 We begin a new module today on surface reactions and this is formally the first lecture although in the last lecture of module 4 I did elaborate on surface reactions a bit. Before I go any further maybe I would like you to have a quick look at this prayer of Ravindranath Tagore where the mind is without fear and the head is held high where knowledge is free where the world has not been broken up into fragments by narrow domestic walls where words come out from the depth of truth where tireless striving stitches its arms towards perfection where the clear stream of reason has not lost its way into the dreary desert sand of dead habit where the mind is laid forward by thee into ever widening thought and action into that heaven of freedom my father laid my country awake. I think this is the right note for country and for that matter every citizen of the world even today. Let us go into the surface reactions formally where I had this whole bunch of missing slides last time but I raised a few questions those are repeated here what factors would distinguish monolay reactions from those in the bulk phase that was the first question. Second question was is the energy of activation the same if a reaction takes place in the surface against that in the bulk and what about catalysis at liquid interfaces those are the questions which we quickly review our responses first the two main differences between the reactions at solid surfaces and those at liquid surfaces could be seen as I remarked earlier it is only at the liquid surfaces that we have equipotential conditions which means all molecules in the surface are at the same chemical potential only at a liquid surface can these two factors orientation and accessibility of the reactant groups be controlled at will if we look at reactions at solid surfaces such control is not possible. And in bulk phase reactions the orientation of reactant groups is completely random this is an important distinction between liquid surfaces and solid surface and bulk phase conditions in a monolayer the orientation of all molecules is nearly identical and depends only on the applied surface pressure. Once we say that orientation of all molecules is nearly identical the same thing holds for the reactive groups present within those molecules they will be equally accessible or inaccessible to the reactant in the liquid phase. There is one more distinguishing feature about this orientation it can be determined precisely from the measurement of the surface dipole moments. So not only can we control the orientation we can also precisely know what it is with these film forming reactant molecules in different configurations and orientations we can measure the corresponding activation energies. It turns out that in many cases by applying sufficiently high pressure surface pressure it is possible to squeeze out the reactive groups from the aqueous reacting phase if that can happen it will mean we will achieve complete inhibition of the surface reaction. We will explain this later with an example near the end of this lecture. Then I also made a comment about photochemical reactions photochemical reactions necessarily mean that we shine light of certain wavelength at a group of molecules present in the bulk or in the surface and those molecules depending on their energy state would be able to intercept and absorb radiation of the particular frequency and then get excited. After getting excited these reactive molecules then could form the products depending on what state they are pushed into. This is possible only if molecules are having certain parts which are capable of absorbing radiation of a particular frequency. We call those parts of molecules capable of absorbing light or radiation of a given wavelength as the chromophores orientation of chromophores will directly depend on the orientation of molecules and that means the interception and absorption of photons will depend on that orientation. Now you see that depending on the applied surface pressure if you can control the orientation of reactive molecules and therefore the chromophores within will be able to control the extent of interception and absorption of the radiant energy thereby we could control the yield of that reaction or efficiency of that photochemical reaction. In photochemical reaction literature the jargon is quantum efficiency. So quantum yield or quantum efficiency could be altered depending on the orientation which is possible to be controlled for surface reactions. Apart from this a factor which is familiar to you especially when I was talking about the large adsorbed surface molecules getting desorbed into the bulk of aqueous phase particularly if we can alter the energy of the electrical double layer by altering the composition of the solution especially pH we could tamper with the electrical factors and that is the same thing which we are talking of again electrical charges are important and some very large effects catalytic effects and kinetic salt factors can result from presence of these charge groups in the monolayer. So kinetic salt factors will reflect on the ability of the medium to change the extent of reaction through elimination of certain reactive species by change of environment. It could be correlated with an understanding of what the salting out effect is you could have a situation like that later in the later part of this lecture we will see what else can come into the picture as the reaction proceeds. Now the rate constants which are dependent on the energy of activation and the pre exponential factor are the next topic that we should be discussing. Similarly the energies of activation in the surface reactions or reactions of monolayers and in the bulk phases are equal but there are exceptions and these exceptions include those reactions which occur at constant surface pressure accompanied by variation of temperature or range over which the film actually undergoes expansion. If you allow for constant area the picture will be different but let us first focus on the constant surface pressure which means now if a reaction is being carried out in the monolayer on a Langmuir trough we are going to allow the movable barrier to be pushed as the reaction occurs so that the surface pressure is maintained constant and this may happen together with a variation in temperature and the film may actually expand. As a result of this expansion there is a possibility that the reactive groups may have increased access to the reactants in the aqueous phase and if that happens then obviously the rate constants and energies of activation would be measured to be different. Effects accompanying this expansion will be superimposed on the normal effect of temperature on the rate of chemical reaction that is why we will have for these special conditions possibly a disagreement with the rate constant or the activation energy for the bulk phase reaction. If you take the other set of conditions if we maintain the area constant the movable barrier is not permitted to move away now then under these constant area conditions it should be expected that we find the similarity of rate constants and activation energy for the surface reaction and the bulk phase reaction. Because this bulk and sometimes we have different reasons and all that. Oh okay now that is I think very very basic we understand that the surface reactions will be monolayer reactions and monolayer reactions are primarily because of the ability of molecules to adsorb with the preferred orientation because of the hydrophobic hydrophilic parts. Hydrophilic part will remain anchored in water hydrophobic part will flip into air whereas if the same molecules were in the bulk then they are equally energetically unacceptable whether they are in an orientation which is horizontal vertical or at any angle does not matter. So that is the orientation which is random in the bulk but in the surface it has got to be at a horizontal surface it has got to be vertical orientation nearly vertical orientation right. So that means that depending on that orientation and the shape of the molecule the symmetry or asymmetry the exact orientation may not always be vertical depends on the asymmetry and if we will take an example later depending on the applied surface pressure we might have the change of orientation if you have very little pressure then we might have an extreme condition we will see what that condition is. The orientation can play a very different very effective role in determining how the reactant group in the monolayer molecules would come in contact with the bulk aqua space. Now that situation is not existent in the bulk phase every reactant group is accessible provided the molecule can be in the bulk it will be all accessible and the pressure will play no part but in surface reaction it is a different thing we will exemplify taking oxidation of oleic acid which I mentioned last time. See it is like this right you have two reactants the there are two reactants one set of reactants is in the monolayer another reactant is in the aqua space for the reaction to occur these two have to come in contact ok. Now the molecules may be anchored nearly vertically under certain condition and the reactive group in the vertical molecules now is not in contact with the aqua space reactant then they cannot be any reaction then you know the reaction rate constant will be 0 whereas the most favorable condition may be the reactant groups reactive groups are all in contact with the aqua space then the reaction rate will be maximum that reaction rate will match with what happens in the bulk phase. So depending on the rate constant magnitude you get you will have a range of activation energy. So under constant surface areas orientation is now fixed more or less the orientation is fixed in the if the area is fixed ok then it is only dependent on what is the applied surface pressure that orientation will be decided by the applied pressure as long as that pressure is let us put it this way I think we have to be careful here under constant area we have given number of molecules in that area the orientation will be fixed by area and not the pressure please remember do not make the mistake here we are going the wrong way. If you you begin with let us say very closely packed molecules they are all vertical and as a result of let us say increase in temperature reaction whatever the film has a tendency to expand and you allow the movable barrier to go away if it goes away then you are what you are doing is you are allowing the surface pressure to change to an extent before it becomes same value at an increase area. Suppose the reactant groups are reactive molecules are reacting and producing a higher surface concentration of species in the surface then as a result of production of larger number of total number of larger number of species total number of species in the surface the pressure will tend to go up higher if the area is fixed if you allow the expansion then the pressure will be constant you just allow that much expansion that the pressure is constant if that is the case then what you are doing is you are allowing for expansion and therefore the change in orientation whereas if the area gets fixed then the orientation of these molecules will remain same ok. So under conditions of expansion or under constant surface pressure we may allow for the accessibility of more reactive groups to the aqueous space all right. So we said like suppose this much is the area and we have the orientation vertical right. Now if now what happens is that that orientation is fixed by that area ok the orientation could change if you allow a different area right supposing that as a result of reaction the surface pressure increases if the surface pressure increases that higher surface pressure is not going to be released and the molecules will be forced to remain at the in that constraint position if you allow expansion then they will be able to change their orientation right. So like we have some particular area the orientation is not perfectly horizontal fine will be more or less equal. So the orientation is somewhat like more vertical or something then the activation Yeah if the orientation is vertical and reactive group is not able to come in contact with the liquid phase reaction the reaction would not be possible reaction. Regarding this particular diagram what reaction can I get out of the surface in constant of the area? Hmm. The markers in similarity will be in any kind of activation. Yeah for bulk phase reaction activation energies do not change right. So for whatever orientation we have if that orientation does not change that rate constant and the corresponding activation energies will remain constant like in bulk phase reaction ok. The values may not be exactly equal there we are not talking about equal but it is only a similarity in terms of constancy alright. Now they can be also deviations arising out of an effect called screening effect the reactive groups could be screened from the reactive groups in the aqueous phase it can happen because of close packing at the surface. Example could be taken I had mentioned this example last time saponification reaction. If you take a vegetable oil being saponified by alkaline solution then the product of reaction will be so. Now if you take the same kind of reaction hydrolysis of tri-laurin monolayers and it is alkaline hydrolysis then the soap molecules may form and if they are able to desorb they could cause retardation of reaction especially when the soap molecules concentration is high that will be at the near the end of the reaction right. The soap layer will be offering an additional barrier between the monolayer molecules and the alkali underneath ok. If that is clear then let us see how we could go about measuring the reaction rate. So experimental methods will need to have some way of tracking the reaction monitoring the concentrations might not be that easy and I had this question to you last time. If we have to monitor the rate of a surface reaction or how the extent of reaction in the surface would proceed what else could be measured and one of you had correctly responded saying that we could measure the interfacial potential. So let us see how we could do that today. If you take a reactant like ethyl pommetate a long chain reactant forming a monolayer and if it is spread on an aqueous alkali the ester will be hydrolyzed into pommetic acid and ethanol right and after sometime t the surface concentration of the reactant is let us say n r number of molecules per centimeter square then the rate of reaction we could express as minus d n r by d t is equal to the rate constant k existing number of molecules in the surface per area n r and surface concentration of hydroxide ions s o h minus right. This is the reaction between the ethyl pommetate and hydroxide k here is the rate constant or velocity constant for hydrolysis s o h minus is the concentration of the hydroxyl ions in the plane of the monolayer and n r is the reactant surface concentration molecules per centimeter square. In such reactions it is always safe to regard this part as a pseudo reaction rate because the surface concentration of hydroxide ions would be practically constant. Can you see why? Can you go on there yeah say it aloud you are coming into mechanisms no the main point is that if hydroxide is in the bulk of the aqueous phase and we are talking of monolayer reaction the simple concentration of proportion of how much reactant is there in the surface monolayer compared to how much is there in the bulk because the bulk will have overwhelmingly large concentration large number of larger number of moles of the other reactant compared to the equivalent in the surface or the depletion of the bulk hydroxide ions will be practically negligible. So, we can always take s o h minus as a constant right. How it comes and what value it will have comes from the second point is the monolayer is a thin one it takes only very little of hydroxide what is available underneath will come and maintain that and we could even say that the surface concentration of hydroxide ions will be practically equal to the bulk concentration right. So, that is what we could say provided the film is uncharged otherwise there are other effects if the film is charged and it can repel the hydroxide ions and then you will have a different concentration. So, because the amount of reactant taken from the bulk phase by the monolayer is quite small the bulk concentration will not change much and if the effects of charge are not there then we could replace s o h minus by b o h minus where b o h minus is the bulk concentration of hydroxyl ions in the water underneath alright. Which means now we can treat this k times b o h minus as constant separating the variables we could integrate and write this ln n r as minus k times b o h minus into t plus constant and if you plot ln n r versus t then the slope will be minus k times b o h minus units of k will be expectedly time inverse minute inverse moles inverse liters right inverse time inverse concentration in practice it is a second order reaction in practice it is much more convenient to measure delta v or contact potential the surface potential rather than n r and that means now we have to go from this expression equation in terms of n r to an equation in terms of the surface potential delta v and you already know how to do it. The next slide shows you if the film is at constant area delta v after any time will be given by think they got this okay this is mutilated equation delta v is 4 pi n r mu r plus 4 pi n p mu p they are done the editing may be I should write here now delta v is 4 pi n r mu r plus 4 pi n p mu p there is certain amount of editing which goes after I have given the finished form and then we have this problem we have delta v delta v is equal to 4 pi n r mu r plus 4 pi n p mu p where n r is the number of reaction molecules per area and n p is the number of product molecules per area mu r is the dipole moment of the reactant molecules and mu p is the dipole moment of the product molecules surface dipole moments of reactant product okay. Now if this is the case you could proceed further may be I like you to work out a bit if the total number of molecules in the surface is constant that means upon reaction the product molecules remain in the surface what could you say about n r and n p adding up together constant n r plus n p will be equal to constant which is let us say n right. So, you should be able to express n p in terms of n r right now if the reaction goes irreversibly from the reactants to products while n r is equal to n minus n p at any time t we could also say something when t goes to infinity about delta v now put this together and convert the basic equation giving n r variation with time minus d n r by d t in terms of delta v and the delta v when time goes to infinity work it out mu r and mu p remain constant the surface dipole moments for reactant and product remain constant. If initially there are these n molecules of reactant per centimeter square and if none of these long chain molecules are lost then we could say n r plus n p is n constant therefore n r can be given in terms of delta v by this equation n r will be delta v minus delta v at time equal to infinity by 4 pi mu r minus mu p right where the delta v at time equal to infinity has been substituted for 4 pi n mu p presuming the reaction goes to completion eventually. So, now from the previous equations 3 and 2 we can write ln n r as ln of this numerator rest is constant minus k b o h minus t plus another constant right. So, if we if we plot ln of delta v minus delta v at infinity versus time we should get a straight line with a slope equal to minus k times b o h minus. So, this is how experimentally you would measure the pseudo first order surface reaction rate constant minus k times b o h minus and this will allow you to determine k for any given surface area and temperature. So, you can do the experiments at constant area and a constant temperature record these surface potentials over time and you would be able to obtain the rate constant reliably provided all the assumptions made here are complied to. There are other ways to track the reaction there is a possibility positive active tracers and the principle here is that radiation which is emitted from greater distances below the surface will be absorbed in water whereas, those those from the surface molecules will be recorded in the Geiger counter. So, if the range of radioactive tracers is selected properly we might be able to see how the reaction occurs and the next table will show you from various considerations that tritium is one of the most effective radioactive tracers. Tritium which is H 3 gives out beta radiation with a range in water about 6 microns and the ratio of activity of the adsorb film to that from the bulk solution is 1.8 when it is a millimolar solution for a 100th molar solution it is about 0.18. Tritium turns out to be very effective in tracking the reaction from the radiation concentration. Next we talk about the steric factors. Steric factor as you might recall is a term used in collision theory and it is the ratio of the rate constant measure experimentally to the rate constant predicted from collision theory. You can see that it could be also defined as the ratio of the pre-exponential factor and the collision frequency and usually it is less than 1, but exceptions can be there. More the complex the reactant molecule is lower is the steric factor. There are some exceptions as I cited Harpoon reactions can exhibit steric factors greater than unity. These are the kind of reactions which exchange electrons and produce ions and the departures from unity can come from several things molecules not being spherical other geometries might be possible. Not all kinetic energy is transferred into the right spot or the presence of solvent when applied to solutions. There is also a solvent cage which can make several collisions take place instead of single collision and therefore, lead to pre-exponential factors far too large and in general we may say row values greater than unity could be ascribed to favorable entropic contributions. The difficulty with steric factors is that it cannot be calculated theoretically and because it cannot be calculated theoretically the usefulness or applicability of collision theory itself is compromised considerably. You have the possibilities of disorder which can cannot be taken care of in the original problem. So, there are effects which are attributed to the entropy which can lead to the factors which are greater than unity. Then we return to the steric factors in monolayers. This should clarify some of those orientation related effects on reaction rates and activation energies through the accessibility of the reactive groups. We take one interesting example that of oxidation of oleic acid by dilute alkaline or acidic permanganate solution. You could see the oleic acid molecule here. There is a double bond and upon oxidation with permanganate you form a dihydroxy derivative. The same thing is shown here in this diagram. We have ME CH2 7 times CH double bond CH, CH2 7 times COOH. On oxidation we get an OH over here and here. So, you get that dihydroxy compound. Now let us see what happens when the area available at the surface for oleic acid molecules is very high. Oleic acid molecules then would tend to lie flat on water surface. So, the double bonds will be in contact with the permanganate aqueous solution, alkaline or acidic. The oleic acid molecules now are in a horizontal position, flat position. Film is expanded. Area is not a constraint here. Double bonds being in contact with the permanganate can be attacked by the oxidizing species and therefore, you get the dihydroxy derivative. However, if we compress this film, we will force some of these double bonds to go away from the surface. Now they can no longer be oxidized by permanganate. We will have to compress this film to an area per chain which is less than what is available here. Only then that crowding will take place and some of the molecules will have to leave the aqueous space and remain at an orientation away. If we keep compressing this film eventually, all the oleic acid molecules will become vertical. That means, all the double bonds will be forced away from the permanganate. Now no reaction should be possible. This is how the surface pressure or compression or decreased area available can create an effect on the observed reaction rate or the major activation energy of a reaction. This is the final position under compression. At very high surface pressures, the rate of oxidation is actually measured to be very small. We have these double bonds now shielded by a very thin membrane of hydrocarbon. These CH groups form a barrier between the double bonds and the permanganate. Thickness of this layer may be just 10 angstroms but the surface pressure is very high and the permanganate ions cannot break through this oriented layer to reach the double bonds. So how do accidental data on rate constant come out? This figure shows you the major rate constant k in minute inverse pseudo first order rate constant versus the number of molecules per area. When the pressure is low, rate constant is quite high. As the pressure becomes very high, rate constant drops very significantly. These are the measurements for oleic acid film which is spread on a 0.003% permanganate ion in 0.01 normal sulphuric acid. So that is the last slide for the lecture today. We conclude here then that the orientation can drastically have an influence on the rate constant. Mainly through the different levels of accessibility, the oriented reactive groups might have relative to the aqueous phase or the liquid phase underneath. We will conclude the lecture here for you.