Structural Properties and Phase Behavior ... 21a strong screening of polyelectrolyte effects with salt as low as 0.1, where
practically the neutral limit is reached (see panel b).
5. MICELLE FORMATION
5.1. Neutral Micelles
Because of their amphiphilic character, DCPs in solution aggregate in the
form of micelles which opens new prospects for the development of drug
delivery carriers having efficient targeting capabilities [29-31]. An example of
a neutral DCP is polyethylene oxide/polypropylene oxide associating high
solubility in aqueous solutions, biocompatibility, loading capacity, retention
and sustained release of drugs. Another good candidate which exhibits the
ability to respond to environmental changes [32, 33], such as pH and
temperature is the DCP made of charged polyacrylic acid/neutral polystyrene
blocks. The relatively small size of DCPs prevents their filtration by the
endothelial system and favors their dispersion into free chains for removal
after the process of drug delivery is completed. The case of partially charged
DCPs is particularly useful for offering the possibility to encapsulate poorly
soluble drugs in the core prior to their delivery on the tumor sight. Here we
briefly discuss some important aspects such as the critical micelle
concentration (in short, cmc) and the aggregation number Nag representing the
number of DCPs per micelle. Depending on the relative size of blocks A and
B, two configurations are possible: star like micelles (see Figure 11a) obtained
when the hydrophobic block in the core is much shorter than the corona
hydrophilic block A (i.e; Na >> Nb). Otherwise, we have the so called crew cut
micelle as shown in Figure 11b [34-36].
Aggregation of DCPs into micelles is accompanied by a drastic reduction
of the free energy which is essentially a sum of contributions from collapsed
B-blocks in the core, stretched A-blocks in the corona and the interface (28,
33, 34). The cmc is obtained in terms of the system parameters by setting the
free energy of the micelle to zero. It is found that in the star like configuration,
the cmc may be represented by the following scaling behavior
2/3 2/5 2 1 /5 2/5
cmc b b ab
3/5
bln ln
NNv v
NN
N