1
Q
What is the difference between PEG and PEO? $$
A
- Low molecular weight polymers, 200 to 20,000 average molecular weight, are poly(ethylene glycols) (PEG) polymerized using base catalysis
- Poly(ethylene oxide)s (PEO) have molecular weights between 100,000 and 5,000,000, and are free-flowing white powders
2
Q
What does the 400 represents in PEG 400? $$
A
9 repeting units of PEG:
PEG MW = 44gr/mol
400/44 = 9 units of PEG
3
Q
Types of biosurfactants ($$test)
A
Fatty acids
Glycolipids
Lipopeptides, Lipoproteins
Polymeric surfactants
Phospholipids
Particulate biosurfactants
4
Q
The main advantages of biosurfactants $$
A
- lower toxicity,
- biodegradable nature, and
- effectiveness at low as well as high temperatures.
5
Q
Froces in self-assembly $$
A
Attractive Driving Force: Brings Self-Assembly building units together. (Hydrophobic tails)
Repulsive Opposition Force: Balances Self-Assembly building units at a certain point. (hydorphilic heads)
Directional/functional Force: Guides the direction of self-assembly/ Provides functionality (such as Magnetic fields, geometry, gravity )
6
Q
Case A: Describe the self-assembly when only arractive driving and repulsive opposition forces are present, $$
A
- self-assembly is a random and usually one-step process.
- aggregates show nonhierarchical structure.
7
Q
Case B: Describe the self-assembly when third class of force is involved $$
A
- the self-assembly processes are now directional
- in many cases they occur as multi-stepwise processes
- show hierarchical structure.
8
Q
Is self-assembly always a single step process? $$
A
- Self-assembly is not always a single-step process;
- it can occur in a double-, triple-, and multi-step patern
- show hierarchical structure.
9
Q
Force balance: $$
A
Total net potential can be described as the net total of all of the arractive and repulsive potentials Involved in each step of the self-assembly as follows:
Utotal (x) = fp | UA,P (x)+UR,P (x) | + fS ⋅| UA,S (x)+UR,S (x) | + fT ⋅| UA,T (x)+UR,T (x) | +Uext (x)
£ fp + fs + f3 + … = 1 ==> Are the fractional coefficients of the contribution of the net potential of each self-assembly step to the total net potential
fp | UA,P (x)+UR,P (x) | = Arractive & repulsive potentals for self-assembly of **primary building units **
fS ⋅| UA,S (x)+UR,S (x) | = Arractive & repulsive potentals for self-assembly of **secondary building units **
fT ⋅| UA,T (x)+UR,T (x) | = Arractive & repulsive potentals for self-assembly of **tertiary building units **
10
Q
Self-assembly is the force balance process between three classes of forces:
A
- Attractive Driving Force: Brings Self-Assembly building units together.
- Repulsive Opposition Force: Balances Self-Assembly building units at a certain point.
- Directional/functional Force: Guides the direction of self-assembly/ Provides functionality
11
Q
Force Balance Approach for Sufactant Micelles
A
Attractive Driving Force: Brings Self-Assembly building units together. (Driving force:** hydrophobic arraction** )
<====>
Repulsive Opposition Force: Balances Self-Assembly building units at a certain point. (
Opposition force electrostatic repulsion and/or solvation force, Steric, hydration and electric double-layer)
12
Q
self-assembly is a _______ and usually ________ process.
- aggregates show ______________ structure.
it can occur in a __________, _______________, _________-_______ patern
A
random
one-step
nonhierarchical
double-, triple-, and multi-step
13
Q
Force Balance Approach for Colloids
A
Attractive Driving Force: Brings Self-Assembly building units together (Van der Waals force)
<=====>
Repulsive Opposition Force: Balances Self-Assembly building units at a certain point. (Electric double-layer)
14
Q
Case A: Describe sefl-assembly when onlly attractive driving and repulsive opposition forces are present:
A
- Sefl-assembly is a random and usually one-step process
- Aggregates show non-heriarchical structure
15
Q
Formation of surfactant or polymer mesophases (i.e. liquid crystals) When:
- fp > fs
- fp = fs
- fp < fs
A
- fp > fs the primary self-aggregate will be dominate over the secondary aggregate
- fp = fs coexistance of primary self-aggregate and secondary aggregate
- fp < fs favors assembly proceeding to the secondary aggregate
16
Q
Total net potential in self-assembly can be described as:
A
the net total of all of the arractive and repulsive potentials Involved in each step of the self-assembly:
Utotal = |Arractive & repulsive potentials for self-assembly of primary building units| + |Arractive & repulsive potentials for self-assembly of secondary building units | + |Arractive & repulsive potentials for self-assembly of tertiary building units|
17
Q
Type I: Self-Assembly and examples $$
A
occurs through only the primary self-assembly step (fp = 1)
- The interplay between the arractive and repulsive forces between the primary building units solely determines the self-assembly process.
- Examples:
- micelle forma^on of surfactants or amphiphilic polymers at low concentration
- Formation of vesicles or microemulsions
- Stable colloidal suspensions
18
Q
Type II: Self-Assembly and examples $$
A
Assembly occurs through both primary and secondary self-assembly steps (fp +fs= 1)
- Examples: FormaTion of surfactant or polymer mesophases (i.e. liquid crystals)
19
Q
Type III: Self-Assembly and examples $$
A
Assembly occurs through primary, secondary, and tertiary and above steps ∑fp + fS + fT +⋅⋅⋅=1
- Examples: quaternary formation of proteins.
20
Q
Type IV: Self-Assembly and examples $$
A
Type IV: Assembly involving external forces and primary, secondary, and or tertiary steps
external forces = magnetic force, electric force, flow stress, capillary force, gravity, substrate interactions
21
Q
When Utotal >0 Uext(X)= 0
The self-assembly is
A
Not possible
22
Q
When Utotal (X) < 0 Uext (X) = 0
The self-assembly is ___________ driven $$
A
kinetically driven.
- self-assembly continues until most of the building units are exhausted - self-aggregates have indefinite sizes and less-defined shapes.
- Examples: coagulated colloidal precipitates, bilayers, gels, macroemulsions
23
Q
When Utotal (X) = 0 with Uext (X) = 0
The self-assembly is ___________ driven $$
A
Thermodinamically
- the building units are in equilibrium with the self-aggregates
- self-aggregates have finite sizes and defined shapes.
- Examples: polymer or surfactant micelles, vesicles, microemulsions
24
Q
Forces involved in Self-Assembly
A
Weak and long-ragne forces play an important role in self-assembly
25
Attractive Forces
1. Van der Waals
2. Solvation
3. Depletion
4. Bridging
5. Hydrophobic
6. π-π Stacking
7. Hydrogen bond
8. Coordination Bond
26
Repulsive Force
1. Electric double-layer
2. Solvation
3. Hydration
4. Steric
27
**Van der Waals Forces** $$
Types of Van der Waals
are **transient, weak electrical attraction** of one atom for another. They **originate from dipole or induced-dipole interactions** at the atomic and molecular level.
1. Keesom: permanent dipole-permanent dipole interaction
2. Deby: permanent dipole-induced dipole interaction
3. London: (or dispersion) interaction: induced dipole-induced dipole interaction
28
* The combined expression of van der Waals attraction and the repulsive force are called: $$
* What is the energy level of van der Waals interactions?
* Van der Waals interactions are always ______ at atomic and molecular levels.
Under certain conditions they can be ________ between colloidal objects.
* the **12-6** power law or the **Leonard-Jones potential. **
* **0.4 - 4 Kj/mol**
* **attractive, repulsive**
29
What type of interactions Van der Waals are? $$$
Van der Waals interactions are always **attractive at atomic and molecular levels.**
Under **certain conditions** they can be **repulsive between colloidal objects. **
30
In water most surfaces are electrically charged, due a number of different mechanisms:
1) Adsorption of an ionic surfactant from solution
2) Surface ionization, due to surface acid-base reactions, e.g. silica in a pH range
SiOH → SiO- + H+
At neutral pH most oxides have negatively charged surfaces.
3) Differential solubility of cation and anion in an insoluble salt
31
**3 importan micelle parmeters: $$**
1. Critical micelle Concentration (cmc)
2. Aggregation number (n)
3. degree of counterion binding on the surface of micelles (α)
32
Van der Waals equation of repulsion and attraction
* The **atractive** interaction energy is proportional to the **sixth power** of the distance while
* The **repulsive** interaction energy is inversely proportional to the **twelth power** of the distance
EVan der Waals = ER + EA = (A/r12) - (B/r6)
33
Critical micelle concentration (CMC) ? $$$
The monomer concentration where the **first micelle begins to appear** is defined as the critical micelle concentration (cmc).
Since pre-micellar aggregates are formed in many cases, this point for the cmc **is not always clear** cut.
34
Surfactant concetration \< CMC $$$
Individual surfactant molecules in solution **No micelles**
35
Surfactant concetration \> CMC $$$
surfactant molecules associate to **form micelles**
36
What are the typical ranges for cmc? $$$
Typical cmc ranges are from 10−5 to 10−2 mole/liter for most of the single - chain surfactants and the amphiphilic polymers, such as the Pluronic series.
37
Typica Diameter of micelles
2-20 nm
38
Forces involved in self-Assembly
39
Electrostatic Force: Electric Double layer: $$
* The electrical double layer (EDL) is the result of t**he variation of electric potential** near a **surface **
40
Describe the movement of ions during electric ion layer formation. $$$$
1. negatively charged ions from solution will be attracted to the positively charged surface
2. - While the counterions (anions) will be strongly concentrated near the surface the co-ins (cations) will deplete the vicinity of that surface.
3. - At a distance farther from the surface, positively charged ions will build up
4. - Far away from the surface both concentrations approach the bulk concentration
The electric double layer is made up of the Stern layer, where cations are adsorbthe surface and the **diffuse layer**, **where the number of counterions exceeds the number of anions**
At pH above the isoelectric point, the cations are adsorbed within the Stern layer; there is an excess of cations in the diffuse layer.
41
Why is the double layer formed? $$$
The double layer is formed in order **to neutralize** the **charged surface** and, in turn, **causes** an **electrokinetic potential between** the **surface** and any point in the mass of **the suspending liquid. **
42
Electric Double Layer between 2 surfaces with the same type of charge. $$
When two surfaces with the same type of charge are approaching together, the **first contact** area is the **outermost region** **of** the **diffuse layers.**
As two surface are coming closer, **the diffuse layers (the potentials)** from each surface come to **overlap**. This generates **excess pressure** that originates the repulsive force between the two surfaces**: electrostatic double - layer repulsion.**
The **potential in the diffuse layer is** mainly **changed by**
1. the geometry of the **surface**, and
2. the **concentration** and **type of electrolyte.**
43
DLVO theory
(Derjaguin, Landau, Vervey, and Overbeek (DLVO)
The **total force acting on the colloidal objects** in solution is the sum of the **electrostatic Double layer force** and the **van der Waals force.**
**U(x) = Electric double layer forces - Van der Waals Forces**
One forces has to be cancelled to allow self-assembly.
44
1) What **induces Steric Forces** between two surfaces? $$
2) What type of range has **Long or short**? $$
3) What **3 factors affect** steric forces between **polymer-coated** surfaces?
4) What types of **interactions/parameters** can affect Steric and Depletion Forces?
5) When **Steric repulsive** force arise?
6) When **Attractive forces** arise?
**1) Steric (or overlap**) force is mainly induced by
* polymers,
* polyelectrolytes, or
* biomacromolecules that are **adsorbed or grafted** onto the **surface of colloidal** objects.
2) **Long** range force whose interaction can reach up **to ∼ 10\* Rg** (where Rg is the radius of the gyration of the polymer chain) in aqueous solution.
3) 3 factors affect steric forces between polymer-coated surfaces: (1) **Coverage of polymer on each surface, **(2) **Reversible adsorption** or 3) **irreversible grafting** onto the surface,
4) What types of interactions/parameters can affect Steric and Depletion Forces:
Chemical/ physical conditions such as:
1. solvent (good or poor in a given polymer)
2. temperature
3. nature of charge (especially in the case of polyelectrolytes)
() Interactions
1. polymer – polymer interaction
2. polymer – solvent interaction
3. polymer–colloid interaction
5) **Steric repulsive force arise when**:
1. Solvent in the system is a **good solvent to the polymer.**
2. The **number density of the polymer chains per unit area** of the surface is in the range where the interaction between the polymer chains restricts the molecular motion or orientational freedom.
3. **The elasticity of polymer coils** is _large enough to oppose the compression_ by the approaching of two surfaces.
6) **Attractive forces arise when**:
1. **Solvent is a poor** solvent..
2. The **polymer** chain has **enough affinity** with the **surfaces.**
3. When the colloidal particles are interacting in the solution with nonadsorbing polymer, **attractive depletion interaction** can occur.
45
Attractive forces arise when:
1. Solvent is a poor solvent..
2. The polymer chain has enough affinity with the surfaces.
3. When the colloidal objects are interacting in the solution with nonadsorbing polymer, attractive depletion interaction can occur.
46
What types of interactions/parameters can affect Steric and Depletion Forces:
Chemical/ physical conditions such as:
1. solvent (good or poor in a given polymer)
2. temperature
3. nature of charge (especially in the case of polyelectrolytes)
Interactions
1. polymer – polymer interaction
2. polymer – solvent interaction
3. polymer–colloid interaction
47
Bridging Flocculation
When a high molecular weight (i.e. very long chain) polymer is present in a very Small amount (parts per million, ppm), and adsorbs onto the colloidal particles.
Parts of the polymer may adsorb onto different colloidal particles and draw them together.
1. One theory suggests that the polymer attaches to two particles simultaneously by both ends
2. Another theory assumes that the polymer attaches at several points leaving loops projecting which attach to other particles
48
Depletion Flocculation
**The polymer is not adsorbed to the particle, but remains free in solution.**
## Footnote
polymer chains are distributed event between colloidal particles when the distance between the colloids is large enough.
When the **distance between the approaching colloids becomes smaller,** **polymer** chains in the intercolloidal region are **squeezed out,** thus **depleting this region.**
When individual polymers or nanoparticles cannot enter the space between the two surfaces, they tend to ‘‘deplete’’ between the colloidal surfaces to gain more conformational conformational entropy, which results in an **osmotic pressure imbalance (P) inside and outside the gap and leads to a depletion attraction between the two surfaces.**
49
Strength of depletion force is mainly dependent on: $$$
- concentration of polymers
- molecular weight of polymers.
50
Why solvation forces are generated when two surfaces are brought close together?
When **liquid molecules** are confined between two surfaces, they tend to achieve a higher ordering. The **increase** of this **order** with the **decrease** in **surface separation** generates the solvation force. **Liquid molecules (up order) + Surface separation (down) --\> solvation forces**
Solvation forces can have an attractive, repulsive or oscillatory nature, becoming dominant at short range.
51
Solvation force depends on:
* the **physical factors** both **of solvent** molecule and **of colloidal** surface.
1. From the **solvent** side: it includes **shape, size, and polarity of solvent** molecule.
2. From the **surface** side, it includes surface properties such as **hydrophobicity, hydrophilicity, surface roughness, and surface homogeneity. **
52
Describe solvation force and equation.
**Solvation repulsion** is a short - range force that operates **below ∼ 3 nm** of separation between the surfaces
Fsolvation(D) = Ke-D/L
D = distance between surfaces
L = correlation length of the orientation ordering of the molecules
* *K \> 0** related to **hydrophilic repulsion** forces
* *K \< 0 hydrophobic attraction** forces
53
why **Hydrophobic** molecules aggregate in aqueous solution?
1. Nonpolar molecules tend to aggregate in water causing the phenomenon the **hydrophobic effect.** Because **water** molecules **cannot form hydrogen bonds** **_with_ nonpolar** substances they **form "cages" of rigid H-bonded pentagons and hexagons _around nonpolar molecules_**. This is **energetically favorable** because it **increases entropy **of the water population.
54
Hydrogen bonding:
1) Donor groups,
2) Accpetor groups
An interaction between a **covalently bonded hydrogen** atom **o_n a donor group_** and a pair of **non-bonded electrons** **on an _acceptor group._**
1) Donor: N-H, O-H
2) Acceptor: O, S, N
55
Important Noncovalent Interactions & their bond energies
* van der Waals interactions:
* Hydrogen bonds:
* Ionic bonds:
* Hydrophobic interactions:
* van der Waals interactions: 0.4-4.0 kJ/mole
* Hydrogen bonds: 12-30 kJ/mole
* Ionic bonds: 20 kJ/mole
* Hydrophobic interactions: \<40 kJ/mole
56
The main attractive driving force for micelle formation is? $$
hydrophobic interactions
57
The aggregation number (N)
1. N \< CMC
2. N = CMC
3. N \> CMC
4. The value of N depends on
is the **number of surfactant** **or polymer** molecules **within a micelle** once the critcal micelle concentration **(CMC)** has been reached.
1. N \< M. Too small micelles have too large area per head group.
2. N=M
3. N \> M. Too large micelles have head groups too closely packed together. Difficulties also in packing hydrocarbons chains. Some head groups forced inside the hydrophobic core.
4. the **type of surfactant**, **temperature** and **electrolyte concentration**
58
Sketch describe how the monomer and micelle concentration change vs. the total amphiphile
At **low surfactant** (amphiphile) concentration there is **only monomer,** and the concentration of **monomer** in solution **increases.**
The amount of the monomers adsorbed at the air – liquid interface and liquid – solid interface (for some of the systems) typically ranges below **10− 8 mole/liter.**
**At the cmc micelles begin to form.** Since the monomers and micelles are at equilibrium, the additonal amount of surfactants forms the micelles after the cmc .
Addition of more monomer above the cmc makes the **micelle concentration increase linearly** with the total concentration, while the monomer concentration in solution remains almost constant.
The concentration of micelles is corrected at the point where the **spherically shaped micelles** are **transformed into higher - order structures** such as **rod** - shape or **wormlike** micelles.
59
Counterion Binding and ratio. $$$
When an ionic micelle is formed, its surface becomes either ca,onic or anionic, depending on the nature of surfactant molecules.
**Some** of the **counterions are free** in the solution, but **some are bound on** the surface of the **micelle**.
The _ratio of counterion bound on the micelle surface to the whole concentration of counterion in the system_ is defined as the **degree of counterion binding ( α ).**
**α ranges 0.2 – 0.8.**
1 − α , which is usually defined as β , is the degree of counterion dissociation.
60
Thermodynamics of Micelle Formation
Mass-Action Model - This approach treats micelle formation as a reaction of *n *(aggregation number) monomers to form a micelle.
nD \<---k---\> Dn
ΔG = RT ln(CMC)
61
What is a covalent bond?
• **Sharing of electrons** (in outer atomic orbitals)
A covalent bond involves the sharing of electron pairs **between two different atom**s (with each bonding electron derived from its respective atom).
* Holds atoms together in molecules
* Stable : take large energy to break
62
What is an ionic bond?
Ionic bond = **electrons transferred **
Attraction between oppositely charged ions (positively and negatively charged ions)
63
What is an coordinate bond?
A coordinate bond (also called a dative covalent bond) is a covalent bond (**a shared pair of electrons)** in which **both electrons come from the same atom **
Self-Assembly
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