Showing papers by "Yeshayahu Talmon published in 2000"

Sphere-to-Cylinder Transition in Aqueous Micellar Solution of a Dimeric (Gemini) Surfactant

Abstract: The dimeric (gemini) surfactant 12-2-12 (dimethylene-1,2-bis(dodecyl dimethylammonium bromide)) has been known to form threadlike micelles at relatively low concentrations. We investigated the micellar growth of this surfactant in aqueous solutions by the much-improved cryo-TEM technique (transmission electron microscopy at cryogenic temperature) in the concentration range between 0.26 and 1.5 wt %. The digitally acquired electron micrographs of solutions, with concentrations up to about 1 wt %, show the coexistence of spheroidal micelles and long, threadlike micelles, the number and length of the latter increasing with concentration at the expense of the former. The micrographs show very few elongated micelles of intermediate sizes. Also, the endcaps of the elongated micelles can be seen to be of a larger diameter than the cylindrical body of those micelles. These results lend support to the theories, developed by various workers, that predicted these features. Some branching is observed at a surfactant ...

Micellar Growth, Network Formation, and Criticality in Aqueous Solutions of the Nonionic Surfactant C12E5

Abstract: We used cryo-TEM and light scattering to study the micellar growth and subsequent network formation in aqueous solutions of the nonionic surfactant C12E5 Cryo-TEM shows the first direct evidence for the existence of connected topology in the vicinity of the critical point and the two-phase separation curve of the C12E5/water micellar system The coexisting phases within the two-phase region consist of one concentrated and one dilute network of interconnected cylindrical micelles These findings are consistent with the recent theoretical explanation of criticality and phase separation in certain nonionic surfactant systems as resulting from entropic attraction between network junctions Away from the two-phase separation curve, we have identified uniaxial micellar growth, with increasing temperature and concentration, into long threadlike micelles From the power-law dependence of the radius of gyration, RG,z, and the hydrodynamic diameter, DH, on the molecular weight (RG,z, DH ∼ Mwν), we find that thes

Cryo-TEM Imaging the Flow-Induced Transition from Vesicles to Threadlike Micelles

Abstract: Transition of vesicles to micelles induced by flow of dilute aqueous cetyltrimethylammonium chloride (CTAC)/sodium 3-methylsalicylate (CH3Sal-) dispersion was directly imaged by rapidly vitrifying small ultrathin specimens after varying periods of postflow on-the-grid relaxation, and then examining them by cryogenic-temperature transmission electron microscopy (cryo-TEM). Vesicles prevail in quiescent equimolar 5 mM dispersions of CTAC and NaCH3Sal at equilibrium. When the dispersions are deformed and strained by rapid drainage of the excess from the specimens, the vesicles transform into an entangled network of threadlike micelles. The micelles revert to vesicles with time after drainage flow ceases. Intermediate structures during the transition were also captured by cryo-TEM. We hypothesize that the straining actions of flow disrupt vesicles, distort the distribution of, or strip, the counterions around vesicle fragments, shift the binding and dissociation equilibrium of counterions, alter the local pre...

Microstructures in Aqueous Solutions of Mixed Dimeric Surfactants: Vesicle Transformation into Networks of Thread-Like Micelles

Abstract: The microstructures in mixtures of two dimeric (gemini) surfactants, the dimethylene-1,2- and eicosamethylene-1,20-bis(dimethyldodecylammonium bromide), referred to as 12-2-12 and 12-20-12, have been investigated at 25 °C by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmission electron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 forms vesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentration then rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlike micelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % and progressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity data clearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was strongly adsorbed by the vesicles. Spectrophotometry and DLM...

Unusual effects of counterion to surfactant concentration ratio on viscoelasticity of a cationic surfactant drag reducer

Abstract: One-to-one molar ratio sodium salicylate (5 mM)/Arquad S-50 (5 mM) solution was investigated in comparison to the sodium salicylate (12.5 mM)/Arquad S-50 (5 mM) or 2.5:1 solution, which was previously reported to be drag reducing yet non-viscoelastic [9] . Results showed a dramatic influence of the counterion to surfactant concentration ratio on the viscoelasticity of the surfactant drag reducing systems. The 1:1 solution has normal surfactant drag reducer characteristics such as viscoelastic properties and thread-like micellar networks and branches. This is the first cationic surfactant/counterion drag reducing system found, for which excess counterion makes the solution non-viscoelastic, while the microstructure of thread-like micellar networks is apparently unchanged.

Novel Organized Structures in Mixtures of a Hydrophobically Modified Polymer and Two Oppositely Charged Surfactants

Abstract: In solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC) and two oppositely charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium chloride (DoTAC), an organized structure of micelles has been observed by using cryogenic temperature transmission electron microscopy (cryo-TEM). On the cryoelectron micrographs the micelles appear to be thread on strings, arranged in a cell-like pattern. This organized structure is induced by the polymer when the average number of polymer hydrophobic tails per mixed micelle is about 3−5, allowing hydrophobic tails from different polymer molecules to bind to the same surfactant micelle. The organized structure was not observed either in the surfactant mixture alone or in a low-viscous solution with a low number of polymer hydrophobic tails per mixed micelle. The oppositely charged surfactant mixture was studied at a total surfactant concentration of 30 mm in the presence and in the absence of 1% w/w polymer. The number of polymer hydrop...

Experimental studies on drag reduction and rheology of mixed cationic surfactants with different alkyl chain lengths

Abstract: Experimental studies of the effects of mixtures of cationic surfactants on their drag reduction and rheological behaviors are reported. Cationic alkyl trimethyl quaternary ammonium surfactants with alkyl chain lengths of C12 and C22 were mixed at different molar ratios (total surfactant concentrations were kept at 5 mM with 12.5 mM sodium salicylate (NaSal) as counterion). Drag reduction tests showed that by adding 10% (mol) of C12, the effective drag reduction range expanded to 4–120 °C, compared with 80–130 °C with only the C22 surfactant. Thus mixing cationic surfactants with different alkyl chain lengths is an effective way of tuning the drag reduction temperature range. Cryo-TEM micrographs revealed thread-like micellar networks for surfactant solutions in the drag reducing temperature range, while vesicles were the dominant microstructures at non-drag reducing temperatures. High extensional viscosity was the main rheological feature for all solutions except 50% C12 (mol) solution, which also does not show strong viscoelasticity. It is not clear why this low extensional viscosity solution with relatively weak viscoelasticity is a good drag reducer.

Polyelectrolyte Micelles: Self-Diffusion and Electron Microscopy Studies

Abstract: We report the results of a study on aqueous solutions of hydrophobically modified polyelectrolytes (charge-neutral diblock copolymers) by different techniques which have been specifically chosen fo...

Direct-Imaging Cryo-Transmission Electron Microscopy in the Study of Colloids and Polymer Solutions

Abstract: Transmission electron microscopy at cryogenic temperatures is by now a well established technique, applied in the study of a wide range of microstructured liquids, including solutions and dispersion of polymers. In a comprehensive review in the context of this volume centered on amphiphilic block copolymers, we start with the rationale for using cryo-TEM, the basics of the technique, its limitations and its potential. We continue with a survey of the work that has been done on synthetic surfactants in various states of aggregation, emphasizing the type of information one can get from the technique, and how that information is used together with data from non-imaging techniques to completely characterize the microstructure of a liquid or a semi-liquid system. We next survey the work on biologically originated systems, excluding biological systems such as cellular organelles and viruses. Throughout the review we try to elucidate the flexibility of the technique and how its applicability can be extended by proper design of the experiments. Finally, we describe how cryo-TEM has been applied in the study of water-soluble polymers and polymer-surfactant systems, including experiments which involve on-the-grid processing and time-resolved cryo-TEM. Gelling systems are prime candidates to be examined by the latter techniques.