Max Moncada Cohen

All Publications

• Mechanisms of the viscosity decrease and increase of aqueous CsCl. Proceedings of the National Academy of Sciences of the United States of America Cohen, M. M., Kacenauskaite, L., Heck, T. R., Pan, J., Fayer, M. D. 2026; 123 (2): e2531278123

  Abstract

  Aqueous salt solutions occur in many aspects of chemistry, biology, and geology. Increasing the concentration of most aqueous salt solutions increases the viscosities. In contrast, adding CsCl to water initially decreases the viscosity, but at moderate concentration further addition increases it. While this phenomenon is well known, the molecular mechanisms for the reduction and increase have not been elucidated. We used ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE) and IR pump-probe experiments, as well as density functional theory to investigate the impact of Cs+ ions on water dynamics, interactions, and structure. OHD-OKE experiments demonstrated that the dynamics of the water hydrogen bond (H-bond) network underpin the viscosity of CsCl solutions. Transient IR spectra of HOD in H2O interacting with Cs+ showed a significant blue shift, a hallmark of hydrogen bonds weaker than those of pure water. Due to its low charge density, Cs+ is distinct from high charge density cations, e.g., Na+ and Li+, which have been observed to strengthen water hydrogen bonds and drive a large, monotonic increase in viscosity with concentration. The results showed that water hydrogen bonds in the Cs+ second solvation shell are weaker than typical water-water hydrogen bonds, and these weak hydrogen bonds give rise to faster collective structural dynamics, leading to reduced viscosity. However, at sufficiently high salt concentrations, the low number of water molecules per ion pair leads to water clusters. Water confined in small clusters slows H-bond rearrangement, leading to an increase in viscosity.

  View details for DOI 10.1073/pnas.2531278123

  View details for PubMedID 41512023

• From Molecular Dynamics to the Conductivity of Sulfuric Acid: Ultrafast Optical Kerr Effect Experiments and Ab Initio Molecular Dynamics Simulations. Journal of the American Chemical Society Kacenauskaite, L., Chen, S., Cohen, M. M., Voth, G. A., Fayer, M. D. 2025; 147 (31): 27720-27730

  Abstract

  Extensive use of sulfuric acid in technological applications calls for knowledge of its molecular scale properties. Here, we report a study of aqueous sulfuric acid solutions across a broad concentration range using optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments and ab initio molecular dynamics (AIMD) simulations. The OHD-OKE experiments measured the time derivative of the polarizability-polarizability correlation function (PPCF). By comparison of distinct components of the OKE signal to the excess proton identity correlation functions calculated from AIMD simulations, it was found that the experimental t3 components quantitatively agreed with the proton hopping time from one water to another, which suggested that the origin of t3 was associated with proton hopping. The proton hopping distances within the t3 time scale were used to determine the proton hopping diffusion constants at several concentrations. Using information from the literature and the Nernst-Einstein conductivity equation, it was shown that the vehicular mechanism was insufficient to describe the conductivity. The experimental concentration-dependent conductivities were reproduced by adding the AIMD proton hopping contribution to the conductivity, and proton hopping was shown to be the dominant component of the conductivity. Finally, the experimental concentration-dependent function, C3/t3, where C3 is the amplitude coefficient of the third PPCF component, closely tracked the concentration dependence of the hopping component of the conductivity.

  View details for DOI 10.1021/jacs.5c06194

  View details for PubMedID 40770737

• Fast Structural Dynamics in Concentrated HCl Solutions: From Proton Hopping to the Bulk Viscosity. Journal of the American Chemical Society Kacenauskaite, L., Moncada Cohen, M., Van Wyck, S. J., Fayer, M. D. 2024

  Abstract

  Concentrated acid solutions, particularly HCl, have been studied extensively to examine the proton hopping and infrared spectral signatures of hydronium ions. Much less attention has been given to the structural dynamics of concentrated HCl solutions. Here, we apply optical heterodyne detected-optical Kerr effect (OHD-OKE) measurements to examine HCl concentration-dependent dynamics from moderate (0.8 m) to very high (15.5 m) concentrations and compare the results to the dynamics of NaCl solutions, as Na+ is similar in size to the hydronium cation. Both HCl and NaCl OHD-OKE signals decay as triexponentials at all concentrations, in contrast to pure water, which decays as a biexponential. Two remarkable features of the HCl dynamics are the following: (1) the bulk viscosity is linearly related to the slowest decay constant, t3, and (2) the concentration-dependent proton hopping times, determined by ab initio MD simulations and 2D IR chemical exchange experiments, both obtained from the literature, fall on the same line as the slowest structural dynamics relaxation time, t3, within experimental error. The structural dynamics of hydronium/chloride/water clusters, with relaxation times t3, are responsible for the concentration dependence of microscopic property of proton hopping and the macroscopic bulk viscosity. The slowest time constant (t3), which does not have a counterpart in pure water, is 3 ps at 0.8 m and increases by a factor of ∼2 by 15.5 m. The two fastest HCl decay constants, t1 and t2, are similar to those of pure water and increase mildly with the concentration.

  View details for DOI 10.1021/jacs.3c11620

  View details for PubMedID 38682723

• Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties. The journal of physical chemistry. B Kacenauskaite, L., Van Wyck, S. J., Moncada Cohen, M., Fayer, M. D. 2023

  Abstract

  We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetra-exponentials. The fastest decay (t1) is the same as pure water's at all concentrations within error, while the second component (t2) slows slightly with concentration. The slower components (t3 and t4), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperature-dependent studies (15-95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τcslow, the correlation time of the slow dynamics (weighted averages of t3 and t4). Remarkably, all four viscosity vs 1/τCslow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

  View details for DOI 10.1021/acs.jpcb.3c07711

  View details for PubMedID 38118403

• Infrared compatible rapid mixer to probe millisecond chemical kinetics REVIEW OF SCIENTIFIC INSTRUMENTS Itani, R. C., Cohen, M., Tokmakoff, A. 2023; 94 (3)

  View details for DOI 10.1063/5.0121817

  View details for Web of Science ID 000943256000006