Long‐Lived Foams Stabilized by a Hydrophobic Dipeptide Hydrogel

Abstract

A hydrogel of hydrophobic dipeptides can be used to create a wet foam with long‐term stability. The dipeptide molecules self‐assemble into fiber‐like networks (due to the presence of metal ions) both at air–water interfaces and in the continuous phase. The former creates an interfacial film stabilizing the air bubbles while the latter forms a bulk gel, which prevents bubble movement and retards growth. If the storage modulus (G′) of the bulk hydrogel is sufficiently high it can stop the coarsening of the air bubbles and thus dramatically improve the stability of the foam. Cryogenic scanning electron microscopy and Raman spectra reveals the width of the fibers (200 nm) and that they are held together by hydrogen bonds. In the absence of bubbles, phase separation is observed between a hydrogel and a water‐rich phase; in the foam this can be suppressed provided that the concentration of dipeptides and metal ions are sufficiently high. It is speculated that the resistance of the bubble arrangement to compaction and hence further drainage arrests the process of phase separation. This foam system has the advantages of long stability, low cost, as well as easy preparation; therefore, it has potential applications in food manufacturing, drug delivery, and personal care industries.

Stabilization of foams by the combined effects of an insoluble gas species and gelation

“…In the absence of any insoluble species in the gas mixture (Z oM = 0), coarsening due to capillary pressure differences can then be counterbalanced only by the elasticity of the continuous phase. Eqn (15) shows that the minimum radius for which a bubble can be stable is given by the elasto-capillary radius:…”

“…Drainage can be arrested when using for instance a concentrated particle suspension as the continuous phase where the particles are jammed in the liquid interstices. 13,14 Moreover, drainage and coalescence can be slowed down by using a foaming liquid 8,11,15 with either large bulk viscosity or specific interfacial rheological properties 9,16 that provide large surface viscosity or surface elastic modulus. Stable highly ordered monodisperse solid foams have been obtained combining a milli-fluidic generator and a gelifying liquid phase.…”

Stabilizing bubble and droplet interfaces using dipeptide hydrogels

“…2 This tends to be the more challenging route. (2) These small molecules may self-assemble into larger structures which retain their order at the interface; [2][3][4][5][6][7][8][9] the trapping at the interface can be much stronger than for individual proteins. Proteins have also been used as subunits within robust composite particles which themselves are interfacially active.…”

“…Recently, the behaviour of very short peptide sequences at liquid/air interfaces has begun to be explored. 6,7 Here naphthalene-protected diphenylalanine (2NapFF) molecules have been investigated most. One approach was to investigate the self-assembly of the dipeptide at the air/water interface prepared via drop-casting.…”

“…The dipeptide concentration as well as the type and concentration of metal ions, affects the foam stability. 7 When the bulk hydrogel had a yield stress larger than the Laplace pressure of the bubbles, ripening was suppressed.…”