Formation of normal micelles

Micelles in water are formed by the self-association of surfactants, also called surface-active agents or detergents. These amphiphiles contain both hydrophilic and hydrophobic (lipophilic) moieties, so that one part of them tends to associate with water and the other part is repelled by water, but readily interacts with apolar molecules [11,12], 

Ci2H250S03Na Sodium dodecylsulfate, SDS Sodium laurylsulfate, NaLS 8 XlO  

C 4H29NMejBr T etradecyltrimethylammonium bromide Myristyryltrimethylammonium bromide, MTAB 3.5X10  

CieHjjNMejBr Hexadecyltrimethylammonium bromide, HTAB Cetyltrimethylammonium bromide, CTAB 9 XlO 

CijHjsNMejO R0(CH2CH20) CH2CH20H Dodecyldimethyl amine oxide Brij, Igepal, Triton 2 XlO  

---

Figure 4. Formation of normal micelles (left), the surfactant phase (middle), and inverse micelles (right) may be referred to the relative size of the interfacial tensions against the oil (yo/n) and the water (jw/p) a plane interface (32)...

The formation of normal micelles in water, or in diols or triols or sulfuric acid is well established, as is the formation of aggregates in apolar solvents containing traces of water. But there are examples of aggregation in dipolar aprotic solvents or in aqueous-organic solvents of relatively low water content. In some cases reaction rates have been studied in these media, but generally only physical studies have been reported. 

As surfactant is added to the second system of Figure 10, however, the tieline that terminates at the CMC for formation of inverted micelles in the oleic phase is reached before the normal CMC is encountered. Once the former tieline is reached, further additions of surfactant merely increase the concentration of inverted micelles. Only with much larger additions of surfactant will normal micelles begin to form. If only aqueous phases were studied, this could lead to the belief that normal micelles and the associated wettability would be encountered in a flow experiment, when, in fact, inverted micelles and different wettabilities would actually occur. 

Micelles (normal and reverse) are aggregates of surfactants, which are formed spontaneously in a liquid phase when the surfactant concentration is increased than the critical micelle concentration (CMC). Micellar solutions are formed in aqueous continuous systems, whereas, the reverse micelles are formed in oily continuous systems. In micelles (oil-in-water micelle), the polar heads of the surfactant lie outside in the aqueous phase, whereas the lipophilic hydrocarbon chains lie inside (Figure 58.3). When the surfactant concentration is increased, the free energy of the system increases due to the inauspicious interactions between the water molecules and the lipophilic portions of the surfactant. The water molecules around the oil droplets structures themselves, thereby, resulting in the decrease in the entropy. The reverse micelles (water-in-oil micelle) have opposite structure, that is, the polar heads lie at the centre, while the lipophilic tails are present outside in the oil phase (Figure 58.3). The surfactant concentration need not necessarily be higher than the CMC for the formation of reverse micelle. Many scientists reported formation of reverse micelles by lecithin in different oil phases. " ... 

Reverse micelles have an inverted structure in comparison to the conventional normal micelles in aqueous systems. Therefore, they are often known as inverse or inverted micelles. In reverse micelles, the micellar cores consist of a hydrophilic polar component and the shells consist of lipophilic nonpolar part of the surfactant molecules. The dipole-dipole interaction between the hydrophilic headgroups acts as one of the driving forces for the formation of reverse micelles in organic solvents. Reverse micelles are mostly observed in the ternary mixtures of surfactant/ water/oil, mostly in oil-rich regions [1-3]. Furthermore, reverse micelles have also been observed in aqueous systems of UpophUic surfactant in surfactant-rich regions [4, 5]. In most of the studies carried out in the past, water was regarded as an essential component in the formulation of reverse micelles. Only a few reports exist in the literature of surfactant science that describe the formation of reverse micelles in organic solvents without water addition [6-10]. 

The dissolution of HM-HEC in water is coordinate with hydration and expansion of the cellulose backbone as described above. The hydrophobic effect results in the formation of pseudo-micelles along the polysaccharide chain. These pockets of concentrated hydrophobe diminish the internal energy of the polysaccharide, stabilizing the presence of the hydrophobes in the water. These transient cross-links build viscosity and impart a yield value to the solution whereas normal HEC solutions cannot suspend water-insoluble oils, HM-HEC solutions can. 

---