Surfactants chain length

Ionic surfactants actually only form micelles when their hydrocarbon chains are sufficiently fluid, that is at temperatures above their chain melting temperature. Below a specific temperature for a given surfactant, the Krafft temperature, the surfactant becomes insoluble rather than self-assembles. For CTAB this temperature is around 20 °C and only above this temperature are micelles formed. In general, the longer the hydrocarbon chain length, the higher the Krafft temperature. For this reason, shorter-chain-length surfactants or branched chain soaps... 

Generally a carbon chain length of 12 to 14 carbon atoms or a coco-type distribution of approximately 50% Cn is used for the primary surfactant in shampoos because of the excellent foam character and surface-active properties provided by such chain lengths. Longer or shorter chain length surfactants are used only in specialty systems. 

Robbins et al. [29] have shown that washing monofunctional cationic surfactants like cetrimonium chloride from hair with normal alkyl sulfates or alcohol ether sulfates does not remove all the cationic from the hair, and, in addition, the anionic detergent can build up with the cationic. Adsorption complexes formed in this manner adsorb to hair with the potential for building up. However, this type of buildup generally levels after five to six treatments. Shorter chain-length surfactants like deceth-2 or -3 ether sulfate do not build up in the same manner. In addition, hair matting has been reported in vivo and attributed to the adsorption of cetrimonium bromide on hair [42]. 

Some limited eontrol of pore size is possible simply by changing the length of the surfactant chain. A good example is for trimethylammonium bromide (CwTMABr, n = 8,10, 12, 14, 16,18) surfactants, where the pore size increases by roughly 2.25 A for each increase of one carbon in the surfactant. This means that even-numbered carbon chain length surfactants can be used to increase the pore size by increments of 4.5 A. A limitation of this method of pore size control is that the shortest chain surfactant from which mesophases can be made is n = 8 (imposing a lower limit of pore size of 15 A). An upper limit of possible pore size is 45 A due to the limited solubility of surfactants with n > 18 carbons which cannot therefore be used. 

Surfactant Template Structure. The structure of the surfactant also affects the mesostmcture in the final film. Changing the surfactant tail length adjusts the d-spacings and pore diameters found in the film, but well-ordered structures stable to calcination are produced only for Cn and longer chain length surfactants (Klotz, 2000b). 

Straight-chain anionic detergents are degraded in the rat by a co, oxidation, the metabolic products being carboxylic acid derivatives of short-chain alkyl sulphates. Anionic surfactants with even numbered chains are metabolized to butyric acid 4-sulphate and this is excreted in the urine [19-22]. The odd-chain length surfactant undecyl sulphate is eliminated in the urine as propionic acid 3-... 

This form is obeyed fairly well above x values of 5-10 dyn/cm in Fig. Ill-15c. Limiting areas or a values of about 22 per molecule result, nearly independent of chain length, as would be expected if the molecules assume a final orientation that is perpendicular to the surface. Larger A values are found for longer-chain surfactants, such as sodium dodecyl sulfate, and this has been attributed to the hydrocarbon tails having a variety of conformations [127]. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

An important application of foams arises in foam displacement, another means to aid enhanced oil recovery. The effectiveness of various foams in displacing oil from porous media has been studied by Shah and co-workers [237, 238]. The displacement efficiency depends on numerous physicochemical variables such as surfactant chain length and temperature with the surface properties of the foaming solution being an important determinant of performance. 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Within a series with a fixed hydrophilic head group, detergency increases with increasing carbon chain length, reaches a maximum, and then decreases. This behavior frequentiy reflects a balance between increased surface activity of the monomer and decreased monomer concentration with increased surface activity. Similar effects are seen in surfactants in biological systems. 

The conditions for surfactants to be useful to form Hquid crystals exist when the cross-sectional areas of the polar group and the hydrocarbon chain are similar. This means that double-chain surfactants are eminently suited, and lecithin (qv) is a natural choice. Combiaations of a monochain ionic surfactant with a long-chain carboxyHc acid or alcohol yield lamellar Hquid crystals at low concentrations, but suffer the disadvantage of the alcohol being too soluble ia the oil phase. A combination of long-chain carboxyHc acid plus an amine of equal chain length suffers less from this problem because of extensive ionisa tion of both amphiphiles. 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

Fluorocarbons with a hydrophilic functional group are very active surfactants [23]. Less than 1% of ionic or nonionic surfactants with perfluoroalkyl groups can reduce the surface tension of water from 72 to 15-20 dyne/cm, compared with 25-35dyne/cm for typical hydrocarbon surfactants [24] Perfluoroether surfactants are about as active as their perfluoroalkyl counterparts of similar chain length [25, 26], but fluorosurfactants with more polar alkyl end groups are considerably less active than their perfluoroalkyl analogues (Table 7)... 

As the example indicates, these liquids contained a complex mixture of surfactant actives. The LAS used in these products was of higher molecular weight (C,2 5-i3 average carbon chain length) than previous laundry liquids. Typical LAS active levels were 12% or less, presumably due to solubility constraints. 

Figure 20 shows the plot of the surface tension vs. the logarithm of the concentration (or-lg c-isotherms) of sodium alkanesulfonates C,0-C15 at 45°C. In accordance with the general behavior of surfactants, the interfacial activity increases with growing chain length. The critical micelle concentration (cM) is shifted to lower concentration values. The typical surface tension at cM is between 38 and 33 mN/m. The ammonium alkanesulfonates show similar behavior, though their solubility is much better. The impact of the counterions is twofold First, a more polarizable counterion lowers the cM value (Fig. 21), while the aggregation number of the micelles rises. Second, polarizable and hydrophobic counterions, such as n-propyl- or isopropylammonium and n-butylammonium ions, enhance the interfacial activity as well (Fig. 22). Hydrophilic counterions such as 2-hydroxyethylammonium have the opposite effect. Table 14 summarizes some data for the dodecane 1-sulfonates.

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