Bionanoscale engineering chapter 01

Bio-nanoscale Engineering Course
M.Sc. in Biochemical Engineering
School of Chemical & Bioengineering
AAiT, Addis Ababa University
Inst.: Shimeles Shumi (PhD), Assis. Professor
Office:N-220, Samsung Bldg
January 2021
1

Bio-nanoscale Engineering
Chapter 0 :Introduction
Nanoscale engineering
• Process of developing novel
technologies and assays at the
nanoscale~1/1000th of human hair
Nanomaterials
• Materials at least with one
dimension in the nanometer scale
https://www.wichlab.com/nanometer-scale-comparison-
nanoparticle-size-comparison-nanotechnology-chart-ruler/

Chapter 0 : Introduction

Chapter 1: Surface and Interface Phenomena
 Surface tension  Interface: the boundary between two or
more phases
 Several types of interface can exist
― Liquid-solid (ls)
― Liquid-vapor (lv)
― Solid-vapor (sv)
― Liquid-liquid (L1L2)
 Do miscible liquids possess interface?
A. Vorobev / Current Opinion in Colloid & Interface
Science 19 (2014) 300–308: miscible
J. Chil. Chem.
Soc. v.53 n.2 Concepción jun. 2008:immiscible

Surface tension phenomena
 In bulk fluid: an isotropic distribution of molecular
forces
 At the interface, a definite force acts towards
either of the two substances
 Net force acts on the water molecules points
towards vapor or water bulk?
 Stronger interactions: vapor-water or water-
water?
 Molecules at the interface: unequal distribution
of energies>>>interfacial energy

 Interfacial energy=surface energy: no of molecules on the surface
 Interfacial energy per surface area is know as surface tension (ɣ)
Surface tension=in SI units [Joules/m2]=[N/m]
 Consider a liquid forming a spherical droplet of radius r floating in a gravity free vacuum
environment (neglect vapor pressure). What would the pressure be inside the droplet?
We can use two approaches to answer this question
a) Thermodynamic approach: the interfacial energy is given by Ei = 4∏r2ɣ
― what does this mean in thermodynamic sense?
―Work, dW = pdV, meaning the work done should be = Ei , i.e., dW = dEi
―Taking the ɣ as a constant, dEi = 8∏rɣdr
―dV= 4∏r2dr, the volume of the sphere

 Thus, the pressure inside the droplet would be
P = dW/dV = dEi/dV = 2ɣ/r
b) Mechanistic approach: consider the balance of
forces occurring on any mid-cross section of the
droplet
 Surface tension force acts inward
on the surface
 It distributed around the perimeter
of the cross section
 Hence the force is 2∏rɣ and is
balanced by the droplet pressure
force (P∏r2)
 Implies that P = 2ɣ/r as before
 If the droplet is floating in air at Pa,
the effect will be P-Pa = 2ɣ/r.
Hence, P-Pa = ɣĸ can be generalized
for any shape.
Where ‘ĸ’ is the curvature of the
surface

Surface interactions with solids
 Liquids contact with solids in most applications
 At the macro scale many of those interactions are inconsequential, but are essential at the
micro scale
 For instance, in the deposition of a small liquid droplet on a clean solid surface, if
― gravitational forces < surface tension forces, then the shape will be semi-spherical
 Contact angle (Ɵ)
― a quantity closely related to surface tension
―specifies equilibrium situations in solid surface contact
―defined as the angle measured in the liquid that is formed at the junction of three
phases

 Consider the three interfaces acting along the contact line with the solid: liquid-solid,
liquid-vapor and vapor-solid interfaces
 For each point on the three-phase line there are three vectors: one for each interface
 It acts perpendicular to the three-phase line and tangential to their corresponding
interface
 The equilibrium relation b/n these vectors is known as Young’s equation (a)
ɣ𝑙𝑣𝑐𝑜𝑠Ɵ + ɣ𝑙𝑠 − ɣ𝑠𝑣 = 0 … … (𝑎)
Ɵ= 𝑐𝑜𝑠
− 1 [
ɣ𝑠𝑣
−ɣ𝑙𝑠
ɣ𝑙𝑣
]
Surface interactions with solids

Wettability and capillary phenomena
 The contact angle (Ɵ) determines the wettability behavior of liquids on solids
―Ɵ = 0° : perfect wetting
―Ɵ = 180° : perfect hydrophobicity , i.e., no wetting at all!
 This has important consequences in many engineering applications
―Surface treatment given to car, or aircraft, windshields….hydrophobic surface
―For painting…good wetting is sought here
 Capillary rise
―Spontaneous flow of liquid to fill small capillaries or pores
 To see the role of wetting properties on solids, consider the following figure
―A small straight tube with open ends and radius R
―Immersed in a fluid with surface tension ɣ and density ρ

 At Ɵ < 90° , the liquid wets the solid tube material
 Thus, there will be a net surface tension force acting upwards Fɣ = 2∏RɣcosƟ
Capillary rise phenomena
 At the static equilibrium, the liquid climbs to a height h and the surface tension force
balances the weight of the liquid column in the tube
 This weight is given by Fg = mg = ρVg = ρ∏R2hg

 While the balance yields an equation for the column height, ℎ =
2ɣ𝑐𝑜𝑠Ɵ
ρ𝑔𝑅
 This equation holds the right dependences of h with R, weakens/ strengths the
capillarity
 There is a pressures difference b/n the top of the column and the bottom. The
bottom coincides in height with the liquid surface level in the open container
 If the gas pressure is Pa, then the liquid pressure in the container is also Pa. This
need to be the pressure at the bottom of the column in the tube too!, they are all at
the same level.
 Let’s apply the hydrostatic equation to this case: P-Pa = ρg (y0-y)

 Taking yo = 0 as the reference level in the liquid container, we see that the pressure at the
top of the column is
P= Pa – ρgh
 This pressure is lower than the atmospheric pressure. Where is the negative P-Pa pressure
coming from?
 Recall from surface tension P-Pa = ɣĸ, so the negative pressure is generated by the curved
surface : ĸ = -ρgh/ɣ
 Hence, we could use directly the hydrostatic equation to find the column height once the
surface curvature is known, h = |ɣĸ/ρg|

Biological applications of surface tension and wetting
 Many biological performances and natural processes demand an understanding of
wetting and interfacial tension
 Different research works show the applications of surface tension in biological and other
sectors
― Most biochemical rxns occur @ the surface & interface
not in solution
― Wetting of substrate_binder and spreading of
binder_substrate determines the performance of
granulation in the tablet formation

 Work of adhesion (Wa) controls the morphology of granule
Ɣ1 : surface free energy of the binder
Ɣ2 : surface free energy of the substrate
Ɣd , ɣp: non-polar and polar contributions of surface free energy
Wa = 4[
ɣ1
𝑑
ɣ2
𝑑
ɣ1
𝑑
+ɣ2
𝑑 +
ɣ1
𝑝
ɣ2
𝑝
ɣ1
𝑑
ɣ2
𝑝]
 Surface tension values of some of the vehicles used in pharmaceutical industry
 Tablet coating via film coating
―If ɣ is too high, wetting process will
be hindered

 Surface tension appears to be a critical factor in the
stabilization of proteins
 Surface tension increases by co-solvent, but does not
ensure increased stabilization
 Correlation of an increase in surface tension and protein
stabilization, transition temperature, properties of co-
solvent are crucial

Surfactant Molecules
 Surfactant is a blend of surface-active agent
―spontaneously bond with each other to form sealed bubbles
―lowers the surface tension/interfacial tension
―act as detergents, wetting agents, emulsifiers, foaming agents or dispersants
 Based on their charge characteristics the surfactant may be
a)Anionic surfactants-soluble in water @ RT & used in pharmaceutical. Used
as a component of emulsifying wax. Example, Sodium Lauryl Sulphate BP- a
mixture of sodium alkyl sulfates, the chief of which is SDS
b)Cationic surfactants-example, quaternary ammonium and pyridinium
cationic surfactants. Used against a wide range of G+ve and some G-ve
bacteria. They are also used for cleaning of wounds and contaminated utensils

c) Non-ionic surfactants-are insoluble in water and used as water -in-oil emulsifiers as
well as wetting agents. Examples, polysorbates and poloxamers
Key points
― Surfactants have two distinct regions in their chemical structure: 1) water liking or hydrophilic 2) water-
hating or hydrophobic
― These molecules are referred as amphiphilic or amphipathic molecules or simply as surfactants or
surface-active agents
DOI: 10.1080/10408436.2013.808985
https://commons.wikimedia.org/wiki/File:Lipid_bilayer_and_micelle.png
Surfactant Molecules

Key applications of Surfactant Molecules
 Some materials do not mix well, results a lots of tension
 Surfactants can reduce such tensions b/n the materials, this helps to stay apart and
facilitates the emulsion process
 Different surfactants work for different materials
 The surfactant positions itself on the molecule stabilizing the emulsion
How to choose a
suitable
surfactant to
reduce a ɣ?

HLB scale showing classification of surfactant function
Hydrophilic-Lipophilic Balance (HLB) Scale  HLB of a surfactant is a measure of the degree
to which it’s hydrophilic or lipophilic
 Determined by calculating values for the
different regions of the molecules which if 1st
described by Griffin in 1949
 Griffin’s method for non-ionic surfactants
described as
HLB = 20*Mh/M
Where Mh : molecular mass of the hydrophilic
portion of the molecule while
M is the molecular mass of the whole molecules.

 An HLB=0, completely lipophilic/hydrophobic molecule
 An HLB=20, completely hydrophilic/lipophobic molecule
 The HLB value can be used to predict the surfactant properties of a molecule
― < 10: lipid-soluble (water-insoluble)
― > 10: water-soluble (lipid-insoluble)
― 1 to 3: anti-foaming agent
― 3 to 6: W/O (water in oil) emulsifier
― 7 to 9: wetting and spreading agent
― 13 to 16: detergent
― 8 to 16: O/W (oil in water) emulsifier
― 16 to 18: solubiliser or hydrotrope

Davies’ method
 A method based on calculating a
value on chemical groups of the
molecule
 It accounts the effect of stronger
and weaker hydrophilic groups
 𝑯𝑳𝑩 = 𝟕 + 𝒊=𝟏
𝒎
𝑯𝒊 − 𝒏 ∗ 𝟎. 𝟒𝟕𝟓
Where :
m-no of hydrophilic groups in the
molecule
Hi –value of the ith hydrophilic
groups (see tables)
n-no of lipophilic groups in the
molecule

Biomolecules as Surfactant
 Biomolecules used as surfactants are commonly called biosurfactant
 Most biosurfactants are either anionic, neutral or cationic (containing amino groups)
 Hydrophobic moiety- has long-chain fatty acids while the hydrophilic moiety can be
carbohydrate, cyclic peptide, amino acid, and phosphate carboxyl acid or alcohol
 Their molar mass generally ranges from 500 to 1500 Da. Biosurfactants can be categorized
by their microbial origin and chemical composition
―Glycolipids : rhamnolipids, sophorolipids, and trehalolipids-mainly by bacteria
―Fatty acids, phospholipids, and neutral lipids-both by bacteria and yeast
―Polymeric biosurfactants: emulsan, lipomanan, alasan,and polysaccharide protein cmplx
―Particulate biosurfactants-forms a microemulsion that influence alkane in microbial cells

Biomolecules as Surfactant

Micelles and 3D structures
 Micelles are assembled colloidal dispersions having a small diameter (5-100nm)
 Its molecules aggregated either by cationic, anionic, zwitterionic or non-ionic groups
 Morphology of
micelles
• spheres
• Rods
• Tubules
• Lamellae
• vesicles
Inverse micelle

Micelles and 3D structures
Critical Micelle Concentration (CMC)
 CMC : is the surfactant concentration at and
above which micelles are formed
 It can be determined for surfactant solutions by
measuring the surface tension at different
concentrations
 Below the CMC , the surface tension decreases
with increasing concentration
 Above the CMC, the surface tension of the
solution is constant due to the concentration
does not change

Adsorption phenomena and Its application
 Adsorption-is a surface phenomena that is characterized by the concentration of
―a chemical species called adsorbate from its vapor
―a solution onto/near the surface/pores of a solid called adsorbent

Tips in graphical: adsorption, absorption, sorption

Physisorption vs. Chemisorption
Types of adsorptions

Adsorption thermodynamics
 When a molecule adsorbs on a surface, it can be either
 activated or non-activated
 direct or precursor mediated (presence of physisorption well)
 molecular or dissociative

Adsorption thermodynamics
 Dissociative adsorption is often activated
 Not all physisorption leads to chemisorption

Adsorption model: Langmuir isotherm
 Isotherm: the amount of adsorbate on the adsorbent as a
function of its pressure (if gas) or concentration (if liquid)
at constant temperature
 Langmuir’s assumptions
1. All the adsorption sites are equivalent, and each site
can only accommodate one molecule.
2. The surface is energetically homogeneous and
adsorbed molecules do not interact.
3. There are no phase transitions.
4. At the maximum adsorption, only a monolayer is
formed. Adsorption only occurs on localized sites on
the surface, not on top of other adsorbates.

Adsorption model: Langmuir isotherm
https://slideplayer.com/slide/13826811/#.YGbGQyj7w8s.gmail

Corrections to Langmuir isotherm
 Lateral interactions
• The behavior of sticking probability vs coverage depends on the strength of the
pairwise interaction energy
 Precursor mediated adsorption
• Precursor diffuses for finite time before finding a vacant site
• Intrinsic precursor: above vacant sites
• Extrinsic precursor: above other adsorbates

Other isotherms
Brunauer-Emmett-Teller (BET)
• is an extension of Langmuir theory
• physical adsorption of gas molecules
• multilayer adsorption
• molecules only interact with adjacent layers
Freundlich
•surface roughness
•adsorbate-adsorbate interactions
•Inhomogeneity
 Kisliuk (precursor-mediated)

Production of high vacuum
Gas masks
Control of humidity
Removal of coloring matter from
solution
Heterogeneous catalysis
Softening of hard water
In curing diseases
Separation of inert gases
De-ionization of water
Cleaning agents
Froth floatation process
Adsorption indicators
Chromatographic analysis
Application of adsorption

Bionanoscale engineering chapter 01

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