Chem.Engg. R Kumar Distinguished Award Lecture Series. talk-3 Prof. Arthi Jayaraman

Multiscale molecular dynamics simulations to understand the effect of the extent & pattern of methylation on structures within aqueous methylcellulose solutions. 

Methylcellulose (MC) is a cellulose derivative where some or all of the three hydroxyl groups (-OH) are replaced by methoxy (-OCH3) groups. The degree of substitution, DS, quantifies the extent of methylation in MC chains; DS = 0 represents cellulose and DS = 3 represents complete methylation in all three positions. Commercial MC chains have a random pattern of methylation and DS ~ 1.8. These MC chains are soluble in water at low temperatures and form a gel network of semi-flexible fibrils at temperatures above 323K. The MC chains’ fibrillar structure imparts favorable physical properties that make these materials ideal for applications as thickeners or stabilizers in food and drug formulations. To characterize the MC chains’ fibril structure in solution, researchers have conducted small angle scattering experiments that revealed that MC fibrils exhibit a consistent diameter across varying chain concentrations and molecular weights. [1] To understand why MC chains exhibit consistent fibril diameters regardless of concentration and molecular weights, we used coarse-grained (CG) molecular dynamics (MD) simulations to study assembly of MC chains from a dispersed state into fibrillar state. We found that the MC chains align in parallel as they assemble into semi-flexible fibrils with some MC chains on the outside of the fibril adopting twisted conformations due to hydrophobic interactions. [2] Given this understanding of commercial MC solutions, we are now answering two questions– 1. Is there a benefit to go beyond commercial MC chains, and tailor the pattern of substitution (random, homogeneous, blocky) and value of DS? 2. How do these parameters – patterns of substitution and DS – affect the MC chains’ assembly? – using a multi-scale simulation approach. First, in atomistic (AA) MD simulations of pre-assembled MC fibrils with explicit water we find that increasing DS and/or temperature promotes twisting of MC chains and changes water arrangement within and around fibrils. Next, we take AA MD configurations to optimize CG models of MC chains at varying DS and patterns of substitutions. Using these optimized CG models, we are simulating MC chains’ assembly and showing how varying DS and patterns of substitution affect fibril diameters and assembly with increasing temperature.