Nanoconfined Amphiphile Aqueous Solutions Studied By Molecular Dynamics Simulations

The association states of amphiphiles in aqueous solutions play the fundamentalrole in various physical and biological phenomena, such as interfacialwetting/dewetting, ligand binding, drug delivery and protein aggregation ordenaturation. Whether amphiphiles are in dispersion or aggregation states directlyinfluences many physical or biological processes. For instance, dispersed surfactantsare easy to adsorb at the liquid-solid interface thus reduce the interfacial tension andenable the wettability of contaminants; Aggregated phospholipids can form variousself-assembled structures, especially the bilayer membrane in cell; Graphene oxide indispersion allows the quenching of DNA probes labeled with dye and can be appliedinto the detection of DNA in biological systems.Generally, solute molecules can stay in either stable dispersion or stableaggregation states at the macroscopic scale. Nanoscale systems always exhibitinteresting phenomena different from that in bulk, such as the extra fast water flow inthe carbon nanotube, gating effects of the carbon nanotube and the incompletewettability of surface. As confinement environment at the nanoscale is widely exist innature including nanopores in the rock or minerals, space between the functionalproteins in cells or tissue and gaps in soil, questions are raised: What are associationstates of amphiphiles in the nanoconfined aqueous solutions? Are there anydifferences of behaviors of solute in between the nanoscale and bulk?In this thesis, we select amphiphilic molecules, pentanol and hexanol, solvated inwater in nanoconfined geometry as the archetypical example and investigatedynamical association behaviors of solute in the nanoconfined aqueous solutions.Using molecular dynamics simulations, we find a reversible transition between thedispersion state and aggregation state in the aqueous solutions confined in thenanoscale geometry, which is not observed in the macroscopic systems. Thenanoscale geometry also leads to a significant increase of critical aggregationconcentration, above which the stable aggregation state appears. To further understand mechnisms underlying these phenomena, firstly, we develop an idealmodel describing the association states of amphiphiles in the nanoconfined geometry.In this model, we assume that solute molecules can be classified into two groups: onegroup consisting of molecules aggregated into a cluster, and the other groupconsisting of dispersed molecules. The cluster is treated as a spherical aggregatewhere hydrophobic tails of amphiphiles densely pack in the internal region, and bothhydrophobic tails and hydrophilic heads occupy the cluster surface. Thus, theGibbs-free-energy of the system is the sum of contributions from the cluster anddispersed molecules outside the cluster. Secondly, by analyzing the systemGibbs-free-energy according to our model, we further find that the Gibbs-free-energycurve shows from a single minimum, through two minima, finally to a singleminimum again as the increase of solute concentration. This trend Gibb-free-energychanges with ensures that the association states of solute molecules transform fromthe stable dispersion, through “reversible state transition” and finally to stableaggregation states. It indicates that the reversible state transition is attributed to thelow free energy barrier (of order kBT) in between two energy minima correspondingto the dispersion and aggregation states. The enhancement of the critical aggregationconcentration results from the fact that at lower concentrations the number of solutemolecules is not large enough to allow the formation of a stable cluster in theconfined systems.It should be amphasized that we add the Tolman length related correction termsinto the general Gibbs-free-energy formule of a cluster in macroscopic aqueoussolutions to characterize the deviation of microscopic interfacial tension from themacroscopic value due to the curvature of an interface. Moreover, we introduce theGibbs-free-energy of dispersed solute molecules in deriving the Gibbs-free-energyformula of the solute molecules in the nanoconfined aqueous solutions. Thesemodifications expand the calculation of Gibbs-free-energy of a cluster in themacroscopic aqueous solutions into that of systems in the nanoconfined aqueoussolutions. We demonstrate the effect of nanoconfinements on the solute association states indetail and find that the dispersion or aggregation of solute molecules will drasticallyinfluence the concentration of solute molecules dispersed around the clustersubsequently results in the “reversible state transtion” where the solute molecules canswitch between the dispersion and aggregation states. These findings enrich the theoryof the association behavior of the solute molecules to the nanoscale and may giveinsights into the fields relevant to the dissolution property of material in confinedaqueous environment, such as the drug absorption, toxicity of nanomaterial, oilextraction and restoration of carbon element in soil.