RICHARD PAYNE
3598
The Electrical Double Layer in Amide Solvents by Richard Payne Air Force Cambridge Research Laboratories, L. G . Hanscom Field, Bedford, Massachusetts (Received February 81, 1969)
Double-layer measurements are reported for a mercury electrode in the solvents N-ethylformamide (NEF) N-t-butylformamide (NBF), N-methylacetamide (NMA), N-ethylacetamide (NEA), N-n-butylacetamide (NBA), dimethylacetamide (DMA), and N-methylpropionamide (NMP), The behavior of N E F resembles that of the previously studied formamide and N-methylformamide (NMF) systems; a broad capacity hump appears on the cathodic branch of the capacity-potential curve. This hump, however, is absent in NBB. For KPF, solutions in the N-monoalkylacetamides and N M P two capacity humps occur on opposite sides of the point of zero charge (pzc). The cathodic hump resembles the formamide hump and is a property of the solvent. Study of the anodic hump is complicated by specific adsorption of PFa- ions. This hump is probably also a solvent property but may be due to specific adsorption. The capacity generally decreases with increasing molecular weight of the solvent but is not directly dependent on the macroscopic dielectric constant. The interfacial tension a t the electrocapillary maximum in the high dielectric constant solvents NMA, KEA, and N E F is close to 380 ergs/cm2 and is not significantly temperature dependent.
Introduction The physical properties and electrochemistry of the aliphatic amide solvents have been described in a recent comprehensive review.* A large number of these solvents exist, many of them liquid at ambient temperatures, but the properties of only a few have been investigated. Double-layer studies reported for formamide and its N-methyl derivatives can be summarized as follows.2 The form of the capacity potential curve is related to the structural properties of the solvent; capacity humps occur in the high dielectric constant solvents, formamide, and N-methylformamide but not in dimethylformamide where the dielectric constant is relatively low. Specific adsorption of anions is inversely proportional to the ability of the solvent to solvate anions through N-hydrogen bonding. It is the object of the present work t o test the general validity of these broad conclusions by extending the measurements to a number of related solvents. I n most investigations of the influence of the solvent on the properties of the double layer undue emphasis has been placed on the significance of the macroscopic dielectric constant of the solvent. However, as Frumkin's experimental work3 of 1923 showed, the doublelayer capacity is not directly related to the macroscopic dielectric constant nor is such a relationship to be expected on theoretical grounds. However, there is no doubt that the unusual structural properties of high dielectric constant solvents are also reflected in the double-layer properties of these solvents. Probably more important factors which have received little attention are the specific chemical interactions of the solvent molecules with both the ions and the electrode, and the effect of the size of the solvent molecule on the mean thickness of the inner region of the double layer. The relative importance of the effective dielectric The Journal of Phgsical Chemistry
constant and thickness of the inner region is at the heart of the current controversy4J concerning the choice of electrical variable in adsorption studies. I n this work the significance of the thickness factor is emphasized for solvents ranging in molecular weight from 45 (formamide) to 115 (N-n-butylacetamide). Measurements are described fo? seven new solvent systems in addition to new measurements in formamide, N-methylformamide, and dimethylformamide.
Experimental Section The capacity of the electrical double layer at a growing mercury drop was measured by the ac bridge technique described previously.6 Measurements were normally conducted a t a single frequency of 1 kHz, but in solutions of low conductance a frequency of 400 Hz was used. Electrocapillary measurements were made in a simple version of the Lippman capillary electrometer also described previously6 but modified by the use of a water thermostat bath controlled to rt0.01". I n all the measurements described the reference electrode used was a normal calomel electrode isolated from the nonaqueous solution by a long electrolyte path and a solution-sealed stopcock. Solvents supplied by Eastman Organic Chemicals were normally purified by repeated distillation through a 40-cm column packed with glass helices under reduced pressure. No chemical or other pretreatment (1) D. S. Reid and C. A. Vincent, J . Electroanal. Chem., 18, 427 (1968). (2) R. Payne, Advan. Electrochem. Electrochem.Eng., in press. (3) A. N. Frumkin, 2.Phys. Chem., 103,43 (1923). (4) A. N. Frumkin, B. B. Damaskin, and A. A. Survila, J. Electroanal. Chem., 16,493 (1968). (5) E. Dutkiewioz, J. D. Garnish, and R. Parsons, ibid., 16, 506 (1968). (6) R. Payne, J . Amer. Chem. Soc., 89,489 (1967).
SYMPOSIUM ON INTERFACIAL PHENOMENA
3599
was used. Solvents prepared in this way had the following typical specific conductances : N-methylacetamide, 1.5 X ohm-' cm-l; N-ethylacetamide, "E 2.6 x 10-6 ohm-' cm-l; N-ethylformamide, 5 X e + a ohm-' cm-l; N-methylpropionamide, 1 X lo-' ohm-I, ohm-' cm-l; N,N-dimethylacetamide, 5.5 X em-'; N-t-butyformamide, 1.4 X low4ohm-' cm-'; and N-n-butylacetamide, 1.3 X 0hm-l These values are similar t o previously reported values for NMA, DMA, and NMP. There appear to be no published data for NEA, NBA, NEF, and NBF. The high conductance of N E F and NBF is typical of formamide and the N-alkyl formamides and is probably due 10 1 I t o ionization of a volatile trace impurity (e.g., formic b.5 0 -0.5 -1.0 -1.5 acid or ammonia) which is not completely removed by Potentiol, v (nce) distillation undler these conditions. This impurity had Figure 1. Double-layer capacity for 0.1 M KPF6 in no noticeable effect on the measurements. However, formamide showing pseudocapacity peak a t -0.15 V due to extreme care is necessary in assessing the effect of such discharge of traces of ammonia. impurities on capacity measurements, as shown by the behavior of forrnamide. The residual high conductance of formamide distilled from CaO or other alkali is due 44 to the presence of traces of ammonia, which are eviKPF, in N-methylocetamide ,30° dently almost impossible t o remove by distillation. This was attempted in the present work by distillation of formamide through a 50 plate column under carefully controlled conditions. The lowest conductance obtained was 1.0 X 10-5 ohm-l em-l under the following conditions : column head pressure 0.5mm, temperature 64", reflux ratio 1, distillation rate 6-7 ml/hr. This product gave the capacity curve shown in Figure 1. The small peak at E = -0.15 V is due to a pseudocapacitance associated with anodic discharge of traces of ammonia as was shown by adding more ammonia. I n previously reported measurements for K F solutions in f ~ r m a m i d eanodic ~ , ~ polarization beyond 0.2 V was Potentiol , v ( m e ) evidently not possible because of discharge of ammonia Figure 2. Double-layer capacity for KPF6 solutions of introduced in substantial concentrations by treatment indicated concentration in N-methylacetamide a t 30". of the solvent with alkali in the purification procedure. Vertical lines indicate potential of zero charge. This clearly illustrates the danger of chemical pretreatment of solvents which may increase rather than decrease the impiirity level as stressed recently by B ~ t l e r . ~ KPF6 (30 and SO"), 0.01 M KPFs (30"), 0.1 M KPF6 in mixtures of NAIA with water (25") and 0.1 M KPF6 Solutions were made up gravimetrically using ACS is NMA containing cyclohexane, n-heptane, and isoGrade salts, in general, dried a t 150" for 24 hr without octane (30"). Electrocapillary curves were obtained further purification. Practical Grade KPF6 supplied for 0.5 and 1 M KPF6 (30"), 0.1 M KPF6 (30 and 40") by Matheson Coleman and Bell was recrystallized from andO.l M CsI (30"). water and vacuum dried before use. Tetra-n-propylTwo capacity humps on opposite sides of the pzc ammonium bromide and tetraethylammonium peroccur in NMA solutions of KPF6, LiBF4, LiC104, and chlorate (East man) and LiC104 (Alfa Inorganics) were NEt4C104, This has not been observed previously in a used as received. Cyclohexane and isooctane (Fisher two-component solution of an inorganic electrolyte. Spectranalized) and n-heptane (Eastman) were disCurves for three concentrations of KPF6 are shown in tilled a t atmospheric pressure before use. Figure 2. The cathodic hump is typical of the hump Results (7) B. B. Damaskin, R. V. Ivanova, and A. A. Survila, Elektrokhim1 . N-Methylacetamide. Capacity measurements are iya, 1,767 (1965). reported for 0.1 M solutions of KPF6, LiC104, LiBF4, (8) E. Dutkiewicz and R. Parsons, J. Electroanal. Chem., 11, 196 KNOs, CsI, NHIC1, NEt4C104, and NPr4Br a t 30". (1966). Further capacity measurements were obtained for 1 M (9) J. N. Butler, Advan. Electrochem. Electrochem. Eng., in press. I
I\
Volume 78,Number 11 November 1860
3600
RICHARD PAYNE
found in other amide solvents whereas the anodic hump is more pronounced (as in aqueous solutions). The capacity at both humps decreases with increasing temperature as usual.2 The effect is larger at the anodic hump. In 1 M KPFa the temperature coefficient is -0.19 kF/cm2 deg for the anodic hump compared with -0.04 pF/cm2 deg for the cathodic hump. The latter agrees precisely with the formamide value.2 The frequency dependence of the capacity for the 1 M KPFa solution was investigated carefully in the range 0.5-4 kHz. The capacity showed a small but significant frequency dependence at both humps. At the cathodic hump the capacity was 0.4% higher (compared with the 1kHz value) at 0.5 kHz and 2,1% lower at 4 kHz. At the anodic hump the capacity was 1.8% higher at 0.5 kHz and 2.4% lower at 4 kHz. The cathodic hump therefore is essentially independent of frequency below 1 kHz whereas the anodic hump remains somewhat frequency dependent even a t 0.5 kHz. The dispersion of -2% between 1 and 4 kHz is somewhat larger than the effect normally found in aqueous and other solutions of comparable conductance, but can probably be attributed, as in these other solutions, to imperfect geometry of the electrodes and electrolyte penetration effects. Strong specific adsorption of anions from NMA is suggested by the capacity curves shown in Figure 3. The anodic hump appears in LiBF4, LiC104, and NEt4C104 but is absent in other solutions. The effect of
Table I : Interfacial Tension (re) and Potential (E,) a t the Electrocapillary Maximum for Various Solutions Solvent
Solute
NMA NMA NMA NMA NMA NMA NMA NMA NMA NLIA NMA X’MA NMA NEA
0.01 M KPFe 0.1 M KPFe 0 . 1 MKPFe 1 M KPFe 0.1 144 LiClO4 0.1 M LiC104 0 . 1 M LiC104 0.1 M LiBFd 0 . 1 M CSI 0 . 1 M KNOs 0.1 M NHiCl 0 . 1 AT NEtrClOi 0.1 M NPr& 0.1 M KPFe
NEA
0 . 1 M CSI
NBA DMA DMA DMA NEF NEF NEF NEF NBF NMP
0 . 1 M KPFe 0.01 KPFe 0.1 M KPFe .I?
Satd. KPFa 0 . 1 M KPF6 0.1 M CsI 0 . 1 M LiC104 0 . 1 M LiClO4 0.1 M KPFa 0.1 M KPF6
The Journal of Physical Chemistry
Temp,
YZl
OC
erga/cma
30 30 40 30 30 40 45 30 30 30 30 30 30 25 25 25 25 25 25 25 25 25 35 25 25
378.2 377.8 383.4
Ez,V
-0.333 -0.345 -0.383 -0.335
377.8 377.5 369.8
369.7 378.8 378.8
-0.320 -0.635 -0.332 -0.374 -0.340 -0.465 -0.33 -0.649 -0.310 -0.20 -0.25 -0.32 -0.36 -0.643 -0.349 -0,.313 -0.342
0.5
0
-0.5
-1.0
Potential, v (nce)
-1.5
Figure 3. Double-layer capacity for 0.1 M solutions in N-methylacetamide at 30 O.
anion adsorption is reflected in the negative shift of the pzc (Table I). The order of specific adsorption of anions is evidently I- > Br- > C1- > PFa > c1O4-, NOo-, BR-. The negative shift of the pzc with increasing concentration of KPFa is consistent with specific adsorption of PF6- ions. This is not, however, confirmed by the electrocapillary measurements which show an increase of yawith concentration. The cathodic hump depends only slightly on the electrolyte for inorganic salts. The capacity is -0.4% lower in 0.1 M LiC104 compared with KPFe. Tetraalkylammonium ions, however, change the shape of the hump and shift it to more anodic potentials probably due to specific adsorption of the cation, which, however, appears to be much weaker than in water.ll No significant shift of the pzc attributable to specific adsorption of N E h + or N P n + could be detected. Specific adsorption of Cs+ and NH4+ cations on the far cathodic side is indicated by the sharp rise of the capacity (Figure 4). This is confirmed for the Cs+ ion by a marked lowering of the interfacial tension (Figure 5 ) . NMA is strongly adsorbed from aqueous solution (Figure 6). Partial desorption occurs on the far cathodic side in dilute solutions. There is no indication of desorption on the anodic side, however, suggesting that the NMA molecule is most strongly adsorbed on a positively charged electrode. The NMA anodic capacity hump is absent for concentrations 5 50 mol %. The electrocapillary measurements in NMA solutions at 30” were marked by unusual experimental difficulties which may be of interest to other workers in this area. The meniscus could normally be set to the required mark (0.9 mm from the capillary tip) without difficulty, but would quiclrly become unstable and move (10) R. DeLevie, J . Electroanal. Chem., 9,117 (1965). (11) A. N. Frumkin and B. B. Damaskin, “Modern Aspects of Electrochemistry,” Vol. 3,J. O’M. Bockris, and B. E. Conway, Ed., Butterworth Inc.,Washington, D. C., 1964, p 149.
3601
SYMPOSIUM ON INTERFACIAL PHENOMENA 4 o F
0.IM KPF6 in NMA / water mixture:
25'
0.41
0.5
0
-0.5
-1.0
- 21
-1.5
Potential, v. (nce)
I
-1.2
-1.3
I
I
I
I
-1.4 -I 5 -I 6 -1.7 Potential, v (nce)
Figure 6. Capacity curves for 0.1 M KPFe solutions in mixtures of N-methylacetamide and water a t 25'. Vertical lines indicate potential of zero charge.
,
-1.8
Figure 4, Cat>hodicbranch of the capacity-potential curve _-r 0.1 M solutions in N-methylacetamide a t 30".
I
4cF
I
I
I
I
I
I
N-methylocetomide , 30"
Fotenfial v. (nce)
-1.0 Potential, v fnce)
- 1.5
I
- 2.0
Figure 5. Electrocapillary curves in N-methyIacetam.de at 30'.
rapidly up or down the capillary. The instability was usually accompanied by formation of an occlusion in the mercury co1um.n a few tenths of a millimeter above the meniscus which would eventually break the thread. The occluded material appeared to be solution but proved to be a gas bubble. The problem disappeared in 0.1 M LiC104 raised to 40" but reappeared in a severe form on lowering the temperature to 35". It was attributed to supersaturation of the solution with nitrogen (used for deaeration) which tends to form gas bubbles in the annular film of solution in the capillary. 2. N-Ethylacetamide and N-n-Butylacetamide. Capacity curves for 0.1 M KPF6 and 0.1 M CsI in NEA and 0.1 M K P R in NBA at 25" are shown in Figure 7. The behavior in both solvents is similar to the NMA system
Figure 7. Double-layer capacity for 0.1 M solutions of (1) KPFe and (2) CsI in N-ethylacetamide and (3) KPFe in N-n-butylacetamide at 25'.
but the capacities are somewhat lower. Double humps are found for KPF6 solutions in both NEA and NBA as in NMA. The cathodic hump becomes sharper and shifts in the anodic direction in the series NMA, NEA, NBA. The anodic hump decreases in size in the same order. In NEA and NBA a small peak occurs a t -0.3 V which is absent from the NMA system. The origin of this is unknown. It is not accompanied by significant direct current or polarization resistance and is presumably therefore a double-layer effect rather than a pseudocapacity peak due to adsorption or discharge of an impurity. Strong specific adsorption of both I- and Csf ions is indicated as in NMA by the negative shift of the pzc (I- adsorption) and sharp increase of capacity at the extremes of polarization. The cathodic rise shows an inflection a t - 1.7 V which is unusual. 'Volume 78,Number 11 November 1960
RICHARD PAYNE
3602
40 nl
5
\
'c
=4
I
-0.5
0
- 1.0 Potential , v.
(ncel
-1.5
Figure 8. Double-layer capacity for 0.1 M solutions of KPF6, LiCIO,, and CsI in N-ethylformamide at 25". Vertical lines indicate potential of zero charge.
I 0.5
I
0
-1.0 Potential, v ( n c e )
-0.5
-1.5
I
- 2.0
residue of the cathodic hump centered at -0.5 V (Figure 9). The similar effect on the far cathodic side at -1.4 V and the sharp rise of the capacity at - 1.6 V are hard to explain. The KPF, and LiC104 in N E F curves are almost superimposable except a t far cathodic potentials where the KPF6 curve is somewhat lower as usually found in nonaqueous solutions. Strong specific adsorption of I- ions is evident but Cs+ is apparently less strongly adsorbed from N E F than from NMA and NEA. The interfacial tension for 0.1 M LiC104 in N E F (378.8 ergs/cm2) is close to the value for the same solution in NMA (377.8 ergs/cm2 at 40") and as in NMA is almost independent of the temperature. In contrast to the extreme instability of the electrometer measurements in the acetamides, the behavior of NEF was unusually good and excellent electrocapillary curves were obtained. 4. Dimethylacetamide. Capacity curves for three concentrations of KPF6 in DMA at 25" are shown in Figure 10. As in D M F the cathodic hump is absent but there is a hump on the anodic side of the pzc. The capacity a t the hump is somewhat dependent on frequency (+leg% at 0.5 kHz and -5.9% at 4 kHz referred to the 1kHz value) as in NMA. Most of this is believed due to solution penetration in the capillary because of the high mobility of DMA. At potentials far cathodic of the pzc a stable dropping mercury electrode in DMA solution could be maintained only with difficulty owing to the solution penetration problem. No increase in the series resistance or direct current was found in the region of the capacity hump. The dielectric constant of D N A is relatively low (37.8 at 25"13). Consequently, the effect of the min-
Figure 9. Double-layer capacity for 0.1 $1 KPFe solutions in formamide, N-methylformamide, N-ethylformamide, and N-t-butylformamide at 25".
Satisfactory electrocapillary curves in NEA could not be obtained owing to sticking of the meniscus in the capillary and the instability problem described for NMA. The interfacial tension a t the ecm for 0.1 M KPF6 in NEA a t 25" is -378 ergs/cm2. 3. N-Ethylformamide and N-t-Butylformamide. The capacity was measured for 0.1 ?&f solutions of KPFe, LiC104, and CsI in N E F (Figure 8) and 0.1 M KPF6 in NBF at 25" (Figure 9). N E F resembles formamide and N M F insofar as a single capacity hump occurs on the cathodic side. However, the hump occurs at a less negative potential and is flatter. I n NBF the characteristic hump disappears and is replaced by a trough. The depression close to the pzc may be due to the effect of the diffuse layer capacity although this seems unlikely in v i m of the high dielectric constant (102 at 25"1 2 ) , From the trend of the curves in the series NMF, NEF, NBF this effect seems more probably due to the The Journal of Phyaical Chemietrg
0
I
0.5
0
I
I
-0.5 -1.0 Potential, v ( m e )
I
- 1.5
Figure 10. Double-layer capacity for KPFe solutions in dimethylacetamide at 25". Vertical lines show potential of zero charge. (12) R. Payne and I. E. Theodorou, unpublished data.
SYMPOSIUMON INTERFACIAL PHENOMENA
3603
Table I1 : Physical Properties and Double-Layer Capacity in Amide Solvents a t 25' Cmina
Solvent
sF/ama
Ref
Formamide N-Methylformamide Dimethylformamide N-Methylacetamide N-Ethylformamide Dimethylacetamide N-Methylpropionamide N-Ethylacetamidei N-t-Butylformamide N-n-Butylacetamide
12.5 (0.1 M K C l ) 8.5 (0.1 M KCl) 6.8 6.7 (30') 9.5 6.0 5.6 5.6 8.4 5.2
7 25 This work This work This work This work This work This work This work This work
Mol wt
73' 73 >isomers 73 87' 87 ,isomers 87,
Length ofb molecule, A
Dieleatrio
5.6 6.4 6.7 6.8 7.7 6.9 7.8 7.7 7.5 9.6 or 7.0"
109 5 182.4 36.7 178.9
constant I
37.8 172.2 129.9 101.7
' Cminis measured a t the minimum on the cathodic branch of the capacity-potential curve for 0.1 M KPF6 solutions except where otherwise stated. Measured along axis of the dipole assuming a planar molecule (R. J. Kurland and E. B. Wilson, J . Chem. Phys., 27, 585 (1957)). n-Butyl group and oxygen atom in cis (7.0 %L) or trans (9.6 A) configuration.
'
imum in the diffuse layer capacity is clearly visible in the 0.01 M solution (Figure 10). The marked concentration dependence of the capacity and corresponding negative shift of the pzc suggests substantial specific adsorption of PF6- ions. Satisfactory electrocapillary measurements in DMA could not be made. However, y I is close to 372.4 ergs/cm2 for 0.01 M KPFe at 25'.
Discussion 1. Xolvent Dependence of Double-Layer Properties in the Absence of Specific Adsorption of Ions. The effect of the solvent on the capacity is summarized in Table 11. The cathodic capacity hump is present only in solvents with unsubsti tuted N-hydrogen. Anodic capacity humps occur in DMA solutions of KPF6 whereas the Nmonoalkylaceta,mides and NMP show both anodic and cathodic humps. The capacity at the minimum on the far cathodic side generally decreases with increasing solvent molecular weight. N E F and NBF, however, are exceptions. Close agreement of the capacity is found between the structural isomers DMF, NMA and DMA, NMP, NEA. It should be noted that N E F is also isomeric with DMF and NMA. The macroscopic dielectric constant varies by a factor of 5 within this group of solvents and bears no direct relationship to the capacity. This further confirms Frumkin's early work3 and is to be expected since the unusual structural effects responsible for the high dielectric constants of solvents like water and formamide are presumably absent in the adsorbed state. The behavior of such solvents in the double layer should resemble that of normal polar solvents ( i e . ) nonassociated) with possible complications due t o lateral interaction, and interaction with solvent in the diffuse layer. The effective dielectric constant of even normal polar solvents in the double layer is impossible to predict. It seems reasonable to assume that for strong electrode polarization the dipoles are permanently aligned in the field and that the effective dielectric constant of the
layer will be close to the high-frequency value. According to measurements on bulk liquids this does not vary much among solvents of similar chemical type and is therefore unlikely to exert an important effecton the double-layer capacity. A factor which seems potentially more important and which is usually neglected is the effect of the size of the solvent molecule on the thickness of the inner region. According to Table I1 the capacity generally decreases with increasing size of the solvent molecule as would be expected. Similar correlations are found for other series of solvents.2 However, the selection of the minimum capacity on the cathodic side of the curve as the reference point is arbitrary although the correlation of capacity with solvent molecular weight seems fairly general over the whole curve. If it is assumed that reorientation of solvent dipoles occurs as the applied potential is varied, then variation of the capacity could be attributed to variation of the thickness since the solvent molecule is generally nonspherical. Variation of the thickness of the inner layer would seem to be a t least as important as variation of the dielectric constant, a possibility which has often been ignored in the interpretation of adsorption from aqueous solution. The cathodic capacity decreases in the alkali metal cation series in the order Li+ < Na+ < K + in the amides as in all other nonaqueous solvents. This is opposite to the order found in aqueous solution^.'^ The smallness of the effect in all solvents is difficult to understand. Many factors enter into the discussion including ionic solvation and structure, compressibility, and polarizability of the adsorbed solvent. Since little is known of any of these factors, discussion of the minor variations of the capacity seems pointless. However, it can be noted that the trend in the capacity in nonaqueous so(13) G.R. Leader and J. F. Gormley, J. Amer. Chem. Soo., 73, 6731 (1961). (14) D.C.Grahame, J . Electrochem. Soc., 98,343 (1951).
Volume 73, Number 11 November 1969
RICHARD PAYNE
3604 Table 111: Lowering of the Interfacial Tension at the Electrocapillary Maximum for 0.1 N Solutions in the Amides and Water at 25' AYE,erdcrna
,----
Salt
DMFa
NMF~
Formamidec
LiCl LiBr CSI KI
7.3 9.5 15 14.6
0
0 . 2 (KC1)
7
waterd
NMAe
7.5
8.9 (30') 3.9
NEF'
1 . 5 (NaC1, 18') 3 . 4 (CaBrs, 18') 9.1
12.2 (18')
a Data of Bezuglyi and Korshikov (Elektrokhimiya, 1, 1422 (1965); 3, 390 (1967)) relative to 0.1 N LiC104. Data of Damaskin and Ivanova (Zh. Fix. Khim., 38,176 (1964)) relative to 0.1 N CSZCOS. ' Data of Payne (unpublished) relative to 0.01 N KCl. Data of Gouy (Ann.Chim. Phys., 29,145 (1903)) relative to pure solvent. ' This work.
lutions is generally consistent with the trend in the Specific adsorption of alkali metal cations in the order K + < R b f < Cs+ is evident from the capacity and crystallographic radii of cations, but not the Stokes radii (obtained from conductance data), whereas in electrocapillary data in all of the systems studied. I n aqueous solutions the opposite is true. The signifithe formamides the order of increasing adsorption is cance of these observations is obscure. evidently formamide < N E F < N M F < DMF. In W. Specisc Adsoyption of Ions. According to current D M F published capacity curvesz1 for CsCl solutions ideas on ionic solvation, anions are strongly solvated show a sharp, concentration-dependent rise on the cathby solvents like water and formamide which are capable odic side indicating strong specific adsorption of Cs ions, of forming a hydrogen bond with the ion. Specific adwhich is confirmed by the corresponding depression of sorption of anions from such solvents should therefore the interfacial tension (10 ergs/cm2 at - 1.8 V against be relatively weak. This is confirmed for halide anions sce for 0.1 M CsC1). Adsorption of Cs+ ions from formin the formamides where, according to Table 111, the amide although evident from the capacity curves of Damaskin, Ivanova, and Survila7 is weak. The obserstrength of adsorption increases in the order formvation by these workers and also by Nancollas, Reid, amide < NMF < DMF. In D M F and in other aprotic and Vincentz2that E(+ ions are strongly specifically adsolvents (dimethyl sulfoxide for example) adsorption of sorbed from formamide does not seem consistent with simple anions is unusualy strongj2consistent with weak solvation indicated by other measurements. Adsorpthe experimental results. Thus Damaskin, et al., observed deviations from diffuse-layer behavior (for a tion of halide ions from water is weaker than from D M F nonspecifically adsorbed electrolyte) on the far cathbut appreciably stronger than from N M F and formamide. This is consistent with the o b s e r ~ a t i o nthat ' ~ ~ ~ ~ odic side and concluded therefore that Na+ and Cs+ ions are both specifically adsorbed to an equal and apheats of solution of alkali metal halides in N M F and preciable extent. However, their capacity curves for formamide are generally much larger than in water. NaCl and KC1 (Figure 2 of ref 7) are almost superimHowever, cation adsorption is generally stronger in the formamides than in water. Damaskin, et ~ l .have , ~ sugposable and show no evidence of specific adsorption of cations. In any case it seems extremely unlikely that gested therefore that the more negative heats of soluNa+ and Csf ions would be specifically adsorbed to the tion in the formamides is due entirely to enhanced solsame degree. Nancollas, et al., reported even larger vation of anions. deviations from diffuse layer behavior for 0.1 M KC1 in Polyatomic anions like C104-, PFB-, NOg-, and formamide. Positive adsorption of the anion over the SCN- are by contrast considerably more strongly adwhole range of potential was reported amounting to sorbed from water than from the amides including DMF.2 According to Bezuglyi and Korshikov,18 the interfacial tension at the ecm actually increases with (15) See, for example, A. J. Parker, Quart. Rev. (London), 16, 163 concentration for LiC104 and LiN03 in DMF. The (1962). surface activity of SCN- was likewise shown to be much (16) R. P. Held and C. M. Criss, J . Phys. Chem., 69, 2611 (1965). smaller in D M F than in water. This was attributed to (17) Yu. M. Povarov, Yu M. Kessler, A. I. Gorbanev, and I. V. Safonova, Dokl. Akad. N a u k SSSR,155, 1411 (1964). strong solvation in DMF, which however is not sup(18) V. D. Bezuglyi and L. A. Korshikov, Elektrokhimiya, 3, 390 ported by the conductance data.lg Adsorption of poly(1967). atomic anions from formamide (PFs-, Nos- 20), NMA (19) J. E. Prue and P. J. Sherrington, Trans. Faraday SOC.,57, 1796 (1961). (NOa-, Clod-, PFe-, BF4-), NEA (PFa-), and N E F (20) R. Payne, unpublished data. (C104-, PF6) also appears to be generally weak. It (21) S. Minc, J. Jastrzebska, and M. Brzostowska, J . Electrochem. seems likely that coadsorption of the solvent and posSoc., 108, 1160 (1961). sibly two-dimensional solvation of adsorbed ions are (22) G. H. Nancollas, D. S. Reid, and C. A. Vincent, J. Phys. Chem., 70, 3300 (1966). important factors. T h e Journal of Phgsieal Chemistry
3605
SYMPOSIUM ON INTERFACIAL PHENOMENA
-
r
KCi in formamide
E+ 1
, 25'
--4
(V
-8
-1.0-12
I
I
CI
- 0.2
I
-5 -10 Charge on the metal
,
I
I
-0.1 ( 2RT/ F)In at (v. J
I
-15
,uC/cm2
Figure 11. Ionic surface excesses for 0.09 M solutions of K I (closed circles) and KC1 (open circles) in formamide a t 25". Broken line indicates limiting repulsion of anions (2.1 &/oma) calculated from diffuse-layer theory for e nonadsorbed electrolyte.
-4 pC/cm2 at the extreme of cathodic polarization (Figure 5 of ref 22). If this were assumed wholly present in the diffuse layer (i.e., no specific adsorption of C1- ions) the cation would have to be specifically adsorbed to the extent of no less than 20 pC/cm2. Capacity and electrocapillary measurements reported elsewhere23 for K I solutions in formamide are consistent with the absence of specific adsorption of K + ions even at concentrations approaching 1 M . This is confirmed by similar measurements20 for KCl and KNOa solutions which are also in good agreement with the predictions of diffuse layer theory for a nonadsorbed electrolyte at charges up to -16 pC/cm2 (Figure 11). Esin and Markov plots (which are independent of electrocap illary measurements) are shown to have zero slope a t all concentrations confirming the conclusions reached from the surface excesses. The results for the KC1 system, which ,are typical, are shown in Figure 12. Since the slope of the E and M plot is related to the charge dependence of the surface excess of anions through the well-known relationship
(1) P
it follows from t'he constancy of the slope a t sufficiently negative values of p that r- has reached a constant value, which as shown by the curves in Figure 11 is close to the diffuse layer limiting value. Finally it should be noted that Nancollas, et al., find -4 pC/cmZ of neutral KC1 adsorbed at the ecm from 0.1 M KC1 in formamide compared with t,he reported valuez4 for aqueous 0.1 M KCI of,only 1.2 pC/cm2. This is in direct disagree-
,
I
0
Figure 12. Esin end Markov plots for KCl solutions in formamide at 25". Broken line of slope 0.5 is predicted by diffuse layer theory for a nonadsorbed electrolyte at the potential of zero charge.
ment both with the results in Figure 11 and those of Damaskin, et al. (who report 0.6 pC/cm2 for 0.1 M NaCI), and is also in conflict with the generally lower level of specific adsorption of anions in formamide compared with water (Table 111). It seems reasonable to conclude therefore that no significant specific adsorption of alkali metal cations other than Cs+ occurs from formamide. Specific adsorption of Cs+, Rb+, and possibly K + ions from NMF,26 Cs+, NH4+ and K f from M I A (Figure 4) and Cs+ from KEF (Figure 8), NEA (Figure 7) and DMF,21however, is clearly evident from the capacity curves in these systems. Specific adsorption of tetraalkylammonium cations from amide solvents (NBu4+ from NMF,25NEt4+, and NPr4+ from NMA) seems to be considerably weaker than from aqueous solutions probably due to stronger interaction of these ions with the organic solvent. 8. Nature of the Capacity Humps. The origin of the capacity humps in aqueous and nonaqueous solutions of inorganic ions is probably the most controversial aspect of double-layer theory. It has been variously suggested that the aqueous solution hump is due to reorientation of solvent dipoles, specific adsorption of anions, interaction of adsorbed solvent, and anions and competitive adsorption of solvent anions. While there is evidence to suggest that all of these effects play a part in different systems the fact remains that the capacity hump occurs in the absence of specific adsorption of (23) R.Payne, J.Chem. Phgs., 42,3371 (1965). (24) D. C. Grahame and R. Parsons, J. Amer. Chem. SOC.,8 3 , 1291 (1961). (25) B.B.Damaskin and Yu. M. Pavarov, Dokl. Akad. Nauk SSSR, 140, 394 (1961).
Volume 73,Number 11 November 1QBO
RICHARD PAYNE
3 606 anions (or when the effects of such adsorption have been taken into account). It must be concluded therefore that the hump is either a property of the solvent alone or the solvent and nonspecifically adsorbed (Le,, diffuse layer) ions. This conclusion is reinforced by the occurrence of humps on the cathodic side of the pzc in solvents like formamide and NMF in a potential region where specific adsorption of ions can certainly be discounted. Since these humps are also virtually independent of the identity of the cation (except for specifically adsorbed tetraalkylammonium ions) it can be further concluded that if ions are involved a t all they are involved in a nonspecific way. The most widely accepted interpretation of the hump is the solvent dipole reorientation theory of WattsTobin26 and Macdonald and bar lo^,^' according to which the permanent dipoles of the adsorbed solvent layer are able to rotate in the potential region of the hump, but become permanently oriented in the applied field at more positive or negative potentials. Since the hump is rarely found a t the potential of zero charge the dipoles evidently possess a preferred orientation, ie., positive (toward the metal) for solvents like water and DMSO (anodic hump) and negative for solvents like formamide and N M F (cathodic hump). For solvents exhibiting anodic humps (e.g., DMSO, butyrolactone) the preferred orientation (as inferred from the shift of the pzc relative to water) appears to be positive as predicted by the theory, but independent evidence suggests that the orientation of the water dipole itself is small and negative.28 The potential of zero charge in formamide (measured against aqueous nce) is close to that of ~ a t e r ,indicating ~,~ little or no preferential orientation of the dipole, while N M F shows a positive shift6j26consistent with positive orientation. I n both formamide and KMF the hump occurs at --0.5 V with respect to the pzc; a strong negative preferred orientation of the dipole is therefore predicted by the theory. The occurrence of two humps in some systems raises a more serious question since it does not at first seem possible to explain both humps in terms of solvent dipole reorientation. However, it has not yet been established that both humps arise from the same cause. It is reasonable to assume that the cathodic hump in NMA for example, is like the characteristic formamide hump (which it closely resembles). The anodic hump however could be due to specific adsorption of anions. The specific adsorption of PFe- ions in NMA was therefore investigated using standard methods. The negative shift of the ecm with increasing concentration (Table I) suggests appreciable adsorption of anions, which, however, is not confirmed by the electrocapillary measurements (Figure 13). y. actually increases with concentration indicating a surface deficiency of electrolyte at the ecm. However, as noted earlier, the electrocapillary measurements are not very reliable especially on the anodic branch of the curve. Comparison with The Journal of Physical Chemistry
I
-
-0.5 1.0 Potential, v (nce)
0
-1.:
Figure 13. Electrocapillary curves for KPF6 solutions in N-methylaoetamide at 30".
the doubly integrated capacity curves revealed discrepancies of up to 3 ergs/cm2 on the anodic side in the 0.1 M KPFBsolution. Discrepancies on both sides of the ecm in the 1 M KPF6 solution were consistent with a small liquid junction potential difference between the two sets of measurements. However, both sets of data, although not entirely independent, were in reasonable agreement showing evidence of some specific adsorption of PFs- on the far anodic side. As a further test for anion adsorption the capacity of the inner layer (Ci) was calculated, assuming the absence of specific adsorption, using the series capacitor formula of GrahameeZ9
1
1
1
c=G+& The diffuse layer capacity (C,) was calculated from the relationship
cd = 19.14(314.8~+ q2)"2
(3) which is appropriate for a solvent of dielectric constant 178.9 at 30°.30 The results of this calculation are shown in Figure 14. Ci depends strongly on the concentration for q >, 6 pC/cm2 indicating the breakdown of the assumption of no specific adsorption. It is not possible to determine the amounts adsorbed in the (26) R. J. Watts-Tobin, Phil. Mag.., 6, 133 (1961). (27) J. R. Macdonald and C. A. Barlow, J . Chem. Phys., 36, 3062 (1962). (28) R. Parsons, Ann. R e p . Chem. Soc., 61,BO (1964). (29) D. C. Grahame, Chem. Rev., 41,441 (1947). (30) 6.J. Bass, W. I. Nathan, R. M. Meighan, and R. H. Cole, J . Phys. Chem., 68,509 (1964).
SYMPOSIUMON INTERFACIAL PHENOMENA
3607
KPF6 in N-methylacefamide , 30"
H
O 20
IO
0 L
-10
-20
L
Potential, v
Charge on the metal ( q , ) , ,uC/cm2
Figure 14. Capacity of the inner region of the double layer for KPF6 solutions in N-methylacetamide a t 30" calculated from eq 2.
absence of therrnodynamic data. The electrocapillary measurements, however, suggest that it will be small. As a direct test of the dipole reorientation theory an idea originally due to Bockris, Devanathan, and Muller31 was employed. According to Bockris, et ai., the solvent dipole should be least strongly adsorbed on the electrode at the potential of reorientation, L e . , in the vicinity of the hump. This test was recently applied by Dutkiewicz and ParsonsE to the formamide system in which diethyl ether was adsorbed from a solution of KF. Maximum lowering of the capacity (and hence maximum adsorption) occurred at the hump, from which the correctness of the dipole reorientation interpretation was inferred. Since the ether itself is polar, however, some ambiguity is introduced. For this reason a search for a nonpolar adsorbate was made. Aliphatic hydrocarbons are appreciably soluble in NMA and seemed suitable, but unfortunately are not strongly adsorbed. Capitcity measurements were obtained for 0.1 M KPF6 solutions with added cyclohexane, nheptane, and isooctane a t -1 M concentration. Isooctane and w-heptane produced no significant effect on the cathodic; hump but cyclohexane lowered the capacity in this region (Figure 15). This confirms Dutkiewicz and Parsons' findings for the formamide system. The effect on the anodic hump in all cases was to increase the capacity; the cyclohexane curve in Figure 15 is typical. This can be interpreted as enhanced specific adsorption of PFs- ions due to a reverse of the salting-out effect, i.e., increase in the activity of the electrolyte due to the added hydrocarbon. However, changes in the shape of the hump are evidence of some adsorption of the hydrocarbon so that no clear interpretation can be made. The test seems to confirm the validity of the dipole reorientation interpretation of the cathodic hump whereas the origin
(nce)
Figure 15. Effect of added cyclohexane on the capacity for 0.1 M KPFe solution in N-methylacetamide a t 30".
I
300
0.5
0
-0.5 Potential, v fnce)
-1.0
Figure 16. Electrocapillary curves for KPFe solutions in dimethylacetamide a t 25' calculated by double integration of the capacity curves of Figure 10. y z is arbitrarily taken as 372.4 ergs/cm2 independent of concentration.
of the anodic hump is still uncertain. All the tests applied indicate some specific adsorption of PFa-. However, it seems unlikely that this is sufficiently extensive to be wholly responsible for the hump although it undoubtedly has some effect. The anodic hump in the DMA system is accompanied by much more substantial specific adsorption of PFGions according to the electrocapillary curves in Figure 16. An adsorption hump is therefore a stronger possibility in this system. Comparison of the DMA curve with a two-hump system (e.g., NMP) shows that the anodic hump in the two systems (and also in NMA, (31) J. O'M. Bookris, M. A. V. Devanathan, and K. Mbller, Proc. Roy. SOC.,A274,55 (1963).
Volume 7.3, Number 11
November 1969
RICHARD PAYNE
3608
4 0 9 I , I of O.IMKPF, solutions
1
0.5
I
0
25”
1
-0.5 -1.0 Potential, v ( n c e )
-1.5
Figure 17. Capacity curves for 0.1 M KPFe solutions in dimethylformamide, dimethylacetamide, and N-methylpropionamide at 25”. Vertical lines show potential of zero charge.
NEA, and NBA) occur at the same potential and are generally similar, suggesting a common origin. A marked inflection in the KPFa-DMF system reported p r e v i o ~ s l yalso ~ ~ coincides with the anodic hump in the other systems (Figure 17). The question of whether the hump is a property of the solvent or the result of anion adsorption must therefore remain open. In theoretical treatments of the interaction of the solvent molecule with the electrode it is usually assumed implicitly that the dipole will be preferentially oriented on an uncharged electrode with at least a component of the dipole moment normal to the interface. Such preferential orientation has been attributed somewhat vaguely to the effect of a “natural” field arising from overlap of electronic wave functions and other factor^.^' The weakness of this approach is that it is purely electrostatic and takes no account of the specific chemical properties of the solvent molecule which are often a far more important factor than the dipole moment. This is shown rather clearly in the amides where the double-layer properties are related to the physical and chemical properties of the solvent rather than the dipole moment which is almost independent of the solvent. The preferred orientation of solvent molecules on an uncharged mercury electrode is therefore more probably due to specific chemical interaction of one or more sites in the molecule with the mercury.
The Journal of Physical Chemiatru
Furthermore, such “specifically adsorbed” solvent molecules would not necessarily be oriented with a the dipole moment normal to the intercomponent face, although this would generally be the case. Reorientation of a dipolar molecule would normally be expected to occur at sufficiently high applied field strengths except where the strength of the chemical binding is unusually large (e.g., for the thiourea mole~ule~~), This concept removes at least one serious difficulty of the reorientation theory, namely the anomalous orientation of dipoles at the pzc. The small surface potential generated by formamide on mercury can be attributed to parallel orientation of the dipole which is adsorbed by specific chemical interaction with the mercury. That such chemical interaction occurs is suggested by the unusually large work of adhesion of formamide on mercury,2 which also explains why such a large polarization (-0.5 V) is required to reorientate the molecule (at the hump). The positive shift of the pzc in N M F can likewise be seen as the incidental consequence of a preferred orientation of the dipole dictated by (‘chemical” rather than electrostatic forces. The existence of double humps in some solvent systems also follows naturally since the molecule can realign with the dipole oriented positively or negatively according to the polarity of the applied field. The nature of the strong interaction of molecules like formamide with mercury is not obvious in the way that the interaction of thiourea, for example, is. The unexpectedly small temperature dependence of the interfacial tension in NMA and NEF may be significant in this respect (the temperature dependence in formamide itself has unfortunately not been measured). This suggests that the surface excess entropy in these solvents is small whereas in water it is large and positive.34 This has been attributed to disruption of the solvent structure when the solvent molecules become adsorbed, an effect which should be even more drastic for highly structural solvents like formamide and NMA. It must be assumed, therefore, that the adsorbed molecules are also in a highly ordered state which implies two-dimensional association. This may be related to the extraordinary stability of these solvents on mercury and their unusual double-layer properties. (32) R. Payne, J. Phys. Chem., 71,1648 (1967). (33) R. Parsons, Proc. Roy. SOC.,A261,79 (1961). (34) G. J. Hills and R. Payne, Trans. Faraday Soc., 61,326 (1965).
