4 research outputs found
Self-Aggregation of Amphiphilic Dendrimer in Aqueous Solution: The Effect of Headgroup and Hydrocarbon Chain Length
The self-aggregation of amphiphilic
dendrimers G<sub>1</sub>QPAMC<sub><i>m</i></sub> based on
polyÂ(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
Dilational Properties of Novel Amphiphilic Dendrimers at Water–Air and Water–Heptane Interfaces
In this work, a series of novel amphiphilic dendrimers
taking polyamidoamine
dendrimer as the core with different hydrophobic tails QPAMC<sub>m</sub> were synthesized and the dilational properties were studied as monolayers
by dilational rheological measurements at the water–air and
water–<i>n</i>-heptane interfaces to explore the
nature of adsorption behaviors. The results showed that the maximum
values of the dilational modulus seemed to have no obvious variation
in a wide change of hydrophobic chain length at the surface. However,
there was considerable variability in the tendency of the influence
of bulk concentration on the dilational modulus at the two different
interfaces. It was interestingly found that the diffusion-exchange
process slowed down with the increase of alkyl chain length leading
to more elastic nature of adsorption film, which was contrary to the
tendencies of conventional single chain and gemini surfactants. It
is reasonable to consider that, in the case of the molecule having
short chain length such as QPAMC<sub>8</sub>, the alkyl chains are
too short to overlap across the headgroup, enable the intermolecular
hydrophobic interaction to be predominant with increasing of surface
concentration, which enhances the elasticity and shows the slowest
diffusion-exchange process. Whereas, when the chain length increases
to 12 or 16, the alkyl chains are long enough to act intramolecularly
to form intracohesion conformation, which results in enhancing the
diffusion-exchange process. In conclusion, the interfacial behaviors
are dictated by the size ratio between the tail and headgroup. A reasonable
model with respect to the molecular interaction was proposed on the
basis of experimental data. The results of interfacial tension relaxation
and dynamic light scattering (DLS) experiments, in accord with the
proposed mechanism, also present the unusual tendency comparing to
the traditional single or gemini surfactants
Calorimetric and Theoretical Study of the Interaction between Some Saccharides and Sodium Halide in Water
Dilution enthalpies and mixing enthalpies of sodium halide
and
some saccharides (glucose, galactose, xylose, arabinose, fructose,
and sucrose) in aqueous solution were determined by calorimetric measurements
at 298.15 K. The values were used to determine enthalpic pair interaction
parameters. Combined with Gibbs energy pair parameters, entropic pair
interaction parameters were also obtained. Theoretical calculations
at the B3LYP/6-311++GÂ(d,p) level were carried out to provide the information
of structures and thermodynamic functions. The information reveals
the thermodynamic essence of the interactions between sodium halide
and saccharides in aqueous solutions. The experimental results and
theoretical calculations show that the sign of enthalpic pair interaction
parameter 2<i>Ï…h</i><sub>ES</sub> is determined by
the direct interaction between saccharides and ions, whereas the difference
in value of 2<i>Ï…h</i><sub>ES</sub> for different
saccharides or electrolytes depends on the partial dehydration of
saccharides or anions in aqueous solution. The difference in value
of entropic pair interaction parameters depends partly on the different
dominant interactions in the process of partial dehydration of saccharides
or ions. An enthalpy–entropy compensation relationship was
observed for the sodium bromide–aldopyranose–water systems.
Remarkably, it can be conjectured that the hydration entropy of glucose
is lower than for other monosaccharides. Perhaps it is one of the
reasons why glucose plays an important role in living organisms rather
than other monosaccharides
Antibacterial Activity of Geminized Amphiphilic Cationic Homopolymers
The
current study is aimed at investigating the effect of cationic
charge density and hydrophobicity on the antibacterial and hemolytic
activities. Two kinds of cationic surfmers, containing single or double
hydrophobic tails (octyl chains or benzyl groups), and the corresponding
homopolymers were synthesized. The antimicrobial activity of these
candidate antibacterials was studied by microbial growth inhibition
assays against <i>Escherichia coli</i>, and hemolysis activity
was carried out using human red blood cells. It was interestingly
found that the homopolymers were much more effective in antibacterial
property than their corresponding monomers. Furthermore, the geminized
homopolymers had significantly higher antibacterial activity than
that of their counterparts but with single amphiphilic side chains
in each repeated unit. Geminized homopolymers, with high positive
charge density and moderate hydrophobicity (such as benzyl groups),
combine both advantages of efficient antibacterial property and prominently
high selectivity. To further explain the antibacterial performance
of the novel polymer series, the molecular interaction mechanism is
proposed according to experimental data which shows that these specimens
are likely to kill microbes by disrupting bacterial membranes, leading
them unlikely to induce resistance