Surface Pressure−Area Isotherms and Fluorescent Behavior of a ω-6-(N-Methyl-N-alkylamine) Naphthoylalkanoic Acid (MANA)

Abstract

This paper reports the surface pressure−area isotherms for the new fluorescent probe ω-[6-(N-methyl-N-ethylamino)naphthoyl]undecanoic acid (MANA10-2) obtained by changing experimental variables such as the subphase conditions, pH, temperature, and the compression rate. The surface monolayer technique was used in combination with surface fluorescence measurements. Changes in pH and temperature of the aqueous subphase were found to significantly influence isotherm shape. The fluorescent properties of the molecule were studied both in the monolayer and in the subphase. This allowed us to determine the orientation of MANA10-2 on the surface and revealed that the monolayer is dissolved at both low and high pH values and high temperatures. Monolayers were also examined by Brewster angle microscopy. The morphology and size of the domains formed are clearly governed by temperature and the pH of the aqueous subphases. Only with monolayers spread on pure water at 20 °C was the transition from the liquid phase to the condensed phase observed. This was also the only occasion when the monolayers exhibited high macroscopic uniformity. A large number of small bright nuclei (typical of a densely packed two-dimensional phase or a three-dimensional phase) are formed at low surface pressure in the presence of HCl and NaOH subphases (pH 1 and 9, respectively).