Surfactants
are molecules containing a hydrophobic long-chain (tail) with a polar head
group and the head group may be ionic or neutral. If these molecules are
dissolved in water, they have a tendency to form aggregates spontaneously, that
is, surfactant molecules will arrange themselves into organized molecular
assemblies, also called normal micelles (see Figure 1) [1, 2]. There is a
particular concentration (the critical concentration) characteristic of the
surfactant (in a given solvent) at or above which these will form micelles and
this is called the critical micellar concentration (CMC). Before the critical
concentration is reached aggregates may still be formed, which are premicellar aggregates
[3]. The micellar aggregates have a certain number of monomer units on the
average, which is known as aggregation number.
The formation of micelle from monomer with aggregation
number n could be represented by the following scheme:
Micelles are highly dynamic aggregates and rates of uptake
of monomers into micellar aggregates are nearly at the diffusion-controlled
limit [4, 5]. Initially, when the concentration is very low these monomers
behave like independent molecular entities in water. As the concentration is
increased they tend to come close to each other and form aggregates of various
sizes as the process is favored thermodynamically. The formation of micelles
can be followed by several methods based on the fact that in the vicinity of
the CMC there is a sharp change in the experimental observables. There are many
methods available in literature for the determination of the CMC [6, 7]:
surface tension, spectrophotometric, kinetics, conductivity, osmotic pressure,
etc.
Fluorescence spectroscopy [8] is a sensitive technique
where we can utilize the environment sensitivity of a molecular probe situated
in the micelle and this could also be an accurate method for CMC determination.
The environment sensitivity can be quantified by the Stokes shift, measured by
from the difference in emission and excitation maxima expressed in energy
(wavenumber) scale, that is,
the larger the Stokes shift the higher sensitive
the probe is. We choose a polar probe where the Stokes shift is very sensitive
to solvent environment (polarity) and which can preferentially bind to micelle.
It may be noted that not all fluorescent probes are sensitive to solvent. With
increase in monomer concentration more and more aggregates (micelle) are formed
and so more of the probe molecules leave water and bind to the aggregates.Thus,
the Stokes shift of the probe molecule will vary considerably as a function of
the monomer concentration in the vicinity of the CMC. This is because the
solvent environment (polarity) in bulk water is different than in aggregates
(organized surfactant assembly), which brings about a change in
emission/excitation frequency maximum and hence different Stokes shifts. When
the monomer concentration is well below and above the CMC the Stokes shift does
not change significantly with concentration of monomers.
The success of this experiment depends on two important
factors: (i) the Stokes shift of the chosen probe molecule must be sensitive to
environment (polarity) (not all probes can show sensitivity in Stokes shift)
and (ii) it should preferentially attach to the aggregate. The surfactant
molecules may be or may not be charged, but in the present case, we used
Figure 1. Micellar aggregate (An aqueous CTAB
micelle).
(a)
(b)
Figure 2. Stick diagram for (a) CTAB, (b)
C-343 Na Salt.
hexadecyltrimethyl ammonium bromide, (cetyl
trimethyl ammonium bromide, CTAB) a charged one (See Figure 2a).
Here the micellar aggregates formed in water are
positively charged. Naturally, a negatively charged probe could bind to this
positively charged micelle and micellar interface will be the preferred
location of the negatively charged probe because of the positive zeta-potential
and it should not look for the nonpolar hydrocarbon interior. Thus, the
electrostatic force of attraction will bind the probe together with the
aggregate [9]. As we approach the CMC, aggregates start to form and the Stokes
shift starts to change indicating that the probe has bound to the micellar
aggregate. With further increases in monomer concentration the aggregate will
grow larger and larger with a further change in Stokes shift until a full grown
micelle is formed. There will then be no further change in the Stokes shift.
Here, the sodium salt of coumarin-343 (C-343) (see Figure
2b) has been used, which is a negatively charged polar molecule showing
considerable change in Stokes shift when compared with water and aqueous
micellar mediums like CTAB.
Surfactants are the powerhouse of detergents. They have
wide application in many consumer products in our day-to-day life and in
industry. The type of surfactant used depends on the applications, which
include removal of dirt and grease from fibers and surfaces and the conditioning
of fibers.
Fabric conditioners are primarily cationic surfactants
dispersed in aqueous systems. In water, cationic surfactants tend to form a
lamellar liquid-crystal structure in which the surfactants form layers. Curling
up of these layers in water leads to formation of small particles called
liposomes. These are attracted to charged surfaces of the fabric materials and
softening occurs in two ways: (i) surfactants tend to reduce the surface
tension which causes fibrils to get pulled out easily, (ii) the spreading of
surfactants to form a surface layer on fabrics acts to lubricate the fibers
resulting in a soft and smooth feel of the fabric.
Foaming agents, emulsifiers, and dispersants are also
surfactants, which suspend gases, immiscible liquids, or solid particles in
water or in some other liquid, respectively. Although there is a similarity in
these systems, in real practice surfactants required to perform the functions
of these systems differ widely. An emulsion can be either oil droplets suspended
in water, an oil-in-water (o/w) emulsion, water suspended in a continuous oil
phase, a water-in-oil (w/o) emulsion, or a mixed emulsion.
Each of these three functions is related to the adsorption
of surfactant at a surface with hydrophilic ends of the molecules oriented to
the water phase. The surfactants form a protective coating around the suspended
material, and these hydrophilic ends associate with the neighboring water
molecules. Solubilization is another phenomena closely related to
emulsification. As the size of the emulsified droplet becomes smaller, a
condition is reached when this droplet and the surfactant micelle are of the
same size. At this stage, an oil droplet can be thought of as being in solution
with the hydrophobic tails of the surfactant (hydrocarbon core of micelle) and
in this case, the term solubilization is used. Emulsions are milky in
appearance, whereas, solubilized oils are clear.
The phenomena of detergency or cleaning by a
detergent is a complex combination of all the above functions. The surface to
be cleaned and the soil to be removed must initially be wet and the soils
should be suspended, solubilized, dissolved, or separated in some way so that
the soil will not just redeposit on the surface in question. In the case of
surfactants, oil first gets distributed in the hydrophobic core of the micelle
and remains encapsulated there so that it cannot return. From the above
discussion it should be clear that the aggregates of surfactant monomers are
the key ingredients for the various functions. A knowledge of the CMC of a
surfactant is very essential and its determination is of great importance.
Nonionic surfactants are important soil removers; they combine good detergent
action with the ability to penetrate and soften fatty and oil-containing soil
from tightly woven fabrics and hard surfaces.
Experimental
Materials and Methods. C-343 was
purchased from Exciton and CTAB from Aldrich and were used as received. The
salt of the dye was made by neutralizing it with aqueous NaOH. The concentration
of the dye was maintained at ~1 × 10–6 M in the final mixture. A set
of solutions were made with varying concentrations of CTAB and a fixed
concentration of the dye. The CTAB concentration in the solutions was varied up
to 1.5 mM with an increment of 0.1 mM. For this, a varying amount of 10 mM CTAB
solution was mixed with a varying amount of water to reach the required
concentration of CTAB, keeping the total volume of mixture fixed, and to all
prepared
Figure 3. Normalized emission spectra of C-343
Na salt with varying concentrations of CTAB in water.
Figure 4. Normalized excitation spectra of
C-343 Na salt with varying concentration of CTAB in water.
solutions a fixed small amount (1 mL, this
volume does not bring about any change of CTAB concentration practically) of
the stock dye solution was added to reach the target dye concentration in the
final mixture. All solutions were prepared with deionized or distilled water
and then allowed to equilibrate for 15 to 20 minutes. Emission spectra were
recorded at 440 nm and excitation spectra were monitored at 620 nm using a
1-cm-path-length cuvette and a Perkin Elmer MPF 44B spectrofluorimeter.
Safety.
Coumarin 343 is an irritant
to the respiratory system and skin, an eye irritant, and harmful if absorbed
through skin (use safety goggles and gloves). CTAB is an irritant to the respiratory system and skin with a risk
of damage from eye contact. Care must be taken to avoid inhalation and use of
safety goggles is recommended. Sodium
hydroxide can cause serious damage upon eye contact, use of safety
goggles is recommended. An eye-wash station should be available in the
laboratory.
Results and Discussion:
Emission spectra for the C-343-Na salt in a CTAB/water
system are shown in Figure 3. The emission maxima were shifted to the higher
frequency near the CMC with increasing surfactant concentration. There was no
significant shift in emission maxima when the surfactant concentrations were
much below or above the CMC. Similar effects were observed in the excitation
spectra (Figure 4): the maxima shifted to the higher-energy side. This means
that possibly the ground state of the probe was also destabilized energetically
as a result of its incorporation into less polar aggregates. Water, being a
highly polar hydrogen-bonding solvent, could bring about significant
stabilization of the electronic state of the polar probe as opposed to the less
polar environment inside the aggregate.
It was observed that the shift in excitation maximum was
more than that of emission, which resulted in a net increase (positive) in the
Stokes shift. This increase in Stokes shift continued with surfactant
concentration until the CMC and remained almost constant with further increase
of monomer concentration signifying that micelle formation was complete.
Analysis of Figures 5 and 6 showed the Stokes shift gradually move to a higher
value with surfactant concentration until a certain concentration was reached,
and it remained practically constant with further addition of monomer. The
point of inflection corresponds to the CMC value.
To determine the point of inflection and hence the CMC
from Figure 6, the rising part and the plateau were fitted with linear
functions. These two fitted lines cut each other at a point corresponding to
the inflection [10], which was found to be at ~0.75 mM and this value is in
good agreement with the literature reported value from other methods [11].
Determination of the maxima of emission or excitation spectra was crucial for
getting a good result and could be accurately assigned manually by careful
examination, otherwise, one might fit the individual spectrum with a lognormal
function, especially when the spectrum looked broad it was difficult to find
the exact position of the maximum manually. By examining the laboratory reports
from a group of students it was found that the typical error for locating the
maxima of emission and excitation ranged from 40 to 90 cm–1 .The
analytical form for the lognormal function [12] is
The fitting function can be used in
scientific graphing software, like Origin or similar software. It should also
be noted that the Stokes shift can be equated to the reorganization energy
using the formula D
(Stokes) = 2 × reorganization energy [13]. So, one
can calculate the reorganization energy from the Stokes shift using this
relationship. This gradual change in Stokes shift until the CMC is reached may
be attributed to the fact that as surfactant is added gradually the size of the
aggregate increases and the probe bound to the aggregate experiences a changing
environment until the CMC is reached. After reaching the CMC there is no
further change
Figure 5. Emission and excitation maxima of
C-343 Na salt with varying concentration of CTAB in water. Vertical lines with
cap indicate error bars.
Figure 6. Stokes shift of C-343 Na salt with
varying concentration of CTAB in water. Vertical lines with cap indicate error
bars.
in the micellar aggregate and hence the Stokes
shift stops changing.
Conclusion
The reorganization energy results from the creation of a
transient solute dipole (by an exciting photon) in a solvent atmosphere. This
induces a reorganization of the polar solvent molecules (for nonpolar
solvents―no solvent reorganization) about the dipole in a manner that
stabilizes the energy of the system. The energy term so involved may be called
the reorganization energy. The Stokes shift is monitored as a function of
monomer concentration. When micelle formation is complete there is no further
change in Stokes shift, that is, no further reorganization of the solvent
atmosphere is evident. Thus, the Stokes shift may be regarded as a marker for
probing the micellization process. This experiment is not only a new technique
for CMC measurement, but students learn the new concept of solvent
reorganization during formation of an organized molecular assembly in a
solvent, and this is reflected on the spectra of the probe dye.
Similar experiments can be carried out with
other cationic surfactants, such as cetyl pyridinium chloride (CPCl) using the
same C-343 Na salt as the probe. For anionic surfactants, we can, in principle,
apply a similar method. This experiment has been regularly conducted in the
Iowa State University physical chemistry laboratory for senior undergraduates
since last year. The experiment is appropriate for undergraduate courses
particularly for seniors who have acquired at least a basic knowledge of
spectroscopy. Based on the results obtained from a group of undergraduate
students at Iowa State University, it was found that the reported results (CMC
value) are within ±10% of the literature reported value for CTAB from other
methods.
Acknowledgment. The author is
thankful to Professor J. W. Petrich, the Chemistry Department Chair, Iowa State
University, for carefully going through the manuscript and for his valuable
comments.
Supporting Materials. Instructions for
the instructor and for students are available as supporting materials ((http://dx.doi.org/10.1333/s00897071097a)
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The
Chemical Educator, Vol. 12, No. 1,
Published on Web 2/1/2007, 10.1333/ s00897071097a, © 2007 The Chemical
Educator