Time-Resolved Fluorescence Quenching and Electron Paramagnetic Resonance Studies of
the Hydration of Lithium Dodecyl Sulfate Micelles
Barney L. Bales* and Antoine Shahin
Department of Physics and Astronomy, California State UniVersity at Northridge,
Northridge, California 91330-8268
Cecilia Lindblad and Mats Almgren
Institute of Physical Chemistry, UniVersity of Uppsala, S-75121 Uppsala, Sweden
ReceiVed: September 15, 1999; In Final Form: NoVember 1, 1999
A spin-probe method to study the surface hydration of sodium dodecyl sulfate (SDS) micelles (Bales, B. L.;
Messina, L.; Vidal, A.; Peric, M.; Nascimento, O. R. J. Phys. Chem. 1998, 102, 10347; referred to as I) is
applied to lithium dodecyl sulfate (LiDS) micelles in order to test both the method and a model of micelle
hydration. The method is based on the fact that the hyperfine spacing between the low- and center-field
resonance lines, A
+
, varies linearly with a polarity index, H(25 °C), which is the volume fraction occupied
by water in a solvent mixture that contains only water as a source of OH dipoles. The model successfully
employed in I predicts that H(25 °C) is determined only by the geometry of the micelle; the amount of water
associated with the micelle is determined by the volume available to house the water. Thus, SDS and LiDS
micelles of the same aggregation number, N
A
, ought to yield the same value of H(25 °C) (and, therefore A
+
)
because, apart from their waters of hydration which are already taken into account by the geometrical model,
neither Li
+
nor Na
+
occupies significant volume. Over the range of aggregation numbers N
A
) 50-110,
values of H(25 °C) determined from measurements of A
+
in LiDS micelles were found to be within (2% of
those in SDS micelles. These data support the geometric model and show that specific interactions due to Li
+
or Na
+
which might affect A
+
are unimportant. The aggregation numbers of LiDS micelles are measured by
time-resolved fluorescence quenching and are well described by N
A
) κ
2
([Li
+
]
aq
)
γ
with κ
2
) 112 ( 2 and γ
) 0.180 ( 0.005, where [Li
+
]
aq
is the concentration of lithium ions in the aqueous phase whether supplied
by LiDS or by both LiDS and LiCl. Thus, LiDS micelles grow according to an empirical formula identical
in form to that for SDS micelles, but at a slower rate. The aggregation numbers at the critical micelle
concentration in the absence of added salt (cmc
0
) are the same for SDS and LiDS micelles, but above this
concentration, LiDS micelles are significantly smaller than SDS micelles for a given ionic strength. By applying
a simple model of a spherical hydrocarbon core surrounded by a polar shell and assuming that the spin-probe
samples all portions of the shell, values of the polarity index H(25 °C) may be converted into values of
N(H
2
O), the number of water molecules per surfactant molecule residing in the polar shell. This conversion
involves no adjustable parameters because the geometrical parameters are fixed from small-angle neutron
scattering measurements. As the micelles grow in the range N
A
) 50-110, N(H
2
O) decreases from 9.6 to 5.4
water molecules per surfactant molecule because the volume per surfactant molecule in the polar shell decreases
allowing less water to reside within the shell. The sphere-rod transition previously observed in I for SDS at
N
A
) 130 cannot be reproduced in LiDS, because LiCl is not sufficiently soluble to achieve an aggregation
number of 130; however, interesting small, unidentified transitions appear to occur near N
A
) 112 and 121.
A byproduct of this work is that relative aggregation numbers for LiDS micelles may be determined from
measured values of A
+
with a precision of about one molecule from the following: A
+
(N
A
) ) (15.468 (
0.004) - N
A
(3.45 ( 0.06) × 10
-3
where A
+
(N
A
) is in gauss. Since a given value of N
A
may be prepared by
choosing different combinations of SDS, LiDS, NaCl, and LiCl concentrations, neither interactions between
the micelles nor the ionic strength influence the value of A
+
.
Introduction
It was previously shown
1
that sodium dodecyl sulfate (SDS)
micelles grow as a power law of the concentration of sodium
ion in the aqueous phase. One purpose of the present work is
to show that lithium dodecyl sulfate (LiDS) micelles also obey
a power law as follows:
where [Li
+
]
aq
is the molar concentration of lithium ion in the
aqueous phase whether it is provided by LiDS, or by both LiDS
and added LiCl and to determine the constants κ
2
and γ. The
contributions to [Li
+
]
aq
from the surfactant and added salt may
be found from the conventional pseudophase ion exchange mass
balance relationship,
1
where the brackets indicate molar concentrations, R is the
apparent degree of counterion dissociation, ) 1 -R, [LiDS]
free
* Corresponding author. E-mail: barney.bales@emial.csun.edu.
N
A
) κ
2
([Li
+
]
aq
)
γ
(1)
[Li
+
]
aq
)R([LiDS] - [LiDS]
free
) + [LiDS]
free
+ [Li
+
]
add
)R[LiDS] + [LiDS]
free
+ [LiCl] (2)
10.1021/jp993289j CCC: $19.00 © xxxx American Chemical Society
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