Influence of Surface Roughness on Cetyltrimethylammonium Bromide Adsorption from Aqueous Solution

Abstract

The influence of surface roughness on surfactant adsorption was studied using a quartz crystal microbalance with dissipation (QCM-D). The sensors employed had root-mean-square (R) roughness values of 2.3, 3.1, and 5.8 nm, corresponding to fractal-calculated surface area ratios (actual/nominal) of 1.13, 1.73, and 2.53, respectively. Adsorption isotherms measured at 25 °C showed that adsorbed mass of cetyltrimethylammonium bromide per unit of actual surface area below 0.8 cmc, or above 1.2 cmc, decreases as the surface roughness increases. At the cmc, both the measured adsorbed amount and the measured dissipation increased dramatically on the rougher surfaces. These results are consistent with the presence of impurities, suggesting that roughness exacerbates well-known phenomena reported in the literature of peak impurity-related adsorption at the cmc. The magnitude of the increase, especially in dissipation, suggests that changes in adsorbed amount may not be the only reason for the observed results, as aggregates at the cmc on rougher surfaces are more flexible and likely contain larger amounts of solvent. Differences in adsorption kinetics were also found as a function of surface roughness, with data showing a second, slower adsorption rate after rapid initial adsorption. A two-rate Langmuir model was used to further examine this effect. Although adsorption completes faster on the smoother surfaces, initial adsorption at zero surface coverage is faster on the rougher surfaces, suggesting the presence of more high-energy sites on the rougher surfaces.

Observing the Effects of Temperature and Surface Roughness on Cetyltrimethylammonium Bromide Adsorption Using a Quartz‐Crystal Microbalance with Dissipation Monitoring

“…When comparing the isotherms above the CMC in Fig. a, b, on average the equilibrium values were lower on the rough surface than the smooth surface, a result our group reported previously (Shi et al, ; Wu et al, ). This decrease in adsorption is caused by disruption of intermolecular tail‐tail interactions by surface roughness (Fragneto et al, ).…”

“…Other studies have shown that gold and various other hydrophobic surfaces have supported the formation of bilayer films characteristic of a hydrophilic surface, although in some cases this was done purposefully by the addition of cosolutes (Fragneto et al, ; Gutig et al, ; Shi et al, ). The hydrophilic nature of the surface used in our experiments was found to be caused by the high concentration of oxygenated sites and the adsorption of halide ions, which have been reported previously by ours and other groups to create a negative charge on the surface (Knag et al, ; Mivehi et al, ; Wu et al, ). The gold surface provided by the manufacturer displays hydrophilic character even on the uncleaned surface, although further oxygenation and hydrophilicity is caused by the recommended cleaning procedure (RCA‐1 solution), which has been shown to increase the hydrophilicity of silicon as well (Bordes et al, ; Cademartiri and Ozin, ; Gutig et al, ; Hermansson et al, 1991).…”

“…Our group reported maxima in the adsorbed amount near the surfactant CMC previously (Shi et al, ). Maxima reported in our previous work were much more substantial than those shown here, although the same surfactant and similar surfaces were used (Wu et al, ). However, previously sonication and helium sparging of the surfactant solutions were not used.…”

“…When kinetics results are viewed on a large time scale, Fig. a, b, there appear to be only two regions of adsorption, a region of fast adsorption (~0–200 s) and a region of slow adsorption (time > 200 s), which were characterized previously through the use of kinetic models (Biswas and Chattoraj, ; Wu et al, ). When the fast region is viewed on a smaller time scale (Fig.…”

Manipulated wettability of a superhydrophobic quartz crystal microbalance through electrowetting

“…The superhydrophobic coating causes a more substantial increase in the level of insertion loss to 8 dB and increased the bandwidth to 6.19 kHz. This can be interpreted as an additional dissipation of energy in air due to the high level of surface roughnesses [35]. All SH coated devices continued to operate with well behaved single mode resonance and good sensitivity.…”