Carlos Vega

I do not believe my biography is particularly interesting, but I hope that it may at least be entertaining, and that anyone reading it may find in it at least one sentence that proves inspiring, especially those researchers who are just beginning their scientific careers. It is only fair to acknowledge that life is an adventure, and that we never quite know what lies ahead.

Nitrate salts are widely used in agriculture, industrial chemistry, and food preservation. Using computer simulations based on the intermolecular parameters of the extended Madrid-2019 force field [V. M. Trejos et al., J. Chem. Phys. 159(22), 224501 (2023)], we examine the effect of intramolecular partial charge distributions of the nitrate anion on the temperature of maximum density (TMD) of aqueous NaNO3, KNO3, and NH4NO3 solutions. A specific internal charge distribution improves agreement with experimental TMD data relative to the original force field and enables predictions for systems lacking experimental measurements. Densities are largely insensitive to the charge distribution, while transport properties (diffusion coefficients and viscosities) show only minor variations. In contrast, structural properties are sensitive to the choice of the charge distribution. The original force field provides better agreement with neutron diffraction structural data than the model yielding the most accurate TMD. An intermediate charge distribution is therefore selected as a compromise, providing a balanced description of all properties. Empirical force fields cannot reproduce all properties simultaneously; however, an appropriate parameter selection, including the intramolecular charge distribution as an additional degree of freedom, yields a consistent description of thermodynamic, transport, and structural properties.

The melting point of water is a fundamental thermodynamic property and a key benchmark for molecular simulations of phase behavior. Here, we estimate the melting temperature of the TIP4P/2005 and TIP4P/Ice models at ambient pressure using the direct coexistence method. We simulate systems containing up to 8640 water molecules and treat long-range interactions with the particle mesh Ewald approach for both electrostatics and dispersion [Lennard-Jones Particle-Mesh Ewald (LJ-PME)], thereby substantially reducing finite-size and cutoff-truncation artifacts. We find that, once these effects are properly accounted for, the melting point of both models increases by nearly 1 K relative to the values obtained from truncating the dispersion interactions at 8.5 Å. The resulting melting temperatures are 250.25 and 270.75 K for the TIP4P/2005 and TIP4P/Ice water models, respectively, thereby bringing the latter significantly closer to experiment. We also evaluate melting enthalpies at coexistence and find them largely insensitive to system size and the long-range treatment. Overall, these results highlight the importance of accurate long-range interactions in phase-coexistence simulations of water.

The long-standing classification of ions as water “structure-makers” or “structure-breakers” is fundamentally reevaluated through advanced Molecular Dynamics simulations employing the accurate Madrid-2019 force field. We quantitatively study the local hydration structures of 1 m aqueous solutions of LiCl, NaCl, and KCl (through local entropy and Kullback-Leibler (KL)-divergence analysis). We demonstrate that all ions act as net disruptors of water’s tetrahedral network, this change being more severe in solutions with high charge density ions. While hydrogen bonding (HB) is diminished within the first hydration shell (FHS) of ions, it returns to bulk-like levels beyond it. From an energetic viewpoint, water–water interactions in solution are destabilized by about 15% compared to pure water, confirming all ions as “structure-breakers” in this regard. Two factors contribute to the energy loss of water: Roughly half arises from a 5% reduction in the total number of HBs, localized primarily within the ions’ hydration shells (plus a small contribution from HB distortion); whereas the other half originates from the destabilization of dipole–dipole correlations beyond the FHS. This latter effect, which scales with ionic charge density, is absent for uncharged solutes. Finally, we identify a correlation between the total energy of water (including the ion–water contribution) in solution and the Jones-Dole B coefficient—a correlation that also holds for divalent cations and hydrophobic solutes. The transition from “structure-maker” to “structure-breaker” (i.e., the change of sign in the B coefficient) occurs near the locus where the total energy of water in the solution is similar to that of pure water.

Perchloric acid (HClO4) is widely used to prepare perchlorate salts with applications in propellants, industry, environmental chemistry, and biology. In this work, we used the intermolecular parameters from the extended Madrid-2019 force field for the perchlorate anion (ClO4-) and the oxonium cation (H3O+) together with TIP4P/2005 water to model perchloric acid solutions. The force field uses scaled charges of ±0.85e for monovalent ions and has been widely applied for aqueous ionic systems. We used the model to predict thermodynamic properties [densities and temperatures of maximum in density (TMD)], structural features (ion-water correlations: ion-hydrogen and ion-oxygen), and transport properties (self-diffusion coefficients and viscosity) of perchloric acid solutions at several concentrations. Experimental densities are predicted in excellent agreement up to 10 m. We also performed molecular simulations over a wide range of temperatures in order to determine the TMD of perchloric acid at different mol lities. Predicted viscosities at 298.15 K and 1 bar are in good agreement with experimental data for concentrations below 4 m. Results are discussed in terms of model strengths and limitations.

Gas hydrates are of great relevance to both the oil industry and the environment. Understanding how these solid structures nucleate from aqueous solutions is essential for controlling their formation. Experimental studies have often suggested that hydrate nucleation originates at the interface between the aqueous phase and the guest-molecule reservoir. To assess this hypothesis, we perform molecular dynamics simulations of CO₂ hydrate nucleation. First, we place hydrate seeds at different positions relative to the interface and monitor their evolution, finding that seeds embedded in the bulk are more likely to grow than those located near or at the interface. Second, we analyze spontaneous nucleation simulations with and without an interface. Our previous work showed that nucleation rates are indistinguishable in both systems, strongly indicating that the interface does not play a role. Here, trajectory analysis reveals that hydrates nucleate in regions of locally high CO₂ concentration, which arise spontaneously in the bulk and are not associated with the interface. Our results indicate that hydrate nucleation does not preferentially occur at the interface, at least under the deep supercooling conditions explored in this work. Further work at higher temperatures, and considering alternative nucleation locations, is needed to reconcile experiments and simulations, and thereby reach a deep understanding of the mechanism of hydrate formation.

Electrolyte solutions at high concentration are indispensable and yet poorly understood. In particular, the extent of speciation─the formation of complexes composed of multiple species─in concentrated ionic solutions is very challenging to obtain theoretically and experimentally, but can have a strong effect on solution properties. The literature is rife with contradictory estimates of speciation from experiments. We find that speciation affects transport properties and is therefore a prerequisite to accurately model concentrated solutions. We turn this to our advantage by showing that the viscosity can be used to determine the extent of complexation in concentrated aqueous solutions. Results of simulations as well as experimental measurements are presented. The atomistic Madrid-2019 force field is extended to model FeCl₂. Solutions of FeCl₂ and MgCl₂ are compared, and the observed difference in viscosity is explained by more complexation in the former, a conclusion supported by recently reported X-ray absorption and neutron scattering experiments.

In this work, we evaluate the interfacial free energy, γ, between ice III and liquid water along the coexistence line for the TIP4P/Ice model using the mold integration technique. The calculated γ values exceed 40 mJ/m2 across all the studied pressures. We observe a non-monotonic pressure dependence with a minimum appearing near 4000 bar, analogously to that observed for hexagonal ice at negative pressures. Furthermore, the interfacial free energy was determined for two different crystal planes at one pressure, revealing an anisotropy of less than 1%.

Crystal nucleation studies using hard-sphere and Lennard-Jones models have shown that the actual (mechanical) pressure within the nucleus is lower than that in the surrounding liquid. Here, we use the mechanical route to obtain the pressure for an ice nucleus in supercooled water (TIP4P/Ice) at 1 bar and 247 K. From this pressure, we obtain the interfacial stress using a thermodynamic definition consistent with mechanical arguments. Moreover, we compare the mechanical pressure with the thermodynamic pressure of bulk ice at an equal chemical potential and the interfacial stress with the interfacial free energy. Furthermore, we investigate these properties on the basal plane. We find that unlike in hard-sphere and Lennard-Jones systems, mechanical and thermodynamic pressures agree for the nucleus, and the interfacial stress and free energy are comparable. However, the basal interface displays an interfacial stress nearly twice its interfacial free energy, suggesting that this agreement may be dependent on the system, underscoring the limitations of mechanical routes to solid-liquid interfacial free energies.

Carbon dioxide (CO2) hydrates hold promising applications in capturing and separating CO2 for climate change mitigation. Understanding their behavior at the molecular level is, therefore, essential, and computer simulations have become powerful tools for exploring their formation and stability, providing valuable insights into their underlying mechanisms. In this work, we perform molecular dynamics simulations to compute the three-phase coexistence line involving the stability region where CO2 is in the vapor phase: CO2 hydrate–liquid water–vapor. This computation was previously inaccessible using the traditional three-phase direct coexistence technique. To achieve this, we employ a novel solubility-based method, which allows us to accurately evaluate the coexistence line. Our results exhibit excellent agreement with experimental data and, for the first time, accurately reproduce the hydrate–liquid–vapor equilibrium line of the CO2–water phase diagram. Finally, we have determined the upper quadruple point (Q2), where the four phases, namely hydrate, liquid water, liquid CO2, and vapor, coexist. Our pioneering result for the Q2 value shows remarkable agreement with experimental observations, validating the accuracy of our findings and representing a significant milestone in the field of gas hydrate research.