Spacers , gemini

A theoretical explanation of experimental results, obtained for modified (non rigid spacer) gemini surfactants, was presented by Andelman and Diamant.47 They noted that the aggregate morphology for dimeric surfactants is related mainly to the influence of the spacer on the specific area X. The geometrical parameter, that determines aggregate shape, is the packing parameter denoted by the formula 48,49... 

Fig. 2.11.35. General structural formula of anionic gemini surfactants (R = spacer).

Beside the conventional surfactants with one polar head group and one nonpolar tail, dimeric and oligomeric surfactants have attracted considerable interest in academia and industry [521], Dimeric surfactants, also called Gemini surfactants, are made up of two amphiphilic moieties connected closely to the head group by a spacer group (Fig. 12.1). In bolaform surfactants the connection is in the middle of the alkyl chain or close to the end, so that they can be considered as two polar head groups connected by a long hydrophobic chain. 

The technology supporting gemini surfactants has been in existence for more than 20 years and they are so-called because they have two hydrophobic head groups and two hydrophilic groups in the same molecule. The two portions of the molecule are linked by a spacer which can be attached directly to the two ionic groups or can be positioned down the hydrocarbon chain as shown in Figure 5.3, positions 1 and 2 respectively. 

A gemini surfactant is a dimeric molecule consisting of two hydrophobic tails and two head groups, linked together with a short spacer [14]. This is illustrated below for a molecule containing two cationic head groups (separated by two methylene groups) with two alkyl chains ... 

Trimeric and oligomeric surfactants have also been prepared (Zana, 1995 Sumida, 1998 In, 2000 Onitsuka, 2001). Their CMC values are even smaller than those of the analogous geminis. As the number of hydrophobic groups per molecule increases for gemini quaternary C12 ammonium compounds with polymethylene -(CH2) j spacers, their surface layers become more dense, their micellar microviscosity increases, and their micellar shape changes from spherical to wormlike, to... 

J. E. Klijn, M. C. A. Stuart, M. Scarzello, A. Wagenaar, J. B. F. N. Engberts, pH-dependent phase behaviour of carbohydrate-based gemini surfactants. The effects of carbohydrate stereochemistry, head group hydrophilicity, and nature of the spacer, J. Phys. Chem. B, 2007, 111, 5204-5211. 

The second of the above compounds is an example of a so-called gemini surfactant. Recently there has been considerable interest in these surfactants which are doubleheaded cationic compounds in which two alkyl dimethyl quaternary ammonium groups per molecule are linked by a hydrocarbon spacer chain. (These are denoted as n-m-n... 

The term gemini surfactant was coined by Menger inl991. A gemini surfactant may be viewed as a surfactant dimer, i.e. two amphiphilic molecules connected by a spacer. Figure 17.1 shows the general structure. Gemini surfactants are also referred to as twin surfactants , dimeric surfactants or bis-surfactants . 

Figure 17.2 shows examples of gemini surfactants, which have been somewhat randomly taken from the literature. Compounds 1-3 are cationics differing in the type of spacer unit connecting the two ionic moieties. The spacer of Compound 1 is hydrophobic and flexible, that of Compound 2 is hydrophilic and flexible, while that of Compound 3 is hydrophobic and rigid. Compound 4 is a typical nonionic gemini and Compound 5 is an anionic one, based on the same backbone structure. Compounds 6 and 7, finally, are examples of heterogemini surfactants. 

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... 

Geminis with flexible spacers, both hydrophilic and hydrophobic, generally show lower Ycmc values than the corresponding surfactants with rigid spacers. This is probably due to a better packing of the former at the air-water interface (see below). 

Figure 17.5. Variation of the area, a, per surfactant molecule at the air-water interface against the number of methylene groups, Sy in the spacer for the series of gemini surfactants Ci2H25N(CH3)2-(CH2),-N(CH3)2Ci2H252+-2Br-. (From R. Zana, Dimeric (gemini) surfactants, in Novel Surfactants (Ed. K. Holmberg), Surfactant Science Series 74, Marcel Dekker, New York, 1998)...

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