Probing Confinement Effects on Fluid Behavior in Nanotubes

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The behavior of a fluid changes significantly upon confinement within smaller and smaller space. An example for a common consequence is the continuous depression in melting point, which results from the increasing importance of the interfacial energy. However, when the confinement reaches the nanometer scale, a radically different picture emerges as it can no longer be ignored what the fluid consists of—namely atoms, molecules, and/or ions. The behavior of fluids at this scale still defies our full understanding, and this is true even for a substance as mundane as water. For example, at the inside of carbon nanotubes (CNTs), exotic ice phases can be stabilized with melting points far exceeding the one of bulk water. Water transport through CNTs is found to be frictionless, while it is not through nanotubes made of boron-nitride. Currently, no consistent and universally applicable theory is available to satisfactorily describe such behavior of nanoconfined fluids and to make reliable predictions. But also experimental efforts lag behind in providing systematic data sets from fluids confined in individual, well-characterized nanopores or -channels.The main goal of this project is to experimentally determine the phase behavior of fluids confined within isolated nanotubes. All studies will first focus on water, and subsequently on other fluids including cyclohexane, n-hexane, and ionic liquids. Both carbon nanotubes (CNTs) as well as boron-nitride nanotubes (BNNTs) will be deployed. Despite their structural similarity, differences in the behavior of confined fluids are expected, resulting from their distinct electronic properties. Experimental results will be compared to the existing literature, with the intention to gauge different modelling approaches and to motivate new theory to advance the field.Specifically, in the case of CNTs, I will use Raman spectroscopy and photoluminescence in conjunction with electrical measurements to probe fluid phase behavior in individual, high-quality, freely suspended nanotubes. Transmission electron microscopy will serve as a complementary characterization method. Besides systematically assessing the diameter-dependence of optical properties as a function of filling state, temperature, and pressure, an objective is to measure the temperature-dependent specific heat of individual filled vs. unfilled CNTs.BNNTs will be purified and isolated by fabricating dispersions starting from commercially available material. Subsequently, the optical properties of these BNNTs will be studied, especially regarding the photoluminescence of defect states. In planar hexagonal boron-nitride, quantum emission from defect states currently receives wide attention but is less studied in BNNTs. It is an objective of this project to increase our understanding of these defects states and to deploy them as sensors for a fluid interfacing with the nanotube.