Surface and interfacial tension as per PCI sylabbus

Defintion and measurements of surface and interfacial tension, surface free energy, spreading coefficient, adsorption at liquid interface, surfactants/ surface active agents, HLB scale, wetting phenomena, solubilization, detergency, adsorption at solid/liquid interface,

1. SURFACE AND INTERFACIAL PHENOMENA B y: Ms. Ashiya Najfath Assistant Professor PA College of Pharmacy

2. INTRODUCTION The boundary that forms between 2 phases, say solid and liquid is called Interface . The properties of the molecules forming the interface are often different from those in the bulk of each phase that they are referred to as forming an interfacial phase The term surface is normally used to denote interface when one of the phases is gas. OR Surface is the term used to describe either a gas-solid or gas-liquid interface.

3. Different types of surfaces and interfaces:

4. Application of surface and interfacial phenomenon in pharmacy Adsorption of drug on solid adjunct in dosage form Penetration of molecules through biological membrane Formation of emulsion and its stability Dispersion of insoluble particle in liquid media forming suspension Few process such as wetting of powders and detergency are based on the principles of interfacial phenomenon.

5. SURFACE TENSION The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid , which tends to minimize surface area is known as surface tension. OR Surface tension is defined as force in newtons , acting on the surface of the liquid at right angles to any line of length of the surface, 1 metre . Units of surface tension: In CGS system- dynes/cm In MKS system- Newtons /meter

6. Consider a pure liquid state. Like molecules experience cohesive forces, whereas unlike molecules experience adhesive forces with adjacent forces with adjacent molecules. The molecules in the bulk are surrounded in all directions with equal forces of attraction. But the molecules at surface of liquid had little attractive forces, then the molecules present within the liquid. But the molecules at the surface experience net inward pull in the bulk of liquid state. This force experienced by surface molecule is called surface tension.

7. The phenomenon of surface tension is responsible for the following process: Formation of spherical globules in emulsions. Formation of nearly spherical shape of falling water droplets. Formation of spherical shape pf mercury particles on flat surface. Rise of liquid in capillary tube.

8. INTERFACIAL TENSION   Interfacial tension is defined as the force per unit length that exist at the interface between two immiscible liquid phases. 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. The units of interfacial tension are same as that of surface tension.

9. Example: The surface tensions of some common liquids and their interfacial tensions against water at 200 C Liquids Surface tension (dynes/cm) Interfacial tensions against water (Dynes/cm) Water 72 - n-Octanol 27 8.5 CCl4 27 45 Olive oil 36 33 n-Hexane 18 51

10. SURFACE FREE ENERGY It is defined as the small amount of work done to increase the area of the liquid by 1 metre 2 . (Derivation)

11. Consider 3 dimensional soap film over the rectangular wire ABCD. The side AD=L is movable. A drop of soap solution is placed on the frame, so that it forms a film within the frame. When force is applied downwards, the film gets stretched as the movable bar AD goes down until the film breaks. If force ‘f’ is applied on AD it shifts the movable wire to distance ‘d’ to A’D’. The work done ‘W’ is given by Work done (W)=force (f) × distance moved (d) - (1) To increase the surface of the soap film small amount of work is done against the surface tension. f= γ × 2L - (2) Substituting eqn (2) in (1) W= γ × 2L × d -(3) (2L × d =∆A, increase in surface area) W= γ × ∆A ∆G= γ × ∆A (∆G=Surface free energy expressed in Ergs)

12. MEASUREMENT OF SURFACE AND INTERFACIAL TENSIONS Capillary rise method Ring detachment method (Du Nouy tensiometer ) Drop weight & drop count method Wilhelmy plate method Bubble pressure method

13. Capillary Rise M ethod Used for the determination of surface tension. When a capillary tube is placed in the liquid contained in a beaker, the liquid rises in the tube for certain distance (height=h). This is because the force of adhesion between the liquid molecules and the capillary wall is greater than the force of cohesion between the liquid molecules. The rise in tube continues until the upward movement is balanced by the downward force of gravity (due to the weight of the liquid).

14. Upward component (a) Radius of capillary tube is ‘r’ The upward force represents surface tension γ , p ulling liquid against gravitational force is a= γ cos ϴ - (1) γ - surface tension of liquid ϴ - contact angle between surface of liquid & capillary wall The total upward force around the circumference of the capillary tube is a= 2 r γ cos ϴ -(2) For water( ϴ =0), because water completely wets the capillary wall. For other liquids used in pharmacy ϴ is very small ( ϴ is nearly=0). So Cos ϴ =1 Thus equation (2) becomes, a= 2 r γ -(3)

15. Downward component (b) The counteracting force is gravity and depends on the weight of the liquid in the capillary rise. Downward component (b)= mass of liquid × acceleration b=(mass upto lower meniscus + w) × acceleration Where w- additional mass of water present above meniscus, it can be disregarded owing to its small quantity. b=mass × acceleration =volume × density × acceleration =cross sectional area × height × density × acceleration = r 2 × h × ƍ × g -(4) At equilibrium, upward component = downward component a=b 2 r γ = r 2 × h × ƍ × g γ = rhƍg  

16. Method: A clean and dry capillary tube is selected and fixed horizontally and fixed horizontally to a stand. The diameter of the capillary tube is determined by travelling microscope & then radius is calculated. About 50ml of water is placed in 100ml beaker. The capillary tube is dipped in water, so that it touches the water surface. The liquid rises in the capillary tube u pto particular height (h 1 ) – determined by travelling microscope. The beaker is then removed, without disturbing the assembly of capillary tube. Again the height of the liquid is noted (h 2 ) Difference in the heights (h=h 2 -h 1 ) denotes rise of liquid in capillary tube. Density of liquid is determined by specific gravity bottle pycnometer (ƍ) Calculate surface tension by substituting all the required data in formula.

17. Ring detachment method/ DuNouy Tensiometer Used to measure surface and interfacial tension . Convenient especially when liquid sample is small. Apparatus : DuNouy tensiometer OR torsion balance. Principle: The force required to detach the platinum iridium ring immersed at the interface or surface is measured. The force is equal to surface or interfacial tension.

18. Method A cleaned platinum-iridium ring is attached to the hook at the torsion lever arm. A more dense liquid is transferred into a clean glass vessel & placed on the table. The table is moved until the ring is immersed in the liquid. The torsion lever arm is released by rotating the knob & instrument is adjusted to 0 reading. The light liquid is poured on the surface of the heavier liquid (in interfacial tension). It consists of tension wire ‘w’ with fixed end ‘A’ and the other end ‘B’ connected to knob ‘K’ that rotates.

19. The rotation produces tension in wire that is transmitted to ring R ( DuNouy Ring). Reading gives the value of force (F) – direct measure of surface tension.

20. Drop formation method D etermination of surface tension . Mechanism is same in both method. Apparatus used- Stalagmometer Upward force= Surface tension on outer circumference of tube = 2 r γ Downward force (on drops)= weight of drop × acceleration = mg If upward force = downward force, then drop breaks 2 r γ = mg - (1)

21. Drop weight method By using clean dried stalagmometer , 20 drops of given unknown liquid are taken in weighing bottle & drop weight is determined (m 1 ). Then 20 drops of reference liquid are taken in weighing bottle & drop weight is determined (m 2 ).

22. Drop count method By using clean dried stalagmometer , the unknown liquid is filled till mark A & number of drops are counted from point A to point B. Similarly, the stalagmometer is filled with reference liquid till mark A & number of drops are counted from point A to point B.

23. Wilhelmy plate method Determination of surface tension . The apparatus consist of a thin mica, glass or platinum plate attached to a suitable balance. It is attached in such a way that it can be immersed in the liquid whose surface tension is to be measured. The plate is then carefully immersed into the liquid, ensuring that it is completely wetted by the liquid. The force required to detach the plate from the liquid is measured by the balance.

24. Where, WL= Reading on the balance prior to detachment W= Weight of plate Length of the plate T= Thickness of plate

25. SPREADING COEFFIECIENT The ability of one liquid to spread over another is calculated as “spreading coefficient”. It is defined as the difference between work of adhesion and work of cohesion. S=WA-WC Work of Adhesion- Energy required to break attraction between the unlike molecules. Work of cohesion- Energy required to break attraction betwwen like molecules . If WA >> WC → then, S is + ve , A spreads over B If WA << WC → then, S is – ve , A forms globules on surface B → No spreading

26. Application of spreading coefficient Absorption of medicament Stabilization of emulsion Coating of tablets

27. ADSORPTION AT LIQUID INTERFACE Absorption is a process wherein a substance is uniformly distributed on surface as well as bulk. Cb- Bulk concentration Cs- Surface concentration The process of absorption is Cb=Cs Adsorption is a surface phenomenon wherein the molecules accumulates on surface. There is positive adsorption & negative adsorption. In positive adsorption , molecules accumulate more on surface than in the bulk, ie ., Cs is much greater than Cb. In negative adsorption , molecules get accumulated in bulk more than on surface, ie ., Cb>>Cs.

28. Adsorbate – Molecules which adsorb Adsorbent – The surface on which molecules adsorb

29. SURFACE ACTIVE AGENTS/ SURFACTANTS Molecules and ions that are adsorbed at the interface are termed as surface active agents or surfactants. In physical sense , Surfactants are defined as, substances which get adsorbed at the interface & exhibit self association in the bulk of the solution & reduce the interfacial tension. In chemical sense , surfactants are defined as polymer like substances that have polar & non polar group . Thus they are termed as amphiphiles indicating they have hydrophilic and lipophilic portions on their structures.

30. Functional groups: Classification based on number & nature of groups Predominantly hydrophilic : More polar groups Predominantly lipophilic : More nonpolar groups

31. Air-Water Oil-water

32. Classification of surfactants Anionic surfactants It ionizes in aqueous solution to yield a large anion and small cation. On dissociation , the long chain anion of these surfactants imparts surface activity , while cation is inactive . Not suitable for internal use because of their unpleasant taste & irritant action on the intestinal mucosa . Examples: oleic acid, sodium lauryl sulphonate (SLS)

33. 2. Cationic surfactants: They dissociate to form positively charged cations . Uses: Disinfecting agent, antiseptics, secondary emulsifying agents for external application . Examples: Quaternary ammonium compounds such as cetrimide , benzalakonium chloride. 3. Ampholytic surfactants: Their ionic characteristics depends on the pH of the system . Below certain pH – these are cationic Above certain pH – these are anionic Intermediate pH – zwitter ions Example: Lecithin & N-dodecyl alanins

34. 4. Nonionic surfactant They do not ionize in aqueous solution. Largest group of surface active agents used pharmaceutically . ☺ Compatible with both anionic & cationic surfactants, low irritancy, Resistance to pH change.  Inactivates preservatives having phenolic or carboxylic groups. Example: Glycerol monostearate , spans, tweens

35. HLB SCALE HLB- Hydrophilic L ipophilic Balance HLB is an arbitrary scale that indicates the extent of hydrophilic & lipophilic balance. It is denoted by simultaneous attraction of the hydrophilic or lipophilic group towards water or oil phase . In general molecules that are oil soluble (lipophilic in nature- produce in W/O type of emulsion ) have low HLB values, those that are water soluble (hydrophilic in nature- produce in O/W type of emulsion ) have high HLB values .

36. Methods of estimation Griffins HLB Scale: An arbitrary scale of surfactants was devised by griffin. The values are the measure of hydrophilic-lipophilic balance of surfactants. It ranges from 0-18. 2-3 → Antifoaming agent These avoid the formation of foam when molecules are agitated. 3-6 → W/O emulsifying agents Used in W/O emulsion to reduce interfacial tension at the interface.

37. 7-9 → Wetting and spreading agents Reduce the contact angles and facilitate uniform spreading of liquids on surfaces. 8-18 → O/W Emulsifying agents Used in O/W emulsion to reduce interfacial tension at the interface . 13-15 → Detergents These agents wet the d irt in clothes, leading to emulsification of dirt, entrapment of d irt within foam , that is easily washed by water. 16-18 → Solubilizing agents They disperse water insoluble substances by micellar solubilization .

38. b) David’s formula: The structure of surfactant molecules is split into different component groups & each group is assigned a number. HLB= Σ (hydrophilic group numbers) – Σ ( lipophilic group numbers) + 7 Applicable for ionizing type of surfactant (like SLS ). c) Formula for surfactants containing polyoxyethylene chains: HLB= Where, E → % by weight of ethylene oxide chains P → % by weight of polyhydric alcohol groups Eg : Beeswax & lanolin  

39. d) Formula for ester type of surfactant: HLB = 20 (1 − S/A ) Where, S → Saponification number of ester A → Acid number of fatty acid Eg : Tween 80 Saponification number: Number of milligrams of potassium hydroxide required to saponify a certain amount of fat or oil. Acid number: N umber of milligrams of KOH required in terms of normality to neutralize the free acid in one gram of sample .

40. WETTING PHENOMENA Wetting is adsorption process in which an intimate contact of the solids with liquid phase is achieved. Surfactants are used to aid wetting of powders. They lower interfacial tension, contact angle and displace air & permit intimate contact . Contact angle (  ) is defined as angle between the liquid droplet & the surface over which it spreads .

41. Application: In preparation of suspensions and emulsions. In case of granulation, prior to tabeletting , the powders are mixed with liquid binders. Film coating requires the wetting of liquids over the tablet surface. Dissolution of tablet.

42. 5 categories  = 0 This means liquid is miscible with one another. 2)  = 90 The 2 immiscible liquids or solid & liquid – equilibrium.

43. 3)  < 90 It indicates wetting of the surface is favourable & fluid will spread over a large area on surface. 4)  > 90 It indicates wetting of the surface is unfavourable & fluid will minimize its contact with surface.

44. 5)  = 180 This indicates absolute no wetting of liquid on surface.

45. SOLUBILIZATION It is the process in which, solubility of organic compound is increased in aqueous medium with the help of surface active agents (surfactant), this phenomenon is know as solubilization . OR It is defines as the preparation of thermodynamically stable isotropic solution of substance normally insoluble or very slightly soluble in a given solvent by addition of components ( eg : surfactants) or by suitable methods(complexation or micronization ) It is used in many industries for the mixing of two immiscible liquids and help in making of drug.

46. Mechanism of micelle formation Amphiphiles (surfactants ) at low concentration exist as monomers , as the concentration increases the monomers aggregate themselves & these aggregates are termed as micelles . CMC :- Critical micelle concentration – is defined as the concentration range of surfactant at which micelles starts forming .

47. Amphiphiles are represented as Below CMC , Surfactants get adsorbed at the interface (air-water interface). As the concentration increases, molecules get accumulates at the interface. At one point the interface & the bulk of the solution are saturated with monomers. At CMC , the formation of micelles starts ( ie . The aggregation of monomers to micelles) B eyond CMC , any further addition of surfactants enhances the formation of micelles only.

48. DETERGENCY Surfactants in aqueous solutions are used to remove the dirt from substrates such as glass, fabric, skin etc . The principle of detergency is based on the formation of micelle . The HLB requirement for the detergency is about 13 to 16 . Examples: Cationic type: Zephiran , Cetrimide Anionic type: Soaps, Sodium lauryl sulphate

49. Steps: 1 . Initial wetting of the dirt from the surface 2. Solubilizing of the dirt 3. Removing the insoluble dirt as deflocculation particles 4. Suspending the particles in the detergent solution 5. Removing the oil soluble materials and convert into emulsion 6. Converting the dirt into foam so as to wash easily

50. Mechanism of detergency

51. ADSORPTION AT SOLID INTERFACE Solid surface when exposed to gaseous molecules or liquid molecules , adsorption takes place forming a uniform layer on the solid surface. The process by which gaseous or liquid molecules get concentrated on the surface, the process is termed as Adsorption . The solid surface is referred as Adsorbent . The gas or liquid molecules are referred as Adsorbate . Depending on the nature of interactions between adsorbate & adsorbent , adsorption is classified into physical & chemical adsorption .

52. PHYSICAL ADSORPTION CHEMICAL ADSORPTION The adsorbate molecules retains the original bond within it. The original bonds within adsorbate molecules are lost & new bonds are formed within the adsorbent. Vanderwaal’s forces Chemical bonds Reversible Irreversible Nonspecific More specific Common at low temperature Occurs at high temperature

53. Heat of adsorption - low Heat of adsorption - high Increase in pressure, adsorption increases No effect of pressure change Forms multimolecular layer on adsorbent Forms unimolecular layer on adsorbent Ex: adsorption of gases on charcoal Ex: adsorption of oxygen on silver or gold

54. Sorption: It is type of adsorption where both type of adsorption occurs simultaneously . Desorption: The phenomenon opposite to adsorption is termed as desorption. In this the adsorbed molecules or ions are removed from solid surface .

55. Applications of solid/gas interface: Removal of objectionable odors from rooms. Prevention of obnoxious gases entering into the body (gas masks) Estimation of surface area & particle size of powders.

56. Adsorption isotherm In the study of adsorption, the amount of gas adsorbed per unit area, or unit mass of solid is measured at different pressures of the gas . The study is usually conducted at constant temperature & graphs are plotted . These plots are called adsorption isotherm. OR The relationship between the adsorbate (gas) physically adsorbed on adsorbent (solid) (Y axis) as a function of its pressure or concentration (X axis), at constant temperature yields an adsorption isotherm . The term isotherm refers to a plot or graph at constant temperature . The number of moles, grams, or milliliters x of gas adsorbed on m grams of adsorbent at standard temperature and pressure is plotted on the vertical axis against the equilibrium pressure of the gas in mmHg on the horizontal axis.

57. C oncepts of adsorption isotherm : Freundlich adsorption isotherm Langmuir adsorption isotherm BET equation

58. Freundlich adsorption isotherm Freundlich proposed a relationship in the form of mathematical equation given by, 1/n -(1) Where, x- amount of gas molecules adsorbed m- mass of solid surface k & n- constants & depends on temperature and nature of gas.  

59. When we plot a graph of x/m with P, we get curved plot which indicates that linear relationship is obtained at low pressure . As pressure increases curvature is seen in the plot.

60. When we take log on both sides, equation (1) changes Log = log k + log P When a plot is drawn w.r.t. log values, we get straight line when pressure is low . Further, increase in pressure curvature is seen in the graph. This indicates freundlich isotherm established the relationship of adsorption with pressure at lower values, but failed to predict value of adsorption at higher pressure .  

61. Langmuir adsorption isotherm Assumptions: The surface of solid possess fixed number of active sites for the adsorption of gases At maximum adsorption , the gas layer is found around the solid and only one molecule thick The rate of adsorption  number of sites unoccupied The rate of evaporation (desorption)  number of occupied sites

62. Closed vessel is taken with the solid surface in which gas molecules are introduced at constant pressure. Initially the concentration of free gas molecules is on the greater side, but with the time they get adsorbed on the solid surface. Simultaneously some gas molecules get evaporated from the surface to free molecule state. This at equilibrium. Adsorption = desorption

63. A plot of P/Y against P should yield a straight line, and y, and b can be obtained from the slope and intercept. 1/ Ymb 1

64. Brunauer , Emmett and Teller (BET) equation BET equation is an extension of the Langmuir theory with the following hypotheses : Adsorptions occur only on well-defined sites of the sample surface Gas molecules physically adsorb on a solid in layers and form multilayer adsorption During multilayered adsorption, it is not necessary for a layer to be completed before an upper layer formation starts Gas molecule can interact with adjacent layers To describe the each layer of adsorption, the Langmuir theory can be used.

65. Types of adsorption isotherm Type I This type of isotherm represents increase in adsorption with increasing pressure & followed by levelling off. This levelling off is due to saturation of the surface with adsorbate. Ex: adsorption of N 2 gas at 78K or oxygen at 83 K on charcoal.

66. 2. Type-II This isotherm is sigmoidal in shape & occurs when gases undergo physical adsorption onto nonporous solids. The first inflection point represents the formation of monolayer, When the pressure is increased multilayer is formed. This isotherm is explained by BET equation. Ex: Adsorption of N 2 on iron/platinum at 78K.

67. 3. Type-III A rarely seen case, wherein the heat of adsorption of gas in first layer is less than the latent heat of condensation of subsequent layers. Ex: Adsorption of bromine on silica or alumina.

68. 4. Type-IV This plot represents the adsorption of gas on porous solids. Initially monomolecular is obtained but it is seen that more amount of gas molecules gets condensed within the pores of the solid. Ex: adsorption of benzene on silica gel

69. 5. Type-V This is seen rarely & indicates capillary condensation. Ex: adsorption of water vapour on charcoal at 100 C

70. Applications of adsorption at solid/liquid interface Gas mask Antidote in poisoning R emoval of coloring matter from solution C hromatographic analysis

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