Читать книгу Surface Science and Adhesion in Cosmetics - Группа авторов - Страница 40

Another way to improve the hardness of oil-wax gel structure is to utilize the crystallization of wax in polar oils. Oil type and oil polarity have strong influence on the solubility of waxes which will affect the crystallization kinetics, crystal morphology, crystal size, crystal density and hardness of the oil-wax solid systems. For example, Imai and coworkers investigated the effect of various oil polarities on the hardness of the paraffin wax in oil-wax systems. They found that by increasing the polarity of oil, the gels became harder because the surfaces of crystals were rougher and the interconnection between wax crystals was tighter in the harder oil-wax gels, as revealed through SEM [17].

Table 2.5 is the summary of wax crystal morphologies observed from optical light microscopy (OLM) and polarized light microscopy (PLM) for some natural waxes in various oils reported for the oleogel systems [7, 8, 10, 24–30, 32]. Depending on the solubility and chemical composition of oils, natural waxes can develop different morphologies which contribute to the hardness of the oil-wax gels.

For example, beeswax exhibits 2 different types of crystals: spherulite and needle-like morphologies depending on the oil. Rice bran wax crystal has a thin long needle-like shape in olive oil, but exhibits a dendritic shape in rice bran oil and a round shape in paraffin oil.

Table 2.5 Crystal morphology of natural waxes in oil-wax gel systems.

Natural wax	Solvents/natural oils	Oil-wax crystal morphology
Beeswax (BW)	TriglycerideCanola/soybean/sunflower oils	Large spherulite crystals
	Corn oil/olive oil	Needle-like crystals
Sunflower wax (SFW)	Acetone/benzene/octyl acetateSunflower/soybean oilsCanolaRice bran oil	Thin plate-like crystals Long fibrous crystals Tiny needle-like crystals
	Triacetin/ethyl acetate/hexane/heptane	Spherical crystals
Carnauba wax (CRW)	Olive oilCanola	Small spherulites
Candelilla wax (CLW)	Safflower oilOlive oilCanolaRice bran oil	Small crystals Small spherulites
Rice Bran wax (RBW)	Olive oilSalad oil (canola/soybean)	Thin long needle-shape crystals
	Rice bran oil	Dendritic shape crystals
	Liquid paraffin oil	Round crystals

Blake and Marangoni [30] reported that the needle-like morphology was observed for waxes with homogeneous compositions such as beeswax (BW), sunflower wax (SFW) and rice bran wax (RBW). Carnauba (CRW) and candelilla (CLW) waxes with heterogeneous composition were used to develop a microplatelet morphology. The DSC and XRD studies reveal that wax morphology is strongly dependent on wax composition. A wax with a chemical homogeneous composition forms a lamellar structure which organizes into needle-like crystal. A wax with a chemical heterogeneous composition develops a less ordered and smaller crystal structure.