Abstracts

Abstract: The energetics and dynamics of the growth of nonionic n-alkyl polyethyleneglycol ether surfactant micelles in the isotropic L1 phase have been analyzed on the basis of a recently described, concentration dependent thermal transition.1 This highly cooperative endothermic transition is assigned as a sphere-to-rod transition. We analyze the thermodynamic data in terms of aggregation-fusion of complete micelles and also discuss the kinetics in the frame of this 'random micelle aggregation' model. The analysis of the calorimetric data leads to a cooperative unit of about 160 detergent molecules undergoing simultaneously the transition from spheroidal to cylindrical structure. We investigate the aggregation kinetics in the ms to ms range using the temperature-jump (TJ) method with observation of scattered and transmitted light and the stress relaxation dynamics monitoring transient electric birefringence (TEB). As expected from the model, the aggregation dynamics gets faster with increasing temperature, while the stress relaxation dynamics slows down with temperature, leading to apparent negative activation energies. At the transition temperatures the dynamics of the structural transition for C14E8 is characterized by an association and dissociation rate constant of 4.4x10^6 M-1s-1 and 1.8x10^3 s-1, respectively, compared to the much slower dynamics of C16E8 with the corresponding rate constants 8.5x10^4 M-1s-1 and 29 s-1, respectively. The rate constants of micelle fusion increase, while those of micelle scission decrease with increasing temperature. The TEB-results are discussed using the relations derived in literature for the competition between rotational motion and chain scission. The fast dynamics of C14E8 compared to the much slower dynamics of C16E8 provides an understanding of the differences in stress compliance and viscosity for these systems. As a major result of the study, we present a consistent mechanism of micellar growth which involves the aggregation-fusion of two micelles in a single, rate-limiting reaction step and not the stepwise incorporation of single surfactant molecules into pre-existing micelles. The speeding up of the TJ-kinetics in C16E8 at higher temperatures indicates a further structural transition, possibly network formation.

Abstract: Differential scanning calorimetry (DSC) studies of micellar, 60 mM solutions of the octaethyleneglycol alkylethers C₁₄E₈ and C₁₆E₈ provide evidence for a narrow endothermic transition at 41 and 32°C, respectively, characterized by an enthalpy change of 2 kJ mol^-1 for both detergents. The observed thermal transition is indicative of a concerted transition of the surfactant molecules, as illustrated on the basis of a simple molecular model. The effect of co-solvents such as different alcohols on the thermal transition is investigated. Glycerol markedly lowers the transition temperature whereas the transition is absent in the presence of a least 10% ethanol. The calorimetric transition correlates with the temperature dependent increase of viscosity and static light scattering as well as with changes observed by small-angle neutron scattering (SANS). The SANS results provide clear evidence for a distinct structural change occuring at the transition temperature, which is interpreted as a sphere-to-rod transition of the detergent micelles. Moreover, the rod length increases with increasing temperature. We suggest that the process causing the thermal transition acts as the prerequisite of the growth process.

Key words: CMC, DSC, detergent, growth, light scattering, neutron scattering, non-ionic micelle, poly(ethyleneglycol) alkylether, thermal transition, viscosity

Abstract:
The dynamics of the sponge structure of the L₃-phase in the ternary H2O-C10E4-n-decanol (C10E0) system has been examined. Temperature-jump-relaxation employing scattered-light detection, pressure jump with conductivity detection, time-resolved electric birefrigence, and dynamic light scattering experiments were performed on the same system at varying surfactant volume fraction Φ at constant temperature. The observed relaxations were all found to be single exponentials. The time constant τ^-1 obtained by these different methods vary over several orders of magnitude. They reveal in part rather strong dependencies on Φ obeying effective scaling laws τ^-1~Φⁿ, with n = 9 for the temperature and pressure jump relaxation and n = 1 for dynamic light scattering. The time constants from electriuc birefringence yield n = 3. The origin of the various exponents can theoretically be understood in terms of passage formation, concentration fluctuations, and relaxation of shape anisotropy due to elastic deformation.

Abstract: We investigate the effect of high electric fields on the percolation behaviour of AOT water-in-oil microemulsions in the L₂-phase and examine the existence of critical fields as function of droplet radius and droplet concentration. It is shown that there is no direct correlation with critical fields as discussed in electrorheological fluids. Very low critical fields are found in certain ranges of droplet radii. Clustering of droplets with material exchange may be postulated under these conditions.

Key words: microemulsions, high electric fields, percolation, electrorheological fluids

Percolation in Alkyl Polyglycosides (APG*) Microemulsions (C_8/10G_1.5/Octan/Water/Octanol)
G. Mönch, G. Ilgenfritz
Progr. Colloid Polym. Sci.X, in press.

Abstract: Mixtures of oil, water, alkyl polyglucosides (APG) and long chain alcohols form almost temperature invariant microemulsion. The phase behaviour depends on the content of co-solvent, usually long chain alcohols. In this paper we will show that the system C8/10G1.5/octane/water/octanol exhibits co-solvent induced percolation phenomena. The percolation transition from an electric conducting oil-in-water microemulsion to an electric non-conducting water-in-oil microemulsion with increasing co-solvent content will be observed by measurements of the electric conductivity and the time-resolved electric birefringence. The field-off relaxation time delivers information concerning the length scale inside the system. The scaling behaviour of field-off relaxation times and Kerr-constants towards the percolation point leads to new insights to the influence of co-solvents on phase behaviour.

Abstract: The influence of oligo- and polyethylene glycols (PEG) on ionic aerosol-OT water-in-oil (w/o) microemulsions has been small-angle neutron scattering (SANS). The scattering curves can be described with a model of polydisperse spheres, using an Ornstein-Zernicke structure factor, regardless of the PEG relative molecular mass. The results obtained at a constant temperature difference from the percolation point indicate that the droplet structure is maintained, the core radius is decreased and the polydispersity is increased with increasing PEG chain length. There is good evidence that long-chain PEGs induce aggregation of these droplets. The results support the model of polymer adsorption, which was deduced from the influence of PEGs on the percolation behaviour.

Abstract: This work reports on the dynamics of L2-microemulsions stabilized by Aerosol-OT. Time-resolved luminescence quenching measurements using the probe Tb(pda)_3^{3-} show the existence of clusters in water-in-oil (w/o) microemulsions. The fast exchange appearing over several microseconds is attributed to intra-cluster quenching, whereas the slow exchange on the millisecond time-scale corresponds to inter-cluster exchange. The fast exchange is decelerated when the temperature is increased and is related to a temperature-induced cluster growth. The slow exchange, conversely, is strongly accelerated within the one-phase region. Below the percolation threshold, the corresponding rate constant obeys an Arrhenius relation. The activation energies increase with the droplet size. In the percolation domain, strong deviation from linearity in the Arrhenius plot occur, which are interpreted by a kinetic scheme considering the limited quenching reaction rate and aggregate collision disturbing the exchange transition state. Enthalpy-entropy compensation is established from the intercepts of the Arrhenius plots. The activation entropy is discussed in terms of the clustering entropy due to an aggregation equilibrium prior to exchange, yielding \Delta S_{cl} in very good agreement with the results from other methods. \Delta S_{cl} increases with the droplet size and solvent hydrocarbon length. The percolation phenomenon is discussed as an entropy-driven droplet aggregation accompanied by a facilated exchange.