Bola-surfactant

Generally, both types of dimeric surfactants still show well-defined, clear CMCs, though in selected cases the CMC is less obvious to standard methods, in particular to conductometric methods [415, 424, 425]. The values of CMCs determined by different methods agree well, and the counter-ion effects are the usual ones [426,427]. But, strikingly, CMC values of both bola-surfactants and of gemini-surfactants are lowered by one order of magnitude or more in... 

From surface tension studies of bola-surfactants, it is concluded that they exhibit wicket-like conformations at the gas-water interface [239,428,430,431], In micelles and liquid crystals however, a stretched conformation is preferred [436,437] this implies that surface tension data and interfacial tension data do no more describe the micellar interface with all the implications for solubilization (compare Sect. 3.4). In fact, some reports stress the extremely low solubilization capacity of bola-surfactants [431,432, 437,438], although others obtain capacities comparable to the ones of the monomers [430]. Also noteworthy, solubilized fluorescence probes indicate a more polar environment for the solubilizates than in micelles of the monomers [430-432], but micellar aggregation numbers of the bola-surfactants are comparable or only slightly lower [429,432, 438, 439]. In exceptional cases, very high aggregation numbers and the existence of an additional pre-CMC are observed [440]. 

NMR-studies indicate a reduced segmental mobility and higher order parameter for the central hydrocarbon fragments of bola-surfactants [437] compared to the tail ends of the corresponding monomers. The differences however are less pronounced as for oligomeric poly soaps [68, 69],... 

Host-guest chemistry can be applied. For example, bola molecules with crown ether heads can host ionic surfactants such as SDS such a catanionic bola/surfactant system is expected to be responsive to additional salt or specific metal ions and, in fact, the sodium ions released from the SDS can enter the cavity of the crown ether, charging the bola, so as to form ion pairs with the anionic SDS molecules. ... 

Exploiting the bipolar nature of the bolaamphiphiles, multilayers of oppositely charged bola/surfactants can be produced using the layer-by-layer method, that is, by dipping a mica or silica wafer into the solution of a bola and an oppositely charged polyelectrolyte multilayer. Perspectives in advanced functional materials can be foreseen in... 

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

Fig. 40a-g. Various types of oligomeric surfactants a bola dimer b gemini dimer c linear hexamer ( tail end type) d linear hexamer ( head type) e cyclic hexamer, f star-shaped hexamer ( tail end type) g star-shaped hexamer ( head type)... 

Fig. 41a-d. Examples of oligomeric surfactants a bola dimer [426-428] b gemini dimer [435, 445, 448] c cyclic hexamer of tail end type [419] d star-shaped trimer of tail end type [420]... 

The term dicephalic surfactants was introduced by Sommerdijk et al. [141] to describe surface-active compounds that contain one hydrocarbon chain and two polar head groups in their molecule. However, unlike bola-amphiphiles, dicephalic surfactants contain the two hydrophilic groups at one end of the hydrophobic group. Some structures of nonionic dicephalic surfactants are shown in Fig. 18. The first examples of such compounds were probably patented and described by Ulsperger et al. [142-144], who reacted 1,5-d-... 

Valivety, R. Gill, I.S. Vulfson, E.N. Enz5unatic synthesis of novel bola- and gemini-surfactants. J. Surfactants and Detergents 1998, 1, 177-185. 

VaUvety R, GUI IS Vulfson EN. 1998. AppUcation of enzymes to the synthesis of amino acid-based bola and gemini surfactants. Journal of Surfactants and Detergents 1(2) 177-185. van Rantwijk F, Woudenberg-van Oosterom M Sheldon RA. 1999. Glycosidase-catalyzed synthesis of alkyl glycosides. Journal of Molecular Catalysis B Enzymatic 6(6) 511-532. 

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