We present results of an extensive x-ray diffraction study of a monolayer of C10F21CH2COOH spread on water (pH2) at 19.1 °C. Lever-rule analysis of the in-plane scattering is used to show that there is a coexistence region between ordered condensed islands and a dilute disordered phase. The coexistence region is found to be bounded by molecular areas of 29 Å2 and 2000±600 Å2, in agreement with the pressure-area isotherm. The molecular tilt of the ordered phase remains unchanged from closest packing (near collapse of the monolayer) throughout the coexistence region, and has a value of 2±3 degrees with respect to the normal to the liquid surface. These results are contrasted with those for hydrocarbon monolayers in which the onset of order in the coexistence region is close to the condensed phase boundary, and the ordered phase is compressible with a continuously variable tilt angle ranging from 30 to 0 deg at closest packing. The differences are attributed to the enhanced chain stiffness of the fluorinated chain. To illustrate this interpretation, preliminary scattering results are presented for a monolayer of C8F17(CH2)4COOH on water, which has some of the features of the aliphatic lipid monolayers. Recent molecular dynamics simulations have been found to reproduce all the important qualitative features of these systems [S. Shin, N. Collazo, and S. A. Rice, J. Chem. Phys. 96, xxxx (1992)].

The structure of a Langmuir monolayer of methyl eicosanoate as determined by xray diffraction and Brewster angle microscopyWe have studied the structure ofa monolayer ofC 2J H 43 0H on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the 11"a plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for 11" = 30 dyn/cm the transition temperature is in the range 16.3 < T < 21.3 ·C. We interpret this observation, by comparison with the properties of the lamellar crystalline n-paraffins, as a hexagonal-topseudohexagonal structural transition analogous to the crystal rotator II-to-rotator I transition. Our results imply that the hydrocarbon tails of the amphiphile molecules dominate the properties of the monolayer.

We have previously observed that the hydrophobic polyelectrolyte poly(2-ethylacrylic acid) solubilizes lipid membranes in a pH-dependent manner, and we have exploited this phenomenon to prepare lipid vesicles that release their contents in response to pH, light, or glucose (Thomas, J. L., and D. A. Tirrell. Acc. Chem. Res. 25:336-342, 1992). The physical basis for the interaction between poly(2-ethylacrylic acid) and lipid membranes has been explored using surface tensiometry and fluorimetry. Varying the polymer concentration results in changes in surface activity and membrane binding that correlate with shifts in the critical pH for membrane solubilization. Furthermore, the binding affinity is reduced as the amount of bound polymer increases. These results are consistent with a hydrophobically driven micellization process, similar to those observed with apolipoproteins, melittin, and other amphiphilic alpha-helix-based polypeptides. The absence of specific secondary structure in the synthetic polymer suggests that amphiphilicity, rather than structure, is the most important factor in membrane micellization by macromolecules.

The conception, fabrication, and utilization of highly organized mono-and multilayer assemblies of alkyltrichlorosilanes and their derivatives have been the focus of several research groups within recent years.1-12 Once formed by either Langmuir-Blodgett transfer from the air water interface1-3 or self-assembly4-12 from solution, the hydrolyzed alkylsilane monolayers linearly polymerize laterally leaving each monomer with a Si-Olinkage directed toward the supporting substrate.4-5 This often results in the formation of a covalent bond between the film and its substrate,1-12 providing additional adhesive stability. It is hoped that such films will be exploited for such uses as electrode modification,3-7 selective perme-

The proton-donating and -accepting abilities of water and methanol to various nitrogen-containing compounds have been determined by calorimetric analysis. Water is a better proton donor than methanol but methanol is a better proton acceptor than water. The interactions by water and methanol at the nitrogens of the diazines follow trends expected from relative proton affinities, pK's, and theoretical calculations. Electrostatic interactions by the diazine nitrogens with pyridine parallel the interactions found for methanol and water at these nitrogens.