Dynamic Properties of Self-Assembled Monolayers of Mercapto Oligo(ethylene oxide) Methyl Ether on an Oscillating Solid–Liquid Interface

The dynamic properties of chemisorbed soft matter on a solid–liquid interface oscillating at megahertz were investigated using a quartz crystal microbalance (QCM). As a chemisorbed soft matter, we employed the self-assembled monolayers of mercapto oligo(ethylene oxide) methyl ethers, HS(CH2CH2O) n CH3 (n = 5, 11, 12, 19, 27, 35, and 43), where those molecular weights had unity. The systematic analyses on the basis of the Voight model revealed that the molecular thickness moving with the solid–liquid interface oscillating at megahertz frequencies, the resonant length, is 8.8 nm, where the frequency dependence of the resonant length is not considered. On the other hand, the analyses based on the Debye process revealed that the logarithm of the resonant length linearly decreases with the logarithm of 2πF, where F is the frequency of the QCM, and varies from 17.3 (9 MHz) to 12.4 nm (81 MHz). Those values in the Debye process were within twice that of the Voight model and were approximately consistent with that in the Voight model. On the basis of the experimental data, we proposed the equation with the resonant length of HS(CH2CH2O) n CH3 as a function of frequency. Moreover, we discussed the difference between the chemisorbed and the physisorbed molecules on the solid–liquid interface oscillating at megahertz frequencies.