Stability of dimethyldioctadecylammonium bromide Langmuir-Blodgett films on mica in aqueous salt solutions—implications for surface force measurements

Langmuir-Blodgett (LB) monolayer films of dimethyldioctadecylammonium bromide (DDOA) on muscovite mica have been studied using Wilhelmy plate type wetting measurements, surface force measurements, atomic force microscopy (AFM), and Brewster angle microscopy (BAM) on insoluble monolayers of DDOA before deposition. In particular, the effect of exposure to aqueous KBr salt solutions was investigated. BAM shows a heterogeneous monolayer with small condensed domains of dendritic shape under conditions normally used for deposition. A stick-jump behaviour of the meniscus is seen during deposition, leading to a large-scale heterogeneity measurable in wetting studies. These also show breakdown and hydrophilization of the LB film at the three-phase contact line (meniscus) and when exposed to salt solutions of approximately 10⁻² M concentration. The advancing contact angle against water is approximately 105°. Surface force measurements show long-range attraction in water, but also a surface charge which depends on salt concentration, and breakdown when surfaces are brought into contact in high salt concentrations. AFM images of untreated films show small holes, and breakdown when exposed to salt solution, especially at the three-phase line. The LB film is judged to be less suitable as a model hydrophobic surface owing to its heterogeneity and instability in salt solution.