Spontaneous Formation of Fractal Structures on Triglyceride Surfaces with Reference to Their Super Water-Repellent Properties

Abstract

Alkylketene dimer (AKD), a kind of wax, has been known to form fractal surfaces spontaneously and show super water-repellency. Such formation of water-repellent and fractal surfaces was also found in this work for triglycerides. Since the crystal phase transitions of these waxes were well studied, we studied the formation of their fractal surfaces through contact angle measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). From time-dependent contact angle measurements, it was found that the formation of super water-repellent surfaces with fractal structures occurred spontaneously also on the triglyceride surfaces at different temperatures. The freshly solidified triglyceride surfaces were almost transparent, and their initial contact angles of water were close to 110 degrees. The surfaces then became rough and clouded after being incubated for a certain time at a specified temperature. The super water-repellent surfaces were quite rough and showed fractal structures with the dimension of ca. 2.2 calculated from the scanning electron microscopic (SEM) images by the box-counting method. The phase transformation from a metastable state to a stable cystalline one after the solidification from the melt of triglycerides was clearly observed by DSC and XRD measurements. The fractal crystalline structures and the super water-repellency resulted from this phase transformation and the crystal growth. Ensuring the initial sample solidified into the metastable state and curing the surface at an appropriate temperature are key factors for the successful preparation of fractal triglyceride surfaces by the solidification method.

Formation mechanism of super water-repellent fractal surfaces of alkylketene dimer

“…13 In this system, the phase transition from the metastable α-form to the stable β-form crystal is essential for spontaneous formation of fractal surfaces.…”

“…In the former trial, fluorinated compounds with low surface energy were usually used, while several nano-or micro-textured or fractal structures on the surfaces were formed in the later case. Various approaches to prepare super water-repellent surfaces have been reported including melt-solidification of a wax, [11][12][13][14] silica and oxidized alumina films through the sol-gel method, [15][16][17] plasma-based etching or deposition, 18,19 gel-like roughened polypropylene, 20 anodically oxidized aluminum, 21,22 etched copper and electrodeposited copper, 23 electrochemical polymerization, 24 layer-by-layer assembly together with electrodeposition, 25 solution-immersion, 26 polymer/nanoparticle composite films, 27,28 films of aligned carbon nanotubes, polymer nanofibers and inorganic nanorods. [29][30][31][32][33][34][35][36] Effects of geometric length scales, surface roughness and surface energy on the wettability of super water-repellent surfaces were extensively discussed from physical and chemical points of view.…”

“…[37][38][39][40][41][42][43][44][45] It has been well known that fractal structures can effectively magnify the real surface area compared with the projected one, and then enhance the wettability of a solid surface. [11][12][13]21,22,46 Alkylketene dimer (AKD), a kind of wax, was found to form fractal structures spontaneously with a dimension of 2.29 and to give super water-repellent surfaces with the water contact angle as large as 174 o . 11,12 Super water-and oil-repellent surfaces 21,22 with fractal dimension of 2.19 and fractal super water-repellent poly(alkylpyrrole) film 24 with a dimension of 2.23 were also successfully made.…”

“…In our recent work, 13 the spontaneous formation of fractal structures on the wax surfaces of triglycerides, trimyristin, tripalmitin and tristearin, and their super water-repellent property have been studied. The spontaneous formation of the fractal structures on the triglyceride surfaces has been found to be originated from the phase transition from the metastable α-form to the stable β-form crystal.…”

Characterization and structural investigation of fractal porous-silica over an extremely wide scale range of pore size

“…For instance, fractal surface structures emerge super-water repellency [8][9][10][11][12][13][14] and super oil-repellency [15]. The super water-repellent fractal structure is spontaneously formed on the surfaces of alkylketene dimer (AKD) and triglycerides waxes [8][9][10][12][13][14]. The super-water repellency is caused by the enhancement of the real surface area of waxes arising from the surface roughness.…”