Intermolecular Forces and Phase Equilibria PDF

1  Attractive forces between molecules.  Occur when there is a variation in e-distribution in a molecule. – Attractive forces between atoms are responsible for the formation of molecules. – Attractive forces between molecules are responsible for the state of substances, that is, s...

1  Attractive forces between molecules.  Occur when there is a variation in e-distribution in a molecule. – Attractive forces between atoms are responsible for the formation of molecules. – Attractive forces between molecules are responsible for the state of substances, that is, solid, liquid, or gas. The strengths of intermolecular forces are generally weaker than either ionic or covalent bonds 2  Types of attractive intermolecular forces  Van der Waals forces Dipole - dipole attractions (Keesom forces ) Dipole - induced dipole attractions (Debye forces) Induced dipole - induced dipole attractions (London forces or dispersion forces)  H-bonding  Ion – dipole and ion – induced dipole forces  Ion- ion interaction 3 Dipole – dipole attractions  Occur between the dipoles of 2 polar molecules.  Caused by the permanent, uneven distribution of electrons, which is caused by the electro -vity differences of atoms in the molecule. +HCl----- +HCl- dipole-dipole attraction Dipole - induced dipole attractions  Type of attraction that occurs between a polar molecule and non polar molecule. 4  – The non polar molecule temporarily become polarized. Induced dipole - induced dipole attractions  Force in which non polar molecules induce instantaneous polarity in one another. “Electrons are shifted to overload one side of an atom or molecule”. 5 Ion - dipole & Ion - induced dipole forces  Account in part for solubility of ionic crystalline substances in water.  The cation attracts the relatively -ve O of H2O & the anion attracts the H-atoms of the dipolar H2O molecules.  Ion-induced dipole forces are presumably involved in the formation of the iodide complex. I2 + k+I- = K+I-3 – This reaction accounts for the solubility of iodine in a solution of potassium iodide. 6 Ion-dipole interactions (e.g., a salt dissolved in water) cation Polar molecule anion Ion – Ion interactions  A cation on one compound will interact with an anion on another compound, giving rise to an intermolecular association.  May be intermolecular (e.g., a hydrochloride salt of a drug) or intramolecular ( e.g., a salt-bridge interaction between counter ions in proteins) 7 H - Bonds/ Bridge  A force b/n H & an e-ve atom (O, N & F).  The strongest type of dipole-dipole attractions. (e.g, exists in ice & liquid water) which accounts for many of unusual properties of water including:  High dielectric constant.  Abnormally low vapor pressure.  High boiling point. 8  Roughly 1/6 of H-bonds of ice are broken when H2O passes into liquid state & essentially all the bridges are destroyed when it vaporizes.  H-bonds can also exist between alcohol molecules, carboxylic acids, aldehydes, esters & polypeptides.  It will be noticed that intra- as well as inter-molecular H- bonds may occur (as in salicylic acid). 9  Phase: homogeneous, physically distinct portion of a system that is separated from other portions of the system by bounding surfaces.  The 3-primary phases (solid, liquid & gaseous) of matter are defined individually under different conditions, but in most systems we encounter phases in coexistence. – E.g, a glass of ice water on a hot summer day comprises three coexisting phases: ice (solid), water (liquid) & vapour (gaseous). 10  J. Willard Gibbs formulated “a relationship for determining the least number of intensive variables’’ (independent variables, e.g., Tº, pressure, density, and conc.) – that can be changed without changing the equilibrium state of the system, or – alternatively, the least No required to define the state of the system.  This is critical number (F) = number of degrees of freedom of the system.  The rule is expressed as follows: F = C-P+2 Where, C: number of components and P: number of phases present. 11  F (degrees of freedom):-The least No of intensive variables that must be fixed/known to describe the system completely.  A mixture of ice, liquid water & water vapor is a 3-phase system.  C is the No of constituents of each phase in the system and can be expressed in the form of a chemical formula or equation. – Eg., C in the mixture of ice, liquid water and water vapor is 1. B/c the composition of all the 3 phases is described by the chemical formula H2O. 12 Examples 1. A given mass of gas ,say, water vapor, confined to a particular volume F = 1-1+2 = 2 2. A system comprising a liquid, say, water in equilibrium with its vapor F = 1-2+2 = 1 3. Suppose we cool liquid water and its vapor until a third phase/ice/ separates out F = 1-3+2 = 0 13 14  In any one of the 3 regions p=1, the phase rule gives. F = 1-1+2 = 2  Therefore, we must fix 2 conditions: To & P.  Along 3 of the curves where 2 phases exist, F = 1.  If the system contains both liquid water & water vapor at 100 oC , we need not specify Pressure, for the vapor pressure can have no other value than 760 mm Hg.  Hence, only one condition need to be given.  At the triple point, where the 3 phases:-Ice, liquid water, and water vapor – are in equilibrium, we say F = 0. 15  2-C system  A max of F = 3 is possible. (E.g, Tº, P & conc.)  B/c in practice we are primarily concerned with liquid and/or solid phases in the particular system under examination.  We frequently choose to disregard the vapor phase & work under normal conditions of 1atm (760 mm Hg) of pressure.  In this manner we reduce the number of F by 1.  In a 2-C system, therefore, only 2 variables (Tº & conc.).  Systems in w/c the vapor phase is ignored & only solid and/or liquid phases are considered are termed condensed systems.  Most appropriate for solid & liquid dosage forms. 16  2-C systems containing liquid phases – Water & ethyl alcohol = miscible in all proportions, – Water & mercury = immiscible. – Water & phenol exhibit partial miscibility (or immiscibility). Phenol – water systems 17  Curve gbhci shows limits of Tº & conc. within w/c 2 liquid phases exist in equilibrium.  Outside this curve contains systems having one liquid phase.  Once the total conc. of phenol exceeds 63% at 50 oC, a single phenol-rich liquid phase is formed.  Critical solution, or upper consolute Tº: The max Tº at which the 2-phase region exists.  In the case of phenol water system, this is 66.8 oC.  All combinations of phenol & water above this Tº are completely miscible and yield one-phase liquid systems. 18  The line bc is termed a tie line; always parallel to base line. All systems prepared on a tie line, at equilibrium, will separate into phases of constant composition. – This phases are termed conjugate phases whose compositions are b & c.  However, relative amounts of the two layer of phases vary.  Thus: 19 Applications of phase diagram  To formulate systems containing >1-C where it may be advantageous to achieve a single phase product.  E.g, handling of solid phenol, a necrotic agent, is facilitated in pharmacy if a soln of phenol with water is formulated.  A number of solns containing different conc. of phenol are official in several pharmacopeias.  Unless the FP of phenol-water mixture is sufficiently low some solidifications may occur at low ambient Tº.  This will lead to inaccuracies in dispensing &loss of convenience. 20  Mulley determined the relevant portion of phenol-water phase diagram & suggested that most convenient formulation of a single liquid phase solution was about 76% w/w.  This mixture has a FP of about 3.5 oC compared with liquified phenol, USP, w/c contains 90% w/w of phenol and freezes at about 17 oC.  It is not possible, therefore, to use the official preparation much below 20 oC.  The formulation proposed by Mulley from consideration of phenol-water phase diagram is therefore to be preferred. 21  3-component systems (ternary system)  For 3-component systems at constant Tº & pressure, the composition can be expressed in terms of coordinates of an equilateral triangle. 22  In figure each corner represents a pure component, i.e. 100% A, 100% B & 100% C.  Each side represents a binary mixture & the interior represents all ternary compositions.  A line parallel to one side of the triangle represents a constant percentage of one component.  These lines intersect at K which must be 20% of A, 50% of B, and therefore 30% of C. – E.g, In the formulation of pharmaceutical solutions containing 2-immiscible liquids plus a mutually soluble liquid (cosolvent or blending agent) the triangular diagram provides a convenient means of expressing the data. 23  In medicine they are significant factors that affect: – emulsion formation & stability. – dispersion to form suspensions. – adsorption of drugs onto solid adjuncts in dosage forms. – penetration of molecules through biological membranes.  Interface: boundary between two phases.  Surface: interface between liquid and gas or solid and gas. 25 Surface/Interfacial tension  In the bulk portion of each phase, molecules are attracted to each other equally in all directions, such that no resultant forces are acting on any one molecule.  At the boundary between phases, however, molecules, are acted upon unequally because they are in contact with other molecules exhibiting different forces of attraction. – Thus, molecules situated at the interface experience interaction forces dissimilar to those experienced in the bulk phase. 26  In liquid systems such unbalanced forces can be satisfied by spontaneous movement of molecules from the interface to the bulk phase. 27  This leaves fewer molecules per unit area at the interface (greater intermolecular distance) and reduces the actual contact area between dissimilar molecules.  Any attempt to reverse this process causes the interface to resist expansion & behave as though it is under tension everywhere in a tangential direction.  The force of this tension per unit length of interface generally is called the interfacial tension/surface tension.  Its unit in the cgs system is dyne/cm and in SI system = N/m 28  Surface free energy (G)  G is proportional to the size of free surface.  Each molecule near the surface of liquid possesses a certain excess of potential energy as compared to the molecules in the bulk of the liquid. – The higher the surface of the liquid, the more molecules have this excessive energy. – Therefore, if surface of liquid increases (e.g., when H2O is broken into a fine spray), the energy of the liquid also increases. 29  Each molecule of the liquid, has a tendency to move inside;  therefore, the liquid takes form with minimal free surface and with minimal surface energy. E.g, liquid droplets tend to assume a spherical shape because a sphere has the smallest surface area per unit volume.  The r/n ship b/n interfacial tension & surface energy can be. Where, ∆G is change in surface energy, ∆A is change in surface area and ϒ is surface tension. 30 Spreading  If a small quantity of an immiscible liquid is placed on a clean surface of a second liquid, it may spread to cover the surface with a film or remain as a drop or lens.  Which of the 2 (a drop or lens) applies depends on the achievement of a state of minimum free energy.  The ability of one liquid to spread over another can be assessed in terms of the spreading coefficient (S):  A +ve or zero value of S is required for spreading to occur. 31 Adsorption at liquid interfaces  Certain molecules & ions, when dispersed in the liquid, move of their own accord to the interface. – Their conc. at interface > their conc. in the bulk of the liquid.  Obviously, G & ϒ of the system are automatically reduced. – Such a phenomenon, where the added molecules are partitioned in favor of the interface, is termed adsorption, or, more correctly, +ve adsorption.  Other materials (e.g., inorganic electrolytes) are partitioned in favor of the bulk, leading to -ve adsorption & a corresponding increase in G & ϒ. 32 Surface active agents (surfactants) or amphiphile  Molecules & ions that are adsorbed at interfaces.  Based on number & nature of polar & non polar groups present, amphiphile may be predominantly hydrophilic (water-loving), lipophilic (oil-loving), or reasonably well balanced between these two extremes. – Eg, straight-chain alcohols, amines, & organic acids change from being predominantly hydrophilic to lipophilic as the number of C-atoms in the alkyl chain increased. 33 Hydrophilic-Lipophilic Balance (HLB) System  Griffin devised arbitrary scale (1 to ~ 50) of HLB of surfactants.  The more hydrophilic have high HLB (in excess of 10).  Whereas, those with numbers 1-10 are lipophilic.  HLB of polyhydric alcohol fatty acid esters such as glyceryl monostearate may be obtained from equation: – S = saponification value of ester & weight in mg of KOH required to saponify 1g of fat or oil – A = acid value of the fatty acid & the weight in mg of KOH required to react with 1g of fatty acid. 34 Note this formula can also be used to determine HLB of polysorbates & sorbitan esters.  For polysorbates (Tweens®) & sorbitan esters (Spans®) E = percentage by weight of oxyethylene chains P = percentage by weight of polyhydric alcohol group (sorbitol or glycerol)  This formula can be also used to determine HLB of beeswax & lanolin derivative. 35 36  Alternatively, HLB value can be calculated by 37  Adsorption at solid interfaces  The applications of adsorption of gases on solids: – Removal of objectionable odors from rooms and foods. – Operation of gas masks. – Measurement of the dimension of particles in a powder.  The principles of solid-liquid adsorption are used in: – Decolorizing solutions. – Adsorption chromatography. – Detergency. – Wetting. 38  Degree of adsorption of a gas by a solid depends on: – Chemical nature of the adsorbent (the material used to adsorb the gas) & the adsorbate (the substance being adsorbed). – the surface area of the adsorbent. – the temperature. – the partial pressure of the adsorbed gas.  The types of adsorption are generally recognized as: – physical or van der Waals adsorption and – chemical adsorption or chemisorption. 39  Physical adsorption, associated with van der Waal forces, is reversible, the removal of the adsorbate from the adsorbent is known as desorption. – E.g, physically adsorbed gas can be desorbed from a solid by increasing Tº & reducing pressure.  Chemisorption, in w/c the adsorbate is attached to the adsorbent by 1o chemical bonds, is irreversible unless the bonds are broken.  R/n ship b/n amount of gas physically adsorbed on a solid & the equilibrium pressure or conc. at constant Tº yields an adsorption isotherm.  The term isotherm refers to a plot at constant temperature. 40  The number of moles, grams, or milliliters, x, of gas adsorbed on, m, grams of adsorbent at standard Tº & pressure is plotted on the vertical axis against equilibrium pressure of the gas in mmHg on the horizontal axis. Type I isotherm 41  Adsorption isotherms 1. Freundlich suggested a r/n ship, Freundlich isotherm:  Y: mass of gas, x, adsorbed per unit mass: m, of adsorbent, P: pressure, K & n: constants. 42  The equation is handled more conveniently when written in logarithmic form. 2. Langmuir developed an equation based on the theory that ‘molecules or atoms of gas are adsorbed on active sites of a solid to form a layer one molecule thick (monolayer)’.  The fraction of centers occupied by gas molecules at pressure, P is represented by θ, and fraction of sites unoccupied is 1- θ. 43  Rate, r1 , of adsorption or condensation of gas molecules on surface is proportional to unoccupied spots, 1- θ & to the pressure, P, or r1 = K1(1- θ)p …1  The rate, r2 ,of evaporation (desorption) of molecules bound on the surface is proportional to the fraction of surface occupied, θ, or r2 = k2θ …2  At equilibrium, r1 = r2 or K1(1- θ)p = k2θ..…3 44  By rearrangement, we obtain ……………4  We can replace K1/k2 , by b and θ by y/ym , where y is the mass of gas adsorbed per gram of adsorbent at pressure p and at constant temperature and ym is the mass of gas that 1 g of the adsorbent can adsorb when the monolayer is complete.  Inserting these terms into equation (4) produces formula ………….5 ( Langmuir isotherm) 45  By inversing equation (5), and multiplying through by p, we can write this for plotting as ………………6  A plot of p/y against p yield a straight line, and ym and b can be obtained from the slope and intercept.  For multilayer adsorption, Langmuir equation was modified by Brunauer, Emmett and Teller (BET equation) 46 Where P = Pressure of the adsorbate. po = vapor pressure when the adsorbent is saturated with adsorbate vapor. ym = quantity of vapor adsorbed per unit mass of adsorbent. b = constant proportional to d/c b/n heat of adsorption of gas in the first layer & the latent heat of condensation of successive layers. – the saturation vapor pressure po is obtained by bringing excess adsorbate in contact with the adsorbent. 47 Get ready for Quiz! 48