Hydrophilic “head,” of surfactant

The incorporation of surfactants is also reported to improve the compatibility. The hydrophilic head of surfactant adsorbs on the cellulose surface, whereas its hydrophobic tail finds proper solvency conditions in the matrix, thus deterring aggregation of the cellular inclusions via steric stabilization. 

Although the hydrophilic head groups of surfactants usually fit into one of the four categories described earlier, there are a number of exotic hydrophobic tail groups, both synthetic and natural, that populate the niches of specialty surfactants. Hydrophobes based on telomers of tetra-fluoroethylene or polydimethylsiloxane ° bring unique surface-active properties to all classes of surfactants, reach extraordinarily low air/water and interfacial tensions, and enhance consumer and industrial product performance at amazingly low levels of use. ... 

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

Figure 1.16 Temperature dependence of the water/oil interfacial tension

The most common and widely accepted classification of surfactants is based on the nature of the hydrophilic (head) group of the surfactants. They can belong to either of the following classes ... 

The most popular classification of surfactants is based on its ionic characteristics they are either anionic, cationic, or nonionic. Since ions are normally found in aqueous fluids, ionic characteristics of surfactants are found in the hydrophilic head thus, an anionic surfactant would have negatively charged species in the hydrophilic heads. Because of the requirement of charge neutrality in an overall fluid system, an anionic surfactant would have its positive counterion within its vicinity. This counterion is subject to various ion-exchange mechanisms either with other types of counterions in the fluid or on solid surfaces. It is possible for nonionic species to be hydrophilic, because of their polar and hydrogen-bonding interactions with water molecules. 

Other studies [85] have shown that cationic bacitracin, anionic AOT, and nonionic Tween 80 all enhanced cellulose hydrolysis, implying that the charge of the surfactant was not an important consideration. In fact. Tween 80 (0.1%) increased the rate and extent of saccharification by up to 40% [82,85]. The structure of the hydrophilic head group of the surfactant also had little significance [85], Those with a sugar group, sophorolipid and rhamnolipid, worked well, as did bacitracin, which has a peptide hydrophilic group [85]. 

Thousands of surfactants are produced commercially worldwide, with applications ranging from pharmaceuticals and food additives to soaps and detergents. Surfactants are typically classified as either anionic, cationic, nonionic or anq>hoteric depending upon the nature of their hydrophilic head group. Nonionic surfactants are widely used in agricultural applications because of their relatively low cost, generally low toxicity, and tolerance to varying solution conditions (e.g., electrolyte concentration). The most common nonionic... 

The hydrophilic head group of nonionic surfactants contains either hydroxyl groups, or more commonly, ethylene oxide (EO) polymer chains. The relative polarity of nonionic surfactants is often characterized by the hydrophile-lipophile balance (HLB), defined as (i) ... 

The close agreement with Aw/eo = 3 obtained for the quaternary system A might prima facie lead us to conclude that the degree of hydration in surfactant-based systems depends on neither the length of the hydrophilic head group of the ethoxylated surfactant nor the presence of pen-tanol (or dodecane) [2,11],... 

But in the case of CTAB, because of its long chain length (about 1.8 nm) and electrical repulsion between the particle surface and hydrophilic head groups of the surfactant, only a few surfactant molecules can adsorb at the particle surface. So the surfactant molecules carmot stabilize the silver nanoparticles in colloid solution. 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

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