Monolayers. Factors affecting the adsorption from solution. Adsorption of amphiphilic molecules on solid support

Transcription

1 Monolayers Adsorption as process Adsorption of gases on solids Adsorption of solutions on solids Factors affecting the adsorption from solution Adsorption of amphiphilic molecules on solid support Adsorption of polymers on solid support Monolayers: two dimensional self-organized systems Langmuir-Blodgett technique

2 Adsorption as process Gibbs equation applied to all interfaces πa = σ ni RigT = k B T π γ = 0 γ Can be used to determine the molecular weight of proteins Low concentration: γ = γ = 0 bc lim π 0 ( π A) = Model for ideal gas films 2dimensional analogue of ideal gas model R ig M T (1) Protein films gazeous at low compression ( < 1 mn/m) (1) can be used

3 Adsorption as process M obtained from surface tension experiment fit the ones obtained in bulk Differences dissociation of protein into units. Application of Gibbs eq: surface tension = f(conc) determination of adsorbtion (mole/area) calculation of the molecular area ( A molecule -1 ) from γ = f(conc) A = N A Γ Γ i = 2 dγ 2R T d ln c ig i calculation of 2-dimensional adsorption equation of state of films

4 Adsorption of gases on solids 2 way of adsorbtion physical & chemical Langmuir equation (gas adsorption) adsorption rate f (conc) adsorption rate f (fraction of non-covered area of adsorbent) desorption rate f (fraction of free non-covered area of adsorbent)

5 Adsorption of gases on solids Equilibrum adsorption rate = desorption rate Adsorption surfactant / amphiphile molecules on solid surfaces solubilisation detergency flotation Gibbs isotherm not good!!!

6 Adsorption of gases on solids Simplifications of Langmuir adsorbtion theory Useful adsorbtion at low p and hight T ideal surface clean, homogenous, non-porous active centers on surface can be occupied by only 1 adsorbed molecule no interactions between adsorbed molecules all centers: equivalent same heat of adsorbtion no solute solvent interactions solute & solvent same size dilute solutions monolayer coverage

7 Adsorbtion isotherm Langmuir theory for gas solid adsorbtion p B p B V V m + = = 1 θ m V m B V p V p + = 1 p straight line ( ) p f V p = B V m 1 V m 1 V p p

8 Langmuir theory for gas solid adsorbtion Adsorbtion behaviour Adsorbtion when p Adsorbtion when T

9 Langmuir theory for gas solid adsorbtion Langmuir plots Adsorbtion exothermic S adsorption < 0 molecules confined in 2-dim (monolayer) more sophisticate Brunauer-Emmett-Teller

10 Adsorption of solutions on solids Langmuir theory for solution-solid adsorption Adsorption of compounds from solutions θ = Γ Γ max surface sites occupied by solute / total number of active centers k c θ = 1 + k c Previous PC lectures Γ = Γmax k c 1+ k c c Γ = c Γ max + k 1 Γ max c = Γ f ( c) straight line

11 Langmuir theory for solution - solid adsorption c = Γ f ( c) straight line 1 Γ max 1 k Γ max Specific surface area of the solid A spec = Γ N max A A 0 Gibbs energy of adsorption G ads = R ig T ln ( k )

12 Langmuir theory for solution solid adsorbtion Langmuir adsorbtion isotherm k c θ = 1 + k c high solute/adsorbant conc (c ) or high surface affinity (k ) kc >> 1 θ 1 the surface is saturated low solute/adsorbant conc (c ) or low surface affinity (k ) kc << 1 θ kc surface sites occupied is proportional with c

13 Adsorption of gases/solutions on solids Adsorbtion interrelations:

14 Factors affecting the adsorption from solution Dilute solutions Role of solvent Competition solute - solvent Polar compound polar surface (solution of non-polar solvent) Interaction: solvent alkyl chains non-polar solvent + (acids) longer chains more interaction adsorption

15 Factors affecting the adsorption from solution Polar compound non-polar surface (solution of polar solvent) Decrease of solubility with the solvent Increase interaction with the surface adsorption polar compounds chain length solvent compete for sites (on solid substrate) solvent compete with surface (for solute molecules) Not significant constant

16 Factors affecting the adsorption from solution Concentrated solutions Role of solvent Competition for active sites Concentration of the solvent ct Common behaviour adsorption of solutions on solids composite isotherm

17 Factors affecting the adsorption from solution composite isotherm Relative conc adsorbat (solute) at the surface c surf c sol x Theoretical models complicated Competition: o Solute solvent o Components of the mixtures

18 Adsorption of amphiphilic molecules on solids Adsorption of surfactants adsorption = f (CPP) CPP Adsorption on hydrophobic surfaces

19 Adsorption of amphiphilic molecules on solids Adsorption on hydrophobic surfaces Hydrophobic chain T Hydrophilic length Addition salts Addition surfactants of opposite charges Adsorption on non-polar surfaces surfactants cylinder shape

20 Adsorption of amphiphilic molecules on solids Adsorption on polar surfaces interaction hydrophilic head surface head group towards the surface 2nd layer adsorption hydrophobic chain hydrophobic chain Interaction of the head group surface < hydrophilic interactions Micelles at the surface Polarity of the surface area occupied by surfactants

21 Adsorption of amphiphilic molecules on solids Adsorption of 2 surfactants Competitive adsorption Hydrophobic non-ionic surfactants displace polymers from hydrophobic surfaces Ionic surfactants & polymers synergy co-adsorp on polar surfaces Adsorption on polymer surface : CPP adsorption hydrophobicity of the surface adsorption

22 Adsorption of amphiphilic molecules on solids adsorption = f (CPP) CPP hydrophobic surfactant Adsorption on hydrophobic surfaces adsorption of non-ionic surfactants = f (ph) Langmuir equation fit adsorption isotherms

23 Adsorption of polymers on solid support A. Adsorption of polymers to stabilize colloidal dispersions polymers adsorbed on the surface of colloidal particles will prevent their aggregation Steric repulsion between colloidal particles with polymers adsorbed onto the surface o Polymers can change their conformation depending on the solvent o Example: PEG is used to stabilize emulsions

24 Adsorption of polymers on solid support B. Adsorption of polymers on solid support depends on properties of the polymer and the type of the solvent. 1. Role of the polymer on the adsorption efficiency M W (pol) adsorption on solid support Example: typical Langmuir isotherms for polymers Low polymer concentrations: sharp adsorption Slow process of adsorption for polymers (hours-days)

25 Adsorption of polymers on solid support Factors affecting the adsorption of polymers on solid support: Adsorption when time Presence of electrolytes (changes of ph) Presence of additives Presence of multi-valence ions and changes in the ionic strength 2. Role of the solvent on the polymer on the adsorption efficiency: induces changes in the conformation of the polymer range of the steric repulsion Good solvent Poor solvent - Adsorption of polymers in poor solvents - Adsorption of polymers when salts and heavy alcohols are added

26 Adsorption of polymers on solid support 3. Role of the polymer conformation on the adsorption efficiency: polymers adsorb in different manners on a surface: - with the tails - with the loops - with the loops, tails and trains range of the steric repulsion Good solvent Poor solvent - Adsorption of polymers in poor solvents - Adsorption of polymers when salts and heavy alcohols are added

27 Adsorption of polymers on solid support o Tails determine the thickness of the adsorbed layer Nb R g = 6 2 Nb R g = 12 2 ideal linear chain rod polymer R g radiusofgyration g N numberofmonomers b lengthofamonomer R g = Nb H-polymer o Polymers have in good solvents: g ( ) g R exp R

28 Adsorption of polymers on solid support 4. Lewis acid-lewis base concept for polymer adsorption on solid support o o Acid solvents compete with an acid surface for basic polymers Basic solvents compete with a basic polymer for an acid surface o Maximum polymer adsorption: neutral solvents

29 Monolayers: two dimensional self-organized systems A. Monolayers formed by soluble amphiphiles amphiphilic molecules reorient themselves at the interface of polar solutions Example: SDS at air/water interface µ 2 µ = θ 2 + RigT ln c + RigT ln c + DS Na µ 2 = chemical potential of SDS dγ = Γ dγ d ln c DS d 2 µ 2 = 2R TΓ ig 2 o Surface area/amphiphile molecule, a 0 1 a0 = a Γ 0 = 52Å 2 N A 2

30 Monolayers: two dimensional self-organized systems Example: SDS at air/water interface, excess of electrolyte: d dγ ln c DS = R ig TΓ 2 o Surface area/amphiphile molecule, a 0 in 0.1M NaCl a 0 = 38Å 2 Decrease in the head groups repulsion Important: Equilibrium between the surfactant molecules in solution and the surfactant molecules at the interface Interfacial properties do not vary independently

31 Monolayers: two dimensional self-organized systems B. Monolayers formed by insoluble amphiphiles amphiphiles are disolved in an organic volatile solvent and deposit by droping the solution onto the air-water interface organic solvent evaporates Monolayers Surface pressure to prevent spreading of the film, Π s Π s = γ θ γ θ γ = Surface tension without amphiphiles γ = Surface tension in the presence of amphiphiles Surface pressure at the interface is lower due to the presence of the amphiphiles (pressure of the adsorbed molecules)

32 Langmuir film balance o Langmuir film balance device used to measure 1. Amphiphiles disolved in a volative organic solvent are pippeted onto the water surface. 2. Organic solvent evaporates 3. Surface tension is measured by moving the sweep towards the head Πs 1 = trough 2 = sweep 3 = float 4 = mirror 5 = main torsion wire Fluorescence microscopy Different aspects of the monolayers

33 Fluorescence microscopy: characterisation of monolayers Example: DMPA monolayers containing 1% fluorescent dye (DP-NBD-PE) a 0 solidphase Mixture of solid-like phase and liquid-like phase solid patches sourrounded by liquid environment

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35 Langmuir-Blodgett technique Langmuir-Blodgett technique method to deposit monolayers on solid support Modification of the Langmuir film balance Dipping device to lower or rise the substrate through the monolayer Automated movable barrier, which moves during the deposition process to controll the surface tension Sensor for Π s multilayers Succesive dipping a plate in and out of a liquid covered with a monolayer

36 Langmuir-Blodgett technique Langmuir-Blodgett technique has various types of deposition:

37 Langmuir-Blodgett technique Applications of the Langmuir-Blodgett technique: Micro and macrolythography Insulating films in semiconductors Non-linear optical elements Coating of communication devices Highly selective membranes Limitations: Films have no long-term stability Not all substratesand not all monolayersare suitablefor L-B technique Condition for L-B technique surface pressure should be enough high that the monolayer is a condensed state Π s 20 40mN / m

38 References: G. M. Kontogeorgis, S. Kill, Introduction to applied colloid and surface chemistry, Wiley-VCH, 2016 D. F. Evans, H. Wennerstrom, The colloidal domain, Wiley- VCH, second edition, 2014.

39 Monolayers Adsorption as process Adsorption of gases on solids Adsorption of solutions on solids Factors affecting the adsorption from solution Adsorption of amphiphilic molecules on solid support Adsorption of polymers on solid support Monolayers: two dimensional self-organized systems Langmuir-Blodgett technique