Introduction to Microemulsions

Abstract

U pregledu su prikazane glavne karakteristike mikroemulzija i njihove primjene. Mikroemulzije su termodinamički stabilni i optički izotropni sustavi koji se sastoje od ulja, vode i amfifilnih molekula koje nazivamo surfaktantima i kosurfaktantima. Mikroemulzija može biti sastavljena od mikrokapljica ulja u vodi ili od mikrokapljica vode u ulju, ili može biti tzv. bikontinuirana faza u kojoj se voda i ulje kontinuirano isprepliću u strukturi nalik spužvi. Mikroemulzije su samoorganizirajući sustavi i nastaju bez utroška energije. Mikroemulzije voda-u-ulju važne su za sintezu raznih anorganskih materijala. Reakcije se zbivaju u malom obujmu vode unutar mikrokapljice i zbog toga produkti često imaju poboljšane karakteristike u pogledu čistoće i raspodjele čestica po veličini. Veličinu mikrokapljica moguće je kontrolirati vrstom i omjerom surfaktanta i kosurfaktanta te kemijskim sastavom uljne i vodene faze. Mikroemulzije mogu imati složeno fazno ponašanje, koje se obično prikazuje faznim dijagramom. Osim spomenutih čistih strukturnih tipova, u mikroemulziji može biti zastupljeno i više njih odjednom. Surfaktanti su amfifilne molekule građene od hidrofilne glave i lipofilnog repa, zbog čega imaju afinitet i prema vodi i prema ulju. Njihova glavna svojstva su adsorpcija na međupovršini i samoorganizacija u supramolekulske strukture. Stvaranjem sloja na granici faza surfaktanti smanjuju površinsku napetost i stabiliziraju mikroemulziju. Prema vrsti hidrofilnog dijela surfaktanti se dijele u anionske, kationske, neionske i amfoterne. U pregledu su prikazane kemijske strukture njihovih tipičnih predstavnika. Mikroemulzijski agregati mogu imati različit oblik, kao npr. sferičan, elipsoidan, cilindričan, crvolik ili dvoslojni. Bikontinuirana faza također se javlja u više različitih oblika, npr. u heksagonskom, lamelnom ili kubičnom. Glavni parametri koji određuju mikrostrukturu su zakrivljenost međupovršinskog filma i kritični parametar pakiranja.Hoar and Schulman were the first (in 1943) to describe the transparent water-in-oil dispersion. They noted that a certain combination of water, oil, surfactant and alcohol cosurfactant would produce a clear homogeneous "solution", which Schulman in 1959 termed microemulsion. Schulman’s model of the inverse submicroscopic micelle is shown in Fig. 1. The IUPAC definition describes microemulsion as a dispersion made of water, oil, and surfactant(s), which is an isotropic and thermodynamically stable system with dispersed domain diameter varying approximately from 1 to 100 nm, usually 10 to 50 nm. Water-in-oil microemulsions are important for the synthesis of various inorganic materials. In contrast to inverse micelle, water-in-oil micro- emulsion aggregates have mobile or free water in the core of the aggregate (Fig. 2). Reactions take place in a small volume of water inside a microdroplet, which results in improved properties of the products in terms of purity and particle size distribution. Microdroplet size can be controlled by suitable choice and mixing ratio of surfactant and cosurfactant (Fig. 3) and by the chemical composition of the oil and water phases. Microemulsions may have complex phase behaviour, commonly displayed in a phase diagram (Fig. 5 and Fig. 12). Surfactants are amphiphile molecules with hydrophilic head and lypophilic tail, thus bringing them affinity both to water and oil. Their main characteristics are adsorption at the interface and self-organization into supramolecular structures. By forming the interface layer, surfactants decrease surface tension and stabilize the microemulsion. According to the character of the hydrophilic head, surfactants are divided into anionic, cationic, nonionic, and amphoteric. Chemical structures of typical representatives are given in Fig. 6. Microemulsion aggregates may have various shapes, e.g. spherical, ellipsoidal, cylindrical, worm-like or bilayer (Fig. 7). Bicontinuous phase also occurs in different forms, e.g. hexagonal, lamellar or cubic. The main parameters that determines the microstructure are curvature of the interface film. (Fig. 8) and critical packing parameter (Fig. 10). A microemulsion can be composed of microdroplets of oil in water, or microdroplets of water in oil, or it can be the so-called bicontinuous phase in which water and oil are continuously intermixed in a sponge-like structure (Fig. 9). The solubility of water in microemulsion (Fig. 12) as well as morphology of microemulsion aggregates (Fig. 7) are very important for the synthesis of nano particles using microemulsion technique