Kinetics of small molecule adsorption studied using precision ellipsometry

Abstract

On the basis of a miniature polarisation modulator, a precision ellipsometry system has been made, enabling real‐time measurement of subnanometre thin layers on reflecting substrates. This system monitored the kinetics of adsorption and desorption of propanol, or ethanol, or methanol on oxidised Si substrates. While adsorption of propanol and ethanol increased the thickness, adsorption of methanol showed surprising kinetics: the thickness first increased, then decreased. To explain this, a model of substitutional (or competitive) adsorption has been used, where the target molecule is adsorbed only when it substitutes another one leaving an adsorption site. The model fits the experimental data quantitatively and can predict processes involving several components on solid surfaces. Precision ellipsometry demonstrated its high analytical potential in investigation of surfaces at molecular level.

Optomechanical measurement of single nanodroplet evaporation with millisecond time-resolution

“…Dynamic molecular simulations do predict the existence of such a regime 40 – 42 . Our present experiments are carried out within an environment saturated by the sprayed mist of 2-propanol molecules, hence the sessile droplets we investigate should be accompanied by such extended film 43 – 45 lying on the resonator surface 30 . This thin (precursor) film is essential to understand the wetting of the deposited droplet and the final stages of its evaporation.…”

Optomechanical measurement of single nanodroplet evaporation with millisecond time-resolution

“…2,31,[34][35][36] In such cases, a transition zone close to the contact line ensures the continuity of hydrostatic pressure between the spherical part of the droplet and an underlying thin film of liquid constituted of molecules continuously adsorbing and desorbing on the substrate. 37,38 Dynamic molecular simulations do predict the existence of such a regime, [39][40][41] Our present experiments are carried within an environment saturated by the sprayed mist of 2-propanol molecules, hence the sessile droplets we investigate should be accompanied by such extended film [42][43][44] lying on the resonator surface. 29 This thin (precursor) film is essential to understand the wetting of the deposited droplet and the final stages of its evaporation.…”

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“…And hence, the signal from the phase-lock amplifier is also proportional to the thickness of the layer. The optical and modulation system as well as calibration and measurement principles are described in detail in ref [9]. After demonstration of high sensitivity of PREL in air, we were asked by our colleagues, whether similar experiments could be done in water, since many chemical and bio-interactions occur in liquids.…”

“…Experiments with propanol, ethanol and methanol showed two processes in the adsorption. One process is fast, in the range of dozens of seconds, defined by diffusion of vapor to fill the box; the other is slow, in the range of hundreds of seconds, defined by substitution of previously adsorbed molecules by those from the vapor [9].…”