Alkyl chain length, anionic surfactant, effect

Schick and Fowkes (11) studied the effect of alkyl chain length of surfactants on critical micelle concentration (CMC). The maximum lowering of CMC occurred when both the anionic and nonionic surfactants had the same chain length. It was also reported that the coefficient of friction between polymeric surfaces reaches a minimum as the chain length of paraffinic oils approached that of stearic acid (12). In order to delineate the effect of chain length of fatty acids on lubrication, the scuff load was measured by Cameron and Crouch (13). The maximum scuff load was observed when both hydrocarbon oil and fatty acid had the same chain length. Similar results of the effect of chain length compatibility on dielectric absorption, surface viscosity and rust prevention have been reported in the literature (14-16). 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

Most detergents contain electrolytes, e.g. sulphate, bicarbonate, carbonate or citrate and the presence of these electrolytes increases the adsorption of anionic surfactants at the gas/liquid interface as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration [7] and to a strong decrease of the cmc. The effect can be of several orders of magnitude. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain length or mixtures of anionic and non-ionic surfactants as they are mostly used in detergency [8]. Mixtures of anionic and non-ionic surfactants follow the mixing rule (eqn. 3) in the ideal case ... 

Effect of the Alkyl Chain Length of the Anionic Surfactant. 

Table II displays the PLMA MW in the model system as a function of the alkyl chain length of the anionic surfactant, all other concentrations remaining constant. When the prepolymerization solution contains only C12EO5, the solution is opaque and the PLMA MW is large this result implies the existence of large micelles. Addition of sodium 2-ethylhexyl sulfate to the C12EO5-LMA prepolymerization solution at the concentration used for SDS (0.035 M) dramatically increases the MW. This result is presumably due to an electrolyte effect that increases nonionic surfactant aggregation numbers (3i), because the sodium 2-ethylhexyl sulfate concentration is far below the surfactant s CMC. However, doubling the sodium 2-ethylhexyl sulfate con-...

Table II. Effect of Alkyl Chain Length of Anionic Surfactant on PLMA MW...

Other soils have not been studied so extensively however, Robbins et al. [29] have shown that Cn alkyl sulfates or alkyl ether sulfates, the traditional shampoo surfactants, do not remove cationic surfactants from hair effectively under certain conditions. However, shorter chain-length anionics such as deceth-2 sulfate is more effective for removing cationics. In addition, alkyl ether sulfates are more effective for removing fatty acid soils in the presence of water hardness than alkyl sulfates [29]. 

Our group [38] measured isotherms for the cationic C TAB surfactant in various ILs toward understanding the role of the IL. The ILs studied included those with imidazolium, pyrrolidinium, and ammonium cations with varied anions. This study identified CMC values by solubility measurements in addition to isotherms, revealing the key importance of temperature on micellization in ILs. Further, we also confirmed the logarithmic dependence of the CMC with surfactant alkyl chain length (Fig. 2.10). The relative CMC values for the same C TAB in different ILs was adequately described by a mean-field model, lending support for a dominant role of interfacial energy between surfactant and IL and a minimal role of electrostatic effects [38]. 

HPLC can be effectively used to determine the degree of sulfonation of anionics such as petroleum sulfonates or paraffin sulfonates, where di- and polysulfonated strac-tures are possible. Another use is determination of the total quantity of a particular surfactant. HPLC may be used to characterize an anionic according to its alkyl chain length,... 

Novel fluorescent anionic surfactants of the types 11.33 and 11.34, where R represents alkyl groups of various lengths, have been applied to wool in order to study their distribution and effects on the physical and chemical properties of the fibre. Sections of the treated fibres were examined under a fluorescence microscope. The intercellular and cell remnant regions appeared to be the preferred locations of the adsorbed surfactants, but the distribution pattern was dependent on the length of the R chain of the surfactant and the conditions of application to wool [52]. 

An especially effective reduction of the Krafft Point results from the insertion of ether groups into the molecule of the anionic surfactant. In table I this is examplified with Na dodecyl sulfate and Na-tetra-decyl sulfate in comparison to various n-alkyl ether sulfates of the same chain length (10). As a measure of the Krafft Point, a temperature is deTined at which a 1 7o solution dissolves clearly. By the incorporation of oxyalkylene groups into the molecule, the Krafft -Point and the melting point are greatly depressed. This depression is especially effective if there is branching in the oxyalkylene groups. 

The comparison of CMC data in distilled vs. hard river water shows that the decrease in CMC with hardness has the order anionics cationics nonionics (Rosen et al., 1996). Hardness increases the dependence of the CMC on alkyl carbon chain length of CnE0mS04, indicating that in hard water the influence of additional carbon atoms is the same for CnE0mS04 as for CnEOm surfactants (Rosen et al., 1996). The influence of ionic strength on micellization of nonionic surfactants is due to a salting out effect of the hydro-phobic moiety of the surfactant molecule (Carala et al., 1994). 

The effect of a surfactant on skin depends on the type of surfactant as described earlier. Wilhelm et al. demonstrated the irritation potential of anionic surfactants.21 They evaluated the effects of sodium salts of n-alkyl sulfates with variable carbon chain length on TEWL and found that a C12 analog gave a maximum response. They suggested that the mechanisms responsible for the hydration of SC are related to the irritation properties of the surfactants. Leveque et al. also suggested22 that the hyperhydration of SC is consecutive to the inflammation process. They demonstrated that the increase of TEWL was induced by SDS without removal of SC lipids. SDS might influence not only SC barrier function, but also the nucleated layer of epidermis and dermal system associated with inflammation.23 Recently, no correlation was found between the level of epidermal hyperplasia and TEWL increase on the SDS-irritated skin.23 Further work would be needed to determine the effects of surfactants on skin. 

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