Self-Aggregation of Amphiphilic Dendrimer in Aqueous
Solution: The Effect of Headgroup and Hydrocarbon Chain Length
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Abstract
The self-aggregation of amphiphilic
dendrimers G<sub>1</sub>QPAMC<sub><i>m</i></sub> based on
poly(amidoamine) PAMAM possessing
the same hydrophilic group but differing in alkyl chain length in
aqueous solution was investigated. Differences in the chemical structures
lead to significant specificities in the aggregate building process.
A variety of physicochemical parameters presented monotonous regularity
with the increase in alkyl chain length in multibranched structure,
as traditional amphiphilic molecules. A significant difference, however,
existed in the morphology and the microenvironment of the microdomain
of the aggregates, with G<sub>1</sub>QPAMC<sub><i>m</i></sub> with an alkyl chain length of 16 intending to form vesicles. To
obtain supporting information about the aggregation mechanism, the
thermodynamic parameters of micellization, the free Gibbs energy Δ<i>G</i><sub>mic</sub>, and the entropy Δ<i>S</i><sub>mic</sub> were derived subsequently, of which the relationship
between the hydrophobic chain length and the thermodynamic properties
indicated that the self-assembly process was jointly driven by enthalpy
and entropy. Other than traditional surfactants, the contribution
of enthalpy has not increased identically to the increase in hydrophobic
interactions, which depends on the ratio of the alkyl chain length
to the radius in the headgroup. Continuous increases in the hydrophobic
chain length from 12 to 16 lead to the intracohesion of the alkyl
chain involved in the process of self-assembly, weakening the hydrophobic
interactions, and the increase in −Δ<i>H</i><sub>mic</sub>, which offers an explanation of the formation of vesicular
structures