Integration Of Special Wetting Properties On Single Substrate

The wetting property of a solid surface is one of the most important charactersrelating to the solid surface. Also, it is one of the commonest phenomena in natureand our living. Two extreme cases, namely superhydrophobic surfaces with contactangle (CA) larger than 150o and superhydrophilic surfaces with CA lower than 10o,have attracted extensive interests considering their intriguing promises inself-cleaning materials. Researches on superhydrophobic and superhydrophilicsurfaces have gained much progress in recent years. In general, superhydrophobicsurfaces can be obtained by controlling the roughness and topography ofhydrophobic surfaces, while superhydrophilic surfaces can be achieved by thechemical modification of a compound with strong interaction with water or byutilizing a capillary effect on hydrophilic surfaces.With the development of the research, researchers are not satisfied with surfacesof single wetting property any more. While, the research on surfaces integrated withdifferent wetting properties has become an intriguing direction. To integratesuperhydrophobicity and superhydrophilicity on one substrate, which means thatthese two properties can change from one to another reversibly or these twoproperties coexist on the different position of the substrate, will extend thesuperhydrophobic and superhydrophilic concept, and may have a potentialimportance for both scientific researches and practical applications.Our group has ever developed a method to construct fractal-like gold structuresbased on gold electrodeposition on a "full-of-defect" matrix of dendritic thiolself-assembled monolayer, which has proven to benefit the realization of thesuperhydrophobicity. In this thesis, we combine this rough gold structure and thewell-developed self-assembled monolayer technique to integrate thesuperhydrophobic and superhydrophilic property on one substrate. 1) An acid-base sensitive surface, which can be superhydrophobic for acid andsuperhydrophilic for base was constructed by a simple method of combining afractal-like gold surface and mixed thiol SAM. For an acid droplet (pH=1) thecontact angle can reach as high as 154°, while a base droplet (pH=13) will spread outon this surface. The reversible transition can be repeated many times without anychange in responsive property.Based on the intrinsic contact angle data on the flat gold substrates that weremodified with mixed thiol self-assembled monolayer of different composition, weexplained the origin of the phenomena by Wenzel and Cassie model. We fond thatboth the composition of the mixed monolayer and the rough gold structure werecritical for this responsive property.Furthermore, we put forward a new model, which was based on the speciality ofthe fractal-like rough structure, Extrand S. and Herminghaus theory, to explain thephenomenon that the surface exhibited a superhydrophobic property even when theintrinsic contact angle was smaller than 90°. Although the actual situation may be farmore complex than this simple model, we hope this will provide a clue for furthertheoretical research.We also expect by using mercaptocarboxylic acids with different pKa, it ispossible to shift the switching point between superhydrophobicity andsuperhydrophilictiy. Furthermore, the combination of rough gold structures and thiolmonolayer assembly provides an ideal system for exploring the relationship betweenwettability and surface chemical properties in more detail and should allow acorrelation of the wetting properties with the surface structure.2) We have developed a simple method to fabricate a gradient fromsuperhydrophobicity to superhydrophilicity. It is based on controlled self-assembledmonolayer of thiol molecule on gold surface and the amplifying effect of wettingproperty on rough surface. Utilizing a relative dilute HS(CH2)11CH3 solution(0.05mm/L), the density of the molecule on the surface can be controlled byimmersing time. Slowly adding the dilute solution into the container in which arough gold substrate stand will lead to a density gradient along the surface. Aftercomplementary adsorption of HS(CH2)10CH2OH, the surface exhibit a gradient fromsuperhydrophobicity to superhydropilicity. The slope of the gradient can be tunedcomfortably by varying the adding speed. Cassie-Baxter and Wenzel equation areemployed to explain this special property based on rough structure and the molecularcomposition gradient which has been determined by XPS. This kind of material willprovide a larger oriented driving force for many important biological and physicalprocesses and have potential application in aspects such as, water droplet movement,oriented axonal specification of neurons, protein adhesion and so on.