Spreading of carbosilane dendrimers at the air/water interface

Spreading of carbosilane dendrimers containing trimethylsilyl or hydroxyeth yl end groups was investigated at the air/water interface. Our observations suggest that the monodisperse, globular molecules of the carbosilane dendr imer with hydroxyl end groups ordered into layers on the water surface. In contrast to the hydrophobic trimethylsilyl ends, the hydroxyl-functionalize d dendrimers formed a monolayer at the air/water interface. Surface pressur e versus film area isotherms were collected and showed full reversibility, irrespective of the degree of compression. Three equilibrium states of the OH-terminated dendrimer were identified depending on the molecular area. (i ) The monolayer was compressible over a remarkably broad range of molecular areas from 1200 to 650 Angstrom(2). I, the transition region, the osmotic pressure varied with concentration according to the power law pi/k T simila r to c(m) with m = 15. In combination with the large monolayer thickness of (1.4 +/- 0.1) nm, the steep power law indicates a globular shape for the a dsorbed molecules characterized by dense packing of chain segments. (ii) In the range between 650 and 350 Angstrom(2), the dendrimer underwent a sharp transition, presumably into a bilayer structure. Since the surface pressur e was almost invariant during compression, the transition is considered to be a first-order phase transition. (iii) Compression beyond 350 Angstrom(2) occurred at a constant pressure, as is typical for an isotropic liquid fil m. The spreading behavior of the OH-terminated dendrimer was compared with that of a polydisperse hyperbranched polymer of identical chemical composit ion. In contrast to the dendrimer, the hyperbranched polymer did not show a ny transitions and exhibited a spreading behavior similar to that of isotro pic liquids.