Surfactants hydrophilic regions

Surfactants can be either found or synthesized to contain both hydrophobic and hydrophilic regions and can be cationic, anionic, or neutral. Cationic surfactants can be chosen to extract metals from soil. Of particular interest are those anionic surfactants that are naturally produced by plants and microorganisms because they can easily be broken down in soil once extraction has been accomplished. However, the use of surfactants for extracting metals has found limited use [27],... 

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

A general guide to the suitability of a solvent is the rule that like dissolves like. Soaps and detergents contain surfactant molecules that have both hydrophobic and hydrophilic regions. 

Enhancement of the aqueous solubility by surfactants occurs as a result of the dual nature of the surfactant molecule. The term surfactant is derived from the concept of a surface-active agent. Surfactants typically contain discrete hydrophobic and hydrophilic regions, which allow them to orient at polar-nonpolar interfaces, such as water/air interfaces. Once the interface is saturated, th surfactants self-associate to form micelles and other aggregates, whereby their hydrophobic region are minimized and shielded from aqueous contact by their hydrophilic regions. This creates a discrete hydrophobic environment suitable forsolubilization of many hydrophobic compounds (Attwood and Florence, 1983 Li et al., 1999 Zhao et al., 1999). 

The hydrophobic portions are usually saturated or unsaturated hydrocarbon chains or, less commonly, heterocyclic or aromatic ring systems. The hydrophilic regions can be anionic, cationic, zwitterionic, or nonionic. Surfactants are generally classified according to the nature of the hydrophilic group. Typical examples are given in Box 6.1. 

Pyridine-N-oxide is one of the smallest and most hydrophilic ( x = 4.3 D) probes that has been studied. Consequently, it is located at the surfactant/water interface or, if sufficient water is present, in the water pool itself (20). We chose to study pyridine-N-oxide since it partitions into more hydrophilic regions of reverse micelles than thymol blue and malachite green. All of these probes are complementary. 

After the oil well has been drilled it is usually necessary to ease the access of the oil to it, i.e. to open the collector . The latter can be achieved by pumping solutions of chemisorbing surfactants into the well. Such solutions convert hydrophilic regions of rocks into the hydrophobic ones, and thus make oil transport through the cracks and capillaries of the crust to the bottom of the borehole easier. During further use of the oil well it is important for one to be able to extract from the bed as much oil as possible. Typically, no more than 50-70% of the oil can be recovered, even from the best performing wells, and in most cases the recovered amount is 30-40% or less. 

Surface-active or amphiphilic molecules, in contrast to insoluble compounds, interact with the water phase. There are essentially two types of film-forming, surface-active compounds dry surfactants, in which the major portion of the molecule extends out of the water (Figure 1.10. A), and wet surfactants, in which the bulk of the molecule is within the water phase and only small portions of the structure project into the air (MacIntyre, 1974 Figure I.IO.B). Synthetic detergents (20), long-chain aliphatic compounds with a polar benzenesulfonate tail, are typical dry surfactants, whereas proteins, with many hydrophilic regions and only a limited... 

D19.7 A surfactant is a species that is active at the interface of two pha.ses or substances, such as the interface between hydrophilic and hydrophobic phases. A surfactant accumulates at the interface and modifies the properties of the surface, in particular, decreasing its surface tension. A typical surfactant consists of a long hydrocarbon tail and other non-polar materials, and a hydrophilic head group, such as the carboxylate group. —C07, that dis.solves in a polar. solvent, typically water. In other words, a surfactant is an amphipathic substance, meaning that it has both hydrophohic and hydrophilic regions. 

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]. 

Most are composed of droplets of an oil dispersed in water but the reverse is becoming more widely studied as new applications demand. In either case, the stability of the systems is enhanced by the addition of one or more surfactants, molecules of which have both hydrophobic and hydrophilic regions that transcend the interface between droplet and bulk medium (Figure 28.4). 

The polymer Nafion is an ionomer with < 15% ionizable sulfonate groups per monomer unit. It has a partly hydro-phobic character and a very high affinity for hydrophobic cations [47,48]. Nafion films feature segregation of hydro-phobic and hydrophillic regions, and can be considered as insoluble polymeric surfactants . 

Welin-Klinstroem et al used a null ellipsometer equipped with an automatic sample scanning device for studies of adsorption and desorption of fibrinogen and IgG at the liquid/solid interface on surface wettability gradients on silicon wafers. To follow the processes along the wettability gradient, off-null ellipsometry was used. The kinetics of adsorption and nonionic-surfactant-induced desorption varied considerably between fibrinogen and IgG. In the hydrophilic region, veiy little protein desorption was seen when a nonionic surfactant was used. 

Surfactant is a lipid-protein complex that is synthesized and released hy alveolar type II epithelial cells. This complex surface-active compound contains both hydrophobic and hydrophilic regions to allow the molecule to spontaneously adsorb to and form monolayers along the air-liquid interface. The role of surfactant in pulmonary fluid mechanics depends on its natural ability to disrupt intermolecular forces by interfering with the attractive forces between water molecules at the interfacial surface—thus lowering the surface tension. While this surfactant mixture is largely comprised of lipids (90%), the surfactant proteins (10%) are required for normal function (Hall et al. 1992 Yu and Possmayer 1993). Finally, the molecule dipalmitoyl phosphatidylcholine (DPPC) makes up 80% of the phospholipid and is largely responsible for the ultra-low surface tensions necessary for respiratory function (<5 dyn/cm) (Klaus et al. 1961 Hawco et al. 1981 Tchoreloff et al. 1991). 

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