Surface Energy

Surface energy is a material property that depends on the constructional elements (organic groups) of the material that are on the phase boundary of solid/liquid. Reducing the surface energy increases the hydrophobicity of the material. The wetting of a solid surface is evaluated by the water contact angle, using a simple model given by Young’s equation [Eq. (13.3.1)] (Figure 13.3.6):

cos в = (csv – CslVClv аз. зл)

where ySL, ySV, and yLV are, respectively, the values of the free energy between the planes per unit of surface of the solid-liquid, solid-vapor, and liquid-vapor phases (Figure 13.3.6). This equation only applies for a smooth surface, since the equation does not take into account the roughness of the substrate (see below). On a smooth surface with a regular composition of CF3 groups, the lowest surface energy

measured is 6.7 mJ m~2. This surface gives a wetting angle for water not higher than 120°, which is not considered to be a superhydrophobic surface [31]. As an example, perfluoroalkyl chains are well known for being waterproof (the surface energy for Teflon is 18.5 mJ m~2) but even these smooth surfaces do not give wetting angles for water of в > 150° [28]. Such surfaces are easy to clean and repel water, but this does not mean that they have self-cleaning properties.

The water repellent properties originate from the C-F groups; the short C-F bond contains paired electrons that hinder the formation of hydrogen bonding and, due to their non-polar nature, prevent the establishment of van der Waals dispersive interaction with polar and non-polar liquids. The absence of hydrogen bonding and other stronger electrostatic interactions further increases with the number of individual fluorine atoms (CF3 > CF2 > CF) and also depends on the length of the perfluoride chain [the electron density increases with the shift of the terminal group of the CF3 to the side, to a value of n & 9 CF3(CF2)n-] [32]. This is also true for hydrocarbons; the surface energy decreases from – CH2, to a mixture of – CH3 and – CH2-, to – CH3 at the crystal level [33, 34]. On this basis, it can be surmised that short perfluoroalkyl chains increase the hydrophobicity, while longer perfluoride chains combined with suitably rough substrates are needed to increase oleophobicity. The orientation, packing, and alignment of perfluoroalkyl chains also influence the free surface energy values and the corresponding contact angles of liquids on such surfaces [28, 34].

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