Alexander Patist

Kinetics of micellization: its significance to technological processes

The association of many classes of surface active molecules into micellar aggregates is a well-known phenomenon. Micelles are often drawn as static structures of spherical aggregates of oriented molecules. However, micelles are in dynamic equilibrium with surfactant monomers in the bulk solution constantly being exchanged with the surfactant molecules in the micelles. Additionally, the micelles themselves are continuously disintegrating and reforming. The first process is a fast relaxation process typically referred to as t1. The latter is a slow relaxation process with relaxation time t2. Thus, t2 represents the entire process of the formation or disintegration of a micelle. The slow relaxation time is directly correlated with the average life-time of a micelle, and hence the molecular packing in the micelle, which in turn relates to the stability of a micelle. It was shown earlier by Shah and coworkers that the stability of sodium dodecyl sulfate (SDS) micelles plays an important role in various technological processes involving an increase in interfacial area, such as foaming, wetting, emulsification, solubilization and detergency. The slow relaxation time of SDS micelles, as measured by pressure-jump and temperature-jump techniques was in the range of 10 4 ‐10 1 s depending on the surfactant concentration. A maximum relaxation time and thus a maximum micellar stability was found at 200 mM SDS, corresponding to the least foaming, largest bubble size, longest wetting time of textile, largest emulsion droplet size and the most rapid solubilization of oil. These results are explained in terms of the flux of surfactant monomers from the bulk to the interface, which determines the dynamic surface tension. The more stable micelles lead to less monomer flux and hence to a higher dynamic surface tension. As the SDS concentration increases, the micelles become more rigid and stable as a result of the decrease in intermicellar distance. The smaller the intermicellar distance, the larger the Coulombic repulsive forces between the micelles leading to enhanced stability of micelles (presumably by increased counterion binding to the micelles). The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped-flow and pressure-jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results show relaxation times t2 in the range of seconds for Triton X-100 to minutes for polyoxyethylene alkyl ethers. The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation time t2 was related to dynamic surface tension and foaming www.elsevier.nl:locate:colsurfa

Chain length compatibility effects in mixed surfactant systems for technological applications

Abstract Chain length compatibility is an important factor in systems involving interfacial films. As surface active molecules as well as other hydrocarbon molecules are aligned at interfaces, the properties of the interface are impacted to a large extent upon the matching or mismatching of the alkyl chain lengths. The effects of chain length compatibility are particularly important to interfacial properties and technologies such as: surface tension, surface viscosity, micellar stability, foamability, lubrication, contact angle, bubble size, environmental remediation, corrosion, enhanced oil recovery, microemulsion water solubilization and microemulsion stability. In this article the authors discuss the importance of chain length compatibility on these interfacial properties and related technologies from a practical and fundamental viewpoint.

The importance of sub-angstrom distances in mixed surfactant systems for technological processes

The spacing between atoms and molecules at interfaces and within materials is extremely important in determining the properties of such interfaces and materials. In surfactant monolayers at the air-water interface, it has been shown that small changes in molecular packing lead to large changes in the interfacial properties. Changes in intermolecular distance as small as 0.04 A have been attributed to the effect of mixing surfactants of different chain lengths. This paper discusses the effect of these sub-angstrom distance changes on foaming, micellar stability, melting points, bubble size, surface viscosity, lubrication, environmental remediation, enhanced oil recovery, and microemulsion stability that result from chain length compatibility.