UC Santa Barbara

Surfactants adsorb onto complex fluid-fluid interfaces, altering the interfacial energy and giving unique interfacial properties. When designing a formula containing surfactants or addressing challenges with naturally occurring surfactants, it is important to understand the interfacial properties of each surfactant system. This work investigates structure-property relationships of three surfactant systems: (1) model lung surfactant (DPPC) degraded by PLA2; (2) lipid/fatty alcohol mixtures inspired by surfactant therapies (SRT); (3) lipid-PEG copolymers. The investigations of (1) and (2) are motivated by the need for effective ARDS therapeutics. Lung surfactant (LS) becomes ‘inactivated’ in ARDS and SRTs are ineffective in treating ARDS, however, the mechanisms hindering LS function and SRT efficacy are not well understood. We hypothesize that the interfacial rheology, which is not easily studied clinically, is critical to the function and efficacy of LS and SRT, respectively.

The investigation of (1) studies the evolving morphology and rheology DPPC monolayers being degraded by PLA2. While degrading, domain morphology passes through qualitatively distinct transitions: compactification, aggregation, network percolation, coarsening, solidification, network erosion, and PLA2-rich domain nucleation. The relative activity of the PLA2 sample impacts the order and the duration of morphology transitions. Irrespective of PLA2 activity, all measured linear viscoelastic surface shear moduli showed the same exponential dependence on condensed phase area fraction throughout monolayer degradation. Monolayer rheology is viscous-dominant until the domain solidification transition, at which point the relative surface elasticity begins to increase. As degradation proceeds further, the relative elasticity starts to decrease once network connections start to be severed.

The investigation of (2) studies the relationship between phase behavior, morphology, and surface rheology of DPPC, hexadecanol (HD), and dihydrocholesterol (DChol) mixtures, which are all used in SRT formulas. The morphology and rheology of DPPC:HD mixtures, with and without DChol, track with the condensation of DPPC. As DPPC condenses and domain area fraction increases, |G*| and relative elasticity grows. When monolayers approach fully condensed, |G*| continues to grow, but less strongly, and relative elasticity decreases. DChol-containing monolayers exhibit similar trends in surface rheology as DChol-free monolayers, but are comparatively easier to shear, especially as surface pressure increases.

System (3) is a library of discrete DMG-PEG4-n copolymers, motivated by lipid nanoparticles applications. Using surface pressure v. area isotherms of copolymer monolayers, we examine the dilute, semi-dilute, and desorption properties of each copolymer as a function of PEG headgroup size n. For the range of n examined, interactions between the PEG headgroups, as opposed to the aliphatic chains, contribute to much of the observed surface behavior. At dilute concentrations, DMG-PEG copolymers with larger headgroups occupy more interfacial area. At semi-dilute concentrations, where the PEG headgroups contact and interpenetrate, each DMG-PEG conjugate exhibits traits of a 2D polymer in a good solvent. At sufficiently high concentrations the energy of each monolayer reaches the respective desorption energy, which grows with the size of the PEG headgroup. Overall, the structure of PEG headgroups in lipid-PEG conjugates play a critical role in the resulting interfacial properties.