PFF - Magnetic Materials for Biotechnological Use

Yr 3 Chemical Engineering

Principle of Bioseparation with Magnetic Supports: Labscale Applications

1. Biological feedstock

   • add magnetic adsorbents

2. Selective adsorption

3. Magnetic separation

   • add buffer

   • remove supernatant

4. Wash and elution steps

5. Transfer final supernatant to fresh tube

6. Purified target species, e.g. a specific cell or molecule

Principle of Bioseparation with Magnetic Supports: Labscale Applications

Magnetic Adsorbents

Magnetic particles or beads are functionalised with an appropriate ligand

Principle of Bioseparation with Magnetic Supports: Labscale Applications

Wash and Elution Steps

Numerous cycles of buffer addition

Magnetic separation and supernatant may be required here

Traditional Approach to Downstream Processing (DSP)

4 Main Steps:

1. Recovery/removal

2. Isolation

3. Purification

4. Polishing

Expanded Bed Adsorption (EBA)

Hybrid of chromatography and fluidised bed

Relies on dense specialised media of defined size and density distribution

Applicable to direct product capture from crude particulate containing feedstocks

(Expanded beds = fluidised beds)

Expanded Bed Adsorption: 3 Gradients

1. Particle size

   • for fluidisation stability

   • biggest at base, smallest at top

2. Voidage

   • to allow particles to pass

   • lowest at base, highest at top

3. Linear velocity

   • highest at base, lowest at top

Expanded Bed Adsorption: Particle Movements at Stable Fluidisation

The size of each matrix particle defines its own unique axial position

• the beads find their ideal position in the column and at stable fluidisation, small circular movements of adsorbent beads are seen

The bed is stable when only small circulatory movements of adsorbent are observed

• the bed expands in a stable and uniform manner

When the bed is stable, no particle within it rises or falls

Magnetic Stabilised Fluidised Beds (MSFBs)

Magnetic dipoles induced in the support particles cause them to become spatially fixed, stabilising the bed to form a MSFB

The magnetic dipoles are induced along the field lines, so that particles have a tendency to align vertically and oriented parallel to the flow through the bed

Magnetic Stabilised Fluidised Beds (MSFBs):

Complete Stabilisation

For complete stabilisation to occur, the magnetic interactions between particles must be of sufficient magnitude to counter the fluidising forces, which tend to mix the particles

As particles in the MSFB are essentially motionless, the stabilised condition can easily be determined by visual inspection

Advantages of MSFBs (x4)

Low pressure drop

Resistance to clogging from particulates and colloidal materials in crude feeds

The possibility for continuous counter current operation

Uncoupling bed expansion from fluidisation velocity by controlling the strength of the applied magnetic field

Disadvantages of MSFBs (x2)

Heavily outweighs the advantages:

• Channelling between the particle chains resulting in lower mass transfer rates

• Added complexity and costs of the magnetic system

Magnetic Separation in Biotechnology

Advantages (x4)

Gentle

Fast

Simple

Compatible with dirty suspensions

Magnetic Separation in Biotechnology

Disadvantages (x2)

Expensive adsorbents

Only available in small amounts

Applications of High-Gradient Magnetic Separation (HGMS) (x3)

Wastewater treatment

Purification of clays, glass sands and other minerals

Preconcentration/beneficiation of weakly magnetic ores

HGMS and Fishing Game Analogy Components (x4)

Rod = magnet

Line = magnetic field

Hook = magnetic support

Bait = immobilised ligand

Magnetic Filters for HGMS (x3)

Steel wool

Knit mesh

Regular wire gauze

Blueprint for a Bioprocess Scale Magnetic Adsorbent:

4 Stages

1. Magnetic core material

2. Particle coatings

3. Small scale ligands

4. Finished product

Blueprint for a Bioprocess Scale Magnetic Adsorbent:

Stage 1 - Magnetic Core Material

Ideally monodisperse < 1 μm and composed of ~ 10 nm crystals

Superparamagnetic properties

Highly textured surface

Blueprint for a Bioprocess Scale Magnetic Adsorbent:

Stage 2 - Particle Coatings

Must be very thin

Should not coalesce particles

Neutral hydrophilic exterior surfaces

Easy to derivatise

Protection against corrosion

Blueprint for a Bioprocess Scale Magnetic Adsorbent:

Stage 3 - Small Stable Ligands (Examples)

Ion exchangers

Mixed mode

Pseudoaffinity mimics

Synthetic inhibitors

Blueprint for a Bioprocess Scale Magnetic Adsorbent:

Stage 4 - Finished Product

Cheap

Scalable

Consistent quality

Fine-tuned for separation

Issues to Address for Optimal Use of the Magnetic Support Material

• Particle properties

  • size distribution

  • density

  • porosity with respect to target molecule

• Magnetic properties

• Available surface area for binding

• Material and surface chemistry

• Ligand

  • selectivity

  • how much is required

  • expense

  • robustness (especially to cleaning reagants)

Wet Drum Magnetic Separators

Separates materials by subjecting them to a stationary magnetic field while they are in suspension

Common Natural Polymers: Agarose

An algal polysaccharide composed of alternating galactose and anhydrogalactose residues

Soluble in hot water and forms rigid meso-macroporous gels upon cooling

Micropores Diameter

< 2 nm

Mesopores Diameter

2 - 50 nm

Macropores Diameter

> 50 nm

Most Common Matrix Synthetic Polymer:

Poly(styrene-co-divinylbenzene)

Highly crossed-linked

Cheap

Easy to make small uniformly sized cross-linked polymer particles from styrene and divinylbenzene

Very hydrophobic and unfriendly to proteins

Magnetic Core Materials:

Ferrites

Readily available in large quantities and are characterised by the common chemical structure MO.Fe₂O₃ where M is typically Fe, Ni, Mn, MnZn or MgCu

Magnetic Core Materials:

Magnetite (FeO.Fe₂O₃ or Fe₃O₂)

Most commonly used

Exhibits higher saturation magnetisation and magnetic susceptibility compared with the only other naturally occurring magnetic iron oxide, maghemite

Magnetic Core Materials:

Magnetite Production

Magnetite crystals can be produced by adding alkali to a mixture of Fe²⁺ and Fe³⁺ salt solutions

Magnetite crystals rapidly grow and precipitate between pH 9-14

To obtain a pure magnetite requires that reactions are carried out in oxygen free environments to prevent runaway formation of paramagnetic ferroxyhyte δ-FeOOH

4 Types of Magnetism

1. Ferromagnetism

2. Paramagnetism

3. Diamagnetism

4. Superparamagnetism

Ferromagnetism

Attracted towards the magnetic field lines when placed in a magnetic field, and the induced force experienced has a strong non-linear dependence on the strength of the magnetic field

As ferromagnetic (e.g. Fe, Co and magnetite) particles can become saturated with magnetic lines of force, a residual magnetism may remain when the applied field is reduced to 0

Ferromagnetic materials consist of microscopic domains, each magnetised in the same direction

Due to their residual magnetism, ferromagnetic materials tend to be unsuitable for repeated use, as they tend to aggregate and form clumps

Paramagnetism

Have induced magnetic forces that respond linearly to the applied magnetic field

The magnetic properties are weaker than those of ferromagnetic substances (unless very high magnetic field strengths are applied)

Paramagnetic substances (e.g. Mn and haematite) do not experience any magnetic memory when the field is removed

Diamagnetism

Materials that do not have a form of paramagnetism or ferromagnetism exhibit diamagnetic properties

These materials have no unpaired electrons and their magnetic response opposes the applied magnetic field

Examples: copper, silver, gold and bismuth

Superparamagnetism

A property of substances whose response to magnetic fields is stronger than normal paramagnetic fields, but still do not exhibit magnetic memory

Properties occur when the crystal size of ferromagnetic substances is sufficiently small to consist of only 1 magnetic domain

The transition point between ferromagnetic and superparamagnetic properties is reported to be ~ 30 nm, i.e. to produce particles with superparamagnetic properties, the crystal size must be < 30 nm

Examples: magnetic iron oxides

Advantages of Superparamagnetic Particles (x4)

Easy resuspension

Large surface area

Slow sedimentation

Uniform distribution of the particles in suspension media

Advantages of Ferroparamagnetic Particles (x1)

Very strong magnetic properties

Wüstite, FeO

Jet black powder

Feo adopts cubic ‘rock salt’ structure where Fe and O atoms are octahedrally coordinated to one another

Non-stoichiometric iron deficient compounds

Usually prepared by thermal decomposition of iron (II) oxide oxalate in inert atmosphere to avoid formation of ferric oxides

Iron Oxides and Oxyhydroxides:

Colours

Intense colours

Exact colour is very sensitive to partcile size, morphology and internal structure (especially cation substitutions), and a level of purity (presence of impurities)

The Common α and γ Phase Iron (III) Oxides:

Hematite, α-Fe₂O₃

Blood red colour in powder form

HCP lattice (trigonal crystal system / hexagonal plate or rhombohedra crystal)

Most common form of Fe₂O₃

Most stable form of iron oxide in nature

Serves as main feedstock for production of iron, steel and many alloys

The Common α and γ Phase Iron (III) Oxides:

Hematite, α-Fe₂O₃ Preparation (x2)

Nucleation and growth of nano-sized ferrihydrite aggregates

Thermal transformations from ferroxyhyte, maghemite and goethite

The Common α and γ Phase Iron (III) Oxides:

Maghemite, γ-Fe₂O₃

Brown colour

CCP lattice (cubic or tetragonal crystal system / cube shaped crystals)

Metastable

Ferrimagnetic ultrafine particles (<10 nm) are superparamagnetic

The Common α and γ Phase Iron (III) Oxides:

Maghemite, γ-Fe₂O₃ Preparation (x3)

Thermal Dehydroxylation of γ-FeOOH (lepidocrocite)

Careful oxidation of Fe₃O₄ (magnetite)

As ultrafine particles by thermal decomposition of iron (III) oxide

The Rare β and ε Phase Iron (III) Oxides:

β-Fe₂O₃

Mustard yellow colour

HCP lattice (cubic body centred crystal structure)

Metastable phase, converts to α-Fe₂O₃ at >770 K

The Rare β and ε Phase Iron (III) Oxides:

β-Fe₂O₃ Preparation (x3)

By reduction of hematite by carbon

Pyrolysis of FeCl₂ solution

Thermal decomposition of FeSO₄

The Rare β and ε Phase Iron (III) Oxides:

ε-Fe₂O₃

Metastable, converts to α-Fe₂O₃ between 770 and 1020 K

Structure and properties intermediate between α-Fe₂O₃ and γ-FeOOH

Magnetic properties employed in precision data storage electronics

Very challenging to prepare in pure form (contamination with α and γ)

The Rare β and ε Phase Iron (III) Oxides:

ε-Fe₂O₃ Preparation (x2)

Oxidation of iron in electric arc (very energy intensive for tiny quantities)

Sol-gel precipitation from Fe(NO₃)₃

Iron Oxyhydroxides

May occur in anhydrous FeOOH or hydrated FeOOH.nH₂O forms monohydrate FeOOH.H₂O

Also described as: iron (III) hydroxide, hydrated iron oxide, yellow iron oxide

Iron (III) oxyhydroxides occurs naturally as 4 mineral polymorphs denoted by the greek letters: α, β, γ, δ

Iron Oxyhydroxides:

α-FeOOH (Goethite)

Yellow-brown colour used as pigment since prehistoric times

Hexagonal close packed, orthohombic crystal system, needle or lath shaped crystals

Iron Oxyhydroxides:

α-FeOOH (Goethite) Preparation (x2)

Via solution from dissolved partly hydrolysed Fe³⁺ ions, e.g. Fe(OH₂)⁺, by nucleation and crystal growth

Form ‘green rusts’ by oxidation

Iron Oxyhydroxides:

β-FeOOH (Akaganeite)

Rusty brown coloured (III) oxyhydroxide/chloride mineral, i.e. FeO (OH, Cl)

Hexagonal close packed, monoclinic crystal system, star shaped crystals

Iron Oxyhydroxides:

β-FeOOH (Akaganeite) Preparation (x1)

By hydrolysis of FeCl₃

Iron Oxyhydroxides:

γ-FeOOH (Lepidocrocite)

Orange colour

Commonly seen as rust on inside of steel water pipes

Cubic close packed, orthorhombic crystal system, needle or lath crystals

Iron Oxyhydroxides:

γ-FeOOH (Lepidocrocite) Preparation (x1)

Oxidation of dissolved Fe²⁺ directly or via a ‘green-rust’ intermediate

Iron Oxyhydroxides:

δ-FeOOH (Feroxyhyte)

Brown colour

Forms under high pressure conditions (ocean beds) but readily converts to hematite or goethite

Also found in poorly drained soils and sediments, formed by the rapid oxidation of iron (II) compounds

Hexagonal close packed, hexagonal crystal systems, lath shaped crystals

Iron Oxyhydroxides:

δ-FeOOH (Feroxyhyte) Preparation (x1)

Rapid oxidation of iron (II) salts

Ferrihydrite, FeOOH.4H₂O

Red-brown colour

Widespread hydrated ferric oxyhydroxide mineral

Metastable pressure of more crystalline minerals such as hematite and goethite by aggregation-based crystal growth (transformation generally inhibited in natural systems by surface adsorbed silica)

Hexagonal close packed, hexagonal crystal system, but exhibits poor crystallinity

Only exists as a fine grained and highly defective nanomaterial composed of small sized (~ 3 nm) individual spherical nanocrystals yielding huge surface areas ( > 300 m² g^-1)

Ferrihydrite, FeOOH.4H₂O:

Preparation (x1)

Direct precipitation from oxygenated aqueous iron rich solutions at pH > 2

‘Green Rusts’, Fe^II_x Fe^III_y(OH)₂

Green-blue coloured Fe^IIFe^III hydroxy compounds containing a certain amount of non-hydroxyl anion (e.g. Cl^-, Br^-, F^-, I^-, SO₄^2-, CO₃^2-)

Layered double hydroxide (LDH) mineral (fougerite) exists in iron containing hydromorphic environments

Exhibits very high sensitivity to oxidation comprises isolation and detection

‘Green Rusts’, Fe^II_x Fe^III_y(OH)₂:

Preparation (x3)

Oxidation of aqueous Fe^II solution

Oxidation/anodic electrolysis of Fe

‘Induced’ hydrolysis from ferrihydrite, goethite or hematite + FeCl₂ solution under reducing oxygen depleted conditions

Order of reactivity to transform into green rust is:

• ferrihydrite > goethite > hematite

The 13 Magnetic Support Designs Afforded by Infiltration, Encapsulation and Coating Techniques

• Infiltrated

  • porous I

• Encapsulated

  • porous II

  • pellicular III

  • non-porous IV

  • non-porous V

• Coated

  • non-porous VI

  • non-porous VII

  • non-porous VIII

  • pellicular IX

  • non-porous X

• Infiltrated and Coated

  • non-porous XI

• Encapsulated and Coated

  • non-porous XII

  • non-porous XIII

Useful Magnetic Support Materials can be Produced by (x4)

1. Encapsulation

2. Infiltration

3. Coating

4. Various combinations of ‘encapsulation and coating’ or ‘infiltration and coating’

Useful Magnetic Support Materials can be Produced by:

Encapsulation

Encapsulation of solid magnetic elements within natural gels, synthetic resins, or liquid-filled microcapsules

Useful Magnetic Support Materials can be Produced by:

Infiltration

Infiltration of porous supports with fine magnetic sub-particles, or with aqueous mixtures of Fe²⁺ and other metal ions (e.g. Fe³⁺, Ni²⁺, Mn²⁺, Zn²⁺, Cu²⁺) capable of forming magnetic ferrrites when triggered to do so

Useful Magnetic Support Materials can be Produced by:

Coating

Coating magnetic core particles with natural or synthetic polymers or inorganic materials, such as silica

Encapsulation:
Commercial Type II Supports

3 commercially available supports based on encapsulation of magnetite in porous cross-linked agarose

Manufacturers have launched improved materials functionalised with metal chelating agents for immobilised metal affinity adsorption or Protein A for the purification of antibodies

Encapsulation:

Preparation of Beaded Agarose Media

Agarose dissolves in boiling water and forms strong, highly macroporoud gels on cooling

Emulsification of aqueous molten agarose solution

• for magnetic versions of beaded agarose media, magnetic particles are added to molten agarose

• for expanded bed supports use larger quartz, glass, steel, alloy particles

Stirring conditions determine particle size distribution

Encapsulation:

Agitation

Agitation disprses the solution, forming beaded droplets, whose size is determined by the agitation conditions

Solidified adsorbents are subsequently formed by polymerisation

The porosity of the resulting supports is influenced by the choice of monomer, co-monomer and polymerisation conditions
Most commercial magnetic materials made in this way behave as non-porous supports (types IV and V)

Encapsulation:

Types IV and V

Polystyrene based magnetic supports

e.g. styrene and divinylbenzene form a highly cross-linked polymer network

Encapsulation:

‘Water-in-Oil’ Emulsion

‘Water-in-oil’ emulsion techniques have been used to fabricate a great many micron and sub-micron sized magnetic supports encased in various synthetic, natural, and hybrid matrices

Techniques permit the rapid production of primary support particles of types II - V and many appear amenable to low-cost, large-scale manufacturing

Estapor:

IV, (M) V, (EM) V

Estapor IV

• extensive range of microspheres based on the Estapor (M) bead

Estapor (M) V

• superparamagnetic crystals uniformly distributed in impervious polystyrene bead

Estapor (EM) V

• superparamagnetic core particle encased in impervious polystyrene

Poly(styrene-co-divinylbenzene) Media:

Based Architecture

Phase separation during polymerisation gives structure resembling a partially fused network of nanoparticles

In most cases, most magnetic versions of beaded polystyrene are made simply by incorporating fine magnetic particles in the pre-polymerisation mix

Poly(styrene-co-divinylbenzene) Media:

Effect of Porogenic Solvent on Pore Size

Low solvency porogen, e.g. higher alcohol

High solvency porogen, e.g. aromatic hydrocarbon

Poly(styrene-co-divinylbenzene) Media:

Agarose vs poly(styrene-co-divinylbenzene) Media

Agarose:

• porosity 90-98%

• medium rigidity

• no brittleness

• hydrophilic and neutral

• 10-200 μm

PS-DVB

• porosity up to 70%

• high rigidity

• some brittleness

• hydrophobic

  • needs surface modification

• 3-30 μm

Poly(styrene-co-divinylbenzene) Media:

General Surface Modification Strategy

Manufacturers modify the surfaces of synthetic hydrophobic polymer based beads in order to make them useful for bioseparation

Create reactive handle, e.g. hydroxyl groups

Covalently attch hydrophilic coating to ‘handles’

Derivatise with ligands usnig conventional coupling procedures

Poly(styrene-co-divinylbenzene) Media:

‘Loop and Train’ Surface Modification

The alternative strategy to covalent coupling

An amphilic coating consisting of alternating blocks of hydrophilic segments (loops) and strongly hydrophobic segments (trains) is applied to the polystyrene/divinyl benzene copolymer base matrix to generate a hydrophilic exterior coat which can be cross-linked in place (at its terminii) and functional with ligands

Encapsulation:

Inorganic Support Materials - Advantages (x3)

High mechanical strength

Thermal stability

Resistance to solvent and microbial attack

Encapsulation:

Inorganic Support Materials

Magnetic sub-particles can also be encapsulated within glass, glass ceramics and other inorganic materials

Encapsulation:

Magnetic Glass Particle (MGP) - Preparation (3 Steps)

1. Mixing a sol consisting of alkoxides of network-forming components (e.g. SiO₂, B₂O₃, Al₂O₃, TiO₂, ZrO₂, GeO₂) and oxides and salts of other components in an alcohol solution, with magnetic sub-particles

2. Spray-drying the mixture to gel the sol layers encasing the magnetic cores

3. Subsequent, high temperature heating in a nitrogen-purged atmosphere yields magnetic cores encased in a substantially pore-free glass

Encapsulation:

Magnetic Porous Glass (MPG) - Preparation

Prepared in a similar way to the manufacture of controlled-pore, glass chromatography adsorbents

1. Iron oxide-rich alkali metal borosilicate glass is melted at extreme temperatures, under oxidising conditions, triggering the formation and growth of minute crystals of magnetic iron oxides within the glassy matrix

2. On reducing the temperature, the glass separates to yield islands of iron-rich crystallites, within a continuous, iron-depleted phase of 2 connected, but chemically distinct vitreous phases phases (alkali metal borate and silicate)

3. After cooling, the glass is then crushed and ball-milled to yield small particles prior to treating with an appropriate mineral acid

Staged Templated Suspension Polymerisation

Small non-porous uniformly sized particles of linear polymers are readily obtained by emulsion polymerisation

The small spheres are used as ‘shape templates’ in processes that not only increase their size many fold, but also introduce pores

A stabilised aqueous disperion of micrometer-size uniform particles is swollen to the target size of the final macroporous product with a combination of solvent, monomers, and cross-linking agents

The swollen particles are then polymerised

The uniformity is retained, but their size is increased considerably, and the presence of solvent during polymerisation leads to desired macroporous structure

In practice, the equilibrium between interfacial energy and swelling forces considerably limits the extent of swelling of the original template

The size of the final bead is no longer determined by the stirring conditions, but by the swelling of the shape-template particles

Infiltration and Coating:

4 Steps

Step 1

• small, smooth-surfaced, monodisperse, macroporous, spherical polymer particles (produced in a classical, multistage, ‘activated swelling’ process, employed in the manufacture of high resolution chromatography adsorbents) are covalently modified with oxidative groups, e.g. -NO₂, -ONO₂

Step 2

• these particles are suspended in an aqueous solution of Fe²⁺ salts

• under appropriate conditions (alkaline pH), Fe(OH)₂ is continuously transported into the pores where it’s oxidised to form iron oxyhydroxide intermediate

Step 3

• on subsequent heating, the iron oxyhydroxide intermediates develop into small crystals of magnetite and/or maghemite, uniformly dispersed throughout the particle

Step 4

• the magnetised supports sre then treated with epoxy compounds or synthetic polymers, which fill most of the pores and coat the exterior bead surface to yield an essentially non-porous support (design XI) with low surface area

Infiltration and Coating:

Step 4 Important Features (x2)

1. Protecting the magnetic elements against possible corrosion, which would result in loss of magnetic properties and possible poisoning of biological molecules by the leakage of soluble Fe³⁺ ions

2. Affording convenient means for chemically attaching appropriate ligands to the exterior surface

Coating:

4 Different Approaches

1. Polymer adsorption

2. Silanisation

3. Graft polymerisation

4. Co-precipitation

Coating:

Polymer Adsorption

Cheap, water-soluble polysaccharide, or synthetic polymers, most often polyamines, such as polyethyleneimine or polyvinyl amine, can be electrostatically adsorbed onto the charged surface of magnetic crystals or particles (design VII) to provide coatings that are easily functionalised

Coating:

Silane Coupling Agents

Silane coupling agents are frequently employed to introduce reactive organofunctionalities onto the surface of inorganic materials such as glass, glass ceramics, silica, metals and metal oxides and, when applied to magnetic supports, the desired ligand is then reacted with the introduced organofunctional moiety (most often a primary amine, epoxy or vinyl groups) by standard methods

Silane Coupling Agents:

Coupling Mechanism

The coupling mechanism or organofunctional silanes depends on a stable link between the organofunctional group (Y) and hydrolysable groups (X) in compounds with the structure X₃SiRY

Silane Coupling Agents:

Organofunctional Groups, Y

The organofunctional groups (Y) are chosen for reactivity or compatibility with the organic species

Silane Coupling Agents:

Hydrolysable Groups, X

The hydrolysable groups (X) are merely intermediates in formation of silanol groups for bonding to mineral surfaces

Coating:

Inducing a Polymer Graft (+ 2 steps)

Inducing a polymer graft to grow at the surface of a magnetic particles is an elegant and relatively inexpensive, 2-step approach to manufacturing stably-coated, non-porous magnetic supports:

1. silane coupling agents, bearing appropriate reactive groups (e.g. vinyl or primary amines) can be employed to prime the particle surface (type VI)

2. using suitable monomers, it is possible to grow thin, tightly-bound synthetic polymer coats at the primed particle surfaces resulting in type VIII supports

Manufacture of Type VIII Magnetic Base Supports:

Factors Affecting Crystal Size and Magnetic Properties (x4)

Rate of addition and mixing

Reactant concentrations

Temperature

Ratio of [Fe²⁺] to [Fe³⁺]

Holding Hydrolysed HOSi Groups at Interface:

Silane Coupling Agents Oxane Bonds

Though oxane bonds of silane coupling agents to some mineral surfaces are easily hydrolysed, it is possible to obtain water-resistant composites of equilibrium conditions can be maintained between a silane-modified polymer and the mineral surface

Holding Hydrolysed HOSi Groups at Interface:

Silane Coupling Agents Equilibrium

Equilibrium conditions are lost if silanol groups are physically removed from the interface

Retention of equilibrium conditions is favoured by proper control of the morphology of the silane-modified coat at the mineral interface

Holding Hydrolysed HOSi Groups at Interface:

Correlated Performance of a Composite with Material Structure at the Interface (x4)

Monomers and oils

Tarry polymers

Flexible or rubbery portions of a polymer

Silanols formed at a rigid interface

Holding Hydrolysed HOSi Groups at Interface:

Correlated Performance of a Composite with Material Structure at the Interface - Monomers and Oils

May be removed physically from the interface as individual links to the surface hydrolysis, thus they are lost for rebonding

Holding Hydrolysed HOSi Groups at Interface:

Correlated Performance of a Composite with Material Structure at the Interface - Tarry Polymers

With viscous flow are self-healing

As silanol bonds are formed at the interface, they flow to an active site and reform oxane bonds

Holding Hydrolysed HOSi Groups at Interface:

Correlated Performance of a Composite with Material Structure at the Interface - Flexible or Rubbery Portions of a Polymer

Retract from the interface as silanol bonds are released through hydrolysis

Water then intervenes and pushes the polymer farther from the surface until complete loss of adhesion results

Holding Hydrolysed HOSi Groups at Interface:

Correlated Performance of a Composite with Material Structure at the Interface - Silanols Formed at a Rigid Interface

Cannot move beyond the dimensions of molecule segments, and so are in a position to reform bonds with the original or adjacent active sites on the surface

Idealised Requirement of a Magnetic Adsorbent for Process Scale HGMF (x3)

Magnetic characteristics requirements

Size, shape, density, uniformity, and stability

Surface architecture and chemistry