Chemistry

Session Surface Chemistry - 1

Introduction Importance of Surfaces Development of Surface Chemistry Surface Sensitivity and Specificity The Surface Science Approach Surface Analytical Techniques The Need for Ultra-high Vacuum (UHV) Experimental Surface Science Systems

Surfaces reactions everywhere Background Surfaces reactions everywhere Washing clothes Brushing teeth Preventing food to stick to pans Surface chemistry important to a wide range of technologies Catalysis Semiconductor Fab Paints & finishes Consumer products

Why are surfaces important Catalysis Pollution Corrosion Sensors Energy Conversion Electronic Devices Surface Processes

Surface Chemistry the study of physical and chemical phenomena that occur at the interface of two phases, including solid-liquid interfaces, solid-gas interfaces, solid-vacuum interfaces, and liquid-gas interfaces

Phenomena in Surface Chemistry Wetting, Spreading and Penetration Foam Breaking in Aqueous Systems Solubilization Rheological Effects in Surfactant Phases

Surface Chemistry in Important Technologies Surface Chemistry in Pharmacy   Surface Chemistry in Food and Feed   Surface Chemistry in Detergency  Surface Chemistry in Agriculture   Surface and Colloid Chemistry in Photographic Technology   Surface Chemistry in Paints   Surface Chemistry of Paper   Surface Chemistry in the Polymerization of Emulsion   Colloidal Processing of Ceramics   Surface Chemistry in Dispersion, Flocculation and Flotation   Surface Chemistry in the Petroleum Industry

Analysis and Characterization in Surface Chemistry Measuring Equilibrium Surface Tensions   Measuring Dynamic Surface Tensions   Measuring Contact Angle Determining Critical Micelle Concentration    Measuring Micelle Size and Shape   Identification of Lyotropic Liquid Crystalline Mesophases   Characterization of Microemulsion Structure   Measuring Particle Size by Light Scattering   Measurement of Electrokinetic Phenomena in Surface Chemistry   Measuring Interactions between Surfaces   Measuring the Forces and Stability of Thin-Liquid Films   Measuring Adsorption

1. Gas-Liquid and Liquid-liquid Interfaces

Surface Tension,  an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet

Surface Tension as a Force caused by the attraction between the molecules of the liquid by various intermolecular forces

تطبيقات كيمياء السطوح والحفز تعد تطبيقات السطوح كثيرة ولاحصر لها ومنها : 1- الحفز 2- المنظفات ( إزالة الأوساخ ) 3- الفصل الكروماتوجرافي ( الفصل اللوني ) 4- طفو المعادن 5- طرد الماء 6- التبادل الأيوني 7- الصباغة 8- الدهانات 9- معالجة المياه هل لديك تطبيقات أخرى؟

مقدمة للجزء الأول (التوتر السطحى) هل فكرت يوماً كيف تقف بعض الحشرات على الماء دون حراك؟ هل فكرت كيف تستطيع وضع إبرة حديدة بحيث تطفو على سطح الماء؟! كل هذا.. تجاوبه ظاهرة طبيعية.. ظاهرة التوتر السطحي.

تعريف التوتر السطحى التوتر السطحي هو ذلك التأثير الذي يجعل الطبقة السطحيّة لأي سائل تتصرف كورقة مرنة. ذلك التأثير الذي يسمح للحشرات . بالسير على الماء، والأشياء المعدنية الصغيرة كالإبر، أو أجزاء ورق القصدير من الطفو على الماء ، وهوالمسبب أيضا للخاصية الشعرية.

هناك تعريفان التوتر السطحى تعريف التوتر السطحى هناك تعريفان التوتر السطحى التعريف الأول : هو الشغل المبذول لزيادة مساحة سطح السائل بمقدار وحدة المساحات و وحداته هى جول/ م2 . التعريف الثانى : هو القوة السطحية المؤثرة عموديا على وحدة الأطوال من سطح السائل. و وحداته هى نيوتن/ متر  و يتضح أن  

سبب ظاهرة التوتر السطحي جزيئات السائل التي في داخل السائل تتعرض لقوى متساوية في جميع الاتجاهات، بينما الجزيئات التي على سطح السائل تتعرض لقوى تجذبها نحو عمق السائل الأمر الذي يجعل جزيئات السطح تتصرف وكأنها غشاء مشدود يحدث التوتر السطحي بسبب التجاذب بين جزيئات السائل بواسطة التغير في قوى الجزئيات الداخلية. معظم السائل كل جزيء يسحب بالتساوي في جميع الإتجاهات بواسطة جزيئات السائل المجاورة ، ومحصّلة هذه القوى صفر.عند سطح السائل تسحب الجزيئات بواسطة الجزيئات الأخرى الأعمق في السائل ولكن ليست الجاذبية كجاذبية الجزيئات المجاورة لها في الوسط من حيث الشدّة (تكون كضغط هواء أو سائل آخر). لذلك كل الجزيئات عند السطح تكون عرضة لقوى داخليّة من التجاذب الجزيئي الذي من الممكن أن يكون متَّزن فقط مع مقاومة السائل للضغط. ولذلك يغير السائل شكله حتى يشغل أقل مساحة سطح ممكنة. A B

Interface is the boundary between two or more phases exist together The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interfacial phase. Several types of interface can exist depending on whether the two adjacent phases are in solid, liquid or gaseous state. Important of Interfacial phenomena in pharmacy: Adsorption of drugs onto solid adjuncts in dosage forms Penetration of molecules through biological membranes Emulsion formation and stability The dispersion of insoluble particles in liquid media to form suspensions.

LIQUID INTERFACES Surface and Interfacial Tensions In the liquid state, the cohesive forces between adjacent molecules are well developed. For the molecules in the bulk of a liquid They are surrounded in all directions by other molecules for which they have an equal attraction. For the molecules at the surface (at the liquid/air interface) Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them. They can develop adhesive forces of attraction with the molecules of the other phase in the interface The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F .

To keep the equilibrium, an equal force must be applied to oppose the inward tension in the surface. Thus SURFACE TENSION [γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of dyne/cm INTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of dyne/cm. Invariably, interfacial tensions are less than surface tensions because an adhesive forces, between the two liquid phases forming the interface are greater than when a liquid and a gas phase exist together. If two liquids are completely miscible, no interfacial tension exists between them. Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST

The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2) erg = dyne . cm Its equivalent to the surface tension γ Thus the greater the area A of interfacial contact between the phases, the greater the free energy. W = γ ∆ A For equilibrium, the surface free energy of a system must be at a minimum. Thus Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume.

Methods for measuring surface and interfacial tension Measurement of Surface and Inter­facial Tensions Methods for measuring surface and interfacial tension 1- Capillary rise method 2- Ring (Du Nouy) tensiometer 3- Drop weight method (Stalagmometer) The choice of the method for measuring surface and interfacial tension depend on: Whether surface or interfacial tension is to be determined. The accuracy desired The size of sample.

Session Objectives Adsorption Adsorption versus absorption Types of adsorption: physisorption and chemisorption Desorption Adsorption isotherms: Freundlich and Langmuir Adsorption isobar Catalysis

Adsorption The phenomenon of higher concentration of any molecular species at the surface than in the bulk Adsorbent The substance on the surface of which adsorption takes place is called adsorbent Adsorbate The substance which is being adsorbed on the surface of another substance. Desorption The process of removal of an adsorbed substance from the surface on which it is absorbed

Adsorbent Materials Activated Carbon Activated Alumina Silica Gel Molecular Sieves (Zeolites) Polar and Non-polar adsorbents

Activated carbon Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate and carbonize (remove volatile components), "Activation" is the process that produces the porous structure essential for effective adsorption by oxidation of carbon with water vapor or CO2. Activated carbon attracts non-polar molecules such as hydrocarbons. Typical surface areas are 300 to 1500 m2/g.

Adsorption vs absorption

Types of Adsorption Positive adsorption occurs when the concentration of adsorbate is higher on the surface of adsorbent than in the bulk. Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk.

Types of adsorption Physical adsorption Chemical adsorption

Factors affecting adsorption Effect of adsorbate: The easily liquifiable gases like NH3, HCl, CO2 etc. are adsorbed to a greater extent than the permanent gases such as H2 ,O2, N2, etc. Effect of specific area of the absorbent: The greater the specific area of the solid, the greater would be its adsorbing capacity. Effect of temperature:adsorption decreases with increase in temperature. Effect of pressure: An increase in pressure causes an increase in the magnitude of adsorption of an adsorbent.

Freundlich Isotherm A graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at constant temperature is called adsorption isotherm At low pressure the graph is nearly straight line At high pressure x/m becomes independent of p Over a narrow range of p

Freundlich Isotherm

Langmuir isotherm f: fraction of surface area covered p: partial pressure of the adsorbate f m: mass of adsorbate adsorbed per unit mass of adsorbent 1-f Rate of adsorption Rate of desorption At equilibrium, ra = rd; Mono-layer coverage

Langmuir adsorption isotherm: Combining equations (1) and (2): a = ka x ka’/kd b = ka/kd The values of constants ‘a’ and ‘b’ depend upon the nature of adsorbate, nature of solid adsorbent and temperature.

Summary of adsorption isotherms Easy to fit adsorption data Chemisorptions and physisorption Freundlich Useful in analysis of reaction mechanism Chemisorption and physisorption Langmuir Note Application Isotherm equation Name

Adsorption isobar Graph between the amount adsorbed(x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar Chemisorption isobar shows an initial increase with temperature and then expected decrease .The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption.

Application of Adsorption In clarification of sugar In gas masks In catalysis In adsorption indicators In chromatographic analysis In softening of hard water In preserving vacuum In paint industry In removing moisture from air in the storage of delicate instruments

Need to make chemicals faster Most Reactions are too slow to be useful...

Ways to Make Chemicals Faster Disadvantage--Too hot! Temperature Pressure Disadvantage--Cause Explosions Catalysts!!!! Disadvantage--Costly Add other Chemicals Disadvantage--Separate chemicals

The Story of Catalysis Why Catalysis ? What is a Catalyst ? How Catalysts Work ?

Role of a Catalyst Catalysts speed up a chemical reaction without being used up... Catalyst + Reactants Catalyst-Reactants Catalyst + Products

Hydrogenation of alkene

Important properties of catalyst Activity: A reasonable rate of reaction is needed. Selectivity: Byproducts should be minimized. Cost: The acceptable cost depends upon the catalyst lifetime and product value.

Catalysis Positive catalysis Negative catalysis Auto catalysis

Catalysis Types of catalysis Homogeneous catalysis Process Products Catalyst 1. Hydrolysis of an ester. Acid and Alcohol H+ 2. Oxidation of SO2 to SO3 SO3 NO in lead chamber process Heterogeneous catalysis Process Products Catalyst 1. Ammonia synthesis NH3 Fe with Al2O3 2. Methanol synthesis MeOH ZnO/Cr2O3

Catalysis Enzyme catalysis Reaction Catalyst Invertase 2. Zymase A coenzyme is an organic non-protein molecule that is a functional part of an enzyme. Coenzymes are not used up in the reactions in which they assist

Do Catalysts Live Forever? NO!!! They can die from: -- poisons which contaminate the catalyst -- large molecules which cover the catalyst -- over heating, over pressurizing -- crumbling/crushing

Automotive Emission Control Automobile catalytic converters need to catalyze several reactions – CHx + O2 CO2 + H2O – CO + O2  CO2 – NOx +CHx  N2 + H2O + CO2 – This is achieved by the use of a supported precious metal catalyst like platinum, palladium etc. – Catalyst needs O2 to operate, CeO2 acts as a temporary regenerable source of O2. CeO2  Ce2O3 + 1/2O2

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