Cationic complexes of nonylphenoxypoly(ethyleneoxy)ethanol

n,.rl

•

.

~ ~trt,

.,I-" k ol Itj pp $fi~ 16q~ Pergamoll P r e ~

Printed m (ire.it Brllam

CATIONIC COMPLEXES OF NONY LPHENOXYPOLY(ETHYLEN EOXY)ETHANOL EXTRACTION

INTO DI('HI.OROMETHANE

ION-SELECTIVE

EI.ECI"RODE

AND

PROPt'RI"IES

A. M. Y. JABER. (i. J. MOt)DY and J. 1). R. FHt)M\S ('hemistr~ l)epartment. UWIST. Cardiff CFI 3.N[.:. Wale,, (Receited 2t) Norenlher 1976)

r~tbstraet--relraphen~,lborate ITPB) salts of calcium, strontium and barium complexes of nonylphenox~,pob(ethyleneoxy)elhanol (NP) have been prepared from the commercially available Anlarox range of NP,, and screened for their suitability as ion-selective electrode sensors. For ,,electivit3 and general ion-selective electrode response behaviour, the best systems are those involving the barium complexes for barium ion response. TPB salt,, of calcium and barium complexes of dicyclohexyl-18-crown-6 have also been prepared. The complexing tendency, of NPs. dicyclohexyl-18-crown-6 and Antarox with alkali metal and alkaline earth metal cations is described in terms of partitioning the picrates of the metal ion complexes from aqueous chloride solutions between water and dichloromethane. As well as percentage picrate extraction, the "'bulk extraction constants" shov, the atfinit~, of dicyclohexyl-18-crown-6 for alkali metal and alkaline earth cations to ',ar~. in the order K > Rb > Cs > Na "~ l.i and Ba - Sr > Be > Ca > Mg. respectively, with the position of beryllium being anomalous and possibl.',, related to the extraction of beryllium into the organic layer even in the absence of coordinating polyether or NP. For the Antarc,x \ P,. the seqt,ences are Rb > K > Cs > Na > I.i and Be %.Ba > Sr > C'a > Mg. respecti,, el!.

INTRODUCTION Nonylphenoxypoly(ethyleneoxy) ethanols [C,,H,,,C,,H,OICH:CH:O).; ,C2H,OH] (NPs) constitute an important series of non-ionic surfactants in which the number of ethylene oxide units can produce important vari;ttlons in w.etting, detergency, emulsification, soluhiliI~ and foam properties. Their complexes with metal ions whose stoichiometries have been scrutinised for se,,eral yearsll-3] form the basis of analytical procedures of the materials[I-6]. Also, tetraphenylborate (Ft~B~ salts of some of the alkaline earth metal ion complexes have recently attracted attention as sensors in =on-selective electrodes [7-121. I-he main objective of the present study has been the as,,essment of the complexing tendency of the Antarox NPs ~ith alkali and alkaline earth metal ions from ,,oJvenl extraction and the "'bulk extraction constants". K,. obtained from partitioning the picrate salts of the complexes between water and dichloromethane. Some complexes have been isolated and screened as prospective ion-selective electrode sensors while the macrocyclic polyether, dicyclohexyl-18-crown-6 (DCHC-6) has been included for comparison.

EXPERIMENTAl. (hemicals. Chemicals were of reagent grade. The Antarox CO ,,eric,, of NP nonionic surfactants were gifts from GAF (Great Brltamt l t d . rilson Road. Roundthorn. Wythenshawe. Manchester. L;.K. Preparation of ,%P and dicyclohexyl-18-crown-6 cation comph'xe.+ and their TPB salts. Alkaline earth cation-NP complex ,,olution,, were prepared in the manner previously described for the barium complexes[7+ I0] by adding metal chloride solution +10cm'of(Llmoledm ' o r Imoledm 'solution) to an aqueous ,olution <1).154g per 100cm') of the appropriate Antarox NP. l'he resulting oxonium km was precipitated with excess aqueous ,,odium TPB. and the filtered precipitate washed well with water

and vacuum dried at 35°C or over phosphorus pentoxide. The white product precipitates were assessed by elemental anal sses [rable I I Complexes of dicyclohexyl-18-crown-6 were similarly prepared (Table I). Attempts to prepare rPB salts of her~,llium, magne,,mm ;aM alkali metal caticm-NP complexes in the above manner were unsuccessful and in the case of the higher alkali metal ions ~,n(I beryllium led to TPB precipitates of the cation,, themselves rather than cation adducts. Attempts at isolation made b,, backextracting into deionised water of the potassium-NP complex picrale contained in the dichloromethane layer of the ,,ol',ent extraction experiments described belov, also failed because the complex dissociated into potassium picrate in the hack extraction to establish a fresh equilibrium. Thus. stoichiometries ,~f her~,llium. magnesium and alkali metal cation complexes remain uncertain. Evaluation of TPB precipitates of NP cation comph'xc.s a~ ion-selectize elettrode wnsors. The "ion-exchangers'" used in electrode membranes comprised the precipitated complex I~0.tl5 g)in an aromatic nitro-solvent mediator (-. ll.4g) [.iquid membrar.e electrodes v, ere based on an Orion Research Inc. liquid membrane electrode (Dpe 92-21)! assembled m the normal wa,,[12], rhe ouler chamber contained the "ion-exchanger'" and the internal reference chamber a 0.1 mole d m ' solution of metal chloride saturated with silver chloride. PV( matrix membrane electrodes v, ere assembled accordin~ to standard pradice[13.141 with master membranes prepared from t).40 g "'ion-exchanger" added to a ,~olution of O.17 g P\. C m 6 cm' tetrahydro[uran followed by controlled evaporation of the tetrahydrofuran solvent. Response lime and freedom from drifl of these electrodes were considerably impro',ed b~. adding a small quantity, of the appropriate Antarox NP I--5~ of the adduct-TPB mass) to the sensor prior to assembly of the ionselective electrode membrane. Both types of electrodes were evaluated b~, their beha'.iom in the appropriate stirred metal chloride calibrating ~,ohlliom, at 25±1).1~( again',t a (orning [No. 4761(i,91 ceramit, plug-type

reference electrode containing 4 mole dm ' pota,,siunl chloride. The e.m.f.s of ,,uch cells v,ere recorded with a Beckman research pH meter or a Radiometer [PHM 64! digital pH meter to (LI m\" in conjunction ',.,.,itha Serve,scribe Model RE 4541 potenUc,n',e~ric recorder.

1689

1690

A.M.Y. JABER et al.

Table I. Tetraphenylborate of alkaline earth metal ion adducts of nonylphenoxypoly ethyleneoxv)ethano s (Antarox series) and dicyclohexyl- 18-crown-6 (DCHC-6)

Cation M2. Mg Ca Sr Ba Ba Ba Ca

Antarox or DCHC-6

No. of EOU in Antarox or of ether oxygens 30 30 30 30 40 20 6

CO-880

CO-890 CO-850 DCHC-6

6

Ba

Stoichiometry of TPB ppt.

Elemental analyses Found Required M.p.(°C) C(%) H(%) M2*(%) B(%) C(%) H(%) M2'(%) B(%)

t Ca(CO-880)o,TPB2 Sr(CO-880)o.,TPB2 Ba(CO-880)o,TPB2 Ba(CO-89O)o~TPB2 Ba(CO-850)o6TPB: Ca(DCHC-6)~TPB2

185-190:]: 197-200:]: 215-217~ 217-220.~ 210-215¢ 157-160

72.6 70.4 66.7 66.8 67.8 74.6

7.98 8.01 7.26 7.44 7.54 7.75

2.95 6.36 9.30 9.62 9.50 2.67

1.6 1.5 1.4 1.5 1.4 1.7

72.3 69.8 67.3 67.0 67.7 74.3

7.60 7.33 7.07 7.01 7.19 7.93

3.09 6.53 9.86 10.0 9.56 2.80

1.7 1.6 1.55 1.6 1.5 1.5

Ba(DCHC-6)2TPB2

210-212

69.4 7.25

10.0

1.7

69.5 7.42

9.03

1.5

tMilkiness produced, but precipitate could not be separated for analysis. ,,-These m.ps are all irreversible with signs of charring ~5°C below the minimum of the quoted ranges. Calculation of "bulk extraction constants" of cation complexes of Antarox NP materials. The determination of "bulk extraction constants" (K,) is based on the picrate extraction method developed for investigating alkali metal cation complexing by macrocyclic polyethers[15] and mactrotetrolide actin antibiotics[16, 171. The experimental method consists of simply shaking an aqueous solution containing various cationic salts of the coloured picrate anion with an organic solvent (dichloromethane) phase containing the neutral Antarox NP ion-binding material and then measuring the optical absorbance of the organic phase to determine the amount of metal-NP complex picrate extracted. In the cases of the macrocyclic polyethers[15] and the macrotetrolide actin antibiotics[16, 17] the salt extraction equilibrium corresponds to Monovalent Neutral Picrate cation + macrocyclic + ion-binding molecule (in aqueous (in organic (in aqueous phase) phase) phase)

over a wide range of conditions[17] and for which the equilibrium constant K~, can be evaluated with appropriate equations from measured concentrations of extracted picrate[17]. A similar procedure has been adopted in this study, but since the molar proportions are no longer unity, eqn (17 takes the more general form of eqn (2) when based on one mole proportion of metal cation.

addition, allowance is made for other possible equilibria, for example, ion-pair formation between the ISL*, species and the picrate anions (reaction (4)) in solvents of low permittivity. ~ is 9.05 for dichloromethane. IS~* + z picrate-* .~(IS6~. picrate~)*.

Reactions (2) and (4) are considered[17. 18] adequate to represent the relevant equilibria. Thus, other equilibria like the extraction of chloride ion and uncomplexed metal ion picrate into the organic solvent have been neglected[U]. ]'he extraction of uncomp[exed ion-binding molecules into the aqueous phase may be similarly neglected[17] and separate experiments in this work on the pattern of the work of Frensdorff[18] show the disCationic complex with macrocyclic molecule

Picrate + (l)

(in organic phase)

(in organic phase)

tribution ratio of Antarox CO-880 to be ~4 x 10-', indicating that over 99% of the NP is extracted by the dichloromethane. K,,,,, for the ion-pair formation (reaction (4)) is given by K~.~ =

, a~,c $ z a.(G,.)

(5)

alpine •

P ' + 6 Z S * + z picrate ~lS~L, , z picrate-*

(4)

, ,~..

K,(a,)(a~D Ca,)

(2)

(6)

n

where I z• is the cation species of valence z, S the neutral ion-binding molecule (Antarox NP material or dicyclohexyl-18crown-6 in this study), n is the number of ethylene oxide units (or polyether oxygens) per mole of S and the factor 6 is the number of ethylene oxide units (or polyether oxygens) taken to be associated with each unit of charge of the ion, I. Thus, for Antarox CO-880 (with 30 ethylene oxide units (EOU) per mole) complexes, n is 30 while z is I for monovalent alkali metal cations and 2 for the bivalent alkaline earth cations. An asterisk (*) denotes quantities characteristic of the organic solvent phase and quantities not so designated refer to the aqueous phase. The "bulk extraction constant". K,. for reaction (2) is given by

Assuming ideal behaviour of all species in the organic solvent phase and of the neutral species in the aqueous phase, activities can be replaced by concentrations so that K, and K,*,p.,.can be written: C,,(Cp,.) K~ - (a,)(a.,c):(C.)6:,. K,.,,,~

, . ,= . . ~,. C,.(G..) K,(a3(ap,¢ (C,)

K,

(8)

Since C*¢ = zC,*,, eqn (77 reduces to K~

a~. (a,,J .... (a,)(a.J(a*)

(7)

(3)

--

( C e J " , , 6,1~ z(a,)(ap,~) (£ ,)

(9)

•

which rearranges to where the a terms are the activities of species denoted by subscript representing the main participants in reaction (2). In

( C * J ÷'= zK,(a,)(ap,J(C*.)~'".

(101

Cutionic complexes of nonylphenoxypolyteth~,leneoxybethanol (~

Bill ( "* ..........

t

+* ,,,. + 2C*,.o,,

I:('*,+"""}

.......

+ oK,*.,,,, K,lu, lla,.,. )'(('*,1"+'".

112)

By expressing a, and a~,, in terms of the aqueous concentrations. C and (',,,,. and (knov+n)acti~il.'. coefficients. 7, and 7,,.,. though the definition: a,a,.,. = C,C,., ',/,'y,,,,. eqn 112) takes the form

( * ........ = {K~C C,,,, I C*, )"'" ( T,?,.,~l}' ' - K,*.,,. K,(',('~,,.(C*I"e'{'/,y,.,!

1131

for montr~.alent cation', and a',,,uming for example+ the complex milh Antarox ('()-88t) is 15,,. nitrate, n being 30. For bi,,alent ,:atit',n,

(:* ....... {2h',C,i(o, ):( "2.I'>" ( -y,)l yo,. I~}'~ ,2K,*.,,,, K.(',(C,.,j(C*)'"""(% t(To,, )

114)

,lnd the complex of al lea.,t calcium..,trontium and barium v,,ilh -\nlarox ('O-g0tl is IS,,, picralc:, n again being 31). Equation', tl~)imd (141 show that the concentration of the chromophore pier,re extracted i,, a function of the ,,ingle variable ICI('~,,,)(C~)""'(T, II'y~,,V] and experimental data need Io he considered as such. For members of the Anlarox range of NPs. 61n takes on values appropriate to the number, n. of EOUs and while it is (:,.2 for -~mtarox CO-8gO with 30 EOL,. it is 0.15 for the CO-89t1 which ha, ..R} EOL", and 0.3 for {'{)-g50 v.ith just 20 EOUs. For complexes of many macroc,,clic polyethers, such as dicyclohexyl-lg-cro~,n-6, n is generall,, 6 when cqn 1121 for con> plexalion with monovalent cations reduces to the simple form derived by Eisenman et ,LilT] C,, C,.,, and C*. related t,, experimental conditions, l h e extraction experiments were carried out; h x equilibrating a known volume. V*, of dichloromethane containing initialb z, knov.n concentration of Antarox or c',clic polyether ion-binding specie,,, ([:.,,.,,. *,~,ilh a known volume. V. of aqueou.,, solution containing knov, n initial concentration of metal chloride. In each case V* = ~," : loom'. I'he equilibrium cuncentratam C*,,..,+,. of the chromophore pier,re wa., measured in the dichloromethane phw, e from the •,pectrophotometric absorbance al 378nm 1376nm for dic~,.,:lohexyl-lg-crov, n-61 of the picrate associated with the complexed metal ion using a I.Ocm path length cell. From this ,.alue. the indi~ idual concentration,,. ('.. C,,,. and C*, ',',ere calculated for lhe equilibrium condition,, l h u s . (

C~ .......

(.,: ,., - - - -

(15)

where _- is I and 2 for monovalent and bivalent comple\ing cation,, respective[,.. For the picrate anion, the amounl extracted ( V*('*r,,.,.,,,} must equal thai initiall3, present in the aqueous phase IVCm~:,,,,.,I+ minus that remaining in the aqtleons phase IV(',,,.) .o that t *( "*p,. ,,,,

l 'Cv...o,,,;, V('+,.,

Hence

('°'

('~ ......

}:*. / , , V

-

6 -

* (', ...... ,.

< 7!

IIII

which with eqn I I(I) and a rearranged eqn (8) gives ( * ........ - { : K , la, Ila,,,,

(..

1691

- o,,..,,+,-

t

16)

It can also be reasoned1171 that ('*. can he expressed in terms of the initial conditions (where the ion-binding molecules are essentially in the organic solvent phase) and the measured value of Cg,,,.,.,:

All the information required to analyse the salt extraction equilibria is therefore contained in eqns (131 and (14) coupled with the quantities in eqns (15) to (17L There i,,. hov..ever, a simplificalion[18] whereby the activip, coefficient of the picrale ion. yo,,. was taken as unip,. For measuring C*,~,,,,, from the spectrophotometric ab,,orhance of Ihe dichloromethane solution,, at 37gnm 1376nm for dic~clohexyl-18-crov, n-6) the molar extinction coefficient taken lot the pier,re was that computed from the metal picrale ",y',tem', associated ~ith each complexing species taken ,,eparatel?,. Ihe absorb,nee as~,ociated with the various picrate cuncentrat,m, had a correlation coefficient of belier than (I.99 and the molar extinction coefficient,, thus calculated and used v, erea,, folhp.,,,, For the ,dkali metals: dic~clohcxyl-18-crov, n-6 ¢17.6111)L Antarox ('()-gR0 11g,3(11)). Antarox ('O-g501 118.6(11)1 and Antart~x ('()-b.9(~ (17.31!I)). For the alkaline earth metal,,: dic.vclohex}l-lg-cro~n-t, (13.000). Antarox Co-8g0 116.200). Antarox (+()-85l~ 1l~.911(t~ .kn. larox CO-890 ( 15.901)1. Check,, were made on the reproducibilits of the ~ariou', ah sorhance measurement', m,tde after equilibrating the aqtleous and the organic phase, l'hus. ,,erie,, of 5 replicate determination,. I'ol potassium chloride and barium chloride used al ,.arious concentration levels (SxlO '. 5x 10 " and 5,":1(1 "mc, ledm ') ,',ill! picrale 19.28 × 10 ' moledn', '~ and '~nlarox ('()-gg0 Ih, 10 ' mule tim 'l had coefficient,, of ',ariation of It:,,,, than 45"; for absorb,nee,, at 37gnm of the dichloromethane la~,ct arid le,., than I.~,c..; for absorb,nee, at 357 nm of the ztqtleOtlS la~.el

RESULTS .~,ND DISCUSSION Strong stoichiometric c o m p l e x e s of alkali and alkaline earth metal cations with neutral m o l e c u l e s h a v e been o b s e r v e d only in recent years, the main interest being with b M o g i c a l materials and m a c r o c y c l i c polyethers. The essential c o m p l e x i n g principle is that the~,e net, tral organic m o l e c u l e s carry a s e q u e n c e of Iocalised charge,, (usually lone pair electrons) of sufficient energy, to form ion-dipole ligands with appropriate cations, l h c elmf o r m a t i o n of the m o l e c u l e is s u c h that it f o r m s a ,,olr a t i o n type shell a r o u n d the cation, effectively, rephtcing the ion h y d r a t i o n shell. T h e charged cationic c o m p l e x thus f o r m e d is electricalb balanced b.,, anion',. T h e mo~,t effective c o m p l e x i n g a g e n t s a m o n g the macrocyclic p o l y e t h e r s contain five to ten o x y g e n alOlllS each s e p a r a t e d f r o m the next b.,, tv, o or more carbon a t o m s . T h e s e c o m p o u n d s normally form I:1 salt-polyether c o m p l e x e s in which the cation encircled bx the o x y g e n a t o m s of the pol.vether ring is held there b,. electrostatic attraction b e t ' s e e n the negatively charged o x y g e n s of the C - O dipoles and the cation. Stob c h i o m e l r i e s other than the ,,tropic I:1 ralio are nov, k n o w n It, e x i s t l l g ] . Similar principles Io the a b o v e hold for certain ,intibiotic materials, s u c h as, the c y c l o d e p s i p e p l i d e , xz, Ii n o m y c i n , which f o r m s the basis of a highly '.,elechx.c p o t a s s i u m ion electrode [20. 21]. A rigid cvclic urr a n g e m e n t of the d o n o r a t o m s e q u e n c e i,, not a prerequisite of the c o m p l c x i n g neutral molecule and ruorc o p e n s t r u c t u r e s have recently attracted interest, for example, the ,,electi,,e calcium i o n - s e n s o r N.N' di[l ll ethoxycarbonyl)undec.,,I] - N,N'.4.5 - t c t r a m e t h } l - 3 . 6 d i o x a o c t a n c amide1221. An e v e n more flexible d o n o r a t o m s e q u e n c e is that of p o l y e t h y l e n e glycols ,,~,ith long chains of e t h y l e n e oxide units (E()IY! v, hich in Ihe NP~ of this stud,, ha',e an a r o m a t i c ring alt:.tched to 1he polymeric s e c t m n of the molecule.

1692

A. M. Y. JABER et al.

TPB salts of NP and dicyclohexyl-18-crown-6 cation complexes Although cationic adducts of polyethylene glycols have been known for several years [ l, 2], the neutral carrier qualities have only recently been exploited in ion-selective electrodes 17-12]. Their special properties in this application have been attributed [7] to the 12 EOU of the N P complex with Ba 2÷ assuming a tight helical conformation with a ring radius of about 1.3 A in which the Ba 2" ion is held by a cage of 12 oxygen atoms (6 in each loop) through ion-dipole interaction. Molecular models indicate this is possible, although absolute confirmation by X-ray diffraction has yet to be made. The stoichiometry of 12 EOU per mole of bivalent cations (M 2') is confirmed in the several TPB precipitates of NPalkaline earth cation complexes prepared in this study (Table I). A similar stoichiometry of 12 (polyether)oxygens per mole of bivalent cations is also indicated for calcium and barium ions in the TPB salts of the cation complexes

with dicyclohexyl-18-crown-6 (Table 1). The mole polyether:mole salt ratio of 2:1 is the same as that for the related complexes of benzo-15-crown-5 with barium [191 and calcium [23]. Concerning ion-selective electrode sensing properties, the length of the ethylene oxide chain (20--40EOUs) appears to be of little consequence (Table 2). Antarox CO-880 is therefore representative as a satisfactory selective barium ion-sensing material in the form of the TPB salt of its adduct with barium dissolved in p nitroethylbenzene for liquid membrane electrodes and in o-nitrophenylphenylether for PVC matrix membrane electrodes. Functional electrodes can be made for calcium and strontium from the appropriate metal ion-adduct, but these are generally less satisfactorily than the barium counterparts. Their response to these ions depends on the absence of barium from test solutions and in these circumstances the strontium electrode has been recommended[9] for monitoring strontium in nuclear waste management programmes.

Table 2. Cation-selective electrode characteristics of TPB precipitates of cation adducts of nonylphenoxypoly (ethyleneoxy)ethanols

Solvent+ mediator

Type of electrode (liquid membrane -LM or PVC)

Linear calibration range to M~' (mole dm -3)

Electrode slope to M~' (mV)

o-ndpe o-ndpe p-neb p-neb o-ndpe

LM PVC LM PVC PVC

1 0 - ' - - 10-~ 10 ~-10-~ I0 ~- ~ 10-3 10 ~-10 3 10 ~-104

- 23 -23 - 23 ~31 27-29

---I

p-neb

LM

10 ~-4x 10-5

~27

Sr

p-neb

PVC

10-J-5 x 10 ~

~ 28

several weeks I

na

o-ndpe p-neb p-neb

PVC LM PVC

10-J-9 x 10-6 10-'-5 x 10 ~ 10-~-10-~

28 to 29 -27 - 29

301 30 / I

o-ndpe p-neb

PVC LM

10 a_ ~ 10..~ 10 t-Sx l0 5

27-28 27-29

- 30 15

p-neb p-neb

PVC LM

10-1-10-~ 10 ~-5 x 10-~

- 24 - 30

2 15

o-ndpe

PVC

10-'-10 -~

- 29

~ 30

Cation (M2")

Code of Antarox adducting material

Ca

Ca Ca Ca Sr Sr~t

CO-880

Ba Ba

Ba / Ba

CO-890

Ba Ba

Lifetime (d)

-

Remarks Erratic behaviour Slightly erratic behaviour Erratic behaviour Poor compatibility of ionexchanger with PVC

-

Poor compatibility of ionexchanger with PVC Good electrodes; discussed in detail in Ref.[10] Good electrode but of very short life; discussed in Ref. 110] Good electrode Similar to electrode with CO-880 Short electrode life Similar to electrode with CO-880 Good electrode

C0-850 Ba

•tp-neb = p-nitroethylbenzene; o-ndpe = o-nitrophenylphenylether. :~Data from Ref.[91. Table 3. Extraction of picric acid in the presence of alkali and alkaline earth metal chlorides into dichloromethane containing Antarox CO-880. Initial conditions: Picric acid 9.28 x 10 -~ mole dm 3. Antarox CO-880 6x 10-'moledm ~. Equal volumes (10 cm 3) of water and dichloromethane. Temperature = 17"C Metal ion concentration (mole dm -~) 5 X 10 ' 2.5 X 10-' 5 x 10-2 5× 10 .3 5 x 10-"

Picric acid extracted from aqueous phase (%) Li'

Na'

K"

Rb"

Cs +

Be 2.

Mg 2~

Ca 2.

SI"2'

Ba2"

--7 6 5

40 32 18 8 5

85 80 69 35 14

--69 38 15

81 79 65 34 13

--60 25 7

--4 4 5

12 9 7 6 6

20 17 10 8 5

48 43 33 23 10

1~'.93

Cationic complexes of nonylphenoxypoly(ethyleneoxy)ethanol

Sohent extraction into dichloromethane It has been shown that addition of an appropriate complexing species, such as a polyether frequently promotes the dissolution of salts in solvents in which the,, arc otherwise substantially insoluble1241. However. extraction is ct~cienl only if the anion is large and highly polarisable. :ts for picrate which also facilitates analysis b~. strong absorptions at 357 nm in water (regardless of the presence of otherwise of metal ions). 337nm in dichloromethane and -~37gnm for metal complex pi;rates in dichloromethane. Accordingly. picrate exmtction ha', been used for expressing relative complexing power[ 15.251. Applied to Antarox CO-880 ('ruble 3). ~t shoas potassium, rubidium and caesium as forming ,trong complexes among the alkali metal ions. with barium leading among the alkaline earth cations. The beha',iour of beryllium, however, is unique. Thus. although for example, picrates are not extracted from aqueous 9.2g x 10 " moledm ' picric acid plus 5× Ill : mole dm ' metal chloride generally into dichIoromethane (absorbance of <0.02 by the dichIoromethanc layer at 339 and at 378nm). beryllium chloride appear', to promote picrale extraction. Such extraction is deduced from the absorbance of 0.16 by the dichloromethane layer at 339nm. Addition of any of ~\ntarox C()-g50, CO-8g0 and CO-890 to the dichIoromcthanc laver produces an absorption maximum of -IL7 ul 378nm (t).10 absorbance at this wavelength in the absence of Antarox or other complexing material) ~ith no evidence of peak or shoulder at 339nm. Dicyclohexyl-18-crown-6 addition gives an even larger absorbance of 1.28 at 376 nm. In these circumstances, the data in Table 3 for beryllium are not strictly comparable with those of the other metal ions. fhe exlent of extraction into an organic phase though normalb u measure of complexing affinity of neutral ion-binding molecules for cations, depends not only on complexing equilibria, but also on the solubilities and partition coefficients of the various uncomplexed and complexed species. Allowance for these features has been made in the salt extraction properties that macrotetrolide antibiotics would he expected to confer on organic solvents and good agreement has been found belween theoretical expectation and experimental observation[ 17. 181. f h e nearest to these systems in the pre,,cnt stud,, i~, dicyclohcxyl-18-crown-6 and the values of K, ITable 41 found for sodium (1.4) and potassium (43)

are of similar order of magnitude to those deduced (sodium = 1.5 and potassium = 78) for the same dichIoromethane solvent extraction system studied by Frensdorff[18] for mixed isomers of the polyether. The observed sequence of K, values for the alkali metal ions with dicyclohexyl-18-crown-6 (K > Rb > Cs 3- Na > Li) (Table 4) is the same as that obtained [171 for picrate extraction of monactin. For the alkaline earth metals, the sequence is Ba > Sr > Be > Ca > Mg (Table 41. Beryllium coming between strontium anti calcium is not strictlx comparable to the other members of the series because of the unique extraction o f its picrate into the dkhIoromethane layer referred to above, although, of course. the shift of the absorption maximum from 339 nm of lhe picrate in the organic layer in the absence of dicyclohexyl-lg-crown-6 to 378nm in the presence t~f the polyether might be considered to bring beryllium into line with other members of the group from the standpoint of complexation. The different unit,; for K, of the alkali metals from those of the alkaline earth metal', mean that the lu, o series cannot be compared. Table 5 summarises the experimental and K, data lor the various Antarox NP materials studied. B,, their nature each set of K, values has different units, hut regardless of this it can be seen that the K, sequence is Rb > K > Cs > Na > l.i in each case f o r the alkali metal ions and Be ~> Ba > Sr - Ca > Mg for the alkaline earth metal cations. The maximum K, at potassium or rubidium in the various cases (fables 4 and 5) is compatible with competition between hydration and complexing, since al least some of the water of hydration must be stripped off the cation to accommodate it within the oxygen cage provided by the dicyclohex3.1-18-crown-6 and the Antarox NPs. With the smaller cations, their high electric charge density greatly attracts both ~ater and the complexing material, thal is, the tendenc~ for hydration becomes greater here. C s , on lhe other hand. ha,, Iou. charge density and there is the matter of larger size that can reduce the complexing tcndcnc~ v, hcn compared with K and R b . With the alkaline earth cations, there it of course a greater charge densib.. Neglecting the :,pccial case of beryllium already discussed above, the balance between hydration, complexing and size favours B u ' !lable,, 4 and 5). The data of lubles 4 and 5 refer to a single ligund.

lane 4. I"XITaction of picrate into dichloromethane b,, dicyclohcxyl-18-crm,~,.n-6( n - 61 Initial conditions: C,,,..,,,,,,.,= 9.28 x 10 "moledm ': C*,,,,~,,= 6x 10 ~moledm ': C ........,~-5x 10 moledm ': ,/: :1).8 (alkali metals~. 0.354 (alkaline earth metalsi. Temperature= 23:C Picrate cxtr:tctcd

K,

I%)

Imole ~dm'l

0.u69 14 43

Metal chloride

ATT~

A,<-

( ,.,.,,,,.: (10 "motedm ')

l.iCI NaCI K(I

0.205 t).714 I.-197

I.II3 11.716 11.11_.2

1.16 4.05 X.50

13 44 92

Rb('!

1.428

(I.15g

X.IO

X7

(,,CI

1.183

0.361

ft.71

'r2

13e('l ,-

1.280

0.298

v.09

eh

(lO" mole ' tim") 750

Mg('L

I).155 0.187

1.138

1.19

I!

1t.21

1.125 0.328 0.235

1.44 ~. 16 7.74

Ifl -" Xt

0.~9 831} 202(I

('a('l, Sr{.'l. t]a{l.,

0.932 I.(X)7

'K, for I',erqlium calculated from A,,. data.

26

8.u

1694

A . M . Y . JABER et al.

T

:6,-4

•

, ~

•

.

.

~ ~

<_~

_

ia

• ,_~ ,.~ ,...;

•

"

~-~

_- "2_ ~,

_

~ m

"o o

~5 o , - - ~

~,:.=

,,..!

~.

o_ ~ 0

!

2 .~'"

:2

,

~

m

~

b

m

~x

~

H

°d

< N N ~ N

.~_ . ~

•§ _~ ~x

~ : ~ , ~ II

~

1695

Cationic complexes of nonylphenoxypoly(ethyleneox.v)ethanol metal ion and picrate concentration. While it is desirable for spectrophotometric reasons to keep the picrate concentration at about the level employed, studies have been made on varying both the ligand and metal ion concentration. Thus. varying the Antarox CO-880 ligand concentration between 1 × 10 ' and 6x 10 ~moledm ' did not alter the sequence of K, values noted above. Indeed, the onl}. notable effect was that while the picrate extracted into the dichloromethane was very little affecled for those cases of low K,, that is, lithium, sodium, magnesium, calcium and strontium, there was enhanced pircrale extraclion with increasing Antarox CO-880 concentration in those cases with a large tendency to complex, that is. potassium, rubidium, caesium and barium with a parallel increase in the K, values. ()n the matter of varying the metal chloride concentration, there is a decrease in picrate extraction with decreasing metal chloride concentration. This is as expected with the trend again being somewhat more pronounced for those case,, with the greater tendency towards complex formation. K, values increase with decrea~,ing metal chloride concentration as previously observed for extraction of alkali metal ions into dichIoromethane by monactinl 17]. h is interesting to look at the theoretical expectations for the alkali and alkaline earth metal cation complexes with Anlarox ('()-g8(I, in the context of eqns (I 3) and (14) respecti,.el.~. For these, the limiting theoretical cxpectations[17] for the ion-pair reaction (4)are for it either Io be negligible whence the log form of eqn (13) is I IogC':L,,, = (-:-~- l l l ° g z ( a ' )(a~,<) " ( C ,*)

-.4 i.~

•~:

,,C,v' gy:>~ ,.o" L :

'

45

7>

g

_1 %'3

.~g;..~" -t#

Fig. I. Plot of log L'*,,,,,,, vs log I(',(',>,,(C*,)":T,] for alkali metal cations and Anlarox Co-880. Slopes of solid linen: l.i 0.053:

Na = 0.32: K = 0.45: Rb - 0.45: ('s = 0.46. Deviations indicated by the dashed lines have been ignored in calculating slope,

....

I

( z - l l I°gK'

(181 •4

,

<~r f~r it to go It) completion whence

log ('.,*,..... = log :(a,)(a,.., r(C*)":"+ log K'p,, K,

(19)

]'lop, of log ('i~ ....... ,. s the first log term on the right-hand ',ide of the equation are shown in Figs. 1 and 2 for the alkali metal and alkaline earth metal cations (excepting magnesium) respectively. Except for lithium and sodium, the slopes for the alkali metals are 0.45 (Fig. I ) a n d among the alkaline earth metals, barit, m has a slope of 0.36 (Fig. 2). Although these deviate somewhat from the respective expected ~ah,es of 05 and 0.33. they indicate eqn (lg) as approximating a description of the data with the extraction into dichloromethane arising through eqn (4) with negligible ion-pairing. I.ithfl,m and calcium give slopes considerably less than the expected value while the unexpected large slope for beryllium may be associated ~ith the experimental factors noted above. The longchain nature of the NPs ma~ be partly responsible for the de',iations and would, of course, be additional to the factors already noted liT] for extractions into dichloromethane by monactin a', responsible for deviations from theoretical behaviour. ('ON(71.LSION lhe

above demonstrates that neutral molecules with

long polyIethyleneoxy) chains, exemplified b.~ various Antarox nonylphenoxypoly(exthyleneoxy) ethanols facilitate the extraction ol salts of monovalent and

"~

/i 2"'d

j _..I

J

i

Log [2C, (Co,¢)P (C~).o 4 ×, }

Fig. 2. Plot of og (',*,, <,:,, ',s log [2C((',,,, )"(C*)""),,] for alkaline earth metal cations and Antarox CO-880. Slopes of solid lines: Be = 0.87: ('a -: 0.094: Sr-- 0.14; Ba = 0.36. Deviations indicated b', the dashed !ines have been ignored calculating slopes.

1696

A.M.Y. JABER et al.

bivalent cations into dichloromethane. This function is related to their use when complexes with cations can be isolated as sensors in ion-selective electrodes.

Acknowledgements--The University of Wales is thanked for a studentship (to AMYJ). Prof. Henry Freiser, University of Arizona (USA) and Prof. T. P. Hadjiioannou, University of Athens (Greecel are thanked for helpful discussions made possible by travel grants from the Scientific Affairs Division of the North Atlantic Treaty Organisation.

REFERENCES 1. R. Neu, Arzneim. Forseh. 9, 585 (1959). 2. T. Uno and K. Miyajima, Chem. Pharm. Bull. (Tokyo) Ii, 75 (1%3). 3. R. J. Levins and R. M. Ikeda, Analyt. Chem. 37, 671 (1%5). 4. W. K. Kimura and T. Harada. Fette Sei[en AnstrMittel. 61. 930 (1959). 5. C. Calzolari. L. Favretto, G. Pertoldi Marietta and L. Favretto Gabrielli, Annali Chim. 64, 463 (1974). 6. L. Favretto and F. Tunis, Analyst 101, 198 (1976J. 7. R. J. Levins, Analyt. Chem. 43, 1045 (1971). 8. R. J. Levins, Analyt. Chem. 44, 1544 (1972). 9. W. W. Bauman, Analyt. Chem. 47, 959 (1975).

10. A. M. Y. Jaber, G. J. Moody and J. D. R. Thomas. Analyst 101, 179 (1976). 11. A. M. Y. Jaber, G. J. Moody and J. D. R. Thomas, Proc. Analyt. Di~'. Chem. Soc. 13, 328 (1976). 12. R. J. Levins, Ger. Often. 2, 264, 721 (1973). 13. G. J. Moody and J. D. R. Thomas. Selectit:e hm Sensitit'e Electrodes. Merrow, Warlord (1971). 14. A. Craggs. G. J. Moody and J. D. R. Thomas. J. Chem. Educ. 51,541 (1974). 15. C. J. Pedersen, Fed. Proc., Fed. Amer. Soc. Exp. Biol. 27, 1305 (1%8). 16. G. Eisenman, S. M. Ciani and G. Szabo, Fed. Proc., Fed. Amer. Soc. Exp. Biol. 27, 1289 (1%8). 17. G. Eisenman, S. Ciani and G. Szabo, J. Membrane Biol. 1, 294 (1%9). 18. H. K. Frensdorff, J. Am. Chem. Soc. 93, 4684 (1971~. 19. C. J. Pedersen, J. Am. Chem. Soc. 92, 386 (1970). 20. L. Pioda, B. Stankova and W. Simon, Anal. Lett. 2, 665 (1969). 21. M. S. Frant and J. W. Ross, Science 167, 987 (19701. 22. D. Ammann, M. Guggi, E. Pretsch, and W. Simon. Anal. Lett. 8, 709 (1975). 23. D. G. Parsons and J. N. Wingfield, lnorg. Chim. Acta, 18, 263 (1976). 24. C. J. Pedersen, J. Am. Chem. Soc. 89, 7017 (1%7). 25. C. J. Pedersen, J. Am. Chem. Soc. 89, 2495 (1%7~: 92. 391 (1970).