Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 5. Self-Association Behavior and Micellar Properties of Ammonium Dodecyl Sulfate

Abstract

Micelles formed in water from ammonium dodecyl sulfate (AmDS) are characterized using time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonance (EPR), conductivity, Krafft temperature, and density measurements. TRFQ was used to measure the aggregation number, N, and the quenching rate constant of pyrene by dodecylpyridinium chloride, k Q . N depends only on the concentration (C aq ) of ammonium ions in the aqueous phase whether these counterions are derived from the surfactant alone or from the surfactant plus added ammonium chloride as follows: N ) N 0 (C aq /cmc 0 ) γ , where N 0 is the aggregation number at the critical micelle concentration in the absence of added salt, cmc 0 , and is equal to 77, 70, and 61 at 16, 25, and 35°C, respectively. The exponent γ ) 0.22 is independent of temperature in the range 16 to 35°C. The fact that N depends only on C aq permits the determination of the micelle ionization degree (R) by employing various experimental approaches to exploit a recent suggestion (J. Phys. Chem. B 2001, 105, 6798) that N depends only on C aq . Utilizing various combinations of salt and surfactant, values of R were obtained by finding common curves as a function of C aq of the following experimental results: the Krafft temperature, N, k Q , the microviscosity of the Stern layer determined from the rotational correlation time of a spin probe, 5-doxyl stearic acid methyl ester, and the spin-probe sensed hydration of the micelle surface. The values of R, determined from applying the aggregation number-based definition of R to all of these quantities, were within experimental uncertainty of the values R ) 0.19, 0.20, and 0.21 derived from conductivity measurements at 16, 25, and 35°C, respectively. The volume fraction of the Stern layer occupied by water decreases as N increases. For AmDS micelles, both the hydration and its decrease are predicted by a simple theory of micelle hydration by fixing the parameters of the theory for sodium dodecyl sulfate and employing no further adjustable parameters. For a given value of N, the hydration decreases as the temperature increases.

Low-Surface Energy Surfactants with Branched Hydrocarbon Architectures

“…21,[44][45][46][47] The effective area per headgroup at the cmc increases significantly on increasing the branching factor (from 53Å 2 for iC18S(FO-180N) to 73Å 2 for iC18S(FO-180N). The parameters in Table 2 indicate that branched and bulky surfactants suffer steric hindrance at the interface which leads to a decrease in the packing efficiency, and therefore higher area per molecule.…”

Specific effects of monovalent counterions on the structural and interfacial properties of dodecyl sulfate monolayers

“…The effect of the counterion to DS − surfactants has on the selfassembly and the structure of the surfactants in aqueous systems has been previously studied using both experimental and simulation methods, particularly for micellar systems [37][38][39][40][41][42] . Molecular dynamics (MD) simulations of Li + , Na + and NH + 4 cations with DS − micelles in aqueous solution by Rakitin and Pack 40 showed that the most compact structure for a micelle occurs with Li + cations that penetrate considerably deeper into the micelle than either Na + or NH + 4 .…”

“…Molecular dynamics (MD) simulations of Li + , Na + and NH + 4 cations with DS − micelles in aqueous solution by Rakitin and Pack 40 showed that the most compact structure for a micelle occurs with Li + cations that penetrate considerably deeper into the micelle than either Na + or NH + 4 . Zana and coworkers used fluorescent measurements to determine that the aggregation number of ADS micelles is larger than those for SDS micelles and similar to caesium dodecyl sulfate (CDS) micelles 39,41 . Sammalkorpi et al used MD simulations to show that ionic strength of the solution affects not only the aggregate size of the resulting DS − micelles but also their structure, where specifically they found that the presence of CaCl 2 induces more compact and densely packed micelles than those in the presence of NaCl 42 .…”

Shaping PEDOT nanoparticles for use in 3D tissue phantoms

“…This suggests that if the concentration of surfactant greatly outpaces the concentrations of the other solutions, there is insufficient material to fill out the template and the excess surfactant will group together in spherical aggregates. Much higher concentrations, as opposed to just ratios, also make longer fiber formation more difficult as it is more favorable to have many short fibers instead of longer ones . This principle of surfactant and iron (III) chloride mediated growth where spheres formed is most clearly illustrated with synthesis 8.…”

“…SDS has an anionic headgroup which ionically binds water, and the hydrophobic tails of SDS orient to form a reverse micelle. The presence of anionic surfactants can improve the conductivity by serving as a counter ion in the PEDOT chains . The second variable in controlling aqueously derived PEDOT NP is the ratio of surfactant to iron chloride as the iron cations are thought to alter the molecular packing of the surfactant by increasing the effective area of the headgroup.…”