Structure of adsorbed molecular layer on fused quartz surface determined sequentially in sodium stearate solution, dry Ar, pure water, and dry Ar by sum frequency generation spectroscopy

Adsorption behavior of sodium stearate molecules at fused quartz/solution interface was studied by sum frequency generation (SFG) spectroscopy. SFG spectra were sequentially obtained at the interface of sodium stearate solution/fused quartz, Ar gas/fused quartz, water/fused quartz, Ar gas/fused quartz. No peak due to sodium stearate was observed in the sodium stearate solution but strong peaks due to CH stretching of CH ₃ group and a small peak due to CH stretching of CH ₂ group appeared after the stearate solution was replaced by Ar, showing the presence of the stearic acid monolayer with very high conformational order under Ar gas flow. No SFG signal was detected when pure water was made contact to the fused quartz surface and SFG signal reappeared again after water was replaced by Ar, showing that the stearic acid monolayer was present on the fused quartz surface but was totally disordered in water. Time dependent SFG measurements showed that the adsorption of stearic acid molecules on the fused quartz surface was very fast and completed within 5 s. Furthermore, it was found that the amount and conformational order of stearic acid molecules on the fused quartz surface determined by SFG measurements under Ar flow depended on the concentration of stearate in solution. They increased as concentration increased and were almost constant when the concentration was higher than 100 μM. A slightly decreased was detected at critical micelle concentration (CMC), 400 μM. No peaks were observed above CMC due to a totally disordered structure.