ACS PRF | ACS
All e-Annual Reports
44637-G9
Effect of Confinement on the Evolution and Morphology of Surfactant Mesophases
The report describes three efforts of research:
1) In the first effort, a research was performed for the determination of the phase behavior of a self-assembling dilute aqueous cetyl trimethyl ammonium bromide (CTAB) and dodecyl benzene sulfonic acid (HDBS) system using flow in microchannels. The diffusional length scales of ~10-100um, and volumes of the order of a few tens of nanoliters allow fast composition and temperature homogeneity compared to "bulk" experiments, where characteristic volumes and length scales are of the order of milliliters and centimeters respectively. Fluorescence emission of a polarity sensitive fluorophore was used with the surfactants for phase characterization. To demonstrate the validity of the new approach, the critical micelle concentrations (CMC) for CTAB and HDBS were first shown to agree with the CMC obtained in the literature under bulk conditions. Subsequently, the microstructures of dilute (less than 0.8wt% total surfactant) aqueous mixtures of CTAB and HDBS were examined. The range of desired concentrations and accurate flow dilutions of the samples were achieved by imposing controlled pressure gradients across the channel network. Marked changes in slopes of fluorescence emission intensity versus composition were used to demarcate phase boundaries. A series of microstructures ranging from mixed micelles (M), vesicles (V) and giant vesicles (GV) were observed in the ternary CTAB/HDBS/water system. Experimental data from the microfluidic method was found to be consistent with the results obtained from "bulk" phase experiments using fluorescence, turbidity, dynamic light scattering and cryogenic transmission electron microscopy.
The determination of phase behavior in aqueous surfactant mixtures is an important problem for consumer product applications. The current methods that use bulk phase measurements are time-consuming, and are serial in nature - that is, vials containing different concentrations of surfactants have to be prepared and equilibrated separately prior to sampling. This process can take several weeks. The microfluidic setup proposed here allows fast sample equilibration, and continuous dilution, so that this process can be speeded up several-fold. This has important implications for the screening of surfactants for specific applications.
2) In the second effort, the effect of confinement on the evolution of a vesicular surfactant mesophase obtained by mixing micellar solutions of CTAB and HDBS has been studied using Small-Angle Neutron Scattering (SANS). The confined spaces have been generated by the random packing of polystyrene beads of radius R_b = 1.5um, 0.25um and 0.1um, creating voids of characteristic dimensions R ~ 0.22 R_b = 330nm, 55nm and 22nm respectively. These length scales are comparable to or less than the size of vesicles formed in the system under conditions of no confinement. Micellar solutions of each surfactant (0.8 wt%) were added sequentially, and the mixture allowed to self-assemble in a scattering cell without beads, as well as three cells with the different sized beads. The SANS data from the sample without confinement was best fitted by a core-shell model, and not by spheres or disks, confirming the presence of vesicles. The data from samples in the confined domains also showed vesicles as the dominant structure, with mean sizes that decreased as the confinement length scale was reduced. A progressively higher polydispersity of vesicle sizes was observed with increasing confinement suggesting the coexistence of surfactant monomers, micelles and other intermediate structures between micelles and vesicles in the system. A simple thermodynamic model accounting for the balance between increased enthalpy when vesicles are formed with curvature higher than the spontaneous one, and increased free volume entropy for smaller vesicles shows trends that support the experimental data. The results of this study are potentially important in the flow of drug delivery vehicles through microcapillaries and in the recovery of oil from fine pores in rocks using surfactant containing fluids.
3) In the third effort, new transient nanostructures of amphiphilic solutions were unveiled by imaging of aggregate structures as soon as they leave the microfluidic chip. By integrating microfluidic chip to a controlled environment vitrification system (CEVS), artifact-free visualization of a vast number samples with different chemical conditions, dispensing and mixing times was made possible. The integration allowed a rapid characterization of macromolecular conformations using cryogenic transmission electron microscopy. A capillary connecting microchannels was passed through the side of the CEVS box and the mixed samples were injected directly on to a holey carbon grid and blotted to remove excess liquid. Subsequently, the grid bearing the sample was plunged into a liquid ethane reservoir and ready to be analyzed by TEM. The self-assembling dilute aqueous CTAB and HDBS system was used to show the effect of evolution timescale and the transient status of aggregate nanostructures. Results showed a time transition of long tubular shaped vesicles to wavy tubular shaped vesicles to spherical shaped vesicles structures. Flow rate and diffusion times were varied to probe the evolution of transient vesicle structures. The creation of the tubular structures was discussed using energy principles.
