Surface-active agents,

Consider a surfactant that is not soluble in the solution and that behaves like an ideal gas on the surface of a fluid with area A. The total surface energy 

The first term is the bare surface energy of the fluid, the second term, the ideal gas entropy of the surfactant whose area density is, cr (aq is a constant with 

7 = Ns/A, where Ng is the number of surfactant molecules on the surface, which is fixed. Taking the derivative including the variation of a, we find that 

This reduction in the tension is due to the fact that as the interfacial area is increased, the translational entropy of the surface active component is increased (for a fixed number of the surface active molecules). This increase in entropy lowers the system free energy and thus reduces y frotit its bare value. This expression is correct for small area fractions of the surface active species, since we considered only the ideal-gas entropy. 

Example Excluded Volume Effects on Surface Tension 

Naturally occurring and synthetic surface-active agents (tensides), some of which are listed in Table 8.16, are used in food processing when a decrease in surface tension is required e. g., in production and stabilization of all kinds of dispersions (Table 8.17). 

Dispersions include emulsions, foams, aerosols and suspensions (Table 8.18). In all cases an outer, continuous phase is distinct from an inner, discontinuous, dispersed phase. Emulsions are of particular importance and they will be outlined in more detail. 

Chocolate tion of starch retrogradation (bread staling) Improvement of rheological 

Instant powders properties, inhibition of fat blooming Solubilization 

Emulsions are dispersed systems, usually of two immiscible liquids. When the outer phase consists of water and the inner of oil, it is considered as an 

Many types of substances act as surfactants, but all share the property of amphipathy the molecule is composed of a nonpolar hydrophobic portion and a polar hydrophilic portion, and is therefore partly hydrophilic and partly hydrophobic. Surfactants may be referred to as either amphiphilic or amphipathic the terms are synonymous. The polar, hydrophilic part of the molecule is called the hydrophilic or lipophobic group, and the nonpolar, hydrophobic part is called the hydrophobic or lipophilic group. Often the hydrophilic part of the molecule is simply called the head and the hydro-phobic part—usually including an elongated alkyl substituent—is called the tail. The presence of a hydrophilic group makes surfactants slightly soluble in aqueous media, and is central to the physicochemical properties of aqueous surfactant solutions. 

Surfactants are classified on the basis of the charge carried by the polar head group as anionic, cationic, nonionic, or amphoteric. Tables 2.1 through 2.4 show the chemical structures of typical examples of these classes. Lecithin, cephalin, and the bile acids are ususally classified as biosurfactants. The bile acids and their conjugates have different properties in solution from surfactants with a long alkyl chain.  

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Soaps of heavy metals have been used but cationic surface-active agents have proved more suitable, notably organic amines of relatively high molecular weight. 

Dimeihylamine, C2H7N, (CH3)2NH. Colourless, inflammable liquid with an ammoniacal odour, mp -96" C, b.p. 7°C. Occurs naturally in herring brine. Prepared in the laboratory by treating nitrosodimetbyl-aniline with a hot solution of sodium hydroxide. Dimethylamine is largely used in the manufacture of other chemicals. These include the solvents dimethylacetamide and dimethyl-formamide, the rocket propellant unsym-metrical dimethylhydrazine, surface-active agents, herbicides, fungicides and rubber accelerators. 

Sorbitol is manufactured by the reduction of glucose in aqueous solution using hydrogen with a nickel catalyst. It is used in the manufacture of ascorbic acid (vitamin C), various surface active agents, foodstuffs, pharmaceuticals, cosmetics, dentifrices, adhesives, polyurethane foams, etc. 

These surface active agents have weaker intermoiecular attractive forces than the solvent, and therefore tend to concentrate in the surface at the expense of the water molecules. The accumulation of adsorbed surface active agent is related to the change in surface tension according to the Gibbs adsorption equation... 

When a surface-active agent is present in a liquid droplet, it can adsorb to the surface, lower the surface energy, and cause the liquid contact angle to increase. This phenomenon, known as autophobicity, was postulated by Zisman many years ago [78, 79]. Autophobicity is quite striking in wetting films on clean... 

The surface-active agents (surfactants) responsible for wetting, flotation and detergency exhibit rather special and interesting properties characteristic of what are called association colloids or, in the older literature, colloidal electrolytes. These properties play an important role in determining, at least indirectly, the detergency of a given surfactant and are therefore considered here... 

In addition to lowering the interfacial tension between a soil and water, a surfactant can play an equally important role by partitioning into the oily phase carrying water with it [232]. This reverse solubilization process aids hydrody-namically controlled removal mechanisms. The partitioning of surface-active agents between oil and water has been the subject of fundamental studies by Grieser and co-workers [197, 233]. 

M. J. Rosen and H. A. Goldsmith, Systematic Analysis of Surface-Active Agents, Wiley-Interscience, New York, 1972. 

I wetting agent for textiles, surface-active agent, [ food additive... 

Chemical Manufacturing. Chemical manufacturing accounts for over 50% of all U.S. caustic soda demand. It is used primarily for pH control, neutralization, off-gas scmbbing, and as a catalyst. About 50% of the total demand in this category, or approximately 25% of overall U.S. consumption, is used in the manufacture of organic intermediates, polymers, and end products. The majority of caustic soda required here is for the production of propylene oxide, polycarbonate resin, epoxies, synthetic fibers, and surface-active agents (6). 

Where surface-active agents are present, the notion of surface tension and the description of the phenomena become more complex. As fluid flows past a circulating drop (bubble), fresh surface is created continuously at the nose of the drop. This fresh surface can have a different concentration of agent, hence a different surface tension, from the surface further downstream that was created earlier. Neither of these values need equal the surface tension developed in a static, equiUbrium situation. A proper description of the flow under these circumstances involves additional dimensionless groups related to the concentrations and diffusivities of the surface-active agents. 

A typical phenohc foam system consists of hquid phenohc resia, blowiag agent, catalyst, surface-active agent, and modifiers. Various formulations and composite systems (65—67) can be used to improve one or more properties of the foam ia specific apphcations such as iasulation properties (63,68—71), flammabihty (72—74), and open cell (76—78) (quahty). 

Docusate sodium is both a surface-active agent for use as a fecal softener, and a wetting agent in industrial, pharmaceutical, cosmetic, and food apphcations. 

Wettable powders are prepared by blending the toxicant in high concentration, usually from 15 ndash 95%, with a dust carrier such as attapulgite which wets and suspends properly in water. One to two percent of a surface-active agent usually is added to improve the wetting and suspensibiUty of the powder. Sprays of wettable powders are used widely in agriculture because of their relative safety to plants. 

Emulsives are solutions of toxicant in water-immiscible organic solvents, commonly at 15 ndash 50%, with a few percent of surface-active agent to promote emulsification, wetting, and spreading. The choice of solvent is predicated upon solvency, safety to plants and animals, volatility, flammabiUty, compatibihty, odor, and cost. The most commonly used solvents are kerosene, xylenes and related petroleum fractions, methyl isobutyl ketone, and amyl acetate. Water emulsion sprays from such emulsive concentrates are widely used in plant protection and for household insect control. 

In water, a particle of lecithin exhibits myelin growth, ie, cylindrical sheets that are formed by bdayers and are separated by water which may break up into liposomes (vesicles with a single bilayer of Hpid enclosing an aqueous space). PhosphoHpids more generally form multilamellar vesicles (MLV) (5). These usually are converted to unilamellar vesicles (ULV) upon treatment, eg, sonication. Like other antipolar, surface-active agents, the phosphoHpids are... 

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