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Interfacial properties and control of foaming by stimuli-responsive protein based formulation
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posted on 24.02.2017by Li, Huazhen
Recently, stimuli-responsive protein surfactants have been developed to control foams with applications in food, household, personal care products and industrial processes. Such protein surfactants can alter their interfacial properties in response to external stimuli such as pH and metal ions, offering a high level of foaming control. However, two main challenges have been hindering the widespread applications of such new generation of bio-surfactants, their high cost and the lack of fundamental knowledge that guides the design of formulation. To address these challenges, this research project aimed to: i) develop formulation for a designed stimuli-responsive protein surfactant DAMP4 for foaming control with lowered overall cost; ii) deliver fundamental knowledge on interfacial physio-chemical properties of the mixture system of protein DAMP4 and chemical surfactants/polymers; and iii) link the interfacial properties with the control of foaming.
A multi-disciplinary approach has been deployed to study the protein surfactants at three different scales: i) at a macro-scale, the foam stability has been determined with foaming assay method while adsorption kinetic has been studied by dynamic surface tension measurements; ii) at a molecular level, dynamic change of interfacial structure of the mixture systems including film thickness and surface excess of individual components has been determined by X-ray and neutron reflectometry techniques, and iii) at a micro-scale, the dynamic three-dimensional microstructure of the foams has been revealed by the X-ray phase-contrast tomography technique.
Two types of formulation have been explored to improve the control of DAMP4-based foaming. The first type of formulation is based on the mixture of DAMP4 and conventional chemical surfactants, and the second type concerns a mixing system of DAMP4 and polysaccharide. Single DAMP4 at low concentration (5 µM) did not give high stability of foams. However, with addition of small amount of sodium dodecyl sulphate (SDS, 18-54 µM, less than 1% of its critical micelle concentration) to low concentration of DAMP4 could significantly enhance the stability of foams. Importantly, foams of the DAMP4-SDS system can still be destabilized by Zn2+ addition, retaining stimuli-response feature of the DAMP4 systems. For the second type of formulation, two types of polysaccharides, which are surface active hydroxypropylmethyl cellulose (HMPC) and surface inactive pectin, have been used for the comparison of their control of DAMP4-based foams. At a polysaccharide concertation of 10-1 wt%, DAMP4-HPMC system showed higher foam stability whereas DAMP4-pectin system established defoaming ability.
Combining fundamental knowledge at multi-scales (macro-, micro- and molecular levels), this research project has identified two key factors, the molecular interactions and subsequent adsorption kinetics, that affect the interfacial properties of the mixture systems and subsequently the foam stability. First, the molecular interactions in the mixture systems affect both adsorption kinetics and compositions of the adsorbed interfacial layer. The adsorption of protein DAMP4 to the air-water interface requires an unfolding of its four-helix bundle in bulk solution to adapt a linear structure at the surface. Upon mixing with SDS, the DAMP4-SDS interactions could promote and stabilize the unfolding of this four-helix bundle, reducing the energy barrier required for DAMP4 adsorption and thus enhancing adsorption kinetics of DAMP4. Due to different surface-activity and charge nature of the polysaccharides used, HPMC and pectin have different interactions with DAMP4. DAMP4-HPMC and DAMP4-pectin polysaccharide mixture systems adsorbed to the air-water interface in different manners. Competitive adsorption between DAMP4 and HMPC was observed while co-adsorption of DAMP4 and pectin occurred at the interface. Second, the adsorption kinetics plays a key role in determining the foam stability. Foaming systems with fast adsorption kinetics, which possess a smaller time constant (t1/2), produce higher stability foams. The addition of small amount of SDS (18 µM) can decrease the adsorption time constant by a factor of 10, significantly enhancing the foam stability of DAMP4-SDS mixture system. On the other hand, additives such as Zn2+ or polysaccharide pectin could prompt DAMP4 to form aggregates or complexes that significantly slowed down the adsorption kinetics. As a result, destabilization of foams occurred. Third, molecular interactions affect not only the foam stability but also the three-dimensional microstructure of the foams. The foams in DAMP4-SDS mixture system adopted a polyhedral structure, which was similar to SDS foams, with a liquid volume fraction lower than that of single DAMP4 but slightly higher than that of single SDS. In the DAMP4-HPMC mixed system, the resulted average bubble size of the DAMP4-HPMC mixture foam was similar to that of single DAMP4 system but the liquid volume was higher than that of both single DAMP4 and single HPMC foams.
In summary, the PhD work has linked the molecular characteristics of DAMP4, chemical surfactant and polysaccharide with their interfacial properties at macro-, micro- and molecular scales. The knowledge discovered could support future development of the next generation of protein surfactants and their formulations. Particularly, it offers opportunities to achieve higher level of foaming control (e.g., stimuli responsive feature) at lower cost through rational formulation design.