Gemini surfactants spacers

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 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... 

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 ... 

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. 

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)...

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). 

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 double-headed cationic compounds where two alkyldimethyl quaternary ammonium groups per molecule are linked by a hydrocarbon spacer chain. (These are denoted as n-m-n surfactants, where n is the length of the terminal alkyl chains and m the length of the spacer chain.) In the second paper (183), Fuller et al. reported the phase behaviour of further m-n-m OCB surfactants and some straight-chain 15-n-15 surfactants (n = 1, 2, 3 and 6). Note that these compounds have terminal hydrophobic chains of the same length as the oxycyanobiphenyl compounds. The m-n-m OCB surfactants all give just a lamellar phase from < 18°- > 100°C. Penetration scans on the 15-n-15 surfactants show that they all exhibit Hi, Vi and La phases to > 100°C. Additionally, a nematic phase is seen for m = 1 and 2, and intermediate phases for m = 1, 2 and 3. 

Gemini surfactants are defined as surfactants made up of two identical amphiphilic moieties connected at the level of the headgroups, or of the alkyl chains, but still very close to the head groups, by a spacer group which can be hydrophobic, flexible, or rigid (Fig. 7.25). 

Before delving into specific examples of gemini surfactants, it should be pointed out that the key feature differentiating them structurally from conventional surfactants is the linkage between individual amphiphilic moieties. The linkages are often referred to as spacers. Although the term... 

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