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■Bio-mathematics, Statistics and Nano-Technologies: Mosquito Control Strategies

can change back to the crystalline state on storage. Spray drying can also affect particle

size; dexamethasone acetate containing PLGA nanoparticles were formulated and spray

dried, the spray drying processes significantly (p < 0.05) increased the nanoparticle sizes.

The mean particle size increased from 200 ± 60 nm to 230 ± 100 nm, however particle

density and size distribution seemed to be unaffected (Gómez Gaete et al. 2008).

12.2.5

Emulsification

Emulsification is a simple and cheap method used to encapsulate bioactives (such as

EOs and extracts) within aqueous solutions. Emulsions comprise two phases, usually oil

and water, which are immiscible (hydrophilic and hydrophobic); the hydrophobic (oil)

phase can be dispersed within an aqueous phase, forming an oil-in-water (O/W) emul-

sion or the opposite can be true forming a water-in-oil (W/O) emulsion (Bakry et al.,

2016). These emulsions are then ready to be further processed using the microencapsula-

tion methods described above. Polymer microcapsules have been created using this method

with polymethyl-methacrylate (PMMA) and jasmine EO; spherical PMMA microcapsules,

with smooth surfaces were achieved when the PMMA to jasmine EO weight ratio was 2:1

and 3:1. At a 1:1 ratio, the polymer capsules could not be achieved, as the capsules com-

prised holes, possibly due to insufficient amounts of PMMA required to coat the jasmine

oil droplets. Encapsulation efficiency was determined to be 72%, as 4.78 mg of jasmine oil

was found, when analysing 20 mg of dried capsule (Teeka et al. 2014).

12.3

CHARACTERIZATION OF MICROCAPSULES

12.3.1

Particle size and size distribution

Microcapsules often come in different sizes and size distributions depending on the

methods used and their size are related to their mechanical properties e.g. smaller mi-

crocapsules will have lower rupture force compared to a larger microcapsule. A study

by Sun and Zhang (2002) showed that the bursting force and deformation of melamine-

formaldehyde, urea-formaldehyde and gelatine-formed microcapsules increased propor-

tionally with their diameter. To determine the particle size of microcapsules, dynamic light

scattering (DLS), laser diffraction and microscopy are used. There are limitations with the

use of laser diffraction, as the refractive index of the shell material must be known for the

measurement. Microscopy has an advantage over laser diffraction as it gives the true image

of the microcapsule but it time consuming and is therefore, mostly used for individual size

analysis rather than size distribution (Gray et al. 2016). DLS method provides the average

particle size and the size distribution within the sample elucidating the homogeneity of

particles being analyzed.

12.3.2

Surface charge

The zeta potential (the charge at the interface between a particle and the surrounding

medium) needs to be controlled to prevent microcapsule aggregation. The zeta potential

should be analyzed to identify what needs to be done to move the zeta potential away from