Effect of molecular branching and surface wettability on solid-liquid surface tension and line-tension of liquid...

Surface nanodroplets have important technological applications. Previous experiments and simulations have shown that their contact angle deviates from Young's equation. A modified version of Young's equation considering the three-phase line tension (τ) has been widely used in literature, and a wide range of values for τ are reported. We have recently shown that molecular branching affects the liquid-vapour surface tension γ_lv of liquid alkanes. Therefore, the wetting behaviour of surface nanodroplets should be affected by molecular branching. This study conducted molecular dynamics (MD) simulations to gain insight into the wetting behaviour of linear and branched alkane nanodroplets on oleophilic and oleophobic surfaces. We aim to examine the Young equation's validity and branching's effect on fundamental properties, including solid-liquid surface tension γ_sl and line tension τ.

The simulations were performed on a linear alkane, triacontane (C₃₀H₆₂), as well as four of its branched isomers: 2,6,13,17-tetrapropyloctadecane,2,6,9,10,13,17-hexaethyloctadecane, 2,5,7,8,11,12,15-heptaethylhexadecane and 2,3,6,7,10,11-hexapropyldodecane. Nanodroplets with a diameter of approximately 15 nm were released onto the surfaces, and their contact angles were measured. Additionally, using a novel approach, the solid-liquid surface tension (γ_sl), the validity of Young's equation and line tension for all alkane and surface combinations are determined.

It was discovered that the calculated γ_sl, deviated from the theoretical γ_sl,Young predicted from Young's equation for all alkanes on oleophilic surfaces. However, this deviation was minimal for branched alkanes on the oleophobic surfaces but more significant for the linear alkane. The findings indicated that γ_sl < 0 for oleophilic surfaces and γ_sl > 0 for oleophobic surfaces. Moreover, it was observed that |γ_sl| was lower for branched molecules and decreased as branching increased. Line tension values were then determined through a novel method, showing τ was positive for oleophilic surfaces ranging from 1.30 × 10^-10 to 6.27 × 10^-11N. On an oleophobic surface, linear alkane shows a negative line tension of -1.15 × 10^-10N and branched alkanes up to two orders of magnitude lower values ranging from -2.09 × 10^-12 to 2.43 × 10^-11N. Line tension values between -1.15 × 10^-10 and + 1.1 × 10^-10N are calculated for various linear alkane and surface combinations. These findings show the dependence of line tension on the contact angle and branching, demonstrating that for linear alkanes, τ is significant, whereas, for branched alkanes, line tension is smaller or negligible for large contact angles.