Nanobubbles and Molecular Scale Phenomena of Vapour-Liquid-Solid Interfaces

Peng, Hong (2013). Nanobubbles and Molecular Scale Phenomena of Vapour-Liquid-Solid Interfaces PhD Thesis, School of Chemical Engineering, The University of Queensland.

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Author Peng, Hong
Thesis Title Nanobubbles and Molecular Scale Phenomena of Vapour-Liquid-Solid Interfaces
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor Greg Birkett
Nguyen, Anh V.
Total pages 148
Total colour pages 27
Total black and white pages 121
Language eng
Subjects 030603 Colloid and Surface Chemistry
090407 Process Control and Simulation
Formatted abstract
Interfaces between aqueous solutions and hydrophobic solid surfaces are important in various areas such as froth flotation and film stability. Many researchers have found that forces between hydrophobic surfaces in aqueous solution are significantly different from the classical DLVO (Derjagin-Landau-Verwey-Overbeek) theory. This long-range attractive force (non-DLVO forces) is thought to be related with the nanoscopic gaseous domains (nanobubbles or nanopancakes) which can exist between aqueous solution and solid surface. However, even though the existence of nanobubbles between liquid and solid surfaces has been confirmed by many experimental data, their formation and stability still raises a lot of questions to be answered. Until now, two major areas remain unresolved: the long-time stability and large contact angle of nanobubbles.

    The aim of this thesis is to investigate the molecular level properties that affect solid, liquid, and gas interfaces, and link these properties to explain the formation and stability of nanoscopic gas domains at solid-liquid interfaces. This will be achieved through the use of molecular simulation methods in conjunction with experimental tests.

    The first outcome of this thesis is to propose a new method (named as weighted testarea method) to calculate surface tension by incorporating the weighting factor from the Bennett method into the free energy perturbation scheme of the test-area method. It is seen that the new method is accurate for all these simulations, giving the same results as the Bennett method, in contrast to the test-area method which cannot calculate the surface tension of a square well fluid. The new method converges as quickly, on the basis of computational time required, as the test-area method and almost twice as quickly as the Bennett method.

    The second outcome of this thesis is to propose a new method of calculating density contours based on atomic density instead of traditional number density. This method results in a much smaller variation in measured contact angle when applying different assumptions compared to using number density for isochoric contours. The most consistent results, across a range of assumptions about the droplet and the contact angle, come from averaging the contact angle from several isochoric density profiles.

    The weighted test-area method was used to determine the interfacial tension between solid-liquid and solid-vapour phases. Cylindrical and spherical nanodroplets were also simulated on the solid surface with contact angles calculated using a precise procedure mentioned in second outcome. Then the line tension values of an LJ fluid interacting with solid surfaces with varying strengths were estimated based on the modified Young’s equation using droplets of different size. The results show that the magnitude of all calculated line tension values are less than 4×10-12 J/m with both negative and positive signs. The best estimate of line tension for this system of Lennard-Jones droplets is 1×10- 13 J/m, which is smaller than the reported estimations in the literature.

    To investigate formation of nanobubbles, we used silicon-nitride tipped atomic force microscope (AFM) cantilevers to probe the graphite-water interface after solvent exchange. The tip detected non-DLVO (no EDL repulsion) forces after solvent-exchange which was not detected before solvent-exchange. The range of the attractive jump-in over the surface is grouped into circular areas of longer range (>40 nm), consistent with nanobubbles, and the area of shorter range (<20 nm). The non-DLVO nature of the area between nanobubbles suggests that the interaction is no longer between a silicon-nitride tip and highly ordered pyrolytic graphite (HOPG). Interfacial gas enrichment (IGE) covering the entire area between nanobubbles is hypothesized to be responsible for the non-DLVO forces. This co-existence of nanobubbles and IGE was not previously been observed.

    Due to the limitation of experiments to characterise IGE, we utilized molecular simulation to investigate the nitrogen gas adsorption on water-graphite interface. The simulation results revealed that a dense gas layer (DGL) with a density equivalent to a gas at pressure of 500 atm is formed and equilibrated with a normal pressure of 1 atm. By varying the number of gas molecules in the system, we observed that several types of dense gas domains exist: aggregates, cylindrical-shape gas domains, and dense gas layers. Spherical shaped gas domains form during the simulation but are unstable and always revert to another type of gas domain. Furthermore, the calculated surface potential of the dense gas-water interface, -17.5 mV, is significantly lower than the surface potential, -65 mV, of normal gas bubble-water interface. The change in surface potential comes from a slight change in the structure of water molecules at DGL-water interface compared with the normal gas-water interface. In addition, the contact angle of the cylindrical-shaped high density nitrogen gas domains is 141°. This contact angle is far greater than the 85° observed for water on graphite at ambient conditions and much closer to the 150° contact angle observed for nanobubbles in AFM experiments.
Keyword contact angle
interfacial tension
Line Tension
Atomic force microcopy
gas enrichment layer
Molecular Simulation
Monte Carlo

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Created: Fri, 24 Jan 2014, 12:26:14 EST by Mr Hong Peng on behalf of Scholarly Communication and Digitisation Service