Wettability characterization of Langmuir-Blodgett films of stearic acid

Wettability analysis was used as a simple method to characterize the surfac e conditions of Langmuir-Blodgett (LB) films. Monolayers of stearic acid (S A) on air/liquid interface were investigated first and then, transferred to a hydrophilic glass surface to prepare LB film of one monolayer. The unifo rmity, stability and molecular orientation of the LB films were studied by the measurement of dynamic contact angle (Wilhelmy plate method) and compar ed with the information inspected by the atomic force microsoopy. The resul ts show that the SA monolayer is more stable on CdCl2 solution than that on the pure water. The transfer ratio for the deposition of LB film was close to unity at low surface pressures, but it increases to a value larger than 2 at higher surface pressures (30, 40 mN/m) as a result of significant col lapse effect. For one layer of SA films obtained at low surface pressures, the advancing contact angles (114 degrees) were equivalent to the theta (a) of a methyl-terminated monolayer, but the receding contact angle (23 degre es) is much smaller than the theta (r) of the methyl-terminated surface (ab out 90 degrees), which reflects that the LB films are not homogeneous but c ontain a significant area ratio of hydrophilic. region. When the transfer i s operated at high surface pressure, the advancing contact angle decreases obviously. This phenomenon is attributed to the significant collapse effect s on the monolayer, which lead to the formation of multiplayer or aggregati ve structure and thus, the area ratio of the hydrophobic tail of SA is decr eased. Due to the small. attractive force of the hydrophilic pole of SA to the glass surface, the SA LB film is not stable during the analysis of dyna mic contact angle (DCA). The surface condition estimated from the surface w ettability is consistent with the results inspected by AFM.