Chapter 19, Saving the planet (Finding Improved Environmental Solutions)

We all try to do our bit to save the planet. Although a few grand gestures can make a difference, mostly we have to do it one step at a time.

HSP are proving versatile in their ability to contribute to lots of small steps. So here we have a selection of examples.

HAP substitutions

The USA lists chemicals considered to be Hazardous Air Pollutants. If you happen to be using one of these as a solvent then there is considerable pressure for you to substitute it with a non-HAP variant. How do you do this sensibly?

The key is functionality. You need to have the same solvating power. It’s unlikely that there is a single solvent that can be used to replace your current solvent, so you need to find a mix of solvents that will do the same job. Without HSP your task is very tricky.

Suppose you are using diethanolamine, with a low (i.e. serious) HAP reporting level. What mixture of solvents will match its particular HSP? The Solvent Optimizer quickly gives you an idea.

Figure 1‑1 Approximating Diethanolamine

The target values for diethanolamine are [17.2, 10.8, 21.2] and by clicking the “2” button you instantly find that the 2-best solvents for giving a match are a 77/23 mix of ethylene glycol and cyclohexanone. Unfortunately, ethylene glycol is also on the HAP list so you want to exclude that from the list. Ctrl-clicking on ethylene glycol grays it out and re-clicking “2” gives you a methanol/benzyl alcohol mix, but methanol is also on the HAP list, so you Ctrl-click that and try again to find that a Dipropylene Glycol (DPG)/ethanol mix is a good match. Now it’s unlikely that you would have thought of such a mix, and you may not even be familiar with DPG. But who knows, this mix might be just what you’re looking for.

The point of this example is that with HSP (and some convenient software tool) it becomes routine to think through these tricky substitution issues in a methodical and environmentally-friendly manner.

VOCs (Volatile Organic Compounds)

The rational approach to reducing the harmful effects of VOCs is to find chemicals of the correct chemical compatibility and with both a low vapour pressure and a low reactivity with ozone (as measured by the Carter MIR) and with OH radicals (Log[OH]).

HSPiP can give you all of this in one module. The Y-MB calculator not only estimates the HSP of a chemical of interest but uses its neural networks to estimate vapour pressure (at 25º) as a convenient judge of how low the vapour pressure might be, plus it calculates the Carter MIR and Log[OH]. As an additional guide, from the vapour pressure an estimate of the RER (Relative Evaporation Rate – nBuAc=100) is made via the empirical formula RER=0.046*MVol*VapourPressure. We’ve been asked why MVol is necessary for calculating the RER. The Vapour Pressure is, essentially, in moles. If you have two chemicals with the same vapour pressure, but one has twice the molar volume, then there will be twice as much of this second chemical in the air. It’s as simple as that! Another question is how the RER based on nBuAc=100 is related to that based on ether. The answer is that the ether-based system is the time needed to evaporate rather than the rate. nBuAc takes 12 times as long to evaporate as ether so

RER-based-on-nBuAc=12/Ether-based-system (or 1200 if ether is 1 in its system).

Bio-solvents

What better way to replace some bad solvent than by some bio-solvent? Well, there are arguments for and against bio-solvents, but at least we can agree on an objective value – their HSP. If the HSP is wrong then however bio-friendly they are, they simply won’t be able to do the job.

A typical example is the proposed use of FAME solvents – Fatty Acid Methyl Esters. We use the data from a typical paper on the subject where FAME solvents were tested for applications of epoxy resins, Y Medina Gonzalez et al, Fatty Acid Methyl Esters as Biosolvents of Epoxy Resins:A Physicochemical Study, J Solution Chemistry, 36, 437-446, 2007.

One of the target resins was an epoxy resin called Bisphenol A diglycidyl ether. The authors used a classic 40-solvent HSP Sphere process to determine values shown in the table. The table includes our S-P estimates of the HSP for two of the FAME materials.

Chemical	δD	δP	δH	R (or Distance)
Epoxy	17.2	6.9	8.2	24
Methyl caprylate (C8)	15.5	2.8	5.6	6
Methyl myristate (C14)	15.4	2.2	4	7.2

Table 1‑1 FAME solvents compared to an Epoxy

This would seem to suggest that FAME would be good solvents for the epoxy and that there wouldn’t be a huge difference in solubility between the C8 and C14 versions. But the solubilities turn out to be disappointingly low (though they at least diminish as chain length gets longer, as would be expected). Is this a failure for HSP? The large R for the Epoxy looks a little odd. With a more typical value the distances would be closer to the border of the sphere and the relative insolubility would be more explicable. And, of course, the rather large molar volumes of the FAME solvents mean that kinetically they will be rather poor – one of the key problems that beset the change to “greener” solvents.

Rational extraction

Work carried out in Professor Staci Simonich’s team in Oregon State University T. Primbs, S. Genualdi, and S.M. Simonich, Solvent Selection for Pressurized Liquid Extraction of Polymeric Sorbents Used in Air Sampling, Environmental Toxicology and Chemistry, 27, 1267–1272, 2008 is a good example of how a complex environmental sampling procedure can be rationalised via HSP. Anthropogenic SOCs “Semivolatile Organic Compounds” travel around the world from their sources in one region to provide pollution in another (often pristine) region. It is a substantial challenge to collect them then to extract them quantitatively. She uses pressurise fluid extraction, but her problem is to ensure extraction of the SOCs without extracting polyurethane (PU) which happen to be one of the key components of the system for trapping particulate and gaseous materials from the air stream.

By plotting a range of typical SOCs, along with typical solvents used in pressurised fluid extraction, choosing PU in the Polymer form and clicking the Solvents button, the 3D plot makes it clear that dichloromethane is too close to the PU to be of use in extraction. The 50/50 hexane/acetone looks, from the H-P plot to be a potential problem with the PU but the RED number shows that it too is safely outside the R of this PU and so should be useable. The favoured solvents, hexane, acetone and 75/25 hexane/acetone seem, from a visual inspection of the 3D plot to be adequate for extracting the SOCs themselves.

Figure 1‑2 Using file OSU Environmental

Predicting problems

The problem of contaminants reaching the Arctic via routes such as the one above is a major one. It has been known for some time that pentachlorophenol [21.5, 6.9, 12.8] not only gets to the Arctic but also accumulates to high levels in polar bears. One of us, many years ago, spotted that the much-used fire-retardant tetrabromo-bisphenol A [20,2, 9.1, 13.8] is very close (Distance=3.5) to pentachlorophenol and would therefore be highly likely to start accumulating in polar bears. Unfortunately for the bears, and those who might eat the bears or the seals with the same problem, this prediction has been vindicated. Of course the bromo flame retardants are now being phased out, but the point of this small section is that HSP predictions can be used with some confidence on issues that take a long time to resolve experimentally. Clearly one can’t make long-term environmental decisions based on HSP alone, but when difficult decisions have to be made, the insights from HSP can be crucial.

Plastic containers

Although the environmental benefits of plastic containers are contentious, there are plenty of times when they are environmentally preferred to glass or tins. If you want to sell something for the first time in plastic, you need to be confident that the contents will not damage the plastic by dissolving it or causing environmental stress cracking. Because it’s relatively easy to test for such issues, there aren’t too many mistakes made. What is not so obvious is when you have aqueous solutions of potent chemicals such as weedkillers. There’s no chance of the solution dissolving the container or producing stress cracking. The problem is that any diffusion of the weedkiller through the walls means potential skin contact for the user or leakage into the environment. A casual examination of a typical tin of weedkiller tells you that one common ingredient is paraquat. From the standard table we find its values are [19.5, 8.8, 5.9] Now let’s try to put this in a PET bottle. There are a number of different PET values, but the R-PET data in the Polymers table is [19.1, 6.3, 9.1, 4.9] and the distance between the paraquat and the PET is 4.1, inside the radius and therefore a warning that PET would not be a good idea. To check out the polymers in general, select paraquat in the solvents table then click Polymers within the Polymers form.

Figure 1‑3 Selecting Paraquat from the main table then clicking Polymers

 

You will find that the other PET values (such as Mylar PET) also have a Score (in this case, the RED number of the paraquat) less than 1. Nearer the top are polymers such as PMMA and PVC ( both REDs~0.35). Near the bottom of the table are the high RED polymers such as Polypropylene and Polyethylene, so you could be confident of no diffusion of the weedkiller through these bottles. If you think that the worry about dilute chemicals in PET bottles is excessive, just think what happens if the lid is left off so that the water evaporates and concentrates the chemical. What remains could be potentially very active in getting through the PET.

 

Bio-polymers

We know that common polymers have land-fill degradation times of 100’s of years. Bio-polymers can have degradation times of months to a few years under the right conditions. But industries aren’t going to shift easily to these new polymers. They have to know much more about them. One key thing that’s needed is an idea of their compatibilities with solvents, plasticizers and other polymers. And the shortest route to identifying these is via HSP. We use, for example, data from the paper A. Agrawal et al. Constrained nonlinear optimization for solubility parameters of poly(lacticacid) and poly(glycolic acid)—validation and comparison, Polymer, 45 (2004) 8603–8612.

Figure 1‑4 Using file PolyLacticAcid

Armed with this correlation, [18.7, 10.1, 6.0, 10.0] and with the solvents sorted by their RED number, it’s easy to determine that a high-boiler such as NMP or a low boiler such as 1,3 Dioxolane would be a good choice of solvent depending on the type of application. If solvent data hadn’t been available, the estimation from the Stefanis-Panayiotou DIY calculator (see the chapter on DIY HSP) gives a creditable [17.2, 9.8, 8.1], which would have been given a reasonable choice of practical solvents. One of us (Abbott) has written a review (Poly(lactic acid): Synthesis, Structure, Properties, Processing and Applications, Eds. R. Auras; L-T, Lim; S.E.M. Selke, H. Tsuji, Wiley, 2010) on how HSP provide a coherent framework for explaining many real-world issues confronting this relatively novel polymer – such as choice of plasticizer, filler dispersion/compatibility and odour/fragrance permeability.

Pervaporation

One way to remove contaminants from water is to apply high pressure to the contaminated liquid as it passes through a tube surrounded by a polymer membrane through which the contaminant can diffuse but the water cannot. After the permeation through the membrane the solvent then evaporates, hence the term pervaporation. A specific example comes from Professor Yoram Cohen’s group at UCLA. They wished to remove tetrachloroethylene and chloroform contaminants from water. The technological innovations in producing a robust membrane supported on a ceramic tube need not concern us here. The key was the choice of the polymer for the membrane.

Figure 1‑5 Pervaporation through PVA

The polymer used was PVA (polyvinylacetate). As the data show, both chloroform and TCE have a RED <1 and are therefore likely to be able to pass through the membrane. Indeed, their pervaporation experiments showed good pervaporation removal of the two contaminants in their system. Note the final solvent in the list. It is not included in the plot but its RED number shows that it too should be removable by pervaporation. Indeed, in a later paper, Cohen’s group showed good speed and specificity for removal of methyl-t-butyl ether which is a modern-day contaminant in groundwater because of its use as an octane enhancer in petrol/gasoline as a “safe” replacement for lead tetraethyl.

Soil chemicals

It’s often important to know whether a chemical will tend to stay attached to soil particles or prefer to partition into the water trickling through the soil. The soil-water partition coefficient (relative to organic carbon), K_oc, has been measured for many compounds and a number of techniques for estimating K_oc have been suggested. A paper by Prof Schüürmann’s group at Leipzig has provided an overview of the various datasets available and compared their own methods to those in the literature: G Schüürmann, R-U Ebert, R Kühne, Prediction of the Sorption of Organic Compounds into Soil Organic Matter from Molecular Structure Environ. Sci. Technol. 2006, 40, 7005-7011.

Using the CAS numbers from their data table it was straightforward to identify 80 chemicals within our own dataset for which there was K_oc data. By using the simple formula (another variation on the χ parameter form)

Equ. 1‑1 log K_oc =-0.327 + 0.0008 * MWt * (4 * (δD_c-δD_s)² + (δP_c-δP_s)² + (δH_c-δH_s)²) ^½

 

a respectable R² of 0.835 was obtained. In this equation, “c” is the chemical and “s” is the soil. δD_s, δP_s and δH_s were obtained by fitting as [13.8, 20.8, 9.2].

Figure 1‑6 Calculation of soil sorption

 

This quick excursion into soil science is not intended to prove that HSP are good predictors of partitioning into soil, nor is any technical explanation being offered - it would need a much more extensive analysis. But it is interesting that once again the same basic HSP equation seems to work adequately when applied to such a complex problem. Note that the fit uses Molecular Weight and not Molar Volume. When the latter is used, the fit is less good.

Radon

Homeowners in some parts of the world are unlucky. Their houses are built on granitic rocks that contain a lot of radon from natural radioactive decay. It is therefore desirable to put a barrier film underneath a house to inhibit the ingress of radon. Can HSP help with the problem? Surprisingly the answer is “yes”. The starting point for thinking through the problem is the well-known fact that EVOH is an excellent oxygen barrier. Why is this very modest polymer such a good oxygen barrier? Nothing about its polymer structure makes it an obvious block to oxygen. And why is something like polyethylene such a poor oxygen barrier? As the Diffusion chapter shows, permeation depends on diffusivity (which depends on polymer structure) and on solubility. And this is why EVOH is so good. If you load Gases.hsd you find the following table:

Figure 1‑7 Gas HSP from Gases.hsd

The HSP of oxygen are [14.7, 0, 0]. The HSP distance from polyethylene (PE) is a modest 5, inside the sphere. The distance from EVOH is a massive 20. No wonder that EVOH is such a good oxygen permeation barrier. The table also includes Radon at [17.2, 0, 0]. The distance from EVOH is 17.5 and from PE is 3. There is a literal take-home message from this: don’t protect your house from Radon with PE film. However, EVOH is a useless barrier for water as it readily swells. So you can’t protect your home with a pure EVOH barrier – you need a water barrier to protect the EVOH. Try a PE-EVOH-PE laminate as the HSP calculations are totally opposite – the distance of PE from water is >20 so the water solubility, and hence permeability is very low. HSP have one more thing to contribute. That laminate is unlikely to exist because the distance between EVOH and PE is so large. Therefore you’ll need a tie layer of intermediate HSP.

It’s also a good idea to load Gases.sof into the Solvent Optimizer, enter the HSP of your polymer as the Target then click on the Sol. Graph button to see the relative solubilities of the different gases in your polymer. It gives you a good overview of the extent to which different gases are likely to want to permeate through your polymer.

Rational substitution and Read Across

It is very difficult to substitute one molecule with another or do a full comparison of two molecules in order to “Read Across” the health and safety values from one known compound to an unknown one. In HSPiP we bring together many of the strands of this and other chapters to make such comparisons powerful and easy. By entering the SMILES values of two chemicals, the Y-MB method provides a whole range of predicted values. Of course comparison of HSP is vital and the HSP Distance is automatically provided. But as Y-MB is based on functional groups, a “Functional Group Distance” is provided which, naturally, is larger the more differences there in functional groups. Important values such as vapour pressure and RER (Relative Evaporation Rate) are key for those interested in VOCs, along with the MIR and OH values for VOC reactivities. Various partition coefficients are estimated: log Kow (octanol/water), log Koc (soil absorbtion) and BCF (Bio Concentration Factor). Remember that the RER calculated is based on nBuAc=100. If you need the ether-based system then the formula is

RER-based-on-nBuAc=12/Ether-based-system (or 1200 if ether is 1 in its system).

Armed with all this information, HSPiP users are in much better shape to make informed choices affecting health, safety and the environment. See the Predictions chapter for more information on each of the predicted parameters.

 

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