Small-angle neutron-scattering study of alkyl polyoxyethylene sulfate micelles. Effect of the number of polyoxyethylene groups on n-dodecyl polyoxyethylene sulfate in deuterium oxide/water mixtures at 25.degree.C

Multilayers formed by polyelectrolyte-surfactant and related mixtures at the air-water interface

The structure and occurrence of multilayered adsorption at the air-water interface of surfactants in combination with other oppositely charged species is reviewed. The main species that trigger multilayer formation are multiply charged metal, oligo- and polyions. The structures vary from the attachment of one or two more or less complete surfactant bilayers to the initial surfactant monolayer at the air-water interface to the attachment of a greater number of bilayers with a more defective structure. The majority of the wide range of observations of such structures have been made using neutron reflectometry. The possible mechanisms for the attraction of surfactant bilayers to an air-water interface are discussed and particular attention is given to the question of whether these structures are true equilibrium structures.

Temperature- and pH-responsive micelles with collapsible poly(N-isopropylacrylamide) headgroups

We have studied the micelle formation and phase behavior of a series of temperature- and pH-responsive surfactants prepared by controlled radical (RAFT) polymerization. These C12NIPAMm surfactants consist of a dodecyl tail, a poly(N-isopropylacrylamide) (polyNIPAM) headgroup with average degrees of polymerization of between 7 and 96, and an ionizable carboxylate group. In the un-ionized state, these surfactants phase separate on warming toward a lower critical solution temperature (LCST), which decreases as the length of the NIPAM group is decreased. This is in agreement with the behavior of conventional nonionic poly(ethylene oxide)-based surfactants but is very different from that of polyNIPAM oligomer solutions. Small angle neutron scattering (SANS) shows that these surfactants self-assemble into micelles consisting of a nearly spherical hydrophobic core surrounded by a "hairy" polyNIPAM shell far below their LCST. Upon warming, the micelles undergo a sphere-to-rod transition induced by the collapse of the polyNIPAM shell, causing a reduction in the headgroup area. In the un-ionized state the demixing follows at the LCST, but a single charge on the free polymer end completely suppresses phase separation, allowing micelles to undergo a shape change but remain dissolved.

Impact of AlCl3 on the self-assembly of the anionic surfactant sodium polyethylene glycol monoalkyl ether sulfate in aqueous solution

Small-angle neutron scattering has been used to study the self-assembly of the anionic surfactant sodium polyethylene glycol monoalkyl ether sulfate in aqueous solution and in the presence of Al(3+) multivalent counterions in the form of AlCl3. The addition of the Al(3+) ions promotes significant micellar growth of the initially globular micelles into highly elongated structures until ultimately lamellar structures form. Increasing the size of the polyethylene oxide, EO, group progressively suppresses micellar growth before lamellar formation. Reducing the alkyl chain length has a similar effect on the structural evolution. Both trends are associated with increased solubility with increasing EO group size and decreasing alkyl chain length. Both the size of the EO group and the length of the alkyl chain affect sodium diethylene glycol monododecyl ether sulfate/Al(3+) complex formation and drive lamellar formation to progressively higher AlCl3 concentrations.

The formation of surface multilayers at the air-water interface from sodium polyethylene glycol monoalkyl ether sulfate/AlCl(3) solutions: the role of the size of the polyethylene oxide group

Neutron reflectivity, NR, and surface tension, ST, have been used to study the surface adsorption properties at the air-water interface of the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate (sodium lauryl ether sulfate, SLES) in the presence of Al(3+) multivalent counterions, by the addition of AlCl3. In the absence of AlCl3 and at low AlCl3 concentrations monolayer adsorption is observed. With increasing AlCl3 concentration, surface multilayer formation is observed, driven by SLES/Al(3+) complex formation. The onset of multilayer formation occurs initially as a single bilayer or a multilayer structure with a limited number of bilayers, N, ≤3, and ultimately at higher AlCl3 concentrations N is large, >20. The evolution in the surface structure is determined by the surfactant and AlCl3 concentrations, and the size of the polyethylene oxide group in the different SLES surfactants studied. From the NR data, approximate surface phase diagrams are constructed, and the evolution of the surface structure with surfactant and electrolyte concentration is shown to be dependent on the size of the polyethylene oxide group. As the polyethylene oxide group increases in size the multilayer formation requires increasingly higher surfactant and AlCl3 concentrations to promote the formation. This is attributed to the increased steric hindrance of the polyethylene oxide group disrupting SLES/Al(3+) complex formation.

Location of Spectroscopic Probes in Self-Aggregating Assemblies. I. The Case for 5-Doxylstearic Acid Methyl Ester Serving as a Benchmark Spectroscopic Probe to Study Micelles

A strategy to locate spectroscopic probes in micelles is presented which involves establishing a "benchmark" probe, i.e., one whose position is well-known and against which other probe positions may be established. Theoretically calculated values of the fraction of the micelle polar shell occupied by water, H(shell), are compared with experimental values measured with the spin probe 5-doxylstearic acid methyl ester (5DSE) for a series of sodium n-alkyl sulfate micelles as functions of both the aggregation numbers and the alkyl chain length. The theoretical values involve one adjustable parameter that may be taken to be the volume in the polar shell inaccessible to water, V(dry). Under the hypothesis that the thickness of the polar shell (5 Angstroms) remains constant as either the aggregation number or the chain length is varied, we find excellent agreement between the theoretical predictions and the experimental results, using the same value of V(dry) for chain lengths 8-12 and for aggregation numbers varying from approximately 38 to 130. We argue that these are compelling reasons that 5DSE follows the zero-order model (ZOM) of probe location. The ZOM applies to any probe that rapidly diffuses within the confines of the micelle polar shell and nowhere else. Thus, 5DSE can serve as a benchmark in the sodium alkyl sulfate micelles. As a further check, results are also presented for ammonium dodecyl sulfate micelles, where 5DSE is also found to follow the ZOM, i.e, no further adjustable parameters are needed to pass from the sodium alkyl sulfate micelles to ammonium dodecyl sulfate micelles. In contrast, results are also presented for a similar spin probe 16-doxylstearic acid methyl ester (16DSE) that is found not to adhere to the ZOM in any of the micelles. A simple first-order correction to the ZOM in which 16DSE is displaced slightly from the polar shell is shown to account for the results well. The necessary displacements, which range from about 0.7 Angstroms outside the polar shell to 1.3 Angstroms inside, are not correlated with either chain lengths or aggregation numbers; however, they correlate rather well with H(shell). Calibrations of 6-, 7-, 10-, and 12DSE spin probes are presented in the Appendix, making them available to measure microviscosities and effective water concentrations.

Structure, Diffusivity and Linear Rheology of Sodium Ether Dodecylsulfate in Aqueous Solutions – (I)

Static and dynamic light scattering measurements have been performed on dilute and semidilute solutions of sodium ether dodecylsulfate (sodium laurylethersulfate, RO(CH