Amphiphilic Siloxane Phosphonate Macromolecule Monolayers at the Air/Water Interface: Effects of Structure and Temperature

Abstract

A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-alphabeta-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-alphabeta-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (pi -A) isotherms of homopolymer at 3-40 degrees C show a clear temperature-induced phase transition (plateaus at pit approximately 17-19 mN/m) below 10 degrees C. The magnitude of the transition substantially increases upon lowering the temperature (partial differential DeltaAt/ partial differential T approximately -0.1 nm2 unit(-1) deg(-1) and partial differential pi t / partial differential T approximately -0.25 mN m(-1) deg(-1)). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the P=O bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 degrees C. The transition occurring in the copolymer at 20 degrees C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.

A Dendritic Amphiphile for Efficient Control of Biomimetic Calcium Phosphate Mineralization

“…This particularly applies to samples at lower temperatures, while at higher temperatures some of the hydrogen bonds may loosen up or break, thus giving rise to the liquid-like behavior of the molecules at the surface. The presence of a quasi-plateau at low temperatures due to the formation of hydrogen bonds has been described before [48,49] and our data confirm these assignments.…”

Cholesteryl Hemisuccinate Monolayers Efficiently Control Calcium Phosphate Nucleation and Growth

“…Indeed, there are several examples showing that the surface pressure to which the monolayers are compressed is important for the observation of such a hysteris. , The occurrence of the hysteresis can also be explained by a small material loss either by dissolution of some amphiphile molecules in the subphase or the formation of small 3D crystals − (although there is no evidence in the BAM experiments for crystallization). Finally, intermolecular hydrogen bonding during film compression has also been held responsible for hysteresis. − …”

“…Hydrogen bonding could also explain our observations, because hydrogen bonds are more pronounced at lower than at higher temperatures, and therefore the hysteresis at lower temperatures is more pronounced. The hydrogen bonds lead to a bridging of the molecules, which in turn may not have enough time to reorganize during the expansion . Possibly, the reorganization of the film during expansion is much slower than the movement of the barriers such that the system is not in equilibrium …”

“…The hydrogen bonds lead to a bridging of the molecules, which in turn may not have enough time to reorganize during the expansion. 90 Possibly, the reorganization of the film during expansion is much slower than the movement of the barriers such that the system is not in equilibrium. 89 To induce a local crystal nucleation at the surface, the monolayer must be enriched in calcium and phosphate ions.…”

Planar Biomimetic Membranes Based on Amphiphilic Block Copolymers

“…Monolayers at the air–water interface are the simplest models of the biological membranes used for investigation of interactions with biomolecules in different conditions, for example, temperature, pH of the subphase and surface pressure of the film, and thus in different physical states of the film (Figure ) . Langmuir monolayer technique is broadly used to study synthetic films of polymers, − mixtures of polymers, and mixtures of polymers with lipids , or with biomolecules . In addition, polymer films at air–water interface are used as scaffolds in biomineralization to control the growth of inorganic particles. − …”