Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics Simulation Study.

Due to the importance of the dolomite mineral in carbonate reservoirs, the wettability characteristics of dolomite surfaces were studied with both experiments and molecular dynamics simulations. Contact angle measurements confirm that the dolomite surface can be rendered oil-wet by carboxylates (acidic components of crude oil) and that the cationic surfactant can reverse the oil-wetness more effectively than the anionic surfactant used in this study. The oil-wetness of an aged dolomite chip was reduced when treated with MgSO4 solution at 80 °C, while CaCl2, MgCl2, and Na2SO4 solutions did not produce any significant wettability alteration. The effects of surfactants and divalent ions, Ca2+, Mg2+, and SO42- (also referred to as Smart Water ions), were simulated with two model dolomite surfaces containing point defects and step vacancies, respectively. The results indicate that the cationic surfactant can weaken the attraction between the oil phase and the carboxylates, while the anionic surfactant tends to maintain the oil-wetness of the dolomite surface by replacing the carboxylates through competitive adsorption. All Ca2+, Mg2+, and SO42- ions can act as potential determining ions, and the detachment of carboxylates is due to the repulsion from SO42- ions drawn close to the surface in the presence of adsorbed Mg2+.

A new parameter-rich structure-aware mechanistic model for amino acid substitution during evolution.

Improvements in the description of amino acid substitution are required to develop better pseudo-energy-based protein structure-aware models for use in phylogenetic studies. These models are used to characterize the probabilities of amino acid substitution and enable better simulation of protein sequences over a phylogeny. A better characterization of amino acid substitution probabilities in turn enables numerous downstream applications, like detecting positive selection, ancestral sequence reconstruction, and evolutionarily-motivated protein engineering. Many existing Markov models for amino acid substitution in molecular evolution disregard molecular structure and describe the amino acid substitution process over longer evolutionary periods poorly. Here, we present a new model upgraded with a site-specific parameterization of pseudo-energy terms in a coarse-grained force field, which describes local heterogeneity in physical constraints on amino acid substitution better than a previous pseudo-energy-based model with minimum cost in runtime. The importance of each weight term parameterization in characterizing underlying features of the site, including contact number, solvent accessibility, and secondary structural elements was evaluated, returning both expected and biologically reasonable relationships between model parameters. This results in the acceptance of proposed amino acid substitutions that more closely resemble those observed site-specific frequencies in gene family alignments. The modular site-specific pseudo-energy function is made available for download through the following website: https://liberles.cst.temple.edu/Software/CASS/index.html.

Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Lin-(C2H4)m Complexes.

Complexes of lithium atoms with ethylene have been identified as potential hydrogen storage materials. As a Li atom approaches an ethylene molecule, two distinct low-lying electronic states are established; one is the 2A1 electronic state (for C2v geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state, and the second is a 2B2 electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge-transfer state. Only the 2B2 charge-transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(C2H4) complex due to charge transfer can bind molecular hydrogen through dipole-induced dipole and dipole-quadrupole electrostatic interactions. Ab initio studies have produced conflicting results for which electronic state is the true ground state for the Li-(C2H4) complex. The most accurate ab initio calculations indicate that the 2A1 van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(C2H4) complex exists in the 2B2 state. Some have suggested that argon matrix effects shift the equilibrium toward the 2B2 state. We report the low-temperature synthesis and IR characterization of Lin-(C2H4)m (n = 1, m = 1 and 2) complexes in solid parahydrogen which are observed using the C═C stretching vibration of ethylene in the complex. These results show that under cryogenic hydrogen storage conditions the Li-(C2H4) complex is more stable in the 2B2 electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics.

Sequence, structure, and cooperativity in folding of elementary protein structural motifs.

Residue-level unfolding of two helix-turn-helix proteins--one naturally occurring and one de novo designed--is reconstructed from multiple sets of site-specific (13)C isotopically edited infrared (IR) and circular dichroism (CD) data using Ising-like statistical-mechanical models. Several model variants are parameterized to test the importance of sequence-specific interactions (approximated by Miyazawa-Jernigan statistical potentials), local structural flexibility (derived from the ensemble of NMR structures), interhelical hydrogen bonds, and native contacts separated by intervening disordered regions (through the Wako-Saitô-Muñoz-Eaton scheme, which disallows such configurations). The models are optimized by directly simulating experimental observables: CD ellipticity at 222 nm for model proteins and their fragments and (13)C-amide I' bands for multiple isotopologues of each protein. We find that data can be quantitatively reproduced by the model that allows two interacting segments flanking a disordered loop (double sequence approximation) and incorporates flexibility in the native contact maps, but neither sequence-specific interactions nor hydrogen bonds are required. The near-identical free energy profiles as a function of the global order parameter are consistent with expected similar folding kinetics for nearly identical structures. However, the predicted folding mechanism for the two motifs is different, reflecting the order of local stability. We introduce free energy profiles for "experimental" reaction coordinates--namely, the degree of local folding as sensed by site-specific (13)C-edited IR, which highlight folding heterogeneity and contrast its overall, average description with the detailed, local picture.

Activation Strain Analysis of SN2 Reactions at C, N, O, and F Centers.

Fundamental principles that determine chemical reactivity and reaction mechanisms are the very foundation of chemistry and many related fields of science. Bimolecular nucleophilic substitutions (SN2) are among the most common and therefore most important reaction types. In this report, we examine the trends in the SN2 reactions with respect to increasing electronegativity of the reaction center by comparing the well-studied backside SN2 Cl- + CH3Cl with similar Cl- substitutions on the isoelectronic series with the second period elements N, O, and F in place of C. Relativistic (ZORA) DFT calculations are used to construct the gas phase reaction potential energy surfaces (PES), and activation strain analysis, which allows decomposition of the PES into the geometrical strain and interaction energy, is employed to analyze the observed trends. We find that SN2@N and SN2@O have similar PES to the prototypical SN2@C, with the well-defined reaction complex (RC) local minima and a central barrier, but all stationary points are, respectively, increasingly stable in energy. The SN2@F, by contrast, exhibits only a single-well PES with no barrier. Using the activation strain model, we show that the trends are due to the interaction energy and originate mainly from the decreasing energy of the empty acceptor orbital (σ*A-Cl) on the reaction center A in the order of C, N, O, and F. The decreasing steric congestion around the central atom is also a likely contributor to this trend. Additional decomposition of the interaction energy using Kohn-Sham molecular orbital (KS-MO) theory provides further support for this explanation, as well as suggesting electrostatic energy as the primary reason for the distinct single-well PES profile for the FCl reaction.

The N-terminal domain allosterically regulates cleavage and activation of the epithelial sodium channel.

The epithelial sodium channel (ENaC) is activated upon endoproteolytic cleavage of specific segments in the extracellular domains of the α- and γ-subunits. Cleavage is accomplished by intracellular proteases prior to membrane insertion and by surface-expressed or extracellular soluble proteases once ENaC resides at the cell surface. These cleavage events are partially regulated by intracellular signaling through an unknown allosteric mechanism. Here, using a combination of computational and experimental techniques, we show that the intracellular N terminus of γ-ENaC undergoes secondary structural transitions upon interaction with phosphoinositides. From ab initio folding simulations of the N termini in the presence and absence of phosphatidylinositol 4,5-bisphosphate (PIP2), we found that PIP2 increases α-helical propensity in the N terminus of γ-ENaC. Electrophysiology and mutation experiments revealed that a highly conserved cluster of lysines in the γ-ENaC N terminus regulates accessibility of extracellular cleavage sites in γ-ENaC. We also show that conditions that decrease PIP2 or enhance ubiquitination sharply limit access of the γ-ENaC extracellular domain to proteases. Further, the efficiency of allosteric control of ENaC proteolysis is dependent on Tyr(370) in γ-ENaC. Our findings provide an allosteric mechanism for ENaC activation regulated by the N termini and sheds light on a potential general mechanism of channel and receptor activation.

Site-specific thermodynamic stability and unfolding of a de novo designed protein structural motif mapped by 13C isotopically edited IR spectroscopy.

The mechanism of protein folding remains poorly understood, in part due to limited experimental information available about partially folded states. Isotopically edited infrared (IR) spectroscopy has emerged as a promising method for studying protein structural changes with site-specific resolution, but its full potential to systematically probe folding at multiple protein sites has not yet been realized. We have used (13)C isotopically edited IR spectroscopy to investigate the site-specific thermal unfolding at seven different locations in the de novo designed helix-turn-helix protein αtα. As one of the few stable helix-turn-helix motifs, αtα is an excellent model for studying the roles of secondary and tertiary interactions in folding. Circular dichroism (CD) experiments on the full αtα motif and its two peptide fragments show that interhelical tertiary contacts are critical for stabilization of the secondary structure. The site-specific thermal unfolding probed by (13)C isotopically edited IR is likewise consistent with primarily tertiary stabilization of the local structure. The least thermally stable part of the αtα motif is near the turn where the interhelical contacts are rather loose, while the motif's center with best established core packing has the highest stability. Similar correlation between the local thermal stability and tertiary contacts was found previously for a naturally occurring helix-turn-helix motif. These results underline the importance of native-like tertiary stabilizing interactions in folding, in agreement with recent state-of-the art folding simulations as well as simplified, native-centric models.

Supramolecular ssDNA templated porphyrin and metalloporphyrin nanoassemblies with tunable helicity.

Free-base and nickel porphyrin-diaminopurine conjugates were formed by hydrogen-bond directed assembly on single-stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptical properties. Large red-shifts of the Soret band in the UV/Vis spectroscopy confirmed strong electronic coupling among assembled porphyrin-diaminopurine units. Slow annealing rates yielded preferentially right-handed nanostructures, whereas fast annealing yielded left-handed nanostructures. Time-dependent DFT simulations of UV/Vis and CD spectra for model porphyrin clusters templated on the canonical B-DNA and its enantiomeric form, were employed to confirm the origin of observed chiroptical properties and to assign the helicity of porphyrin nanoassemblies. Molar CD and CD anisotropy g factors of dialyzed templated porphyrin nanoassemblies showed very high chiroptical anisotropy. The DNA-templated porphyrin nanoassemblies displayed high thermal and pH stability. The structure and handedness of all assemblies was preserved at temperatures up to +85 °C and pH between 3 and 12. High-resolution transition electron microscopy confirmed formation of DNA-templated nickel(II) porphyrin nanoassemblies and their self-assembly into helical fibrils with micrometer lengths.

Temperature dependence of amino acid side chain IR absorptions in the amide I' region.

Amide I' IR spectra are widely used for studies of structural changes in peptides and proteins as a function of temperature. Temperature dependent absorptions of amino acid side-chains that overlap the amide I' may significantly complicate the structural analyses. While the side-chain IR spectra have been investigated previously, thus far their dependence on temperature has not been reported. Here we present the study of the changes in the IR spectra with temperature for side-chain groups of aspartate, glutamate, asparagine, glutamine, arginine, and tyrosine in the amide I' region (in D2O). Band fitting analysis was employed to extract the temperature dependence of the individual spectral parameters, such as peak frequency, integrated intensity, band width, and shape. As expected, the side-chain IR bands exhibit significant changes with temperature. The majority of the spectral parameters, particularly the frequency and intensity, show linear dependence on temperature, but the direction and magnitude vary depending on the particular side-chain group. The exception is arginine, which exhibits a distinctly nonlinear frequency shift with temperature for its asymmetric CN3H5(+) bending signal, although a linear fit can account for this change to within ~1/3 cm(-1). The applicability of the determined spectral parameters for estimations of temperature-dependent side-chain absorptions in peptides and proteins are discussed.

Multivariate analysis of spectral data with frequency shifts: application to temperature dependent infrared spectra of peptides and proteins.

Changes in the amide I' IR band with temperature are widely used for elucidation of peptide and protein conformational transitions and folding equilibria. Since amide I' exhibits inherent temperature dependent frequency shifts, standard mixture analysis methods are not applicable. To reliably extract the true thermodynamic states, frequency shifts of the component spectra must be explicitly taken into account. For this purpose, new methods termed shifted multivariate spectra analysis (SMSA) and parametric SMSA (pSMSA) are developed and tested on sets of synthetic data as well as real experimental amide I' spectra for thermal unfolding of an α-helical peptide. SMSA uses no specific functional form for the transition (soft modeling), while the parametric variant (pSMSA) assumes a thermodynamic model (hard modeling). The implementation is optimized specifically for amide I' IR in that it takes advantage of known, linear dependence of the frequencies as well as intensities on temperature. The synthetic data tests demonstrate the robustness of the methods; the initial test parameters are recovered with a high degree of reliability, although the nonparameteric SMSA is subject to the rotational ambiguity. Application to the peptide experimental amide I' data illustrates additional complications encountered with the analysis of real systems, such as correction for the side-chain spectra and interference of spectral shape changes. Nevertheless, the results are in excellent agreement with the independent control using circular dichroism (CD) data. The general applicability and limitations of the methods are discussed along with potential extensions.

Insight into the packing pattern of ß2 fibrils: a model study of glutamic acid rich oligomers with 13C isotopic edited vibrational spectroscopy.

Polyglutamic acid at low pH forms aggregates and self-assembles into a spiral, fibril-like superstructure formed as a ß2-type sheet conformation that has a more compact intersheet packing than commonly found. This is stabilized by three-centered bifurcated hydrogen bonding of the amide carbonyl involving the protonated glutamic acid side chain. We report vibrational spectroscopic results and analyses for oligopeptides rich in glutamic acid enhanced with (13)C isotope labeling in a study modeling low pH poly-Glu self-assembly. Our results indicate bifurcated H-bonding and ß2 aggregation can be attained in these model decamers, confirming they have the same conformations as poly-Glu. We also prepared conventional ß1-sheet aggregates by rapid precipitation from the residual peptides in the higher pH supernatant. By comparing the isotope-enhanced IR and VCD spectra with theoretical predictions, we deduced that the oligo-Glu ß2 structure is based on stacked, twisted, antiparallel ß-sheets. The best fit to theoretical predictions was obtained for the strands being out of register, sequentially stepped by one residue, in a ladder-like fashion. The alternate ß1 conformer for this oligopeptide was similarly shown to be antiparallel but was less ordered and apparently had a different registry in its aggregate structure.

T1BT* structural study of an anti-plasmodial peptide through NMR and molecular dynamics.

BACKGROUND: T1BT* is a peptide construct containing the T1 and B epitopes located in the 5' minor repeat and the 3' major repeat of the central repeat region of the Plasmodium falciparum circumsporozoite protein (CSP), respectively, and the universal T* epitope located in the C-terminus of the same protein. This peptide construct, with B = (NANP)3, has been found to elicit antisporozoite antibodies and gamma-interferon-screening T-cell responses in inbred strains of mice and in outbred nonhuman primates. On the other hand, NMR and CD spectroscopies have identified the peptide B' = (NPNA)3 as the structural unit of the major repeat in the CSP, rather than the more commonly quoted NANP. With the goal of assessing the structural impact of the NPNA cadence on a proven anti-plasmodial peptide, the solution structures of T1BT* and T1B'T* were determined in this work. METHODS: NMR spectroscopy and molecular dynamics calculations were used to determine the solution structures of T1BT* and T1B'T*. These structures were compared to determine the main differences and similarities between them. RESULTS: Both peptides exhibit radically different structures, with the T1B'T* showing strong helical tendencies. NMR and CD data, in conjunction with molecular modelling, provide additional information about the topologies of T1BT* and T1B'T*. Knowing the peptide structures required to elicit the proper immunogenic response can help in the design of more effective, conformationally defined malaria vaccine candidates. If peptides derived from the CSP are required to have helical structures to interact efficiently with their corresponding antibodies, a vaccine based on the T1B'T* construct should show higher efficiency as a pre-erythrocyte vaccine that would prevent infection of hepatocytes by sporozoites.

Quantitative Predictions and Experimental Validation of Liquid-Vapor Interfacial Tension in Binary and Ternary Mixtures of Alkanes Using Molecular Dynamics Simulations.

Liquid-vapor interfacial properties of alkane mixtures present a challenge for experimental determination, especially under conditions relevant to the energy industry processes. Molecular dynamics (MD) simulations can accurately predict interfacial tensions (IFTs) for complex alkane mixtures under virtually any conditions, thereby alleviating the need for difficult and costly experiments. MD simulations with the CHARMM force field and empirical corrections for the IFT and pressure were used to obtain the IFT for three binary mixtures of ethane (with n-pentane, n-hexane, and n-nonane) and a ternary system (ethane/n-butane/n-decane) under a variety of conditions. The results were thoroughly validated against experimental data from the literature, and new original IFT data were collected using the pendant drop method. The simulations are able to reproduce the experimental IFT to better than 0.5 mN/m or 5% on average and within 1 mN/m or 10% in the worst case. IFTs for the studied three binary and ternary alkane mixtures were predicted for wide ranges of conditions with no known experimental data. Finally, using the MD simulation data, the reliability of the widely used empirical parachor model for predicting IFT was reaffirmed, and the significance of the empirical parameters examined to establish an optimal balance between the accuracy and broad applicability of the model.

Unusual cold denaturation of a small protein domain.

A thermal unfolding study of the 45-residue α-helical domain UBA(2) using circular dichroism is presented. The protein is highly thermostable and exhibits a clear cold unfolding transition with the onset near 290 K without denaturant. Cold denaturation in proteins is rarely observed in general and is quite unique among small helical protein domains. The cold unfolding was further investigated in urea solutions, and a simple thermodynamic model was used to fit all thermal and urea unfolding data. The resulting thermodynamic parameters are compared to those of other small protein domains. Possible origins of the unusual cold unfolding of UBA(2) are discussed.

Dynamics of protein folding: probing the kinetic network of folding-unfolding transitions with experiment and theory.

The problem of spontaneous folding of amino acid chains into highly organized, biologically functional three-dimensional protein structures continues to challenge the modern science. Understanding how proteins fold requires characterization of the underlying energy landscapes as well as the dynamics of the polypeptide chains in all stages of the folding process. In recent years, important advances toward these goals have been achieved owing to the rapidly growing interdisciplinary interest and significant progress in both experimental techniques and theoretical methods. Improvements in the experimental time resolution led to determination of the timescales of the important elementary events in folding, such as formation of secondary structure and tertiary contacts. Sensitive single molecule methods made possible probing the distributions of the unfolded and folded states and following the folding reaction of individual protein molecules. Discovery of proteins that fold in microseconds opened the possibility of atomic-level theoretical simulations of folding and their direct comparisons with experimental data, as well as of direct experimental observation of the barrier-less folding transition. The ultra-fast folding also brought new questions, concerning the intrinsic limits of the folding rates and experimental signatures of barrier-less "downhill" folding. These problems will require novel approaches for even more detailed experimental investigations of the folding dynamics as well as for the analysis of the folding kinetic data. For theoretical simulations of folding, a main challenge is how to extract the relevant information from overwhelmingly detailed atomistic trajectories. New theoretical methods have been devised to allow a systematic approach towards a quantitative analysis of the kinetic network of folding-unfolding transitions between various configuration states of a protein, revealing the transition states and the associated folding pathways at multiple levels, from atomistic to coarse-grained representations. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.

Single-molecule spectroscopy of the temperature-induced collapse of unfolded proteins.

We used single-molecule FRET in combination with other biophysical methods and molecular simulations to investigate the effect of temperature on the dimensions of unfolded proteins. With single-molecule FRET, this question can be addressed even under near-native conditions, where most molecules are folded, allowing us to probe a wide range of denaturant concentrations and temperatures. We find a compaction of the unfolded state of a small cold shock protein with increasing temperature in both the presence and the absence of denaturant, with good agreement between the results from single-molecule FRET and dynamic light scattering. Although dissociation of denaturant from the polypeptide chain with increasing temperature accounts for part of the compaction, the results indicate an important role for additional temperature-dependent interactions within the unfolded chain. The observation of a collapse of a similar extent in the extremely hydrophilic, intrinsically disordered protein prothymosin alpha suggests that the hydrophobic effect is not the sole source of the underlying interactions. Circular dichroism spectroscopy and replica exchange molecular dynamics simulations in explicit water show changes in secondary structure content with increasing temperature and suggest a contribution of intramolecular hydrogen bonding to unfolded state collapse.

Multi-Probe Equilibrium Analysis of Gradual (Un)Folding Processes.

Studies of small proteins that exhibit noncooperative, gradual (un)folding can offer unique insights into the rarely accessible intermediate stages of the protein folding processes. Detailed experimental characterization of these intermediate states requires approaches that utilize multiple site-specific probes of the local structure. Isotopically edited infrared (IR) spectroscopy has emerged as a powerful methodology capable of providing such high-resolution structural information. Labeling of selected amide carbonyls with 13C results in detectable side-bands of amide I' vibrations, which are sensitive to local conformation and/or solvent exposure without introducing any significant structural perturbation to the protein. Incorporation of isotopically labeled amino acids at specific positions can be achieved by the chemical synthesis of the studied proteins. We describe the basic procedures for synthesis of 13C isotopically edited protein samples, experimental IR spectroscopic measurements and analysis of the site-specific equilibrium thermal unfolding of a small protein from the temperature-dependent IR data.

Liquid-Vapor Interfacial Tension in Alkane Mixtures: Improving Predictive Capabilities of Molecular Dynamics Simulations.

The liquid-vapor interfacial properties of hydrocarbons and their mixtures are important factors in a wide range of industrial processes and applications. Determining these properties experimentally, however, is not only practically demanding, but many important properties, such as phase densities and compositions are not directly experimentally accessible, thus requiring the development of theoretical models. Molecular dynamics (MD) simulations, by contrast, are relatively straightforward even for the most complex of mixtures and directly provide all of the microscopic quantities for the studied systems. We have previously applied MD simulations to study the liquid-vapor equilibria of mixtures of hydrocarbons and CO2 that are particularly relevant to hydrocarbon recovery from geologic formations. In this study, we explore in more detail the robustness of the simulation methods with respect to the choice of the model system parameters, investigate the accuracy of the simulations in determining the key quantities: system pressure and interfacial tension (IFT), and, finally, devise a simple correction for achieving a much closer agreement between the simulated and experimental quantities. We perform extensive MD simulations for three mixtures, propane/n-pentane, propane/n-hexane, and CO2/n-pentane, using model systems from 1000 up to 100 000 molecules, and different simulation box dimensions to test for the sensitivity to finite-size effects. The results show that changing the system size and box dimensions does not significantly impact the accuracy of the simulations. Subsequently, we examine the accuracy of the MD simulations in determining the pressure and IFT for two pure hydrocarbon systems, n-pentane and n-heptane. Finally, we propose a simple linear correction formula for the pressures and IFTs obtained from the MD simulations that closely reproduce the experimental values for single components and mixtures of hydrocarbons. Our results enable the MD simulations to provide more accurate and reliable predictions of the interfacial properties, thereby reducing the need for challenging laboratory experiments.

Wettability reversal on oil-wet calcite surfaces: Experimental and computational investigations of the effect of the hydrophobic chain length of cationic surfactants.

HYPOTHESIS: Oil recovery from carbonate reservoirs is often low, in a large part due to the oil-wet state of the constituent rocks. Cationic surfactants are among the most effective compounds capable of reversing the carbonate wettability to more water-wet, which significantly enhances oil recovery. Screening for the most effective cationic surfactants can be facilitated by studying the effects of specific molecular properties, such as the hydrophobic chain length, on the wettability reversal efficiency using molecular dynamics (MD) simulations. EXPERIMENTS AND SIMULATIONS: Wettability reversal by quaternary ammonium cationic surfactants with varying hydrophobic chain length was studied by the combination of MD simulation and experimental contact angle measurements on oil-wet calcite chips. Both experiments and simulations also featured model oils consisting of different size hydrocarbons in order to explore the potential size-specific interactions between the surfactants and oil molecules. FINDINGS: We found strong correlation between the wettability reversal and the surfactant length, with the longer surfactants universally rendering calcite surfaces more water-wet. By contrast, the wettability reversal is independent of the model oil used, implying that the effect is not due to specific hydrocarbon size. Instead, the superior wettability reversal performance of the more hydrophobic surfactants is due to their greater affinity to the oil/brine interfaces.

Surfactant-induced wettability reversal on oil-wet calcite surfaces: Experimentation and molecular dynamics simulations with scaled-charges.

HYPOTHESIS: Surfactant flooding is the leading approach for reversing the wettability of oil-wet carbonate reservoirs, which is critical for the recovery of the remaining oil. Combination of molecular dynamics (MD) simulations with experiments on simplified model systems can uncover the molecular mechanisms of wettability reversal and identify key molecular properties for systematic design of new, effective chemical formulations for the enhanced oil recovery. EXPERIMENTS/SIMULATIONS: Wettability reversal by a series of surfactant solutions was studied experimentally using contact angle measurements on aged calcite chips, and a novel MD simulation methodology with scaled-charges that provides superior description of the ionic interactions in aqueous solutions. FINDINGS: The MD simulation results were in excellent agreement with the experiments. Cationic surfactants were the most effective in reversing the calcite wettability, resulting in complete detachment of the oil from the surface. Some nonionic surfactants also altered the wettability, but to a lesser degree, while the amphoteric and anionic surfactants had no effect. From the tested cationic surfactants, the double-tailed one was the least effective, but the experiments were inconclusive due to its poor solubility. Contributions of specific interactions to the wettability reversal process and implications for the design and optimization of surfactants for the enhanced oil recovery are discussed.