ABSTRACT
GLOSSARY a: surface area occupied by one surfactant at the air-solution interface erne: critical micelle concentration m: number of carbon atoms in the surfactant alkyl chain m-s-m: alkanediyl-a,w-bis(alkyldimethylammonium bromide) dimeric surfactants s: number of carbon atoms in the alkanediyl spacer of m-s-m surfactants nT: total number of carbon and oxygen atoms in the spacer of dimeric surfactant with a poly(ethylene oxide) spacer (see surfactant A3 in Table 1) C: surfactant concentration DT AB, CT AB: dodecyl, hexadecyltrimethylammonium bromide SANS: small angle neutron scattering TRFQ: time-resolved fluorescence quenching r : surface excess concentration
I. INTRODUCTION Dimeric surfactants are 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 headgroups, by a spacer group, which can be of variable chemical nature (Figs. la and b). The first studies of dimeric surfactants, by Bunton et al. in 1971 [1] and Ul'berg et al. [2,3] in 1974, dealt with surface properties and critical micelle concentrations (erne) of alkanediyl-a,wbis(alkyldimethylammonium bromide) surfactants. These surfactants were referred to as "bisquaternary ammonium" [1-4], "gemini" [5], "dimeric" [6], and "siamese" [7] surfactants. Other surfactants of similar structure, with a hydrophobic or hydrophilic, flexible or rigid spacer, were later synthesized and investigated. Still later, the synthesis of this type of surfactants was extended in three directions:
1. Surfactants made up of more than two amphiphilic moieties connected at the level or close to the headgroup, called oligomeric surfactants [8,9]
2. Dimeric surfactants where the two amphiphilic moieties are different and that can be referred to as heterodimeric surfactants [1 0,11]
3. Dimeric surfactants with mixed fluorinated-hydrogenated alkyl chains, such as C8F11C4Hs [12]
In this chapter "dimeric" refers to surfactants made up by the covalent binding of two moieties, which are conventional amphiphiles (made up of one hydrophilic headgroup and one hydrophobic moiety, which may include more than one alkyl chain). It is essential that the spacer, whichever its nature, be located very close to the headgroups. When a fully hydrophobic spacer (polymethylene chain or aromatic group) connects the amphiphilic moieties far from the headgroups, somewhere in the second half of the alkyl chains, the dimeric surfactant is then simply a bolaform surfactant with a branched alkyl chain (Fig. 1c), characterized by a high erne and poor performances [13]. The results may be worse with a hydrophilic spacer.
