Published on Web 01/18/2003
Determination of Critical Micelle Concentration by
Hyper-Rayleigh Scattering
Suhrit Ghosh, Anu Krishnan, Puspendu K. Das, and S. Ramakrishnan*
Contribution from the Department of Inorganic and Physical Chemistry,
Indian Institute of Science, Bangalore 560012, India
Abstract: The critical micelle concentration (CMC) of several surfactants that contain an NLO chromophore,
either at the hydrocarbon tail, or at the hydrophilic headgroup, or even as a counterion, was determined by
hyper-Rayleigh scattering (HRS). In all cases, the HRS signal exhibited a similar variation with surfactant
concentration, wherein the CMC is inferred from a rather unprecedented drop in the signal intensity. This
drop is attributed to the formation of small pre-micellar aggregates, whose concentrations become negligible
above CMC. In addition, a probe molecule, which upon protonation yielded a species with significantly
enhanced HRS intensity, was developed and its utility for the determination of the CMC of simple fatty
acids was demonstrated.
value.4 On the other hand, linking them to the periphery of a
dendrimer was shown to cause an effective lowering of â.5 In
Introduction
Critical micelle concentration (CMC) of surfactants is readily
determined by a variety of simple techniques, wherein an
appropriate property such as osmotic pressure, ionic conduc-
tance, scattered light intensity etc., is monitored as a function
of concentration. A break in the plot of the chosen property
versus concentration often signifies the formation of micellar
aggregates.1 Most of these measurements rely on the change in
the property due to the formation of large aggregates of the
amphiphile molecules (typically, about 50-100 molecules)
above CMC. Any further addition of amphiphiles to the solution,
it is expected, will not alter the concentration of free surfactants
in solution but will only lead to an increase in the number of
such micellar aggregates. Because the aggregates and free
species contribute to different extents toward the measured
property, an abrupt change of slope occurs at the CMC. Both
the size and the shape of these surfactant aggregates are known
to depend on a variety of factors such as the surfactant and salt
concentrations, temperature, etc. However, only a few methods,
such as dynamic light scattering and SAXS, have been utilized
directly to probe such size and shape changes.
the former case, there is a cumulative addition of the individual
chromophoric dipoles leading to a large net dipole along the
helical axis, which in turn results in a large coherent second
harmonic response. Although in the latter, it is due to the near
spherical symmetry attained by the higher generation dendrimers
causing just the opposite effect on the â value. More recently,
significant enhancement in â has been realized in dendritic
wedges in which the NLO chromophores are incorporated
between the branching junctions.6 Other examples in which the
spatial fixation of the relative orientation of the chromophores
has been exploited to modulate the net â value, include those
in which the chromophores are linked to a calixarene skeleton7
or other rigid molecular frameworks.8 The formation of su-
pramolecular H-bonded assemblies in solution has also been
studied using HRS.9
HRS is forbidden in a centrosymmetric structure, and for a
perfect spherically symmetric arrangement of NLO chro-
mophores, it is expected, that â would be zero in the electric
dipole approximation. However, any deviation from the perfectly
spherical arrangement of molecular dipoles would lead to second
(4) Kauranen, M.; Verbiest, T.; Boutton, C.; Teerenstra, M. N.; Clays, K.;
Scouten, A. J.; Nolte, R. J. M.; Persoons, A. Science. 1995, 270, 966.
(5) Put, E. J. H.; Clays, K.; Persoons, A.; Biemans, H. A. M.; Luijkx, C. P.
M.; Meijer, E. W. Chem. Phys. Lett. 1996, 260, 136.
Hyper-Rayleigh scattering2 (HRS) has been shown to be an
effective tool to examine spatially correlated NLO chro-
mophores, which can lead to either a reduction or enhancement
of their hyperpolarizability, â.3 Rigidly tethering the NLO
chromophores onto a helical polymer backbone, such that the
chromophores make a fixed angle (*90°) with respect to the
helical axis has been shown to dramatically enhance the â
(6) Yokoyama, S.; Nakahma, T.; Otomo, A.; Mashiko, S. J. Am. Chem. Soc.
2000, 122, 3174.
(7) Kelderman, E.; Derhaeg, L.; Heesink, G. J. T.; Verboom, W.; Engbersen,
J. F. J.; Hulst, N. F. V.; Persoons, A.; Reinhoudt, D. N. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 1075. Kenis, P. J. A.; Noordman, O. F. J.;
Houbrechts, S.; Hummel, G. J. V.; Harkema, S.; Veggel, F. C. J. M. V.;
Clays, K.; Engbersen, J. F. J.; Persoons, A.; Hulst, N. F. V.; Reinhoudt, D.
N. J. Am. Chem. Soc. 1998, 120, 7875.
(8) Deussen, H. J.; Hendrickx, E.; Boutton, C.; Krog, D.; Clays, K.; Bechgaard,
K.; Persoons, A.; Bjornholm, T. J. Am. Chem. Soc. 1996, 118, 6841.
Hendrickx, E.; Boutton, C.; Clays, K.; Persoons, A.; Van Es, S.; Biemans,
T.; Meijer, B. Chem. Phys. Lett. 1997, 270, 241. Mukhopadhyay, P.;
Bharadwaj, P. K.; Savita, G.; Krishnan, A.; Das, P. K Chem. Commun.
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J. AM. CHEM. SOC. 2003, 125, 1602-1606
10.1021/ja029070r CCC: $25.00 © 2003 American Chemical Society