Dynamical effects of supramolecular aggregates in water-butoxyethanol mixtures studied by viscosity measurements

Association and microheterogeneity in aqueous 2-butoxyethanol solutions

Molecular dynamics simulations are employed to investigate aggregation and microheterogeneity in dilute solutions of 2-butoxyethanol (BE) in water. The BE concentration is varied from near infinite dilution to the mole fraction X(BE) = 0.04. It was found that large systems (32,000 molecules) are necessary to accommodate the BE aggregates that form in this concentration range. Simulations were performed with two different force fields, and similar results were obtained. At very low concentration, BE aggregation is not observed, but evidence is found for intramolecular hydrogen bonds (between the hydroxyl hydrogen and the ether oxygen of the same BE molecule) that form five-member ring configurations, similar to those reported in experimental studies of BE in nonaqueous solvent. Initial signs of BE association appear at X(BE) ≈ 0.005, after which aggregation occurs rapidly, with aggregates that can be described as micelle-like being fully formed at X(BE) ≈ 0.02. This concentration agrees well with many experimental studies of aggregation in BE-water solutions. Between X(BE) = 0.02 and 0.04, the aggregates appear to grow a little in size, but the basic structure remains the same. At long range, the various pair correlation functions show clear density oscillations associated with BE aggregation. This allows us to identify the length scales of the existing microheterogeneity and to estimate the size of the BE aggregates. If we assume spherical aggregates, then our estimate of the radius at X(BE) = 0.04 (~42 Å) is close to estimates obtained from light scattering and small-angle neutron scattering experiments.

Self-association of short-chain nonionic amphiphiles in binary and ternary systems: comparison between the cleavable ethylene glycol monobutyrate and its ether counterparts

In the context of environmental concerns for the production of surface active species, the introduction of a carbonyl function into the skeleton of ethyleneglycol-derived solvo-surfactants is a way to access cleavable compounds with presumed enhanced biodegradability. Ethylene glycol monobutyrate (C(3)COE(1)) was synthesized and compared to its ether counterparts, ethylene glycol monopropyl (C(3)E(1)) and monobutyl ethers (C(4)E(1)), to assess the effect of the insertion of a carbonyl function in the skeleton of short-chain ethoxylated amphiphilic compounds. In aqueous solutions, the ester has intermediate behavior between that of the two ethers with regard to surface tension, solubilization of Me-naphtalene in water, and self-diffusion by PGSE NMR. In ternary systems, C(3)COE(1) and C(3)E(1) have the same optimal oil (EACN = 2.8), which is much more polar than that of C(4)E(1) (EACN = 8.5). With regard to the ability to form structured systems, the behavior in water does not differ significantly for the three compounds, and the transition between nonassociating solvents and amphiphilic solvents, sometimes called solvo-surfactants, is gradual. In ternary systems, however, only C(4)E(1) and C(3)COE(1) form a third phase near the optimal formulation, which tends to show that C(3)COE(1) possesses the minimum amphiphilicity to get a structuration. Self-diffusion NMR studies of the one-phase domains do not, however, allow us to distinguish between different degrees of organization in the three systems.

Junctions and end-caps in self-assembled non-ionic cylindrical micelles

Cylindrical micelles are known to exhibit two types of morphologies: branched networks and linear, worm-like (or thread-like) micelles. These structures correspond to two types of topological defects: end-caps and junction points. Although either type of defect increases the micelle energy (when compared to the cylindrical sections), they are stabilized by an increase in the translational (end-caps) or configurational (junctions) entropy. End-caps reduce the length of the cylindrical micelles, resulting in a suspension of linear, worm-like micelles. Y-junction branch points cause the formation of a network structure that may percolate and coexist thermodynamically with a "sol" of finite cylinders with end-caps. In this paper, we review current experimental and theoretical studies of non-ionic cylindrical micelles in aqueous solutions. We focus on single and multicomponent amphiphiles, and consider both small molecules and macromolecules (polymers), in order to identify the driving forces that determine the type of topological 'defect' and the resulting system morphology.