Defined Oligomeric Surfactants

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], 

Virtually all examples reported for higher oligomeric surfactants than dimer belong to the tail end type , preventing an analysis of the evolution of geometric effects. This is the more true, as the majority of the tail end structures represents stars [416, 417, 420, 422,425, 449] rather than linear or cyclic oligomers [418,419,423], Most attempts to produce head-type trimers suffer from the poor water solubility of the products [450], but recently some water-soluble examples have been reported [427]. 

Although no full picture of the above referred studies can be obtained, some general trends are visible oligomerization of surfactants tends to decrease surface activity and CMC, spacer effects in contrast to simple HLB considerations are evident, and the mobilities of the various molecular fragments change considerably. All these general trends parallel the differences seen between standard surfactants and polysoaps in Sects. 3 to 5. 

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In 1950 the Fischer-Tropsch synthesis was banned in Germany by the allied forces. Sinarol, a high paraffinic kerosene fraction sold by Shell, was used as a substitute. This ban coincided with the rapid development of the European petrochemical industry, and in due time Fischer-Tropsch synthesis applied to the production of paraffins became uneconomic anyway. After the war there was a steady worldwide increase in the demand for surfactants. In order to continually meet the demand for synthetic detergents, the industry was compelled to find a substitute for /z-paraffin. This was achieved by the oligomerization of the propene part of raffinate gases with phosphoric acid catalyst at 200°C and about 20 bars pressure to produce tetrapropene. Tetrapropene was inexpensive, comprising a defined C cut and an olefinic double bond. Instead of the Lewis acid, aluminum chloride, hydrofluoric acid could now be used as a considerably milder, more economical, and easier-to-handle alkylation catalyst [4],... 

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