Nanoscale confinement can be defined as a space confined by interfaces with at least one nanometer-scale dimension. Objects under nanoscale confinement have a large ratio of interfacial area to volume that makes interfacial properties have significant impact. This dissertation examines three cases in which liquids are confined between solids. The main focus (two papers) describes how electrostatic interactions between two interfaces affect ions confined within the liquid. Commonly, the charge distribution near an interface is described by electrical double layer model, where the characteristic decay length of the potential is the Debye length κ^(-1), which is typically 1–100 nm. In a nanoscale confinement, the electrostatic potential from both confining surfaces overlaps, and there is no bulk solution in the confined liquid. If the two surfaces have the same potential in isolation, the potential will increase throughout the liquid phase. I examine two hypotheses for ions under confinement in aqueous solution: (1) diffusion of ions will be hindered by the electrostatic potential; (2) surfactants will form surface aggregates (a form of micelles) that would not occur without the modified potential.
To test the first hypothesis, I studied diffusion of fluorescein sodium salt in the nanoscale water confined between glass surfaces. The confining glass surfaces were fabricated by thermally bonding Borofloat glass wafers. Fluorescence microscopy was used to monitor the amount of fluorescein throughout the confined water, and thereby to understand the diffusion Measurements with done for a variety of different Debye lengths and water film thicknesses. I found that the time for fluorescein to reach equilibrium distribution in the nano-scale confinement could be 10× longer when there was no salt initially present compared to when salt was present. However, even a small amount of salt initially in the confined liquid led to a very weak effect of Debye length on diffusion. Thus, provided that the surface potential inside a thin film is initially screened by even a low concentration of electrolyte inside the confinement, diffusion is unhindered. A practical application of this result is delivery of dissolved species should not be preceded by infusion of pure water into pores if speedy delivery is desired.
For the second hypothesis, I studied adsorption and aggregation of dodecyltrimethylammonium bromide (DTAB), a cationic surfactant, within the same type of nanoscale confinement by Borofloat glass. A fluorescent dye, Nile red, whose fluorescence depends on its solvent environment was used to indicate formation of surface aggregates by the surfactant. We found that surface aggregation of DTAB occurred at a very low surfactant concentration (<1 % of the critical micelle concentration) when the confinement was less than 30 nm, which was about one Debye length of the solution. This finding overturns a major assumption of many surface forces measurements and ideas of colloidal stability. It has been customary to assume that the state of surfactant aggregation is constant when two particles approach, whereas we find that aggregation changes with the solution is confined. The change in aggregation can lead to a change in electrical potential, which affects the surface forces and colloidal stability. Past work that used this assumption will need to be re-interpreted.
The third topic was the study of the displacement of oil trapped in dead-end nanopores by water. This is a model of the process of tertiary oil recovery. Surfactants are used to assist with oil recovery, but the mechanism is not well studied. Three hypotheses were considered for the effect of surfactant on oil displacement: (1) Lowering of the oil–water interfacial tension; (2) Adsorption to the water–solid interface; and (3) Effects on transport rather than thermodynamics. Measurements of three different types of surfactants: sodium dodecyl sulfate (SDS), an anionic surfactant; Aerosol OT (AOT), an anionic surfactant; dodecyltrimethylammonium bromide (DTAB), a cationic surfactant; and no surfactant. Results show that AOT was the only surfactant that led to substantial spontaneous displacement of oil within 12 hours. The effect was attributed to AOT's ability for form reverse micelles in the oil phase that could deliver water to the hydrophilic solid walls, thereby displacing oil. No prior literature describing this mechanism has been found.
Doctor of Philosophy
Nanoscale confinement are domains contained by interfaces with at least one dimension on the nanometer scale level. This dissertation describes very thin (1–100 nm) layers of water between solids. Such thin layers of water are important in oil recovery, cellular processes, delivery of sham-poo to hair, drug delivery, etc. I studied the transport and adsorption of ions in these thin layers, particularly when the solid walls were charged. Results show that (1) Diffusion of ions could be se-verely hindered by unscreened electrostatic potential within the thin film of water. Diffusion times were increased by up to 10 times; (2) Surfactant aggregation occurred in the thin film, even when it did not occur in bulk solution at the same concentration; (3) Water could not displace oil in a thin film, even when assisted by a variety of surfactants. One particular surfactant, Aerosol OT could displace the oil, which I attribute to its ability to transport water through the oil and onto the solid.