Diffusion behavior of pharmaceutical O/W microemulsions studied by dynamic light scattering |
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Authors: | Anuj Shukla Reinhard H H Neubert |
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Institution: | 1. Department of Pharmacy, Institute of Pharmaceutics and Biopharmaceutics, Martin-Luther-University Halle-Wittenberg, 06120, Halle/Saale, Germany 2. Lehrstuhl für Physikalische Chemie II, Universit?t Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany
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Abstract: | Dynamic light scattering experiments have been performed at various concentrations, of pharmaceutical oil-in-water microemulsions
consisting of Eutanol G as oil, a blend of a high (Tagat O2) and a low (Poloxamer 331) hydrophilic–lipophilic balance surfactant,
and a hydrophilic phase (propylene glycol/water). We probe the dynamics of these microemulsions by dynamic light scattering.
In the measured concentration range, two modes of relaxation were observed. The faster decaying mode is ascribed classically
to the collective diffusion D
c (total droplet number density fluctuation). We show that the slow mode is also diffusive and suggest that its possible origin
is the relaxation of polydispersity fluctuations. The diffusion coefficient associated with this mode is then the self-diffusion
D
s of the droplets. It was found that D
c and D
s had opposite volume fractions of oil plus surfactants (ϕ) dependence and a common limiting value D
0 for ϕ=0. Average hydrodynamic radius (R
h=10.5 nm) of droplets was calculated from D
0. R
h is supposed to compose the inner core, a surfactant film including possible solvent molecules, which migrate with the droplet.
The concentration dependence of diffusion coefficients reflects the effect of hard sphere and the supplementary repulsive
interactions which arises due to loss of entropy, when absorbed chains of surfactant intermingle on the close approach of
the two droplets. This mechanism could also explain the observed stability of our systems. The estimated extent of polydispersity
is 0.22 from the amplitude of slower decaying mode. The polydispersity in microemulsion systems is dynamic in origin. Results
indicate that the time scale for local polydispersity fluctuations is at least three orders of magnitude longer than the estimated
time between droplet collisions. |
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Keywords: | O/W microemulsions Dynamic light scattering Dermal drug delivery |
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