REFERESCF: TO THE ELECTRIC-‘LLBEHAITTIOROF lIEIIBRAISESOF POROUS CH-4R.ICTER. I
I. 1IEJIBR.kSES 11- CLASSICIL PHTSIC.IL C“E3fIBTIIT
Pfefier’i (20) measurement ivith ‘.~einipermenl,lc”membranes of thc oimotica pressure of sugar solutions has in the hands of \-aii’t Hoff become a coriicr stonc of modern theory of dilute solutions. The secoiid instance in the history of physical chemistry in \vhich memhrane,~ haye p l a y d an important r81e \vas in connection Ivith Donnan’s ~ o r 011 k menibrane equilibria (9). Donnan and his collnhorntoi~te,sted this theory, using for their experimental work mostly the same kind of membranes as thox, w e d hy Pfcffer, the copper ferrocyanide membrane, vhich originally had been developed by 11. Traube (29). van’t Hoff’s brilliant coi~relationof the osmotic picssure v-ith the other thermodynamic properties of dilute solutions nccouiits for the fact that only a limitecl number of later investigators hare made direct osmotic-pressure measurement.q v i t h solutes of 1011- molecular weight. The much more convenient methods of freezing- and hoiling-point determinations h a w superseded the methods of osmotic-pressure measurement escept in the case of colloidal solutions. On Pfeffer’s and on Donnaii’s n-ork are based the ividelj- used osmotic methods for the determination of the molecular w i g h t of high-niolecular-n.eight substances and colloids. This method is today a physicochemical tool of great importance. In all these instaiices the menibranees were \\.hat is called conventionally “seniipernieable.”:! Pfeff’er’s memhrnnes ivcre impermealile to the solute sugar ~
1 Presented in abstract before the Division of Colloid Chemistry a t the 10Sth IIceting of the American Cheniical Society, S e n . Tork C i t y , Septcmber 1.1, 1944. 2 The term “semipermeable” is used today in a sense irider t h a n the literal iiieniiing of the Ivord. I t can be taken t o be literally correct only if applied t o instances such as thosc originally treated by Pfcffer : namely,n-here onlj- tn-o species of part‘icles are present, solute and solvent, and the one or the other of these molecular species (usually the solvent) penetrates the membrane freely, whereas the other inolecular species cannot permeate through i t . To avoid possible confusion it is advisable t o qualify the term “seniipermenlilc” by the inipernieable to niolccular species B. Such n addition: perincable t o molecular species -1, qualification n-ould also indicate t h a t seniipcrnieability is not a universal propert>-inherent in 3 inembrane as such, h u t a quality which can be defined properly only with respect t o a particular surrounding system. (See also the text beloiv.) I n s y s t e m containing more than two molecular species, the term “semipcrnieable“ loses 47
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KARL S O L L S E K
and permeable to v-atel; Donnan membranes are impermeable to (at least) one ionic constituent! ordinarily referred to as the colloid ion, and permeable to the solvent’ and tn-o or more kinds of ions in solution. These membranes, if considered to be perfect. can act as reversible physicochemical machines and therefore are highly useful in theoretical considerations ; they are experimental realizations of the ~~seniipermeable” rvalk and pistons Tvhich lvere so irnportant in the early (kwlopment of physical chemistry (19). Ideal reversible niembranes. hoxever, rcpresent only a very limited part of the membranes i m f u l in chemical ivork and occurring in nature. Membranes of a someivhat imperfect character are used Jvidely in the laboratory as dialyzing membranes, as ultrafilter+, etc., and are used for similar purposes in industrial operations ; liken-ise mcmbranes of varying degrees of permeability and semipermeability occur u n i w r d l y in plant and animal organisms, constituting there one of tlie fundamental deT.ices ii-hich regulate the exchange of material and thus the flux of life. From a wider point of T-ieiv “semipermeability” is only a limiting case of the possible properties of membranes, and the cquilihrium state n-hich can be observed u-ith membranes becomes n limiting case of the dynamics of membrane systems. It is therefore small \yonder that literally thous:a.nds of papers deal n-ith specific problems of the dynamics of membrane systems. Historically, interest has been focussed largely on the restrictive (barrier) action of membranes under various conditions, that is, on various aspects of simple membrane permeability and the underlying regularities. 11. XATURE -\SD D E F I S I l I O S S O F MEAIBR i S C S ; MEMBRAXES AS PHIBICOCHCI\IIC.iL MLI.\CHIXC6
Only rarely have investigators of membranes made an attempt t o define the term “membrane”. In most indances it is taken for granted that the reader possesses a sufficiently accurate concept of the meaning of this term, though the everyday usage of this word is anything but concisely circumscribed, as can readily be seen from the scientifically pointless definitions given in dictionaries. It seems to be impossible to find a universally acceptable definition for the term “membrane”. X definition of this term n-hich is useful for the mechanical engineer would be rather useless for the physiologist, and the lattei’s definition can be only of limited value to the physical chemist. X formal definition of the term “membrane” vhich would IIC uqcfiil from the physicochemical point of vienhas not yet come to the attention of the ‘author. its literal meaning. It might be advisable to substitute for it another term, such as “sclectopermeable.” In any case, in systems containing three or more species of molecules it is imperative t o s t a t e specifically Tvhich molecular species are able to penetrate themembrane and which molecular species are preventcd from doing so. A membrane ideal for the dialysis or the molecular-wight determination of a particular protein in solution, for instance, can be defined as a “selectopernieable” membrane nhich is impermeable to the protein under consideration and permeable to the solvent as n-ell as to all other molecular species present in the system.
PHYSICAL CHElIISTIZY OF ME3IBR.ISES.
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49
One characteristic featiue of membranes, if considered from the physicochemical viewpoint, or as n-e niay say of “physicocheniicnl membranes”, is the fact that they can be defined properly only in correlation t o their surroundings. For example, a sheet of platinum separating oxygen arid nitrogen does not slio~v any charncteristic membrane features; it is merely an inert separating Ivall. Hoiwver, if the same sheet c;f platicum sepamtes hydrogen from nitrogen, it assumes a typical memiirane function. I t completely prohibits diffusion of nitrogen across itself, whereas it is freely permeaide to hydrogen (particularly if heated) (21). The same material, according to its surroundings, niay or may not assume membrane functions (compare also footnote 2). meaningful definition of physicochemical mernhranes therefore must nccessarily s t r e ~ sthe functional aspects of membranes. Though menibranes are usually thin flat sheets, the ”main extension in two dimensions” ivhich is frequently emphasized is not a necessary and essential characteristic of membranes. It hay nothing to do \\-it11the quality of the processes which occur across membranes ; it only enhances the quantitative rfficiency by increasing the area Tvhich is available for the processes under consider a t ion. ’ One may attempt to define a physicochemical membrane in the follon-ing manner : bbL-l physicochemical membrane is a structure or phase, separating tn-o other phases or Compartments, 71-hich olxtructe or prevents completely gross mass movement between the latter, but permits passage \r.ith various degrees of restriction of one or several species of particles from the one to the other or between the tn-o adjacent phases or compartments, the passage of material across the membrane in some cases being due exclusively to niolecular movement (diffusion) and in other cases to other forces, such as mechanical pressure, electric potential differences, etc. ” I n vien- of the different but very common biological usage of the term “membrane”, it is worth n-hile t o keep in mind that the above definition includes tacitly the following addition : “, physicochemical i membrane n-ill not by expenditure of energy of its on-n bring about any transport of substances betn-een the tn-o phases or compartments separated by it.” (Compare Ilrogh (IA).) The foregoing definition of the term “membrane!’, however, does not take into account explicitly some of the most important aspects of the functional characteristics and the behavior of membranes u-hich give them their main scientific importance and interest. Membranes, while regulating the movement of particlee and the flon- of energy, i.e., n-hile being passively involved in 1-arious energetic processes occurring across their thickness, cause a great variety of effects such a s movement of solute, the closely related dewlopment of (not necessarily .static) hydrostatic pressures, and the partial or complete separation of solutes from the solvent. With electrolytes, in addition, numerous electrical effects may he obserwd, such as static or dynamic ( 2 3 ) membrane potentials, anomalous oaniosis, movement of third ions against concentration gradients, electroosmosis, etc. In all these cases the membranes act as physicochemical machines, transforming hy various mechanisms the free energy of the adjacent phases (or energy applied through them)
50
IiARL SOLLSER
into other forms of energy (mechanical, concentration, electrical, etc.), sometimes in a reversible but ordinal ily in an irreverqible manner. Physicochemical membranes are in thi\ respect stiictly anulogoiis to mechanical machines: they regulate energetic processe+, essentially without thereby being changed, exhausted, or consumed. ,Iccordingly we may attempt non- t o define meni’uraneh in the follon ing manner: ‘..-I membrane as u phase o r s t r u c t w e anterposed betzceen two phases or compartments which obstructs or complctely pretieiits gross mass movement between the latter, but permits passage, uith carioiis degrees of restriction, of one or seceral species of particlesjrom the one to tht other or Dctween the two adjacent phases or compartments, and whzch thereby actiny as a pli ysicochemical macliiize transforms with various degrecs of e&ciency according to i t s oicii nature and the nature and composition of the two adjacent phases or compartments the f r e e encrgy of the adjacent phases or compartments, or energy applaedj’row the outside lo the latter, inlo other forms of energy.” As long as \\-e consider only itleal “semipermeable” membranes and only reversible processes across them, a&x i s done by the classical physical chemists, the special mechanisms by n-hich the membranes act is of no importance n-hatsoexrer from the point of TTien of thermodynamic t h e ~ r y . ~ soon, homver, as n-e consider the dynamics eithcr of ideal semipermeable or of (non-ideal) membiancs of various degrees oi restricted perrneahility, the questions of nicchanisnis and membrane structure beconie of paramount importance. One can hope to underqtand the dynamics of a membrane system only if the mechanism of the restricting action of the membrane under the given situation is kn0n.n. The machine function of membranc~can be revealed fully only by model studiep. On account of their gieat importance one vi11 ha\-e to pay particular attention to the simple and complex clectiical machine actions of membranes. The construction of propel model.; constitutcq, so t o speak, a problem oi membrane engineering n-hich can he solved only by the use of membranes of carefully predetermined properties. 111. 31E3IBR.lSE MECHISISL\IS ; COLLODIOS SIEMRR.1SES AS E X l M P L E S O F
3 I E h I B R h S E S O F POROC-S CHAR-1CTER
Con\-rntionallg one recognizes t v o main classes of membranes,--homogeneous phase (“oil”) membranes and membranes of porous character. Honiogeneous phase memhranes exert their typical membrane functions by means of selective difkential solubility; particles, molecules, or ions which are solnble in an oil phase can penetrate and pass across. Membranes of porous character act as sieves. This is quite obT-ious n-ith The attitude of many of the older classical physical eiieniists towards the problem of osmotic mechanisms can bc characterized by van’t IIofi’s statenlent (Z. physik. Chem. 9, 485 (1892) : “.$gain we have tlic bnsically incaningless question: n-hat exerts the osniotic pressure? Indeed, as emphasized before, I care in the long run only about its iiiagiiitude. As i t has been shoivn to be equal to gas pressure one is inclincd to think of a similar mechanism of its origin as n-it11 gases. W h o , lloivever, is led astray by this should rather cut o u t a n y thought of a mechanism.” Compare, horwver, the different attitude of S e r n s t (reference IO, page 138).
PHlsIC'.iL CHEMISTRY O F JIEJIBRISES.
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51
porous diaphragms and other microscopically porous structures, and has hardly ever been doubted for siic11 membranes as parchment or collodion membranes of great ]rater content. The molecular siew character of the copper ferrocyanide membrane, already postulated by 11.Traube (29), was verified by Collander ( 7 ) J 71-ho also demonstrated the molecular sieve action of the *,dried"collodion membane (8),at least with solutions of non-electrolytes and weak electrolytes. The ion permeability of the latter membranes, according to Michaelis (17) must liken-ise lie explained by 5ici.e action. Object ions, hon-eT , ha-,-e lieen raised against the concept' of the ''molecdar sieve", the controversy centering around the .,dried" collodion membrane. Some investigators prefer the view that the dried collodion membrane behaves more like a continuous phase, exerting its membrane function by means of a solubility mechanism (see. e.g., reference 3 ) ; some authors (30) attempt to reconcile the tn-o antagonistic viem. Some new experiments and considerations TT-hich prove that the electrolyte permeability of dried collodion membranes must lie explained on the Iia of a microheterogeneous structure of these membranes v-ill be described in a s Several years ago the author and his collaborators started n-ork on mendmines irith the idea in mind of liuilcling physicochemical model systems inrolving membranes i\.hich n-ould he helpful to the biologist in clarifying the structure and functions, in particular the electrical behavior, of microscopic and macroscopic memliranes as they occur in living systems. From the outset it \vas necessary to make a decision as to Jvhich class of meml,rane., those of b'oil" or those of porous character, should be made the starting point of this TT-ork. 11-e decided to start with membranes of the latter type. Ncmbrnnes of porous character are at present much more suitable and convenient ior experimental work, s n d al:'o can be prepared n-ith a great variety of characteristics. In addition, so fiects of great interest to the biologist, such as (electrical) anomalous os , electroosmosis, and (ultra) filtration cannot be obtained v?th membrancs of the homogeneous-phase type.3 Collodion has for a long time been recognized to be a particularly desirable material for the preparation of membranes of porous character on account of its ready availability and the case n-ith n-hich sturdy membranes of various degrees of porosity can be prepared. Collodion membranes, furthermore, can give rise to very pronounced electrical effect,?:such as membrane potentials and anomalous osmosis, and therefore seemed to be particularly desirable. They have the ad1 The choice of membranes of porous character as the object of our work does not iiiiply a n y tlecision as to v,-hether membranes of homogeneous phase o r of porous character are of greater importance i n living systenis. The questicn is probably riot one which can he decided with a simple either-ur. The nvailablc evidericc indicates t h a t in single cells as well 2s in macroscopic structures both types of membranes frequently are containedin some sort of complex mosaic. With indiridual cells, iri most instances, t h e exchange of nonelectrolytes is governed primarily l>y the solubility in the protoplasmic cell membrane, whereas electrolytes seem frequently t o enter through some porous structure or its functional equivalent. The typical function of macroscopic biological niembranes, such as the intestinal wall, the capillary blood vessels, and the filtering structures of the kidneys seems t o be due primarily t o a microheterogeneous porous structure.
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KARL SOLLSER
ditional advantage that thcii ui.e vi11 allon linking 1111tlivi TI ork clobcly to many of the best investigations on membrane, can undoubtedly be applied n ith certain modifications t o other membrane.; than those investigated so far VI. THE O R I G I S O F T H L ELECTROCHEJIIC.-LL CHAR.LCTER O F COLLODIOS
Collodion membrane, of different porosity, prepared from various commercially available collodion solutions, all behaved alike and in normal fashion ton-ardq non-electrolyte .elutions, but large differences, characteristic of the different preparation>. appeared Trhen the membranes were tested for their electiical behavior eithei Y ith the anomalous osmosis or the concentration potential method ( 2 5 , 2 G ) . Considerable differences in the electrochemical behavior of various collodion prepaiations had already been found by Michaelis and Perlzn eig (18) and by Wilbrandt (30). The most characteristic electrical effects were obtained n ith certain brands of imported (Kahlbaum) preparations. It hac. been assunied geneially, though not unirersally, that the electrokinetic charge of collodion is duc to ion adsorption (11, 16). This, hon-ever, hardly seemed compatible TI itli the fact that membranes prepared from different collodion pieparations, though they behave evactly alike if tested n ith non-electrolytes, qholy profound differences in contact TI ith electrolyte solutions (25, 26). Thece difference., on the l m i s of the ion adsorption hypothesis, can be evplained only by the as.uniption that indifferent, non-specific ions are adsorbed by the x-niious collodion preparations t o very different degrees. Such an assumption, hon ever. i- not i n harmony n-ith the general experience on adsorption phenomena .5 The clarification of this controversial point could be achieved by comparison of seveial collodion preparations of different degrees of electrochemical “activity”. K e ncre fortunate in .ecming a t the time of the outbreak of the n a r a sufficient supply of the x arious electrochemically active German preparations, 11hich had heen used pieferentially 11y previ0u.j investigators, t o carry out such a Michaelis (I, ?\lithaelis Iiolloid-Z. 62, 2 (1933)) indicated t h a t the purest collodion preparation seemed t o shun the most pronounced electrical properties, b u t left a n y decision on this question t o the e\perts i n iiitrocellulose chemistry Wilbrandt (30) tried t o explain the differciices bet:veen differeiit brands of collodion preparations b y the assumption of differences 111 the nuinher and distribution of polar groups (O-KOz groups) His v i e m n e r e clitically considered a n d caiiiiot he accepted (26).
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&ARL SOLLh-ER
comparison. It would mean too much of a deviation into the field of nitrocellulose chemistry to give all the details. Empirically we found with the various preparations a very pronounced parallelisni between ash content and activity, sulfate content and activity, fluidity in solution and activity, and opacityin solution and activity. The electrochemically active brands shoned high ash and high sulfate content and their solutions w r e of low viscosity and somewhat turbid. The electrochemical activity of collodion preparations therefore seemed t o go parallel n i t h the impurities present and the degree of degradation of the nitrocellulose molecules in solution ( 2 5 , 26). On the basis of further work it could be concluded that the electrochemical activity of collodion membranes depends entirely upon the presence of impurities of an acidic (anionic) nature contained in the collodion used for their preparation. The active acidic impurities are largely due to partial oxidation which occurs in the manufacturing process, and partially due t o acidic groups which are present in the native cellulose fiber (26). It is easy t o see that this view of the origin of the electrochemical properties of collodion is compatible with the observation that various collodion preparations may behave strictly similarly towards solutions of non-electrolytes but very differently when tested with solutions of electrolytes. The impurities present in the collodion are high-molecular-weight organic compounds of anionic, acidic character, such as "nitrocellulosic acids", i.e., nitrocellulose molecules carrying one or several carboxyl groups. These impurities must be assumed to act in the following manner: They constitute an esential part of the membrane skeleton; some of them are located n i t h their acidic dissociable groups in the pore walls of the membrane. Thus fixed, dissociated or potentially dissociable units arise on the pore walls of the membrane. They are built permanently into the collodion structure, the corresponding counter-ions being actually or potentially dissociated off into the adjacent solution which fills the pore. The frequency of the occurrence of the dissociable groups on the pore walls,-in other words, the charge density on the structural elements of the membrane Jyhich are accesqible to the solution,-determines the degree of electrochemical activity of a membrane. Upon contact of the membrane with an electrolyte solution the counter-ions of the fixed anionic groups exchange with the cation. of the former. This is obviously the mechanism which determines the actual electrochemical activity of a membrane in contact with various electrolyte qolution,. If the counter-ions in solution are the cations of a strong inorganic base, complete or nearly complete dissociation of the surface compounds can be assumed. From this TI^ can conclude that the base-exchange capacity of the pore suifaccs of a memhrniie determines its electrochemical actiT ity.6 It will he necewiry t o discuss this point in greater detail later. GThis concept, of course, does not apply n i t h o u t restrictioii t u all situations I'or example, if a n y one of the ions present i n solution is stionglv atlsorbahle, it must ,-trongly influence the ionic structure of the interfacial layer. I n uueli cases n e undoubtedly approach the situation formerly assumed also for strong electrolyres I t is conceivable t h a t with a iiienibrane iiinterial n hich is completely or nearly completely
PH‘;.SIC.iL
CHEJIISTRT O F JIEJIBR.LSEB.
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57
The above concept of the electrochemical nature of collodion membranes links -them closely to the *bproteinizcd”membranes of Loeb and others. In both cases the electrochemical character of the membrane is determined predominaptly by the presence of high-molecular-\reight dissociable compounds located in the microsurfaces of the membrane structure, not hy non-specific ion adsorption. K i t h the collodion membranes the electrochemically active compounds happen to be contained in the collodion itself; v i t h the proteinized membranes the elcctrochemically active molecules cover the structural elements of the membrane in the adsorbed state-the end result from the electrochcmical point of vieIv is hardly distinguishable. VII. T H E ACTIV.iTIOX
O F BULK COLLODIOS .\SD COLLODIOK
3IEMGR.ISES; JlEGIPERJISELECTIVE COLLODIOS MEJIBRASES
In the case of the electronegative membranes the immediate experimental problem \vas t o increase by some method the surface concentration of fixed dissociable acidic groups present on the pore walls of collodion membranes, since the various collodion preparations which are commercially available a t present in this country are too pure to yield membranes of pronounced electrochemical activity.; Collodion can be activated by oxidation, for oxidation increases the number of dissociable anionic groups (carboxyl groups) in it ( 2 5 , 2 7 ) . The oxidation method of activating collodion may be applied to fibrous collodion in bulk as well as to membranes. Oxidizing agents irhich n-ere found t o be satisfactory are sodium and calcium hypochlorites and sodium hypobromite in various concentrations and a t various pH T-alues. The most effective oxidizing agents, however, are solutions of sodium or potassium hydroxide. The alkalies cause a complicated decomposition of nitrocellulose Trith the formation of nitrites and probably other nitrous compounds. The nitrous compounds act upon the collodion, causing thorough oxidation. Detailed oxidation procedures have been worked out for bulk collodion and for highly porous and for dried membranes, the optimum conditions being different in the three cases. void of any dissociable groups! even in solutions of strong electrolytes, preferential ion adsorption comes into play t o a decisive extent. However, no case of this nature has so far been described for collodion membranes. Acetylcellulose membranes were found by Meyer and Sievers (If.: ;\rch. ;inat. I'hysiol., p. 87 (1867); Gcsnrnmeltc k~bhaildlurlgc~l, l'tcyer and Muller, Berlin (1899). (80) W I L B R A S ~IT.: T , ,I. C h i . Physiol. 18, 933 (1035).
THE S ~ ~ S ' l ' l ~LICErJl(' :~I ~ ~ ( ~ I ~ - ~ ~ ~ I l ~ ~ I I ~ ~ , ~ ~ ~ I H . S,VAS KLOOSTEIt
IVIKSTOX A . DOUGLAS
1)cpicrlment o j Chcmisir.!/, Rcnssclnei I'olytcchnic I n s l i f u t c , 'f'~,o!/~A'L~cY'oI,/;
Iicceiccd Soucniber 6, IO&
In coiitrnst to the more thuri three thoiisand minimum azeotropes :vhicli h ~ v e l w n recorded in the literittiire, there are only about t:vo hundred and fifty mnsimimi iizeotropes. The hcst linoux of this t,ype are systems having water :1.s one component and r ~ i i inorganic acid (hydrofluoric ncid, hydrochloric acid, hydi,ol,romic acid, hydriodic acid, nitric acid) as the second constituent. Les,s fumi1i:tr :we maximum azeotropes foi-ined f i m i acetic i~cidand organic bases (pyridine, picoline, t i k t hylaminc), first dc ibccl hy Gmtlner (2) in 1S91, working in 1-ictor Jleyer's luhratorg. On the assumption that definite conipoundv \\.ere fornied, Gardnei. ascribed t o one of his azeotropes the formula 4CH3COOI-I. S(C12HB')B h i t admitted that) in t>hevapyr stale, :tt the boiling point of thymol, This **Compound"was completely tlissociat'ed into acetic acid and triethylamine nioleculr,s. Since n o other inl'orination besides the boiling point (162°C.) of' the b'compouiidOwas svailable, it was considered worbhwhile to study the boiling point-vapor composition diagram for the acetic acid-trietliylaminc systein at atmospheric pressure. In the coui'se of this investigation it was discovered that the tn-o components are not, miseihle in all proport'ions. In this respect the y s t e m rescmhles t o sonic cistent. the \\ater-hydrochloi.ic acid and the water1iydrol)rornic acid systems, i\,hei-c there is little miscibility on the acid side in spite of the fact that' considerable 1ie:tt evolution and cion t i x t i m occur when one of these acids is added t o ivater. I~XPP;l
