Wetting Behavior Of Alcohol Molecules On SiO₂ Surface Studied By Molecular Dynamics Simulations

Wetting phenomenon is a common phenomenon in nature.The wettability of the liquids have a wide range of applications in self-cleaning materials,self-assembly of amphiphilic molecules,micro-electromechanical systems,oil extraction,surface treatment,and so on.The wetting behavior of liquids on solid surfaces at the nanoscale often has unique properties.For example,water molecules could form an ordered monolayer on a solid surface with particular charge patterns.In addition,a liquid droplet could form on top of the monolayer.This phenomenon is called“ordered water monolayer that does not completely wet water”.As a typical amphiphilic molecule,alcohols have a very wide range of applications.For example,ethanol is often used as a typical amphiphile as a disinfectant,fuel cell fuel,organic solution,and organic chemical,and methanol is often used to make formaldehyde.Understanding the properties of alcohol molecules on mineral surfaces has many applications such as environmental protection,biological science,and industrial production.Previous studies in our group have found that ethanol molecules could spontaneously assemble to an ordered monolayer with an ethanol droplet on top of it on a specific charged model solid surface,thus found the phenomenon of“ordered ethanol monolayer that does not completely wet ethanol”.The mechanism of this phenomenon is similar to the aforementioned mechanism of“ordered water monolayer that does not completely wet water”.So far as we know,although the orientation of ethanol molecules on the SiO₂ and calcite surface has been experimentally studied,it is still unknown that the whether the alcohol molecule have“ordered alcohol monolayer that does not completely wet alcohol”phenomenon on the realistic surface.In addition,the family of alcohol molecules have the same OH polar head,which can form hydrogen bonds,and various lengths of the hydrophobic nonpolar tail,which indicates the more complex adsorption behavior on solid surfaces than that of water.The length of the hydrophobic tail can affect the solvation behavior of the amphiphilic molecules at the nanoscale,as the hydrophobic effect is sensitive to the hydrophobic tail length.So far,the effect of the length of the hydrophobic tail on the wetting behavior of the family of alcohol molecules is not yet understood.In this thesis,we investigated molecular wetting behaviors of ethanol molecules on a hydroxylated SiO₂ surface using molecular dynamics simulations.It has been found that ethanol molecules could form an ordered monolayer on the SiO₂ surface,and an ethanol droplet could form on the monolayer,which provided evidence for the theoretic prediction of“ordered ethanol monolayer that doesn't wetting ethanol”phenomenon on the realistic surface.The first layer of ethanol molecules on SiO₂surface arranged to an ordered tetragonal structure,which was consistent with the distribution of the adsorption sites of SiO₂.The hydroxyl groups?OH?of the ethanol molecules in the first layer pointed toward to the SiO₂ substrate while the methyl groups?CH₃?pointed away from SiO₂ substrate.The CH₃ group of the second layer of ethanol molecules pointed toward to the first layer of ethanol molecules,which formed to a face to face arrangement of CH₃ group.Since the first layer of ethanol molecules was adsorbed vertically on the surface of SiO₂,the ethanol molecules could only form hydrogen bonds with the SiO₂surface,which would reduce the number of hydrogen bonds formed by the first layer of ethanol molecules and the droplet.This is the mechanism which ethanol droplet can stay stably on the ethanol monolayer.In addition,In addition,we found similar structures of ordered ethanol monolayer and ethanol droplet on Al₂O₃?001?surface and Calcite?104?surface.Our study found that methanol and propanol molecules could also form an ordered monolayer and droplet structure on the SiO₂ surface.As the methanol molecules were relatively small,the ordering of the first layer of methanol molecules was relatively weak.While the length of propanol molecules was relatively large,the propanol molecules in the first layer would be constrained to each other,so the orientation of the propanol molecule in the first layer was the most ordered.Similar to the ethanol molecules,the second layer of propanol molecules formed to a face to face arrangement of CH₃ group with the first layer of propanol molecules,but this arrangement didn't appear in the methanol molecules.Our calculations show that the contact angles of methanol,ethanol and propanol droplets are 36°,58°and 54°,respectively,where the contact angle of ethanol droplets is the largest.This is mainly due to the weakest interaction between ethanol droplet and ethanol monolayer.From the Young-Dupre equation,we can obtain the largest contact angle of ethanol droplet.Further studies have found that changing the surface morphology of SiO₂ has a significant effect on the contact angle of ethanol droplet.Increasing the lattice constant of SiO₂ surface,reducing the polarity of SiO₂ surface,or generating defects on SiO₂ surface will increase the contact angle of ethanol droplet.This is mainly because changing the surface morphology of SiO₂ will destroy the ordered structure of the first layer of ethanol molecules,which will increase the interaction energy between the first layer of ethanol molecules and the ethanol droplet.These results provide the basis of judgment for the formation of ordered ethanol monolayers and droplets on the surface of one material.That is,the distance between the adsorption sites on the material surface of the material must matches the size of the ethanol molecule,and the polarity of the material surface should be large enough,and the surface cannot have defects.It is worth noting that water molecules could significantly affect the infiltration behavior of ethanol on the surface of SiO₂.Mainly because the water molecules disrupted the ordered structure of the first layer of ethanol molecules on the surface of SiO₂ and caused defects in the arrangement of the first layer of ethanol molecules.This will increase the interaction energy of the first layer of ethanol molecules with the droplet above it.These studies indicate that the wetting ability of ethanol molecules on the SiO₂ surface can be adjusted by adding water molecules.Our research results give a microscopic physics picture of the wetting ability of alcohol molecules on the surface of mineral materials at molecular scale,and deepen the understanding of the wetting behavior of alcohol molecules on a strongly polar surface.In addition,our research results will have a certain enlightening significance in the designing of micro-nanometer fluid systems and the preparation of self-cleaning materials.