Abstract
Two anionic surfactants, dodecylsulfate (DDS) and dodecylbenzenesulfonate (DBS), were intercalated into layered zinc hydroxide salts (LHS) using the direct alkaline co-precipitation method, and characterized by powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) and thermogravimetric analysis/differential thermal analysis (TGA/DTA). Different UV-absorbing organic molecules, like salicylates, cinnamates and benzophenones, were adsolubilized in the LHS interlayer following two different procedures (conventional microwave treatment and microwave with hydrothermal treatment). The adsolubilized products were investigated by PXRD, FTIR, diffuse reflectance UV-Vis (DRUV-Vis) and luminescence spectroscopies before and after exposure to UV radiation. Most of the products showed a good absorption in the UV region, from UVC to UVA, and good stability under UV radiation. The photodegradation tests showed that DDS-intercalated compounds were more stable than those intercalated with DBS. Adsolubilization in LHS can be an interesting alternative to immobilize neutral molecules with UV absorption capability, to prepare materials to be used in sunscreen formulations.
Highlights
Layered hydroxide salts (LHS) have a structure based on that of brucite, Mg(OH)[2], where a fraction of the hydroxide anions of the layers have been replaced by appropriate anions or water molecules
The basal distances calculated from the powder X-ray diffraction (PXRD) patterns were 32.6 and 31.11 Å for layered zinc hydroxide salts (LHS)/DBS
The PXRD patterns of the surfactant-intercalated LHS remained almost unchanged after adsolubilization of benzophenone, but basal spacing were somewhat larger
Summary
Layered hydroxide salts (LHS) have a structure based on that of brucite, Mg(OH)[2], where a fraction of the hydroxide anions of the layers have been replaced by appropriate anions or water molecules. Different organic molecules which absorb UV radiation, like salicylates, cinnamates and benzophenones, are frequently used as sunscreen components, but they can penetrate into the corneous stratus, causing different skin reactions, such as contact dermatitis, allergies, photoallergic reactions, and others.[14,15] Layered inorganic compounds (clays, layered double hydroxides and others) can absorb, scatter and reflect the UV radiation; if they host the UV absorbers in their interlayers these continue being active, but will not interact with the skin. Immobilization of organic UV absorbing molecules within inorganic matrixes is an interesting alternative to reduce the absorption of organic sunscreens by human bodies
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