Publications

[13] Surfactant in a Polyol–CO2 Mixture: Insights from a Classical Density Functional Theory Study

Published in Langmuir, 2022

Silicone–polyether (SPE) surfactants, made of a polydimethyl-siloxane (PDMS) backbone and polyether branches, are commonly used as additives in the production of polymeric foams with improved properties. A key step in the production of polymeric foams is the nucleation of gas bubbles in the polymer matrix upon supersaturation of dissolved gas. However, the role of SPE surfactants in the nucleation of gas bubbles is not well understood. In this study, we use classical density functional theory to investigate the effect of an SPE surfactant on the nucleation of CO2 bubbles in a polyol foam formulation. We find that the addition of an SPE surfactant leads to a ∼3-fold decrease in the polyol–CO2 interfacial tension at the surfactant’s critical micelle concentration. Additionally, the surfactant is found to reduce the free energy barrier and affect the minimum free energy pathway (MFEP) associated with CO2 bubble nucleation. In the absence of a surfactant, a CO2-rich bubble nucleates from a homogeneous CO2-supersaturated polyol solution by following an MFEP characterized by a single nucleation barrier. Adding a surfactant results in a two-step nucleation process with reduced free energy barriers. The first barrier corresponds to the formation of a spherical aggregate with a liquid-like CO2 core. This spherical aggregate then grows into a CO2-rich bubble (spherical aggregate with a vapor-like CO2 core) of a critical size representing the second barrier. We hypothesize that the stronger affinity of CO2 for PDMS (than polyether) stabilizes the spherical aggregate with the liquid-like CO2 core, leading to a lower free energy barrier for CO2 bubble nucleation. Stabilization of such an aggregate during the early stages of the nucleation may lead to foams with more, smaller bubbles, which can improve their microstrustural features and insulating abilities.

Recommended citation: Sriteja Mantha, Huikuan Chao, Andrew S. Ylitalo, Thomas C. Fitzgibbons, Weijun Zhou, Valeriy V. Ginzburg, and Zhen-Gang Wang (2022). " Surfactant in a Polyol–CO2 Mixture: Insights from a Classical Density Functional Theory Study ." Langmuir . 51, 16172-16182. https://doi.org/10.1021/acs.langmuir.2c02913

[12] Bottom-up Construction of Dynamic Density Functional Theories for Inhomogeneous Polymer Systems from Microscopic Simulations

Published in Macromolecules, 2020

We propose and compare different strategies to construct dynamic density functional theories (DDFTs) for inhomogeneous polymer systems close to equilibrium from microscopic simulation trajectories. We focus on the systematic construction of the mobility coefficient, Λ(r,r′), which relates the thermodynamic driving force on monomers at position r′ to the motion of monomers at position r. A first approach based on the Green–Kubo formalism turns out to be impractical because of a severe plateau problem. Instead, we propose to extract the mobility coefficient from an effective characteristic relaxation time of the single chain dynamic structure factor. To test our approach, we study the kinetics of ordering and disordering in diblock copolymer melts. The DDFT results are in very good agreement with the data from corresponding fine-grained simulations.

Recommended citation: Sriteja Mantha, Shuanhu Qi, and Friederike Schmid (2019). " Bottom-up Construction of Dynamic Density Functional Theories for Inhomogeneous Polymer Systems from Microscopic Simulations ." Physical Review Materials . 53, 3409-3423. https://doi.org/10.1021/acs.macromol.0c00130

[11] How ill-defined constituents produce well-defined nanoparticles: Effect of polymer dispersity on the uniformity of copolymeric micelles

Published in Physical Review Materials, 2019

We investigate the effect of polymer length dispersity on the properties of self-assembled micelles in solution by self-consistent field calculations. Polydispersity stabilizes micelles by raising the free energy barriers of micelle formation and dissolution. Most importantly, it significantly reduces the size fluctuations of micelles: Block copolymers of moderate polydispersity form more uniform particles than their monodisperse counterparts. We attribute this to the fact that the packing of the solvophobic monomers in the core can be optimized if the constituent polymers have different length.

Recommended citation: Sriteja Mantha, Shuanhu Qi, Matthias Barz, and Friederike Schmid (2019). " How ill-defined constituents produce well-defined nanoparticles: Effect of polymer dispersity on the uniformity of copolymeric micelles ." Physical Review Materials . 3, 026002. https://doi.org/10.1103/PhysRevMaterials.3.026002

[10] Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals

Published in Journal of Physical Chemistry B, 2018

The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na+, K+, NMe4+) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion–water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Recommended citation: Grayson L. Jackson, Sriteja Mantha, Sung A. Kim, Souleymane O. Diallo, Kenneth W. Herwig, Arun Yethiraj, and Mahesh K. Mahanthappa (2018). " Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals ." Journal of Physical Chemistry B . 122, 10031-10043. https://doi.org/10.1021/acs.jpcb.8b05942

[9] Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies

Published in Journal of Physical Chemistry B, 2018

The dynamics of confined water is of fundamental and long-standing interest. In technologically important forms of confinement, such as proton-exchange membranes, electrostatic interactions with the confining matrix and counterions play significant roles on the properties of water. There has been recent interest on the dynamics of water confined to the lyotropic liquid crystalline (LLC) morphologies of Gemini dicarboxylate surfactants. These systems are exciting because the nature of confinement, for example, size and curvature of channels and surface functionality is dictated by the chemistry of the self-assembling surfactant molecules. Quasielastic neutron scattering experiments have shown an interesting dependence of the water self-diffusion constant, Dα, on the identity (denoted α) of the counterion: at high hydration, the magnitude of the water self-diffusion constant is in the order DTMA < DNa < DK, where TMA, Na, and K refer to tetramethyl ammonium, sodium, and potassium counterions, respectively. This sequence is similar to what is seen in bulk electrolyte solutions. At low hydrations, however, the order of water self-diffusion is different, that is, DNa < DTMA < DK. In this work, we present molecular dynamics simulations for the dynamics of water in the LLC phases of dicarboxylate Gemini surfactants. The simulations reproduce the trends seen in experiments. From an analysis of the trajectories, we hypothesize that two competing factors play a role: the volume accessible to the water molecules and the correlations between the water and the counterion. The excluded volume effect is the largest with TMA+, and the electrostatic correlation is the strongest with Na+. The observed trend is a result of which of these two effects is dominant at a given water to surfactant ratio.

Recommended citation: Sriteja Mantha, Grayson L. Jackson, Mahesh K. Mahanthappa, and Arun Yethiraj (2018). " Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies ." Journal of Physical Chemistry B . 122, 2408-2413. https://doi.org/10.1021/acs.jpcb.7b12034

[8] Electrostatic Interactions Govern “Odd/Even” Effects in Water-Induced Gemini Surfactant Self-Assembly

Published in Journal of Physical Chemistry B, 2016

Gemini surfactants comprise two single-tailed surfactants connected by a linker at or near the hydrophilic headgroup. They display a variety of water-concentration-dependent lyotropic liquid crystal morphologies that are sensitive to surfactant molecular structure and the nature of the headgroups and counterions. Recently, an interesting dependence of the aqueous-phase behavior on the length of the linker has been discovered; odd-numbered linker length surfactants exhibit characteristically different phase diagrams than even-numbered linker surfactants. In this work, we investigate this “odd/even effect” using computer simulations, focusing on experimentally studied gemini dicarboxylates with Na+ counterions, seven nonterminal carbon atoms in the tails, and either three, four, five, or six carbon atoms in the linker (denoted Na-73, Na-74, Na-75, and Na-76, respectively). We find that the relative electrostatic repulsion between headgroups in the different morphologies is correlated with the qualitative features of the experimental phase diagrams, predicting destabilization of hexagonal phases as the cylinders pack close together at low water content. Significant differences in the relative headgroup orientations of Na-74 and Na-76 compared to those of Na-73 and Na-75 surfactants lead to differences in linker–linker packing and long-range headgroup–headgroup electrostatic repulsion, which affects the delicate electrostatic balance between the hexagonal and gyroid phases. Much of the fundamental insight presented in this work is enabled by the ability to computationally construct and analyze metastable phases that are not observable in experiments.

Recommended citation: Sriteja Mantha, Jesse G. McDaniel, Dominic V. Perroni, Mahesh K. Mahanthappa, and Arun Yethiraj (2017). " Electrostatic Interactions Govern “Odd/Even” Effects in Water-Induced Gemini Surfactant Self-Assembly ." Journal of Physical Chemistry B . 121, 565-576. https://doi.org/10.1021/acs.jpcb.6b06882

[7] Dynamics of Water in Gemini Surfactant-Based Lyotropic Liquid Crystals

Published in Journal of Physical Chemistry, 2016

The dynamics of water confined to nanometer-sized domains is important in a variety of applications ranging from proton exchange membranes to crowding effects in biophysics. In this work, we study the dynamics of water in gemini surfactant-based lyotropic liquid crystals (LLCs) using molecular dynamics simulations. These systems have well characterized morphologies, for example, hexagonal, gyroid, and lamellar, and the surfaces of the confining regions can be controlled by modifying the headgroup of the surfactants. This allows one to study the effect of topology, functionalization, and interfacial curvature on the dynamics of confined water. Through analysis of the translational diffusion and rotational relaxation, we conclude that the hydration level and resulting confinement length scale is the predominate determiner of the rates of water dynamics, and other effects, namely, surface functionality and curvature, are largely secondary. This novel analysis of the water dynamics in these LLC systems provides an important comparison for previous studies of water dynamics in lipid bilayers and reverse micelles.

Recommended citation: Jesse G. McDaniel, Sriteja Mantha, and Arun Yethiraj (2016). "Dynamics of Water in Gemini Surfactant-Based Lyotropic Liquid Crystals ." Journal of Physical Chemistry B . 120, 10860-10868. https://doi.org/10.1021/acs.jpcb.6b08087

[6] Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor

Published in Journal of Chemical Physics, 2016

The properties of water under confinement are of practical and fundamental interest. In this work, we study the properties of water in the self-assembled lyotropic phases of Gemini surfactants with a focus on testing the standard analysis of quasi-elastic neutron scattering (QENS) experiments. In QENS experiments, the dynamic structure factor is measured and fit to models to extract the translational diffusion constant, DT, and rotational relaxation time, τR. We test this procedure by using simulation results for the dynamic structure factor, extracting the dynamic parameters from the fit as is typically done in experiments, and comparing the values to those directly measured in the simulations. We find that the de-coupling approximation, where the intermediate scattering function is assumed to be a product of translational and rotational contributions, is quite accurate. The jump-diffusion and isotropic rotation models, however, are not accurate when the degree of confinement is high. In particular, the exponential approximations for the intermediate scattering function fail for highly confined water and the values of DT and τR can differ from the measured value by as much as a factor of two. Other models have more fit parameters, however, and with the range of energies and wave-vectors accessible to QENS, the typical analysis appears to be the best choice. In the most confined lamellar phase, the dynamics are sufficiently slow that QENS does not access a large enough time scale.

Recommended citation: Sriteja Mantha, and Arun Yethiraj (2016). "Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor ." Journal of Chemical Physics . 144, 084504. https://doi.org/10.1063/1.4942471

[5] Conformational properties of sodium polystyrenesulfonate in water: Insights from a coarse-grained model with explicit solvent

Published in Journal of Physical Chemistry B, 2015

Polymer solutions present a significant computational challenge because chemical realism on small length scales can be important, but the polymer molecules are very large. In polyelectrolyte solutions, there is often the additional complexity that the molecules consist of hydrophobic and charged groups, which makes an accurate treatment of the solvent, water, crucial. One route to achieve this balance is through coarse-grained models where several atoms on a monomer are grouped into one interaction site. In this work, we develop a coarse grained (CG) model for sodium polystyrenesulfonate (NaPSS) in water using a methodology consistent with the MARTINI coarse-graining philosophy, where four heavy atoms are grouped into one CG site. We consider two models for water: polarizable MARTINI (POL) and big multipole water (BMW). In each case, interaction parameters for the polymer sites are obtained by matching the potential of mean force between two monomers to results of atomistic simulations. The force field based on the POL water provides a more reasonable description of polymer properties than that based on the BMW water. We study the properties of single chains using the POL force field. Fully sulfonated chains are rodlike (i.e., the root-mean-square radius of gyration, Rg, scales linearly with degree of polymerization, N). When the fraction of sulfonation, f, is 0.25 or less, the chain collapses into a cylindrical globule. For f = 0.5, pearl-necklace conformations are observed when every second monomer is sulfonated. The lifetime of a counterion around a polymer is on the order of 100 ps, suggesting that there is no counterion condensation. The model is computationally feasible and should allow one to study the effect of local chemistry on the properties of polymers in aqueous solution.

Recommended citation: Sriteja Mantha, and Arun Yethiraj (2015). "Conformational Properties of Sodium Polystyrenesulfonate in Water: Insights from a Coarse-Grained Model with Explicit Solvent ." Journal of Physical Chemistry . 119, 11010-11018. https://doi.org/10.1021/acs.jpcb.5b01700

[4] Molecular dynamics simulation of a single-stranded DNA with heterogeneous distribution of nucleobases in aqueous medium

Published in Journal of Chemical Physics, 2013

The DNA metabolic processes often involve single-stranded DNA (ss-DNA) molecules as important intermediates. In the absence of base complementarity, ss-DNAs are more flexible and interact strongly with water in aqueous media. Ss-DNA–water interactions are expected to control the conformational flexibility of the DNA strand, which in turn should influence the properties of the surrounding water molecules. We have performed room temperature molecular dynamics simulation of an aqueous solution containing the ss-DNA dodecamer, 5′-CGCGAATTCGCG-3′. The conformational flexibility of the DNA strand and the microscopic structure and ordering of water molecules around it have been explored. The simulation reveals transformation of the initial base-stacked form of the ss-DNA to a fluctuating collapsed coil-like conformation with the formation of a few non-sequentially stacked base pairs. A preliminary analysis shows further collapse of the DNA conformation in presence of additional salt (NaCl) due to screening of negative charges along the backbone by excess cations. Additionally, higher packing of water molecules within a short distance from the DNA strand is found to be associated with realignment of water molecules by breaking their regular tetrahedral ordering.

Recommended citation: Kaushik Charaborty, Sriteja Mantha, and Sanjoy Bandyopadhyay (2013). "Molecular dynamics simulation of a single-stranded DNA with heterogeneous distribution of nucleobases in aqueous medium." Journal of Chemical Physics. 139, 075103. https://doi.org/10.1063/1.4818537

Sriteja Mantha