Advances in Colloid and Interface Science (v.193-194, #C)

The moving contact line of a dilute polymer solution that advances over, or recedes from a solid substrate, is a fundamental problem of fluid dynamics with important practical applications. In particular, the case of droplets impacting on hydrophobic surfaces received much attention in the recent past. Experiments show that while the advancing motion proceeds as with Newtonian liquids, recession is severely inhibited. This phenomenon was initially understood as an effect of elongational viscosity, which was believed to cause large energy dissipation in the fluid. Later on, a hydrodynamic mechanism was proposed to suggest that the slowing down of the contact line is due to non-Newtonian normal stresses generated near the moving droplet edge. Recent experiments however ruled out the role of elongational viscosity, showing that the fluid velocity measured inside the droplet during retraction is the same in water drops and polymer solution drops. Direct visualization of fluorescently stained λ-DNA molecules showed that polymer molecules are stretched perpendicularly to the contact line as the drop edge sweeps the substrate, which suggests an effective friction arises locally at the drop edge, causing the contact line to slow down.Display Omitted
Keywords: Dynamic wetting; Dilute polymer solutions; Drop impact;

The fascinating and serendipitous discovery, in 1976, of the characteristic viscoelastic behavior of wormlike micelles of cetyltrimethylammonium salicylate (CTASa) surfactant solution at ~ 2 × 10− 4  M CTASa became a catalyst for an increasing interest in both industrial application and mechanism of the origin of micellar growth of this and related wormlike micellar systems. It has been perceived for more than three decades, based upon qualitative evidence, that the extent of the strength of the counterion (X) binding to ionic micelles determines the counterion-induced micellar structural transition from spherical-to-small rodlike-to-linear long stiff/flexible rodlike/wormlike-to-entangled wormlike micelles. This perception predicts the presence of a possible quantitative correlation of counterionic micellar binding constants (KX) with counterion-induced micellar growth. The quantitative estimation of counterion binding affinity to cationic micelles, in terms of the values of the degree of counterion binding (βX), is concluded to be either inefficient or unreliable for moderately hydrophobic counterions (such as substituted benzoate ions). The values of KX, measured in terms of conventional ion exchange constants (KX Y), can provide a quantitative correlation between KX or KX Y (with a reference counterion Y = Cl or Br) and counterion-induced ionic micellar growth. A recent new semi-empirical kinetic (SEK) method provides the estimation of KX Y for Y = Br as well as ratio of counterionic micellar binding constants KX/KBr (= RX Br) where the values of KBr and KX have been derived from the kinetic parameters in the presence of cationic spherical and nonspherical micelles, respectively. The SEK method has been used to determine the values of KX Br or RX Br for X = 2-, 3- and 4-ClC6H4CO2 . Rheometric measurements on aqueous CTABr/MX (MX = 2-,3- and 4-ClBzNa) solutions containing 0.015 M CTABr and varying values of [MX] reveal the presence of spherical micelles for MX = 2-ClBzNa and long linear as well as entangled wormlike micelles for MX = 3- and 4-ClBzNa. The respective values of KX Br or RX Br of 5.7, 50 and 48 for X = 2-, 3- and 4-ClBz give a quantitative correlation with the rheometric measurements of the structural features of micelles of 0.015 M CTABr solutions containing 2-, 3- and 4 ClBzNa.Display Omitted
Keywords: Cationic micelles; Ratio of counterionic micellar binding constants KX/KBr (= KX Br or RX Br); Determination of KX Br or RX Br; MX = 2-, 3- and 4-ClC6H4CO2Na; Rheometric measurements; Spherical micelles; Long linear and entangled wormlike micelles;

Adsorptive removal of dyes from aqueous solution onto carbon nanotubes: A review by Vinod Kumar Gupta; Rajeev Kumar; Arunima Nayak; Tawfik A. Saleh; M.A. Barakat (24-34).
Adsorption is a widely used technique for the separation and removal of pollutants from wastewaters. Carbon nanotubes (CNTs) are emerging as potential adsorbents because of its well defined cylindrical hollow structure, large surface area, high aspect ratios, hydrophobic wall and easily modified surfaces. In this review, dye adsorption capability of CNTs and CNT based composites from aqueous system has been compiled. This article provides the information about the defect, adsorption sites on CNTs and batch adsorption studies under the influence of various operational parameters such as contact time, solution pH, temperatures etc. and deals with mechanisms involved in adsorption of dyes onto CNTs. From the literature reviewed, it is observed that single walled carbon nanotubes (SWCNTs) show higher adsorption capacity than multi walled carbon nanotubes (MWCNTs) and functionalized and CNT composite have better sorption capacity than as grown CNTs. It is evident from the literature that CNT based nanosorbents have shown good potential for the removal of dyes from aqueous solution. However, still more research work should be focused on the development of cost effective, higher efficient and environmental friendly CNT based nanosorbents for their commercial applications.Display Omitted
Keywords: Carbon nanotubes; Active sites; Dyes; Adsorption; Desorption;