Surface tension Gemini 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. 

The dynamic surface tension is an important property, relevant to many practical, non-equilibrium processes such as emulsification and foaming. Dynamic surface tension is a measure of how fast, in the millisecond range, a surfactant decreases the surface tension from the value of pure water (around 70 mN/m) to values in the range of 30 mN/m. It has been found that the type of spacer governs the dynamic surface tension of geminis to a considerable degree, i.e. the longer and... 

Figure 17.3. Surface tension versus log concentration plots of the gemini surfactant Ci2H25N(CH3)2-(CH2)3-N(CH3)2Ci2 H25 ". 2Br"( ) and the monomeric surfactant Ci2H25N(CH3)3 +. Br (B). (From R. Zana, Dimeric (gemini) surfactants, in Novel Surfactants (Ed. K. Holmberg), Surfactant Science Series 74, Marcel Dekker, New York, 1998)...

It is observed that the Gemini surfactants are 10-100 times more efficient in decreasing the surface tension... 

A series of cationic gemini surfactants, alkanediyl-ot,a)-bis[di(2-hydro-xylethyl) dodecylammonium] dibromide (abbreviated as 12-S-12 (OH), with s = 4, 6, 8 and 10 methylenes) have been synthesized, and their aggregation properties in aqueous solution have been studied by surface tension, calorimetry, H NMR, DLS, and TEM. Two critical aggregation concentrations are observed for these surfactants. These surfactants start to form dimers at concentrations well below their critical micelle concentrations (CMC). Above the CMC, these surfactants can form both micelles and vesicles spontaneously with a micelle-to-vesicle transition.The formation and characterization of catanionic vesicles by newly synthesized lysine- and... 

Figure 6 (a) Dynamic surface dilational moduli for pure gelatin and gelatin/[C -i-C im]Brj systems, (b) Static surface dilational viscoelasticity as a function of [C -5-C im]Brj concentrations, (c) Surface tension as a function of surfactant concentration for pure [C -5-C im]Brj and gelatin/[C -5-C im]Br2 systems, (d) A schematic illustration of the interaction between gelatin and imidazolium gemini surfactant on the air/solution surface. The surface dilational modulus. Reproduced from Ao et al. [94] with permission from the Royal Society of Chemistry. 

Extensive surface tension measurements have been performed on aqueous solutions of gemini surfactants with the purpose of investigating their behavior at the air solution interface (measurement of surface area a occupied by one surfactant molecule at the interface) and determining CMCs. The surface areas a were obtained from the slope of the variation of the surface tension 7 with In C (C = surfactant concentration) using the Gibbs expression of the surface excess concentration T ... 

The effectiveness of gemini surfactants in lowering the surface tension of water is close to that of the corresponding monomeric surfactant. Indeed, the values of the surface tension at the CMC, ycMc, are close for monomeric and gemini surfactants, as illustrated by the results shown in Fig. 3 for 12-3-12, 2Br [4] and its corresponding monomeric surfactant, dodecyl-trimethylammonium bromide (DTAB) [72]. However, the former are always more efficient surface-active agents than the latter because their CMCs... 

FIG. 3 Surface tension vs. concentration of the gemini surfactant 12-3-12, 2Br" ( ) and of dodecyltrimethylammonium bromide ( ). (Adapted from data in Refs. 4 and 72.)... 

For instance, the values of a for C Eg and CioEg are 63 and 70 A, respectively. This suggests that the monolayer formed by gemini surfactants [Q] is relatively closely packed. For surfactants [Q] with X = S04Na, the surface tension at the CMC ("Ycmc) increases with the polyoxyethylene chain length, a phenomenon that is also observed with C E surfactants... 

Dynamic surface tension (DST) studies of the cationic surfactants [A] with Y = CHOH showed that their adsorption at the air-solution interface is controlled by diffusion [83]. On the contrary, for gemini surfactants [Q] (Fig. 5), the results suggest that at the beginning (short times) the adsorption is essentially diffusion controlled. However, close to equilibrium (long times) the DST decays are not consistent with a diffusion-controlled adsorption... 

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