamate, the outer layers exhibit less of an organization degree, since the surface effect in this region is almost imperceptible.

On the perfluorodecyl coating, the droplets of n-decane and water have an appreciable amount of liquid molecules that are able to penetrate the coating and in the bulk of these droplets welldefined adsorption layers were encountered due to the effect of the interactions with the solid surface. The configuration of the liquid droplets on the fluorocarbamate coating does not show any stratification. The thickness of the multiple layers of the coating prevents the liquid from being energetically affected by the solid surface and the n-decane and water molecules interact only with the outer, less organized layers of this surfactant. As a result, the density profile for both liquids on this coating resembles that of a fluid-fluid interface.

Both surfactants evaluated exhibit attractive interactions with the n-decane and water molecules. However, the liquid molecules favor remaining in the bulk of the droplets since the liquid-liquid attractive forces overcome the surfactant-liquid interactions. The higher value of the 22 ACS Paragon Plus Environment

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perfluorodecyl-water attractive energy generates a smaller contact angle for the water droplet on this coating.

The

perfluorodecyl

surfactant

produces

intermediate-wetting

coatings,

while

the

fluorocarbamate alters, more efficiently, the wettability of the surface to water. The presence of sulfur, nitrogen and oxygen atoms in the molecular structure of the fluorocarbamate results in the charge distribution of this surfactant being significantly different than the charge distribution in the perfluorodecyl. This causes differences in the electrostatic interactions with the water molecules. Also, the heteroatom content in the fluorocarbamate molecule generates an additional attractive energy on the n-decane molecules, giving a smaller contact angle for this fluid than on the perfluorodecyl coating.

The contact angle results obtained with molecular dynamics simulations present, in most cases, deviations of less than 3° with respect to reported experimental measurements. With this, it can be established that the models and methodologies used in the present study are adequate to represent the wettability alteration phenomenon.


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ACKNOWLEDGEMENTS Financial support for this work was provided by Agencia Nacional de Hidrocarburos (ANH) and Colciencias (call for proposals 721-2015, project 111872150012, Contract No. FP44842016-2016). The authors also thank the Universidad Nacional de Colombia- Sede Medellín for allowing simulations in the advanced numerical computation unit (UNICA).

Ivan Moncayo-Riascos also thanks the scholarship provided by the Administrative Department of Science, Technology and Innovation – Colciencias - call for proposals 727-2016.

AUTHOR INFORMATION Corresponding Author * Tel.: +57-4-4255280, +57 300 7775 5629; Email address: [email protected]

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript


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FIGURES

Fluorocarbamate

CF

Tail

Perfluorodecyl

NonCF

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

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Figure 1. Fluorocarbonated organosilanic surfactants and the sections of each molecular structure selected: Tail, CF and NonCF.


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Figure 2. Droplets of n–decane and water on the uncovered silica surface. Above: top view. Below: front view.

Figure 3. Contact angle of the n-decane (top) and water droplets (bottom) on the perfluorodecyl and fluorocarbamate coatings.


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Figure 4. Top view of the monolayer coating for each surfactant and the multilayer coating for the fluorocarbamate (cyan: fluorine, white: hydrogen, red: oxygen).

Figure 5. Radial distribution function and coordination number of the silica atoms for a monolayer of coating with perfluorodecyl and fluorocarbamate.


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Figure 6. Density profiles of a perfluorodecyl monolayer on the surface before the addition of water or n-decane.

Figure 7. Density profiles of the perfluorodecyl monolayer upon the addition of water and ndecane droplets.

Figure 8. Density profile of the fluorocarbamate coating: monolayer (top) and multilayer (bottom).


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Figure 9. Density profile of the n-decane droplet on the perfluorodecyl monolayer coating and on the fluorocarbamate multilayer coating.

Figure 10. Density profile for the water droplet on the perfluorodecyl monolayer coating and the fluorocarbamate multilayer coating.


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

TABLES

Table 1. Interaction energies between sections of each surfactant and the liquid phase. Tail (T), fluorocarbonated chain (CF), non-fluorocarbonated chain (NonCF), n-decane (d) and water (w). Interacting groups T-T d-d T-d CF-d NonCF-d w-w T-w CF-w NonCF-w

Interaction energy [kcal/mol] Perfluorodecyl Fluorocarbamate 15.085 36.040 -2.878 -6.476 -0.610 -0.762 -0.531 -0.583 -0.079 -0.179 -10.055 -8.744 -0.125 -0.104 -0.083 -0.044 -0.021 -0.081


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COMPARISON OF LINEAR AND BRANCHED MOLECULAR

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