Metastability in Monolayer Films Transferred onto Solid Substrates by the Langmuir-Blodgett Method: IR Evidence for Transfer-Induced Phase Transitions FAZALE R. RANA, SUCI WIDAYATI, BRIAN W. GREGORY, and RICHARD A. DLUHY* Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556 The rate at which a monomolecular film is deposited onto a solid sub- strate in the Langmuir-Blodgett process of preparing supported mono- layer films influences the final structure of the transferred film. Atten- uated total reflectance infrared spectroscopic studies of monolayers transferred to germanium substrates show that the speed at which the substrate is drawn through the air/water interface influences the final conformation in the hydrocarbon chains of amphiphilic film molecules. This transfer-induced effect is especially evident when the monolayer is transferred from the expanded region of surface-pressure-molecular-area isotherms at low surface pressures; the effect is minimized when the film molecules are transferred from condensed phases at high surface pres- sures. This phenomenon has been observed for both a fatty acid and a phospholipid, which suggests that these conformational changes may occur in a variety of hydrocarbon amphiphiles transferred from the air/ water interface. This conformational ordering may be due to a kinetically limited phase transition taking place in the meniscus formed between the solid substrate and aqueous subphase. In addition, the results ob- tained for both the phospholipid and fatty acid suggest that the structure of the amphiphile may help determine the extent and nature of the transfer-speed-induced structural changes taking place in the mono- molecular film. Index Headings: Monomolecular films; Infrared spectroscopy; Surface chemistry; Langmuir-Blodgett transfers; Phase transitions. INTRODUCTION In recent years, there has been increased interest in the structure and properties of organic thin films deposited onto solid substrates. This interest is due largely to the potential application of ordered nano-structures in a va- riety of fields, including the areas such as molecular elec- tronics, nonlinear optics, and sensors, l The most com- mon method of preparing these films involves transferring a substrate vertically through a monomolecular film ad- sorbed at the air/water (A/W) interface. This procedure was developed by Blodgett and Langmuir nearly 60 years ago; 2,3 however, many aspects of this process have yet to be fully characterized. One of the less understood aspects of the Langmuir- Blodgett (L-B) process is the effect that the rate of de- position (i.e., the rate at which the solid substrate is passed through the monomolecular film at the A/W interface) has on the formation and structure of the transferred monolayer film. It has been observed that deposition of a monolayer onto a substrate will occur at transfer speeds that are below a maximum critical velocity (Vmax)which is on the order of a few centimeters per second. 4 Vm,x has Received 21 January 1994; accepted 6 July 1994. * Author to whom correspondenceshouldbe sent. been found to be relatively large under the following con- ditions: (1) when the charge of the film molecule head- groups are opposite in sign to those on the substrate sur- face; (2) when zipper-like hydrogen bonding occurs between the substrate and the undissociated head-groups of the film molecules; or (3) when there is minimal lateral sep- aration between the head-groups of film molecules? In addition, when the charge of the substrate and adsorbed monolayer film is the same, Vmax can be increased by the addition ofcounterions to the subphase. Buhaenko et al. 6 have shown that above a critical film viscosity there is a correlation between sharp decreases in Vmaxand an in- crease in the film viscosity. Vmax appears to be indepen- dent ofmonolayer age for a pure H20 subphase; however, the presence of either monovalent or divalent cations in the subphase will lead to a large decrease in Vmax as the monolayer film ages. This effect is presumably due to either phase changes or molecular reorganizations occur- ring in the film induced by the counterions. 7,8 De Gennes has developed expressions relating the transfer rate to the contact angle formed at the triple point line which are based on interfacial tensions and hydrodynamic argu- ments and naturally yield a Vmax .9 It has been suggested that the magnitude of Vmax is determined by an activation energy barrier which arises from the work required to disrupt the molecular interactions taking place in the film at the triple point line, and therefore at deposition rates exceeding Vmax the triple point line advances faster than film adsorption can take place. 1°-12 While these studies have provided a limited under- standing of the factors which determine the maximum deposition rate for L-B film formation, very little atten- tion has been paid to the changes which occur in the structure of L-B films when the transfer speed is varied at rates that are less than Vmax. Only two studies have been reported in which the latter question has been ad- dressed. Transmission electron diffraction, X-ray, and neutron reflectometry data s,13indicate that multilayer L-B films formed with the use of relatively fast transfer speeds are highly ordered. In contrast, multilayered films pre- pared at slow transfer speeds exhibit substantial inter- mixing between the layers in comparison to the films prepared at faster speeds. These results have important implications for materials fabrication. To the best of our knowledge, the effect of the deposition rate on the mo- lecular structure of the first monolayer transferred from the A/W interface to a solid support has not been char- acterized. In addition to providing important insight into fundamental aspects of the L-B process, understanding 1196 Volume 48, Number 10, 1994 0003-7028/94/4810-119652.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy