Spontaneous Spreading of Liquid Droplets on Mixed Alkanethiol Monolayers:  Dynamics of Wetting and Wetting Transition

Abstract

We report and analyze the static and dynamic wetting properties of mixed alkanethiols self-assembled monolayers on gold substrates. These were prepared using end-methylated (undecyl mercaptan) and end-hydroxylated (11-mercapto-1-undecanol) compounds of the same length to avoid physical roughness of the resulting monolayer. The characterization of the surface composition was achieved by X-ray photoelectron spectroscopy measurement, showing that the surface composition is statistically not different from the composition of the grafting solution. The critical surface tension γc varies linearly within the range 20.3−39.3 mN/m as the hydroxylation of the surface increases. Contact angle hysteresis is maximum on the 50% hydroxylated surface. The relaxation of the shape of a squalane drop to its equilibrium configuration is well described by the molecular-kinetic theory of wetting, attesting that the energy dissipation during the spreading phenomenon is mainly controlled by the friction of the liquid molecules on the solid substrate. Versus the fraction of methylated/hydroxylated compounds on the surface, we observe a wetting transition for squalane. Below this wetting transition, the friction coefficient ζ between the squalane and the substrate is a linear combination of the friction coefficients measured on the corresponding pure substrates; above the wetting transition, the friction remains quasi-constant.

Evaporation of Water Microdroplets on Self‐Assembled Monolayers: From Pinning to Shrinking

“…This type of surface design exhibits at least three advantages for wetting and evaporation studies: [8] 1) the monolayer films fully cover the surfaces with highly oriented and closely packed molecules; 2) they introduce no changes in substrate roughness but controlled chemical heterogeneity on the surface; 3) the surface energy (i.e., the wetting properties) can be tuned as a function of the relative concentrations of the components. As mentioned above, similar systems have already been studied in the past, however, previous investigations focused on their structure, wetting properties (both structural dependence and hysteresis), and stability.…”

“…[3±5, 8,15] The values ranged from approximately 1108 to 508, which demonstrates that the surfaces were successfully designed to vary from hydrophobic (> 908) to hydrophilic (< 908). Figure 2(B) shows the contact area corresponding to the contact angle in Figure 2(A) as a function of the evaporation time.…”

Analysis of Self-Assembled Monolayers by Thermal Desorption Mass Spectrometry: Neighborhood Interaction and Hydrogen/Deuterium Exchange

“…17 Semal also reported a wetting transition phenomenon when the degree of surface hydroxylation was about 70%, described as a shift from the partial-wetting regime to the complete wetting regime. 18 Ulman proved this concentration-driven surface transition in the wetting of mixed alkanethiol monolayers on gold by a Monte-Carlo simulation and mean-field calculations on a simplified lattice-gas model. 19 For random mixing, the fraction of asymmetric dimer was mathematically as follows: …”

Spreading on Heterogeneous Substrates

“…The experimental conditions are described elsewhere [8]. An a posteriori characterization of such substrates was carried out using X-ray reflectivity.…”

“…They range from 21 to 39 mN/m, for the fully methylated and the fully hydroxylated surfaces, respectively. As discussed in [8], the Fox-Zismann approximation is not per se entirely justified, because the liquid used does not belong to a homologous series of liquids such as the alkanes, but can be understood as a first-order approximation of the Good-Girifalco equation [10,11], or of related theories [12], for a surface tension of the liquid γ close to the critical surface tension of the solid. More explicitly, a better approximation is to plot cos θ versus γ -1/2 according to:…”