Tentamen. TFYA47 Ytor och gränsskikt, TEN2. 17 oktober 2017 kl Skrivsal: TER3

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1 Thomas Ederth IFM / Molekylär Fysik ted@ifm.liu.se Tentamen TFYA47 Ytor och gränsskikt, TEN2 17 oktober 2017 kl Skrivsal: TER3 Tentamen omfattar 5 problem som vardera kan ge 10 poäng. För godkänt krävs totalt 22 poäng samt minst 3 poäng per uppgift. Tentamen består av 3 sidor (inklusive denna). Lösningar läggs ut på kurshemsidan efter skrivtidens slut. Skrivningsresultat meddelas senast 12 arbetsdagar efter tentamenstillfället. Tillåtna hjälpmedel: Physics Handbook Räknedosa (med tömda minnen) Kursansvarig: Thomas Ederth, som ca kl. 16 svarar på frågor i skrivsalarna, och i övrigt finns tillgänglig på ankn eller telefon under skrivtiden. Kursadministratör: Jenny Ählström, ankn. 4091, jenny.ahlstrom@liu.se. Lösningar skall om möjligt åtföljas av figur, införda beteckningar skall definieras, ekvationer motiveras och numeriskt svar alltid skrivas ut med enhet. Orimligt svar medför noll poäng på uppgiften. Lycka till!

2 Tentamen i TFYA47 Ytor och gränsskikt, TEN2, 17 oktober a) Ange index för de markerade riktningarna i figuren nedan. [3p] b) Ange Millerindex för planen nedan. Origo är markerat med en punkt. [3p] c) Krom har bcc-struktur, och den kubiska enhetscellen har längden 287 pm. Beräkna ytdensiteten av atomer i ett (111)-plan hos krom. [4p] 2. a) Molekyler enligt figuren har bildat ett monolager på en yta. Beskriv tre olika sätt att avgöra hur de adsorberade molekylerna är orienterade på ytan. [6p] b) Hur fungerar Brewstervinkelmikroskopi (BAM), och vad kan man använda det till? [4p] 3. a) Beskriv två typer av krafter som kan uppstå mellan partiklar eller ytor i vattenlösningar i närvaro av polymerer. [4p] b) Vad är ett elektrostatiskt dubbelskikt, och hur uppkommer det? [4p] c) Vatten är en associerad vätska, vad betyder det? [2p] 4. a) Förklara begreppen hydrofil, hydrofob och oleofob, samt ange ett exempel för varje typ. [3p] b) Vattentrycks uturändenpåettrör(medtunnaväggar)enligtfiguren. i) När är tryckskillnaden över den fria vätskeytan störst? ii) Om rörets diameter är 1 mm, uppskatta hur hög vattenpelaren kan vara i röret utan att vattnet rinner ut. [3p] c) Förklara hur man kan använda en Zisman-plot för att bestämma den kritiska ytspänningen (critical surface tension) utgående från allmänna principer för om en vätska sprids på en yta eller inte. [4p] 5. a) Beskriv hur självorganisation av alkantioler på en yta går till, och ange ungefärliga tidsskalor för olika delar av processen. [4p] b) Ange minst tre viktiga skillnader mellan klorosilaner och alkoxysilaner. [4p] c) Hur hindrar poly(etylenglykol) som är fäst på en yta att t.ex. proteiner fastnar på ytan? [3p]

3 Examination, TFYA47 Surfaces and Interfaces, TEN2, 17 October a) Assign indices for the directions shown in the figure below. [3p] b) Give the Miller index for each of the planes below. The origin is indicated with a dot. [3p] c) Chromium has bcc structure, and the cubic unit cell has the length 287 pm. Calculate the surface density of atoms in a (111)-plane in chromium. [4p] 2. a) Molecules of the type in the figure have formed a monolayer on a surface. Describe three ways of determining the orientation of the adsorbed molecules on the surface. [6p] b) How does Brewster angle microscopy (BAM) work, and what can it be used for? [4p] 3. a) Describe two types of forces that can occur between particles or surfaces in aqueous solutions in the presence of polymers. [4p] b) What is an electrostatic double layer, and how does it emerge? [4p] c) Water is an associated liquid, what does that mean? [2p] 4. a) Explain the terms hydrophilic, hydrophobic and oleophobic, and give at least one example of each type. [3p] b) Water is pressed out of the end of a tube(with thin walls) according to the figure. i) When is the pressure difference over the free liquid surface largest? ii) If the tube diameter is 1 mm, estimate the maximum height of a water column in the tube before the water falls out. [3p] c) Explain how a Zisman plot can be used to determine the critical surface tension, using general principles determining whether a liquid spreads over a surface or not. [4p] 5. a) Describe the self-organization of alkanethiols on a surface, and indicate approximate timescales for the different parts of the process. [4p] b) Specify at least three important differences between chlorosilanes and alkoxysilanes. [4p] c) How does poly(etylene glycol) attached to a surface prevent, for example, proteins from sticking to the surface? [3p]

4 Lösningsförslag, TFYA47 Ytor och gränsskikt, 21 oktober a) i) [210], ii) [010], iii) [1 1 1] b) i) (212), ii) (100), iii) (01 1) c) The (111) plane of a BCC structure is shown to the left below (note that the center atom in the cubic cell is above the indicated plane. If the side of the cubic cell is a, the dimensions of the shown triangle are as given on the right in the figure. The height h is obtained via Phythagoras theorem, and since this triangle contains exactly one half atom (1/6 atom in each corner) the surface density ρ s, i.e. the number of atoms per area unit is obtained as ρ s = 1 1 = /a = 287 pm/ = 7, atoms/m 2 2 2a h 1/2 2. a) For example, angle-resolved XPS (just measuring the chemical shifts will inform about which atom binds to the surface, but only limited orientation information; it will not tell whether the molecule is upright or in a flat orientation), IRAS, STM, or LEED (they provide different information, but all can yield some details about how the molecules are adsorbed onto the surface). b) See the course literature for an account of how BAM works. It is used to study, among other things, thickness and organization of thin films on liquid surfaces. 3. a) For example, depletion forces, bridging, or steric stabilization... please refer to the literature for details(lecture notes on intermolecular and surface forces)! b) An electrostatic double layer occurs on a surface immersed in water (or any other liquid with high dielectric constant) and is composed of a layer of tightly bound ions, and outside this layer a diffuse layer of ions with opposite charge. c) An associated liquid is a liquid where there is short-range order, so that the position and orientation of a molecule in the liquid is not independent of the position and orientation of its neighbour molecules. In most cases this is a result of strong hydrogen-binding between molecules in the liquid. 4. a) Hydrophilic literary means water loving, and refers to a molecule that is attracted to, or dissolved by, water (or absorbs water, if it is a body). Hydrophobic refers to molecules, surfaces, or materials which repel water, or do not absorb water. Oleophobic similarly refers to a molecule or surface that repels oil, of which fluorocarbons are common examples.

5 b) i) The (Laplace) pressure is greatest when the radius is smallest, since P = 2γ/r. If the liquid surface remains attached to the tube at the end, this occurs when the liquid surface forms a half sphere, with the diameter equal to the diameter of the tube. ii)itisthesurfacetension thatkeeps thewater inthetube, andifthediameter of the tube is d, the vertical force exerted by the surface tension along the periphery of the tube end is at most F γ = γ l = 72,8 mn/m πd = 2, N, equivalent to the weight of 2, g = 2, cm 3 water. The cross sectional area of the tube is πr 2 = 7, cm 2, and the calculated volume will thus fill a height of 2,97 cm in the tube. If we take into account that some water is suspended below the tube end, and approximating this with a half sphere, the remaining height in the tube is 2,90 cm. If the height is calculated using capillary rise, the obtained answer is very similar, this is because it is the surface tension that holds the water in place in both cases, but with the capillary rise method the calculation fails if the radius of the tube is too large, since the meniscus will no longer be spherical. c) In general low-energy (meaning low surface energy) liquids spread over high-energy surfaces. Thus, in the Zisman method, a series of liquids with increasing surface tension (decreasing spreading coefficient) are used, and in principle the surface tension of the liquid which just barely spreads over the surface, is taken to be the critical surface tension of the substrate. In practice, cosθ of the liquids is plotted versus the surface tensions of the liquids, and a line is fitted to the data; the value of the surface tension where this line has cosθ = 1 (zero contact angle) is taken as the critical surface tension. 5. a) The adsorption of thiols onto (metal) surfaces is diffusion controlled; almost all molecules reaching the surface form a covalent bond. Within seconds, the surface is covered in a disordered layer(unless the solution is very dilute). After this, the molecules start organizing themselves, van der Waals forces between chains drive the organization of the chains into a crystalline layer, since this maximizes chain-chain interaction. This takes several hours, or perhaps up to a day, until the chains form a crystalline layer. b) Chlorosilanes are very reactive at room temperature, whereas alkoxy silanes have to be heat treated to react with an oxide surface. Chlorosilanes cannot have polar functional groups, since the chlorosilane groups react with polar residues, but alkoxysilanes can have any type of functional group - which also makes them easier to chemically modify after layer formation. Correctly handled, chlorosilanes result in more homogeneous layers with better chemical stability. c) By steric repulsion and/or strong hydration. Steric repulsion means that the poly(ethylene glycol) (PEG), which is hydrophilic and has taken up water, is compressed as a protein attaches to the surface. Thus increases the osmotic pressure in the compressed region, and drives an influx of water to this region. This restores the hydrated structure of the polymer. The process can, alternatively, be understood such that water binds strongly to PEG chains, and approaching proteins do not have the capacity of displacing these water molecules.