Structure of three-component microemulsions in the critical region determined by small-angle neutron scattering

Abstract

The intensity distribution of the critical scattering from sodium di-2-ethylhexylsulfosuccinate AOT -D20 -n-alkane water-in-oil (W/0) microemulsions has been measured over an extensive range of droplet volume fractions (3 -30 vol %) and temperatures (22 to 43'C) in the critical region.The water/surfactant molar ratio of the microemulsion was kept at a constant value of 40.8, for which previous experiments on the temperature variation have been well documented. A structural model of W/O microemulsions based on well-defined surfactant-coated water droplets is firmly established up to a volume fraction of about 20 vol % for all temperatures studied. Data analysis assumes that the cloud points and subsequent phase separation are caused by concentration fluctuations of polydisperse droplets. The major conclusions drawn from the analysis are as follows. (1) The order parameter of the critical phenomenon can be taken to be the volume fraction of the dispersed droplets. (2) The size and polydispersity of the droplets remain essentially constant in the vicinity of the critical point (for a fixed water/surfactant ratio). (3) The critical phenomenon is driven by an increased attraction between the droplets as the critical point is approached. (4) The critical point can be approached by either raising the temperature at fixed volume fraction or by varying the carbon number of the oil solvent at fixed volume fraction and temperature. (5) The nature of the droplets does not change upon a phase separation into two coexisting microemulsions. The data also gives some evidence that the droplet picture of the microemulsion breaks down at sufficiently high concentrations of water and surfactant.

Simultaneous SAXS and SANS Analysis for the Detection of Toroidal Supramolecular Polymers Composed of Noncovalent Supermacrocycles in Solution

“…Increasing the number of naphthalene units in the molecular building block yields greater rigidity, as evidenced by a larger toroid and a reduction in solvent penetration into the shell. The detailed structural analysis demonstrates the applicability of SAS to monitor complex solution-based self-assembly.Self-assembly processes range from the micellisation of amphiphiles [1][2][3] to supramolecular polymerization [4] and onwards to the more elaborate folding of DNA origami.[5] When apolar molecules associate, solvent-solvent and solvent-solute interactions play a key role in the assembly process.[6] Upon solvent removal, the assembly structure is rarely unaffected, necessitating the probing of the sample structure in solution in order to form an accurate assembly model. Unfortunately, this is often difficult to do, and, while cryo-electron microscopy can reveal high-resolution structures in (frozen) solution, [1,7] there are only a few reports of its use in non-aqueous solutions.…”

Self-association of disc-like molecules in hexadecane

“…Schultz distribution is skewed to the larger sizes, tending to a Gaussian form at large z value and approaches to a delta function as z approaches infinity. Since j-th moment of the Schultz distribution can be easily calculated by the polydispersity index p can be defined and derived to be The weighted intra-particle structure factor for spherical particles with Schultz distribution in sizes can be expressed as [25] This integration can be carried out analytically as [26], where Appendix B : Particle form factor of stacked disk-like molecules.…”

Fluorinated microemulsions as reaction media for fluorous nanoparticles

“…The influence of system composition on temperature-dependent phase behavior and nanodroplet structure has been investigated by neutron scattering, representing one of the first in-depth studies on F-microemulsion structure by SANS. Interestingly and significantly, these F-MEs stabilized by fluorinated analogues of AOT are found to behave in a very similar way to other AOT-based water-in-hydrocarbon oil, [33][34][35][36][37][38] water-in-CO 2 (e.g. ref.…”

“…Such critical-type behavior has been noted in hydrocarbon-based microemulsions on nearing the high temperature phase boundary. 35,37 Clearly (Fig. 1) for higher w value diHCF4-stabilized F-MEs at 40 C, the high-T boundary is much closer than that for the equivalent diCF3-stabilized samples.…”