Non-aqueous Foam Stabilized By Particles Based On Conjugated Linoleic Acid

Foam is a thermodynamically unstable system,which can be divided into aqueous foam and non-aqueous foam according to the type of continuous phase.Different from the well-known aqueous foam,the stable non-aqueous foam is difficult to be obtained due to the low dielectric constant,low surface tension of non-aqueous solvents,and thus hydrocarbon surfactants are difficult to adsorb on the surface of the gas-liquid interface of non-aqueous foam.Although non-aqueous foam is widely applied in foods,daily chemicals,and petroleum industry,it is until the last decade that the fundamental research of non-aqueous foam is paid attention to,but mechanism to stabilize non-aqueous foam is not clear.Performance of non-aqueous foam is influenced by foam stabilizers,solvent types,viscosities and surface tensions of gas-liquid dispersions,in which foam stabilizers are the key factor.Therefore,the research is focused on selecting suitable foam stabilizers and exploring mechanism to stabilize non-aqueous foam.At present,special surfactants,modified solid particles,and crystalline particles are commonly used as foam stabilizers for non-aqueous foam.The non-aqueous foam stabilized by special surfactants and modified solid particles is only suitable for some specific fields due to safety concerns of fluorocarbon compounds.Moreover,the modified solid particles are easy to create foam in solvents with lower surface tension while difficult in polar solvents with high surface tension.Crystal particles of lipids are generally obtained during cooling down to cause high melting point substances precipitated,but the preparation process is complicated and has to be completed directly in the target non-aqueous solvent,making it difficult to extract and characterize crystal particles of lipids from oil phases.Conjugated linoleic acid（CLA）is a good molecular building block.In the early stage of our laboratory,O/W Pickering emulsion and aqueous two-phase emulsion were successfully stabilized by vesicles,micelles,fatty acid lamellar liquid crystals（FALLC）were constructed with CLA and solid particles modified by CLA.As mentioned above,crystalline particles of fatty acid are good non-aqueous foam stabilizers,so can these fatty acid particles with an ordered structure stabilize non-aqueous foam?What is the foam stabilization mechanism?Based on the above questions and the summary of the literature,three types of CLA-based particles were constructed to be foam stabilizers:Fe₃O₄nanoparticles modified by sodium conjugated linoleate（SCL@Fe₃O₄ NPs）,multi-morphologic self-crosslinking sodium conjugated linoleate micelles（SSCLM）and FALLC.The main research contents and results of this paper are as follows:1.SCL@Fe₃O₄ NPs promote foaming in the glycerol/water systemSCL@Fe₃O₄ NPs were prepared by SCL modifying the surface of Fe₃O₄ and used to promote glycerol and water system foaming.The results show that SCL@Fe₃O₄ NPs can effectively reduce the surface tension of the glycerol/water system.By changing the ratios of glycerol to water,the wettability of SCL@Fe₃O₄ NPs can be effectively adjusted,resulting that the SCL@Fe₃O₄ NPs can be adsorbed easily at the gas-liquid interface of non-aqueous foam and form a film on the surface to prevent the coarsening and coalescence of the liquid film,which enhancing the stability of foam.2.Polymorphic SSCLM promote foaming in the glycerol/water systemCoupled with self-assembly and self-crosslinking strategy,polymorphic SSCLM were prepared by concentration-driven method and used to promote foaming in glycerol/water system.The results show the morphology of SSCLM are spherelike,rodlike,disclike,and threadlike with the increasing of SCL concentration.SSCLM of different morphologies have properties of foaming and stabilization in the glycerin/water system,and the foam film is constructed by glycerin and water.SSCLM have good surface activity under alkaline conditions,they can be adsorbed on the liquid film,effectively reducing the surface tension and providing the possibility of foaming of the system.3.SCL/H₂O FALLC stabilizes vegetable oil foamUsing CLA as model fatty acid,the SCL/H₂O FALLC was constructed through p H-response assembly process.Polarizing microscopy proved FALLC was successfully prepared.The foam systems of olive oil,perilla oil,and sunflower oil is stabilized by FALLC with good foaming properties and the foaming foam stability.The addition of FALLC can produce small surface tension gradients which can be of sufficient magnitude to give weak Gibbs-Marangoni effects and generate foam.Moreover,the addition of FALLC increases the viscosity of the system,which is beneficial to the stability of the foam.The results of the comprehensive forward fluorescence microscope,confocal laser scanning microscopy and polarizing microscopy together demonstrate that foam liquid film is constructed by vegetable oil and water,and the FALLC adsorbed on the plateau border between the foam enlarging the curvature radius of this region,which can reduce the Laplace pressure,and increase the stability of foam.On this basis,the influence of water content in the system was investigated,it was found that with the increasing of water content,the foam overrun and half-life increased significantly,the location of the foam is changed from above the liquid to the inside of the liquid.