Lateral Mobility of Amphiphiles Adsorbed at the Air/Water Interface
Dynamic properties of the air/water interface were explored using Langmuir monolayer methods, ring and drop shape tensiometry, Brewster angle microscopy, and above all, cyclic voltammetry with 2D line and barrier microelectrodes. The goal is to gain insight into the rate of lateral self-diffusion of water molecules in the air/water interfacial region, defined as the space in which water transitions from 90 to 10 percent of its bulk density.
Because measuring the rate of water self-diffusion directly is experimentally impossible, 2,2,6,6-tetramethylpiperidnyl-1-oxy (TEMPO) was employed as a surfactant probe molecule. The dynamics and kinetics of TEMPO partitioning to the interface were thoroughly investigated under a variety of conditions. TEMPO was found to have a partition constant of 380 ± 30 M-1 in solutions of 1 mM HClO4 and 2 mM LiClO4. The partition constant of TEMPO can be adjusted by synthesizing derivatives of varying hydrophobicity at the C-4 position in the carbon ring. Placing a hydoxy group results in a compound that does not partition to the air/water interface. Adding an ethyl group results in approximately an order of magnitude increase of the partition constant.
The first attempts to determine the lateral diffusion coefficient of TEMPO (Dsurf) employed 2D line microelectrodes. However, these measurements were found to be insufficient for determining Dsurf of TEMPO for several reasons. The kinetics of TEMPO partitioning to the interface are fast relative to the experiment. This allows the surface population of TEMPO, as it is oxidized to TEMPO+, to be replenished by the solution population of TEMPO, thereby enhancing the surface current. It is therefore impossible to independently determine Dsurf without knowing the desorption rate constant, kdes. Numerical simulation with COMSOL Multiphysics allowed us to obtain a number of Dsurf, kdes pairs, and a calibration plot was created showing possible values of Dsurf as a function of kdes for 2D line microelectrodes.
The limitation of 2D line microelectrodes is that they exhibit degradation of the voltammetric signal over the time scale of our experiment (15-45 s), burdening the experiment with a systematic negative error. The characteristics of the signal decay indicate that it stems from a loss of the gold/air/solution triple phase line, thereby preventing electrooxidation of TEMPO from occurring precisely at the microline. A variety of mechanisms were hypothesized and tested to determine the exact cause of signal decay, with the aim of either eliminating it or finding a correction term to account for it. No hypothesis was successfully confirmed as the cause of decay. Because decay was a function of time the line electrode was in contact with solution, faster scan rates were preferred, typically 50 mV/s. The calibration curve obtained
from 2D line microelectrodes was taken as a lower bound value for Dsurf. For values of kdes > 103 s-1, the dependence of the value of Dsurf on kdes becomes small, and the calibration curve obtained for 2D line microelectrodes gave a value of 8 ± 4 cm2/s for Dsurf in this region.
Further experiments were designed to more accurately determine the value of Dsurf. This was done by modifying the electrode geometry. Thin barrier films were placed over the gold surface that prevented direct electrooxidation at the microline. Surface adsorbed TEMPO is still able to influence the voltammetric signal by desorbing to replace TEMPO that has been oxidized in the bulk solution. This allowed the creation of a set of independent calibration curves and the identification of a point of intersection, thereby determining both Dsurf and kdes. Two types of barrier films were employed: spin coated SU-8 photoresist and vapor deposited silicon monoxide. The first type of film employed was the SU-8 photoresist. SU-8 barrier films provided high reproducibility, but there was uncertainty as to the rigidity of the polymer film after breaking. Vapor deposited silicon monoxide was chosen as an alternative barrier film due to its ease of fabrication and greater confidence in its rigidity. Silicon monoxide barrier films were thoroughly characterized and found to break unevenly at the point of contact with solution. Due to their unusual breaking characteristics, SiO barrier electrodes had to be calibrated with a non-partitioning electroactive analyte. Both types of films generated calibration plots that intersected to yield Dsurf values of 7 ± 3 × 10-5 cm2/s. However, calibration curves obtained from simulating experiments performed with SU-8 barrier electrodes intersected at a kdes value of 104 s-1, while the curves obtained using results from SiO barrier films intersected at a kdes value of 103 s-1. Resolving the discrepancy between these two values is a possible direction for a future project. It is encouraging to note the remarkably good agreement between the values of Dsurf obtained from the 2D microline experiments and the barrier electrodes. This value for Dsurf is approximately a factor of 4 greater than the value obtained by Pohorille and Wilson in MD simulations. Applying the 2D diffusive model described by Hughes and coworkers, the viscosity of the air/water interfacial region is estimated to be one third of the viscosity of bulk water.