Simulating the effect of different surfactants on stain removal from a cellulose surface
Surfactant molecules which possess a polar head and a nonpolar tail part are known as amphiphiles and they spontaneously aggregate to form association structures such as micelles, bilayers and vesicles in aqueous solution. At a concentration above the critical micelle concentration (CMC), a spherical micelle forms. It is also known that the shape of the micelle changes from spherical to cylindrical, hexagonal and also lamellar phases by increasing the concentration
Determination of the phases and the structural transition of the micelles is very important in predicting the washing and cleaning mechanisms of surfactants. Also, a particular form of micelle, liposome,which is a vesicle formed from lipid molecules, can interact with biomembranes, and the liposomes play very important roles in many applications such as drug delivery and artificial oxygen carrier. However, the measurement of phase behaviors and characterization of various micelle structures are difficult and time-consuming. The elucidation of the surfactant mesostructures at atomic resolution is not currently possible because the time and length scales at which these phenomena occur are not accessible by molecular dynamics (MD) simulation. However, DPD simulations allow the study of such systems.
In the DPD method molecules of a fluid are grouped together to form fluid elements, or beads, that interact via soft, short-range forces. Figure one illustrates this difference between atomistic and mesoscopic modeling by showing a phospholipid molecule in atomistic and bead representation.
DPD was introduced in 1992 by Hoogerbrugge and Koelman, who applied it to measure the hydrodynamic drag on a cylinder in a moving fluid. The algorithm was modified by Groot and Warren and used to study the phase separation of immiscible polymeric fluids. Their scheme has since been used to investigate pore formation in amphiphilic bilayers, to follow the self-assembly and behavior of vesicles and to calculate the material properties of other polymeric systems, for example the agglomeration of Carbon nanotubes in polymers.
In this study we compare the effect of two different surfactants, the ionic SDS surfactant and the nonionic cocamide. Both surfactants are commonly used in in detergent formulations, hence we study the effect of removal of an oil film from a cellulose surface.
Parametrization of the repulsive parameters for the surfactants, the oil and the cellulose were performed according to the Warren and Groot scheme based on the solubility parameters from atomistic LAMMPS molecular dynamics simulations, the parametrization for the SDS molecule in water was taken from, except that the self interaction parameter between same pairs was set to 25 instead of 78. Figure 2 shows a typical snapshot of the cocamide/water/oil/cellulose system after 100000 equilibration steps, Figure 3 shows the same for the SDS/water/oil/cellulose system.
It should be noted, that in the case of SDS some oil diffuses through the system from one surface side to the other, whereas in the case of cocamide a micelle forms and stays on the surface. This effect can be quantified by looking at the mean square displacement and diffusion coefficients for both systems, which are displayed in Figure 4.
As can be seen in the mean square displacement for the SDS/ oil micella, there is a constant diffusion in the system, whereas the mean square displacement for the cocamide/ oil micellle is completely static.
Mesoscale models for an ionic and a non- ionic surfactant have been developed, and the different effect of these surfactants on a layer of oil on a cellulose surface has been studied. It was noticed, that the ionic SDS surfactant allows diffusion of the oil/ surfactant micella through the system, whereas in the case of non- ionic cocamide the micelle stays static on the cellulose surface. Hence SDS should give a better performance in washing powder formulations than cocamide.
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