A computer simulation study of the segregation of amphiphiles in binary immiscible matrices: short asymmetric...

The bulk and interfacial properties of ternary mixtures with asymmetric amphiphiles (A2B8) in A2 and B2 matrices and in A2 and B10 matrices are investigated by the dissipative particle dynamics type of molecular-dynamics simulations. The monomer concentrations of A2B8(phiA2B8) studied are below the critical micelle concentration (phiA2B8(cmc)) for the formation of micelles in the presence of an adsorbed amphiphilic monolayer at the interface. Macrophase separation from the mixed phase to the segregated state with A-rich and B-rich coexisting phases and the segregation of A2B8 at the interface are thermodynamically gradual but are accompanied by a pronounced stretching and orientation of the constituent chains. The segregation of A2B8 at the interface broadens the interfacial region and reduces the interfacial tension. The chain conformation of the asymmetric amphiphilic molecules and the interfacial properties are dominated by the majority block in the amphiphilic chain and dependent on the composition of the matrix in contact with the majority block. In the A2 and B2 matrices, the B8 blocks in A2B8 chains at the interface resemble a wet brush swollen by short B2 chains. Swelling is responsible for the pronounced stretching and orienting of the amphiphilic chains and the reduced interfacial amphiphile enrichment. At the same interfacial amphiphile excess, however, swollen amphiphiles are more efficient in reducing the interfacial tension than nonswollen amphiphiles.