Portal de Periódicos da CAPES

Why do we need chemical derivatization?

2017; Wiley; Volume: 14; Issue: 7 Linguagem: Inglês

10.1002/ppap.201700044

1612-8869

Andreas Holländer,

Various Chemistry Research Topics

"Die organische Chemie kann einen jetzt ganz toll machen. Sie kommt mir wie ein Urwald der Tropenländer vor, voll der merkwürdigsten Dinge, ein ungeheures Dickicht, ohne Ausgang und Ende, in das man sich nicht hinein wagen mag."1,1 Friedrich Wöhler – one of the pioneers of organic chemistry – wrote these words in a letter to the Swedish chemist Jöns Jacob Berzelius in the year 1835. At that time, organic chemistry as we know it today was in its infancy. From there it took several decades until chemists developed the concepts of chemical bonds and molecular structures which opened the door to understanding the nature of organic substances.2 It was almost 50 years after Wöhler's letter that the first infrared absorption spectra were reported.3 It took another 50-60 years until instrumental analytical techniques for molecular analysis like infrared spectroscopy4 or mass spectrometry5 started to become routine technologies in chemical laboratories. Before instrumental techniques were used, substances were analyzed by their physical properties and by chemical reactions. Their reactivity was used to classify them. In the German language, chemists used the words Kationentrennungsgang and Anionentrennungsgang (procedure to separate cations and anions, i.e., qualitative analysis of inorganic substances by identifying the ions they consist of.), which describes very well this part of the qualitative inorganic analysis. Similar procedures had been developed for identifying organic substances.6 These procedures were very time consuming. The analysis of one sample could take several days. Today we have sophisticated instrumental techniques which can do the job in minutes or even in seconds. However, knowing that we have powerful equipment we are used to ask much more complicated questions than our predecessors did in the past. Some years ago, Stefan Kröpke, Falko Pippig, and I wrote a paper on the status of the chemical analysis of functionalized polymer surfaces.7 We started by describing what we actually have to deal with. Polymers are quite complicated substances and they tend to get even more complicated the closer we look at them. They comprise a mixture of molecules with a rather broad distribution of molecular masses, sometimes ranging from oligomers with less than 1000 g/mol to several million g/mol. In many cases the low molecular weight fraction gets enriched at the surface during melt processing. Moreover, all polymers contain irregular structures as residues of the polymerization catalyst, products of polymerization side reactions, chain ends, and degradation products, to mention only a few of them. Beside these intrinsic components of polymers, the technical materials mostly contain additives. At least a fraction of these chemical species, which do not appear in the paper formula of the polymer, can be found in higher concentrations at the surface than the bulk. This description is brief and incomplete but it points out that the polymer materials we encounter in everyday life and in various industrial applications are complex substances. And these are the materials that we use to do surface treatments. Electrical discharges are versatile tools to convert the inert surfaces of polymer materials very efficiently. Millisecond exposure times suffice to activate a polyolefin surface with a corona in large scale industrial applications. The reason for this efficiency lies in the radical chemistry on which the conversions are based. The electrical discharge generates highly energetic species which are able to dissociate virtually any chemical bond. As a result, very reactive radicals are formed. In the case of activation, for example, oxygen is involved and it gives rise to an oxidation chain reaction. As the flip side of the coin, radicals can react with a wide variety of chemical structures that give rise to a large number of different reaction products, even "exotic" ones when looking at it from the viewpoint of classic organic chemistry. Adding other gases to the discharge opens up more reaction paths and gives rise to even more complexity. In the case of plasma polymerization there can be a very large number of such possible reaction pathways, which result in a wide variety of chemical structures. If we consider the different penetration depths of the various plasma components, we come to the conclusion that there can be, and will be, a vertical stacking of different chemical structures from the outer surface inwards toward the bulk. In real-world materials we will encounter a convolution of the plasma treatment profile with the chemical profile of the original material including layers of adsorbed substances which can result in a very complex overall profile. The analysis of these features is virtually impossible with today's routine technology. If we look at the picture presented in the previous paragraphs, we can well imagine Wöhler's feelings when he wrote the words cited at the beginning. But as he did many years ago, we can try to find ways to improve our tools and to thereby extend our knowledge. Chemical derivatization surely is one way to improve the tool kit of our instrumental analysis. Derivatization is based on converting compounds into a derivative which can be better analyzed in one way or another. Usually this conversion is specific for a functional group or a class of functional groups. In the early years of organic chemistry, properties like the melting point or the color of the derivative were tabulated and used to identify substances. Today, derivatization techniques are used extensively in gas chromatography and mass spectrometry.8 For example, polar compounds are derivatized to obtain less polar derivatives which are more volatile and can, therefore, be more readily analyzed. Another set of derivatization reactions is used to improve the separation and to ease the identification in chromatographies (gas as well liquid). In mass spectrometry, derivatization allows one to obtain more information about the structure of the analyzed molecule, like the position of a particular functional group in the molecule.9 These are only a few examples of the applications of derivatization techniques. Technology providers and chemicals suppliers offer a wide range of derivatization chemicals for various purposes, mainly for chromatographic techniques but also for mass spectrometry. Despite the fact that derivatization techniques are widely applied, there is an important and remarkable difference between the instrumental analytical techniques and the supporting derivatization: State of the art analytical instruments are largely standardized and often automated. The processing and interpretation of measured data often run automatically with sophisticated programs which are supported by comprehensive digital libraries. Scientists can obtain reliable results without knowing very much about the actual measuring technology.2 These techniques expanded from the domain of experienced specialists and later became tools in routine laboratory work. Applying derivatization techniques, in contrast, generally requires much more knowledge of the instrumentation, the actual measuring technique, the sample, and the chemistry behind it all. It often is far from being routine. Also in the field of surface analysis, derivatization techniques have been used to improve instrumental techniques in order to obtain more specific information about an organic sample material such as a polymer. Most efforts have been undertaken for supporting XPS measurements. Examples were shown in the early work of Hollahan 1969,10 the later extensive investigations by Everhart et al.,11 or in reviews by Batich12 and by Chilkouti et al.13 The latter paper also included derivatization for SIMS. More recent and more general reviews also cover derivatizations for other techniques like UV-, fluorescence-, and IR spectroscopies.14-16 The derivatization of functional groups at polymer surfaces with fluorescent dyes was reported by Holmes-Farley et al. in 198617 and later by Ivanov et al.18 Derivatizations and infrared spectroscopy were applied for surface analyses in various ways, such as IRRAS,19 IR reflectance spectroscopy,20 and ATR-IR.21 The literature cited here represents only a rather small fraction of the overall body of published work. Unfortunately, there exists no review or monograph that covers the complete state of the art in this field, and which could serve as a guide or textbook. However, the recent paper by Klages and Kotula22 covers part of this subject in an excellent manner for the very particular field of nitrogen-based plasma-functionalized organic surfaces. It provides a comprehensive, critical overview not only about the published literature in the field, but also about its historical progression. The authors discuss in depth the chemistry underlying derivatization reactions, and they explore the various pathways of possible reactions. They present a thesis and discuss on the basis of a formidable variety of experimental data the pros and cons of possible explanations for the findings. All in all it is a commendable piece of work summarizing the state of the art and providing a critical evaluation of current knowledge. As such, it represents a fine example for similar work in other fields of surface analysis. The paper clearly demonstrates that derivatization reactions can be powerful tools to obtain valuable information, but need to be handled wisely and with great caution. The issues regarding side reactions are discussed in great detail, while certain other issues appear only as short remarks. Such more general considerations for the application of derivatization reactions were discussed in a recent paper by Girard-Lauriault et al.23 with a focus on XPS only, although many of them are valid for other techniques, directly or by analogy. In particular, the discussion of the pros and cons of gas- and liquid-phase reactions, the importance of reactant diffusion, reaction kinetics, the chemical composition profile and, of course, the crucial importance of possible side reactions have to be taken into account for the case of all surface-analytical derivatization reactions. The authors also suggest using all available data to check whether the reactions proceeded as expected. In the case of XPS this implies that processing the elemental analysis data shall be scrutinized by the high resolution spectral data. For complex samples or in the case where doubt exists, it is advisable to use complementary techniques, as was done by Klages and Kotula. If different instrumental techniques are combined, their specificities have to be considered. Each technique probes different features of a sample and analyses a different part of the sample. XPS gives an elemental analysis and information about binding partners of the constituent atoms (except hydrogen, of course). Resulting values represent volume concentrations averaged over the sampled volume, where the latter depends on the photoelectrons' mean free path. If we assume it to be 2 nm, 95% of the signal comes from the outer 10 nm of the sample (analysis depth), with an exponential sensitivity gradient from the outer surface toward the bulk of the sample. In the case of a derivatized sample, this profile is likely a convolution of the original sample's compositional depth profile with another profile due to the conversion with the derivatization agent. For IR reflectance measurements and for ATR-IR the situation is similar, but the analysis depth depends on measurement conditions and on hardware such as the ATR crystal. It lies in the range between several 100 nm and several μm, that is, very much larger than the value for XPS. Consequently, features which are clearly visible in XPS may just barely appear in the IR spectrum. IR spectroscopy probes groups of atoms, structural features rendered detectable by their absorptions due to molecular vibrations. If the absorption coefficients are known for the particular groups under study, quantitative information can be obtained. However, often these coefficients are not available and they must be estimated from similar samples, or the particular absorption band is related to another band for a relative comparison of different samples. In contrast to the techniques discussed above, fluorescence labeling yields area concentrations; this means that the complete sample thickness (in the case of typical film samples) contributes to the signal within the analyzed area. In this case the analysis depth is determined by the compositional depth profile of the sample, and by the profile possibly created via reactions with the fluorescence label. If calibrated properly, absolute concentrations are thereby accessible. Considering the last paragraphs, it can be quite challenging to relate information obtained from different techniques with one another. Wilken et al.24 provided an example for combining various techniques, including derivatization, to obtain the quantitative chemical composition evolution of olefin polymers during their VUV irradiation. The samples they investigated had a relatively simple chemical composition since the experiments were done in high vacuum. But it can be seen that obtaining a sound picture of the surface chemistry evolution of a polymer in the course of a treatment requires considerable efforts. Since such extensive investigations are not always necessary for a particular task the most important question we have to ask before we do any experiments is: What do we actually want to know? Answering this question as precisely as possible will help a lot to select the analytical tools and to find a way through the jungle of organic chemistry. Despite remarkable improvements of instrumental techniques in the surface analysis of organic substances, there are still serious limitations in their ability to answer numerous questions relating to functionalized surfaces. Derivatization reactions have proven to be valuable tools for obtaining additional information. But it also has been shown that as much background knowledge as possible about the sample and positive scepticism are required to use this information wisely. Further research in this field is therefore highly desirable.