Direct measurement of surface forces as a novel means of investigating supramolecular assemblies

Abstract

We review our recent research on the interactions of supramolecular assemblies at the nanometer level. Our aim is to illustrate how surface force measurements can be extended to material science. The approach employed in our laboratories involves the preparation of well-defined surfaces by functionalized amphiphiles and the use of the Langmuir-Blodgett technique. We routinely characterize these films by different physical and chemical methods (Fourier Transform Infrared Spectroscopy, FTIR, and Atomic Force Microscopy, AFM, for example). Brush layer structures of polyelectrolytes such as poly(methacrylic acid) and poly(glutamic acid) were inferred from the analyses of the steric components of the force profiles. These analyses permitted, for the first time, the determination of the elastic compressibility modulus, Y, of an oriented, single polymer brush layer in any solvent. In particular, in water Y = 0.2 ∼ 0.6 MPa was obtained for a poly(glutamic acid) brush in which the chains were ionized (polymerization degree, n = 21 ∼ 44). In contrast, Y was found to be two orders of magnitude larger for the brush layer with a regular secondary structure ( Y = 39 MPa for the α-helix rich layer of n = 44, for example). Another set of studies revealed an unusually long-range attraction (> 200 nm) between hydrophobic surfaces and apposed monolayers of nucleic acid bases in their electrically neutral states. Forces regulating molecular recognition were also studied. We found stable attractions between monolayers of a complementary pair of nucleic acid bases, adenine and thymine, in the 5.6 to 9.1 pH range. The applications of the surface forces measurement discussed in our renew provides a new insight into supramolecular assemblies at the molecular level.