Surface Phenomena - Module 1 PDF

Interface phenomena Module 1 1. Surface phenomena Diana Silva September 2024 Evaluation & Bibliography Class attendance To be able to attend the assessment test: 1/3 of the classes; If 1/3 and 2/3 of the classes are attend a bonus of 0.5 points will be given in the final mark; If > than 2/3 of the classes are attend a bonus of 1.0 point will be given in the final mark. Test One written test (multiple choice and open questions) Contact Duration: 2 h 00 min Date: WEEK 21th – 25th of October E-mail: [email protected] Richard M. Pashley, Marilyn E. Karaman - Applied Colloid and Surface Chemistry - 2004 John Wiley & Sons, Ltd H. Yildirim Erbil - Surface Chemistry Of Solid and Liquid Interfaces - 2006 by Blackwell Publishing Ltd Peter Atkins & Julio de Paula – Physical Chemistry - 2006 by Oxford University Press, Eighth Edition Interface Phenomena - Surface phenomena 2 Interface Phenomena – Module 1 - PROGRAMME 1. Surface/Interface Phenomena 4. Adsorption Definition of surface and interface. Definition. Surface tension and surface energy. Difference between absorption versus adsorption. Wetting and spreading. Adsorbate and adsorbent. Characteristics of the surface. Desorption. Methods for the measurement of surface tension of liquids Methods of adsorption analysis (direct and indirect methods). and solids. 2. Adhesion 5. Polymer Blends Definition and theory. Definition. Fundamental and practical adhesion. Miscible and immiscible polymers. Strength of adhesion bond. Characterisation of polymer blends. Failure of adhesion. Theories of adhesion. Friction, wear and lubrication 3. Polymer Surface 6. Polymer Composites Definition. Surface rearrangement. Factors affecting properties of polymer composites Surface modification of polymers. Reinforcement materials Chemical characterisation of polymer surfaces. Nanocomposites Characterisation Nanocomposites Interface Phenomena - Surface phenomena 3 Interface Phenomena – Module 1 - OBJECTIVES Acquire knowledge in the field of surface phenomena, necessary for the development new and improved polymers. Identify the surface of materials at different states of matter. Know how the applicability of the materials can be affected by the properties of their surface. Know the main methodologies for processing and modify the materials surface. Understanding the interactions at the interface. Be aware of possible surface characterisation methods. Interface Phenomena - Surface phenomena 4 Surface/Interface phenomena Interface Phenomena - Surface phenomena 5 Interface Phenomena – Module 1 1. Surface phenomena Surface of polymers The behaviour of polymers at interfaces is of fundamental importance in a wide range of technologies and applications: Industrial processes imply surface active components such as catalysts, colloids, surfactants. Adhesive and coating properties (e.g. heat sealing, reinforcing etc.). Polymer processing and machinery handling (e.g. friction, self-adhesion, welding of two polymers by thermal or solvent bonding as a common example of strength development by polymer-polymer interface). Development of various products are based on the unique properties of interfaces (photographic films, membranes, composites etc.). Interface Phenomena - Surface phenomena 6 Surface Gas Liquid Solid No distinct shape Shape of the container Holds its shape Volume of the container Fixed volume Fixed volume Interface Phenomena - Surface phenomena 7 Surface Involving part of a macroscopic object (bulk) in contact with its environment. deﬁnes the physical limit of a solid or liquid. The structure of a surface is given by the atomic and molecular composition and arrangement of the atoms in space. Liquids → easy to “identify” the surface Interface Phenomena - Surface phenomena 8 Surface Solids → Roughness sometimes makes it difficult to define a surface line! An ideal solid surface is atomically ﬂat and chemically homogenous. BUT, in reality solid surfaces show a surface roughness over varying length scales and are chemically heterogeneous to a degree due to the presence of impurities or polycrystallinity differences. h y x structure topography On surface chemistry studies the surface of solids is regarded as the top ~100 nm of the solid. Interface Phenomena - Surface phenomena 9 Surface In large objects with small surface area A to volume V ratio (A/V) the physical and chemical properties are primarily defined by the bulk (inside). In small objects with a large A/V-ratio the properties are strongly influenced by the surface. A 24 96 384 V 8 64 512 A/V 3.0 1.5 0.75 Chemical heterogeneity of a surface → adhesion, adsorption, wettability, biocompatibility, printability and lubrication behaviour. Affects gas and liquid adsorption capacity of a substrate and also the extent of a catalysis reaction. Interface Phenomena - Surface phenomena 10 Surface versus Interface Surface Interface Limit of a phase. Boundary between two adjacent phases. It is a part of an interface! Exists in all cases, since every single phase is in contact with another phase such as solid–gas, liquid–gas contacts, etc. EXCEPT in absolute vacuum conditions. At the border of a solid or liquid in contact with vapor there is usually no abrupt change in density, but a more or less continuous transition from high density to low density. The interface consists either of evaporating bulk material or condensing material from the gas phase. INTERFACE Vapor Liquid ρ A is a chemical compound composed of the two surrounding phases B z Interface Phenomena - Surface phenomena 11 Different Surface and Interface scenarios Liquid - Gas Solid - Gas Liquid(A) - Liquid(B) Solid - Liquid Solid(A) - Solid(B) Liquid molecules Molecules highly Both highly mobile; Liquid may dissolve Both materials may are mobile and immobile; Shape of the solid surface react forming a disordered; Crystalline solids surface controlled atoms; new compound at Evaporation and re- highly ordered by Surface Tension; Liquid molecules the interface; condensation structure Solubility of are more ordered At high occurs. Chemical reactions molecules → at the surface than temperatures may occur. controlled by in the bulk. interdiffusion is chemical potential. possible. Interface Phenomena - Surface phenomena 12 Surface/Interface Interactions: Adhesion vs. Cohesion A A Adhesion Surrounding medium B B Attractive interactions Interfacial region between two different media Substrate Cohesion A Attractive interactions A within a phase A Interface Phenomena - Surface phenomena 13 Surface Energy (Liquids) In the liquid bulk the molecules are surrounded in all directions by other molecules, to each individual molecule there is equal attraction. We only have cohesive forces ! Vapour At the surface, molecules only form cohesive forces with molecules adjacent and below them. The resultant forces on molecules in the bulk and at the surface is DIFFERENT. Liquid There is an excess of ENERGY in the surface → Surface Energy energy per unit area, J/m2 in the SI system mJ/m2 Mercury 465 is the work that should be supplied to bring the molecules from the interior bulk Paraffin 22 phase to its surface to create a new surface having a unit area (1 m2). Water 72 NOTE: mJ/m2 is generally used in Liquid molecules tend to move from the surface to the surface science to keep the numerical bulk → When surface increases → liquid tends to form values the same as for the previously used ergs/cm2 values minimal surface, aka minimal surface energy. Interface Phenomena - Surface phenomena 14 Surface Tension (Liquids) The molecules that are located at the surface behave differently to those in the bulk phase. There is an orientation effect for molecules at ﬂuid surfaces: the molecules at or near the ﬂuid surface will be orientated differently with respect to each other than the molecules in the bulk phase. Any molecule at the ﬂuid surface would be under an asymmetrical force ﬁeld, resulting in surface tension (γ). 𝛾 Increases Closer the molecule is to the surface → magnitude of the force Imbalance of forces Interface Phenomena - Surface phenomena 15 Surface Tension (Liquids) Surface tension is a concept allied to surface energy. Force pulling the molecules of the surface together Force per unit length applied parallel to the surface, N/m in the SI system 1 𝛾 = 𝑛𝐴 ∙ 𝑛 ∙ 𝜀 4 Where: nA- number of molecules per unit area N – number of closest neighbours ε – energy of interaction of a pair of molecules Interface Phenomena - Surface phenomena 16 Surface Tension Classical thermodynamics can be applied to determine the relationship between surface tension and other macroscopic variables. In practice, a series of mathematical treatments of the combination of the first two laws of thermodynamics are used in solving engineering problems where mass and energy are continuously exchanged; in these cases local thermodynamic equilibrium is assumed. A system is said to be in equilibrium if its measurable properties do not change over time. Thermodynamic systems can consist of either a single phase or multiple phases and may include one or more components. Interface Phenomena - Surface phenomena 17 Surface Tension The first law of thermodynamics gives us the conservation of energy: 𝑑𝐸෠ = 𝛿𝑄 − 𝛿𝑊 where Ȇ is the total energy of the closed system, δQ is heat transferred to a system and δW is the work done by the system to the surroundings. Ȇ is due to internal energy (E), like kinetic, potential, electromagnetic, surface tension. Other forms are negligible so Ȇ = E. Q is positive if the W is positive if the system receives heat system does work Internal Energy of a system can be increased by: Energy cannot be created or destroyed, but it can be transformed from one form to another. In an isolated 1. Adding energy to the system (heat). Q positive system the sum of all forms of energy is constant. 2. Doing work on the system. W negative Interface Phenomena - Surface phenomena 18 Surface Tension The second law of thermodynamics can be described by the Clausius inequality: 𝛿𝑄 𝑑𝑆 ≥ S is the entropy (measures the molecular 𝑇 degree of freedom of a system) where dS is the change of entropy of the closed system, T is the temperature. Entropy of a system can only increase! As S can only increase, it defines what processes are The system tends toward a state of thermodynamic irreversible. equilibrium where S is highest at the given E. Interface Phenomena - Surface phenomena 19 Surface Tension Combining the first and the second laws of thermodynamics results in an equation very useful for determining the conditions of equilibrium and stability of systems: 𝑑𝐸 ≤ 𝑇𝑑𝑆 − 𝛿𝑊 The thermodynamic property (Gibbs free energy) G, tells us about the spontaneity of a process, and it is given by: H is the enthalpy (amount of internal ∆𝐺 = ∆𝐻 − 𝑇 ∙ ∆𝑆 energy contained) ∆𝐺 < 0 Spontaneous reaction (Exergonic reaction) ∆𝐺 > 0 Not spontaneous reaction ∆𝐺 = 0 Equilibrium Interface Phenomena - Surface phenomena 20 Surface Tension 1 2 COLD HOT 1 2 1 2 Interface Phenomena - Surface phenomena 21 Surface Tension Thermodynamic concept of surface tension (γ), defines it as the partial differential of the Gibbs free energy (G) of the system with the respect to the area (A), at constant temperature (T) and pressure (P): 𝜕𝐺 Surface tension is the tendency of surfaces at rest 𝛾= 𝜕𝐴 𝑇𝑃 to shrink into the minimum surface area possible! Unfortunately, there are several limitations to the above arguments for the Gibbs convention. First of all, we should keep in mind that Gibbs’s treatments apply only to plane interfaces. If the interface has a ﬁnite curvature, the problem is considerably more complex. If the surface is a solid, it can withstand an anisotropic stress, and it is a very complex matter to apply thermodynamics to this case. Also, in this convention the system is in thermodynamic equilibrium. However, this is not always true. Interface Phenomena - Surface phenomena 22 Surface Tension The formation of curved liquid surfaces such as spherical liquid drops in air, or curved liquid meniscuses in thin capillary glass tubes, is the consequence of the surface area minimisation process due to the existence of liquid surface free energy. If two phases are in hydrostatic equilibrium, they can be separated by a ﬂat curvature-free interface. is rarely encountered and if a liquid interface is curved, this means that the pressure is greater on the concave side (the inside of a bubble, for example) than on the convex, by an amount, ∆P, which depends on the liquid surface tension and on the magnitude of the curvature. The phenomena due to the presence of curved liquid surfaces are called capillary phenomena Interface Phenomena - Surface phenomena 23 Surface Tension 𝑃1 > 𝑃0 𝑑𝑟 → infinitesimal decrease in bubble radius dr water air 𝑑𝐺 = −𝛾 4𝜋𝑟 2 − 4𝜋(𝑟 − 𝑑𝑟)2 + 𝑃1 − 𝑃0 4𝜋𝑟 2 𝑑𝑟 r P0 P1 = −8𝜋𝑟𝑑𝑟𝛾 + ∆𝑃4𝜋𝑟 2 𝑑𝑟 𝑑𝐺 2𝛾 The Young–Laplace equation for a =0 ∴ ∆𝑃 = 𝑑𝑟 𝑟 single spherical interface In general, that is for any curved interface, this relationship expands to include the two principal radii of curvature, R1 and R2 : 1 1 ∆𝑃 = 𝛾 + 𝑅1 𝑅2 R2 The Young–Laplace equation is the expression that relates the pressure difference, ∆P, to the curvature of the R1 surface and the surface tension of the liquid. Note: spherical surface R1 = R2 = r Interface Phenomena - Surface phenomena 24 Surface Tension Table. Surface tensions for different liquids Types of liquid Liquid Temperature (°C) Surface Tension (mN/m) Helium -271 0.26 Nonpolar liquid Nitrogen -153 0.20 Ammonia -40 35.4 Hydrogen-bonded liquid (polar) Water 20 72.9 Mercury 20 484 Metallic liquid Silver 1100 878 Interface Phenomena - Surface phenomena 25 Methods of determining the surface tension of liquids Wilhelmy plate Platinum plate is used as standard A plate hanged to a balance (Wilhelmy balance) touches a liquid surface. As it moves downward the plate needs to work to overpower the Liquids’ surface tension. The force (F) required to move the plate downward is converted to surface tension through: 𝐹 𝛾= L ∙ cos 𝜃 L → plate perimeter θ → contact angle Interface Phenomena - Surface phenomena 26 Methods of determining the surface tension of liquids Noüy Ring method Platinum ring is used as standard ring A platinum ring is hanged on the balance and the bottom of the ring is place on the liquid surface. Liquid The ring is immersed in the liquid and it is repeatedly pull up and down. A tensile force measured and the surface tension is calculated through: 𝐹 𝛾= L ∙ cos 𝜃 Interface Phenomena - Surface phenomena 27 Methods of determining the surface tension of liquids Pendent drop method The liquid is pushed out of the needle of a syringe (a micrometric syringe is often used to adjust the volume of the drop). This method involves the determination of the profile of a drop of one liquid suspended. The shape of the drop is determined through: droplet volume, density of the liquid, surface tension. 1 𝛾= ∆𝜌𝑔𝑑𝑒2 ∆𝜌 Density difference 𝐻 𝑔 Gravitational acceleration Neddle 1 Correction coefficient determined by 𝑑𝑒 /𝑑𝑒 𝐻 Liquid Interface Phenomena - Surface phenomena 28 Methods of determining the surface tension of liquids Maximum bubble pressure method Involves flow of a gas bubble (typically air or nitrogen) at a constant rate and blows them through a capillary with a known diameter which is submerged in the sample liquid. The pressure inside of the gas bubble increases until the bubble becomes hemispherical and its radius corresponds to the radius of the capillary. 𝑟𝑏𝑢𝑏𝑏𝑙𝑒 > 𝑅 𝑟𝑏𝑢𝑏𝑏𝑙𝑒 = 𝑅 𝑟𝑏𝑢𝑏𝑏𝑙𝑒 < 𝑅 ℎ ∆𝑃𝑚𝑎𝑥 𝑅 ∆𝑃𝑚𝑎𝑥 → maximum pressure difference 𝛾= 2 gas 𝑅 → Radius of the capillary ∆𝑃𝑚𝑎𝑥 liquid The method is based on the continuous measurement of the applied pressure versus bubble rate formed at the end of the capillary. Interface Phenomena - Surface phenomena 29 Methods of determining the surface tension of liquids Capillary rise method θ Is based on measuring the penetration time needed for a liquid to rise to a certain height when the end of a capillary is immersed into the solution. Uses Laplace equation : 2𝛾 cos 𝜃 ∆𝑃 = 𝑟 However, this will be accurate only if the liquid wets the walls of the glass tube. ∆𝑃 = ℎ𝜌𝑔 𝜌 Density 𝑔 Gravitational acceleration IMPORTANT: if θ > 90° (e.g. mercury on glass), the liquid will actually fall below the reservoir level and the meniscus will be curved in the opposite direction, making it impossible to calculate the surface tension through this method ! Interface Phenomena - Surface phenomena 30 Surface tension in solids The molecules on a solid surface are generally fixed. A solid cannot spontaneously contract to minimise its surface area. This situation is completely different from that of liquids. There are many theoretical difficulties in defining the surface free energy of solids, mainly arising from the deviations from ideality due to the immobility of surface atoms and molecules. Even so, there is a need to pre-determine the average surface tensions of solids in order to predict their usage in many industrial applications. As such mostly indirect experimental methods are applied to determine the surface energy of solids. Interface Phenomena - Surface phenomena 32 Surface tension in solids When solids are deformed by external forces at ambient temperature, they generally react elastically (recover their initial shape after the applied load is removed). But, if we continue to apply the load the material will start to deform plastically until failure. And we achieve complete break of the test specimen. IMPORTANT Two new solid surfaces are obtained, but the breakage force is not equal to the cohesion forces between the solid molecules because the break occurs at a mechanically weak point! Moreover there is some last-minute viscous ﬂow of the test specimen which alters the interfacial area. Sample’ crack or defect Overall, the measured surface tension force depends on the history of formation of the test specimen. Interface Phenomena - Surface phenomena 33 Surface tension in solids Temperature At high temperature some solids, such as sintering powders, become viscous and gain mobility so that they can diffuse laterally under their melting points. 2𝛾 cos 𝜃 They lose their elastic responses and thus capillarity equations can be applied for such cases. ∆𝑃 = 𝑟 Laplace equation Some polymers also behave similarly; as capillary equations by me used, above the polymers’ melting points and show elastic properties below their glass transition temperatures. IMPORTANT Overall, it is possible to melt a solid, and calculate its surface tension while it is in a liquid state using the technique measurements discussed previously. The obtained values are then extrapolated to room temperature by means of suitable semi-empirical equations. Interface Phenomena - Surface phenomena 34 Methods of determining the surface energy of solids Atomic force microscopy The central part of the AFM is the sharp probe that scans the surface of the sample. It is closely monitored in each movement by the piezoelectric materials with a fine subnano- precision. Usually short range scans, typically with the same x-y dimensions, are done and alteration of the tip movement are detected and result in a contrast image. Interface Phenomena - Surface phenomena 35 Methods of determining the surface energy of solids Atomic force microscopy AFM is capable of analysing by force modulation local chemical (but does not give quantitative data about chemical information) and mechanical properties of the surface, such as adhesion and elasticity. Even forces between molecules can be measured. The force required to pull the tip off the substrate surface is called pull-off or adhesion force (F) which is directly related to the surface tension of the sample: 𝐹 𝛾= 2𝑐𝜋𝑅 R is the radius of the particle (probing tip) c constant dependent on the AFM model Interface Phenomena - Surface phenomena 36 Interface Tension Similar to surface tension as cohesive forces are also involved. However the main forces involved in interfacial tension are adhesive forces (tension) between the phases. Several types of interface may exist depending on state of the two adjacent phases Phase Interfacial tension Examples Gas-Gas - No interface possible* Gas-liquid γLV Liquid surface (e.g., water exposed to air) V from vapour has a gas does not have a limit Gas-Solid γSV Solid surface (e.g., Table) Liquid-Liquid γLL Liquid-Liquid interface (e.g. Lava lamp) Liquid-Solid γLS Liquid-solid interface (e.g. water drops on a metal) Solid-Solid γSS Solid-Solid interface (e.g. a phone in a table) *Gas-Gas → ease of miscibility of different gas molecules in free space does not allow any interface formation. Interface Phenomena - Surface phenomena 37 Interface Tension When we consider two immiscible phases (A and B) and an interface between them, we should deﬁne the interfacial tension, as the force that operates inwards from the boundaries of a surface perpendicularly to each phase, tend to minimise the area of the interface. Interface tension (γAB) can be given through: Force per unit length existing at the 1ൗ interface between two immiscible 𝛾𝐴𝐵 = 𝛾𝐴 + 𝛾𝐵 − 2𝜙 𝛾𝐴 𝛾𝐵 2 phases, N/m in the SI system where φ is a Good-Girifalco interaction parameter (entropic contributions and non-dispersive forces across interfaces). If dispersion forces are the dominant bonds in both phases A and B the molecules have similar properties and φ will be close to unity. Interface Phenomena - Surface phenomena 38 Interface Tension The interfacial tension is further lowered by the existence of other forces (e.g. polar forces) that act at the interface 1ൗ 1ൗ 𝑝 𝑝 𝛾𝐴𝐵 = 𝛾𝐴 + 𝛾𝐵 − 2 𝛾𝐴𝑑 𝛾𝐵𝑑 2 −2 𝛾𝐴 𝛾𝐵 2 Lifshitz-van der Waals forces (LW) represent all the van der Waals interactions 𝛾 𝐿𝑊 = 𝛾 𝑑 + 𝛾 𝑝 Fowkes suggested that the principal non-dispersion interactions are not polar acid-base (ab) interactions 𝛾 = 𝛾 𝑑 + 𝛾 𝑎𝑏 Interface Phenomena - Surface phenomena 39 Interface Tension Lewis added the concept of acid (γ+) and base (γ-) parameter of surface energy 1ൗ 1ൗ 1ൗ 𝛾𝐴𝐵 = 𝛾𝐴 + 𝛾𝐵 − 2 𝛾𝐴𝐿𝑊 𝛾𝐵𝐿𝑊 2 − 2 𝛾𝐴+ 𝛾𝐵− 2 − 2 𝛾𝐴− 𝛾𝐵+ 2 Surface energy acid-base components will be zero if the material is mono-functional or inert. The interface will be stable when: 𝛾𝐴𝐵 = 𝛾𝐴 + 𝛾𝐵 − ∆ where Δ depends on the type of bonding across the interface (e.g., dispersion forces, polar forces, LW etc.) Interface Phenomena - Surface phenomena 40 Method of determining the interfacial energy Sessile drop method If θ < 90º, the liquid is said to wet the solid. If θ = 0º complete wet. If θ > 90º then it is said to be non-wetting. Is based on the analysis of the profile of the drop placed on a solid substrate. The liquid is contained in a syringe from which a droplet is deposited onto the substrate, and a high resolution camera captures the image. The drop can then be analysed either by eye (using a protractor) or using image analysis software to calculate contact angle, surface and interfacial tension, wettability and absorption The sessile drop technique can be used to measure contact angle between solid, liquid and vapor phases and characterise the solid surface properties by solving Young’s equation: Leaves us with two variables to determine 𝛾𝑆𝑉 𝛾𝑆𝐿 − 𝛾𝑆𝑉 and 𝛾𝑆𝐿 → Thermodynamic models of adhesion cos 𝜃 = are used to determine the surface tension, γSV of 𝛾𝐿𝑉 the wetted solid. where equilibrium contact angle, cos θ is related to the interfacial energy of the three involved surfaces; solid-liquid, 𝛾𝑆𝐿 , solid-vapor, 𝛾𝑆𝑉 and liquid- vapor 𝛾𝐿𝑉. Interface Phenomena - Surface phenomena 41 Contact angle Directly from our concept of surface energies, it is clear that we would expect a liquid to spread on a substrate if: 𝛾𝑆𝐿 − 𝛾𝑆𝑉 − 𝛾𝐿𝑉 > 0 As such, one my define the spreading coefficient as: 𝛾𝐿𝑉 𝑆𝐿𝑆 = 𝛾𝑆𝐿 − 𝛾𝑆𝑉 − 𝛾𝐿𝑉 Air 𝛾𝑆𝑉 𝜃 Liquid 𝛾𝑆𝐿 Solid So if 𝑆𝐿𝑆 > 0 The liquid will spread across the surface of the solid. 𝛾𝐿𝑉 Liquid Air 𝜃 𝛾𝑆𝑉 Solid In water the wetted solid is termed ‘hydrophilic’, whereas the non-wetted solid is 𝛾𝑆𝐿 ‘hydrophobic’. Contact angle is greatly affected by a substrate’s surface roughness and chemical micro-heterogeneity. Interface Phenomena - Surface phenomena 42

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