Micelles of the anionic surfactant sodium dodecylbenzenesulfonate (NaDoBS) in dilute aqueous solution can be transformed into lamellar aggregates by the addition of alkali metal chloride (LiCl to CsCl) and tetraalkylammonium chloride (alkyl is methyl or n-butyl) salts. Depending on the type of cation, concentration of salt, and isomeric purity of the alkyl chain of the surfactant, different types of phases are observed: large unilamellar vesicles, multivesicular vesicles, and flocculated multilamellar vesicles (lamellar droplets). Over limited concentration ranges, some salts induce phase separation in a surfactant-rich and a surfactant-lean phase. The formation of the different phases was monitored by turbidity and fluorescence depolarization measurements, whereas the phases were characterized by light microscopy, freeze-fracture electron microscopy, and confocal scanning laser microscopy. Thermodynamic aspects of aggregation, in particular the counterion binding characteristics, were studied by microcalorimetry and conductivity. On a molecular level, the packing in a lamellar array can be explained largely in terms of a change in counterion binding and, to a lesser extent, by a decrease of the hydration of the headgroup and the counterion. A better counterion binding is facilitated by a less hydrated cation or by an increase of the electrolyte concentration. The formation of different types of lamellar aggregates is due to different types of interactions between lamellar layers or between aggregates: largely repulsive for stable dispersions of unilamellar vesicles to attractive down to short distances for the flocculated lamellar droplets.
The self-association of copolymers of N-isopropylacrylamide and N-n-octadecylacrylamide (Pnipam-C18) in aqueous solutions was studied by means of time-resolved fluorescence quenching. The discrete domains consist of several polymer chains interacting through their hydrophobic side chains, since the number of aliphatic side chains involved in the microdomain formation (aggregation number) is larger than the number of aliphatic side chains per polymer. By means of titration microcalorimetry, the interaction of the copolymer with surfactants was studied. Strong association between the copolymer and the cationic surfactants N-cetylpyridinium chloride (C16PyCl) and cetyltrimethylammonium bromide (CTAB) occurs by partitioning of the surfactants in a noncooperative mechanism. Prior to mixed-micelle formation, individual surfactant molecules adsorb to collapsed polymer coils as can be seen from the large exothermal contribution in the enthalpy curves which result from microcalorimetric titration of surfactant into aqueous Pnipam-C18 solutions.
The lyotropic phase behavior of (technical grade)
dodecylbenzenesulfonates (DoBS) is strongly influenced
by the type of counterion and the relative amount of water. Phase
diagrams are composed for the following
systems: HDoBS/water, NaDoBS/water, (HDoBS + NaDoBS 1:1)/water,
LiDoBS/water, KDoBS/water,
CsDoBS/water, Ca(DoBS)2/water, NaDoBS/water/NaCl, and
NaDoBS/water/CsCl. The phases were
characterized by light microscopy, freeze-fracture electron microscopy,
X-ray diffraction, and macroscopic
appearance. The phase diagrams all contain large areas of lamellar
phases. The appearance of the lamellar
phases differs along this series, in particular regarding swelling
behavior, either with or without a micellar
phase next to the lamellar phase, and formation of large, rather
irregular lamellar units versus smaller,
perfectly spherical lamellar topologies (so-called lamellar droplets).
LiDoBS shows, in addition, a hexagonal
phase between 25 and 50 wt %. Explanations for the occurrence of
the different phases are given in
molecular terms and in terms of interactions between bilayers and
between aggregates.
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