Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 150

bending modulus of the interfacial layer (Walz, 1998; Yaghmur et al ., 2002 ), with increased
molecular compatibility reducing the bending modulus of the interfacial layer (Yaghmur
et al ., 2002 ).
An increase in the alkyl chain length of the compound in the oil will typically decrease
its solubility in water (Anton et al ., 2008), and consequently reduce the total area of the
microemulsion monophasic region. Using the BSO (Bansal-Shah-O'Connell) equation, the
maximum solubilization for ionic microemulsions occurs when (Bansal et al ., 1980 ; Garti
et al ., 1995 , 2001 ; Shiao et al ., 1998 ; Yaghmur et al ., 2002 ):

l += l l (5.5)

o a s

where l o = oil alkyl chain length, l a = alcohol alkyl chain length and l s = surfactant alkyl
chain length.
However, this equation should be used with caution. Shiao and co-workers (1998)
reported that the BSO equation failed if the microemulsion contained components with
branched alkyl chains, as these decreased the packing order of the hydrocarbons in the inter-
face compared to linear alkyl chains. Garti and co-workers (1995, 2000a) showed that the
BSO equation was valid for non-ionic surfactants in five-component microemulsions only
when the surfactant (alcohol) exhibited limited solubility in both the aqueous and oil phases.

5.10 CHARACTERIZATION TECHNIQUES

As a first approximation, the presence of microemulsions in a mixed system is often gauged
using ternary phase diagrams (TPDs) combined with visual observations. Further charac-
terization can be performed with a number of techniques, including DLS, cryo-transmission
electron microscopy (cryo-TEM), small angle X-ray scattering (SAXS), small angle neutron
scattering (SANS), nuclear magnetic resonance (NMR) and TRFQ. Others include conductance,
viscosity, thermal conductivity, ultrasonic absorption, transient electrical birefringence and
vibrational spectroscopy (infrared and Raman).

5.10.1 Ternary phase diagrams
TPDs are used to identify and characterize microemulsion regions (Figure 5.4). The three
components (an aqueous phase, an organic phase and a surfactant phase) that compose the
system are located at the triangle's apexes, where their corresponding volume fraction is
100%. Moving away from that apex reduces its volume fraction and increases the volume
fraction of one or both of the other two components. Thus, each point within the triangle
represents a unique combination of the three components, unless positioned along an axis
where the mixture is binary. Varying the composition leads to the determination of phase
boundaries, and hence phase behavior. As with the surfactant phase, the aqueous and organic
phases may also be mixtures of multiple compounds. Where four or more components are
used, pseudo-ternary phase diagrams (PTPDs) are used with one or more apexes represent a
defined mixture of two or more components (Flanagan and Singh, 2006).
Within a putative TPD (Figure 5.4), any point within the triangle represents a three-
component mixture of the apexes. For point “X”, the proportion of water (A) can be
calculated by drawing a line through “X”, parallel to the surfactant-oil (BC) axis. As apex
A in the TPD represents 100% water, point X shows a water concentration of 70%. By