- Critical micelle concentration (1) (remove)
- Single Molecule Study of Polymer-Surfactant Interactions (2007)
- The interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and a hydrophobically modified non ionic polymer, methylcellulose (MC), have been investigated in aqueous solution by fluorescence correlation spectroscopy (FCS), cryo-TEM, turbidity and rheology. The micelle formation of SDS is followed with cationic Cresyl Violet perchlorate dye via diffusion time. The opposite polarity of dye is suitable to aggregate with micelles and act as a labeled dye. Two major studies focused in the research work are concentration dependent and temperature measurements to understand the interactions of MC/SDS aggregates. The concentration of SDS is varied to a wide range in the mixture by fixing the concentration of MC. By this approach, the changes in the aggregation and the conformations of MC chains are being studied. Similar studies have been repeated at various temperatures in the range of 25-60 °C to understand the changes in gelation properties of MC. To understand the results of the above mentioned studies of MC/SDS, the behavior of MC and SDS is analyzed individually. In this line, the critical micelle concentration (CMC) of SDS has found with FCS measurements is in good agreement with literature value obtained from ‘classical methods’. The hydrodynamic radius of SDS micelle around CMC is found to be ~ 2.0 nm. The shape of the autocorrelation curves and number of dye particles in the focal volume also supports to follow the SDS micelle. When varying the concentration of MC, slight changes in the diffusion time of dye are observed. The MC/SDS mixtures show huge increase in the diffusion time compared to the individual components MC and SDS. At constant MC concentration the diffusion time of single aggregates increases gradually up to a certain SDS concentration and decreases to a minimum when the SDS concentration is further increased. This behavior coincides with the behavior of the zero shear viscosity. Two different fractions viz fast diffusing fraction of dye molecule along with the larger aggregates are observed in between the critical aggregation concentration (CAC) and end of aggregation (EOA). FCS is used to follow the dynamics of single aggregates of the different populations. At very high concentration of SDS, MC/SDS mixtures show the worm like structure in cryoTEM measurements. A model is proposed based on FCS, cryoTEM and rheology measurements to explain the effect of surfactant concentration on polymer conformation and aggregation size. While varying the temperature, MC/SDS mixtures show changes in the diffusion time only at room temperature. MC has the tendency to form thermoreversible gel upon heated above 50 °C. The presence of SDS alter the intensity of MC gelation. Before CAC, the addition of SDS promotes the MC gelation. The MC-SDS mixture giving maximum aggregation at room temperature shows decreasing tendency in its diffusion time upon increasing the temperaure. We have shown that a single molecule technique like FCS can be successfully used to follow the dynamics of single aggregates in polymer/surfactant systems. We can identify single inter-chain aggregates, the hydrodynamic size of which changes in a characteristic way as a function of surfactant concentration. These changes are reflected in the behavior of the macroscopic viscosity. The present results show the large potential of single molecule experiments as a complement to the classical macroscopic techniques for a characterization of polymer solutions and polymer/surfactant mixtures. In addition to the large aggregates dominating the macroscopic rheology of the system, the single molecule approach can identify considerably faster aggregates as well, which are not accessible by conventional techniques. Thereby the single molecule approach is able to monitor what may be called a micro viscosity of the solution, i.e. the potential of small aggregates to diffuse rather fast through a network of slowly diffusing chains. This study also shows that the diffusion behavior of polymer-surfactant systems can be followed by FCS without covalent labeling with dye molecules. Moreover, FCS is only sensitive to the dye concentration; therefore these investigations can be applied over a wide range of polymer concentrations.