8. Surface interactions Effect of polymers

Transcription

1 CHEM-E2150 Interfacial Phenomena in Biobased Systems 8. Surface interactions Effect of polymers Monika Österberg Spring 2017

2 Content 1. Example of the effect of polymers on the surface properties 2. Polymers in solution 1. What determines the solubility and radius of gyration? 3. Theory of polymer (polyelectrolyte) adsorption 4. More examples of effect of polymers a) Flocculation and stabilization 5. Summary of surface interaction forces

3 Learning objectives You are familiar with the main surface forces and their origin You understand how surface interactions can be modified by adsorption of polymers You understand the effect of surface charge, polymer charge and properties of the media (I, ph) on the conformation of adsorbed polymers And how polymer conformation affects interactions

4 Bioinspired lubricating films of cellulose nanofibrils and hyaluronic acid Read Valle-Delgado, JJ, Johansson, L-S, Österberg, M, (2016) Bioinspired lubricating films of cellulose nanofibrils and hyaluronic acid Colloids and Surfaces B: Biointerfaces dx.doi.org/ /j.colsurfb Think about what type of forces are present in the systems What is the effect of electrolyte concentration in the media?

5 HA attached to CNF films by esterification reaction between hydroxyl and carboxyl groups Objective: Durable lubricating layer

6 Atomic force microscope (AFM) and colloid probe technique AFM Measurement of forces between a colloid probe and a substrate using an AFM. 20 μm Glass colloid probe Surface force measurement Friction force measurement

7 What forces are present? Van der Waals Forces? Electrostatic Double- Layer Forces? Steric forces? Fig. 2 PBS, ph ~7 Increase in steric repulsion What is the reason for the steric repulsion? PBS = phosphate buffered saline dx.doi.org/ /j.colsurfb

8 Effect of ionic strength What is the effect of ionic strength on van der Waals attraction? Double-Layer repulsion? Electrosteric repulsion? Hydrated layer, high repulsion Low friction Phosphate buffered saline (PBS):10 mm Na 2 HPO 4, 1.8 mm KH 2 PO 4, 137 mm NaCl, 2.7 mm KCl High I Phosphate buffer (PB): 10 mm Na 2 HPO 4, 1.8 mm KH 2 PO 4 Low I Fig dx.doi.org/ /j.colsurfb

9 Effect of ionic strength II Low I Hydrated fibril and polymer layers Very low friction High I Collapsed polymer layer Higher friction

10 Reflections on the previous example Lubrication was achieved by attachment of polymers Hydrated polymer layer Extended polymer chains (good solvent) Similar approach of surface modification can be used for: Steric stabilization of nanoparticles Antifouling surfaces (grafting of PEG chains) In composites for better alignment of reinforcing fibers Summarize forces

11 Surface forces recap DLVO: F tot = F vdw F DL Hydration force: Additional short range steric repulsion due to strongly bound water molecules. Ca Ionic strength - ø (Surface - potential) - ε 1, n - 1 ε 3, n 3 ε 2, n 2 vdw forces: Always present. Electrostatic Double Layer forces: Present between charged surfaces. ø (Surface potential) Forces due to adsorbed polymers: Attractive bridging force or steric repulsion. Depends on coverage, conformation and interactions between polymer and solvent. Laitoksen nimi 2/7/

12 The forces were affected by the conformation of the polymer In this case the polyelectrolyte was covalently bound Surfaces can also be modified via polymer adsorption We need to understand how polymers adsorb on surfaces We need to understand polymer conformation in solution

13 Polyelectrolytes in solution

14 Polyelectrolytes The term polyelectrolyte is used for polymers consisting of a macromolecule carrying covalently bound anionic or cationic groups, and low-molecular counterions securing for electroneutrality. Polyacids Polybases Ampholytes Most important acid groups: - Carboxylate. -COOH fi -COO H e.g. lignin, hemicellulose, starch, many other polysaccharides - Sulfonate,. -OSO 3 H fi -OSO 3 H e.g. polystyrenesulfonate, lignosulfonate Most important basic groups: - Amines: primary: RNH 2 H fi R-NH 3 secondary: R 2 NH 2 H fi R 2 NH 3 quaternary: R(CH 3 ) 3 N Cl Polyelectrolytes containing both anionic and cationic groups covalently bound, e.g. proteins 14

15 Cationic flocculation polymers Polyacrylamide CH CH 2 CH CH 2 CH Modified (cationised) PAM, C-PAM cationic at all ph:s of practical interest O O O H C H 2 C H C H 2 C H C H 2 C C H H 2 H C C H 2 C H 2 C NH 2 NH 2 NH 2 Neutral in neutral and alkaline, cationic in acid soutions O NH 2 O NH 2 O NH 2 O NH O NH 2 O NH CH 2 CH 2 CH 2 CH CH 2 H 3 C N CH 2 CH CH 2 CH 3 Diacryloethylidimethylammonium chloride (DADMAC) H 2 C N CH 2 C H 2 N H 3 C CH 3 H 3 C CH 3 CH 2 Poly(N,N-dimethyl-3,4-diethyl pyrrolidonium chloride (Poly-DADMAC) CH 2 CH 2 H 3 C CH 3 H 3 C CH 3 N CH 3 CH 2 CH 2 N CH 3 C-PAM is extensively used as a flocculant in papermaking and water treatment 15

16 Characteristics of polyelectrolytes Polyelectrolytes 1. The ionic groups are classified as weakly and strongly acid and basic groups. Different ph dependence 2. The average distance between the adjacent ionic charges along the chain is an important parameter determining the polyelectrolyte behaviour, especially in solution. 3. Location of the charges. In integral type polyelectrolytes the ionic sites are part of the polymer backbone. In pendant type polyelectrolytes the ionic site is attached to the backbone with a spacer. 4. The species of low molecular counterion influences strongly on the solution properties. 16

17 Parameters that describe the electric properties of polyelectrolytes Degree of substitution, DS = fraction of monomers in a homopolymer that are modified by some substitution (for example, by adding an ionic group). the amount of modified monomers DS = degree of polymerization DS=0-100 %, The DS of cationic synthetic polymers vary for end use Charge density, CD: Often given as equivalents/mass (mmol/g or mekv/g) number of dissociated groups Degree of dissociation, α a = total number of ionizable groups α is only important ph is near pka (pka of COOH is ~4.5 so carboxylic groups are charged only when ph is basic ph >7) (pka of NH2 is ~9.5 so the amine groups are only charged when ph is acidic ph <7) 17

18 Polyelectrolyte swelling: the Donnan equilibrium Swollen polyelectrolyte with free counterions All ions except those chemically bound to the polyelectrolyte are freely mobile between the solution in the coils (f) and the external phase (e) Ion distribution is determined by mass equilibrium and electroneutrality conditions (Donnan equilibrium) Both the coil phase and the external phase are electrostatically neutral. Hence, the polyelectrolyte coils contain more freely mobile counterions than co-ions External solution 18 The solution consists of 1) Polyelectrolyte coils that are swollen with an aqueous solution containing small, mobile cations and anions 2) A surrounding solution that contains only small, mobile cations and anions

19 Distribution of ions The polymer coils swell with water until the chemical potentials of water in the coil and in the surrounding solution are equal. At equilibrium, for any ion with charge z i l = distribution coefficient [X] f = l z [X] e Multivalent ions distribute more strongly into the coils than monovalent ions! The higher the ionic strength of the solution, the smaller will be the concentration differences of small ions between the coils and the external solution. Hence, swelling decreases with increasing ionic strength. i.e. l depends on ionic strength. The ionic strength is given by I = 1 2 i z i 2 c o,i Multivalent ions increase ionic strength more effectively than monovalent ions size of polymer coil is more effectively decreased by multivalent ions 19

20 Based on the Donnan equilibrium predict how Degree of substitution Electrolyte concentration Degree of dissociation Affects the polymer coil size The higher the DS the larger the coil, due to repulsion between like charges and more counter ions in the coil The higher the electrolyte concentration of the media the smaller the coil size. Smaller difference between concentration of counter ions inside coil and outside, smaller osmotic pressure. At high enough I the polyelectrolyte behaves as a neutral polymer. The higher the degree of dissociation, the larger the coil size. α affects the effective charge

21 Polyelectrolytes in solution Polyelectrolytes generally dissolve well in water. A dissolved polyelectrolyte molecule acquires a charge by dissociation. The (small) counterions of polyelectrolytes are mobile, but are influenced by the strong electric field created by the polyelectrolyte. The counterion concentration inside the polymer coil is high. The chain becomes more rigid (it is stretched) because of osmotic repulsion between counterions of neighbouring segments. 21

22 Polyelectrolytes in solution The effective charge of a polyelectrolyte is lower than the stoichiometric charge because of counterion condensation. The swelling of a polyelectrolyte with water depends strongly on ionic strength (osmotic pressure). The mean dimensions of the coil increases as the charge density of the polyion increases. At high ionic strengths the polyelectrolyte behaves more or less like a neutral polymer. 22

23 Polymer Molecular weight (millions) Polyethyleneimine Cationic polyacrylamide DS 20% DS 59% ,4 Polyelectrolytes in solution Hydrodynamic radii of cationic polymers in aqueous solution Radius nm How does a) the length of the polymer chain b) the amount of charged groups (DS) affect the size of the polymer coil in water? DS 80%

24 Polymers in solution Why polymers dissolve? Why polyelectrolytes dissolve? Size and molecular weights are determined from polymer solutions

25 Solubility Thermodynamics 1) The free energy of mixing (ΔG) should be negative In ideal case the enthalpy ΔH is zero (by definition). The entropy (-T ΔS) is always negative so in ideal case the polymer always dissolves DG mix = DH mix -TDS In the case of normal polymers (polyethylene, polystyrene) the enthalpy term is positive which means that the solvent has to be carefully chosen (Qualitatively like dissolves like ) For polyelectrolytes in water the enthalpy term is negative and polyelectrolytes dissolve easily mix

26 Polymers in solution In solution a polymer behaves like a long, more or less mobile chain that changes its shape (conformation) more or less randomly due to thermal motions. If there are no restrictions the polymer forms a random coil, whose size depends on the solvent In a good solvent the chain will expand interaction between the polymer and the solvent is favored, and solvent-monomer contacts are maximised In a poor solvent the chain will contract, to reduce interactions with the solvent Between the extremes is so called θ(theta) solvent (and the polymer behaves like an ideal polymer) Good solvent: Theta solvent: Poor solvent: 26

27 Polymer solution theory Three concentration ranges can be distinguished: I. Mean intermolecular distance >> R g, - colloidal solution II Mean molecular distance R g,. -Strong interactions III Mean molecular distance < R g, -Polymer chains form solvent-swollen polymer network. 27

28 Flory-Huggins theory Predicts the solubility of polymers Is based on a lattice theory, which makes certain assumptions: The mixture of solute and solvent is completely random. One solvent or solute molecule in each lattice site. The total volume of the system remains constant when solute and solvent is mixed, V a V b = V solution. The number of neighbours is constant. Only interactions between nearest neighbours are considered. 28

29 The basis of Flory-Huggins theory This theory predicts the solubility of polymers, mixing of solutions and solvent-swelling of polymer. A polymer solution is stable if for the process N A solvent molecules N B solute molecules Solution N = N A N B Gibbs energy DG M < 0. The Flory-Huggins theory is based on the theory of regular solutions, which assumes: - "Ideal" mixing entropy DS M -That the changes in contact energies between polymer segments and solvent, the enthalpy of mixing, DH M, describes these interactions DG M = DH M - TDS M < 0 29

30 Entropy of mixing B A A B There are many more ways of arranging the molecules so that they form a solution. Hence, the solution is a more probable state than the state in which solvent and polymer molecules are separately. Formation of solution is accompanied by an entropy increase. If nothing else affect the process of solution, polymer dissolves in solvent completely. 30

31 Enthalpy of mixing Regular solutions Solubility of polymers Before mixing After mixing e AA interaction energy between two molecules of type A e AA e BB e AB e BA e BB similarly If mixing on a molecular level, there s a different molecule as the nearest neighbour D H H mix 0 Hf e e e AA 2e AB - AB BB ( e e ) AA 2 BB DH mix = N A N A NB N B Z 1) 2) 3) 4) Ø e AA e Œe AB - º 2 BB ø œ ß 1) Number of molecules of type A 2) Probability to achieve molecule of type B as the nearest neighbour to type A 3) Number of nearest neighbours (coordination number) 4) Contact energy 31

32 Mixing enthalpy The volume fractions of the polymer and the solvent are: f A = N A V A N A V A N B V B f B = N B V B N A V A N B V B V A = the volume occupied by a solvent molecule V B = the volume occupied by the polymer N A = the number of solvent molecules N B = the number of polymer molecules The basic assumption is: the volume of a solvent molecule and a polymer segment is equal so that V B = rv A 32

33 Mixing enthalpy Pairwise segmentsegment interaction e BB Pairwise solventsolvent interaction e AA Pairwise segmentsolvent interaction e AB When A-A and B-B contacts are replaced by A-B contacts the change in contact energy per A-B-bond formed is De AB = e AB e AA e BB Assume that each solvent molecule has z neighbours. In the solution, on the average, the fraction of polymer neighbours that are polymer segments is f A and the fraction of segment neighbours that are solvents is f B Hence, the number of A-B-contacts in the solution is N B zrf A = N A zf B and the Enthalpy of mixing is 33 DH M = N A zf B De AB = N B zrf A De AB

34 The interaction parameter of Flory and Huggins, chi parameter The dimensionless parameter is defined by c = zde AB kt which is called Flory-Huggins interaction parameter or c- (chi) parameter c = 1 when zde AB = kt, i.e. the interaction energy is equal to the thermal energy. c-parameter can be calculated in solubility of polymer or activity of solvent in solution (pressure of gas). Respectively the solubility of polymer can be estimated if c-parameter in solution is known. if c < 1/2 the polymer is completely soluble The temperature in which c = 1/2 is called the q-temperature (theta temperature). And the polymer behaves ideally. -> the polymers float in the solution without interference from each other or from the chains the polymers are formed 34

35 The interaction parameter: chi Good solvent 0< c <0.5 Theta solvent c = 0.5 Poor solvent c >0.5 2/7/

36 Summary of polymers in solution DG M = DH M - TDS M < 0 The entropy of mixing positive The enthalpy term will vary depending on interactions between polymer and solvent Polyelectrolytes in aqueous media: Conformation dependent on effective charge and electrolyte (salt) concentration Laitoksen nimi 2/7/

37 Adsorption of polymers and polyelectrolytes 2/7/2017 Laitoksen nimi 37

38 A typical adsorption isotherm for a monodisperse polymer G(mg/m ) The plateau adsorbed amount is a function of molecular weight and solvency. The higher the molecular weight the higher the adsorbed amount is in general. Adsorbed amounts are as a rule higher in theta solvents than in good solvents. C Net adsorption depends both on the properties of the solvent and the surface 38

39 Structural aspects tail loop tail tail train Polymers in solution have large number of internal degrees of freedom. Adsorption leads to changes in conformation and loss in conformational entropy for the polymer. At low coverage chains have a tendency to flatten (lowest energy). 39

40 Description of the interfacial region An increase in concentration of solute in the interfacial region is called adsorption. Chemisorption adsorption involving formation of chemical bonds Physisorption only physical interactions Depletion negative adsorption, reduce in solute concentration near the interface Adsorption depends on the net adsorption energy for polymer segments, i.e. difference between free energy ΔG of segment/surface contact and solvent/surface contact. Should be sufficiently negative

41 Scheutjens-Fleer theory (lattice model) Application of Flory-Huggins theory to adsorption Calculate DG for the process: polymer in solution DG =DH adsorbed polymer -TDS Segment/solvent interaction parameter χ polymer Free energy Change in enthalpy Change in entropy solvent surface ΔG<0 for adsorption Segment/surface interaction parameter χ s

42 Factors affecting the enthalpy, ΔH: Interactions between polymer segments and surface Interactions between solvent and surface Interactions between polymer and solvent in solution Interactions between polymer and solvent at the surface Based on these facts give some examples of surface, solvent or polymer properties that will promote adsorption.

43 Factors that affect the entropy, ΔS Polymer concentration at interface increases of mixing for polymer decreases entropy Large amount of solvent molecules are released from polymer network entropy of mixing increases Total entropy of the system increases force for adsorption one driving

44 Exercise What is the conformation of the adsorbed polyelectrolyte in the following cases and how does the situation change if the salt concentration in solution increases? 1. The surface is neutral and the polymer is charged. 2. The surface and the polyelectrolyte have the same sign of charge. 3. The polyelectrolyte and the surface have opposite charge. The surface charge density is high and the charge of the polyelectrolyte is low. 4. The polyelectrolyte and the surface have opposite charge. The charge density of both the surface and the polyelectrolyte is high. 44

45 1. Polyelectrolyte adsorption on uncharged surface Electrostatic contributions opposes the adsorption due to repulsion between segments Salt promotes adsorption 2. Polyelectrolyte adsorption on charge surface with same sign Repulsion both between segments and between segments and surface Salt promotes adsorption 3. Polyelectrolyte adsorption on oppositely charged surface (pure electrosorption) No chemical affinity Salt has a negative effect 4. Polyelectrolyte adsorption on oppositely charged surface Polymer charge higher than surface charge Some surface affinity or poor solvency Salt promotes adsorption 5. Polyelectrolyte adsorption on oppositely charged surface Polymer charge lower than surface charge Salt has a negative effect 45

46 Conformation of polyelectrolytes effect of ionic strength Low ionic strength High ionic strength 46

47 The effect of different parameters on polymer adsorption Properties of surfaces - Surface charge, surface area, chemical consistency of surface Structure of polymer - Solubility, molecular weight, degree of substitution, structure (linear/ branched) Properties of solvent - ph, ionic strength

48 Practical applications of polymer adsorption Retention aids in papermaking Strength additives in papermaking and composites Surface modification Steric stabilization Charge reversal Layer-by-layer deposition

49 Layer-by-Layer deposition Theory Adsorption of charged polymers (polyelectrolytes) or charged particles on a surface with an opposite charge leading to charge reversal. - H2O H2O Driving force for build-up of multilayer: Electrostatic attraction, Donor/acceptor interactions, Hydrogen bonding, covalent bonds, or specific recognition. For polyelectrolytes: Entropy gain by the release of the counterions

50 Examples Breathable and water resistant textiles ~160º LbL cationic polymer CNF: Effect on paper strength and ductility Cationic PLL anionic wax particles Conformation of polymer affected adsorption Wågberg s group: dx.doi.org/ /j.carbpol

51 Adsorption of C-PAM on bleached kraft pulp. M w 1 milj., ph 7 DS 7.8, 15.3, 27.8, NaCl DS What is the effect of degree of substitution on adsorption? What is the effect of electrolyte concentration on adsorption in each case? Explain both the rise and decrease in adsorption upon adding salt. Lindström, T. and Wågberg, L. Tappi J.66:6 (1983) 83 2/7/

52 The effect of time on the adsorbed layer Conformational changes: very important in practical use of flocculants Time Adsorption on a porous surface; slow changes Time 06/52

53 Summary of polymer adsorption at surfaces The conformation in solution affects the conformation of adsorbed layer: extended col flat conformation Entropy of polymer decreases upon adsorption but entropy of solvent molecules increase net positive effect Polyelectrolytes: Effect of ionic strength: High I coiled polymer in solution more loops and tails upon adsorption other factors affecting adsorption

54 Next lecture: Thursday 9.2 Surface sensitive techniques: Adsorption Quartz crystal microbalance with dissipation monitoring (QCM-D) Surface plasmon resonance (SPR) Ellipsometry Surface chemical composition X-ray photoelectron spectroscopy (XPS) Surface structure and surface forces Atomic force microscopy (AFM)