Describe the principle behind what differentiates a nano-emulsion from a micro-emulsion,

with particular reference to:

a. thermodynamic stability and kinetic stability
b. solubilisation, micelles and critical micelle concentration (note that the critical micelle
concentration is the concentration of surfactant molecules at which micelles form)

Micro-emulsion thermodynamically and kinetically stable under a range of conditions. As free
energy in micro-emulsion has a lower energy state than separated phrase, it will tend to remain in
micro-emulsion state.

Nano-emulsion is thermodynamically unstable. Free energy in nano-emulsion has a higher
energy state than separated phrase. It will tend to breakdown and moves towards the lowest
energy levels, which is a separated phrase. Metastability (Kinetic stability) depends on

[1]
.
Higher

between nano-emulsion state and separated phase, longer nano-emulsion stability.

Figure 1: Schematic diagram of the free energy of microemulsion and nanoemulsion systems

When surfactants is added to water, surfactants molecules will form a layer on the surface of
water, with hydrophilic group facing towards the water surface, and hydrophobic group facing to
air. By increasing the amount surfactant added to water, beyond Critical Micelle Concentration
(CMC), the surface would saturate with surfactants, causing surfactants with hydrophobic group
to clump together, forming micelles. Adding oil to water and surfactant solution, micelles will
incorporate oil droplet within hydrophobic group, forming micro-emulation. As more oil added
into surfactant solution, micelle will grow and reach saturation where by no more oil can be
absorbed, forming nano-emulation
[1]
.

The paper by Rao and McClements discusses results related to nano and micro emulsions.
The so called micro-emulsions are reported as being smaller than those of the nano-
emulsion droplets. Making reference to specific results in the paper, on what basis is this
distinction made?

From the paper
[2]
, section 3.4, Rao and McClements describes emulation particle size of lemon
oil in SMP and Tween 80, surfactant with two different set-up. Figure 9a, lemon oil
concentration of 1.5% and Figure 2, 0.5%. Figure 3, having an emulation particle size of 20nm,
defined as a micro-emulation. Figure 2, particle size of 130nm, defined as nano-emulation.

Figure 2: nanoemulsion of lemon oil concentration of 1.5%

Figure 3: microemulsion of lemon oil concentration of 0.5%

Defining a smaller particle size as micro-emulation and bigger particle size as nano-emulation
based on thermodynamical and kinetical stability
[1]
. Referring to figure 2, particle diameter
increases as the number of storage size increases. This shows that lemon oil within the micelle
grows due to Ostwald Ripening where larger micelle grows at the expense of smaller micelle,
due to the difference in chemical potential
[3]
. Lemon oil nano-emulation will eventually
breakdown to form up two-separated phrase, which is the lowest energy level.
Figure 3 displays a stable particle size, through out the experimented storage days. Variation of
particle size depends on the concentration and proportion of the surfactant added. The surfactant
used is able to stabilize a particle size of 20nm, forming a thermodynamically and kinetically
stable micro-emulation.
Referring to the paper by J. Soma, K. D. Papadopoulos, paying particular attention to Fig
3, what can be commented about the stability of emulsions made from these chemicals, are
they micro or nano-emulsions based upon the definition by McClements.

Ostwald Ripening (OR) is a process where large particles grow at the expense of smaller particle
due to difference in chemical potential. Based on McClements definition, Free energy in nano-
emulsion has a higher energy state than separated phrase. It will tend to breakdown and moves
towards the lowest energy levels, which is a separated phrase, given an appropriate amount of
time.


Figure 4: Ostwald Ripening rate as a function of surfactant concentration
Referencing from figure 3 in literature
[3]
, surfactant chemical used, sodium dodecyl sulfate
(SDS) promotes the formation of nano-emulation. Referring to paper
[3]
, figure 3 shows that
when sufficient SDS added in to oil-in-water solution, above CMC concentration

[3]
where micelles are formed, the rate of OR increase when more SDS added into oil-in-water
solution. Additional surfactant lead to a excess amount of surfactant molecules in the solution,
the surfactant having a hydrophobic side will tends to congregate around the existing micelles,
forming larger droplets. The micelle formed by SDS does not stabilize oil-in-water solution,
instead promoting OR when more SDS is added into solution. Free energy,

, between nano-
emulation state and separated state for SDS is low.


Figure 5: normalize average diameter as a function of concentration of SDS above the CMC
Additionally, figure 5 also shows an increase in particle size against time, regardless of SDS
concentration above CMC concentration, favoring nano-emulation.











Part B
Describe the principle of how an ion selective electrode functions, with special reference to:
a. The use of a membrane (18 marks)
b. Internal and external reference electrode (18 marks)
c. Show the equation that can be used to determine the concentration or activity of an
anion or cation (14 marks)

Ion selective electrode works by diffusing selective ions across the membrane, into the analyte
solution. Electrode will measure the chemical potential differnece between filling solution and
analyte solution

The ion selective membrane is thin sheet hydrophobic organic polymer, impregnated in viscous
organic solution containing an ionophore, which is soluble inside the membrane. Ionophore
chosen to have high affinity for specific type cation, lower affinity with other ions. To neutralize
the positive charge within the organic solution, a hydrophobic anion, which is soluble in the
membrane layer, is introduced.

When ion-selective probe and external reference probe is immerse into analyte solution, cations
impregnated within the membrane layer will diffuse through into analyte solution. This will
generate an excess positive charge in the analyte solution and an excessive negative charge in
membrane layer. Activity of an anion or cation can be determine by Nernst Equation and Gibbs
energy where

) (1)
From Nernst equation
(2)

Substitute equation (1) into (2)

) (3)
By diffusing cation ions from membrane into analyte solution, this will create a positive charge
in the analyte solution, near to the membrane surface. The charge separation creates an electric
potential difference (

.

The potential difference between analyte solution and inner filling solution is equal to

(4)

Substitute equation (3) to (4)

(5)

The first three terms can be expressed as a constant. Activity of cation (

) in the membrane is
very near to a constant is due to only minute amounts of cations can be diffused into analyte
solution, due to hydrophobic anion that is found in the membrane which is insoluble in water.

is a constant as the activity of cation in the filling solution is constant.

depends on accessible surface area and solvation parameter of atom[5], hence a constant.
Equation 5 can be rearranged and expressed as,

(6)
To find the activity of the outer analyte solution

, simply re-arrange the equation to

) (7)
To calibrate ion selective electrode, standard buffer solution, can be used to standardize the
reading obtained from the electrode.








Nomenclature

is the activity of the inner membrane

is the activity of the outer analyte solution

E is the electric potential difference between the inner membrane and the outer analytesolution
E
outer
is the electric potential of the outer solution
E
inner
is the electric potential of the filling solution
F is the Faraday constant
is the difference in Gibbs energy between solution.

is the change in free energy of solvation of a solute molecule

n is the charge of the ion
R is the Universal gas constant
T is the temperature

References
1. D. J. McClements, Nanoemulsions versus microemulsions: terminology, differences, and
similarities. Soft Matter 8, 1719 (2012).
2. J. Rao, D. J. McClements, Lemon oil solubilization in mixed surfactant solutions:
Rationalizing microemulsion & nanoemulsion formation. Food Hydrocolloids 26, 268
(2012).
3. J. Soma, K. D. Papadopoulos, Ostwald Ripening in Sodium Dodecyl Sulfate-Stabilized
Decane-in-Water Emulsions. Journal of Colloid and Interface Science 181, 225 (1996).
4. Quantitative Chemical Analysis by Daniel C, Harris, 8th Edition (2010), ISBN-10: 1-
4292-1815-0. Chapter 14, pages 314-331
5. Implicit solvation - Wikipedia, the free encyclopedia. 2013. Implicit solvation -
Wikipedia, the free encyclopedia. [ONLINE] Available at:
http://en.wikipedia.org/wiki/Implicit_solvation. [Accessed 09 May 2013].