Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 153

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20 30 40 50

Droplet size (nm)

Figure 5.5 Cryo-TEM of a microemulsion consisting of soybean oil, glycerol monooleate, polysorbate
80 and water (left) and the corresponding size distribution obtained via DLS (right) (unpublished results).

micelles and striated (lamellar) structures. The authors also noted that samples were
susceptible to electron beam damage.
Fast Fourier Transform cryo-TEM has been found to be a suitable replacement to SAXS
to study the nano-structure of microemulsions (Sagalowicz et al ., 2006 ). This technique
offers the advantage of being able to study individual particles as well the co-existence of
particles with varying structural details in the same sample (Leser et al ., 2006 ; Sagalowicz
et al ., 2006 ).
Finally, a novel technique is freeze-fracture direct imaging, where sensitive samples
such as microemulsions can be studied without the need for a replica. This technique does
not have a blotting step (as per cryo-TEM) that can damage microemulsion structures
(Belkoura et al ., 2004 ; Gradzielski, 2008 ).

5.10.4 Dynamic light scattering
Dynamic light scattering (DLS) is a commonly used technique to determine droplet size
and polydispersity in microemulsions. When a coherent light beam interacts with colloidal
particles undergoing Brownian motion, the particles scatter light. The original measurement
is a time correlation function of the scattered intensity of the particles within the micro-
emulsion. The decrease of this correlation function with time (lag time) is used to extract
the diffusion coefficient of a particle or droplet in solution (Goddeeris et al ., 2006 ).
The measured diffusion coefficient can be used to calculate a hydrodynamic radius ( R h ) of
the droplet using the Stokes-Einstein equation:

h R T/ 6D

= πη (5.6)

is the viscosity
(cP) of the continuous phase, and D (cm 2 /s) is the diffusion coefficient (Provencher, 1979;
Goddeeris et al ., 2006). For proper interpretation of results, it is very important to establish
whether the R h values are based on unimodal or multimodal distributions.

where k is the Boltzmann constant, T is the absolute temperature in Kelvin, h