Hemimicelle self assemblies

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

Several papers reported on the spatial determination of photoactive molecule in organized assemblies (76-79), and Pallavicini and coworkers reviewed on the use of luminescence as probe in self-assembly of multicomponent fluorescent sensors (80). Also, luminescence quenching studies on [Rufbpyls] in sodiiun dodecyl sulfate (SDS) micelles and hemimicelles by using a variety of quenchers were reported by Tiuro and coworkers (81) and then reviewed by De Schryver and coworkers (82). 

Several different experimental techniques, such as fluorescence decay, electron spin resonance (ESR) spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, neutron reflectome-try, calorimetry, Fourier-transform infrared (FT-IR) adsorption spectroscopy, small-angle neutron scattering (SANS), ellipsometry and surface force measurements, have been used to study self-assembled surfactant structures at the solid-liquid interface (11). These measurements, although providing insight into the hemimicellization process, critical aggregation numbers... 

Figure 10.24. Schematic representation of the proposed self-assembled surfactant films at concentrations corresponding to regions A-F in Figure 10.23 (A) individual surfactant adsorption (B) low concentration of hemimicelles on the surface (C) higher concentration of hemimicelles on the surface (D) hemimicelles and spherical surfactant aggregates formed due to increased surfactant adsorption and transition of some hemimicelles to spherical aggregates (E) randomly oriented spherical aggregates at the onset of steric repulsive forces (F) surface fully covered with randomly oriented spherical aggregates (after ref. (11)). ( Reproduced with permission from Langmuir, 2001, 17, 468-473, Copyright (2001) American Chemical Society )...

An alternative route is to use diblock copolymers in a selective solvent. In this case, one of the blocks (say, the A block) is not soluble. This has two consequences. In solution, free chains are not stable they self-assemble into structures such as micelles or lamellae where the insoluble A chains form the core they are screened from the solvent by a corona (which is, of course, some kind of spherical brush) of B chains. On the surface, several situations may arise. In case the A blocks make contact with the surface, either they can form a homogeneous (complete) wetting film from which a brush of B chains protrudes or a heterogeneous layer of packed hemimicelles can arise. It is also possible that the B chains adsorb, without any contact between A and the surface. In that case the micelles remain almost intact. The polymer-solvent interaction of the A chain has a large influence the worse the solvent, the less solvent is taken up in A domains, and the denser the B chains are packed at the A-solvent interface. Since A segments can adsorb in the form of an insoluble film, where many of them have no direct contact with the substrate, their crowding limits the brush density much less severely than when they would be soluble. 

SF calculations for adsorption from a selective solvent were carried out by van Lent and Scheutjens [39]. Of course, the calculations had to deal with the self-assembly process as well. For the case of a rather poorly soluble block connected to a soluble block and not too extreme block lengths the critical micellization concentration (CMC) is extremely low. For any practical situation it then can be safely assumed that adsorption takes place from a solution of almost constant chemical potential, so in equilibrium there is no significant effect of concentration. The longer the A blocks (and hence the shorter the B blocks), the thicker the A film and the higher the density of the brush of B blocks. These calculations make clear that it is favorable to use a strongly insoluble block as anchor, and that the wettability of the solvent-substrate pair by A is important. The case of a heterogeneous layer of hemimicelles requires a more elaborate two-dimensional SF scheme, which was not considered by Van Lent. 

---