We made direct force measurements by atomic force microscopy (AFM) at surfaces of polymer brushes comprised of low-polydispersity poly(methyl methacrylate) (PMMA) chains densely end-grafted on a silicon substrate by living radical polymerization. These brushes are characterized by an exceptionally high, nearly constant graft density (approximately 0.4 chains/nm 2 ) and a wide range of molecular weights of the graft chains. This graft density is at least 10 times larger than those of the previously studied polymer brushes prepared by the adsorption of block copolymers. Force measurements were made in toluene with a silica particle-attached cantilever. The hysteresis behavior of a piezo actuator used in the AFM was corrected by simultaneously measuring its piezo current. The true distance D between the substrate surface and the silica probe, which usually is difficult to define in AFM experiments, was successfully determined by AFM imaging across the boundary of a scratched and an unscratched region on the sample surface. In this way, we could obtain quantitative force-distance profiles. The repulsive force was observed to rapidly increase with decreasing separation. The equilibrium thickness Le of the brushes, i.e., the critical distance at which a repulsive force was detectable, was found to be proportional to the weight-average chain contour length Lc,w, giving Le/Lc,w ) 0.6. This indicates formation of a homogeneous polymer layer with highly stretched graft chains. Unlike the previously reported results for lower density polymer brushes, the force-distance profiles for different graft chain lengths were not scaled by the reduced distance D/Le. Longer brushes were more resistant to compression: for example, the longest studied brush was compressible only to D/Le ) 0.8, which was about 3 times as large as the scaled dry thickness Ld/Le.

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The micellar structures formed by the perfluorooctylsulfonate ion (FOS -) in combination with the tetraethylammonium (TEA + ) and lithium (Li + ) counterions are studied by measuring the NMR chemical shifts and self-diffusion coefficients of both TEA + and FOS -at 30°C. Spherical micelles generated by LiFOS are characterized by a lifetime of 10 -6 s, an aggregation number of about 36, and a cmc value of 7.16 mM. The fluorine spectrum of the FOS -ion in the micellar solutions of LiFOS consists of one set of peaks whose chemical shifts are averaged between the monomer and micellar states. When combined with the hydrophobic TEA + counterion, the FOS -ions show the 19 F spectra which consist of two sets of peaks whose chemical shifts and intensities are concentration dependent. The 19 F spectroscopic features are discussed by assuming the appearance of small aggregates at high surfactant concentrations. The binding coefficient (0.80) of the TEA + counterions to FOS -micelles is determined on the basis of the self-diffusion coefficients, measured for both TEA + and FOS -. The micellar self-diffusion coefficient in the TEAFOS system (8.4 × 10 -12 m 2 s -1 ) is found to be 1 order of magnitude lower than that in LiFOS system (5.7 × 10 -11 m 2 s -1 ), reflecting the dynamic slowdown due to the formation of threadlike micelles.

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The interaction forces between two hard surfaces in the presence of particle-containing aqueous systems were measured with an atomic force microscope. Bare silica and sodium dodecyl sulfate (SDS)-modified paraffin were used for the surfaces. SDS micelles and colloidal silica were used for the particles. Increasing the number of micelles between two silica surfaces increased the number and magnitude of the oscillations. The effect of the compression speed was seen only with fast speeds g3420 nm s -1 , probably because of a hydrodynamic force. The decay length of the observed electrostatic repulsion decreased with a SDS concentration increase, as a result of an increase in the number of free ions above the critical micelle concentration resulting from a finite dissociation of ions from the surfactant. The effect of using a silica particle instead of a micelle did not show any measurable differences in the oscillations of the force curves. The effect of using an asymmetric system (silica-solution-SDS-modified paraffin) instead of a symmetric system (silica-solution-silica) showed differences only in the Derjaguin-Landau-Verway-Overbeek forces. In all cases, the forces could be explained well by a summation of an electrostatic, van der Waals, and a structural force.

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The forces between two silica surfaces in a surfactant-containing apolar solvent were studied by atomic force microscopy, as a function of the silica surface type, surfactant concentration, compression speed, and effect of water presence. In the apolar system, no electrostatic repulsive forces were measurable between two hydrophobized silica surfaces in dodecane. However, the introduction of the anionic surfactant of sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) into the dodecane caused a measurable surface potential because of the adsorption of the hydrophobic bis(2-ethylhexyl) sulfosuccinate (AOT) anions to silica surfaces. Increasing the NaAOT concentration from the critical micelle concentration of 7.6 to 500 mM caused a maximum in the surface potential at 100 mM because of the subsequent adsorption of the sodium cations onto the AOT anions that have already adsorbed. Oscillations in the force curves were only observed for the NaAOT concentration of 500 mM; a smaller concentration did not give enough reverse micelles to cause measurable structuring effects. The experimental results in this study are well-explained by an extended Derjaguin−Landau−Verway−Overbeek theory, which includes electrostatic, van der Waals, and structural forces.