Surfactant polar head group

Model calculations of interface-solute electrostatic interactions reproduce well the view of microenvironment polarities of micelles and bilayers obtained from experimental data [57]. According to molecular dynamics simulations, at 1.2 nm from a bilayer interface, water has the properties of bulk water. At shorter distances, water movement slows as individual water molecules become attracted to the interface. At the true interface, which is a region containing both H2O molecules and the surfactant polar head groups, the water molecules are oriented with... 

The short dielectric relaxation process (presented here by 12) is associated with the anisotropic motion of the monomer alcohol species in a chain cluster (149). In microemulsions, the short process is the superposition of several dielectric relaxation processes, which have similar relaxation times such as movement or rotation of the alcohol monomers, hydrate water, and surfactant polar head groups. The short relaxation time is barely affected by the alcohol concentration in the mixture since it is less sensitive to the aggregation process. 

Falcone et al. used the anionic sodium l,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant, benzyl-n-hexadecyldimethylammonium chloride (BHDC) to prepare IL microemulsions [48]. The ILs chosen were l-butyl-3-methylimidazolium trifluoromethanesulfonate ([C mim][CF3S03]) and l-butyl-3-methylimidazolium trifluoroacetate ([C mim][CF3COJ see Scheme 13.2). DLS experiments reveal the formation of microemulsions. Besides, the FTIR results suggest that the ionic interactions (with the surfactant polar head groups, surfactant counterions, or IL counterions) are substantially modified upon confinement. These interactions produce segregation of IL s ions, altering the composition of the microemulsion interfaces. 

Previous studies performed in our group have shown very peculiar and interesting water properties inside RMs that emerges from the confinement effect and the interaction with the surfactant at the interface [27,39,40,42]. For example, water properties differ between RM systems formed with anionic and cationic surfactants. The water molecules entrapped inside the AOT/benzene RMs show enhanced electron donor ability compared with bulk water, while water entrapped inside the BHDC/benzene RMs appears nonelectron donating due to its interaction with the cationic surfactant polar head group [27,38, 39, 42]. These results have tremendous impact when RMs are used as nanoreactors [40,47]. 

Adsorption of surfactants on polar surfaces like pigments will only occur if there is an interaction between the surfactant polar head group and the... 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

Fig. 10 Cross sections of (a) spherical and cylindrical micelles, and (b) cylindrical and lamellar vesicles in aqueous solution. Each surfactant molecule making up the structures has a polar head-group, depicted as a circle, and a nonpolar, hydrophobic chain, depicted as a zigzag.

The electrostatic and steric effects can be combined to stabilize nanoparticles in solution. This type of stabilization is generally provided by means of ionic surfactants such as alkylammonium cations (Scheme 9.3). These compounds bear both a polar head group which is able to generate an electrical double layer, and a lipophilic side chain which is able to provide steric repulsion [14, 15]. 

Micelle-forming surfactants typically have structures which are constituted from a polar head group and a straight chain of the alkyl group, usually 8-18 carbons in number. 

Numerous books and reviews have been published on this subject (e.g. Fendler and Fendler, 1975 Mittal, 1977). Therefore, the structural characteristics of micelles will be presented only to the extent that is necessary for the subsequent discussions. These surfactants form micelles at concentrations above the cmc (critical micelle concentration). Such micelles have average radii of 12-30 A and contain 20-100 surfactant molecules. The hydrophobic part of the aggregate forms the core of the micelle while the polar head groups are located at the micellar surface. Micelles at concentrations close to their cmc are assumed to possess spherical and ellipsoidal structures (Tanford, 1973, 1978). A schematic representation of a spherical ionic micelle is shown in Fig. 1. 

One characteristic feature of surfactants is their amphiphilic nature. These molecules present two moieties the hydrophobic moiety (usually a hydrocarbon chain) interacts with the nanotube sidewalls, while the hydrophilic part, called polar head group, is generally charged or has zwitterionic character. It has the double function of helping solubility in aqueous solvents and of providing additional stabilization towards tubes aggregation by coulombic charge repulsion. 

The majority of studies have used surfactants that wrap around nanocarbons via van der Waals interactions [37]. For instance, surfactants such as sodium dodecylsulfate (SDS) are commonly used to disperse CNTs in aqueous solutions [38,39] while other surfactants, such as Pluorinc-123, are used to mechanically exfoliate graphene from graphite flakes (Fig. 5.4(a)) [40,41]. The polar head group of the surfactant can be used to further hybridize the nanocarbon via a range of covalent or noncovalent interactions [42]. For example, nanoparticles of Pt [43,44] and Pd [45] have been decorated onto SDS-wrapped MWCNTs. Similarly, Whitsitt et al. evaluated various surfactants for their ability to facilitate the deposition of Si02 NPs onto SWCNTs [46,47]. As an exam-... 

Amphiphilic tertiary phosphines have their phosphorus donor atom located somewhere in the hydrophobic part of the molecule and should have at least one long alkyl or alkyl-aryl chain carrying a polar head group (Scheme 4. 10). Some of them, such as the sulfonated derivatives, are quite well soluble in water, others, such as Ph2P(CH) COOH (n = 3, 5, 7, 9, 11) are practically insoluble, however, can be easily solubilized with common surfactants (SDS, CTAB etc.). 

The physical chemical properties of the surfactants that contain an ester bond between the hydrophobic tail and the polar head group are very similar to those of alcohol ethoxylates of the same alkyl chain length and the same number of oxyethylene units. The CMC and the cloud point values of the linear ester surfactant 1 of Fig. 4 are approximately the same as those of the straight chained alcohol ethoxylate tetra(ethylene glycol)monooctyl ether (C8E4), i.e., around 10 mM and 40 °C, respectively. Thus it appears that the... 

Biodegradation tests have shown that surfactants containing a carbonate bond between the hydrophobic tail and the polar head group are readily biodegradable. In comparative tests such carbonate surfactants biodegrade somewhat faster than the corresponding surfactants containing an ester bond [35]. The carbonate bond is not only susceptible to alkaline hydrolysis... 

The hyperactivity of, for example, lipases at low w -values (shown in Fig. 5) is explained by the water-shell-model [2]. The activity of the enzyme at w -values higher than 5 corresponds to its activity in bulk aqueous solutions. There exist two aqueous regions within a reverse micelle, schematically shown in Fig. 6. One is located in the inner part of the reverse micelle and has the same physical properties as bulk water the other is attached to the polar head groups of the surfactant and differs in its physical properties strongly from bulk water. 

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