The initial study focused on quantification of the drift in the readings taken with the Wilhelmy plate method. This was critical to understanding the problem and its effect on monolayer characterisation. For the initial characterisation, the trough was filled with distilled water, and a new piece of filter paper was hung on the Wilhelmy balance. Once the water had visibly saturated the filter paper, the sensor was ‘zeroed’; this data point was used as t = 0. The surface pressure was then recorded over a 3 h period. The experiment was repeated 3 times, on 3 consecutive days. The data collected during the 3 experiments are shown in figure 1B.
Figure 1B shows that the form of the drift is repeatable, with the measured surface pressure taking approximately around 150 min to reach equilibrium. There is a clear variation between each run, with the average drift equating to approximately 10% (3–4 mN m-1) of the surface pressure that would be typically used during transfer of a monolayer onto a substrate during Langmuir-Blodgett deposition.
As the drift was observed to approach a stable plateau, it was assumed that this problem was caused by one or more parts of the system requiring time to equilibrate. Given that the system is comprised of water and a sensor, this limits the number of possible causes. The two key hypotheses considered were the presence of contamination at the liquid surface that causes the surface pressure to take time to equilibrate, and instabilities in the Wilhelmy balance mechanism.
Firstly, the possible contamination of the liquid surface was considered. A control experiment was undertaken to ensure that the measured surface pressure had reached equilibrium. The filter paper was then removed and replaced with a new piece. This replacement of the filter paper was undertaken carefully to minimise disturbance of the water’s surface. Once fully wetted, the Wilhelmy sensor was again ‘zeroed’ and the resulting drift in the surface pressure readings was recorded. The data collected are shown in figure 1C.
Figure 1C shows little difference in the magnitude of the drift in the surface pressure readings between the two experiments. If water (or contamination thereof) was the cause of the drift, then with the first plate we might expect to see a slow change in the surface pressure as the interface layer forming contaminate reached an equilibrium. However, once formed this layer should not be significantly disrupted by the addition of a new plate as it has been already reached in equilibrium (typified by a decrease in the rate of change in surface pressure). However, in figure 1C, the drift with the first and second plates is similar in magnitude, which indicates that the change is not due to a layer forming from the contaminant in the sub-phase. This indicates that the drift is a property of the Wilhelmy sensor system and not of water.
Figure 1D shows a reduction in the magnitude of the drift of the Wilhelmy surface pressure, when the filter paper plate had been pre-soaked in water. As discussed, the Wilhelmy plate measures the weight of the plate to determine the surface pressure of water. The drift observed after the Wilhelmy plate has been wetted suggests the filter paper is increasing in the weight. The reason for this effect could be that, although appearing fully wetted, the filter paper continues to absorb more water when immersed in the trough, increasing the overall weight, which was recorded by the pressure sensor.
To test the swelling theory, the change in weight of the filter paper was measured using an Ohaus GA200D balance (±0.2 mg precision). 3 filter paper plates were wetted in a small vial of distilled water and weighed immediately. Once weighed, the plates were returned to the vial of distilled water, where they were soaked for 30 min. During this period, they were weighed at the intervals to record any change in the wetted weight. The results are shown in figure 1E.
Figure 1E shows that the plates gain weight at a similar rate to the drift observed in the measurement of surface pressure by the Wilhelmy balance, indicating that drift is the result of the filter paper plates slowly gaining weight.