Gemini surfactants values

Generally, both types of dimeric surfactants still show well-defined, clear CMCs, though in selected cases the CMC is less obvious to standard methods, in particular to conductometric methods [415, 424, 425]. The values of CMCs determined by different methods agree well, and the counter-ion effects are the usual ones [426,427]. But, strikingly, CMC values of both bola-surfactants and of gemini-surfactants are lowered by one order of magnitude or more in... 

A highly unexpected result, first reported in 2(X)3, was the efficient adsorption of hydroxide ions at pH values slightly above 7.5 to (almost) uncharged hydrophobic snrfaces and vesicular surfaces formed by reduced-sugar based gemini surfactants. The origin of this phenomenon has been investigated in some detail. ... 

For conventional surfactants with a long alkyl chain (m > 16) and dimeric (gemini) surfactants with m>8, the entry of a surfactant in a micelle is slower than for a diffusion-controlled process. The surfactant residence time and the micelle lifetime can become long with respect to the values found for conventional surfactants. 

Gemini surfactants have been obtained from a trisiloxane amine, D-gluconic acid 5-lactone and oligoethylene glycol diglycidyl ethers [89] (Scheme 6). These surfactants reduced the surface tension of water to approximately 21 mN/m at a concentration of around 10 mol/L and showed two critical aggregation concentration values. 

Arguably the most important parameter for any surfactant is the CMC value. This is because below this concentration the monomer level increases as more is dissolved, and hence the surfactant chemical potential (activity) also increases. Above the CMC, the monomer concentration and surfactant chemical potential are approximately constant, so surfactant absorption at interfaces and interfacial tensions show only small changes with composition under most conditions. For liquid crystal researchers, the CMC is the concentration at which the building blocks (micelles) of soluble surfactant mesophases appear. Moreover, with partially soluble surfactants it is the lowest concentration at which a liquid crystal dispersion in water appears. Fortunately there are well-established simple rules which describe how CMC values vary with chain length for linear, monoalkyl surfactants. From these, and a library of measured CMC values (35-38), it is possible to estimate the approximate CMC for branched alkyl chain and di- (or multi-) alkyl surfactants. Thus, most materials are covered. This includes the gemini surfactants, a new fashionable group where two conventional surfactant molecules are linked by a hydrophobic spacer of variable length (38). 

While the gemini surfactants discussed above have many properties favorable for EOR and remediation applications, like all surfactants their use will be determined by their behavior under conditions specific to the application. In general their adsorption values were lower than those exhibited by many other surfactants and their salinity tolerance has been demonstrated. The similar behavior exhibited by the CIO MADS and CIO DADS components onto alumina further illustrates the importance of pore size in influencing adsorption. For this particular suite of surfactants the components which are not true gemini surfactants tend to... 

The CMC values of Gemini surfactants are at least one order of magnitude lower than that of the corresponding conventional single-tail surfactants with a similar head group. The CMC values of commonly reported Gemini surfactants and some classic conventional surfactants are listed in Table 11.7. 

Table 11.7 CMC values of representative single-tail and gemini surfactants[26-28].

As discussed previously, the cmc values for gemini surfactants are always lower than their single long-chain counterpart due to more favorable (AG ). As can be seen in Figure 4.5, there is also a decrease in cmc as a function of increasing nC (slope = -0.89 0.03). Concerning the influence of the number of carbons in the... 

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