III. SURFACE PROPERTIES III.A. SURFACE TENSION GOAL: To investigate the influene of the solution onentration and/or the kind of the solute on the surfae tension INTRODUCTION Liquids tend to adopt shapes that minimize their surfae area, for then the maximum numbers of moleules are in the bulk and hene surrounded by and interating with neighbors. Droplets of liquids therefore tend to be spherial, beause a sphere is the shape with the smallest surfae-to-volume ratio. However, there may be other fores present that ompete against the tendeny to form this ideal shape, and in partiular gravity may flatten spheres into puddles or oeans. Surfae effets may be expressed in the language of Helmholtz and Gibbs energies. The link between these quantities and the surfae area is the work needed to hange the area by a given amount, and the fat that the da and dg are equal (under different onditions) to the work done in hanging the energy of the system. The work needed to hange the surfae area, of a sample by an infinitesimal amount and we write: d is proportional to d dw d is alled the surfae tension; its dimensions are [energy/area] and its units are typially joules per meter squares [J/m 2 ]. However, values of are usually reported in newtons per meter [N/m]. The work of surfae formation at onstant volume and temperature an be identified with the hange in the Helmholtz energy, and we an write: da d
Beause the Helmholtz energy dereases if the surfae area dereases surfaes have a natural tendeny to ontrat. This is more formal way of expressing what was already desribed. The minimization of the surfae area of the liquid may results in the formation of a urved surfae, as in the bubble. Generally there are two onsequenes of urvature, and hene of the surfae tension, that are relevant to the properties of the liquids. One is that the vapour pressure of a liquid depends on the urvature of its surfae. The other is the apillary rise (or fall) of liquids in narrow tubes. The surfae tension an be measured quantitatively by various methods, we an measure the height to whih the liquid rises in a apillary tube, we an pulling a wire ring from the surfae of a liquid, we an weigh drops whih fall from a speial glass tip et. The ontat angle In many ases there is a nonzero angle between the edge of the menisus and the wall. The origin of the ontat angle an be traed to the balane of fores at the line of ontat between the liquid and the solid. If the solid-gas or the solid-liquid, liquid-gas surfae tensions are denoted sg, sl, respetively the horizontal fores are in balane if: sg sl This expression solves to: os sg sl if we note that the work of adhesion of the liquid to the solid (per unit area of ontat) is
wad sg sl Finally we an onsider that: os w ad 1 now one an see that the liquid ompletely wets the surfae fully, orresponding to 0, when o w ad 2. The liquid does not wet the surfae ( 90 ) when w ad. For merury in ontat with glass o whih orresponds to w / 0. ad 23 140, indiating a relatively low work of adhesion of the merury bto glass on aount of the strong ohesive fores within merury. Influene of temperature on surfae tension Surfae tension depends on temperature; for that reason, when a value is given for the surfae tension of an interfae, temperature must be expliitly stated. The general trend is that surfae tension dereases with the inrease of temperature, reahing a value of 0 at the ritial temperature. The orrelation between the temperature and the fore tension is desribed by Eötvös rule. There are only empirial equations to relate surfae tension and temperature: V is the molar volume of that substane T is the ritial temperature k is a onstant for eah substane. 2 / 3 V k( T T) Surfae tension in everyday life Some examples of the effets of surfae tension seen with ordinary water: Beading of rain water on the surfae of a waxed automobile. Water adheres weakly to wax and strongly to itself, so water lusters in drops. Surfae tension gives them their near-spherial shape, beause a sphere has the smallest possible surfae area to volume ratio
Formation of drops ours when a mass of liquid is strethed. Water adhering to the fauet gaining mass until it is strethed to a point where the surfae tension an no longer bind it to the fauet. It then separates and surfae tension forms the drop into a sphere. If a stream of water were running from the fauet, the stream would break up into drops during its fall. This is beause of gravity strething the stream, and surfae tension then pinhing it into spheres Surfae tension has a big influene on other ommon phenomena, espeially when ertain substanes, surfatants, are used to derease it: Soap Bubbles have very large surfae areas for very small masses. Bubbles annot be formed from pure water beause water has very high surfae tension, but the use of surfatants an redue the surfae tension more than tenfold, making it very easy to inrease its surfae area. Emulsions are a type of solution where surfae tension is also very important. Oil will not spontaneously mix with water, but the presene of a surfatant provides a derease in surfae tension that allows the formation of small droplets of oil in the bulk of water (or vie versa). GIBBS ISOTHERM Starting from the onsideration of thermodynami equilibrium, Josiah Willard Gibbs proved that surfae tension and onentration are linked through surfae onentration, Γ, represents exess of solute per unit area of the surfae over what would be present if the bulk onentration prevailed all the way to the surfae, it an be positive, negative or zero. It has units of mol/m 2. In the derivation of the equation it is assumed that the solution is ideal, (so μ = μ o + RTlnC) and surfae onentration of the solvent is zero, so it is only valid under these assumptions. It is also onsidered that the interfae is bidimensional, whih is not true, further models as Guggenheim's orret this flaw. Gibbs isotherm is 1 RT ln T, p
is the onentration of the substane in the bulk solution, R is the gas onstant, T the temperature and γ is the surfae tension of the solution. Therefore inorgani salts have negative surfae onentrations (whih is logial, beause they have strong attrations with the solvent) and surfatants have positive surfae onentrations: they adsorb on the interfae. SURFACE TENSION MEASUREMENTS BY STALAGMOMETRIC METHOD Introdution Gravity fore an be desribed using formula: F g m g V n d g where m is the mass of the drop, d is the density of the liquid, g is a earth aeleration. Beause the surfae tension fore is equal: F 2r the surfae tension we an alulate using formula: V d g 2 r n In the stalogmometri method to avoid the volume of the ontainer and the radius of the apillary we use the model liquid with the well known surfae tension-water The surfae tension of the examinated solution we alulate aording to the formula: where: m the surfae tension of model liquid nm d m. n d n m the number of drops flow out for the model liquid n the number of drops flow out for the examinated liquid d density of the examinated liquid d m density of the model liquid. m The surfae tension of water at a given temperature an be alulated using formula:
m 3 [72.9 0.155( t 18)] x10 PROCEDURE: 1. Prepare solutions of substanes advisabled by the TA. 2. Flush the stalagmometer before eah measurement using the liquid whih will be examinated. 3. Count the drops flowing out of the stalagmometer (start your ounting when the menisus of the liquid ahieves the upper ontration, stop your ounting when the menisus ahieves the lower ontration). 4. Enlose your results in the table. CALCULATIONS: 1. Calulate the surfae tension for water and eah solution. 2. Density of solutions read off form the plots performed using delivered tables of densities. 3. Plot σ=f(ln) and using the slop alulate the Gibbs isotherm.
III.B. ADSORPTION GOAL: To investigate the influene of the solution onentration on the proess of adsorption, to learn how to use the Freundlih isotherm INTRODUCTION The attahment of partiles to a surfae is alled adsorption. The substane that absorbs is the adsorbate and the underlying material is adsorbent or substarte. The reverse of adsorption is desorption. Chemial reations at solid surfaes may differ sharply from reations in the bulk, for reation pathway of muh lower ativation energy may be provided, and hene result in atalysis. The basi theory of adsorption of gases on solids is due to Langmuir, who onsidered the surfae of a solid to be made up of elementary spaes eah of whih ould adsorb one gas moleule. Furthermore, it was assumed that all the elementary spaes were idential in their affinity for a gas moleule and that the presene of a gas moleule on one spae did not affet the properties of neighboring spaes. At adsorption equilibrium the rate of evaporation of the adsorbed gas is equal to the rate of ondensation. PHYSISORPTION AND CHEMISORPTION Moleules and atoms an attah to surfaes in two ways. In physisorption (an abbreviation of physial adsorption ), there is a van der Waals interation (for example, a dispersion or a dipolar interation) between the adsorbate and the substrate. Van der Waals interations have a long range but are weak, and the energy released when a partile is physisorbed is of the same order of magnitude as the enthalpy of ondensation. Suh small energies an be adsorbed as vibrations of the lattie and dissipated as thermal motion, and, a moleule bouning aross the surfae will gradually lose its energy and finally adsorb to it in the monitoring the rise in temperature of a sample of known heat apaity, and typial values are in the region of 20 kj/mol. This small enthalpy hange is insuffiient to lead to band breaking, so a physiosorbed moleule retains its identity, although it might be distorted by the presene of the surfae.
In hemisorption (an abbreviation of hemial adsorption ), the moleules (or atoms) stik to the surfae by forming a hemial (usually ovalent) bond, and tend to find sites that maximize their oordination number with the substrate. The enthalpy of hemisorption is very muh greater that that for physisorption, and typial values are in the region of 200 kj/mol. ADSORPTION ISOTHERMS a) The Langmuir isotherm The simplest physially plausible isotherm is based on three assumptions: 1. Adsorption annot proeed beyond monolayer overage. 2. All sites are equivalent and the surfae is uniform 3. The ability of a moleule to adsorb at a given site is independent of the oupation of neighboring sites. The dynami equilibrium is: with rate onstants A ka for adsorption and g Msurfae AM surfae kd for desorption. The rate of hange of surfae overage due to adsorption is proportional to the partial pressure p of A and the number of vaant sites N 1, where N is the total number of sites, and is the frational overage. The rate of hange of due to desorption is proportional to the number of adsorbed speies, N : d k dt At equilibrium there is no net hange, and solving for gives the Langmuir isotherm: d N Kp 1 Kp K k k a d
b) the BET isotherm If the initial adsorbed layer an at as a substarte for further (for example, physial) adsorption then, instead of the isotherm leveling off to some saturated value at high pressures, it an be expeted to rise indefinitely. The most widely used isotherm dealing with multilayer adsorption was derived by Stephen Brunauer, Paul Emmett, and Edward teller, and is alled the BET isotherm: V Vmon z p z 1 p z1 1 z * in this expression, * p is the vapour partial pressure above a layer of adsorbate that is more than one moleule thik and whih resembles a pure bulk liquid, V mon is the volume orresponding to monolayer overage, and is a onstant whih is large when the enthalpy of desorption from a monolayer is large ompared with the enthalpy of vaporization of the liquid adsorbate. ) the Freundlih isotherm When the temperature is onstant the amount a [mol] of the substrate adsorbed by the mass unit of the adsorbate is proportional to the onentration of the substrate in the solution. The Freundlih isotherm is given by the equation: n a K the onstants K and n that haraterize the substrate-adsorbate system an be named experimentally aording to the formula: log a log K n log PROCEDURE: 1 Prepare solutions of aeti or oxali aid
2 Weigh a sample of ative arbon and add this sample to the flask 3 To the flask ontaining ative arbon add 50ml of prepared aid s solution, mix eah sample10 minutes, after this time filtrate the sample 4 Determine the onentration of NaOH solution using HCl solution 5 Using NaOH denote the onentration of initial aid solutions 1 6 Using NaOH denote the onentration of strained aid solutions 2 7 Enlose your results in the table CALCULATIONS: 1 Calulate a a 1 2 m V where: m -mass of the ative arbon [kg] V - volume of solution used for the adsorption [dm 3 ] 1 and 2 onentrations of the solutions before and after adsorption respetively [mol/dm 3 ]. 2 Plot log f log a 2, the slope is equal n, interept is equal log K