The surface pressure-area (pi-A) isotherms of enantiomeric polylactides and their equimolar blend reveal differences in the appearance of features and compression rate effects. Surface potential data indicate that the poly(L-lactide) monolayer interacts differently than the 50:50 poly(L-lactide)/poly(D-lactide) blend film with the water subphase. In the case of the 50:50 blend at 25degreesC, compression- expansion experiments confirm that the change in the monolayer is irreversible within the time frame of the experiment. In the poly(L-lactide) case, hysteresis is also observed, but the film readily reverts to its original state. Infrared spectra of the polymer monolayers at the air-water interface were obtained using the polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique. PM-IRRAS spectra of a compressed poly(L-lactide) monolayer are consistent with the polylactide helices lying in the plane of the air-water interface. No absorbances are observed in the PM-IRRAS spectra Of poly(L-lactide) prior to the plateau in the pi-A isotherms, suggesting that the helix formation is linked to the increase in surface density which accompanies film compression. The PM-IRRAS spectra establish that the conformation of the poly(L-lactide) helices with respect to the air-water interface is different than the conformation of the D/L blend helices.

Ultrathin Films of behenic acid methyl ester (BAME) deposited by spin coating on solid substrates or spread at the air/water interface were studied by infrared spectroscopy and Brewster angle microscopy. Anisotropic optical constants of BAME were determined from a transmittance spectrum at normal incidence and a parallel-polarized reflectance spectrum at grazing incidence. A transmittance spectrum recorded at oblique incidence (60degrees) was used to validate these optical constants. The anisotropic extinction coefficients and the polarized ATR spectra were used to calculate the tilt angle of several transition moments and also the molecular tilt angle of BAME molecules assuming an all-trans conformation of the alkyl chain. The results indicate that the alkyl chain of the fatty acid ester is tilted and that the molecular tilt angle is close to 30degrees. PM-IRRAS spectra of BAME at the air/water interface were also recorded as a function of the surface pressure. The splitting of the methylene bending mode shows that the all-tans alkyl chains are packed in a quasi-crystalline structure at the air/water interface, even at low surface pressure. This Finding was further confirmed by Brewster angle microscopy. Furthermore, the band progression due to the methylene wagging modes was observed for the first time in infrared spectra of a Langmuir monolayer. The simulation of the PM-IRRAS spectrum of a BAME monolayer recorded at 30 mN/m using the optical constants obtained from films prepared by spin coating indicate that, as opposed to the orientation on solid substrates, the alkyl chain of BAME is nearly perpendicular to the air/water interface in Langmuir films.

The polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique has been used in situ to determine the orientation and molecular structure of an equimolar mixture of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) spread at the air-water interface. The characteristics of the compression isotherm and of the PM-IRRAS spectra give clear evidence for the presence of a PLLA/PDLA stereocomplex. One of the most striking features in the PM-IRRAS spectra of the stereocomplex is the derivative shape of the band due to the C=O stretching vibration, providing a spectral signature of the presence of polylactide helices oriented parallel to the water surface. The positive and the negative components of the C=O band observed at 1749 and 1765 cm(-1) are assigned to the A and E modes of the helical structure, respectively. This assigment was confirmed by recording transmission spectra of the transferred stereocomplex at normal and oblique incidence. Compression of the monolayer past 17 Angstrom (2)/repeat unit results in the formation of a bilayer structure. The surface pressure-area isotherm and the PM-IRRAS features suggest that the structure of the film at the air-water interface is similar to the three-dimensional crystal structure of the PLLA/PDLA stereocomplex. In the bulk crystalline structure, the molecules adopt a 3(1)-helix conformation, and a segment of a PLLA molecule is paired with a segment of a PDLA molecule, resulting in a racemic unit cell. The PM-IRRAS technique is thus shown to provide detailed insight into the structure of these polymeric Langmuir films and definitely shows that helical polymeric structures can be directly observed at the air-water interface.

A parallel investigation of the conformational changes of dipalmitoylphosphatidylglycerol (DPPG) monolayers induced by surface pressure has been studied in situ at the air water interface using polarization modulation external infrared reflection absorption spectroscopy (PM-IRRAS) and on solid substrate by attenuated total reflection (ATR) spectroscopy. For both the ATR and PM-IRRAS spectra of DPPG, the non-linear increase of the intensity of the antisymmetric methylene stretching band of DPPG with molecular surface density, indicates that the compression of the DPPG monolayer induces a change of the conformation and/or orientation of the acyl chains of the phospholipid. At surface pressures between 7 and 20 mN/m, the acyl chains of the DPPG monolayer are found to be more ordered on the germanium substrate than at the air–water interface. At higher surface pressures, the conformation of the lipid acyl chains of DPPG on both substrates are almost similar. In the liquid-condensed phase, the acyl chains are in all-trans conformation and their tilt angle with respect to the normal of the film is approximately 30°. Our results also suggest the presence of DPPG solid domains in the liquid-expanded phase. Investigation of polar head group region indicates that at low surface pressures, the carbonyl groups were more oriented on the water surface than the acyl chains. Finally, the present study shows that intermolecular hydrogen bonding probably occur between the glycerol hydroxyl and the phosphate or carbonyl groups of the phospholipid.

Spread monolayers of a cellulose alkyl ether (HPC-C16) have been investigated at the air/water interface by polarisation modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Infrared spectra exhibit bands associated with the symmetric and anti-symmetric methylene stretching vibrations, as well as with the methylene scissoring vibration, at 2850, 2917 and 1471–1463 cm−1, respectively. All of these bands appear as positive peaks in the PM-IRRAS spectra, indicating that the hexadecane sidechains have a net orientation perpendicular to water surface. The relatively low frequencies of the methylene stretching bands suggest that the hydrocarbon sidechains adopt a near all-trans conformation. The intensity of these bands increases with decreasing molecular area during monolayer compression. This increase is due not only to an increase in the surface density of the absorbing groups but also to orientational changes. These results indicate that alkyl sidechain orientation at the air/water interface is, to a certain degree, dependent on the surface pressure. Furthermore, no change in orientation is observed during compression through the plateau typically present in surface pressure–area isotherms of these polymers.