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PM Physico-Chemical
Characterization
Determination of critical micelle
concentration (CMC)
Pyrene fluorescence for the determination of
CMC and studying the interior of the PM
One of the mostly used methods in PM characterization.
For a pyrene molecule P;
P > (excitation)> P*
P* + P > P (excimer)
Pe/Pm: measure of the ease of excimer (e)
formation from the monomer (m)
Excimer formation is function of microviscosity of
the micelle core,
Excimer formation is sensitive to pyrene
concentration because it involves an interaction
between two pyrene species.
The emission spectrum of the pyrene monomer in
the 350- to 420-nm region consists of five
primary vibronic bands, usually designated as
I1–I5, from shorter to longer wavelengths.
band 1: shows significant intensity enhancements
in polar environments;
band 3: shows minimal variation in intensity with
polarity changes.
General Method
Dissolve Pyrene in organic solvent,
Add to concentration series of the polymer
solution*,
Evaporate the organic solvent,
Measure fluorescent spectra using fluorescence
spectrophotometer.
* the final concentration of pyrene is in the 10−7 M range.
CMC is determined using the ratio of peak
intensities at 338 and 333 nm (I338/I333) from
pyrene’s excitation spectra.
A number of I338/I333 values is been obtained by
varying the polymer concentration.
When the polymer concentration is low, the I338/I333
value is the same as that of pyrene in water.
When the polymer concentration increases, the
red shift from 333 to 338 nm in the pyrene
excitation spectra indicates the movement of
pyrene into a more hydrophobic environment.
Figure: Plot of the intensity ratio I338/I333 and I1/I3, which were
obtained from the excitation and emission spectra, respectively, as a
function of log C. The cmc was taken from the intersection of the
horizontal line at low polymer concentrations with the tangent of the
curve at high polymer concentrations.
Measurement of lower critical solution
temperature (LCST) or the cloud point
Turbidity method using UV–vis
Spectrophotometer by monitoring the
transmittance at 500 nm at preset heating
rate.
LCST, why?
Hydrophilic polymer + water
 hydrogen bond (exothermic),
Hydrophobic polymer,
 Surrounded by water clusters (low entropy),
At higher temperatures
 Release of water molecules (increase in S),
 Hyrophobic interaction (increase in S)
 polymer precipitation.
Figure - Schematic representation of hydrophobic interaction
Ref.: Physical Pharmacy Book
J.X. Zhang et al. / Colloids and Surfaces B: Biointerfaces 43 (2005)
123–130
Particle size distribution of PM in aqueous solution of 0.5%
at: (a) 25 ◦C; and (b) 45 ◦C. LCST=32.6 C.
Self-assembly and thermally-induced change of a copolymer in
aqueous solution.
Note: the students get confused from the above figure, we should
distinguish between increase in concentration and increase in
temperature.
Diameter changes of a PM as a function of temperature
Micelle size: can be determined using light
scattering methods,
Micelle morphology: can be observed using
transmission electron microscopy (TEM),
……..
Micellar drug solubilization
Surfactants and amphiphilic block copolymers can
greatly affect the aqueous solubility of compounds
by providing a hydrophobic reservoir where they
can partition.
Water solubility of some hydrophobic drugs was
enhanced by a factor of 300 when incorporated
into the core of some PM.
The partitioning of some chemotherapeutic agents
into the hydrophobic PM phase was highly favored
with partition coefficients as high as 5.0x104.
G. Gaucher et al. / Journal of Controlled Release 109 (2005) 169–188
Measurement of drug solubilized in the PM:
By dissolving the lyophilized PM in
organic solvent like DMSO.
Drug concentration is then measured using
suitable analytical method.
Micelle stability
Involves:
Storage stability;
Dilution stability;
In vivo stability: Adsorption of Protein at PM
surface >>PM clearance from the blood.
OR
PM-protein binding>>disrupt micelle
cohesion>> premature drug release.
Storage stability
lyophilized drug-loaded PM are stored at specific
temperature and humidity conditions and the
samples are monitored for time-dependent changes in
particle size and
drug content
during the storage period.
Dilution stability
The effect of dilution on the micelles can be studied by
incubating the micelles in buffer solution at say 10-fold
dilution at the required temperature for certain period.
After filtration, the incubation solution is then analyzed
for the presence of the drug.
In vitro Drug release from the PM
In vitro release profiles of the loaded drug from
the PM is examined at specific conditions
(vehicle, temperature, pH, ) using dialysis
membrane of suitable MWCO.
At predetermined time intervals, the amount of
released drug is determined using suitable
analytical method.