Here,
we study one-component and mixed n-alkyl-poly(ethylene
glycol) (CmEn) micelles with varying poly(ethylene glycol) (PEG) chain lengths n using coarse-grained molecular simulations. These nonionic
alkyl-PEG surfactants and their aggregates are widely used in bio
and chemical technology. As expected, the simulations show that increasing
the PEG chain length decreases the alkyl-PEG micelle core diameter
and the aggregation number but also enhances PEG chain penetration
to the core region and spreads the micelle corona. Both the core and
corona density are heavily dependent on the PEG chain length and decrease
with increasing PEG length. Furthermore, we find that the alkyl-PEG
surfactants exhibit two distinct micellization modes: surfactants
with short PEG chains as their hydrophilic heads aggregate with the
PEG heads relatively extended. Their aggregation number and the PEG
corona density are dictated by the core carbon density. For longer
PEG chains, the PEG sterics, that is, the volume occupied by the PEG
head group, becomes the critical factor limiting the aggregation.
Finally, simulations of binary mixtures of alkyl-PEGs of two different
PEG chain lengths show that even in the absence of core-freezing,
the surfactants prefer the aggregate size of their single-component
solutions with the segregation propelled via enthalpic contributions.
The findings, especially as they provide a handle on the density and
the density profile of the aggregates, raise attention to effective
packing shape as a design factor of micellar systems, for example,
drug transport, solubilization, or partitioning.