Let's discuss the techniques that center around the measurement of adsorption onto and desorption from surfaces.

Finished coatings that protect surfaces often represent the end of a long process of design and testing. In use, these products provide the surface finish, which, apart from being visible, also acts as the system boundary, ideally closing it off or protecting it from aspects of the environment. So it is ironic that the coatings industry uses open system or 'flow-through' instrumentation to help understand surface chemical phenomena associated with pigments, fillers, additives, and protection.

This article discusses the techniques that center around the measurement of adsorption onto and desorption from surfaces. A new instrument, developed to make these measurements, is called a flow microcalorimeter (FMC). It is an ultrasensitive flow-through microcalorimeter designed to measure the small energies associated with preferential sorption of molecules onto (or off) surfaces and the number of molecules involved in such interactions, all in one automated experiment.

Technically speaking, a sample, which can be a powder, fibers, filaments, strips or small pieces of solid (the adsorbent), is placed in the FMC cell, where it is environmentally isolated. The adsorbent can be evacuated (optionally) before it is wetted by an appropriate fluid (called the carrier). The energy changes resulting from these processes can be measured before initiating a flow of the carrier through the sample, which is retained by an appropriate filter/membrane system. The carrier can be gaseous or liquid, pure or a mixture, depending on the application.

A unique feature of FMC flow-through design is that the fluid flow can be fed from the FMC cell to a downstream detector so that the composition of the FMC cell effluent can be monitored and recorded. While the carrier flow acquires a stable equilibrium with the adsorbent, the Down Stream Detector (DSD) allows users to monitor dissolved material eluting from the sample.

By switching from a flow of carrier to one containing a probe, the effect this has on the surface can be observed. Originally, the only measurement made was the thermal one, but the option to use a DSD connected to the FMC cell effluent is now available so that an online measure of probe concentration emerging from the FMC cell can be made. From the profiles of these two data streams, the energy associated with the interaction and the number of molecules that were involved can be determined. These numbers can then be combined to determine the molar enthalpy of sorption, which categorizes the interaction. As a bonus, the heat and concentration profiles, plotted against time, provide a visual representation of the interaction kinetics. Time-slicing these data quantifies the kinetics.

The FMC also allows for the adsorption isotherms to be determined by adsorbing increasing concentrations of a probe onto a surface in discrete steps. Isothermal measurements can be made at temperatures from ambient to 240°C, and at pressures from 10-9 bar to 50 bar, using appropriate hardware. The FMC automation systems can add ease and precision to the experimentation as well.

The benefits of flow adsorption microcalorimetry are derived from the high thermal sensitivity it offers, the rapid detector response and the fact that adsorption and desorption measurements can be made in a constantly flowing stream of fluid. This translates to an ability to measure heats of adsorption or desorption of small quantities of probe on low surface area materials, to quantify the kinetics of those interactions, and to quantify the reversibility of those interactions, all in one experiment.

Recent developments in FMC technology have increased the maximum sensitivity of the Series 4 models by a factor of 16, which, combined with the new automation systems, have broadeneded the scope for FMC application into areas such as hydrophilic/hydrophobic surface categorization,¹ the characteristics of fly ash as a filler,² how donor-acceptor interactions affect the mechanical properties of wood,³ fullerene adsorption onto active carbons,⁴ gas and vapor phase adsorption on carbons,⁵ properties of lubricants in magnetic media,⁶ and alkyd resin adsorption on titanium dioxide.⁷

In application terms, there are a variety of ways the system can be used to examine paint and pigments: it has been successfully applied to the study of adhesion,⁸ magnetic media,⁹ and surfaces of colloidal particles.¹⁰

Application 1: Acid Sites in Anatase

A feature of the flow-through design is that users can saturate to the capacity of a surface, for the adsorption of a probe, and measure the energy and number of molecules involved in the process. Assuming that the probe concentration is sufficient to fully saturate the surface, the saturation profile that the FMC generates describes the kinetics of the interaction both in terms of the rate of heat evolution and the rate of probe uptake. By time-slicing these profiles, the kinetics in terms of surface coverage can be analyzed as the active surface sites become occupied by probe molecules. This shows how accessible the surface is and how the sites are distributed.

In this work, anatase is saturated with a monobasic molecule, ammonia (5% by volume in nitrogen), to determine the level and distribution of surface acidity.

A gas phase system (FMC-4020) has been applied to the study of acid sites on a titanium dioxide surface (anatase) using ammonia to titrate the acidic surface sites, at 132 degrees C. Acidic sites, their number and strength in TiO₂, have been linked to the ability of resins to bond onto the pigment and crosslink - both important in a paint's performance and manufacture. By saturating the pigment with ammonia, we are effectively titrating its surface and neutralizing the acid sites with ammonia. The number of those sites is a function of the quantity of ammonia removed from the stream (lower trace in Figure 1), while their distribution and accessibility on the surface is a function of the thermal data profile (top trace in Figure 1).

Figure 1 shows two complete adsorption/desorption cycles of ammonia on TiO₂ stabilized in a flow of nitrogen at 1ml/min. Qualitatively, it illustrates that the first exposure to ammonia produces more energy and ammonia uptake than the second exposure (following desorption). By subtracting the second set of integral values from the first, the irreversible (chemisorbed) and reversible (physisorbed) components of the interaction can be qualified. So, in this example, the adsorption of ammonia is almost exactly 50% reversible (first exposure to ammonia yields 3.51J/g, second exposure 1.76J/g). The example also shows that the anatase surface is very accessible to the ammonia molecule, since the adsorption peaks (both heat and matter transfer) are quite sharp and symmetrical, while the kinetics of desorption show a slower process.

Application 2: TiO₂and Water

For pigments such as TiO₂, dispersed in aqueous media, the properties of the dispersions depend on the strength of interaction between the TiO₂ surface and water. They also depend on the interaction with the pigment surface and dispersing agents, which are capable of displacing water from the surface. It is therefore important to know how strongly water is adsorbed on different samples of TiO2 and how it can be displaced by agents, which are required to produce a stable dispersion.

In this example we can see how the affinity of water for TiO2 surface can be determined over the whole range of surface saturation. Both the integral heat and the amounts of adsorption for this experiment have been measured by the FMC. By time-slicing these integral values across the duration of the peaks, the integral and differential quantities for the adsorption of water can be determined, as shown in Figure 3.

This technique is a new feature of FMC data processing, which permits the rapid comparison of a large number of samples of the same type of material, without the need to perform a full adsorption isotherm experiment on each sample. The only requirement is that the adsorption be performed at a probe concentration that is sufficient to saturate the active surface present in the sample. This may require a separate experiment to establish what is the appropriate concentration of probe to achieve valid conditions for this 'one peak' kinetic measurement.

Application 3: Corrosion Inhibition

A few years ago, we were asked if we could develop a technique to assess the efficiency of a corrosion inhibitor. The 'flow-through' design of the FMC allows users to examine how a surface is attacked, how it can be protected and how efficient that protection is, in one experiment. This technique combines the 'saturation' approach with 'loop injections' (whereby a small aliquot of a probe is introduced into the fluid stream). The advantage of loop injections is that the small quantity of probe introduced is not sufficient to saturate the sample and can not dwell for long in contact with the surface before it is 'washed away' by the stream of carrier. As a result, the contact time (and hence interaction time) is much shorter than with saturation events and only some sites are given the opportunity to interact with the probe contained in the aliquot. This technique has been applied (along with saturation exposure of the inhibitor) in the experiment shown in Figure 4, page 54.

This approach shows how injected materials interact, at the solid-fluid interface, with a solid surface before and after treatment. Since users can also watch the treatment with the inhibitor itself, all the aspects of the heat and matter transfer can be monitored in one experiment.

In this example, a sample of iron milled in pentane is at a stable thermal and matter balance in a flow of n-heptane, to the left of both traces. An injection of 20 µl 0.5% (w/v) acetic acid dissolved in n-heptane appears as the first heat peak on the top trace (thermal data), while a simultaneous trace below shows the acid that does not get adsorbed, appearing slightly delayed, on the refractive index detector trace below it.

There follows a saturation adsorption (and then desorption) of 0.25% (w/v) SWI corrosion inhibitor (peaks 2 and 3 on both traces). Finally there is a repeat of the 20µl acetic acid injection. Remember that the top trace is FMC thermal data, the lower trace shows matter transfer. The acid interacts very strongly with the iron without the inhibitor present, and much less once the surface is protected (see the loop injections - peaks 1 and 4 - on both traces, before and after). At the same time the quantity of acid arriving at the RI detector is smaller before inhibitor protection, and much larger when the SWI prevents its sorption on the iron. The thermal trace indicates that less than half the heat was evolved from the second acid pulse while the RI detector indicates that much more acid is getting there and therefore not reacting with (or being adsorbed on) the iron.

Conventionally, this data is reported as kilojoules per mole adsorbed (or displaced), and this can be calculated directly from the data by normalizing the output from each trace in heat per unit gram of sample, and number of moles per gram of sample.

Application 4: The Thermodynamics Of Cement Hydration

The previous three application examples show how flow-through experiments can measure the acidity of a surface, how water sorbs onto a pigment, and how loop injection and saturation techniques can combine to measure the level of surface protection offered by an inhibitor. In this example, the FMC was used in 'Static Mode' to quantify the heats of evacuation, wetting and hydration of a commercial cement.

The cement, in powder form, was placed in the FMC cell, evacuated in-situ (top trace, peak 1), wetted by pure water (top trace, peak 2), and allowed to stand for 32 hours while the cement cured (top trace, peak 3). The cured sample was easily removed from the cell following the experiment since it contacted only smooth PTFE.

The top trace in Figure 5 shows the entire evacuation, wetting and hydration cycle of a 44.1mg sample of commercial Portland cement (top), with the evacuation and wetting cycles displayed in detail on the middle trace and the subsequent hydration of the sample over 32 hours on the lower trace. This is an example of the FMC operating in 'Static Mode' where one time point corresponds to eight seconds.

The same 'static' instrument configuration can be used for measurements of heats of mixing or dilution, wetting, and diffusion, and requires minor additional hardware to adapt from a 'flow through' instrument.

In these examples of studies on materials used in the paint, corrosion and construction materials industries, we have tried to demonstrate the versatility of the flow microcalorimetric techniques and how these industries could benefit from the new instrumentation and techniques that have evolved. We continue to work with our customers to develop new, faster and easier techniques to investigate surfaces and the interactions which take place upon them.

North American distribution by Gilson Co. Inc., phone 800/444.1508; or e-mail FMC4bss@aol.com

For more information on flow-through instrumentation, contact Microscal, 79 Southern Row, London W10 5AL; phone intl. +44-208-969-3935; Web site www.microscal.com; e-mail info@microscal.com.

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