Plant surfaces play a major role in protection against multiple potential biotic and abiotic stress factors . To adapt to these multiple functions, the plant epidermis has developed various characteristics, including specialised cell types such as trichomes or stomata . Epidermal cells are surrounded by a cell wall, which plays a crucial structural and physiological role in plant development and survival .
Differentiation and maintenance of the epidermis are essential for plant growth and survival and require continuous cross-talk between epidermal cells and their immediate environment . Epidermal cells also provide mechanical support by adhering strongly to each other via a strengthened cell wall, which is usually noticeably thicker on the external surface. In addition to the asymmetrical deposition of cell wall material, epidermal cells secrete a lipid-rich cuticle specifically into the thickened external cell wall matrix . Therefore, the cuticle may be considered a cutinised cell wall, emphasizing the heterogeneous nature of this layer and its interconnection with the cell wall beneath . The main protective role of the cuticle is related to the prevention of uncontrolled exchange of water and gases between the plant and the surrounding environment . The functional relevance of the cuticle to plant growth and survival is evidenced by the significant commitment of epidermal cells to cuticle production .
The cuticle is made of a bio-polymer matrix, waxes that are deposited on to (epicuticular) or intruded into (intracuticular) this matrix, and variable amounts of polysaccharides and phenolics [4, 7]. It is an asymmetric membrane  generally comprising three distinct layers from the outer to the inner side of the organ, namely: (i) the epicuticular wax layer, (ii) the “cuticle proper” containing waxes and cutin and/or cutan, and (iii) the “cuticular layer” composed of cutin and/or cutan and a high polysaccharide content .
Waxes commonly constitute 20 to 60% of the cuticle mass and are complex mixtures of straight chain aliphatics . Wax composition and structure can vary among different species, organs, states of development, and environmental and stress conditions during growth [10, 11]. The mechanisms of epicuticular wax formation and regeneration have been assessed in some studies  and it has been proposed that cuticular transpiration is the driving force behind wax movement through the cuticle [13, 14].
The cuticle matrix is commonly made of cutin, which is a biopolymer formed by a network of inter-esterified, hydroxyl- and hydroxy-epoxy C16 and/or C18 fatty acids . At least six different types of cuticular ultrastructures have been identified by transmission electron microscopy (TEM) , but their relationship to cutin monomer composition remains unclear [7, 16]. The formation of cutinsomes, which are spherical nanoparticles resulting from the self-assembly of cutin hydroxyacid monomers in a polar environment, has been demonstrated; cutinsomes have been proposed as building units of the bio-polyester cutin .
While cutin is depolymerised and solubilised upon saponification, cuticles from some species contain a non-saponifiable and non-extractable polymer known as cutan, which yields a characteristic series of long chain n-alkenes and n-alkanes upon flash pyrolysis . Cutin has been found to be the only polymer present in the cuticles of many fruits and leaves of Solanaceae and Citrus species , while different proportions of cutin and cutan have been determined in cuticular membranes extracted from leaves  and fruits such as peppers, apples or peaches [19, 20].
Major differences in surface topography have been observed in different species and organs, but three hierarchical levels of structuring may occur in association with: (i) the general shape of epidermal cells, (ii) cuticular folds, and (iii) epicuticular wax crystals . For example, the presence of papillae  or trichomes  can have a major effect on surface topography and wettability at the microscale level. Also, increased surface roughness and surface hydrophobicity have been reported owing to the occurrence of nano-scale structures provided by epicuticular wax crystals [22, 23].
Different degrees of wettability of leaves from various species have been reported by measuring water contact angles (e.g., [21, 24–26]). In addition, phyllosphere-related factors such as the deposition of aerosols or microorganisms can lead to plant surface heterogeneity [27, 28], especially in urban or polluted habitats . However, non-wettable surfaces have been observed to accumulate particles more slowly than wettable ones .
Recently Fernández et al.  estimated the surface free energy, polarity and work of adhesion of a model pubescent surface and proposed the implementation of membrane science approaches to exploring the physical-chemical properties of plant surfaces. It has been suggested that the cuticle acts as a “solution-diffusion” membrane for the diffusion of some solvents and solutes [31, 32]. To analyse the permeability of the plant cuticle to solutes and solvents, both the solubility and diffusivity of the compounds must be taken into consideration. While diffusivity is a kinetic parameter associated with the molecular size of a compound in relation to the structure of the matrix, solubility is a thermodynamic parameter that indicates the affinity of a given chemical for the cuticle. Therefore, and as a preliminary step towards the evaluation of plant cuticle permeability, we have analysed for the first time the solubility of model plant surfaces and chemical constituents in relation to agrochemicals of commercial significance, following a thermodynamic approach. Prediction of solubility parameters is commonly used, for example, in the design and fabrication of polymeric membranes [33, 34], in the coating industry  and also in pharmacology . However, with the exception of the human skin [37, 38], this procedure has not so far been applied to estimating the properties of biological surfaces.
As model plant surfaces, peach and pepper fruits were selected since they contain alkanes as major wax constituents but have significantly different surface topographies. Juvenile Eucalyptus globulus leaves, which are covered with a dense layer of nano-tubes and contain β-diketones as dominant waxes, were also evaluated for comparison.
For model plant surfaces, cuticular constituents and agrochemicals, the following hypotheses were tested: (i) is it possible to predict the solubility of plant surface constituents and the affinity of agrochemicals for plant surfaces? and (ii) can solubility parameters be used to estimate the properties of the plant cuticle?