J. B. Peng

Kinetics of a structural phase transition in Langmuir monolayers studied using x‐ray diffraction

Frequently, time‐dependent effects are seen in monolayers of amphiphilic molecules (Langmuir films) when they are compressed, so that the pressure after some time is different from that recorded immediately after compression. We have identified for the first time a microscopic relaxation mechanism in monolayers of heneicosanol (C21H43OH): namely, a transition from a uniaxially distorted (‘‘pseudohexagonal’’) structural, formed upon compression, to an undistorted hexagonal structure. For T>20 °C we observe only an apparently hexagonal phase, while at T=5 °C we observe only an apparently stable pseudohexagonal phase. When 10≤T≤20 °C, the monolayer structure changes with time from pseudohexagonal to hexagonal. The rate at which this transformation occurs is strongly temperature dependent. We propose that the observed temperature dependence is determined by the rate of nucleation of a hexagonal phase from a metastable shear‐induced structure.

Contact angle of lead stearate-covered water on mica during the deposition of Langmuir-Blodgett assemblies☆

Abstract We have measured the dynamic and static contact angles between the monolayer-covered water surface and a mica substrate during the deposition of successive monolayers of lead stearate. The substrate was suspended from a Cahn RG Electrobalance in order to measure the contact angle-dependent force during deposition of the film; both dynamic and static angles were calculated from the output of the balance. The difference between the dynamic contact angles of the film- covered subphase on clean mica and during polar deposition on a lead stearate multilayer is approximately 18°. The static contact angle on the upstroke (polar deposition, StPb/PbSt) averages 6° greater than the dynamic angle. On the other hand, there is very little difference on the downstroke (PbSt/StPb deposition) between the static and the dynamic angles. The contact angles appear to be independent of surface pressure in the range 15–24 dyn cm-1.

Verification of epitaxial growth and determination of chain positions in Langmuir—Blodgett films of lead stearate on mica

Abstract We have determined from X-ray diffraction that lead stearate, which like many hydrocarbon compounds does not form good crystals in bulk, can be grown epitaxially on mica using the Langmuir—Blodgett technique. The strong alternation in intensities of the (11 l ) peaks cannot be explained by considering the lead ions only, and imply that the hydrocarbon chains do not close-pack into a triangular array but reproduce the in-plane structure of the lead ions.

A wide temperature range hermetically sealed langmuir-blodgett apparatus

Abstract We have built a Langmuir-Blodgett system situated inside a double-walled enclosure through which water or refrigerant can be circulated. This permits the temperature to be varied over a wide range without temperature gradients or condensation on the sides of the enclosure. Since the enclosure can be hermitically sealed, oxygen-free or other selected environments may be introduced.

Synchrotron diffraction studies of lead octadecanoate, tetracosanoic acid and 1-eicosanol monolayers on water

Abstract We have used diffraction of synchrotron radiation to study Langmuir monolayers of several different materials. Lead octadecanoate (stearate) was seen to undergo first order melting, although there is no clear indication of this in its isotherm. 1-eicosanol, which has a “kink” in its isotherm, shows diffraction peaks of similar size both above and below this kink. Tetracosanoic acid shows, at high pressure, a second peak close to the first.

Measurement of contact angle relaxation during the deposition of Langmuir-Blodgett films of cadmium stearate and valinomycin

Abstract We studied the contact angle relaxation during the Langmuir-Blodgett deposition of cadmium stearate and valinomycin films. For cadmium, we found that the relaxation time of the receding contact angle depends strongly on the deposition type; the static value that the receding angle relaxes to depends on the concentration of Cd 2+ ions in the subphase. For 4.0 × 10 -2 M Cd 2+ subphase, the deposition begins and remains Y-type. However, at a lower concentration of 2.5 × 10 -3 M the deposition changes from Y- to X-type. Prior to this transition, the static receding angles for the two different Cd 2+ concentrations are different but the relaxation times are about the same. After the transition, the relaxation process is more than 20 times faster. For valinomycin, the deposition is predominantly Z-type.

Using the transfer of a Langmuir monolayer as a probe of wetting

Abstract We observe a spontaneous transfer of a Langmuir monolayer of valinomycin to a hydrophilic substrate as soon as the edge of the substrate touches the water surface. By measuring the total monolayer area transferred, the capillary rise h can be determined. We find that the dependence of h on the spreading pressure is well accounted for by an expression due to Joanny and de Gennes but that the actual values of h are five orders of magnitude too small. “Prewetting” the substrate before the monolayer was allowed to spread did not affect the observed capillary rise.

Apparatus for making A/B superlattice Langmuir–Blodgett films

We report a modification recently made to a Langmuir–Blodgett superlattice apparatus previously described in this journal. The modification permits the preparation of so‐called A/B superlattices consisting of the sequence ABAB⋅⋅⋅, where A and B are monolayers of two different film‐forming molecules deposited into a multilayer structure.

Diffraction Studies of Langmuir Films

Insoluble organic molecules that are amphiphilic, i.e. with both hydrophobic and hydrophilic sections, can usually be spread on the surface of water to form monomolecular films. Although these films have been known for thousands of years, Irving Langmuir’s name is now attached to them in recognition of a vital contribution [1]: he found in 1917 that the two-dimensional pressure (π) vs. specific area (A) curves for these films had apparent discontinuities suggesting the existence of multiple monolayer phases. Such studies are now easily performed using commercial apparatus, and π-A data for thousands of materials exist in the literature, along with structural speculations based on this indirect and ambiguous evidence. Unfortunately, the films have proved relatively inaccessible to many of the usual condensed-matter probes; in particular, until very recently [2, 3] there had been no direct structural information (i.e. no diffraction data). In this report, we summarize the results to date of our diffraction studies of mono layers of three ‘simple’ compounds: lead octadecanoate (stearate), tetracosanoic acid, and 1-eicosanol.