Fatty Acids and Surfactants

Comparison of the adsorption of lauric and caprylic acid as a function of the total bulk concentration. At pH 4, this concentration is made up predominantly of fatty add molecules and at pH 9 predominantly by their conjugate bases. ( ) Points at equilibrium (d6/dt = 0). ( ) Points at which the equilibrium is not yet attained. 

The adsorption of fatty acids on the non-polar hydrophobic surface (Hg) is dominated by their hydrophobic properties. The extent of adsorption increases with increasing chain length. The following relationship of the free energy of adsorption, AGacis. and the number of C atoms, nc, of the fatty acids can be established  

As seen in Fig. 4.8, the adsorption of lauric acid (C12) is slow because of slow transport (diffusion) at concentrations smaller than 10 6 M. In case of Na+-caprylate (Cs) the attainment of equilibrium is delayed most probably by structural rearrangement at the surface. In case of anions, such association reactions are slower than with free acids. 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

Mixtures of Adsorbates. The adsorption behavior of mixtures of adsorbates is more complicated. In the most simple case, concentrations C1 and C2 are very low and surface coverage is 61 + 6g 1. Then, when interaction between the adsorbed species is negligible, the adsorption of both types of adsorbates occurs independently, and we can write Langmuir isotherms 

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Figure 1 Fatty acid and surfactant type organogelators.

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Polyoxyethylene Esters. This series of surfactants consists of polyoxyethylene (polyethylene glycol) esters of fatty acids and aUphatic carboxyhc acids related to abietic acid (see Resins, natural). They differ markedly from mono- and diglycerides in properties and uses. 

Carboxylic Amides. Carboxyhc amide nonionic surfactants are condensation products of fatty acids and hydroxyalkyl amines. 

Ethoxylation of alkyl amine ethoxylates is an economical route to obtain the variety of properties required by numerous and sometimes smaH-volume industrial uses of cationic surfactants. Commercial amine ethoxylates shown in Tables 27 and 28 are derived from linear alkyl amines, ahphatic /-alkyl amines, and rosin (dehydroabietyl) amines. Despite the variety of chemical stmctures, the amine ethoxylates tend to have similar properties. In general, they are yellow or amber Hquids or yellowish low melting soHds. Specific gravity at room temperature ranges from 0.9 to 1.15, and they are soluble in acidic media. Higher ethoxylation promotes solubiUty in neutral and alkaline media. The lower ethoxylates form insoluble salts with fatty acids and other anionic surfactants. Salts of higher ethoxylates are soluble, however. Oil solubiUty decreases with increasing ethylene oxide content but many ethoxylates with a fairly even hydrophilic—hydrophobic balance show appreciable oil solubiUty and are used as solutes in the oil phase. 

The solubihty characteristics of sodium acyl isethionates allow them to be used in synthetic detergent (syndet) bars. Complex blends of an isethionate and various soaps, free fatty acids, and small amounts of other surfactants reportedly are essentially nonirritant skin cleansers (66). As a rule, the more detersive surfactants, for example alkyl sulfates, a-olefin sulfonates, and alkylaryl sulfonates, are used in limited amounts in skin cleansers. Most skin cleansers are compounded to leave an emollient residue on the skin after rinsing with water. Free fatty acids, alkyl betaines, and some compatible cationic or quaternary compounds have been found to be especially useful. A mildly acidic environment on the skin helps control the growth of resident microbial species. Detergent-based skin cleansers can be formulated with abrasives to remove scaly or hard-to-remove materials from the skin. 

Adsorption. Many studies have been made of the adsorption of soaps and synthetic surfactants on fibers in an attempt to relate detergency behavior to adsorption effects. Relatively fewer studies have been made of the adsorption of surfactants by soils (57). Plots of the adsorption of sodium soaps by a series of carbon blacks and charcoals show that the fatty acid and the alkaU are adsorbed independently, within limits, although the presence of excess aLkaU reduces the sorption of total fatty acids (58). No straightforward relationship was noted between detergency and adsorption. 

In a study of the adsorption of soap and several synthetic surfactants on a variety of textile fibers, it was found that cotton and nylon adsorbed less surfactant than wool under comparable conditions (59). Among the various surfactants, the cationic types were adsorbed to the greatest extent, whereas nonionic types were adsorbed least. The adsorption of nonionic surfactants decreased with increasing length of the polyoxyethylene chain. When soaps were adsorbed, the fatty acid and the aLkaU behaved more or less independently just as they did when adsorbed on carbon. The adsorption of sodium oleate by cotton has been shown independently to result in the deposition of acid soap (a composition intermediate between the free fatty acid and the sodium salt), if no heavy-metal ions are present in the system (60). In hard water, the adsorbate has large proportions of lime soap. 

Considerable interest arose during the 1970 s and 1980 s in the use of micro-organisms to produce useful fatty adds and related compounds from hydrocarbons derived from the petroleum industry. During this period, a large number of patents were granted in Europe, USA and Japan protecting processes leading to the production of alkanols, alkyl oxides, ketones, alkanoic adds, alkane dioic acids and surfactants from hydrocarbons. Many of these processes involved the use of bacteria and yeasts associated with hydrocarbon catabolism. 

The C10-C,8 fatty acids and fatty acid methyl esters, important intermediates for the production of surfactants, are almost exclusively produced by hydrolysis or a transesterification reaction of animal or vegetable fats and fatty oils. 

The few examples of deliberate investigation of dynamic processes as reflected by compression/expansion hysteresis have involved monolayers of fatty acids (Munden and Swarbrick, 1973 Munden et al., 1969), lecithins (Bienkowski and Skolnick, 1974 Cook and Webb, 1966), polymer films (Townsend and Buck, 1988) and monolayers of fatty acids and their sodium sulfate salts on aqueous subphases of alkanolamines (Rosano et al., 1971). A few of these studies determined the amount of hysteresis as a function of the rate of compression and expansion. However, no quantitative analysis of the results was attempted. Historically, dynamic surface tension has been used to study the dynamic response of lung phosphatidylcholine surfactant monolayers to a sinusoidal compression/expansion rate in order to mimic the mechanical contraction and expansion of the lungs. 

Ethoxylation of alcohols, alkylphenols, fatty acids, and many other organic raw materials is also a very important reaction for the surfactant industry, used to produce a broad variety of surfactants, most of which belong to the non-ionics group. The reaction with propoxylation (PO) is also practised, although to a lesser degree. The product used for ethoxylation is EO, or PO in the case of propoxylation, and is conducted using alkaline catalysts such as NaOH or NaOCH3. 

Fatty acid and unsaturated fatty acid mono- and diethanolamides Fatty acid mono- (FAMA) and diethanol amides (FADA) with the general formula (C,32n+1-C(0)N(HyCH2-CH2-0H)2 x = l or 0) are presented with their general structural formulae in Fig. 2.9.28(a) and (b). These surfactants have found a widespread application in household and personal care formulations because of their quite good... 

In principle, the iterative experimental approach one would take today is no different than the one pursued by the product developers at Lever Brothers Co. in the early 1950 s, though possibly accumulated data about surfactants, fatty acids, and soaps in the public domain might make ingredient selection some-... 

The Gibbs equation relates the extent of adsorption at an interface (reversible equilibrium) to the change in interfacial tension qualitatively, Eq. (4.3) predicts that a substance which reduces the surface (interfacial) tension [(Sy/8 In aj) < 0] will be adsorbed at the surface (interface). Electrolytes have the tendency to increase (slightly) y, but most organic molecules, especially surface active substances (long chain fatty acids, detergents, surfactants) decrease the surface tension (Fig. 4.1). Amphi-pathic molecules (which contain hydrophobic and hydrophilic groups) become oriented at the interface. 

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