Starch microemulsions are stabilized suspensions of starch particles in a liquid medium, typically water. The properties of starch microemulsions are influenced by a number of factors, including the type and concentration of stabilizing agents, the size and shape of the starch particles, and the conditions under which the microemulsion is prepared and stored.
- Introduction to Starch Microemulsions
- Starch Microemulsion Preparation
- Applications of Starch Microemulsions
- Application of Starch Microemulsions to Foods
Characterization Methods for Starch Microemulsions
Droplet Size and Size Distribution
The microsphere particle size plays a very important role in the drug-release profile and varies with different microsphere preparation methods. The smaller the particle, the higher the surface area exposed to the release medium. The erosion rates of water-insoluble polymers are also greater for smaller microspheres. The particle size distribution is more meaningful than the average particle size when comparing different microsphere preparation methods because microspheres with the same average particle size but different size distributions may have very different drugrelease profiles.
Particle size can be measured with various methods, including optical microscopy, resistance and light-blocking methods, light scattering, laser diffraction analysis, scanning electron microscopy, and photon-correlation spectroscopy. For example, light-scattering methods are widely used to measure particles and require highly diluted emulsions when droplet sizes are measured. However, in most cases, the addition of water to a microemulsion causes phase separation, and an O/W emulsion is formed. To overcome this limitation, a model has been proposed to correct the scattering data for undiluted microemulsions. With this numerical approach, it is possible to estimate particle sizes in undiluted microemulsions.
As well as size determination, DLS is a good method for measuring the translational diffusion coefficients of starch microemulsion droplets (i.e., colloidal particles) and is useful in the investigation of concentrated dispersions. When a coherent beam of light (such as a laser) interacts with colloidal particles in Brownian motion, the intensity correlation function provides information on the translational diffusion coefficient of the scattering particles and therefore the hydrodynamic radius, according to the Stokes–Einstein equation. The diffusion coefficient can be related to a parameter called the “correlation length” in a more general way than the diffusion coefficient, because it conveniently characterizes both discrete droplets and clusters, providing scope for estimating interparticle interactions. Therefore, the DLS method is potentially important in the analysis of the particle sizes and related physical characteristics of starch microemulsions.
Freeze-fracture electron microscopy (FFEM) delivers images of starch microemulsion structures. The starch microemulsion is rapidly frozen and ruptured, and the fracture face is replicated. Depending on the composition, both droplet-like structures and bicontinuous structures can be identified in a simple three-component system.
In addition to the methods described above, several other methods are used to study starch microemulsions.
Small-angle X-ray scattering (SAXS)
SAXS can be used to measure structures with sizes in the nanometer range and provides information on ordered systems, such as liquid crystalline phases. It has been used to measure the structures of starch microemulsions.
Small-angle neutron scattering (SANS)
SANS uses elastic neutron scattering to investigate the structures of liquid systems in the range of 1–1000 nm. It is used to study the particle sizes, polydispersity, and fluctuations in starch microemulsions. It covers a larger range of wave vectors than SAXS and is therefore suited to the investigation of short- and long-range structures in liquid systems. The scattering data are fitted to models of the starch microemulsion structures.
Nuclear magnetic resonance (NMR)
When characterizing starch microemulsions, NMR spectroscopy is used to measure the self-diffusion coefficients of surfactants, cosurfactants, oils, and water. The results give a clear indication of the structure of the starch microemulsion. O/W, W/O, and bicontinuous-type microemulsions can be distinguished by the selfdiffusion coefficients of the oil- and water-phase molecules.
Characteristics of Starch Microemulsions
Many factors must be considered when producing microspheres with a desirable particle size or size distribution. These factors include, but are not limited to, the polymer type and concentration in the organic phase, the volume fraction of the dispersed phase, the identities and amounts of excipients, the rate of stirring during hardening, the temperature during preparation, the speed of homogenization, and the identity and amount of surfactant. Whether the droplets have approximately the same density and continuous phase must be evaluated to predict their stability. Emulsions that are initially homogeneous, without creaming or sedimentation on visual inspection, are thought to be stable. Microscopic images and droplet size distributions are also used to analyze stability.
Temperature and stability
Starch microemulsions are thermodynamically stable and form spontaneously (or with very low-energy input) under the right conditions. However, thermodynamic stability comes at a price: starch microemulsions require a greater amount of surfactant than emulsions, which may increase their potential irritability.
Because colloidal dispersions are thermodynamically stable systems, their free energy is lower than the free energy of the separate phases (oil and water). Therefore, in principle, starch microemulsions form spontaneously, without energy input. In practice, some energy input (in the form of gentle mixing, stirring, or heating) facilitates their formation because there are kinetic energy barriers that must be overcome or mass transport limitations that retard their formation.
Particle size and stability
The emulsifying ability of starch granules seems inversely proportional to their size. When the diameters of starch nanoparticles (SNPs) were 150–700 nm, the mean droplet size of an emulsion stabilized by the SNPs increased from 29.33 to 48.76 μm.
Ions and stability
Generally, as the salt concentration increases, there is a greater tendency for the particles of a microemulsion to aggregate because the stabilizing repulsive forces are weakened by the increase in the electrolyte concentration. However, NaCl concentrations of 0–90 mM had no obvious effect on the stability of a Pickering emulsion stabilized with cornstarch-based nanoparticles.
Oil concentration and stability
After an emulsion is prepared, the freshly established oil droplets probably undergo limited coalescence during the first hour, until the SNP coverage is sufficient to stabilize the droplets. A marked increase in the oil fractions of corn, tapioca, sweet potato, and waxy cornstarch nanoparticles was observed after 1 month in storage. When the oil fractions were 0.6 and 0.75, the internal oil phase accounted for a great proportion of the emulsified phase, which approached or even exceeded the critical oil fraction for closely packed emulsion systems, and thus easily underwent phase inversion; oiling off occurred after 1 day in storage. A progressive increase in the volume of the emulsified phase was observed after 3 h in storage when the oil fraction was increased from 0.25 to 0.5.
The internal contents of starch microemulsion droplets are known to be exchanged between droplets, typically on a millisecond time scale. They diffuse and collide, and if the collisions are sufficiently violent, then the surfactant film may rupture, facilitating droplet exchange. Therefore, the droplets are kinetically unstable. However, if the emulsions are dispersed to sufficiently small droplets (<500 Å), the tendency to coalesce is counteracted by an energy barrier. The system will then remain dispersed and transparent for long periods (months).
The drops of the dispersed phase are generally large (>0.1 μm), so they often take on a milky, rather than a translucent, appearance. Once the conditions are right, starch microemulsions will form spontaneously. In simple aqueous systems, the formation of starch microemulsions is dependent on the surfactant type and structure. If the surfactant is ionic and contains a single hydrocarbon chain (e.g., sodium dodecyl sulfate), microemulsions only form if a cosurfactant (e.g., a medium-sized aliphatic alcohol) and/or electrolyte (e.g., 0.2 M NaCl) is also present. With doublechain ionic surfactants (e.g., Aerosol-OT (Sinopharm Chemical Reagent Co., Ltd., China)) and some nonionic surfactants, a cosurfactant is unnecessary. This results from one of the most fundamental properties of microemulsions, the ultralow interfacial tension between the oil and water phases.
Starch microemulsions can display percolation phenomena at specific volumetric fractions of water. According to the percolation theory, phase transformation from reverse structures (W/O) to normal-type systems (O/W) through the emergence of bicontinuous systems and other aggregates may occur as the aqueous content of the system increases. This is generally accompanied by an increase in the system’s electrical conductivity, which has often been used as a method of characterizing the internal structure of emulsions. Below a critical water volumetric fraction (Φc), water droplets are embedded in a low-conductivity oil medium and isolated from each other. As the volumetric fraction of water reaches Φc (the percolation threshold), an increase in the interlinking of the aqueous droplets occurs, and other structures form. This is usually apparent as a sharp increase in conductivity. Transition to a water-continuous system then follows as the water fraction continues to increase.
The flow behavior of starch microemulsions is an important factor for their technical applications. Processes such as pumping, spraying, or spreading each require a different flow behavior. One example is the use of gels for skin treatments. On the one hand, they must spread easily but, on the other hand, must not run off the skin. For this application, a microemulsion requires low viscosity at high shear rates and a significantly higher viscosity at low shear rates. The flow behavior can be optimized by varying the composition of the microemulsion.