Temperature-Dependent Phase Behavior of Langmuir Films of 10,12-Pentacosadiynoic Acid at the Air/Water Interface and Its Effects on Chromatic Stability of the Polymerized Langmuir-Schaefer Films

Abstract

The effect of temperature on the two-dimensional phase of 10,12-pentacosadiynoic acid (PCDA) Langmuir films at the air/water interface was investigated. The temperature of the Langmuir films was precisely controlled from 5 to 50 °C and their surface isotherms and in-situ visible absorption spectra were acquired. Depending on the temperature, the PCDA Langmuir films were found to be classified into a liquid-condensed (low temperatures, 5 and 25 °C) and a liquid-expanded phase (high temperatures, 40 and 50 °C). After polymerizing the PCDA Langmuir films with 254 nm UV light at the specific temperatures, the films were transferred to hydrophobic glass using the Langumir-Schaefer (LS) method. Upon thermal and pH stimuli, their chromatic transition characteristics were analyzed by visible spectroscopy. The liquid-condensed films were found to be more susceptible to thermal stimulus than the liquid-expanded films. The latter also showed remarkable chromatic stability against the pH in the region from 2 to 11, compared to the former. Thus, when sturdy films are required, the multilayered PCDA LS films prepared at the liquid-expanded phase are more suitable than the liquid-condensed films, and vice versa. This result is expected to be very useful for controlling the sensitivity and stability of polydiacetylene-based sensory systems simply by changing the polymerization temperature, even without synthesizing new monomers.