FTIR-ATR studies on Langmuir-Blodgett films of stearic acid with 1-9 monolayers

Abstract

Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectra have been recorded of Langmuir-Blodgett (LB) films of stearic acid deposited on a germanium plate with 1, 2, 3, 5, and 9 monolayers. Examination of the CH2 scissoring band suggests that the hydrocarbon chain of stearic acid in the first monolayer is in a hexagonal or pseudohexagonal subcell packing where each hydrocarbon chain is freely rotated around its axis oriented approximately perpendicular to the surface. In the LB films thicker than 2 monolayers, on the other hand, the molecules in the upper monolayers other than the first monolayer crystallize with the monoclinic form where the hydrocarbon chains are packed alternately and are inclined at an angle of about 30°with respect to the surface normal, showing a tendency to align their a crystal axes parallel to the direction of the withdrawal of the germanium plate in the film preparation. It is also concluded from frequencies and intensities of the progression bands due to the CH2 wagging vibrations that stearic acid occurs as the cis configuration for the C=0 and Ca-€ß bonds in the 1-monolayer film but the trans configuration starts to appear in the 3-monolayer film. A striking feature in this study is the absence of the C-O stretching band of stearic acid in the first monolayer on the germanium plate. This phenomenon may be interpreted by the short-range image field model for its oscillating dipole parallel to the germanium surface.

Molecular structure of surfactant-coated surfaces

“…When the monolayer is cooled well below room temperature, the in-plane lattice distorts to a nonhexagonal arrangement with fine 600 twinning of oriented regions much less than 103 A across (S.G., H. W. Deckman, J. H. Dunsmuir, and M. S. Alvarez, unpublished results). In the second case, in which another layer is deposited on top of the monolayer, the net is again distorted into a nonhexagonal symmetry with 600 twinning now occurring on a larger length scale (on the order of 10 ,tm) (16)(17)(18)(19)(20); S.G., H.W.D., J.H.D., and M.S.A., unpublished results). In fact, a progression from hexagonal to orthorhombic symmetry has been observed as the number of layers is increased from 1 to 10 (19,20).…”

“…In the second case, in which another layer is deposited on top of the monolayer, the net is again distorted into a nonhexagonal symmetry with 600 twinning now occurring on a larger length scale (on the order of 10 ,tm) (16)(17)(18)(19)(20); S.G., H.W.D., J.H.D., and M.S.A., unpublished results). In fact, a progression from hexagonal to orthorhombic symmetry has been observed as the number of layers is increased from 1 to 10 (19,20). These few layer structures again do not always exactly fit the known structures of bulk chain molecules but approach this limit as the number of layers is increased.…”

“…These "free-volume-allowed" tilts can be much smaller than the ratcheted tilts of the densest phases and must compete with the formation of gauche conformers for excess free volume. In the literature on monolayers, the tilt of the molecules is shown to depend on the particular material under examination (3,4,19,20,(30)(31)(32)36). Often, the molecules of monolayers are reported to be roughly normal to the surface; in multilayer assemblies the larger angles characteristic of the densest structures are reported (16)(17)(18).…”

“…Often, the molecules of monolayers are reported to be roughly normal to the surface; in multilayer assemblies the larger angles characteristic of the densest structures are reported (16)(17)(18). In fact, a slow crossover as a function of number of layers in an LB assembly has been discussed (19,20).…”

Interfacial Organization of Y‐Shaped Rod–Coil Molecules Packed into Cylindrical Nanoarchitectures

“…Also, the relative intensity of the asymmetric CH 2 to symmetric vibration increases in LB films of the Y34 molecule compared with that of the Y21 molecule, which indicates that the alkyl chains are more inclined to the substrate surface in the LB films of Y34 than in that of Y21 transferred at high surface pressure. [37,38] …”