1 Introduction

The assembling and wetting behavior of small molecules, such as water and alcohol molecules, are important for applications in various fields including interfacial science [1,2], energy conversion [3], environmental protection [4,5], biological science [6-8], and nanodevices [9]. Owing to its importance, the adsorption of alcohol molecules on various mineral surfaces such as SiO₂ [10-13], mica [14], Al₂ O₃ [15,16], calcite [17,18], and sapphire [19], has been extensively investigated.

Our previous study showed that the small water molecules could form an ordered monolayer on a solid surface with particular charge patterns. In addition, a liquid droplet could form on top of the monolayer [20-24]; it has been referred to as "ordered water monolayer that does not completely wet water" at room temperature. Similar phenomena have been observed on several real solid surfaces such as metals (Pt [25,26] and Pd [26]), sapphire [27], talc [28], hydroxylated Al₂ O₃ and SiO₂ [29], etc. The family of alcohol molecules have the same OH polar head, which can form hydrogen bonds, and various lengths of the hydrophobic nonpolar tail, which indicates the more complex adsorption behavior on solid surfaces than that of water. The length of the hydrophobic tail can affect the solvation behavior of the amphiphilic molecules at the nanoscale, as the hydrophobic effect is sensitive to the hydrophobic tail length [30,31]. The effect of the length of the hydrophobic tail on the wetting behavior of the family of alcohol molecules is not yet understood, though it has been shown that an ethanol droplet can form on an ordered monolayer on a SiO₂ surface [32,33].

In this study, we investigated molecular wetting behaviors of alcohol molecules with various hydrophobic tail lengths (0.143 nm, 0.297 nm, and 0.451 nm for methanol, ethanol, and propanol, respectively) on a hydroxylated SiO₂ surface using molecular dynamics simulations. The results showed that methanol, ethanol, and propanol could form ordered monolayers on the SiO₂ surface, and that corresponding droplets could form on the monolayers. The contact angle of the ethanol droplet is the largest, attributed to the weakest adhesion interaction between the ethanol droplet and monolayer.

2 Computational methods

The (111)-face of β-cristobalite SiO₂ was used as the substrate, as in our previous study [33]. The CLAYFF force field was used to parameterize SiO₂ [33,34]. The Lorentz–Berthelot mixing rules were used for the unlike-pair interactions. First, alcohol droplets of pure methanol, ethanol, and propanol, were deposited on SiO₂ substrates. Each of the pure methanol, ethanol, and propanol droplets contained 1,500 molecules of the corresponding alcohol. The parameters of the alcohol molecules were taken from the OPLS-AA force field [35-37]. The MD simulation software GROMACS version 5.0.2 was employed for the simulations [38]. The simulation box size was 12.350 nm × 14.176 nm × 20.000 nm. The total simulation time of each system was 20 ns; the data of the last 10 ns of the trajectory were used for the analysis. All other simulation parameter settings were the same with those in our previous study [33]. The criteria for the hydrogen bonds were: distance (O−O) smaller than 3.5 Å and angle (H−O···O) smaller than 30°.

3 Results and discussion

Our simulations show that all three types of alcohol molecules exhibit similar behaviors on the SiO₂ substrate. Figure 1 shows side-view snapshots of the simulation results for methanol, ethanol, and propanol. All three types of alcohol can assemble to an ordered monolayer with a droplet on top of it. The detailed configurations of the monolayers in Fig. 1 show that the alcohol molecules in the monolayers stand upright on the SiO₂ substrate, with the OH groups pointing downwards and CH₃ groups pointing upwards. The top-view snapshots in Fig. 2 show that each of the three types of alcohol molecules in the monolayers are arranged in an ordered rhombic structure.

Fig. 1Full-screen View

(Color online) Side-view snapshots of the simulation results at 20 ns. Monolayer and droplet of (a) methanol, (b) ethanol, and (c) propanol on SiO₂. The bottom figures show detailed configurations of the monolayers.

Fig. 2Full-screen View

(Color online) Top-view snapshots of the simulation results at 20 ns. Monolayer and droplet of (a) methanol, (b) ethanol, and (c) propanol.

The numbers of hydrogen bonds formed between alcohol molecules and between alcohol molecules and SiO₂ are shown in Fig. 3. The results showed that each methanol, ethanol, and propanol molecule in the monolayers can form 1.84, 1.92, and 1.93 hydrogen bonds with the SiO₂ surface. Each alcohol molecule could form at most two hydrogen bonds with SiO₂. Therefore, hydrogen bonds could not be formed between alcohol molecules in the monolayer or between the monolayer and droplet. In other words, the formation of hydrogen bonds between the monolayer and SiO₂ surface prevents the formation of hydrogen bonds between the monolayer and droplet. This explains why the alcohol droplet can form on the monolayer, which is consistent with the results in our previous study [33].

Fig. 3Full-screen View

(Color online) Average number of hydrogen bonds formed between alcohol molecules of (a) methanol, (b) ethanol, and (c) propanol and SiO₂. The black, red, and blue curves represent the hydrogen bonds formed between alcohol molecules in the monolayer, alcohol molecules in the monolayer and droplet, and alcohol molecules in the monolayer and SiO₂, respectively.

In order to describe their morphology, we calculated the density distributions of the droplets, as shown in Fig. 4. First, the droplet was divided into small units with dimensions of 0.1 nm × 0.1 nm × 0.1 nm. Then, the number of alcohol molecules in each unit was calculated according to the geometrical center of each alcohol molecule. The density distribution was averaged over the range of 10–20 ns of the simulation trajectory. The density distributions were employed to determine the liquid-vapor interface of the droplet, as follows: (1) the density of the droplet was analyzed, from its interior to the surface; (2) if the density at a given position was below 1/2 of the bulk density, that position was considered to be at the liquid-vapor interface of the droplet. The contact angle of the droplet was calculated according to the interface of the droplet. Figure 5 shows that the contact angles of the methanol, ethanol, and propanol droplets are 36°, 58°, and 54°, respectively. The contact angle of the ethanol droplet is the largest among the three types of alcohol droplets.

Fig. 4Full-screen View

(Color online) Number density distribution of the alcohol molecules in the droplets of (a) methanol, (b) ethanol, and (c) propanol; r denotes the distance from the center of mass of the droplet in the xy plane, and z denotes the height from the monolayer. The molecular number density is expressed in nm^-3.

Fig. 5Full-screen View

(Color online) Contact angles of the alcohol droplets (black squares) and interaction energies between the droplets and corresponding monolayers (red circles).

The modified Young–Dupre equation can be expressed as [39]:

where ${\gamma }_{\text{L}}$ is the surface tension of the liquid, $\theta$ is the contact angle, and Φ is the interaction potential energy per unit area between the liquid droplet and substrate. The surface tensions of methanol, ethanol, and propanol are very similar (at 25°C, the surface tensions of methanol, ethanol, and propanol are 22.51, 21.82, and 21.22 mN/m, respectively) [40]. Therefore, the contact angle is mainly determined by the interaction potential energy. The interaction potential energybetween the alcohol droplet and monolayer is shown in Fig. 5, which shows that the interaction energybetween the droplet and monolayer of ethanol is the largest, among the three types of alcohol. Based on Equation (1) and the interaction energy, we derive that the contact angle $\theta$ of the ethanol droplet is the largest among the three types, consistent with the simulation results.

Further, we studied the orientation of the alcohol molecules in the monolayer and droplet by calculating the probability distributions of their tilt angle φ. It is defined as the angle between the line that connects the O atom of OH and C atom of CH₃, and z-axis [32]. Figure 6 shows that the probability distributions of φ of the three types of alcohol molecules have a similar profile; they all have a peak at φ= 0°. This indicates that the alcohol molecules in the monolayer favor an upright orientation with respect to the SiO₂ substrate. Owing to the strong attractive force between the OH groups of the alcohol molecules and SiO₂, the OH groups of the alcohol molecules point towards the SiO₂ surface, while the CH₃ groups point away from the SiO₂ surface. The probability distributions of φ of all three types decreases to zero with the increase of φ; the rate of decrease is different. For methanol, ethanol, and propanol the probability distributions of φ decreases to zero for φ> 120°, 70°, and 45°. The probability distribution of φ of the propanol molecules decreases faster than those of the methanol and ethanol molecules, which indicates that the propanol molecules in the monolayer stand straighter than the methanol and ethanol molecules. The propanol molecules can arrange in a stable configuration on the SiO₂ surface, as the tail chain of the propanol molecules is longer than those of the methanol and ethanol molecules. The methanol molecules have a short chain length, hence they can easily lie flat on the surface. Figure 6 shows that the probability distributions of φ of the alcohol molecules in the droplets are homogeneous, which indicates that the alcohol molecules in the droplets maintain a disordered orientation. The ordered orientation of the alcohol molecules in the monolayers leads to the ordered structures of the monolayers.

Fig. 6Full-screen View

(Color online) Distribution of the tilt angle φ of the (a) methanol, (b) ethanol, and (c) propanol molecules in the monolayer and droplet. The black squares and red circles represent the tilt angle φ distributions of the alcohol molecules in the monolayer and droplet, respectively.

In order to provide further insights into the adsorption behavior of the alcohol molecules near the SiO₂ surfaces, we calculated the probability distributions of the CH₃ and OH groups of the methanol, ethanol, and propanol molecules along the z-axis (perpendicular to the SiO₂ surface). The results are shown in Fig. 7. The density distributions of the three types of alcohol molecules are similar. The distance between the first peaks of OH and CH₃ of the propanol molecules is 0.30 nm, larger than those of the methanol (0.09 nm) and ethanol (0.20 nm) molecules, as the size of the propanol molecule is larger than those of the methanol and ethanol molecules. Our previous study showed that the second peak of the C atom of CH₃ was near the first peak of the ethanol molecules, which indicates that a face-to-face orientation of CH₃ formed between the monolayer and droplet [33]. In addition, Fig. 7 shows that for the propanol molecules a face-to-face configuration of CH₃ is also formed between the monolayer and droplet. However, this face-to-face configuration of CH₃ for the methanol molecules is indistinct, mainly due to the small length of the methanol molecules.

Fig. 7Full-screen View

(Color online) Probability distributions of the CH₃ and OH groups of the (a) methanol (b) ethanol, and (c) propanol molecules along the z-axis (perpendicular to the SiO₂ surface). The black and red curves represent the probability distributions of the C atom in CH₃, and O atom in OH, of the alcohol molecules, respectively.

4 Conclusion

Molecular dynamics simulations showed that the methanol, ethanol, and propanol molecules could form an ordered monolayer on the (111)-oriented hydroxylated β-cristobalite SiO₂ substrate and, in addition, a droplet could form on top of this ordered monolayer. The alcohol molecules in the monolayer favored the formation of hydrogen bonds with the SiO₂ substrate; this prevented the formation of hydrogen bonds with the droplet, and led to the droplet formation. The contact angles of the methanol, ethanol, and propanol droplets were 36°, 58°, and 54°, respectively. The contact angle of the ethanol droplet was the largest among the three types of alcohol droplets, owing to the weakest adhesion interaction between the ethanol droplet and monolayer, consistent with the Young–Dupre equation.