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Anion-Specific Binding to n-Hexadecyl Phosphorylcholine Micelles
Jacks P. Priebe,† Bruno S. Souza,† Gustavo A. Micke,† Ana C. O. Costa,† Haidi D. Fiedler,†
Clifford A. Bunton,‡ and Faruk Nome*,†
†
´
Departamento de Quımica, Universidade Federal de Santa Catarina, Florianopolis-SC 88040-900, Brazil, and
ꢀ
‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California
Received July 8, 2009. Revised Manuscript Received August 12, 2009
Hexadecyl phosphorylcholine (HPC) micelles incorporate anions rather than cations in the interfacial region, giving
an anionoid micelle with a negative zeta potential. Hydronium ion incorporation in the micellar pseudophase parallels
the increase in the negative zeta potential, and salts increase the rate of A1 hydrolysis of 2-(p-heptyloxyphenyl)-1,3-
dioxolane in micellized HPC and inhibit the reaction of OH- with naphthoic anhydride. The kinetic effects are larger
with NaClO4 than with NaCl. The increased micellar negative charge with added salts increases the repulsion between
headgroups and decreases the aggregation number. These observations are relevant to understanding the behaviors of
biological phosphorylcholine amphiphiles.
Introduction
preferred over that of anions. Dediazonization trapping of Cl-
and Br-, examined at high salt concentration with both head-
group sequences, shows that these ions are incorporated into both
sulfobetaine and phosphorylcholine micelles with a cation depen-
dence, although the effects on micellar morphology were not
considered.10 In view of the importance of understanding ion
interactions with phosphorylcholine amphiphiles in biological
systems, we examined salt effects on zeta potentials, aggregation
numbers, and rates of hydroxide ion and hydrogen ion-mediated
hydrolyses of micelle-bound substrates as physical models of ion-
specific interactions at biological interfaces.
The interaction of ions with micelles of aqueous ionic surfac-
tants is generally described in terms of electrostatic and ion-
specific nonelectrostatic interactions.1 Anions rather than cations
bind to zwitterionic sulfobetaine micelles, and binding increases
with soft anions (Pearson definition) following the Hofmeister
series.2-8 Less-hydrophilic anions (e.g., ClO4-) bind much more
strongly than halide ions that follow the sequence I- > Br-
>
Cl- and OH- and are only weakly bound.7-9 The softer, less-
hydrophilic anions can enter the headgroup region and interact
with the quaternary ammonium ion, whereas the sulfonate ion
should be strongly hydrated and should not interact strongly with
cations. Anion addition makes the sulfobetaine micelle anionoid
with increasing negative surface charge, as given by zeta poten-
tials, following the Hofmeister series, and the micelle then takes
up cations (e.g., H3Oþ). Anion binding, estimated by various
methods, is much weaker than with cationic micelles but follows
the same ion order in dilute salt.3-10
The ion-headgroup sequence in n-hexadecylphosphorylcho-
line, HPC, micelles is opposite to that in sulfobetaine, SB3-14,
micelles, and at the simplest level, the binding of cations should be
Results and Discussion
Salt Effects on the Aggregation Number of HPC. For
zwitterionic sulfobetaine surfactants (SB3-n), the aggregation num-
ber, Nagg, is only slightly sensitive to 0.01-0.05 M salts.8,11,12 We
measured Nagg of HPC micelles without and with the salts, which
have significant electrophoretic effects on SB3-n micelles.6-9
The inset of Figure 1 shows a typical anion effect upon changes
in fluorescence intensity against quencher concentrations, accord-
ing to eq 1,13
*Corresponding author. E-mail: faruk@qmc.ufsc.br.
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ꢀ
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I0
I
½DPCꢀNagg
ln
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ð1Þ
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1008 DOI: 10.1021/la902469d
Published on Web 08/27/2009
Langmuir 2010, 26(2), 1008–1012