Aggregation polar group factor

The micelles formed in the non-polar solvents would seem in general to be relatively small, the aggregation number being of the order of 4 to 30. When the micelles are nearly spherical, as they seem to be at low concentrations, the low aggregation numbers are due to steric factors. The space in a spherical soft-core micelle permits only the accommodation of a limited number of polar groups this also applies to the number of bulky hydrocarbon groups in the outer parts of the micelle (Eicke, 1980 Ekwall, 1972 Kertes and Gutman, 1976 Rounds, 1976). 

The polar group of surfactants is one of the essential factors determining the size of soft-core reverse micelles. In general, reverse micelles have moderate aggregation numbers carboxylates in toluene (5 to 20), calcium cetylphosphate plus calcium alkylphenolate in lube oil (20 to 30), tetra-n-alkyl-ammonium salts in benzene (3 to 25), and sodium bis(2-ethylhexyl)sulfosuccinate in dodecane (32), (Bascom et al., 1959). 

Aggregation number Three main factors play a critical role in aggregation of soft-core reverse micelles the interaction between polar groups, the interaction of the hydrophobic (non-polar) part, and environmental factors. Compare aggregation numbers of some surfactants in low polar solvents specified in Table 3.1. 

It is obvious that a different factor must be found to explain the sensitivity to dimer formation for maximum water content in the monomeric surfactant aggregates because this phenomenon Is entirely outside the realm of polymers and their entroplc conformational demands. One such factor is the Interaction between aromatic compounds and the polar group of a surfactant. Two examples of such Interaction are described In the next section. 

The presence of the polar groups introduces another factor capable to enhance the aggregation numbers for the zwitterionic samples in n-decane. Much lower Nw values were observed for the amine-capped copolymers, meaning that the amine groups are not polar enough to enhance the association process. Typical LALLS plots are given in figure 12. 

The perturbations to rod-like molecules created by functional groups can operate also on disk-like ones. When substituted with at least one flexible chain and at least one other (frequently polar) group, preferably in a para orientation, many molecules with benzene or cyclohexane as cores provide nematic and smectic phases [11,12]. Additional substituents can also lead to other mesomorphic aggregation schemes [105]. The shapes of these molecules can no longer be considered disk-like, and they owe their ability to be liquid crystalline to shape anisotropy and electronic factors alluded to previously [18,19]. 

The additive mixtures interact in a variety of ways, both in the bulk oil and on surfaces. Tribochemical interactions of additives in the oil formulation are discussed in Chapter 2. Surfactant molecules, when dissolved in base oil, are capable of self-organization to form aggregates such as soft-core reverse micelles (RMs). The polar or charged head groups of these molecules with the counter ions form the interior of the micelle (core), and the hydrocarbon chains made up its external shell. The most important factor governing the tribochemical reactions under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles discussed in Chapter 3. 

Figure 7 Plot of the change in the product of the coupling and maximum saturation factors as a function of macromolecular structure. At lower pH values, the spin-labelled lipids are present as vesicles and vesicular aggregates, while at higher pH values, micelles are formed. The higher psmax values for the micelles imply greater water accessibility to the radical site. The solid circles represent 16-DS (16-doxyl stearic acid, spin-labelled at the end of the lipid tail) while the open circles represent 5-DS (5-doxyl stearic acid, spin-labelled near the polar head group). Reproduced with permission from Ref. [70].

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