59th Annual Report on Research 2014

The structural properties of fluids confined between solid surfaces are valuable for interpreting fluid behavior as continuum theory breaks down.  When coupled with force measurements in the surface force apparatus (SFA), the changes in the fluid refractive index can shed light on the mechanism of elusive surface interactions such as hydration, hydrophobic, or solvation interactions.   For example, such measurements could be used to determine the properties of a depletion layer near a surface like that found for water near a hydrophobic surface. Likewise, the same approach can be applied to any adsorbed or deposited film to determine the film’s thickness or refractive index and quantify the properties of any polymer, surfactant or thin-film of interest.  Such measurements are challenging due in particular to systematic error which arises from uncertainty in substrate properties and the effects of inhomogeneity in the fluid or film refractive index that is not typically accounted for in optical analysis.

Although several complimentary techniques are useful for measuring properties of the thin films and fluids near surfaces such as elipsometry, quartz crystal microbalance, x-ray and neutron reflectivity, and surface plasmon resonance spectrometry, none can match ultra-high resolution SFA measurements which enable; (1) measurement of the interaction force profile as a function of the absolute distance between the surfaces with sub-angstrom (<0.1nm) resolution; and (2) measurement of the density of a fluid or media confined between the two surfaces, via refractive index measurements, as a function of the absolute surface separation.

Over the past year and a half, we have made significant progress in developing an interferometry data analysis method for determining the optical thickness of thin films or any variation in the refractive index of a fluid or film near a surface. In particular, the method does not require contacting or confining the fluid or film, but importantly, can also be used to measure deviations in refractive index induced or enhanced by confinement, as well. By analyzing data taken at many inter-surface separation distances out to at least 300 nm, the properties of a film can be quantitatively determined. The film can consist of material deposited on the surface like a polymer brush, or variation in a fluid’s refractive index near a surface resulting from, for example, a concentration gradient, depletion in density of water near a hydrophobic surface, confinement induced ordering, or surface roughness. Additionally, we have carried out a thorough investigation of the effects of random and systematic error in the surface force apparatus data analysis and modeling to enable simultaneous measurement of the surface separation (<0.1nm) and intervening refractive index (≤ 0.001) as a function of confinement.  So far, these unique capabilities have been used to determine the density distribution of adsorbed polymer films as shown in Figure 1 and the breakdown in continuum behavior of water and hexadecane under nano-confinement between two hydrophilic surfaces (mica) as shown in Figure 2. With this important foundation, future studies will seek to quantitatively investigate the properties of fluids under nano-confinement and vigorously test current theories of the origin of the hydrophobic force which will impact our fundamental understanding of hydrophobic interactions at the nano to macro-scale.

Figure 1. (A) Schematic of the optical layers of an adsorbed polymer layer in a fluid and the refractive index profile of the system (heavy black line). (B) Refractive index or mass fraction of the polymer (here cationic poly ethyleneimine, PEI) in water as a function of distance from the surfaces.

Figure 2. Preliminary data of the (A) refractive index of water confined between two hydrophilic surfaces (mica) as a function of surface separation or confinement. (B) Relative density of water based on refractive index measurements in (A). (C) Relative density of hexadecane confined between two hydrophilic surfaces (mica) as a function of surface separation or confinement.  We are in the process of repeating these measurements with our new, more accurate data analysis algorithm detailed in Figure 3.

Figure 3. Schematic of analysis algorithm routine used to enable simultaneous measurement of the surface separation (<0.1nm) and intervening refractive index (≤ 0.001) as a function of confinement.