The Surfaces of the Ceratonia siliqua L. (Carob) Leaflet: Insights from Physics and Chemistry

Abstract

The production of superhydrophobic coatings inspired by the surface of plant leaves is a challenging goal. Such coatings hold a bright technological future in niche markets of the aeronautical, space, naval, building, automobile, and biomedical sectors. This work is focused on the adaxial (top) and abaxial (bottom) surfaces of the leaflet of the Ceratonia silique L. (carob), a high-commercial-value Mediterranean tree cultivated in many regions of the world. The adaxial and abaxial surfaces feature hydrophobic and superhydrophobic behaviors, respectively. Their chemical composition is very simple: monopalmitin ester and palmitic acid are protuberant in the epicuticular and intracuticular wax layers of the adaxial surface, respectively, whereas 1-octacosanol dominates in the abaxial wax layers. In both surfaces, epicuticular wax is organized along a randomly oriented and intricate network of nanometer-thick and micrometer-long plates, whose density and degree of interconnection are significantly higher in the abaxial surface. The measured tilting angles for the abaxial surface (12–70°) reveal unusual variable density and water adhesion of the nanostructured plate-based texture. Optical measurements demonstrate that light reflectance/absorbance of the glaucous abaxial surface is significantly higher/lower than that of the nonglaucous adaxial surface. In both surfaces, diffuse reflectance is dominant, and the absorbance is weakly dependent on the light incidence angle. We show that the highly dense nanostructured platelike texture of the epicuticular abaxial layer of the C. siliqua leaflet works as a sophisticated light and water management system: it reflects solar radiation diffusely to lower the surface temperature, and it has superhydrophobic character to keep the surface dry. Such attributes enable efficient gas exchange (photosynthesis and respiration), transpiration, and evaporation. To mimic for the first time the abaxial surface, a templation approach was adopted using poly­(dimethylsiloxane) (PDMS)/poly­(methylphenylsiloxane) (PMPS) positive/negative replicas and a soft polymer/siloxane negative replica produced by the sol–gel process. Because high topographical variations of the biotemplate and wax adhesion to the biohybrid film affected the replication quality, the reproduction of the wax texture via the synthesis of 1-octacosanol-grafted siloxane-based hybrid materials is proposed as a suitable route to duplicate the abaxial surface with high fidelity. The natural chemical/physical strategy adopted by the C. siliqua leaflet to face the harsh Mediterranean climate is a powerful source of bioinspiration for the development of diffuse reflecting and superhydrophobic material systems with foreseen applications as dual-functional antiglare and water-repelling coatings.

Vicariance Between Cercis siliquastrum L. and Ceratonia siliqua L. Unveiled by the Physical–Chemical Properties of the Leaves’ Epicuticular Waxes

“…Let us now compare the data obtained here for the C. siliquastrum leaf with those reported previously for the leaf of C. siliqua (Rodríguez et al, 2021).…”

“…With this goal in mind, we have examined the chemical composition, morphology, and wettability of the epicuticular (and in some cases, also the intracuticular) waxes of the abaxial and adaxial surfaces of the C. siliquastrum leaf, and also the optical properties of the leaf (Figure 3). The results obtained have been subsequently compared with those reported by some of us recently for the C. siliqua leaf (Rodríguez et al, 2021). We provide in this work clear proofs that the abaxial surfaces of the C. siliquastrum and C. siliqua leaves share a number of quite remarkable features that confirm their biological proximity: (a) the epicuticular waxes have the same dominant chemical compound (an alcohol whose length differs only in two carbon atoms), plate-like morphologies, and superhydrophobic behavior; and (b) very similar optical features.…”

“…With this goal in mind, we have examined the chemical composition, morphology, and wettability of the epicuticular (and in some cases, also the intracuticular) waxes of the abaxial and adaxial surfaces of the C. siliquastrum leaf, and also the optical properties of the leaf ( Figure 3 ). The results obtained have been subsequently compared with those reported by some of us recently for the C. siliqua leaf ( Rodríguez et al, 2021 ).…”

“…The integrating sphere was used in transmittance and reflectance modes. Technical details of the two configurations adopted may be found elsewhere ( Rodríguez et al, 2021 ). The measurements were performed on a C. siliquastrum leaf section (1 cm × 1 cm) cut from a set of around 4–6 leaves.…”

Durable and Flexible Hydrophobic Surface with a Micropatterned Composite Metal–Polymer Structure

“…Hydrophobic surfaces have attracted extensive research interest in many fields inspired by the self-cleaning behavior of natural plant leaves, such as lotus 1 and carob leaflets. 2 In general, if the water contact angle is greater than 90°, solid surfaces are considered hydrophobic, 3 which typically leads to self-cleaning, 4 antifouling, 5 anti-icing, 6 anticorrosion, 7 and drag reduction 8 behaviors, yielding huge potentials for industrial and engineering applications. For instance, a hydrophobic metallic surface can effectively isolate the corrosive media from the metal, therefore, the corrosion rate may be minimized.…”