Surface Area Sur (French “above”) Face (French “face”) Area (Latin “open empty space”)
Surface Area – what is it? “Surface Area is the means through which a solid interacts with its surroundings, especially liquids and gases.” Surface area is created by division of particles (size reduction) and the generation of porosity. Surface area is destroyed by sintering (exceeding Tg), melting and Ostwald ripening.
How to Create Area • Size Reduction • Grinding, • milling, • nanoscale preparation • Make pores • Partial decomposition • Leach • Gel then lyophilize
How to destroy area • Surface area is destroyed by • melting • sintering (exceeding Tg), and • Ostwald ripening
Surface Area – Importance? Remember that surface area is the means through which a solid interacts with its surroundings. Consider the following three interactions: Solid-Solid: autohesiveness (cohesiveness) eg flow, compactibility etc. Solid-Liquid: wetting, non-wetting, adsorption capacity etc. Solid gas: adsorption, catalysis, etc.
Solid-Solid Interaction Dynamic angle of repose Static angle of repose What happens next?
Liquid-solid interaction Wicking: adhesive forces exceed cohesive forces… liquid wax is drawn up through fibers of wick to the exterior where it evaporates, mixes with air and burns upon ignition from the hot gases above.
Suitable methods of determination • Gas adsorption allows probing of entire surface including irregularities and pore interiors. • The amount adsorbed is a function of temperature, pressure and the strength of attraction or interaction potential. • Physisorption is generally weak and reversible. The solid must be cooled and a method used to estimate the monolayer coverage from which surface area can be calculated.
The gas sorption process - intermolecular forces Lennard-Jones potential function
The gas sorption process • Langmuir[i] described the kinetic behavior of the adsorption process. He postulated that at equilibrium, the rate of arrival of adsorptive (adsorption) and the rate of evaporation of adsorbate (desorption) were equal. Furthermore, the heat of adsorption was taken to be constant and unchanging with the degree of coverage, θ. [i] I. Langmuir, J. Amer. Chem. Soc., 40, 1368 (1918)
Irving Langmuir (1881-1957) Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903 1903-1906 M.A. and Ph.D. in 1906 from Göttingen. 1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey. 1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director 1913 -Invented the gas filled, coiled tungsten filament incandescent lamp. 1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons 1927 Coined the use of the term "plasma" for an ionized gas. 1935-1937 With Katherine Blodgett studied thin films. 1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation. 1932 The Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry" http://public.lanl.gov/alp/plasma/history.html
Langmuirian behavior Confining adsorption to a monolayer, the Langmuir equation can be written where V is the volume of gas adsorbed at pressure P, Vmis the monolayer capacity (i.e. θ=1) expressed as the volume of gas at STP and K is a constant for any given gas-solid pair. Rearranging in the form of a straight line (y=ab+x) gives
Adsorption Process Adsorptive Adsorbate Adsorbent
The Isotherm • The amount of gas adsorbed is a function of • The strength of interaction between gas and solid (intrinsic) • Temperature (fixed) • Pressure (controlled variable)
4) 2 Physisorption Process
Low pressure behavior (monolayer) The “knee”
Choice of gas and temperature • Gases • Nitrogen • Argon • Krypton • Carbon dioxide • Others • Temperatures • Liquid Nitrogen • Liquid Argon • Dry ice/acetone • Water/ice • Others
Measurement Method Classical vacuum, volumetric. Requires that adsorbate be adsorbed by the sample, at some reduced temperature, as a function of pressure of pure adsorptive.
Vacuum-Volumetric • P/Po values are achieved by creating conditions of partial vacuum. • High precision and accurate pressure transducers monitor pressure changes due to the adsorption process.
Vacuum-VolumetricPo, p0, psat Po value can be • measured in a dedicated cell • with or • without dedicated transducer • calculated from atmospheric pressure • input manually (eg Kr 2.63mmHg at 77.4K) • measured in cell over sample
Working Equation PV = nRT nads = ndosed - nvoid nads = (PV/RT)man. - (PV/RT)cell
Sample preparation The adsorbate has to “see” the real surface. • Whilst a little pre-adsorbed moisture wouldn't affect the monolayer capacity, it would affect the strength with which it is adsorbed, so the monolayer would be formed at a different pressure than on a clean surface. • Pores can be easily blocked by moisture. Water undergoes capillary condensation at humidities well below bulk saturation in the confines of the pore (just as nitrogen does as we will see later). • Micropores can be completely filled, not just blocked! • We always quote surface area per unit of dry mass.
Outgassing of Surface Sample is cleaned of adsorbed contaminants, mainly moisture, by the application of vacuum or flow of dry inert gas and preferably some heat. How much heat?
Outgassing of Surface • How much heat? • What is the proper temperature for sample preparation? • Should be high enough to promote rapid removal of surface adsorbed species without changing the surface texture. • Obviously not high enough to melt the solid, nor hot enough to exceed the glass transition point, Tg. Estimate Tg as melting pt 0.7 in kelvin (allow safety margin). • Or no more than melting point 0.5 (kelvin) – Tammann temperature. • Example: • Magnesium Stearate monograph: 40 °C (for 2 hours)
Vacuum or flow • A flow is good at removing large quantities of weakly bonded “wet” water by displacement of the vapor from external surfaces. However in the depths of a pore, water must diffuse out… and in doing so must battle past a much higher concentration of purge gas. Only when it is out of the pore will it be physically swept away.
Vacuum or flow • Vacuum is not so good at removing large quantities of weakly bonded “wet” water since once it has left the sample it must diffuse towards the pump. (A “random walk” will make the actual distance needed to travel MUCH further than it looks to us! It will spend much of its time wandering back towards the sample!). However in the depths of a pore, water must diffuse out… and in doing so does not have to battle past much - certainly not any purge gas.
Traps Traps are used on vacuum outgassing systems to prevent: • Oil backstreaming from pump (use foreline trap of activated alumina or cold trap) (Or use a dry pump system!) • Evolved species off the sample from migrating to the pump and • reducing pump efficiency, • effectively “pausing” the outgassing at the vapor pressure of the evolved contaminants.
Sample PreparationDegassing Parameters • Method • Temperature • Time • Test • Backfill
Is it ready? Test (vacuum) • A sample still outgassing will cause a pressure rise when isolated from the vacuum system • Hot samples outgas faster than cool ones. • Generally less than 50 microns/min hot, 20 microns/min cool will indicate readiness. • (background rise of empty degas station?)
Is it ready? Backfill (vacuum) • Cell should be backfilled to ambient pressure to allow easy removal and to prevent ingress of atmosphere. • Avoid inconsistent use of helium as backfill gas – buoyancy errors. • He buoyancy error approx 1 mg/ml of cell volume. • Backfilled cells cool much faster. • An evacuated cell barely warm to the touch can be hot enough to cause burns when backfilled.
Elutriation Elutriation: The process of separating the lighter particles of a powder from the heavier ones by means of an upward directed stream of fluid (gas or liquid). IUPAC Compendium of Chemical Terminology 2nd Edition (1997)
Elutriation Its how a vacuum cleaner works! Remember, a vacuum doesn’t suck!
Filters, pro’s and cons • The ability of a filter to trap particles depends on its fineness and its tortuosity. • It serves to both sieve out particles and to slow the rate of evacuation, hence to reduce gas flow velocity. • The obstruction to flow can limit effective vacuum. • Filters can become blocked!
Filters, pro’s and cons (continued…) • What effect might a contaminated filter have on resultant data? • Isotherms might appear “open”. • Isotherms may be “shifted” slightly. • Surface area values might be lower than expected.
Sample Cells Analysis Hardware
Sample cells “What are the characteristics of a sample cell?” • Overall length. • Volume of “bulb”. • Diameter of stem. “How do I select the right one?”
Sample cells • Overall length. • Sample must be completely immersed under coolant, as should shoulder part of bulb. Cells are long to allow for the evaporation of cryogenic coolant. A coolant which does not evaporate can be used with a “short” cell. • Volume of “bulb”. • Should be large enough to accommodate sufficient sample for reasonable accuracy (e.g. at least 1 m2) but never more than 2/3 full. Small sample quantities should be accommodated in small bulbs (see void volume control). • Diameter of stem. • Large enough to easily accommodate particle size, but small enough to improve overall sensitivity. “How do I select the right one?”
Void volume control “What is void volume and why bother?” • Quantity of gas adsorbed is determined by (i) waiting for minimal/zero pressure change and (ii) subtracting quantity of gas in internal volume of equilibration “zone” from total quantity “dosed” into cell. • The total volume in which the final equilibrium pressure is monitored is the void volume.
Void volume control “Therefore it should be minimized” “How?” • Close off the manifold after the dose. • Use a small manifold if the valve must stay open. • Use a sample cell with small internal volume. • Cool as little of the cell as is absolutely necessary. • Work at low pressure.
Void volume control “Close off the manifold after the dose” • A pressure transducer on the analysis station monitors pressure drop due to adsorption (rise due to desorption) in the station volume only. • For a given quantity of gas being adsorbed, a smaller volume gives rise to a larger (and more easily and accurately measured) pressure change. n = PV/RT i.e. greater sensitivity This is the standard configuration for all Autosorb gas sorption analyzers.