We present a study of how patterns formed by Langmuir monolayer domains of a stable phase, usually solid or liquid condensed, propagate into a metastable one, usually liquid expanded. During this propagation, the interface between the two phases moves as the metastable phase is transformed into the more stable one. The interface becomes unstable and forms patterns as a result of the competition between a chemical potential gradient that destabilizes the interface on one hand and line tension that stabilizes the interface on the other. During domain growth, we found a morphology transition from tip splitting to side branching; doublons were also found. These morphological features were observed with Brewster angle microscopy in three different monolayers at the water/air interface: dioctadecylamine, ethyl palmitate, and ethyl stearate. In addition, we observed the onset of the instability in round domains when an abrupt lateral pressure jump is made on the monolayer. Frequency histograms of unstable wavelengths are consistent with the linear-instability dispersion relation of classical free-boundary models. For the case of dendritic morphologies, we measured the radius of the dendrite tip as a function of the dendrite length as well as the spacing of the side branches along a dendrite. Finally, a possible explanation of why Langmuir monolayers present this kind of nonequilibrium growth patterns is presented. In the steady state, the growth behavior is determined by Laplace's equation in the particle density with specific boundary conditions. These equations are equivalent to those used in the theory of morphology diagrams for two-dimensional diffusional growth, where morphological transitions of the kind observed here have been predicted.

The phase diagram of dioctadecylamine Langmuir monolayer has been determined from pressurearea isotherms and from direct observations of the monolayer using Brewster angle microscopy. We describe the observed domains, textures, and phase boundaries for the mesophases presented by this system between 5°C and 45°C, at pHϭ3. The phases were named as G, LE, S 1 , and S 2 according to their apparent texture. Contrast between domains of condensed phases is quite low, indicating small tilting of the amine tails. The G -S 1 , G -S 2 , G -LE, and LE-S 1 transitions seem to be of first order; meanwhile, the S 1 -S 2 transition seems to be of second order. We observed how the froths changed when the area density increases at constant pressure, along the G -L, G -S 1 , and G -S 2 phase transitions. In the L -S 1 transition, the shape of the domains of the emerging phase is of sixfold dendrites. The phase diagram obtained is very different from those obtained for single chain amphiphiles. In this system, the extent of the phases depends on the acid used to fix the pH. In addition, all our observations indicate that dioctadecylamine does not form a monolayer above pHϳ3.9.

We show that domain growth of condensed phases from a metastable phase in Langmuir monolayers presents several stages. At the very beginning, depending on the supersaturation level, structures evolve through a tip-splitting dynamics. If supersaturation levels are high, there is a morphological transition, domains grow with needle tips that show as growth proceeds, side branching. The way in which the instability starts at round domains when a small lateral pressure jump is applied to the monolayer is also shown. A model for a monolayer interacting with the subphase is presented. This model can be related to the theory of dynamic phase transitions, where morphological structures and morphological transitions are predicted.

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