Hydrophobicity, alkyl chain length

TX-45, TX-114, TX-100 have the same hydrophobic group (4-(l,l,3,3-tet-ramethylbutyl)phenyl), but have different lengths of hydrophilic poly ethoxy 1 chain, i.e. n = 5, 7.5 and 10, respectively (Scheme 4.6) Tween 20 and Tween 80 have the same molecular structure and EO number, while differentiate in the hydrophobic alkyl chain length. It is well known that the smaller the Henry s constant, the larger the solubility. The Henry s constants of C02 (e.g., 25 °C) in the surfactants with more EO content are smaller (TX-45 39.8 > TX-114 28.7 > TX-100 20.0), while the effect of alkyl chain length on the Henry s constant is very limited (Tween 20 10.7 and Tween 80 10.1). The absolute value of the enthalpy increases considerably with increasing EO content (TX-45 -14.5 2.1, TX-114 -15.9 2.3 and TX-100 -20.4 1.6) [83]. 

Fig. 2. Surface tension of solutions of CnEm (n=12-16 m=4-8) in bmimBF4. a) Effect of hydrophilic ethyleneoxide part b) Effect of hydrophobic alkyl chain length...

The various olefinsulfonates used in this study differed in their alkyl chain lengths and in the position of the ionic head group along the hydrophobe moiety. Furthermore, the composition of some IOS compounds was modulated by using different process conditions. 

Phosphoric acid esters based on alkylene oxide adducts are of great interest. Their properties can be altered by the length and structure of the hydrophobic alkyl chain. But they are also controlled by the kind and length of the hydrophilic alkyleneoxide chain. The latter can easily be tailored by selection between ethylene oxide and propylene oxide and by the degree of alkoxylation. 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

Some of the compounds described in this chapter were studied for specific physical properties. Surface tension measurements with solutions of 9-16 in 0.01 M hydrochloric acid demonstrated that these zwitterionic X5Si-silicates are highly efficient surfactants.21 These compounds contain a polar (zwitterionic) hydrophilic moiety and a long lipophilic z-alkyl group. Increase of the n-alkyl chain length (9-15) was found to result in an increase of surface activity. The equilibrium surface tension vs concentration isotherms for 9 and 16 were analyzed quantitatively and the surface thermodynamics of these surfactants interpreted on the molecular level. Furthermore, preliminary studies demonstrated that aqueous solutions of 9-16 lead to a hydrophobizing of glass surfaces.21... 

In all of these data what requires explanation is the effectiveness of the dodecyl chain, the decreasing activity with increasing ethylene oxide chain length above EiQ-14 the increase in activity when we move from a very hydrophobic surfactant with short ethylene oxide chain to the optimum, and the decrease in activity with increasing lipophilicity of compounds with alkyl chain lengths greater than Cl2. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

The physical chemical properties of the surfactants that contain an ester bond between the hydrophobic tail and the polar head group are very similar to those of alcohol ethoxylates of the same alkyl chain length and the same number of oxyethylene units. The CMC and the cloud point values of the linear ester surfactant 1 of Fig. 4 are approximately the same as those of the straight chained alcohol ethoxylate tetra(ethylene glycol)monooctyl ether (C8E4), i.e., around 10 mM and 40 °C, respectively. Thus it appears that the... 

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