1D17
Gelation of Aqueous Surfactant Solutions Examined by Raman Spectroscopy(Gakushuin University) Ashok Zachariah Samuel, Koichi Iwata
Introduction
Beyond a critical concentration, surfactant (e.g. cetyltrimethylammonium bromide)
molecules aggregate into micelles, vesicle etc. in aqueous solution. The shape of the assembled
structure depends on the S/m ratio (S – surface area of the aggregate and m- number of molecules
per aggregate). Interplay of several intermolecular forces, such as, repulsive interaction between
ionic head groups, van der Waal’s interaction between the hydrophobic chains, selectivehydration of the ionic head group etc. controls the aggregation behavior. Interestingly, addition ofsmall organic molecules (e.g. Na-salicylate, NaSA) induces further aggregation of these micellar
aggregates into worm-like micelle (fibrils)1 and eventually results in the gelation of the aqueoussolution. This behavior is not easily explained by the geometrical argument proposed to explain
the shape of surfactant aggregates in solution.2 A specific intermolecular interaction is believed tobe responsible for the gelation of surfactant solution but the true nature of this interaction remains
unclear. We have used Raman spectroscopy to probe the behavior of a gelator, Na-salicylate, in
the CTAB-gel and to understand the specific intermolecular interaction that leads to gelation.
Experiment
In a typical experiment we prepared the gel samples by mixing 20 mM
cetyltrimethylammonium bromide (CTAB), 20 mM Na-salicylate and water.
The water composition was varied from 20 to 500 weight percent. The samples
were maintained at 40°C for two to five days to equilibrate and ensure
homogeneity. Photograph of a gel-sample is given in Figure 1. Raman spectra
were recorded with a He-Ne laser (632.8 nm, 30 mW) using a Raman
microscope (Olympus microscope and spectrometer from Tokyo Instruments)
Figure 1:
equipped with a 100X oil immersion objective. In a typical acquisition, sample Transparent CTAB
was exposed for 10 min and 5 cycles were averaged.
gel.
Results and Discussion
The present investigation of CTAB gel was initiated to probe the intermolecularinteraction and to understand the nature of gelator in the material. CTAB aqueous solution
transforms to a gel upon addition of NaSA. The ionic interaction between the head groups ofCTAB and NaSA has been predicted as the reason for the gelation of the surfactant solution. Inorder to test this hypothesis we have attempted to prepare gel with salicylic acid (SA) instead ofNaSA. In our study, specific Raman bands for SA and NaSA have been identified, which areused as Raman signatures to address the nature of the gelator. The pKa of SA is 2.9 and in neutral
water SA should exist as carboxylate, which is a requisite for the gel formation if ionicIntensity (a.u.)interactionisresponsible for it. As
10000
expected, addition of9000 CTAB
8000
the acid form (SA)7000
also induced gelation
6000of CTAB solution.
NaSA
5000
However, the Raman
740 760 780 800 820 840 860
spectrum of SA-1
Ramanshift (cm )
CTAB
gel
had
Figure 2. a) Raman spectra of the gel. Relative intensity of these bands indicates the spatialdifferent
distribution of constituents in the gel b) A pictorial depiction of distribution of different features
components in the fibrillar network in the gel.
from the Raman
spectrum of the NaSA-CTAB gel. Detailed analysis revealed that SA remains in the carboxylic
acid form in spite of the neutral pH of water. Such a behavior is unexpected from the acid basechemistry. Moreover, this result disproves the hither to believed ionic interaction mechanism ofgelation.
11000a)NaSAb)

B
DEFG
H
I+

Na

Br

Br

7+NO

H

7+O

Br

HO - OO - O

N+

Br

N+

HO

O

NO

7Micelles

HO

O+

O

H

H

7Salicylic Acid (SA)Worm like Micelles

Raman image recorded for the gel prepared with a mixture of NaSA and SA indicated that
their distribution is not homogeneous within the gel. Self segregation of SA and NaSA along theself assembled fibrils of CTAB (see the introduction) in the gel could be a possible reason(Figure 2). We believe that the difference in hydration of SA and NaSA drives their selfsegregation.
Our findings pose the question, “what interaction
SA Solution
in water
158 cm‐1
causes gelation?” A new Raman band was observed in theCTAB gel
162 cm‐1
low wavenumber region (~160 cm-1) of the Raman spectrum
C(D)TAB gelof the gel, which was absent in the Raman spectrum of solid
163 cm‐1
SA. Our study has revealed that this mode has a large
contribution from aromatic ring vibrations. The nature of thecarboxylic group minimally affects the position of this mode.
Ramanshift (cm‐1)
Moreover, the position of this mode shows a considerable
Figure 3. Raman band in the low
shift and change in bandwidth in the gel compared to its wavenumber
region is shown in the figure.
position and bandwidth in the aqueous solution. Similarly, aconsiderable shift the vibrational mode of trimethylammonium was also observed in gel. Hence it
is possible that the aromatic pi interaction between the head group and the benzene ring of SA (orNaSA) is the intermolecular interaction that leads to gelation. To substantiate our hypothesis we
found that simple benzene molecule also formed of transparent gel type material.
2500

SA solution in water
CSH 20mM 100C(D)SH 20mM 100*

Intensity(a.u.)(a.u.)
Intensity

2000

1500

1000

500

0130

140

150

160

170

180

190

200

References
1. Raghavan, S. R. Langmuir 2009, 25, 8382-8385
2. Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans.
2 1976, 72, 1525– 1568