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Experimental-Theoretical Approach to the Adsorption Mechanisms for Anionic, Cationic and Zwitterionic Surfactants at the Calcite-Water Interface. Agustín Durán-Álvarez, Mauricio Maldonado-Domínguez, Oscar González-Antonio, Cecilia DuránValencia, Margarita Romero-Ávila, Fernando Barragán-Aroche, and Simon Lopez-Ramirez Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04151 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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Experimental-Theoretical Approach to the Adsorption Mechanisms for Anionic, Cationic and Zwitterionic Surfactants at the Calcite-Water Interface.

Agustín Durán-Álvarez†,Ѧ, Mauricio Maldonado-Domínguez§, Oscar González-Antonio§, Cecilia DuránValencia†, Margarita Romero-Ávila§, Fernando Barragán-Aroche†, Simón López-Ramírez†*. †

Universidad Nacional Autónoma de México, Facultad de Química, Departamento de Ingeniería

Química/USIP, Ciudad Universitaria, México, D. F., C.P. 04510, México. §

Universidad Nacional Autónoma de México, Facultad de Química, Departamento de Química Orgánica,

Ciudad Universitaria, México, D. F., C.P. 04510, México. Ѧ

Universidad Nacional Autónoma de México, Facultad de Ingeniería, Departamento de Ingeniería

Petrolera, Ciudad Universitaria, México, D. F., C.P. 04510, México. *) Corresponding author’s e-mail. [email protected]

Abstract. The adsorption of surfactants (DTAB, SDS and CAPB) at the calcite-water interface was studied through surface zeta potential measurements and multiscale molecular dynamics. The ground-state polarization of surfactants proved to be a key factor for the observed behavior; correlation was found between adsorption and the hard or soft charge-distribution of the amphiphile. SDS exhibits a steep aggregation profile, reaching saturation and showing classic ionic-surfactant behavior. In contrast, DTAB and CAPB featured diversified adsorption profiles, suggesting interplay between supramolecular aggregation and desorption from the solid surface, and alleviating charge buildup at the carbonate surface when bulk concentration approaches CMC. This manifests as an adsorption profile with a fast initial step, followed by a metastable plateau, and finalizing with a sharp decrease and stabilization of surface charge. Suggesting this competition of equilibria, elicited at the CaCO3 surface, this study provides atomistic insight into the adsorption mechanism for ionic surfactants on calcite, which is in accordance with experimental evidence, and which is a relevant criterion for developing Enhanced Oil Recovery processes.

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I. INTRODUCTION Enhanced Oil Recovery (EOR) processes are nowadays an important topic due to the growing necessity of increasing oil production and achieving more efficient extractions on challenging geologies. Different methods have been implemented to improve performance in extraction processes. Injection of batches consisting of surface-active substances is one of the main applications. This features intrinsic adsorption phenomena as a major impact factor.1-2 Among all different surfactant-based EOR processes, adsorption within a reservoir leads to chromatographic retardation of surfactants when transported through the mineral matrix. This may render the designed EOR processes inefficient or non-feasible in terms of economy.3-4 Low adsorption is a critical requirement to ensure effective propagation of the surfactants in porous media. Dodecyltrimethylammonium bromide (DTAB), sodium dodecylsulfate (SDS) and cocoamidopropyl betaine (CAPB) are a triad of surfactants widely researched within the context of EOR, emphasizing their importance as archetypical chemical species for surfactants studies.5-13 Elucidating the molecular manner in which these chemicals interact with mineral surfaces is relevant for the oil industry since injection of chemicals must consider the interaction with the reservoir. The chemical identity of the rock present in the reservoir is one of the main variables when studying solid-liquid surface behavior, as suggested by Somasundaran and Zhang14-15 in their monitoring studies related to the effects on wettability. Amongst the main juxtaposed points between the two most important types of reservoirs, silicates and carbonates, is the vast range of porosities present in silicates when compared with carbonates. There are also differences in their porosity-permeability relationship.16 Both mineral families are prone to chemical reactions, requiring, in the case of sandstones, relatively aggressive conditions of acidic chemical weathering to decompose.17 Some silicates resist decomposition and display no acid-base properties.18 Surface charge is also a remarkable difference; in a wide pH range (4-11) pure calcite has a positive zeta potential19-23, while silica has negative values at the same range.24 The majority of EOR-oriented adsorption studies are focused on silicate surfaces; therefore it is only natural to turn towards the other important type of reservoir. Electrokinetic data of the isoelectric points (IEPs) of calcite has been summarized, with pH values ranging from 4.2 to 11. The IEP of calcite depends on the sources of materials, equilibration time and ions in solution. Given the same ionic strength (NaCl, 10-3 M), in the pH range of 7–11, natural calcite was more negatively charged than synthetic calcite.19 In a different study, Vdović found out that only synthetic samples of calcite displayed positive zeta potential values, and exhibited two different isoelectric points: one at acidic conditions (≈ 6.5) and the other at alkaline pH values (≈ 9.7). In this work, it was also ACS Paragon Plus Environment

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remarked that one of the main issues on zeta potential of solid dispersions is focused on the particle dispersion control, since flocculation and precipitation of suspended solids may affect experimental measurements.20 This can be overcome by the porous media preparation to measure zeta potential on solidliquid interface, as applied in our study using the streaming potential technique. Roques and co-workers studied the concept of potential determining ions (PDI), as a major impact variable that affects the zeta potential measurements.21 The main differences between positive and negative values at the same pH conditions are related to substrate clay content and the mentioned PDI (Ca2+ and HCO3− according to Roques). Ma and colleagues22 reported a positive value on synthetic calcite of 4.2 ± 7.2 mV (pH = 9.8), while Planck reported zeta potential for calcite on a pH range of 9 - 12.5 and reported positive values below an IEP at pH = 11.4.23 Nieto-Álvarez used High-Performance Liquid Chromatography (HPLC) to study the surface behavior of cocoamidopropyl hydroxy sultaine; it was coupled to Transmission Electron Microscopy (TEM) to analyze micelles and other complex aggregates. This work shows the importance of dynamic techniques that take into account solution, adsorption, and flow phenomena; this approach renders the study of surfactant aggregation possible.25 Kolev and co-workers26 found considerable modifications of zeta potential on glass spherical particles, with DTAB; in all cases the result was positive values of this parameter. In a different study performed by Haiyian, an hemimicelle adsorption model is described with cetyltrimethylamonium bromide molecules on fumed TiO2.27 Planck and co-workers23 performed studies of zeta potential on calcite, which were influenced by carboxylate anions; in their research, at pH=9, they found monotonic behavior of zeta potential as surfactant concentration increased. Tandem-monitored experiments using zeta potential and spectroscopic techniques have also been approached by Labidi and Djebaili28 to characterize structural changes undergone by the substrate and surfactants before and after the adsorption mechanism has developed. According to the literature, anionic SDS showed a substantially higher adsorption compared with cationic cetylpyridinium chloride on calcite under the same conditions.22 However, if abundant silica and/or clay exist in the carbonated formation, a substantially higher adsorption of cationic surfactants may be observed.29 A positive surface charge does not mean that a unique charge is distributed across the surface of calcite. Reduced, but not null, ionic adsorption of cations may happen on a positively charged surface. This is ACS Paragon Plus Environment

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reinforced by the results obtained by Ahualli and co-workers.30 They measured surface charge on colloidal particles of silica. Aqueous solutions of CTAB and C12E5OH displayed simple surface behavior, whilst SDS yielded a complex profile and negative-charge accumulation. Given the adsorption of anionic SDS on the negative surface of silica, a safe assumption is that cationic surfactants may adsorb on positivelycharged calcite. Somasundaran and colleagues have studied the negative adsorption of surfactants on solid substrates where some supramolecular ensembles, consolidated during the adsorption process, return to the solution bulk, minimizing electrostatic buildup. This phenomenon receives the name of micellar exclusion,31 and one of its possible consequences is a decrease in surfactant load around the CMC region. The importance of hardness and softness as electronic descriptors was analyzed in a tandem theoreticalexperimental study by Vlachy and co-workers.32 They report a Hofmeister-like scale for surfactant cationic and anionic head-groups, positioning the ammonium cation as the softest species in their study, which suggests that the trimethylammonium head-group would be even more polarizable and softer. Alkylsulfates are located in the middle of their scale, being the sulfate group a relatively hard moiety in a surfactant. While hard atoms, like fluorine, oxygen and sodium cation, localize large charge densities, the soft, polarizable alkyl chains delocalize charge through inductive and stereoelectronic mechanisms.33 Hard-hard pairs display strong ionic interaction, while the soft-soft pairs interact in a diffuse manner. Recently, Benková and Cordeiro studied the adsorption modes of poly(ethylene oxide) on silica by means of molecular dynamics (MD), which reinforces the importance of computational methods on the understanding of interfacial phenomena.34 Previous MD research about adsorption behavior was aimed at mineral structure and chemical reactivity.35 The free energy of adsorption of water and metal ions (magnesium, calcium, and strontium) on the (104) calcite surface has also been approached through MD, helping establish ulterior studies on such material, like the one hereby reported.36 In the present work, studies of the surface behavior of calcite as a function of surfactant concentration are reported for three surfactant families, by means of solid-surface zeta potential. Supramolecular aggregation stages, and surface-charge behavior are approached and discussed. The systems, conformed by calcite, water and SDS, DTAB and CAPB surfactants, were built and analyzed by means of quantum-level electronic description of surfactants, followed by MD simulations. In this way, we approached, at an atomistic-level, a description of these arrangements via molecular modelling of the physicochemical phenomena developed by such systems. The results derived from these studies afford a better


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understanding of the interactions between the reservoir and the selected chemical products for EOR applications. II. EXPERIMENTAL SECTION A. Substances CaCO3 was obtained from MEYER (> 99.9%), and sieved with the guide of ASTM Manual on Test Sieving Methods37 with a particle size from 38 to 53 microns. X-Ray diffraction analysis, shows that CaCO3 sample is structured as calcite with no traceable amounts of aragonite (please refer to supporting information for the corresponding PXRD diffractogram). The solutes were dissolved in deionized water obtained from a miliQ system (ASTM-I). Acidic and basic titrations were made with hydrochloric acid (HCl) (Sigma Aldrich > 37.8%) and sodium hydroxide (NaOH) (Sigma Aldrich > 99.5%) diluted at 0.05 mol dm-3. Zeta potential measurements were made using three representative ionic surfactants: sodium dodecylsulfate (anionic) (>95% Sigma Aldrich), dodecyltrimethylammonium bromide (cationic) (> 99.5% Sigma-Aldrich) and cocoamidopropyl betaine (zwitterionic) (30.85% - OSSA; organosintesis.com). The gas used for purging CO2 off was industrial-grade Nitrogen (CRYOINFRA, > 95%). NMR spectra were recorded for each surfactant. The corresponding results were consistent with the chemical identity of each species. B. Methods The surface zeta potential measurements were carried out by streaming zeta potential technique using a SurPASS-Anton Paar analyzer in a controlled temperature room. The methodology corresponds to the SurPASS adsorption test. The sample preparation runs as follows: deionized water was poured into beakers (500 mL) and mixed with 2 grams of granular CaCO3 of grain size lower than 25 microns, during 4 hours (for more details, please refer to supporting information); once solids completely settled, the clarified solution was decanted. This last solid-liquid separation was performed three times to avoid CaCO3 as a dispersed phase. Subsequently, N2 gas was purged into the solution, until pH and conductivity remained constant. Titrations started until temperature stabilization was reached. The amount of substrate used as porous media was 1.2 grams for all tests. A cylindrical cell with filtering screens of 25 microns was used to contain the CaCO3 powder. When tests are carried out with surfactants, it is very important to avoid foam


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formation in order to keep surfactant concentration constant in the bulk phase. The average temperature was 25.03 °C with a standard deviation of 0.71 °C. The correct equipment functioning was verified using polypropylene film, provided by Anton-Paar, and its isoelectric point (IEP) was found at pH = 4.09, which falls within the accepted range (pH=4.0 ± 0.2). The studied surfactant molecules (SDS, CAPB and DTAB) were analyzed using Materials Studio 8 (MS8).38 All the numerical modules employed throughout this work are contained within this suite. Computational experiments were carried out to study the binding of surfactants on the (104) surface of calcite through molecular dynamics. The system was defined as a periodic slab with a neutral surface of calcite. The crystal structure was cleaved at the (104) face, as it is the most probably exposed surface on the calcite form of calcium carbonate. The dimensions of this surface are 48.6 Å X 59.9 Å (2911.1 Å2). The thickness of the mineral was set to 5 layers (15.18 Å) of CaCO3. The block of ionic mineral was optimized with molecular mechanics (MM), using the COMPASS forcefield, with fine convergence criteria. All MM and MD computations were carried out using the Forcite module in MS8. The simulation cell for MD was built by creating a vacuum space of 250 nm on top of this optimized calcite block. In all the simulations, the bottom 4 layers of CaCO3 were fixed at their optimized geometry, while the exposed layer was allowed to freely evolve without any geometrical constraint. The integrity of the mineral surface was kept throughout all MD runs, with all ions remaining near the starting coordinates of their barycenter. Simulation involving a locally positive site on calcite was achieved by removing a carbonate anion from the center of the exposed mineral layer. The electronic structure for the ground state of dodecylsulfate anion, dodecyltrimethylammonium cation, CAPB and water molecules, as well as Na+ and Br- counterions, was obtained at the DFT level using the meta-GGA M06-L functional38 with the double-zeta DNP numerical basis set. Infrared frequencies were computed for every molecular system to ensure that a minimum in potential energy was reached. Atomic charge distribution was obtained through Hirshfeld population analysis.40 This charge partition was used as input for all the MD simulations. To model the intrinsic modes of direct adsorption of surfactants on calcite, molecules were allowed to interact independently through MD simulations. A set of 5 different initial geometries was generated for each surfactant near the cleaved surface of CaCO3, covering different cases of interaction with neutral and


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positive calcite. The thermodynamic ensemble chosen was NVT, that allows energy exchange with a thermal bath, to be imposed with the Nose thermostat.41 To model the behavior of water in these simulations, the O_3 parameterization from the Universal Force Field, UFF, was employed.42 Thus, water molecules were approximated as being nearly tetrahedral, with dummy atoms representing available lone pairs. Atomic charges were derived using the aforementioned DFT methodology. Bond lengths were not fixed during simulation. To simulate the diffusion and accumulation of surfactants near the exposed face of calcite, MD runs were performed using the NVT ensemble, at 298 K, with 1 fs simulation step. Sequential increases in the load of surfactant were applied once equilibration was reached. Total simulation time was set to 2 ns. This is backed up by information found in the literature, where Marrink and coworkers43 determined that the most suitable simulation time is a function of the surfactant concentration. The concentrations used for zeta potential measurements reported in the present study are correctly transferred and described with a simulation time of 1 ns. This approach provided descriptive results for CAPB, precluding the need for explicit solvent molecules. Ionic surfactants SDS and DTAB required explicit solvation due to strong ionic interactions. To explicitly include the dielectric effects of water in the diffusion and aggregation of DTAB and SDS onto the calcite exposed surface, a layer of 100 nm of a 25% solution of surfactant in water was built using the Amorphous Cell module within MS8, with a density of 0.8 g/mL. In order to screen the electrostatic interactions with the periodic block of calcite on the upper face of the simulation cell, an additional layer of 100 nm of pure water was packed into the simulation cell. RESULTS AND DISCUSSION A. Synthetic Calcite As seen in figure 1, the measured overall charge of the synthetic calcite used is positive in a pH from 4 to 11. It showed two different IEP at 4.1 and 10.9. In the pH ranges employed throughout this work, a positive value of 2.3±0.96 mV was the initial zeta potential for all experiments, which is in accordance with results reported in previous works.20-23


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ζ [mV] 5

0

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6

10

pH

-5

-10

-15

-20

Figure 1. Zeta potential of synthetic calcite as a function of pH

B. Electronic Description of Surfactant Molecules through DFT Electron density distribution is a key descriptor for molecular systems, as it influences intersystem interactions due to electrostatic or electrodynamic phenomena. Density Functional Theory (DFT) was chosen since electron density for stationary states is a natural consequence of applying this theoretical framework to ground-state geometry optimization. Calculation of Electrostatic potentials (ESP) results in what is depicted in Figure 2, showing the contrasting electronic nature of the studied surfactants.

Figure 2. Electrostatic potential maps for the studied surfactants, which were computed at the DFT M06-L/DNP level with implicit COSMO water solvation. Yellow surfaces enclose a negative charge, while positive zones are presented as blue surfaces.

The soft sulfur atom in the sulfate group in SDS is surrounded by oxygen atoms, rendering this anion a hard chemical species. The charge is quite localized in sodium dodecylsulfate and, thus, its description as a ACS Paragon Plus Environment

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classic ionic surfactant seems adequate for most applications where mainly Coulombic and hydrophobic forces

dictate

physicochemical

behavior.

In

contrast,

the

soft

electrostatic nature

of

the

dodecyltrimethylammonium cation must be pointed out, as can be seen in its electrostatic potential surface. The positive charge is distributed across the hydrocarbon domain, permeating the hydrophobic tail. This results in a diffuse positive charge, which is poorly solvated by water. Zwitterionic CAPB features a finer structure with locally negative and positive zones distributed along the polar portion of this molecule. Hard ionic loci are thus in coexistence with soft cationic zones. As is clearly depicted in Figure 2, all species are undeniably different in charge distribution. Therefore, the need for a molecular-level description, different from the traditional tail-head approach, arises. This is relevant when analyzing self-interaction and adsorption modes, as will be pointed out later in this work.

C. Adsorption of Sodium Dodecylsulfate (SDS) 1) Experimental Results As shown in figure 3, the overall charge of CaCO3 was modified from its positive initial value into a negative one; this zeta potential shift is associated with an IEP at 0.025 mM. A linear behaviour is observed from 0 mM to 0.6 mM with a pronounced negative slope ζ/mM. Below this point, there is a decrease in the slope rate until it reduces almost to zero. This non-linear decrease in slope rate may be associated to saturation due to charge buildup; near this region a plateau is reached at ≈ 0.7 mM with ζ ≈ 33 mV and remains almost constant until it reaches a minimum in zeta potential in the 4.5 mM - 5.5 mM range with ζ = -34.9 mV. At higher SDS concentrations there is a positive low slope in zeta potential. The minimum zeta potential is reached in the vicinity of SDS critical micelle concentration. This adsorption profile matches the behaviour of carboxylates (hard species) adsorbed on calcite.23


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pH, K*

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SDS Bulk Concentration [mM] Figure 3. Zeta potential, conductivity and pH, as a function of molar concentration. The vertical dashed line indicates SDS CMC=5.95 mM evaluated by conductimetric analysis. CMC previously reported without calcite: 8.08 mM.44

2) Computational Results In order to capture the electrostatic interactions among minerals and ions in solution, molecular dynamics was carried out with a cleaved calcite surface and a film of adsorbed water-surfactant 25% (w/w) mixture (Figure 4). The hardness of the sulfate anionic head reflects on its rapid and strong adsorption onto the hard surface of calcite, displacing adsorbed water molecules to establish ionic chemical contacts. When a positive charge is imposed on this surface, an additional Coulombic component contributes to the overall interaction; although, chemisorption was observed even in neutral surfaces.

Figure 4. Selected frames from MD trajectory for the adsorption of SDS at the water/calcite interface.


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The hard sodium ions, very stable in solvated state, have already been proposed by Stipp to display no significant adsorption onto calcite;45 these cations diffuse freely through the bulk of the solution or around negative loci. Because of this, there is a charge imbalance in the surface of calcite, where negative potential increases until the electrostatic repulsion repels incoming surfactant molecules, which evolve towards aggregation in solution. This charge imbalance is clearly captured by zeta potential measurements. In our experimental setup, the initially positive surface adsorbs dodecylsulfate anions reaching a zero-charge point. Further adsorption occurs, until saturation is reached due to accumulation of negative potential at the mineral surface. D. Adsorption of Dodecyltrimethylammonium Bromide 1) Experimental Results In the cationic surfactant study (Figure 5), zeta potential remains positive in all the concentration range. A linear adsorption profile is observed from 0 mM to 4.5 mM, with a steady increase in positive charge. From 4.5 mM to 9 mM there is an abatement in the slope rate until it reduces to zero; this region may be correlated with more dense adsorption at the interface, such as hemimicellar adsorption, double layer or other aggregation arrangements. The maximum zeta potential modification is reached at 10 mM (0.6 times CMC). This maximum indicates saturation of the solid surface. Beyond this point, there is a linear decline in zeta potential from 10 mM to 13.5 mM. As the CMC is approached, aggregation into solvated clusters may represent a favorable mechanism for surfactant association. These arrangements formed at the calcite exposed-face prior to CMC may detach from surface following the micellar exclusion mechanism proposed by Somasundaran.31 This way, surfactant desorption from the CaCO3 surface, alleviates positive charge accumulation, hence reducing zeta potential. Close to CMC (14.75 mM), there is a transition zone from 14 mM to 17 mM with slope reduction; after this zone, a new linear behaviour is observed. Since the number of micelles in bulk increases with concentration, the last region can be described by the preferable micelle formation of surfactants over substrate adsorption; therefore, a linear decrease of zeta potential is observed as concentration in bulk increases. This mechanism may display an additional contribution from structural shifts at the adsorbed surfactant layer, as contacts between aggregates may lead to coalescence into compact arrangements.


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pH, K*

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*K[mS/m] /30

ζ(C) [mV]

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10

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5

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DTAB Bulk Concentration [mM]

40

Figure 5. Zeta potential, conductivity and pH, as a function of molar concentration. The vertical dashed line indicates DTAB CMC=14.75 mM evaluated by conductimetric analysis on this system. CMC previously reported without calcite: 14.6-16 mM.46

2) Computational Results To understand the contrasting behavior between SDS and DTAB, the soft electron density in DTAB must be considered. With this study, it is clear that the quaternary alkylammonium surfactants, require a more detailed description that includes the high polarizability of organic frameworks. Adsorption of the DTA cation on calcite displays no characteristic directionality, suggesting a soft electrostatic behavior in this molecule. The relatively diffuse electron density in this molecule manifests in a loose adsorption where, according to molecular dynamics, translation and migration of adsorbed surfactants occurs freely at 298 K and at low surfactant loads. By the end of in vacuo MD simulations, it was consistently found that adsorption occurs at the mineral interface up to surfactant loads where buildup of positive charge hinders further accumulation. The incorporation of bromide counterions and water in this loose structure is expected to occur, as suggested by zeta potential measurements. In these experiments, the observed behavior consists on an initial accumulation of positive charge which, upon reaching a maximum value, decreases to a plateau of positive charge. However, the zeta potential profile observed is not described with this approximation, thus explicit solvent must be included during simulation.


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Figure 6. Molecular dynamics simulation of the adsorption of DTAB at the surface of calcite. The diffuse physisorption of supramolecular aggregates is observed, leading to compact packing and coulombic exclusion of charged clusters, which follows a Somasundaran-Fuerstenau mechanism.

If the solvent is explicitly considered during MD, a Somasundaran-Fuerstenau behavior is predicted (Figure 6).15 A detail arising from Figure 6, is the fact that molecules of DTAB are shown to be adsorbed in two main instances. Some of them interact with the surface via trimethylammonium headgroups, and some molecules do it through a more diffuse contact involving both, aliphatic tail and polar headgroup, as depicted in Figure 7. The electronic distribution calculated for this species suggests that a non-negligible amount of positive charge in the molecule distributes along the hydrocarbonated portion of the surfactant. This partly positive tail becomes another adsorption-susceptible area in DTAB; therefore, it will interact with the surface, to a lesser degree, but in the same manner as the headgroup. As a relevant feature, MD runs consistently suggested bromide anions and adsorbed water as key species in DTAB adsorption, screening electrostatic repulsion.


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Figure 7. Close up picture of the equilibrated structure of DTAB aggregates at the calcite-water interface, according to multiscale MD simulations. The diffuse physisorption of submicellar aggregates is observed, illustrating the key role of water and counterions on this adsorption mechanism.

Here lie the main differences between SDS and DTAB. When comparing headgroups and their influence on the rest of the molecule, it can be seen that the electrostatic component remains in the sulfate groups in its entirety for the SDS, while the ammonium portion of DTAB allows charge to distribute through a great portion of the C12 aliphatic chain. Thus, DTAB molecules interact with calcite in a diffuse fashion. In itemizing the adsorption mechanism for DTAB, an initial stage of single-molecule adsorption is followed by the formation of supramolecular aggregates. During this transition, the differential solvation of ions in solution is responsible for the electrical features of these aggregate states, with bromide ions diffusing more easily to the bulk of the solution, resulting in the observed positive charge buildup. Once a certain chemical potential of surfactant in solution is reached, a change in structure towards layer coalescence, may be associated with a denser, more compact array. Further temporal evolution yielded additional trajectories featuring micellar exclusion due to electrostatic buildup at the interface. This suggests that disaggregation and coalescence of adsorbed clusters, are key responsible factors for the change in surface charge observed during streaming zeta potential measurements. E. Adsorption of Cocoamidopropyl Betaine 1) Experimental Results As depicted in Figure 8, for the amphoteric surfactant CAPB, positive values of zeta potential were observed only at concentrations lower than 0.012 mM, where an IEP is found; zeta potential then decreased with a linear negative slope from 0 mM to 0.055 mM, which is congruent with single layer adsorption. From 0.055 mM to 0.18 mM a double layer model may be invoked to account for the observed response; beyond that point, a clearly defined plateau is reached. The first stabilization area exists within the range from 0.1 mM to 0.6 mM with ξ=-17 mV, modifying towards less negative values upon concentration increase; this change in zeta potential may be caused by micellar exclusion31 and structural changes, just as in DTAB case; this behavior manifests as concentration approaches CMC. Near the CAPB´s CMC at 1.65 mM, the slope reduction reached almost zero, with the appearance of a new plateau; at this region, which prolongs until 14 mM, no evidence of substantial variations in zeta potential were observed.


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pH *K[mS/m] /200 +7.3 ζ(C) [mV]

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-18

0

2

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14

CAPB Bulk Concentration [mM] Figure 8. Zeta potential, conductivity and pH, as a function of CAPB concentration. CMC previously reported without calcite: 2.92 mM.9

2) Computational Results The hard carboxylate group localizes negative charge in the anionic area, in a manner analogous to the already-described sulfate group, while positive charge distribution computed for the quaternary ammonium portion of CAPB suggests a soft character, similar to the quaternary ammonium of DTA. The amide linkage represents an additional polar subsystem, with the oxygen atom as a hard negative-charge center and the N-H portion acting as hydrogen-bond donor. Upon MD simulation, the hard carboxylate moiety adsorbs onto the positively charged surface of calcite. The modes of adsorption of this surfactant are varied and complex, as studied through in vacuo MD runs (please refer to Supporting Information for a more in-depth discussion of adsorption modes of CAPB on calcite). Both chemisorption and physisorption interactions may be present within the same molecular unit, depending on the concentration of surfactant molecules near the mineral surface. Since the aggregation of CAPB is predicted to be a slow process (no net charge), sequential buildup of surfactant at the calcite surface was steered by increasing the number of molecules near the interface over time. In our zeta potential measurements, an initial increase of negative charge is observed. With a larger ACS Paragon Plus Environment

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load of CAPB in solution, a decrease in charge manifests, until stabilization at an approximately constant value is reached. In our MD simulations, upon increasing the amount of surfactant near the surface of calcite, the packing of CAPB leads to the growth of rod-like structures around locally positive zones, which are attached to the surface by chemisorbed contacts. The equilibrated simulation cell obtained was duplicated, so the surfactant rod obtained was near the vicinity of an equivalent structure. The resulting system evolved at a constant temperature scanning its supramolecular space. Upon simulation, the structures coalesced into a single aggregate. As a result, they exhibited a structural shift towards a compact packing upon concentration increase, according to the Somasundaran-Fuerstenau mechanism, illustrated in figure 9.

Figure 9. Molecular dynamics simulation of the adsorption of CAPB at the surface of calcite. The formation of rod-like aggregates around positive loci and their coalescence was observed, following a Somasundaran-Fuerstenau mechanism47-48, with a structural shift as surfactant load increases.

On the negative-charge accumulation observed consistently in the zeta potential measurements, Danov and coworkers proposed the trapping of anions in solution by the locally positive groups in CAPB when located at an interface,49 or during aggregation. It is also likely that the hard carboxylate groups, being more easily solvated than soft quaternary ammonium groups, are located at the surface of the adsorbed CAPB clusters. At low concentrations, the hard-hard interaction between anionic heads and the calcium ions exposed on the surface of calcite prevails as shown by the aforementioned adsorption modes. The cationic portion, which is not adsorbed onto the mineral surface, may cooperatively coordinate or trap anions from the surrounding solution, accumulating negative potential. At a given amount of surfactant, a structural transition may occur as suggested by molecular dynamics. This coalescence of neighboring adsorption clusters would decrease the effective surface of the rod-shaped structure and, thus, reduce the surface charge to a limit value. ACS Paragon Plus Environment

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A convenient remark is that all the computations were performed under initial random forces. Shearrelated phenomena are thus not captured within this picture, yet these shear forces may be present in experimental and field setups. This may induce further reorganization or disaggregation.

III. CONCLUSIONS Through streaming zeta potential measurements on packed calcite, the electrokinetic behavior was probed in the presence of aqueous solutions of anionic, cationic and zwitterionic surfactants. A DFT description of surfactants captures the chemical nature (hardness or softness) of each surfactant at a finer level of detail than the classical head-tail description. Through this approach, electron density distributions are become a part of the extended portrait of surfactant-related phenomena presented in this work. The highly oxygenated sulfate head in SDS stands as a relatively hard species in terms of charge distribution, rendering this molecule and homologous alkylsulfates classic head-tail surfactants. This is perceived as steep adsorption and the highest net charge modification on calcite surface, along with a point of surface saturation and a plateau in zeta potential It is noteworthy that cationic surfactants display significant adsorption on positively charged calcite. The soft cationic DTAB features a diffuse charge distribution, involving structural rearrangements led by a combination of hydrophobic aggregation and micellar exclusion mechanisms, as supported by DFTparameterized molecular dynamics. The zwitterionic CAPB molecule exhibits a combined behavior, with a hard adsorption due to the anionic carboxylate moiety and soft cationic quaternary ammonium portion. This duality provides the driving force for cooperative adsorption and coalescence of aggregates as a function of concentration. The factors influencing adsorption of surfactants on the positive surface of calcite were elucidated, along with plausible mechanisms for these processes. This is an important factor to consider when planning and designing EOR processes, since it is an important criterion during selection of surfactant products for this technological application.


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IV. ACKNOWLEDGEMENTS We gratefully acknowledge financial support from ‘Fondo Sectorial CONACYT-SENER Hidrocarburos’ under grant #0185183, ‘PROCESO DE RECUPERACIÓN MEJORADA CON LA TECNOLOGÍA DE INYECCIÓN DE QUÍMICOS (ASP) CON APLICACIÓN MEDIANTE PRUEBA PILOTO EN EL CAMPO POZA RICA’ and we sincerely thank the USAII and USIP from Facultad de Química, UNAM for the support given.

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Figure S13. Energy profile and structural evolution derived from MD simulations on the water-calcite-SDS system. 175x144mm (150 x 150 DPI)


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Figure S14. Energy profile and structural evolution derived from MD simulations on the water-calcite-DTAB system. 183x145mm (150 x 150 DPI)


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Figure S15. Energy profile and structural evolution derived from MD simulations on the water-calcite-CAPB system. 205x125mm (150 x 150 DPI)


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