A Review of Nonionic Surfactant Micellar Kinetics Studies Using
the Temperature-Jump Technique
Paul D.T. Huibers
University of Florida
Dept. of Chemical Engineering
Center for Surface Science & Engineering
P.O. Box 116005
Gainesville, FL 32611-6005
July 5, 1996
Introduction
The study of the kinetics of micellization reached its peak in
the decade of the 1970's. The primary breakthroughs were the
discovery of the existence of two (fast and slow) relaxation times,
and the development of a model [Aniannson, 1976] for the kinetic
process of micelle formation and disintegration. Several reviews
have been written on the measurement of micellar kinetics, covering
such experimental methods as temperature-jump, pressure-jump,
stopped-flow, and ultrasonic relaxation [Muller, 1979] [Gormally,
1980] [Lang, 1987]. The measurement of relaxation kinetics is
covered in a text by Bernasconi [1976] and a book edited by Wyn-Jones
[1975].
The majority of this work was done on ionic surfactants, using
pressure-jump to perturb the solution, and electrical conductivity
as a detection method. The entire literature for nonionic surfactants
is limited to one type, the octylphenol ethoxylates, taking advantage
of specific UV absorbance characteristics associated with the
aromatic structure in that molecule. The detection technique
used for this surfactant could not be generally applied to nonionic
surfactants. For the determination of the slow micellar relaxation
time, the techniques of temperature-jump [Lang, 1972b, 1975] [Tondre,
1975] [Chan, 1977] and stopped-flow [Lang, 1972a, 1975] were used
for the octylphenol ethoxylates.
Discussion
T-jump without dyes. Lang [1972b] determined a linear
relationship between the reciprocal of the relaxation time and
concentration above the cmc. Two octylphenyl ethoxylates (p-1,1,3,3-tetramethylbutylphenyl)
were studied, with an average of 16 (Triton X-165) and 30 (Triton
X-305) ethylene oxide residues. Lang used a electrical discharge
type of T-jump apparatus, discharging a 0.1 microfarad capacitor
charged to 27 kV through the surfactant solution, to which 0.1
M KCl had been added to increase electrical conductivity. Using
this method, a 5oC temperature jump could be achieved.
An optical detection technique was used, measuring absorbance
at 290 nm. It has been established [Gratzer, 1969] [Ikeda, 1970]
that the UV absorption spectra of these octylphenyl ethoxylate
surfactants is sensitive to the formation of micelles. The relaxation
results are assumed to be the slow relaxation process. For Triton
X-165, the relaxation rate varied linearly with concentration
from 5 sec-1 at 0.05 g/l to 35 sec-1 at
0.11 g/l, and for Triton X-305, from 100 sec-1 at 0.16
g/l to 700 sec-1 at 0.30 g/l.
T-jump with dyes. Chan [1977] reviewed the results of
Martell [1976] for the fast and slow relaxation time of Triton
X-100 (octylphenyl 9.5 ethoxylate), taken at 10 and 20oC.
These measurements were taken with NaCl included to increase
ionic strength, and the dye magnesium 8-amino-1-naphthalenesulfonate.
As with Lang's results, the relaxation rate increased linearly
with concentration, indicating that the micelles are less stable
as surfactant concentration increased. Micelles are also less
stable at the higher temperature.
The limits to the use of dyes for the study of micellar relaxation
kinetics is examined by Tondre [1975], where eight different dyes
were used with anionic, cationic and nonionic surfactants. The
influence of the dye on the micellar kinetics was determined,
establishing a minimum allowable surfactant/dye ratio before the
micellar lifetime was influenced by the presence of dye. An excess
of dye generally decreased the micellar lifetime. The only nonionic
surfactant in this study was octylphenol 16 ethoxylate, and a
minimum surfactant/dye ratio of 25 was determined for the dye
eosin. Nonionics were actually the most favorable class of surfactant,
as the ionic surfactants required a much higher ratio(100 or more),
making detection more difficult.
T-jump near the cloud point. More recent T-jump studies
of nonionic surfactants were conducted close to the cloud point.
Platz [1981] used both temperature-jump and dynamic light scattering
to study Triton X-100 solutions with KCl added. For the T-jump
experiments, Platz used light scattering at 350 nm as the detection
method. Strey [1986] also studied cloud point phenomena with
dynamic light scattering, and used laser light scattering at 515
and 633 nm to study the relaxation time of C12E6
over the range 15 to 35oC, for the concentration
range of 10 to 60x CMC. He demonstrated that relaxation rate
(inverse time constant) increases linearly with the square root
of concentration over the range of conditions studied.
Alternate T-jump methods. Laser-heating methods have been
developed for a variety of research projects. A system specifically
for T-jump studies of micellar kinetics has been described by
Grubic [1981], and demonstrated for the measurement of SDS micellar
kinetics. This technique offers the possibility of studying nonionic
surfactant solutions without the need to add electrolyte to the
solution, thus allowing micellar lifetime to be measured for pure
surfactant solutions.
Conclusion
The complete list of nonionic surfactants studied for the determination of micellar kinetics consists of just three octylphenyl ethoxylates (Triton X-100, X-165 and X-305) and a linear alkyl ethoxylate (C12E6). These have all been examined by the temperature-jump technique, using UV absorbance as the detection method for the octylphenyl ethoxylates and laser light scattering for the linear alkyl ethoxylate. The Joule heating T-jump technique requires that up to 0.1 M electrolyte (ex. KCl) be added in order to sufficiently raise the conductivity of the solution. The absorbance detection method (UV absorbance of the phenyl ring) cannot be applied to nonionics other than alkylphenyl ethoxylates, because this method depends on the unique sensitivity of the UV absorption spectrum to the micellar state, and the primary UV absorption is done by the phenyl ring.
A linear relationship between the micellar kinetic rate constant
and concentration has been established for the alkylphenyl ethoxylates,
and between the rate constant and the square root of concentration
for C12E6. Thus, nonionic micelles become
less stable as the concentration of surfactant increases, for
the systems studied. It may be that this relationship holds true
for the kinetics of all nonionic micelles.
References
Aniansson, E.A.G., Wall, S.N., Almgren, M., Hoffmann, H., Kielman, I., Ulbricht, W., Zana, R., Lang, J., Tondre, C. J. Phys. Chem. 80, 905 (1976).
Bernasconi, C.F. Relaxation Kinetics, Academic Press, New York, 1976.
Chan, S.K., Hermann, U., Ostner, W., Kahlweit, M. "On the Kinetics of the Formation of Ionic Micelles. II. Analysis of the Time Constants" Ber. Bunsenges. Phys. Chem. 81, 396-402 (1977).
Gormally, J., Gettins, W.J., Wyn-Jones, E. "Kinetic Studies of Micelle Formation in Surfactants" in Molecular Interactions, Vol. 2, H. Ratajczak, W.J. Orville-Thomas, Eds., Wiley, New York, 1980, Chap. 3.
Gratzer, W.B., Beaven, G.H. "Effect of Protein Denaturation on Micelle Stability" J. Phys. Chem. 73, 2270-2273 (1969).
Grubic, M., Strey, R., Teubner, M. "On the Application of a Laser T-Jump Apparatus for Perturbation of Ionic Micellar Solutions" J. Colloid Interface Sci. 80, 453-458 (1981).
Ikeda, S., Fasman, G.D. "Absorption and Fluorescence of a Nonionic Detergent in Aqueous Solution" J. Polymer Sci. A-1 8, 991-1001 (1970).
Lang, J., Auburn, J.J., Eyring, E.M. "Kinetics of Octylphenyl Polyoxyethylene Alcohol Micelle Dissociation by a Stopped-Flow Technique" J. Colloid Interface Sci. 41, 484-490 (1972).
Lang, J., Eyring, E.M. "Kinetics of the Dissociation of Nonionic Detergent Micelles by a Temperature-Jump Technique" J. Polymer Sci. A-2 10, 89-99 (1972).
Lang, J. "Kinetic Studies of the Dissociation of Nonionic Detergent Micelles" in Chemical and Biological Applications of Relaxation Spectrometry, E. Wyn-Jones, Ed., D. Reidel, Dordrecht, Holland, 1975, pp. 195-200. [Note: this chapter is a review of Lang, 1972a and 1972b with no additional data. -P.H.]
Lang, J., Zana, R. "Chemical Relaxation Methods" in Surfactant Solutions - New Methods of Investigation, R. Zana, Ed., Marcel Dekker, New York, 1987, Chap. 8.
Martell, J., Thesis, Göttingen, Germany, November 1976.
Muller, N. "Kinetics of Micelle Dissociation by Temperature-Jump Techniques. A Reinterpretation" J. Phys. Chem. 76, 3017-3020 (1972).
Muller, N. "Kinetics of Micellization" in Solution Chemistry of Surfactants, Vol. 1, K.L. Mittal, Ed., Plenum, New York, 1979, pp. 267-295.
Platz, G. "Untersuchung von Trübungserscheinungen im Einphasengebiet wäßriger Lösungen nichtionogener Tenside" Ber. Bunsenges. Phys. Chem. 85, 1155-1163 (1981).
Strey, R., Pakush, A. "Critical Fluctuations, Micelle Kinetics and Phase Diagram of Water - Nonionic Surfactant, H2O - C12E6 in " Surfactants in Solution, Vol. 4, K.L. Mittal, P. Bothorel, Eds., Plenum Press, New York, 1986.
Tondre, C., Lang, J., Zana, R. "On the Use of Dyes for the Kinetic Study of Micellar Equilibria" J. Colloid Interface Sci. 52, 372-379 (1975).
Wyn-Jones, E. Chemical and Biological Applications of Relaxation
Spectrometry, (Proc. NATO Study Inst., Salford, England, 1974)
D. Reidel, Dordrecht, Holland, 1975.