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Synthesis and Colloidal Study of a Gemini Amphiphile
Stephanie L. Torcivia and Kevin L. Caran*
Amphiphilic molecules (those with both polar and non-polar components) are typically soluble
in an aqueous environment. Four amphiphiles are shown in figure 1; each has a polar head
group and a non-polar tail. The polar head group can be cationic (1), anionic (2), zwitterionic
(3), or non-ionic (4). The non-polar tail of an amphiphile can also vary in structure – for
example, it can be linear, branched, saturated, unsaturated, steroidal or aromatic. Each structure
in Figure 1 has a linear, saturated tail.
Figure 1. Structures of Amphiphilic Molecules
O
O
N
Br
N
O
P
3
O
1
OH
O
OH
H
N
Na
HO
O
2
OH
OH
4
At low concentrations, these amphiphiles typically form a monolayer at the air-water interface
(with hydrophobic portions extending upwards to minimize contact with water), which is in
equilibrium with monomers in solution. Above a certain concentration (“critical micelle
concentration” or CMC), amphiphiles will form colloidal aggregates with their hydrophobic
tails inward and head groups outward in aqueous solution. The CMC, as well as the size and
shape of the aggregates that form are dependant on the structure of the amphiphile
(hydrophobic/hydrophilic balance, molecular geometry, etc), temperature, concentration of
added electrolytes, and other factors. Generally, as an amphiphile’s hydrophobic tail (or tails)
increases in length, the CMC value decreases. An increase in hydrophobic character from a
longer tail decreases the solubility in solution, and thus causing aggregation at a lower
concentration. A common aggregate morphology for amphiphiles in water is the micelle – a
nano-scale supramolecular structure (typically ~2 nm wide) that can range in shape from
approximately spherical to worm-like, with a spherical cross section (see Figure 2).
a
b
c
Figure 2. Colloidal Aggregation. a. A micelle (~spherical) with polar
headgroups facing out, and non-polar tails in. b. Cross section and side view of a
worm-like micelle (WLM) composed of cationic amphiphiles (with anionic
counterions). Note that the cross section of the WLM is similar to the cross
section of a spherical micelle. c. At high concentrations, WLMs can become
entangled, thus increasing the viscosity of the sample.
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Gemini amphiphiles1 are molecules composed of two polar, hydrophilic head groups with two
non-polar, hydrophobic tails (Figure 3a). The heads are covalently bonded together by a
spacer, which can vary in structure. A short hand used to name gemini amphiphiles with linear
alkyl spacers and linear alkyl tails is “n-s-m” (where n and m refer to the alkyl chains attached
to each head group, and s refers to the number of carbons in the spacer). The gemini we will
use for this lab is 12-2-12 (Figure 3b), which has quaternary ammonium headgroups. The
CMC of a gemini depends on the structure of spacer, head group and hydrophobic tail on the
molecule.
a
tail
head spacer head
group
group
b
N
Br
tail
Figure 3. Gemini Amphiphiles
a. General structure of a gemini amphiphile.
b. Structure of 12-2-12.
N
Br
Direct visualization of micelles is not trivial, due to their nanoscale dimensions (typically ~2
nm wide). However, indirect methods can be used to monitor the physical and chemical
changes that occur during micelle formation. The CMC typically coincides with a sharp break
in the plot of a physical parameter – conductivity, surface tension, or diffusion rate (using
NMR), among others – verses concentration.
In Part I, we will synthesize the gemini amphiphile 12-2-12 according to Scheme 1 below.
N,N,N′,N′-tetramethylethylenediamene, a bis-nucleophile, is reacted with an excess (more than
two equivalents) of 1-bromododecane in refluxing acetonitrile. Two SN2 reactions serve to
install two alkyl chains on the diamine while simultaneously quaternizing the amines in a
Menshutkin reaction. The two quaternary ammoniums serve as the two polar (hydrophilic)
head groups, and the C12H25 chains as the non-polar (hydrophobic) tails. The product is
insoluble in acetone, so addition of this solvent at the end of the synthesis should allow the
gemini product to crash out of solution. We will separate the solid product via vacuum
filtration.
Scheme 1. Synthesis of 12-2-12
H25C12 N
N C12H25
+ 2 C12H25Br
N
CH
CN
N
3
reflux
2 Br
In Part II, we will use the fluorescent dye 4-(Dicyanomethylene)-2-methyl-6-(pdimethylaminostyryl)-4H-pyran (DCM, see figure 4 for structure) to study the colloidal
properties of 12-2-12. DCM is a useful fluorescent dye in that it is soluble in a range of
solvents, and fluoresces at different wavelengths depending on the local environment of the
dye. In Part IIa, we will investigate the
N
fluorescence properties of DCM in a polar
(methanol) and non-polar (hexane)
environment. Then in Part IIb, we will introduce
N
a small amount of DCM to aqueous mixtures
above and below the CMC of 12-2-12.
N
O
Figure 4. Structure of DCM fluorescent dye.
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Laboratory Procedure
Part I – Synthesis and purification of the gemini amphiphile, 12-2-12
1. Set up a magnetic stir plate with a heating mantle attached to a Variac on top.
2. To a 25-mL round-bottom-flask containing a stir-bar, add 1.0 mL acetonitrile, 3.1 mL
1-bromododecane (d = 1.030 g/mL), and 0.65 mL N,N,N’,N’tetramethylethylenediamene (d = 0.770 g/mL).
3. Connect a water-cooled condenser and start the flow of water.
4. Heat the reaction to reflux for 60min. (Note: Once the reaction is steadily refluxing,
you should do part IIa below during the 60-minute reaction.)
5. Remove the round-bottom flask from the heating mantle, and carefully add 15-20 mL
of acetone to the warm crude reaction mixture to cause the product to precipitate from
the solution. Additional acetone may be added if needed to form a precipitate. Cool the
mixture on an ice-bath for 5-10 minutes. Use a spatula to break up any solid product
that forms.
6. Separate the solid product via vacuum filtration, and wash it several times with acetone.
Allow it to dry on the vacuum filter for ~5-10 minutes. If needed, you may further dry
the solid in the oven.
7. Weigh your solid product, and calculate a percent yield.
8. Determine the melting point of your solid, and compare this to the literature value (194196 °C, dec).
Part IIa – DCM fluorescent dye in polar and non-polar solvents
(Note: Part IIa may be done while the reaction in Part I is under reflux.)
1. Measure ~10 mL of each of the following solvents into four separate test tubes:
dH2O, methanol, dichloromethane, toluene
2. Add 15 drops of the DCM fluorescent dye (0.1 mg/mL stock solution in EtOH) to
each test tube, and carefully swirl the tubes to mix.
3. Shine a black-light on each tube. You should use the long wavelength setting (365
nm) on the black-light, as most glass test tubes block shorter wavelength UV light.
With the room lights dimmed, observe and record the color and intensity (i.e. how
much light is emitted) of the fluorescence emission of the dye in each of these
solvents. Save these samples for comparison to samples in Part IIb.
Part IIb – DCM fluorescent dye in aqueous samples of 12-2-12 above and below CMC
1. The CMC of 12-2-12 in water is known to be approximately 0.97 mM. Prepare two
samples of 12-2-12 in 125-mL Erlenmeyer flasks as follows:
Sample A: 0.013 g of 12-2-12 in 100 mL dH2O
Sample B: 0.307 g of 12-2-12 in 100 mL dH2O
Note: Adding warm dH2O or warming the solutions slightly under hot running tap
water will help to fully dissolve the solids 12-2-12..
2. Once the solid has completely dissolved, transfer ~10 mL of each solutions to
separate clean test tubes and add 15 drops of the stock DCM solution to each.
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3. Compare the appearance of the fluorescence of each sample under a black light.
Compare these results to your samples from Part IIa.
Part IIc – Aqueous 12-2-12 at high concentration
1. In a test tube, dissolve 0.310 g 12-2-12 in 5 mL of hot dH2O. You may need to heat
the sample further under a stream of hot tap water to get all of the solid to dissolve.
2. Allow the mixture to cool and observe the physical appearance of the sample.
Prelab questions:
1. Calculate the number of moles of 1-bromododecane, and N,N,N’,N’tetramethylethylenediamene used in part I. What is the limiting reagent?
2. Calculate the theoretical yield of 12-2-12. [Note that you should include the two
bromides (Br -) in your calculation of the molecular mass.]
Postlab questions:
1. Calculate the percent yield of 12-2-12.
2. Describe the difference in the appearance of the fluorescence of DCM in dH2O,
methanol, dichloromethane and toluene (part IIa).
3. Calculate the concentration of 12-2-12 in Sample A and Sample B (part IIb). Describe
the difference in the appearance of the DCM fluorescence in these two samples.
Compare these to the appearance of the solutions in part IIa. Suggest an explanation for
the difference in appearance of these two samples. (Hint-DCM, is relatively
hydrophobic. Think about how the immediate environment of DCM changes above the
CMC.)
4. Describe the physical appearance of your concentrated 12-2-12 sample in Part IIc.
Suggest an explanation for this appearance.
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Menger, F. M., Keiper, J. S. Angew. Chem. Int. Ed. 2000, 39, 1906.