The energy released when tetradecyltrimethylammonium bromide (C 14 TAB) is combined with different derivatives of chlorobenzoates in an isothermal titration calorimeter (ITC) allows a direct evaluation of the spontaneity of the threading of the aromatic anions into the micellar palisade. The comparison between the enthalpimetric curves with the ones for viscosity of the solutions, hydrodynamic radii of the aggregates (dynamic light scattering experiments), and the fraction of aromatic anions incorporated (diffusion-ordered spectroscopy experiments) allows the establishment of the variations of enthalpy with formation, growth, and decrease of the wormlike micelles (WLMs). The formation of WLMs with C 14 TAB is very favored (very exothermic) for titrations of chlorobenzoate derivatives which present the chlorine atom in positions 3 or 4 of the aromatic ring. However, the aggregation is highly unfavorable if chlorine is at position 2 of chlorobenzoate. According to the results, the high potential of the ITC to determine critical concentrations and the energies associated with the aggregation of a cationic surfactant and aromatic anions for the formation of WLMs was demonstrated.
“Smart”
fluids displaying large changes in their
rheological properties in response to external stimuli have been of
great interest in recent years. For example, “smart”
wormlike micelles (WLMs) that respond to pH can be readily formulated
by combining a cationic surfactant such as cetyltrimethylammonium
bromide (CTAB) with an aromatic compound such as 1,2-dihydroxybenzene
(DHB). Here, we show that a pH-responsive aqueous formulation as mentioned
above can be simultaneously made responsive to ultraviolet (UV) light
by incorporating a photoacid generator (PAG) into the system. A commercially
available PAG, diphenyliodonium-2-carboxylate, is used here. Upon
exposure to UV light, this PAG irreversibly photolyzes into iodobenzene
(IB) and benzoic acid (BA), with the formation of BA, leading to a
drop in pH. WLMs formed by mixtures of CTAB, DHB, and the PAG are
systematically characterized before and after UV irradiation. As the
PAG photolyzes, an increase in the viscosity of WLMs occurs by a factor
of 1000. We show that the ratio of the zero-shear viscosity η0 (after UV/before UV) depends on the initial pH of the sample.
The UV-induced increase in η0 can be attributed to
the growth of WLMs in solution, which in turn is influenced by both
the ionization state of DHB and the presence of IB and BA.
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