Langmuir-Blodgett (LB) films have been investigated, extensively, over the past 70 years. Despite considerable efforts aimed at exploiting their nonlinear optical, piezoelectric, pyroelectric, semiconducting, sensing and barrier properties, problems associated with film quality and stability have hampered their practical development. This feature article highlights one recent advance in the LB area (i.e., the ionic cross-linking of LB films or "gluing"), which has resulted in single bilayer membranes possessing extraordinary quality and stability. The gluing of LB films provides new opportunities for an old technology that has been waning in recent years.

Poly(4-styrenesulfonate) (PSS) has been used to ionically cross-link (glue together) single Langmuir-Blodgett bilayers derived from 1,2,4,5-tetrakis[N,N-dimethyl-N-(1-hexadecyl)-ammoniummethyl]benzene (2a) and tris(N,N-dimethyl-N-hexadecylammoniummethyl)mesitylene (3). The resulting films are of high quality as judged by He/N2 permeation selectivities of ca. 100. Such selectivity is well in excess of the Knudsen diffusion limit of 2.6 and approaches that which has been found for glued bilayers of 5,11,17,23,29,35-hexakis[(N,N,N-trimethylamonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexamedecyloxy-calix[6]arene hexachloride (1). The significance of these findings to LB technology, in general, is briefly discussed.

Permeation measurements have been made for He, CO2, and N2 across single Langmuir-Blodgett (LB) bilayers derived from 1,2,4,5-tetrakis[(N-(undecanoamidoethyl)-N,N-dimethyl ammonium) methyl]benzene tetrabromide (2) and 1,2,4,5-tetrakis[(N-(perfluoroundecanoamidoethyl)-N,N-dimethyl ammonium)methyl]benzene tetrabromide (3) in the absence and in the presence of entrapped poly(acrylic acid) (PAA). In the absence of PAA, single LB bilayers of 3 show a higher permeance for He and N2 but a lower permeation rate of CO2, as compared with analogous LB bilayers made from 2. The relatively low permeation rate of CO2 for the former has been attributed to reduced associative interactions with the fluorocarbon-rich bilayer. The same behavior has also been observed for LB bilayers containing PAA, formed under conditions that yield glued bilayers of 2 and 3 having similar diffusional pathways, as judged by He/N2 selectivities. These results, together with the fact that glued bilayers of 2 (having a thinner PAA layer as compared with those made from 3) exhibit lower He and N2 permeances, provide compelling evidence that the main barrier for gas transport is the combination of surfactant plus PAA and not simply a thin PAA layer that is encased within the surfactant bilayer.

The mixing behavior of a phospholipid containing a cis-cyclopropyl moiety (1) with one that contains two myristoyl groups (3a) has been investigated in fluid bilayers via the nearest-neighbor recognition (NNR) method. In the absence of cholesterol, these lipids mix ideally. In the presence of cholesterol, they show a modest preference for homo-phospholipid association. A trans-form of 1 (i.e., 2) was found to have similar behavior, except that the influence of cholesterol in promoting homo-phospholipid association was greater. Similar results have been found in membranes in which 3a is replaced with a phospholipid bearing two palmitoyol chains (3b). In this case, the effect of the kink is approximately twice as great. The implications of these findings, with respect to the "trans-fatty acid debate", are briefly discussed