Viscoelastic Properties of Lipopolymers at the Air−Water Interface:  A Combined Interfacial Stress Rheometer and Film Balance Study

Abstract

Poly(ethylene glycol) (PEG) is a molecule that exhibits unique behavior when compared with polymers in its homologous family. Depending on its environment, it may show hydrophilic, hydrophobic, or amphiphilic properties. We have studied several PEG lipopolymers, where a PEG chain with a molecular weight (MW) of 2000 g/mol or 5000 g/mol is covalently attached to 1,2-dipalmitoyl-or 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, with a Langmuir film balance and a recently developed interfacial stress rheometer. In particular, we have determined how the rheological properties of PEG molecules anchored at the airwater interface change when the polymer chains are forced into highly stretched brush conformations. Pressure-area isotherms of monolayers of PEG lipopolymers exhibit two phase transitions: a desorption transition of the PEG chains from the air-water interface at 10 mN/m and a high film pressure transition at 20-40 mN/m, but the nature of the latter transition is still poorly understood. We have observed a remarkable change of the viscoelastic properties in the range of the high-pressure transition. The monolayer is fluid below the transition, with the surface loss modulus, Gs′′, being larger than the surface storage modulus, Gs′, but becomes remarkably elastic above, with Gs′ > Gs′′. This indicates that a strong correlation exists between the reversible, first order-like high-pressure transition and the formation of a physical gel. Our surface rheological experiments indicate that formation of a physical network can be understood if water intercalates mediate the interaction between adjacent PEG chains via hydrogen bonding.

Lung Surfactant Gelation Induced by Epithelial Cells Exposed to Air Pollution or Oxidative Stress

“…The ISR is a unique oscillating magnetic needle viscometer that measures the dynamic rheology of surfactants at an air-liquid interface ( Figure 1A) (25)(26)(27). The design of the ISR is based on applying a controlled shear stress to an interface while simultaneously measuring the resultant deformation (strain) as a function of time.…”

“…The design of the ISR is based on applying a controlled shear stress to an interface while simultaneously measuring the resultant deformation (strain) as a function of time. Conventionally, a large Langmuir trough filled with a liquid (subphase) is used, on top of which a monolayer of interest is spread (25)(26)(27). However, for our study, a Langmuir trough was modified to smaller size (a rectangular glass trough 7.6 cm long, 5 cm wide, and 0.5 cm tall; volume ϭ 19 ml), to accommodate the relatively smaller amount of cell supernatant (20 ml) for each of our study conditions.…”

“…Such interfaces are termed "viscoelastic," referring to flow responses that are intermediate between a purely viscous response and one that is purely elastic. The interfacial stress rheometer (ISR) is a novel instrument that uses an oscillating magnetic needle to measure the shear viscosity and elasticity of a surfactant monolayer at the air-water interface (25)(26)(27).…”

Dynamic Properties of Poly(styrene)–Poly(ethylene oxide) Diblock Copolymer Films at the Air–Water Interface

“…The obtained conclusions were examined in a number of experimental studies at the solid-liquid (4,5) or liquid-gas interfaces (7)(8)(9)(17)(18)(19)(20)(21)(22). It is noteworthy that although the dynamic surface properties can be especially important from the point of view of applications, only a few studies were devoted to the dynamics of polymer brushes at liquid interfaces (17,20,(23)(24)(25)(26). In most of the studies surface quasielastic light scat- 1 To whom correspondence should be addressed.…”

Emulsions for interfacial filtration.

“…The emulsion filter takes advantage of several characteristics of emulsions to efficiently trap and concentrate materials. First, a wide range of materials are attracted to interfaces between two materials, such as the surface of a droplet in an emulsion including proteins [1], cells including algae and bacteria [2,3], viruses [4], medically relevant materials such as cholesterol and caffeine [5,6], phenols [7], polymers including surfactants and lipo-polymers [8,9,10], hydrocarbons [11]. For many devices, this adsorption is a hindrance that leads to bio-fouling of surfaces and membranes as biological materials collect on the walls of the device.…”