Linking the Continuum and Molecular Scales of Adsorption Modeling for Non-Ionic Small Molecules and Homopolymers

surfactant;rare event sampling;molecular simulation;transport phenomena;free energy calculation;Chemical Engineering;Engineering;Chemical Engineering

Huston, KyleZiff, Robert M ;

University of Michigan

关键词: surfactant;    rare event sampling;    molecular simulation;    transport phenomena;    free energy calculation;    Chemical Engineering;    Engineering;    Chemical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/144112/khuston_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Computational tensiometry and other quantitative adsorption predictions for small molecules and polymers are possible in the foreseeable future, but first, the application of the techniques to surfactant adsorption must be developed, and basic research is needed to identify the set of minimally required features of the molecular model that permits quantitative prediction. We take up the first challenge and apply three methods to three adsorption problems.In the first approach, we simulate poly(ethylene oxide) (PEO) oligomers and a model Tween 80 (polyoxyethylene sorbitan monooleate) molecule at the water/alkane interface. We use the weighted histogram analysis method (WHAM) to calculate interfacial potentials of mean force (PMFs) for PEG and Tween 80 using the atomistic GROMOS 53a6OXY+D and two coarse-grained (CG) MARTINI force fields. Because the force fields have not yet been validated for PEO adsorption to hydrophobic interfaces, we calculate PMFs for alcohol ethoxylates C12E2 and C12E8 and find agreement for the atomistic forcefield with reported semiempirical results, whereas for both CG force fields, PEO adsorbs too weakly to the hydrophobic interface. With the newly validated atomistic force field, we bracket the dilute adsorption free energy for a model Tween 80 molecule at the clean water/squalane interface. We also calculate the pressure–area isotherm and—with molecular thermodynamic theory and a simple transport model—demonstrate the transition from irreversible to reversible adsorption with increasing surface coverage, consistent with past experimental reporting. In the second approach, we sought to explain experiments that show relaxation of oil/water interfacial tension by adsorption of alkyl ethoxylate surfactants from water is delayed relative to diffusion-controlled adsorption. We examine possible causes of this delay. We argue that a theory implicating transient depletion near an adsorbing interface for suppressing interfacial relaxation is invalid. We find that re-dissolution of the surfactant in the oil droplet cannot explain the apparent interfacial resistance at short times. We also perform WHAM with molecular dynamics simulation and do not find any evidence of an energy barrier or low-diffusivity zone near the interface. Nor do we find evidence from simulation that pre-micellar aggregation slows diffusion enough to cause the observed resistance to interfacial adsorption. We are therefore unable to pinpoint the cause of the resistance, but we suggest that ;;dead time” associated with the experimental method could be responsible – specifically local depletion of surfactant by the ejected droplet when creating the fresh oil/water interface.In the third approach, we compute desorption rates for isolated polymers stuck to a solid wall with forward flux sampling (FFS). We interpret computed rates on the basis of a conjecture that a dimensionless desorption time scales with the equilibrium ratio of adsorbed surface amount to bulk concentration. We find that the dimensionless desorption time approaches the expected exponential scaling with the degree of polymerization multiplied by the mean field interaction between the monomer and the wall. However, we also find this strong adsorption scaling only becomes accurate for polymers which adsorb irreversibly on realistic timescales. We also find that excluded volume interactions and bending angle potentials shrink the desorption time and weaken the scaling of desorption time with N. For sufficiently weakly-adsorbing chains, the dimensionless desorption time becomes independent of N, suggesting a diffusion-controlled process overtakes the detachment process in importance.