Despite the rapid growing trend in integration of biological experiments on miniaturized substrates, several technical roadblocks persistently preclude successful translation or adaption of the promising microfluidic toolsets for solving biological challenges. Reliable system-level packaging, multilayer alignment, as well as world-to-chip interfaces are among a few long-standing problems in the to-be-solved list.17, 18, 19 For instance, a majority of microfluidic devices fabrication, deriving from soft lithography-based techniques,20 exploit an irreversible packaging scheme, i.e., oxygen plasma-activated polymer surfaces forming covalent links upon intimate contacts. The plasma-based bonding offers reasonably good repeatability and adaptability; however, it is incompatible with either the standard alignment strategy or biological processing requirements. Simon and his co-workers introduced a reversible packaging technique employing web-like vacuum channels to surround and seal targeted microfluidic channels without any thermal, chemical, or optical treatment to study leukocyte adhesion under shear flow.21 More importantly, the technique can be applied to any single layer of soft lithography-molded devices and is capable of integrating a biological surface with microfluidic environments on demand. Magnetic forces were also used to establish reversible seal of microfluidic devices, which can be highly adaptive to a broad range of biochemically functionalized substrates.22, 23 Recently, our group proposed an automatic packaging process using capillary bridge to self-align and self-engage two polymer surfaces with chemical activations, offering a facile solution to simultaneously align and seal multilayer microfluidic structures fabricated by photolithography or soft lithography.24 Although the latest efforts have been attempted to deliver innovative solutions to address different aspects of microfluidic packaging for biological use, none of the above mentioned offers a systematic approach to overcome the challenges in bio-friendly sealing, multilayer construction and alignment, reversible packaging, and world-to-chip connectivity as a whole.
intermolecular and surface forces solution manual zip
Wet and dry foams are prevalent in many industries, ranging from the food processing and commercial cosmetic sectors to industries such as chemical and oil-refining. Uncontrolled foaming results in product losses, equipment downtime or damage and cleanup costs. To speed up defoaming or enable anti-foaming, liquid oil or hydrophobic particles are usually added. However, such additives may need to be later separated and removed for environmental reasons and product quality. Here, we show that passive defoaming or active anti-foaming is possible simply by the interaction of foam with chemically or morphologically modified surfaces, of which the superamphiphobic variant exhibits superior performance. They significantly improve retraction of highly stable wet foams and prevention of growing dry foams, as quantified for beer and aqueous soap solution as model systems. Microscopic imaging reveals that amphiphobic nano-protrusions directly destabilize contacting foam bubbles, which can favorably vent through air gaps warranted by a Cassie wetting state. This mode of interfacial destabilization offers untapped potential for developing efficient, low-power and sustainable foam and froth management.
Volume 11, Number 2, May 1989, pp.2-10Solvents & Sensibilityby Chris Stavroudis and Sharon BlankPart I: No Teas-ingAt no time in modern conservation have developments in practical,bench-top techniques been more exciting than now. The recentre-evaluation of what it is we are doing when we "clean" a work ofart with solvents is revolutionary. The work pioneered by RichardWolbers at the Winterthur Museum/University of Delaware ArtConservation Training Program has engendered a new interest andawareness in the basic processes of working with solvents.Here we will examine the concept of solvent strength and presenta simplified, but accurate, conceptual model to help understand theinteractions of solvents with materials of interest to conservators.We will also discuss how this information can be used as a tool tohelp solve some of the vexing problems facing conservators. A fewrecipes follow at the end to illustrate the application of theory topractice, but please do not skip ahead to the end--rest assured, thebutler did it.We all have certain notions about solvent strength. Acetone is a"strong" but rapidly evaporating solvent. Water is "weak" and notreally a solvent at all. Petroleum benzine, VM&P naphtha, etc.are "weak" or "very mild" solvents. All of these statements miss thepoint.A solvent is strong or weak relative to the material beingdissolved (and relative to the material we do not want to dissolve).This concept is crucial to the safe exploitation of solvents inconservation. It pertains to formulating solvent carriers foradhesives and varnishes, to the safe removal of old varnish layersfrom the surface of paintings, to the removal of aged masking tapeadhesive residues from anywhere they shouldn't be.The interaction of a solvent with something we want to dissolveor, alternately, something we do not want to dissolve, is governedby a number of molecular interactions. Attempts to characterizethese interactions include Hansen's Solubility parameters, the Teasdiagram, and others, too numerous to count. We modestly propose thatthese schema should henceforth be banned. See figure 2 in Part II--no Teas-ing.<graphic status=omitted> (For those of you, gentlereaders, who cannot live without those technical details, please see"Part II, Teas-Busters", immediatelyfollowing.)All we ever needed to know about solubility, we learned inkindergarten: "like dissolves like". Solubility parameters, in alltheir guises, are merely attempts to quantify this simplenotion.REVOLUTIONOne of the tenets of revolution is that the old order must bedestroyed. Old, insufficiently accurate ideas about solvents andsolutes must be dispatched. In fact, the very nomenclature that weuse to discuss solvents must be re-examined; the terms aremeaningless. Welcome to the revolution.With the exception of ionic interactions, salts and acid-basereactions, the concept that is not only common-sense but rigorouslyand scientifically valid is: like dissolves like.POLARITYPolarity, as we will be using the term, is a way to compare the"like-ness" of materials. The term is used as an empiricalestimate, a mental construct, which is probably how you thinkabout solvents anyway. Polarity is an aspect of all materials andthe concept allows us to fit a material's behavior into the rangefrom non-polar to highly polar.Polarity can be thought of as a line which extends from polarmaterials like water all the way to non-polar aliphatichydrocarbons. Every material can be placed somewhere along thatline. Some interesting effects are observed when the world isbroken down and placed along the polarity line. Most of thematerials which occur naturally are polar. Proteins and mineralsare all polar in nature. Animal and plant derived oils, fats, andwaxes are intermediate in polarity. Natural rubber and petroleumproducts tend to be even less polar. When rubber and petroleumare refined, the non-polar fractions are isolated.Consider a polymer molecule in solution: it is there becausebeing in solution is easier (requires less energy) than not beingin solution. This is because the molecules are similar; i.e.,like dissolves like--their polarities are similar.We all have been using the imprecise term "polarity" for years.Unfortunately, the term "polarity" can be used in two ways. Inthe solubility parameters and Teas diagrams, dipole interactionsare often called polarity--this is confusing and not accurate.Polarity indeed does include dipole interactions; but as polarityincreases, so does hydrogen bonding. For more along these lines,see Part II>.The concept of polarity can be used to avoid harm to objects too.Cellulosic materials and many other materials of biological origin,such as wood, paper, and leather, are very sensitive to water, whichswells and disrupts their structure. In some cases this swelling isinappropriate. To avoid damage to an already degraded object, theconservator will wish to avoid aqueous solutions. As might beexpected, the very polar solvents, such as alcohols, may also provedeleterious. In formulating a conservation treatment, theconservator will deliberately use the least polar system that works.(In other cases, as in the washing of degraded paper objects, theswelling and probable structural changes are used to advantage.)Matching the polarity of materials allows a material, say BocourMagna paints (n-butyl methacrylate), to be put into solution. n-butyl methacrylate has an intermediate polarity; it dissolves wellin toluene and xylene. Water and alcohol are too polar and cannotmix with the more "oily" polymer molecules. A conservator wishing toavoid the use of the more toxic xylene or toluene can inpaint withMagna colors using a mixture of naphtha and acetone. While naphthais not sufficiently polar to dissolve the resin, the addition ofrelatively polar acetone makes the solution energeticallyfavorable.Acryloid B-48N includes an adhesion promoter that assists theless polar acrylic molecules to adhere to a polar metal surface.Consider the problem of formulating an adhesive to mend a stoneartifact. The surface of stone is surprisingly polar. (If you thinkabout it, it makes sense. Stone is a very insoluble ionic salt andalthough it doesn't dissolve, it has a great affinity for water.) Toget the greatest adhesion between the glue and the stone, we want toselect a material that is highly compatible with that polar surface.While the notion of the stone actually being soluble in theadhesive, or vice versa, is clearly ridiculous, the greater theinteraction between the adhesive polymer and the stone substrate thegreater the adhesive strength will be. Does this begin to sound likesurface contact angle; well, it is. Welcome to the revolution.Whether or not we want to make the bond as strong as possible isanother matter; that's conservation. Indeed, we could formulate ouradhesive in such a way that the solvent does not allow the polymerchains to unfurl properly and form a glue with less cohesivestrength, but that is another story.One of the neat things about viewing the world through polar-colored glasses is understanding the way organic materials age. Withexposure to oxygen, organic molecules will oxidize, forming polarfunctional groups. Precept two: as organic materials age, theybecome more polar. What this means to our intrepid conservator isthat more polar solvents are required to remove aged, degradedmaterials.Natural resin varnishes, originally applied in "mild" solventslike turpentine, oxidize and become yellow and "insoluble". Theseare symptoms of aging, increasing polarity. In the bad old days, wewould have to have used "strong" solvents like ethanol or acetone toremove the discolored layer. Using our new knowledge about polarity,we can now reason thus: A combination of a good solvent for freshdammar, like xylene, and a highly polar solvent, water, shoulddissolve aged dammar. Sadly, oil and water don't mix; but readon--emulsifiers exist.In other words, there are two ways to approach dissolving ouraged, more polar varnish: to use progressively more polar solventslike ethyl alcohol or acetone (which are not only more polar but arevery aggressive "strong" solvents); or to blend a customized mixtureof solvents tailored to dissolve just that varnish layer. An exampleof that tailoring is a mixture of xylene and water with a nonionicdetergent.SOAPS, DETERGENTS, SURFACTANTS, EMULSIFIERSAll of the above terms refer to a molecule that can insinuateitself between polar and non-polar molecules. The detergent has apolar end that dissolves in water, and a non-polar end thatdissolves in greases, fats, oils, and other icky things.Like dissolves like and your hands are greasy; what can you do?You know the grease on your hands can be dissolved in tallow, butthat really doesn't help. If you react tallow with a strong alkalilike lye, you saponify the acid sites on the stearic acid portion ofthe tallow. The resulting soap can be used to clean your hands. Thenon-polar tallow-like parts of the soap molecules can interact (formVan der Waals bonds) with the grease on your hands, and thepotassium salt end will dissolve in water thanks to ionicinteractions. The best part is your hands will be clean.Now for the Richard Wolbers revolution. You have a painting witha layer of aged dammar varnish on the surface. The varnish no longerdissolves in the "mild" non-polar solvent in which it was applied;the dammar has oxidized, become more polar (precept two). You wantto remove the varnish layer. An old-fashioned conservator would use"strong" solvents (because "strong" solvents are more polar than"mild") to remove the varnish. You are an enlightened conservator.Richard reasoned that it should be possible to dissolve aged(oxidized) dammar into a mixture of xylene and water stabilized by anon-ionic detergent, say Triton X-100, thus allowing oil and waterto mix, varnish to be removed, and the world to be a kinder, gentlerplace.Let's take "like dissolves like" to its wildest implication, thestaggering notion that we can dissolve aged varnishes into anaqueous solution. Further reflection on the nature of oxidizedresins led Richard to formulate an aqueous cleaning system by makinga soap customized to dissolve aged dammar. Ideally, you would liketo isolate the oxidization products of dammar that are present ascarboxylate groups, react them with a base (triethanolamine), andmake a soap. As these groups are present at only small percentagesin dammar, it is wasteful to make such a soap from dammar resin.Abietic acid, an acid component from rosin is cheap, structurallysimilar to dammar, and the triethanolamine salt is commerciallyavailable as a soap. A 4% solution of triethanolamine abietate inwater will dissolve moderately aged (oxidized) dammar films. Formore aged, highly oxidized natural resin films, a soap based ontriethanolamine deoxycholate will perform miraculously.In formulating resin soaps, there are more variables that must beconsidered. One is strongly advised to add Triton X-100 to improvewetting, and hydroxypropylmethyl cellulose or Carbopol to gel thesolution. More importantly, pH must be controlled. Too alkaline asolution (say above pH 8- 8.5) can saponify or attack underlying oillayers.The above cautions emphasize that these new approaches tocleaning require insight and thought. Merely mixing up all the newformulas and using them in testing as you do acetone will bedisappointing or even destructive. You must begin to think like adetergent formulator.The ability to customize a solvent system to the specific natureof the substrate allows the conservator far greater control over theprocess of conservation. It is like the difference between a scalpeland a machete; both are sharp, and either can do lots of damage. Theamount of control possible with a scalpel, however, makes it theinstrument of choice for surgery. Whether this new approach tosolvency is used to unpack (Richard's term) the layers on thesurface of a painting, or to remove masking tape residue fromwherever it oughtn't be, it is a revolution that all practicingconservators will want to join.When given the choice between an aqueous system and the use oforganic solvents, if both are equal in terms of effectiveness andsafety of the object, then the safety of the conservator can becomethe deciding factor. One of Richard's more important contributionsmay be to dramatically extend the lives of his fellowconservators!While an afterthought to this article, it must be emphasized thatthe contributions Richard Wolbers has made to the process ofcleaning artifacts has been built on the equally importantanalytical technique he has brought to the field, reactivefluorescent staining. A problem cannot be solved without firstframing a question.Sharon Blank, Los Angeles County Museum of ArtChris Stavroudis, Private Paintings ConservatorPart II: Teas-BustersIn part I of this suite, we offhandedlydismissed the Teas solubility diagram specifically and solubilityparameters in general. This is a justification of that dismissal.Scientifically inclined readers may wish to get Kleenex, this willnot be pretty; the rest of you can laugh-it-up.The history of trying to understand solvents and why somedissolve some materials and not others is discussed elsewhere. Wewill also ignore the thermodynamic underpinnings of solubilitytheory (entropy, enthalpy, etc.) and cut to the good part. [TheHedley article, ref. 3, and John Burke's work, ref. 12, present much of this information in a mostpalatable form. Read and be enlightened.]Based on the thermodynamics of the solution process, Hildebrandmade a number of simplifying assumptions and showed that theattractive forces holding one molecule to another in a liquid, theCohesive Energy Density, were related to solubility. It was reasonedthat if either the solvent molecules or the solute moleculespreferred sticking to themselves rather than each other, solutionwould not take place. For solution to occur, therefore, theattractive forces between two solvent molecules, between two solutemolecules, and between a solvent molecule and solute molecule mustbe roughly equal--the Cohesive Energy Density (CED). This force isrelated to the energy it takes to separate the solvent moleculesfrom each other, the heat of vaporization, less the mechanical workdone separating the molecules, all divided by the molar volume.Hildebrand defined the square root of the CED as the solubilityparameter, d.Charles M. Hansen, in 1967, developed a set of three solubilityparameters to more accurately describe a solvent, non-ionicemulsifier, pigment, or polymer (ref. 1). The three parameters werederived in part empirically and in part from thermodynamicproperties. He felt that the Cohesive Energy Density, orHildebrand's solubility parameter squared, represented the summationof three component energies, or squared solubility parameters.Hansen isolated and attached numbers to the three component forcesthat hold molecules together: van der Waals forces, dipole forces,and hydrogen bonding forces.Van der Waals forces, also called dispersion forces, areextremely weak forces present between all molecules. They areexplained by quantum mechanics. As electrons zip about in theirorbits, they can induce a distortion in the electrons in neighboringmolecules. The distortion causes the molecules to beever-so-slightly attracted to each other. While not the mostconvincing explanation, it's quantum mechanics, and that's our bestguess at reality. Van der Waals forces are related to the surfacearea of molecules; they only operate between portions of differentmolecules that are in close contact. Van der Waals or dispersionforces are the attractive forces in aliphatic hydrocarbons.Dipole forces, also called polar forces (an unfortunate use ofthe term as polar should include hydrogen bonding too), are theforces between molecules with permanent dipoles. A permanent dipoleis the consequence of a lack of symmetry in the electron cloudssurrounding the molecule. The lack of symmetry causes one area ofthe molecule to have a partial positive charge; another area, acorresponding negative charge. Interaction between molecules takesplace as the partial negative areas of one molecule are attracted tothe partial positive areas of adjacent molecules. Because dipoleforces are electrostatic, they are stronger than van der Waalsforces. The ketones have strong polar intermolecular forces.Hydrogen bonding forces occur between molecules where a hydrogenatom is bonded to a strongly electronegative atom, O, N, or F. Theoxygen or nitrogen atom attracts the hydrogen electron cloud sostrongly that very powerful dipole forces result within themolecule. Hydrogen bonding explains why water is a liquid ratherthan a gas at room temperature. Although water is a small, lightweight molecule, the hydrogen dipole is attracted to an adjacentoxygen dipole, thus increasing the CED. Hydrogen bonding isimportant in alcohols, in addition to water.Hansen assigned a partial solubility parameter to each of theseforces. The calculation of the dipole partial solubility parameterwas based on measurements of refractive index, dielectric constant,and the dipole moment, all of which can be measured for solvents.The hydrogen bonding partial parameter is based on a "reasonablevalue for the OH...O bond" of 5000 calories per mole, as indicatedby infrared spectroscopy and other measurements. The value iscorrected for the molar volume of the molecule and the number ofhydrogen bonding sites present. Having calculated partial solubilityparameters for dipole and hydrogen bonding, the van der Waals ordispersion component could be calculated as what was left over inthe total, Hildebrand, solubility. Then these figures were adjusted,adding a good deal of intuition.Hansen worked with the three parameters in three dimensions usinga computer. The graphs presented in his book are very difficult toread and are projections of two of the three dimensions. Polymersolubility is represented by a central value, the point of bestsolubility (often the solvent or solvent combination that producesthe lowest intrinsic viscosity value), and a radius that representedthe size of a sphere about that point that included solvents thatwould dissolve the polymer.We have all seen, or at least been shown, the infamous triangularTeas diagram (figure 1). The Teas diagramis a graphical short-cut to represent the three dimensions thatHansen's solubility parameters demand onto two dimensional paper.The diagram graphically represents the combined effects of dipoleforces, hydrogen bonding forces, and dispersion forces as a visualindication of a solvent's relative "strength." The three solubilityparameters are not that accurate, although they are among the bestpredictors we have. Compressing the parameters onto a triangulardiagram makes matters worse. [Barton, ref. 6, iseven more disparaging. He says Teas' values have "the disadvantagethat they are completely empirical, without even the limitedtheoretical justification of Hansen's three-component parameters."]In the Teas diagram, the range of solubilities of a polymer isdistorted into an odd shape, some relations between solvents arelost (xylene and dimethylformamide have very similar dispersionsolubility parameters and vastly different polarity and hydrogenbonding values, but the reduction process obliterates thisinformation).In the preparation of the graphics for this article, we noticedthat the formula given by everyone for converting from Hansen'ssolubility parameters to Teas coordinates did not work for a numberof solvents, notably the aromatic and aliphatic hydrocarbons. Thiswas unsettling. The formula is very simple and to get the numberswrong once is embarrassing; repeatedly, down-right demoralizing.After a bit of research, we found the following confession byTeas' own hand: "The coordinates for some seven solvents were alsoarbitrarily adjusted to bring family grouping in more reasonablearrangement." No wonder the paper (ref. 2) onlyreceived $150, fifth prize, in the 1967 Roon Awards competition.Since the Teas diagram is but a predictive tool (often it worksmuch better in hindsight) fraught with problems, we propose to banthe diagram, see figure 2. The diagonalline in the figure represents, in a practical sense, polarity. Tomake the diagram simpler to use and understand, we have removed theedges and all the little triangles, and tipped it on its side. Nowthe polarity line is horizontal, and solvent families can berepresented schematically, as in figure 3.As organic materials oxidize, they move in the direction of thepolarity line, towards the left.One of the weaknesses of the Hansen solubility parameters, andtherefore also the Teas diagram, is that in assigning a hydrogenbonding parameter, an important detail in the nature of hydrogenbonding is eliminated. For hydrogen bonding to occur there must beboth a proton (hydrogen) donor site (eg. an O-H group) and anacceptor site (for example, the electron pairs zipping about anoxygen atom, c=O, in a ketone). Some molecules possess a donor siteand no acceptor, chloroform; others, like MEK, have an acceptorwithout a donor. A single number cannot represent bothaffinities.For any reader further interested in what's new in solubility anddissolving power, see Huyskens et al., ref. 5. Inthe discussion of the new system utilizing the concept of dissolvingpower, symbolized by $, one can appreciate the value of Hansen'soriginal system. [The dissolving power, $, has not been shown tohave any influence on the diminishing buying power of the $.]The concepts presented here do not apply to ionic interactions,the solubility of salts, or acid-base reactions. It is alsoimportant to emphasize that solubility concerns itself only withthermodynamics, not how fast something may go into solution. Kineticsolvent effects are based only on the size of the solvent molecule;the smaller the solvent, the faster it can wiggle its way into asolute to begin thermodynamic dissolution. Large solvent moleculeshave more trouble diffusing and are therefore slower.Thanks to Eric Hansen and Michael Jaffe, both of the GettyConservation Institute; Pieter Meyers, Los Angeles County Museum ofArt; Walter Henry, Preservation Department, Green Library, Stanford;Debra Evans, Fine Arts Museums of San Francisco; and to RichardWolbers, University of Delaware/Winterthur Museum Art ConservationProgram.Sharon Blank, Los Angeles County Museum of ArtChris Stavroudis, Private Paintings ConservatorPart III: What the Butler Did or, Some Tips on FormulationsMere formulae are not enough; the revolution requires a re-thinking about how we remove one material from another.Hopefully, the No-Teas-ing diagram will help. <graphicstatus=omitted> Below are some other helpful starting points.As mentioned in Part I, oxidized varnish,too polar to be soluble in xylene alone, can be dissolved in a gel,formulated by Richard Wolbers, based on xylene, water, and TritonX-100. The formulation published previously in the WAACNewsletter in an article by Dare Hartwell (Vol. 8, no. 2,1986, pp 4-6), reprinted from the WCG Newsletter, hassince been revised slightly by Richard. 20 ml of Triton X-100 isdissolved in 50 ml of xylene. When mixed, add 30 ml of water towhich a small amount of triethanolamine (no more than 1%) has beenadded, and shake vigorously until a gel has formed. The gel iseffective on fairly young resin layers that have not oxidized to thepoint where resin soaps can be used, but that are too polar to bedissolved in xylene alone.When combining a non-polar solvent, water, and a detergent thereare four possible outcomes. The ingredients can: 1) not mix,leaving two or three layers in your beaker (a bad solution). Theycan form an emulsion, either 2) an oil in water, or 3) a water inoil emulsion; or 4) a gel can be formed. The high concentrationof Triton in the above formulation stabilizes the suspension ofxylene and water at a mid-point between oil/water and water/oilemulsion.Such solvent gels have very nice working properties. Theevaporation of the solvent is reduced, the polarity of themixture is quite high because of the water present, and the gelis transparent so its activity on the surface can be monitored.Carbopol Gels:Carbopol is an acrylic acid polymer that can be used to gel bothaqueous and solvent/water cleaning systems. To be of use, theacrylic acid groups have to be neutralized, in effect unfurling thepolymer chain and giving a structure to the solution. If theneutralization is achieved with an amine that also has detergentproperties, like triethanolamine (TEA) or Ethomeen C-25, the largenet of Carbopol provides a backbone for the solution stabilizingemulsifier, allowing the formation of very stable solvent gels.The following tricks aid in the formulation of Carbopol basedgels. Elaborate stirring procedures (air driven paddle stirrers,magnetic stir-bars) are avoided with a commensurate increase in timeto let materials equilibrate. The procedures also allow preparationof small batches with considerably less bother.Stock Carbopol Gels:To make the Carbopol Gel precursor, sprinkle 5 g of dry Carbopol934 onto the surface of 65 ml distilled water while stirringbriskly. Allow the stiff gel to stand until uniform in texture andappearance. Depending on the efficiency of the dispersion, thiscould be from 5 minutes to a day or two (four hours is my average).With all the Carbopol gels, only distilled water should be used asthe resin will react and precipitate with the calcium salts in hardwater.To make the triethanolamine based gel, dissolve 10.2 grams (9.1ml) of triethanolamine (or 12 g, 11.5 ml, of 85% triethanolamine)into 10 ml distilled water. While stirring, add the triethanolaminesolution to the precursor. The above gel should have a pH of about8.0-8.5. You must check the pH yourself with pH papers or a meter;trust no one. After standing for a time, the pH can be raised (morealkaline) with small additions of diluted TEA solution. (The pH canbe lowered by adding additional precursor, but equilibrium requiresconsiderable waiting time.) If the solution is too stiff formeasuring pH with test papers, remove a small amount and mix withdistilled water, then test.To make the Ethomeen based gel, add 10 ml of distilled water tothe precursor and stir until smooth. While stirring, add 12.5 g ofEthomeen C-25. The pH may be adjusted by adding additional C- 25 (ortriethanolamine solution).To make a dual neutralization gel, dissolve 8.8 grams TEA in 10ml water. While stirring, add the TEA solution to the precursor andthen 10 grams of Ethomeen C-25. "Dual neutralization sets up abridge between the oil phase and the water phase by forming[Carbopol] salts soluble in both phases." (Quoted from the Carbopolmanual, ref. 11.)Aqueous Gels:Aqueous gels can be made with either of the Carbopol stock gelsor with hydroxypropylmethyl cellulose (HPMC), or any of the othercellulose ethers like methyl cellulose, Ethulose, or Klucel.Richard usually uses the HPMC for gelling his cleaning mixtures.Carbopol gels have a slight detergency effect on their own andhave a different feel on the swab.HPMC gels are neutral in solution and are a must for enzyme gels,gels containing EDTA, or gels with high ionic strengths. HPMCforms a nicely workable gel at a concentration of 1.5 grams per100 ml of water or aqueous solution. The HPMC can be stirred intosolution on a magnetic stirrer or it can be sprinkled on thesurface of the liquid, allowed to swell overnight, and stirred inthe next day. It should also be possible to pre-make stockHPMC/water gels for use as described for the Carbopol gels.The Carbopol gels can be prepared by cutting a small amount ofany of the stock gels (triethanolamine, Ethomeen C-25, or dualneutralization) into an aqueous solution. 3-4 grams of stock willgel 25ml of water or aqueous solution. A combination of stirringand patience yields very good results. I personally prefer thefeel of the Carbopol gel to the HPMC.Solvent Gels:Solvent gels are more complicated. The solubility of the solventsolution must be compatible with the gelled Carbopol. Generally theEthomeen based gel is more compatible with higher proportions ofsolvent to water. If the Carbopol is not soluble in the solventsystem, it will precipitate out of the solution as a sticky, white,stringy residue. To test the compatibility of the Carbopol with asolvent system, submerse a small piece of the stock gel into thesolution. If the surface of the gel becomes cloudy, the Carbopol isnot soluble in the solution. Try modifying the solvent/watermixture. Generally more water, additional Triton X-100, oradditional polar solvents will make the Carbopol soluble in thesolution.When the solution is compatible with the Carbopol gel, cut thestock gel into the solvent solution. Generally letting the mixturestand for a few hours with occasional stirring will form a smoothgel. Check the pH of the resulting gel; it will very probably havebecome acidic, in which case more Ethomeen C-25 or triethanolamineshould be added.Solvent gels based on immiscible solvents, water and xylene forexample, are more difficult to make. Surfactants help, but veryoften one ends up with a stabilized, thickened emulsion. That's ok,but we would prefer a gel.Solvent gels can also be formulated based on modified celluloseethers like Klucel GF and Ethulose.Resin Soaps:The resin soaps are for dissolving aged (oxidized) natural resinfilms into water-based systems. The triethanolamine abietatetends to work better for less oxidized films. Triethanolaminedeoxycholate seems to be better for more degraded, highlyoxidized films.Stock Solutions (20% w/v):5 g Triethanolamine Abietate0.6 g Triton X-10025 ml distilled waterTriethanolamine3.7 g Deoxycholic Acid (Free acid)1.4 g (1.2 ml) Triethanolamine (1.6 g, 1.4 ml, 85%)0.6 g Triton X-10025 ml distilled waterTriethanolamineIn both cases, the first ingredients should be mixed. Theadditional triethanolamine is added, drop-wise, to obtain thedesired pH (8.0-8.5 as a general rule). For the deoxycholate, anadditional 0.6-0.9 grams of triethanolamine may be required toneutralize and dissolve the non-water soluble free acid. Use eithera magnetic stirrer, or allow sufficient time for equilibrium to bereached (overnight for the first measurement of the deoxycholatesoap) before testing pH and adjusting.To make the working solutions, dilute the stock solution to 4%(or less) w/v solution (i.e. 1 ml stock solution to 4 ml distilledwater). The working solutions may be gelled with eitherhydroxypropylmethyl cellulose (1.5 g/ 100 ml) or Carbopol/TEA gel(adjusted to the same pH as the resin soap). More Triton X-100 maybe added to improve wetting of more non-polar surfaces, butremember; the oil layer is non-polar as well. (The working solutionalready has 0.5% Triton in it.)To further improve solubility of less polar natural resin films(say, with a small amount of drying oil added), 1-3% of benzylalcohol may be included in the formulation. Before adding the benzylalcohol, add additional Triton X-100 to bring the total Tritonpercentage up to 4%.For even higher drying oil content natural resin films, thedeoxycholate soap solution (4%), gelled with hydroxypropylmethylcellulose, can be used as a buffer solution for a lipase enzyme. ApH of 8.5 is recommended. Do not use a gel laden with air bubbles asthe air can denature the enzyme. Sprinkle a small amount of enzymeon the surface of the gel in a wide mouthed container. Only prepareenough of the enzyme gel for the day's work. Allow the gel to standfor 15-30 minutes, then gently stir the enzyme into the gel,avoiding air bubbles. Allow to stand for an additional 30 minutes toan hour. The solubility limit of the enzyme is 10-30 mg/ml; if thegel is slightly hazy, too much enzyme has been added.If there is a varnish film below the oil layer that issusceptible to the deoxycholate soap solution, the Lipase can beprepared in a Tris buffer solution, i.e. Trizma pH 8.4, and gelledwith HPMC. The buffer is made according to directions (0.664 g/100ml), 0.01% Triton X-100 (or more) is added, and 1-2%hydroxypropylmethyl cellulose is used to thicken it. The samecautions about air bubbles apply.The Lipase should be kept refrigerated in a desiccator to prolongits usable life. Before opening the vial, warm it in your hand toavoid moisture condensing on the inside surface.Surface Cleaning StrategiesSurface grime is very polar in nature. It has an enormous surfacearea which is exposed to oxygen and therefore tends to be oxidized.Surface dirt also tends to be weakly acidic (oxidation to carboxylicacid groups) which is why it responds so well to ammonia andtriethanolamine solutions. (Important layers beneath the surfacegrime may not respond so favorably to such alkaline attacks;solutions with a pH exceeding 8-8.5, advises Richard, should beavoided.)We all know the virtues of spit cleaning. Spit is warm, whenfresh, and highly polar. It is also pH and ionic strength balancedand contains amylase, the starch-"eating" enzyme. No conservatorcould live without spit, no matter how embarrassing it is in areport.One strategy for removing surface grime, also published in the WAACNewsletter, Vol. 8, no. 2, developed by the author, is toincrease the polarity of naphtha by adding a small amount of water.To stabilize the mixture, the least amount of detergent possible isadded (because any residue may be difficult to remove). An extremelystable emulsion is formed by the mixture of 9 parts naphtha to 1part 5% Triton X-100 in water. The final concentration of Triton is1/2%. If desired, after an emulsion has formed, the emulsion can bediluted in half again with naphtha, yielding a final formulation of95% naphtha, 4.75% water, and 0.25% Triton X-100. As with thexylene/water/Triton gel, activity can be increased by adding a smallamount of triethanolamine to the 5% Triton solution to make theemulsion slightly alkaline (no more than pH 8-8.5).Other highly effective approaches to the removal of surface grimedeveloped by Richard are soap solutions based on Maypon 4C. Maypon4C is used in the formulation of "baby" shampoos, and is a soapbased on an esterified protein molecule. The soap, at 4% dilution indistilled water and gelled with hydroxypropylmethyl cellulose (orCarbopol triethanolamine, pH 8-8.5), works rather well on surfacegrime. Remarkable results are obtained when 5% (or less) benzylalcohol are added to the gel. The benzyl alcohol functions as aco-solvent, in the solubility context, discussed above, and as a lowHLB detergent, increasing the detergency of the Maypon.For highly oxidized surface grime (very old residue fromcigarette smoke, I believe) that was not entirely or evenly removedby "spit" cleaning, I found 4% triethanolamine deoxycholate soap,gelled with Carbopol-TEA, pH 8-8.5, to be remarkably effective.NOTE:Many of the materials discussed pose safety and health hazards tothe conservator! It is the responsibility of each conservator tofamiliarize themselves with any special handling proceduresnecessary with these materials. Consult Material Safety DataSheets, and read any and all cautions.SUPPLIES:Triethanolamine Abietate (S-85): Chem Service; P.O. Box 3108;West Chester, PA 19381-3108; (215) 692-3026.Deoxycholic Acid, Free acid (D-2510); hydroxypropylmethylcellulose, 4000 centipoise at 2% (H-7509); Lipase, type VII, fromCandida Cylindracae (L-1754); Protease, type XXIII, from AspergillusOryzae (P-4032)[recommended by Bob Futernick, FAMSF]; Trizma buffer,at 0.05M pH 8.4 (T-5253). Sigma Chemical Company; P.O. Box 14508;St. Louis, MO 63178; (800) 325- 3010.Carbopol 934, (Carbopol Water Soluble Resins formulating guide,36pp): BF Goodrich Company; Specialty Polymers & ChemicalsDivision; 6100 Oak Tree Blvd; Cleveland, OH 44131; (800) 331- 1144.Tri Ess Sciences, Inc.; 1020 W. Chestnut St.; Burbank, CA 91506;(213) 245-7685.Ethomeen C-25: Armak; Industrial Chemical Division; NewProvidence, NJ 07974; (201) 665- 2500.Maypon 4C: Inolex Chemical Co.; Jackson & Swanson Streets;Philadelphia, PA 19148-3487; (215) 271-0800.Triton X-100, everything else, and much much more: ConservationMaterials, Ltd.; 1165 Marietta Way; P.O. Box 2884; Sparks, NV 89431;(702) 331-0582.Chris Stavroudis, Private Paintings ConservatorDISCLAIMER:These formulae are presented to provide an exchange ofinformation between conservators. The authors, sources, and/or WAACcannot be responsible for any undesirable outcome experienced byanother practitioner. Further, it is not intended for any person whois not a trained conservator to use these formulae on works ofart.Authors' postscript, 1993:Since publication, some of the formulas and techniques(particularly with regard to solvent gels) have been improvedupon.References (for Parts I, II, and III)1. Hansen, Charles M.; The ThreeDimensional Solubility Parameter and Solvent DiffusionCoefficient, Copenhagen: Danish Technical Press, 1967.2. Teas, Jean P; "Graphic Analysis of ResinSolubilities", Journal of Paint Technology, Vol. 40, no. 516(January 1968), pp 19-25.3. Hedley, Gerry; "Solubility Parameters andVarnish Removal: A Survey", The Conservator, No. 4, 1980, pp12-18.4. Wolbers, Richard C.; "Notes for Workshop onNew Methods in the Cleaning of Paintings," Getty ConservationInstitute Training Program, 1988.5. Huyskens, P.L. et al; "Dissolving Power ofSolvents and Solvent Blends for Polymers", Journal of CoatingsTechnology, Vol. 57, no. 724 (May 1985), pp 57-67.6. Barton, Allan F. M.; CRC Handbook ofSolubility Parameters and Other Cohesion Parameters, BocaRaton: CRC Press, 1983.7. Southall, Anna; "New approach to cleaningpainted surfaces", Conservation News, No. 37 (November1988).8. Payne, John; "Report on PaintingConservation Workshop, GCI, New Methods in the Cleaning ofPaintings", ICOM (ICCM) Bulletin, No. 28 (September1988), p 7.9. Wolbers, Richard; "Aspects of theExamination and Cleaning of Two Portraits by Richard and WilliamsJennys", AIC Preprints, 1988, pp 245-260.10. Wolbers, Richard and Landrey, Greg; "TheUse of Direct Reactive Fluorescent Dyes for the Characterization ofBinding Media in Cross Sectional Examinations", AICPreprints, 1987, pp 168-202.11. "Carbopol Water Soluble Resins", The BFGoodrich Company, Specialty Polymers & Chemicals Division.12. Burke, John; "Solubility Parameters:Theory and Application", The Book and Paper GroupAnnual, Vol 3, 1984, pp 13- 58. 2ff7e9595c
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