Sriteja Mantha

Sriteja Mantha

Postdoctoral Research Associate, Prof. Zhen-Gang Wang Research Group, Caltech

Talks and presentations

[1] Structure of polyelectrolyte and multi-valent ion complexes

March 06, 2023

Conference proceedings talk, American Physical Society March Meeting, Las Vegas, NV

Experiments have established the ability of aqueous polyelectrolytes to passivate and delay the crystallization of limescale (CaCO3). Polyelectrolytes are expected to influence the CaCO3 crystallization by chelating Ca2+ ions from the solution and modifying the crystal growth by preferentially adsorbing to certain crystal surfaces. These processes may potentially delay the onset of nucleation and crystal growth, respectively. However, the mechanism through which polyelectrolytes operate is currently unknown. The binding of a Ca2+ to a charged residue on the polymer backbone is strongly dependent on the charge state of a polymer and its conformation, which are affected by the pH, concentration of Ca2+, and the overall ionic strength of the solution. The ability of a polyelectrolyte to sequester free Ca2+ depends on the polyelectrolyte-Ca2+ binding energy. We conduct molecular dynamics simulations with enhanced sampling techniques to study the interaction of Ca2+ with various polyanions in an aqueous suspension. We will present our findings on the polyelectrolyte-Ca2+ binding energetics, comment on the preferential Ca2+ binding sites on a polyelectrolyte backbone, and discuss their impact on the backbone conformations.

[2] Divalent cation-mediated polyanion attraction in an aqueous solution

March 06, 2023

Conference proceedings talk, American Physical Society March Meeting, Las Vegas, NV

Polyelectrolytes are known to passivate/delay limescale (CaCO3) crystallization from aqueous suspension. Negatively charged functional groups along the polymer backbone chelate cations and form both intrachain and interchain ion bridges. At sufficiently high ionic strengths, these ion bridges lead to attractive interactions between the polyanions and precipitate polymer-ion complexes out of suspension. Polyelectrolyte effectiveness in preventing scale formation depends on its ability to chelate more Ca2+ ions before precipitation. The critical Ca2+ concentration is known as the Ca-tolerance of the polyelectrolyte. Our objective is to design polyelectrolytes with higher Ca-tolerance. We use the thermodynamic stability criterion and relate the Ca-tolerance of a polyelectrolyte to the potential of mean force (PMF) between two polyelectrolyte chains in an aqueous Ca2+ salt solution. We employ well-tempered metadynamics and Hamiltonian replica exchange protocols to calculate the two-chain PMF from molecular simulations. We systematically characterize the effect of solution ionic strength, temperature, functional groups, and molecular weight of the polyelectrolyte on the two-chain PMF. We will clarify the mechanism through which multi-valent ions result in the precipitation of the polyelectrolyte and present design principles for polyelectrolytes with higher Ca-tolerance values.

[3] Bubble nucleation in the surfactant stabilized polyol-CO2 mixtures: Insights from a classical density function theory study

November 06, 2022

Conference proceedings talk, American Institute of Chemical Engineers Annual Meeting, Las Vegas, NV

Polyol based foams are widely sought-after materials for manufacturing thermal insulators, high resilience foam seating, adhesives, hard plastics for electronic instruments, etc. The polyol foams are produced by the reaction of di-isocyanate with polyol to form polyurethane and water. Some amount of isocyanate reacts with water to produce CO2. The generated CO2 nucleates into bubbles within the polymer matrix, forming a foam. Silicone-polyether(SPE) surfactants, made of poly dimethyl-siloxane backbone and polyether branches, are commonly used to stabilize the foam formulation. The SPE surfactants reduce interfacial tension between polyol-CO2 interface, promotes bubble generation and impacts the foam cell size. It is also known that the composition of SPE surfactant significantly influences its role in stabilizing the foam formulation as well. However, the physical mechanism of how these SPE surfactants affect the nucleation and stability of the bubbles is not well understood. In this talk, using classical density functional theory models, we propose design principles for SPE surfactants and elucidate the mechanism through which they lead to foams with improved physical properties.

[4] Bubble nucleation in the surfactant stabilized polyol-CO2 mixtures: Insights from a classical density function theory study

March 06, 2022

Conference proceedings talk, American Physical Society March Meeting, Las Vegas, NV

Silicone-polyether (SPE) surfactants, made of poly dimethyl-siloxane backbone and polyether branches, are commonly used to stabilize the polyol-CO2 foam formulation. The SPE surfactants reduce interfacial tension between polyol-CO2 interface, promote bubble generation and impacts the foam cell size. However, the mechanism through which SPE surfactants affect the nucleation and stability of the bubbles is not well understood. We find that increase in the CO2 concentration in the polyol+CO2+SPE system leads to significant decrease in the SPE CMC. This underscores the importance of CO2 bubble nucleation from a pre-formed micelle. In this work using classical density functional theory we will compare and contrast our observations on the CO2 bubble nucleation from a homogeneous polyol+CO2+SPE mixture and from a preformed SPE micelle in the system.

[5] Bubble nucleation in the surfactant stabilized polyol-CO2 mixtures: Insights from a classical density function theory study

March 06, 2021

Conference proceedings talk, American Physical Society March Meeting, Las Vegas, NV

Polyol based foams are widely sought-after materials for manufacturing thermal insulators, high resilience foam seating, adhesives, hard plastics for electronic instruments, etc. The polyol foams are produced by the reaction of di-isocyanate with polyol to form polyurethane and water. Some amount of isocyanate reacts with water to produce CO2. The generated CO2nucleates into bubbles within the polymer matrix, forming a foam. Silicone surfactants, made of poly dimethyl-siloxane backbone and polyether branches, are commonly used to stabilize the foam formulation. The silicone surfactants reduce interfacial tension between polyol-CO2interface, promotes bubble generation and impacts the foam cell size. It is also known that the composition of silicone surfactant significantly influences its role in stabilizing the foam formulation as well. However, the physical mechanism of how these silicone surfactants affect the nucleation and stability of the bubbles is not well understood. In this talk, using classical density functional theory models, we propose design principles for silicone surfactants and elucidate the mechanism through which they lead to foams with improved physical properties.

[6] Systematic construction of the dynamic density functional theory for inhomogeneous polymer systems

March 05, 2020

Conference proceedings talk, American Physical Society March Meeting, Denver, CO

Time scales predicted by the dynamic density functional theory (DDFT) for an inhomogeneous polymer system are far from accurate. One of the main reasons for this is, approximate local and non-local schemes employed to compute the mobility coefficient,Λαβ(r,r’) . In the DDFT calculations,Λαβ(r,r’), relates the thermodynamic driving force due to the monomer β at r’ to the current of the monomer α at r . In this talk, we will put forward a physically motivated approach to compute the Λαβ(r,r’) with the objective to improve the DDFT predictions. We compute the Λαβ(r,r’) from the relaxation time of the single chain dynamic structure factor. We find that the Λαβ(r,r’) obtained from such an approach captures both the global dynamics and the effective local rearrangements of the chain at relevant length scales. Using this scheme, we conduct DDFT calculations to study two related problems. One is the formation of the lamellar morphology in a symmetric diblock copolymer system starting from a homogeneously dispersed state, and the other is the relaxation of the lamellar morphology into a homogeneously dispersed state. We show that the DDFT predictions for the above problems are in reasonably good agreement with the corresponding fine-grained simulations.

[7] Increasing block copolymer dispersity leads to more uniform micelles

March 05, 2019

Conference proceedings talk, American Physical Society March Meeting, Denver, CO

Synthetic polymers posses some inherent dispersity in their length due to the mechanism of the underlying polymerization reaction. Since nearly every property of the polymers depend strongly on the length of the chain, it is expected that the polymer chain dispersity effects different structural, dynamic and their self-assembly properties in the solution as well as in the melt conditions. In this work we investigate the effect of amphiphilic diblock copolymer chain length dispersity on the size distribution of the spherical micelles formed by them in the solution. Using self-consistent field theory calculations, we show that the monodisperse diblock copolymers form micelles of different sizes in the solution, whereas polydisperse diblock copolymers form micelles which are uniform in size. We attribute this to the fact that the packing of the solvophobic monomers in the micellar core can be optimized if the constituent polymers have different length.

[8] Effect of Polymer Chain Polydispersity on the Size of Spherical Micelles Formed in Solution

March 05, 2018

Conference proceedings talk, American Physical Society March Meeting, Denver, CO

Amphiphilic molecules in solution display a rich diversity of micellar morphologies. Micellar structures and their size distribution are expected to depend on molecular parameters like, chain length of amphiphilic molecules, the solvophobic to solvophilic ratio, the intermolecular interactions etc. In this work we investigate the effect of polymer length polydispersity on the size of spherical micelles formed by diblock copolymers in solution. Using self-consistent field theoretic simulations, we show that monodisperse polymers favor formation of micelles of different sizes, whereas polydisperse polymers favor the formation of micelles with monodisperse size distribution. Differences in the free energetic contributions associated with the chain stretching explains above differences in the size of micelles formed by monodisperse and polydisperse diblock copolymers in solution. In the micelles formed by monodisperse polymers, chains are stretched to different lengths to accommodate micelles of different sizes. On the other hand such a chain stretching is found to be very narrow in micelles formed by polydisperse polymers.