Surface chemistry part 7

Added: 2026-05-12

[00:00:01]so before today i begin the next part i would like to discuss with you little bit more about we can say the tindal effect and little bit more about the electrical properties so that i'll go to the next one because i believe that something more i have to say you now that is tindal effect what is its importance now let me tell you tindal effect was done by zig he devised an instrument called ultra microscope let me tell you and this it an intense beam of light from ah particular lamp we can say the name of the lamp is what arc lamp correct the r clamp is focused on the colloidal solution and that colloidal solution you took it in a beaker or in a glass vessel you can say that you have you will be taking it and when it is being focused you will find it out if it is a true solution then the light will pass through you know but if it is being colloidal solution then the light gets scattered now how do we come to know you'll observe it under a microscope you you'll observe this effect listen carefully you'll observe this effect under a microscope you'll observe it that is you pass the light from the arc lamp into the colloidal solution you will observe it that what is going to observe what is going to happen the light which is scattered by each individual colloidal particles appears as a bright star against a dark background because basically this room this practice experiment is being done in a dark room it's a dark room experiment thus the number of colloidal particles can be counted you can count it because bright starts just like stars in this kind of so of course you cannot count the stars in the sky but here you can count it because the beaker is there but you are passing the light through it and you are observing it under a microscope you will see some particles will be shining there that is nothing but the colloidal particles you can say that clear and the light will get scattered it will get dispersed we can say that so knowing the volume of the solution the number of particles per unit volume also can be determined suppose you have taken one liter solution or two liter solution you can count it that how many particles are shining so this further listen carefully this further helps to find out the average mass of the particle ah which we have noted from the ultra microscope now we do not see the actual particles here mind it we do not see the actual particle what i said you you will see the stars like the light which is scattering the star like object which is nothing but shining thus ultra microscope it not only gives the information about the size and shape of the particles it also confirms the heterogeneous nature of the colloidal solution because what i said you in the beginning itself i said in case of true solution the light passes through simply all right because it is transparent the light passes through simply without getting any deviation but here it gets deviated it gets scattered and if you see it observe it under a microscope then you can see this uh lighted particles you can see it audited which is being floating there which is being moving there got it by the help of the brownian motion it is being striking one particle is striking another is striking another and you will see it is uh or what you can say is exact moment you can see it all right by the help of microscope mind it you have to see it so that is the importance of the tyndall effect and this much i want to say you that ah tyndall effect when you do the tyndall effect you see this particular thing in collateral solution the most important thing i said you that it confirms the heterogeneous nature of the colloidal solution that's very very important now next is if you see the electrical properties of the colloidal solution you know the stability of the colloidal solution is due to the fact that the colloidal particles in the solution ah in the colloidal solutions are electrically charged the stability depends upon that only as it is charged so it is what you can say stable the particles therefore repel one another and do not come closer to each other to form a large non-colloidal particle so that it will get suspended they are repelling the charge which is present in the particles none due to which they repel one another suppose if they would have not repelled they would have come closer to each other they would have formed a big mass and they would have settled down as suspended particles but this is not happening so collateral particles either carry positive or negative charge all the dispersed particle in collateral solution can carry the same charge while the dispersion medium has an equal and opposite charge present in it got it an equal and opposite charge which is being present in it we can say that now next is the question comes here where from the charge appears on the collateral particles from where we are getting this particular charge what is the origin of this electrically charged on colloidal particles so their various theories are given for it one theory is that that is the electron capture by particles from air and during elect what you can say the bredix arc method you can see that that is the particles it captures from the air in the bread is arc method what you are doing you are taking there is nothing but a solvent which is called the dispersant medium and you are taking there is two metal electrodes you are passing electric current through it and when it becomes hot you immersed in a particular solvent let us say you have taken to gold electrodes when it becomes hot you immerse in water you will see an electric arc is being produced i have discussed with you so there what is saying that electron capture so when you are heating this when you are passing electric current when the electrode gets heated some air what you can say surrounding it of course there is a poor conductor of electricity mind it it gets surrounding it surrounding that particular one very near to it got it it gets what you can say charged so due to that also you can say uh that is uh some charge appear on the colloidal particles it is being believed okay this is one theory that when you are charging the electrical rods the metal rods send the air which is surrounding it very small amount of it's not a large amount very small amount of it gets charged now that depends upon also the voltage which you are passing through it suppose if it is low volt voltage no question about it but if it is a high voltage then only a small amount of air it gets charged now all these things you have studied in class 11 in atomic structure in cathode ray experiment and the pressure will be very very low then only it will be possible otherwise not because air is a poor conductor of electricity so these theory it's not so valid we can say that now frictional electricity caused by the mutual rubbing of the collateral particles with molecules of the dispersion medium now this is ah quite convincing that what happens is that when the dispersion medium and the dispersed phase they collide with one another so due to the mutual rubbing of the colloidal particles so some charges appears we can see that some charges appears in it so that is another theory it is being there is proved forward it is being brought forward now dissociation of the molecules followed by the aggregation of ions now i have read it in my cells where the aggregation of the ions is taking place got it now the soap solution now in case of soap i am going to discuss later on about it it gets dissociated from any plus science and have a tendency to aggregate into a cluster carrying negative charge all right it's aggregate so carry a negative charge i'll show you that is the what you can say the color this this is the soap solution you can see the negative and the positive charge is being present here that is here you can see the co minus this is soap molecule you can see fatty acid chain is there co minus is a negative charge and na plus n is the positive charge so in this way what you find is that n a plus and co minus uh that is oppositely charged are being developed now the particles of a dye have a tendon diminished color correct so which is present in the clothes have a tendency to dissociate to form aggregate carrying positive negative charges depending upon its composition all right so when this when you are putting soap up on the clothes so these particles or the color they also are having opposite charges so they aggregate and they form clusters not big clusters so that it will settle down no and hence they carry positive or negative charges depending upon its composition we can say that so this is another theory is being put forward so in this way so many theories are being put forward regarding this uh what you can say the colloidal solution we can say that that word from the origin of the charge takes place in the collateral solution so now out of the various theories which i discussed with you now one is very convincing is that out of the various theories of the origin of the charts which i have discussed with you now preferential absorption absorption of ions is considered as a best explanation now what is preferential absorption of ions suppose you will take an ionic colloid absorbs uh ions common to its own lattice during the preparation of the colloidal solution now you must not be following i'll let me give an example to you let us take if colloidal solution of a g i you want to prepare it what you need to do you will take potassium iodide solution and you will add it to silver nitrate solution simple potassium i did silver nitrate all right it is what is being forming that is nothing but a double decomposition reaction takes place and it forms potassium nitrate and silver added correct now so what is happening now listen here carefully when if you want to prepare colloidal solution of agi then you will add potassium iodide solution to agno3 still sorry till ki is in excess little bit excess that means you want to prepare a saturated solution now iodide ions will be absorbed on the surface of agi you have to get ai in sorry ki in excess so that ki dissociates into k plus i minus and i minus i n will be absorbed in the surface of agi particles thereby giving them a negative charge audit ions which is absorbed on the surface of silver iodide on the other hand if colloidal solution of agi is prepared by agno3 solution to ki solution till agno3 in excess in the previous case i have taken ki in excess now suppose you are preparing same agent o3 plus ki but you are adding silver nitrate in excess you may also ask that suppose if i take silver nitrate and express excess what will happen so very simple what will happen ag plus ions will be absorbed thereby giving positive charge to the colloidal particles if you are taking i k i excess then iodide ions will be absorbed on the surface of agi if you are taking silver nitrite excess then ag plus ions will be absorbed on the surface of agi giving a positive charge we can say that so in either case whether you take potassium iodide excess or agno 3 excess means a saturated solution if you take it in either case the left out ions will remain in the dispersant medium left iron means if you have taken ki then k plus n is left out if you have taken silver nitrate excess then no3 ion is left out now because a g plus sign is being absorbed got it thereby giving equal and opposite charge to the dispersion medium you got in the dispersed phase a gi and i minus so what you can say k plus ion is left out in the dispersion medium if you are taking a gno3 nexus then a g plus is uh absorbed on the surface of a gi so you get no3 ions in the dispersion medium in this way suppose you can take ferric chloride also with ferric what you can say with when it's added to sodium hydroxide you will get ferric hydroxide got it so in this way any any ah what you can say examples you can take it and you can explain it alright so this is quite convincing this particular one it is quite convincing this one is that is when one type of ions of the electrolyte are absorbed on the surface of the colloidal particles it forms a fixed layer that is i minor science it forms a fixed layer or a g plus ions is found forms a fixed layer it attracts the counter ions from the medium forming a second layer which is mobile of course it is i minus is going to absorb the k it is going to pull the k plus ion but k plus ion will be mobile it will be moving throughout the dispersion medium got it so that forms a diffuse layer i minus forms a fixed layer over the agi but k plus ion will form a diffused layer similarly in case of agno3 a g plus ion will form a fixed layer over agi but no3 plus no3 minus sign will form a diffuse layer i hope you are getting in this particular one so double layer of opposite charges thus formed is called helmholtz electric electrical double layer this is on s as a result a difference of potential exists between the fixed layer and the diffuse layer the fixed layer is there on the absorbed surface and the diffuse layer is there on the dispersal medium so difference in potential definitely one is positive another is negative opposite charges are there no so a difference in potential exist and remember it listen carefully this potential difference is known as electro kinetic potential or zeta potential zeta z eta zeta potential it is known as got it so the double layer of opposite charges thus formed is called helmholtz electrical double layer all right one is fixed and there is a diffuse layer already i said you as a result of difference of this potential exists between the fixed layer and the diffuse layer and this potential is known as electro kinetic potential or zeta potential receptor bias i hope you got it the origin and i hope you got it what is electrokinetic potential and the absorption what you can say uh theory that is preferential absorption of ions from the solution i hope you got it in simple words i've explained you now next coming to the next what you can see in your screen hardy schultz law now hardy schultz law ah before that the last class you have read it about uh coagulation flocculation calculating ion flocculating ion i've told you about it i've told you also the smaller the coagulation value of the electrolyte greater is its coagulating power or precipitating power smaller the coagulating value of an electrolyte now that is what is being given in the hardy school's law so the hari shul's law has got two points basically got it it has got two points basically you may write it down the hardy souls law ah the main two points which are stated as follows that is defective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles these ions are called coagulating ions or flocculating ants again i repeat the effective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles if it is negative then it will be positive charge if it is positive then it will be negative charge these ions are called coagulating ions are flocculating ants now second point is much more important greater is the valency of the coagulating of the flocculating iron greater is its power to bring about calculation greater is the valency suppose you are having n a plus c a two plus a l three plus so a l three plus will be having large coagulating power to bring about population then ca 2 plus and na plus got it so greater is the valency of the coagulating or the flocculating iron greater is its power to bring about coagulation just now explained with an example you can see it on the screen also it is being there all right you are having lcl3 bacl2 nacl solution and al 3 plus that is alcl3 solution will bring about calculation easily compared to bacl2 and nacl so to say similarly in case of negative charge also same thing suppose if you are having uh that is ferrocesianite fec and whole six four minus ions and phosphate ions sulfate ions and chloride ions which will bring about coagulation faster that is fec and whole six four minus the ferrocyanide and we can say that clear so higher is the charge higher is the coagulating power we can say that all right higher is the charge higher is the coagulating power we can say so ah hardy soul's law it tells us that the quantity of the electrolyte which is required to calculate a definite amount of colloidal solution depends upon the valency of the coagulating ion it depends upon the valency that it was observed that if you are ah adding aluminium chloride to coagulate and if you are adding barium chloride to calculate so aluminium chloride will calculate more than that of the barium chloride because valency is more so you may require small amount of aluminium chloride to coagulate to precipitate we consider that clear got it that is what the hardy souls are the quantity of the electrolyte which is required to coagulate a definite amount of colloidal solution it depends upon the valency of the coagulating ion that is what i said that is what the hardy souls loss say so based upon that we can say we can have a formula coagulating power of the electrolyte one by coagulating power of the electrolyte two let us say aluminium chloride by barium chloride equal is equal to coagulating value of the electrolyte two by coagulating value of the electrolyte one again i repeat coagulating power of electrolyte one by coagulating power of electrolyte two is equal to coagulating value of electrolyte two by what you can say coagulating ah value of electrolyte one now you must be thinking coagulation value what is this that will be given to you it will be given to you for example i remember it and for that reason i am saying you for sodium chloride it will be 52 for magnesium chloride it is 0.72 in this way that value will be given to you just you need to put the value whatever the electrolyte is given and you will do it accordingly got it coagulating power calculating value you will be given mean three things will be given to you the fourth one you need to find it out that's it what i want to say got it in the formula so in this way so i understood what i said that in case of positively charged uh more is the charge and in case of negative charge also more is the charge all right so if more is the charge mean charge means valencies then more is the higher is the coagulation power we can say that all right so in electrophoresis the particles of the dispersed phase moves towards oppositely charged electrode and gets neutralized if the process is continued for sufficient time these neutral particles unite and grow in size and it settle down we can say that all right clear we have already discussed this about this now next is if you compare the coagulation of lyophobic and live uh felliculoids then lyophobic souls are less stable and hence more easily coagulated listen carefully lyophobic liquid hating all right are more easily calculated than live felliculid this is because the stability of the lifefelix souls is due to two factors what is that lyophilic souls same charge in the collateral particles and solvation of the colloidal particles we can say that and this does to bring about coagulation both these factors has to be removed and this is done by adding electrolyte or by adding suitable solvent now coming to the electrolyte by adding electrolyte ascent understood what i said the lyophobic are less stable than what you can say life illicolites this is because stability of the liver colloids is due to two factors what is stability of life helicoids that is what are the factors i said use the same charge on all the collateral particles that is it will repel it will not come and then it will not coagulate and solvation of the collateral particles it dissolves is indiana in the colloidal particles so that's to if you want to bring coagulation just to bring about coagulation you have to remove this you have to remove at the same charge which is there you have to remove this you have to see that it does not get dissolved completely so how it is being done by adding electrolyte or by adding a suitable solvent we can say that now that adding electrolyte that factor i am coming to you now all right so what is that the first you can see the adding electrolyte that is gold number what is gold number so gold number of a protective colloid is the minimum mass of it in milligrams which must be added to 10 ml of a gold sole so that no coagulation of the gold sole takes place one n1 ml of 10 percent nacl solution is added to it it is there on in the screen we can say that the gold number of protected collide is a minimum mass in milligrams which must be added to 10 ml of the gold sole so that no coagulation of the gold sole takes place when one ml of 10 percent sodium chloride solution is rapidly added to it all right so this is on as protection of collides now you must be thinking why why why should i protect the colloid because if you don't protect the lipo because then it will coagulate just now you said it it is less stable diaphobic colloids are less stable than life electrolytes then it will coagulate got it now to prevent that we need to protect it for example i can give you a very simple example suppose ice creams you'll see the chocolate cover is there over the ice creams the chocolate you can say this cover is there the layer is there over the ice cream the chocolate ice creams the chocolate layer what it does it prevents the ice cream to melt so that does act as a protective colloid the chocolate all right now for you it gives a good taste of course fine but according to chemistry that acts as a protective collide it does not allow the ice cream to melt to coagulate we consider that similarly upon the cake you can see we are putting the cream the chocolate or the cream we are putting it what it does it does not allow the cake to what you can say uh shrink or shrimp we can say that so that the carbon dioxide gas which is being trapped in the duff which makes the cake soft all right that is being retained because if you are putting a layer of chocolate or the cream upon it so carbon dioxide will be trapped in it due to which the cake will be soft otherwise if you don't put the layer what will happen the carbon dioxide slowly and slowly and slowly it will diffuse out by the help of effusion it will diffuse out and hence the cake will become but you can say just like chapati it is going to become after some time you can say that so to prevent the cake to become what you can say chapati or shrink in size what we're doing we are putting a layer of chocolate or the cream upon it so that acts as a protective color understood now what is the use of the protective bloods that is the use of the protective collides you can see here also same thing if you want to protect the gold sole then we have to add that is 10 ml of what you can say 1 ml of 10 percent sodium chloride solution so that the gold sole does not coagulate it does not precipitate now you will say why it will not why if it coagulates not life and see gold solvent is used tell me it is being used as an electrolyte to uh what you can say to that is uh what i should say that electroplate the silver articles basically the silver ornaments if you want to plate with electroplate with gold then we require the gold sole we can say that so hence the gold needs to be protected otherwise suppose if the gold sole coagulates and it is a loss big loss for the industries you know what is the cost of gold now understood clear so that is known as the protective colloids under the proteculites we are adding electrolyte which i just talked to you now this is ah that is nothing but a soap solution see the head of the soap is water loving and the tail of the soap is water heating you can see it here hydrophobic lights which i have discussed with you all the soap particles you can see the head and the tail they come together and forms a cluster which is called as micelles all right you can see it here all the this particular one and you can see the chain that is the hydrophilic the chain is what co minus and after that you are having ch2 ch2 ch2 ch2 ch2 ch2 so this this particular one is hydrophobic the ch2 part and that the one which is hydrophilic is nothing but ah b is your water loving and that is uh you're the the head of the what you can say the soap we can see now next is after that we are cleansing action of the soap uh for already discussed with you just i gave you this particular diagram so that you should go through it but it when you're adding the soap solution to the oiler grease and what it happens it is being discussed here all right it pulls out the oil and grease and uh that is micelles it forms the cluster it forms and hence it cleans the clothes accordingly so soap decreases surface tension of water making it better wetting agent soap converts grease greasy and oily dirt into micelles that become dispersed in water so keeps the greasy muscles in suspension and prevents it from redepositing got it so already this it has been discussed with you that when you are adding this detergent or the soap to soak to the dirt or the grease clothes then what happens is that it pull out the dirt and the grease and it helps to coagulate it and precipitate it but it's and when you are adding water what happens is that that is being washed away with the water because soap decreases surface tension of water and makes it a better waiting agent now this is a reaction which is given so how it is being formed we can see it is being given see ethyl acetate reacts with water it goes hydrolysis you have read it in chemist what you can say organic it forms acidic acid and ethyl alcohol this is known as hydrolysis of esters now this uh ester when it reacts with instead of water it reacts with naoh it forms sodium acetate and ethyl alcohol if it reacts with water it forms acetic acid entire alcohol but if it reacts with sodium hydroxide means the first one is acidic hydrolysis the second equation is basic hydrolysis in the case of basic medium we can say that so it forms sodium acetate and ethyl alcohol now uh what you will be seeing is that that is one way you can get the soap another method here you can see is that we have taken a triglycerides that is the animal fat which i've discussed with you in uh what you can say alcohol chapter when it reacts with nh it forms glycerol and soap is being produced rco na because soap is a sodium salt of higher fatty acids it's a higher fatty acids higher means r value is either c17h35 or c15h33 or c15h31 that is you can take steric acid or oleic acid or palmitic acid all right clear so this glycerides where from you got it we get it from the animal fat when it reacts with naoh in prince of water it forms glycerol and soap is formed as a by-product we can say that but it already have discussed with you all these things so it's being done this particular one thank you very much