Slide 1

Mercury Porosimetry

By Vinodhini B Vishwanathan S K

Pores Classifications-Porosity Methods of Porosity MeasurementsMercury Porosimetry Method HysteresisWashburn EquationAdvantages Disadvantages-ApplicationsContents

What are Porous Materials?

Non-porous solidPorous solid

Porous materials have high specific surface area and pore volume.

3All solids can be classified in to two categories; porous and non-porous solids. Porous solids are those that have high surface area and high pore volume where as non-porous solids are those that have low surface area and low pore volume. In general, all solids to some extend are porous except ceramics fired at high temperatures.Size of Pores (IUPAC Standard)2 nm50 nmMicroporesMesoporesMacroporesZeolite,Activated carbon,Metal organicframeworkMesoporous silica, Activated carbonSintered metals and ceramicsPores are classified according to the sizes: < 2 nm micropores, 2-50 nm mesopores, >50 nm macrospores.4Pores can be open or closed. Open pores are accessible where as closed pores are inaccessible. Open pores Types Of Pores

Open Vs Closed PoresDead end (open)ClosedInter-connected (open)Passing (open)Open pores are accessible whereas closed pores are inaccessible pores. Open pores can be inter-connected, passing or dead end.6Shapes of PoresConicalIntersticesSlitsCylindricalSpherical orInk BottlePore Shapes7Measurement of PorosityPore size & its distributionSpecific Surface Area, m2/g = PorosityParameters used to measure porosity; specific surface area, specific pore volume ,and pore size and its distribution. Mass of the solid, gTotal surface area, m2Specific Pore volume, cm3/gMass of the solid, gTotal pore volume, cm3=Porosity, % = Volume of solid (including pores)Volume of poresX 1008The three measure of a porous solids are surface area, pore size and its distribution and pore volume. Laboratory Measurements Direct Measurement MethodImbibition MethodMercury Porosimetry MethodGas expansion methodDensity MethodPetrographic MethodBulk Volume Measurement Mercury PorosimetryExtremely useful characterization technique for porous materials.

Pores b/w 500m-3.5nm can be investigated.

Only method that provides information about Pore size distribution, Porosity , Skeletal & Apparent density and Surface area of the sample.

Purity of Mercury Hydraulic oil should not be recycled , it contaminate the mercury and changes its dielectric and flow properties.

Contamination of mercury affects the contact angles and surface tension values.

Mercury-Acid washed, dried and distilled preferably doubly- or triply-distilled.

It is always advisable to use fresh mercury for every measurement.

Sample Preparation Porous materials are prone to adsorb water or other chemicals, which should be removed during the initial evacuation of the sample.

One of the key parameter here is the sample weight.mercury amalgamation12InstrumentationReservoir of mercury to which a graduated barrel with a piston is attached.

Penetrometer-sample of known weight is placed.

The sample cell is surrounded by hydraulic fluid.

After evacuating the air around the specimen with the vacuum pump and the penetrometer is filled with mercury up to the reference level.

Schematic diagram

Working PrincipleAt normal pressures Hg will not enter the pores of most samples. From the Hg displacement, bulk volume of the sample is calculated.The pressure on the Hg is then raised further, forcing the mercury into the pores of the sample .At sufficiently high pressure, the mercury will invade all the pores.The amount of mercury lost into the sample provides the pore volume directly. The porosity can then be calculated from the bulk volume and the pore volume.Pressure RangeLow Pressure SystemThe first data point is usually taken at a pressure of 3000 to 4000 Pa or higher.High Pressure SystemPressure given to the hydraulic oil surrounding the sample cell in an isostatic way up to 414 Mpa.Mercury Injection Apparatus

Mercury will enter the capillary at the threshold pressure, but will not advance.The mercury level increase is depend on an excess pressure applied , above the threshold pressureFor the increased level the distance - time relationship is

The distance - time relationship for mercury in tubes of five different sizes is shown fig below, E.g., More than 100 seconds are needed to travel 3 cm in a tube of 0.5m radius.

Washburn equation The relationship between the applied pressure (p) and the pore size (d), d= (4/p) cos where, -surface tension of mercury, -contact angle between solid and mercury

The value of Vi(Hg) at the applied pressure Pi apparently gives the cumulative volume of all available pores of radius d.HysteresisWardlaw at 1988 proposed mercury entrapment leads to hysteresis between the mercury intrusion and extrusion curves.Explanations proposed are: a) Contact angle hysteresis b) Ink bottle theory c) Percolation -connectivity model

Hysteresis

a) Contact angle hysteresis

The surface roughness or the impurities on the mercury or solid surface could certainly affect the contact angle.

Capillary action of a wetting and non-wetting liquid relative to the walls of a capillary . A liquid with low surface tension (low surface energy) resting on a solid surface of higher surface tension will spread out on the surface forming a contact angle less than 90o; this is referred to as wetting. If the surface energy of the liquid exceeds that of the solid, the liquid will form a bead and the angle of contact will be between 90oand 180o24 b) Ink Bottle Theory The opening of a pore is smaller than the actual cavity. So mercury entering into the pore cavity depends on the neck size and not the actual cavity size. c) Connectivity Model Uses network of pores. An extension of ink bottle theory

Percent Porosity

Where, Va -Hg volume intruded at any given pressure Vb -Hg volume intruded at a user-defined Intrapore Filling Pressure Limit Vc -Hg volume intruded at the max. experimental pressurebetween particles within particles27

The PMI Mercury Intrusion PorosimeterEquipment

PMI Mercury Porosimeter Data ReportPore Volume vs. Diameter Delta Volume histogram Pore Distribution (histogram) Percent Total Pore Volume Pore Distribution (pressure and diameter) Particle Size Distribution (histogram) Percent Porosity and Density Cumulative Surface Area Porosimetry Text Data

AdvantagesSpeed and simplicity.grain size and pore throat size distribution of the sample can be calculated.Can analyse lower porosity values than other methods.Low-pressure mercury porosimetry determines macropores (pore diameter 14 200 m).High-pressure porosimetry determines mesopores and macropores (pore diameter 3 nm 14 m).it can be done on small irregular samples.maximum resolution & accuracy31Limitations & DisadvantagesMercury porosimetry does not actually measures the internal pore size, determines the largest connection (throat or pore channel)Cost of distilled Mercury is high.The assumption of a constant value of surface tension and angle of contact for mercury.The Pores are not usually circular in shape, so the results can only be comparativeSample must be disposed of safely after the test.

contact angle may differ due to differences in the surfaces of the samples, Damage or compression of highly porous silica32ApplicationsCharacterization of carbon blacks, pharmaceutical samples.

Distribution of porous bead cellulose.

Determination of Maltose.

Distribution of foamed chitosan.

Determination of Cement Particles.

Thought for the Day

Thank You