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water interface
J. Electroanal. Chem., 151 (1983) 163-177
163
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ADSORPTION OF 2-METHOXYETHANOL AT THE MERCURY/WATER INTERFACE
O. IKEDA, K. K O G O and H. T A M U R A
Department of Applied Chemtstry, Faculty of Engineermg~ Osaka University, 2-1 Yamadaoka, Sulta, Osaka 565 (Japan) (Received 18th September 1979; in final form 30th December 1982)
ABSTRACT Adsorption of 2-methoxyethanol at the mercury/water interface was studied by means of electrocapillary curves. The isotherm was found to be congruent with respect to rational inner layer potential drop AM~/,. The adsorption of 2-methoxyethanol could be described by the Frumkin isotherm with 17s = 6.3 × 10-l0 mol cm -2 throughout the potential, but the interaction parameter was varied with the potential. Standard free energy of adsorption showed quadratic dependence on the potential, the m a x i m u m being observed at a = - 5 . 4 /tC cm -2 and AEM~= --0.22 V. However, the slope of this dependence changed at the m a x i m u m of adsorption, and the cause of this was discussed on the basis of a change in the surface water structure. It was concluded from the small positive effective normal dipole m o m e n t that 2-methoxyethanol takes a horizontal orientation with the positive end of the dipole toward the electrode surface.
INTRODUCTION
It has been shown that 2-(methyltlfio)ethanol [1] takes a vertical orientation with the methylthio (CH3S-) group on the electrode, and it was suggested that a somewhat larger inner layer capacity of 2-(methylthio)ethanol, compared to that of aliphatic alcohols [2], was due to a large electronic polarization of the sulfur atom. In order to make this point clearer, the adsorption behavior of 2-methoxyethanol was studied because the sulfur atom of 2-(methylthio)ethanol is replaced by an oxygen atom with a lower electronic polarizability than that of sulfur atom [3]. Further, 2-methoxyethanol is a derivative of ethylene glycol and its molecular structure resembles that of 1-butanol. The adsorption behavior of ethylene glycol [4] is quite different from that of 1-butanol [2,5]; however, it was of interest to know whether the adsorption behavior of 2-methoxyethanol resembles that of ethylene glycol or 1-butanol. Buckley and Brochu [6] determined the minimum energy molecular conformation and the dipole moment components parallel to three different rotation axes with respect to 2-methoxyethanol. Comparison of these data with the results obtained from the analysis of the double layer structure enabled us to determine a precise orentation of 2-methoxyethanol on the electrode surface. 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
164 Ramanamurti and Apparao [7] have already studied the adsorption of 2methoxyethanol in aqueous 0.1 M KC1 by means of differential capacity measurements. Some of their results were similar to ours, but the interpretation of the adsorption behavior was different. EXPERIMENTAL Commercial reagent grade 2-methoxyethanol was used and was purified by distillation. Sodium fluoride was of high purity reagent grade (Merck, ultrapure), and was used without further purification. Water and mercury were triply distilled before use. Nitrogen was deoxygenated by passing through copper mesh heated to 400°C. Electrocapillary measurement was carried out at 25 +__ 0.05°C by the m a x i m u m bubble pressure method [8,9]. In order to avoid the change in the caracteristics of the glass capillary [10], the measurement was carried out without intervals between runs. Further, the reading was corrected by measuring the standard solution (0.5 M Na2SO 4 [11]) after each 2 runs. Fourteen different concentrations of 2methoxyethanol were prepared to cover between 0.01 M and 2.0 M. A 1 M KC1 calomel electrode (MCE) and a platinum mesh were used as the reference and the counter electrode, respectively. The interfacial tension was measured at 50-mV intervals, except for the positive and negative extremes where 25-mV intervals were chosen. The error of the measurement was + 0.1 m N m - ~ near the electrocapillary maximum, and _+0.2 m N m - l at the positive and negative extremes. The medium effects on the activity of the supporting electrolyte [12,13], and on the activity coefficient of the adsorbate [14] were not taken into consideration. Most of the parameters essential to the analysis of the adsorption were obtained by analyzing the electrocapillary curves with a computer program. RESULTS AND DISCUSSION Electrocapillary curves
Figure 1 shows the electrocapillary curves for 0.4 M N a F containing 2methoxyethanol in various concentrations. The drop in the interfacial tension 7 was most remarkable at potentials around - 0 . 7 V vs. MCE. The potential of the electrocapillary maximum, Eec m (which is summarized in Table 1 with the interfacial tension at the maximum, Yecm) was shifted toward positive potentials with an increase in the concentration of 2-methoxyethanol. This indicates that the adsorbate on the electrode shows a positive effective normal dipole moment (i.e. a positive end of the dipole toward the electrode) or no effective normal dipole moment. Surface excess
Figure 2 shows the relation between the surface excess F and the electrode potential E at different concentrations of 2-methoxyethanol. The m a x i m u m of the
165
420
l
400
Z
380
l
l
0
l
-0.5 E / V
vs
-i.0 MCE
Fig. 1. Electrocapillary curves for 0.4 M NaF solution containing 2-methoxyethanol in various concentrations at 25°C. Concentration: (1) 0 (base solution, 0.4 M NaF), (2) 0.05, (3) 0.1, (4) 0.2, (5) 0.3, (6) 0.5, (7) 0.7, (8) 1.0, (9) 1.4, and (10) 2.0 M.
surface excess was observed at E = - 0 . 7 0 to - 0 . 7 5 V vs. MCE. On the other hand, the charge and the potential of maximum free energy of adsorption can be determined from the point of intersection in the potential-charge ( E - o ) curves [ 15]. In the following analysis, however, the rational inner layer potential drop A~q) was used in place of the electrode potential, because the discussion is focused on the properties of the inner layer. The value of A~q, was calculated from the next relation, A~q) = E - E z - A2~, where E and E z are the measured potential and the potential of zero charge for the electrolyte solution without a specific adsorption of ions ( - 0 . 4 7 2 V vs. MCE [16]), respectively, and A2q) is the potential drop across the diffuse double layer, which was calculated by using the Gouy-Chapman theory [17]. Figure 3 shows the A2Mq)-o curves for several concentrations of 2-methoxyethanol.
166 TABLE 1 Data of Eec m and "Yecm Co~g/mol 1- i
Eecrn/V vs. MCE
Yecm/mN m - 1
0(0.4 M NaF) 0.01 0.02 0.03 0.05 0.07
-
427.3 427.2 427.0 426.7 426.3 425.9 425.1 424.5 423.1 421.4 418.7 416.4 413.7 410.4 406.8
0.473 0.472 0.471 0.470 0.467 0.463 - 0.456 -0.451 - 0.447 -0.429 -0.411 -0.389 - 0.367 -0.345 -0.322
0.1
0.14 0.2 0.3 0.5 0.7 1.0 1.4 2.0
The curve for the base solution intersects those for the other solutions at one point, and the charge and the rational inner layer potential drop of the maximum free e n e r g y o f a d s o r p t i o n , Omax a n d A~,hma x w e r e f o u n d t o b e - 5 . 4 + 0.1 # C c m - 2 a n d - 0 . 2 2 0 ___0.005 V, r e s p e c t i v e l y .
C'4 I L)
1
,-q
2 3
O
4
O
x
/ 7 8 9
.~ i 0
~
~
-0.5
~
2i ~
10 11
-i.0
E / V vs M C E
Fig. 2. Variation of surface excess of 2-methoxyethanol at various concentrations with electrode potential. Concentration: (1) 2.0, (2) 1.4, (3) 1.0, (4) 0.7, (5) 0.5, (6) 0.3, (7) 0.2, (8) 0.14, (9) 0.1, (10) 0.07, (11) 0.05, and (12) 0.03 M.
167
1
-i0
2 ,,,o/
4
< I
3 5
C;max
-5
/ ///// 615
4
3
2
0.2
2#max
1 I
I~
0
-0.2
A~4> /
I -0.4
v
Fig. 3. Correlations between surface charge density and rational inner layer potential drop for several concentrations of 2-methoxyethanol. Concentration: 0 (base solution, 0.4 M NaF), (2) 0.14, (3) 0.3, (4) 0.5, (5) 1.0, and (6) 2.0 M.
The adsorption of organic c o m p o u n d s having no net dipole m o m e n t or showing no effective normal dipole m o m e n t gives Omax at o = --2 to - - 4 /~C cm -2, for example, - 2 / ~ C c m - 2 for pyrazine [18] and - 3 . 5 / ~ C c m - 2 for ethylene glycol [4]. Both the more negative value of Omax c o m p a r e d to values for the above two c o m p o u n d s and the positive shift of E e c m suggest that the positive end of the dipole of 2-methoxyethanol is directed toward the electrode surface. Surface excess at saturation
The value of the surface excess at saturation F s is necessary for the determination of the type of isotherm, and for the elucidation of the orientation at the interface. The estimation of this value was done by extrapolating the plot of F - i against Corg1 at constant m ~ to Corg - i = 0, where Corg is the concentration of 2-methoxyethanol in mol 1-1. Such a plot is shown in Fig. 4 and the estimated F s was about 6.2 to 6.3 x 10 -1°
168
o.71 ,~!
0.6
o u
0.5
o ! 0
M
/
/
o
0.4
I
0.3
0.2
0.1 t 0
I
I
!
!
I
1
2
3
4
5
Corg
-1
/
M-1
Fig. 4. Estimation of F s at different AM2~. T h e value of AM~ is indicated by each line.
mol cm -2 irrespective of A ~ . Then the mean value of 6.3 × 10-l0 mol cm -2 was employed as the FS. The same Fs was also estimated by the similar plot at constant charge density, though the plot showed some curvature.
Selection of electrical variables There are several types of analyses to select whether the free energy of adsorption is a function of charge density or of potential as the electrical variables [ 19]. One of them is based on the linearity of the plot for the change in a at constant AM~ due to adsorption (Fig. 5) or that for the change in A2M~ at constant a due to adsorption (Fig. 6). The linearity of the plot in Fig. 5 is better than that in Fig. 6 with respect to the adsorption of 2-methoxyethanol. This indicates that the free energy of adsorption of 2-methoxyethanol is a function of AM2~. A relation between the slope of the straight lines in Fig. 5, ( 0 o / 0 F ) Ar~ and AM2~
169
-10
-0.3
-5
o-o-o-o-~--~ --~
.
v
~
-0.2
I 0 U
~
0
o2
*
D
10
I
I
I
2
4
6
F x l O I0 / m o l
c m -2
Fig. 5. Change in surface charge density due to adsorption of 2-methoxyethanol at constant A ~ . The value of AM2~is indicated by each line.
is shown in Fig. 7. The slope in Fig. 7 can be correlated to quadratic dependence of the free energy of adsorption upon A2Me?[20]. but showed an inflection at the point of Ar~@r,ax. This was quite similar to the case of succinonitrile [21].
Adsorption &otherm In order to determine the adsorption isotherm of 2-methoxyethanol, several isotherms were examined, and it was found that the adsorption of 2-methoxyethanol could be described by the Frumkin isotherm
fix = [0/(1 - 8)] e x p ( - 2 a 8 )
(1)
170
AM4p=-O.
-5
.
o
.
"
_5 ~
o
o
.
o
V
"-- - 0 , 4 ( - 0 . 0 6 ) o---..o---o__-0.5(-0.06)
" o
3
(-0.06)
o
W
"
"
-0.2(-0.06)
o
:~
- 0 . 1 (0) 0 (0.i)
A (D
I
_5 "~ n
o v
•
.
.
u
,-4
0.2(0.25)
-4
0.3(0.25)
-3
0
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Fig. 6. Change m rational inner layer potential drop due to adsorption of 2-methoxyethanol at constant charge density. Charge density is indicated by each line.
r~ I
-2
L)
CD
I
o X
co
0
I
I
0.3
0
I
-0.3
~/v Fig. 7. Relationbetween(0o/OF) and A~@.
I
-0.6
171
where fl is the adsorption coefficient related to the standard free energy of adsorption AG° by fl = exp(-AG°/RT), x is the molar fraction of the adsorbate, 8 is the coverage, and a is the interaction parameter. Figure 8 shows the Frumkin isotherm plot using Fs = 6.3 × 10-10 mol cm -2 which has already been estimated. The value in the parentheses represents a at each Aft+. In the region of AM~ < m2MdPmax , the value of a was very close to zero. In fact, the adsorption of 2-methoxyethanol in this region could also be described by the Langmuir isotherm (i.e. a = 0), as already reported by Ramanamurti and Apparao [7]. The value of Fs evaluated from the fit to the Langmuir isotherm was 6.2 × 10-10 mol cm-2, which was almost the same as that used for the Frumkin isotherm. In the M . . . . however, the adsorption of 2-methoxyethanol could not be region of Ar~¢~> A2¢ described by the Langmuir isotherm, because the Langmuir isotherm plot ( x / F vs
-0.6
~=-8 pC cm-2 _
~
-7
-0,4
-0.2
"
>
~
~
~
-5
-4
0.2
2
4
0.4
I
I
i
2
4
6
F xlOI0 / mol cm-2 Fig. 8. Frumkin isotherm plot at different AM~. The value of AMq and the estimated interaction parameter a are indicated by each line and in the parentheses, respectively.
172 x) showed a deviation f r o m linearity at points for lower x, and the F s evaluated from the plot at higher x increased with an increase in A ~ , that was in conflict with the result of a constant F s deduced from Fig. 4. O n the contrary, the fit of the adsorption in the region of AM2q~> AM~bmax to the F r u m k i n isotherm results in an increase of a. Recent interpretation of positive a in the F r u m k i n isotherm due to Pulidori et al. [22] is as follows: A - S interactions are weaker than 1 / 2 ( A - A + S - S ) interactions, where A and S denote the adsorbate and the solvent, respectively. Furthermore, Amadelli et al. [23] proposed a parallelism between the interpretation of Mohilner et al. [24] that a positive value of a in the F r u m k i n isotherm corresponds to a positive deviation from the Raoult's law or the association of adsorbates in the surface solution, and the interpretation of Pulidori et al. cited above. These interpretations lead to a speculation that 2-methoxyethanol in AMEq,> A~q'ma × adsorbs through the formation of clusters or making w a t e r - w a t e r interactions somewhat stronger than those in A~,p < A ~ m a x.
Free energy of adsorption The free energy of adsorption expressed as In fl can be obtained from the extrapolation of the straight lines in Fig. 8 to 0 = 0. Figure 9 shows the plot of In fl against (A~q, - Aff~max) 2. Quadratic dependence of In fl on A~q, seems to be obeyed for the adsorption of 2-methoxyethanol, but the dependence is different in the two regions, namely A r ~ > AM~max and A r ~ < AMdPmax.
"''O." "'O..
(a)
O~
".. (b)
,-'4
3
"00
I
I
0.i
0.2 M
M
(A2~-A2~ma x)
2
/
0.3 V2
Fig. 9. Dependence of In fl on ( A 2M M ~ -- A2t~max ) 2. Dotted lines, (a) and (b) represent the slopes estimated from the two linear relations in Fig. 7 and eqn. (3). The points near the lines (a) and (b) correspond to the experimental values obtained for the region of AMe/,< AM~maxand A2~b M > A2d? M . . . . respectively.
173
The slope of the straight lines in Fig. 5 is related to the change in In fl due to A~q~
[20] ( a o / o r ) AM,~ = _ g T ( a In
B/OAMq,)
(2)
When In fl is a quadratic function of (AMq~-- AM~max), the following equation can be derived from eq. (2) 8(~o/~F)/8(AM~b -- AM~bmax)= -- 2 R T [8 In fl/8(AMd? -- AMqSmax)2].
(3)
One can estimate the value of the left hand side of eqn. (3) from the slope of the linear line in Fig. 8, and then elucidate the value of [3 In fl/8(A~2q~ - m M 2 ~ m a x ) 2 ] . The dotted lines in Fig. 9 correspond to the slopes obtained in the above way. The experimental points obtained from the fit to the Frumkin isotherm are well on the two dotted lines. This suggests that the experimental results are reliable and the analysis is self-consistent.
Orientation model of 2-methoxyethanol Figure 10 shows the top (a) and the side (b) views with respect to the scale drawing of a 2-methoxyethanol adsorbed on the electrode. In drawing these pictures, Fs of 6.3 x 10 - l ° mol cm -2, an estimated orientation of the positive end of the dipole toward the electrode, and the minimum energy conformation of 2methoxyethanol determined by Buckley and Brochu [6] were taken into consideration. The area of a rectangle enclosing the molecule in Fig. 10a is 0.624 nm 2 or Fs = 6.3 x 10-10 mol cm -2. The height of the molecule from the electrode surface was 0.44 nm. The dipole moment of 2-methoxyethanol (CH3OCH2CH2OH) determined from the measurements of the microwave spectrum [6} was 7.87 + 0.10 × 10 -30 C m. This is the maximum value obtained by adding vectorially the dipole moment components parallel to three rotation axes. Thus, the components parallel to the C H 3 - O axis ga, C H 2 - O (ether) axis/z b, and C H 2 - O (hydroxyl) axis/~c are
(a)
0.6 n m (b)
0.4
0.2
//////I/11/I////////11///11/ Electrode
Fig. 10. Scale drawing of a 2-methoxyethanol molecule adsorbed on the electrode: (a) top view, and (b) side view. The rectangle enclosing the molecule in (a) corresponds to the occupied area.
174
6.77, 3.84, and 0.8 X 10 -3o C m, respectively. When 2-methoxyethanol takes a vertical orientation where the methyl group is on the electrode, its effective normal dipole moment is expected to be /~a = 6.77 × 10 -3o C m. On the other hand, the dipole moment for the orientation where the hydroxyl group is directed toward the solution side and the hydrophobic groups are on the electrode is presumed to be about 3.3 × 10 -3° C m, because the angle of the C H 2 - O (ether) axis to the electrode surface is about 60 ° .
Evaluation of the dipole moment of an adsorbed 2-methoxyethanol Parsons et al. [25] have shown in the adsorption of urea that the dipole moment of the adsorbate at the interface can be evaluated from an analysis of the rational inner layer potential drop at constant charge density. We applied the analysis to the adsorption of tetramethylthiourea [26], where the difference in AzMq,was evaluated between the surface layer saturated with water and the adsorbate. In such a case, the analysis seems to be independent of the congruency of the isotherm. In analogy with tetramethylthiourea, the dipole moment of 2-methoxyethanol was obtained as a function of charge density, assuming the horizontal orientation shown in Fig. 10. The result is summarized in Table 2 with other inner layer parameters, where K l, e, and P are the integral inner layer capacity, the permittivity of the inner layer, and the effective normal dipole moment, and subscripts 0 and 1 mean the surface layer saturated with water and 2-methoxyethanol, respectively. The evaluated dipole moment PI is in the range of 4.7 to 3.0 × 10 -3o C m. These values agreed approximately with that estimated for the horizontal orientation in Fig. 10, 3.3 × 10 -3° C m. Figure 11 shows the variation of integral inner layer capacity K i due to adsorption of 2-methoxyethanol, which was used to obtain K~. The line for o = - 5 ~C cm -2 near the m a x i m u m adsorption was linear, but the lines for other charge densities were curves and those for o < - 5 #C cm -2 showed a negative deviation while those for o > - 5 /~C c m - 2 showed a positive deviation. Similar trends were also observed for the same plot of K i at constant A~q~ against F. If one assumes a constant orientation throughout coverage, the curve for o = 2 # C cm -2 means that
TABLE 2 Various inner layer parameters at saturation with water and 2-methoxyethanol o//~C cm -2
2
1
0
-l
-2
-3
-4
-5
-6
-7
-8
K~/#F cm -2 K6('°n) a / # F cm -2 K~/FF cm -2
29.7 24.1 16.0 7.0 4.8
29.3 23.7 16.0 7.0 4.8
28.8 23.4 16.0 7.0 4.7
28.2 23.0 15.8 7.0 4.6
27.6 22.7 15.5 6.8 4.7
26.9 22.2 15.0 6.6 4.4
26.2 21.8 14.3 6.3 3.9
25.5 21.3 12.6 5.5 3.4
24.7 20.8 11.1 4.9 2.9
23.8 20.2 9.5 4.2 3.1
23.2 19.7 8.8 3.9 3.2
elxl011/F m - ~ Plxl03°/C m
a Here K6 ('°") represents the integral inner layer capacity due to free charges, which is free from the contribution due to water dipoles.
175
o
o ~ ~ O
-"~-...~.,, 0=2 HC cm-2
30
¢q I O
20
10 ~L
0
I
I
1
2
4
6
r xl010 / mol cm-2 Fig. 11. Variation of K' at constant a due to adsorption of 2-methoxyethanol. the permittivity of the surface layer at lower coverage is larger than that for original pure water layer. Such a phenomenon seems to resemble that observed with respect to the refractive index in the mixing of water and 2-methoxyethanol [27].
Interpretation of adsorption behavior The self-consistent results so far obtained indicate that 2-methoxyethanol adsorbs horizontally and its orientation does not vary with the electric field. However, it is difficult to explain on the basis of these findings the presence of two different quadratic dependences of In/3 on A2Mq),or the inflection a t AM~max with the line in Fig. 7, which Battisti et al. encountered already in the case of succinonitrile [21]. The cause of the inflection seems to be related to two characteristic changes occurring around the maximum adsorption, namely (i) the positive deviation of K ' in the region of o > - 4 / ~ C cm -2, and (ii) the increase of the interaction parameter in the region of AMq,> AMq)max. The first seems to be related to the property of surface water structures. Especially, the values of K i for small coverage of 2-methoxyethanol at o = 2/zC cm -2 are larger than K~. From the definition of the integral inner layer capacity, this seems to suggest that preferential orientation of water molecules at this charge density is lowered by the presence of 2-methoxyethanol, that is to say, E e c m moves toward more positive potentials than those expected from only the orientation of 2-methoxyethanol owing to lowering in the orientation of water showing negative effective normal dipole moment.
176
Recent water adsorption models at the mercury/aqueous solution interface assume a water structure consisting of free molecules with [28,29] and without [30] chemisorption with the electrode, and cluster molecules. The cluster is in equilibrium with free water molecules, and the amount in the surface layer depends on the electric field. Moreover, the clusters are considered to have smaller [28,29] or not [30] effective normal dipole moment, compared to that of free water molecules. In a mixed solution of 2-methoxyethanol and water, Kurtz et al. [27] suggested that 2-methoxyethanol forms no hydrogen bonds with water. The above information also leads to the speculation, which is similar to that previously described, that adsorption of 2-methoxyethanol at positive electric field occurs through clusters of 2-methoxyethanol and accelerates the formation of water clusters with the consumption of free water molecules. This speculation may explain the second change in the interaction parameter in the Frumkin isotherm, if we follow the previously cited interpretation of a due to Pulidori et al. [22].
Comparison with other related compounds 2-Methoxyethanol takes a horizontal orientation, in which the hydrophobic groups contact to the electrode surface and the hydrophilic groups are directed toward the solution side. This orientation was different from that of 2(methylthio)ethanol, in which the methyl group and the sulfur atom contact to the electrode surface at the same time. Such a difference in the manner of contact seems to result from the fact that sulfur is a more polarizable and more hydrophobic atom than oxygen. Further, the permittivity of the inner layer saturated with 2methoxyethanol was 3.9 to 7.0 X 10- ii F m - 1 in the range of o = - 8 to 2 #C cm -2, and it was smaller than those of 2-(methylthio)ethanol which showed 7.1 to 8.8 × 10 -11 F m -~ in the same range of o. An exact theory of the permittivity of the double layer has not yet been given, because it contains problems somewhat different from those usually encountered in the dielectric theory, for example, a variable electric field and very thin layer of less than a nm. Therefore, the permittivity of the double layer with organic adsorbates has been explained in a qualitative way [21,31,32]. However, the somewhat lower inner layer permittivity with 2-methoxyethanol is thought to be due to the lower electronic polarizability of the oxygen than of the sulfur atom [3]. The adsorption behavior of ethylene glycol [4] has been studied by Trasatti. This compound showed a horizontal orientation with no effective normal dipole moment. The dipole moments obtained from permittivity measurements in solutions for both 2-methoxyethanol and ethylene glycol [33] are nearly the same, and are 6.8 and 7.7 x 10 -3o C m, respectively. Irrespective of these large dipole moments, both compounds showed a horizontal orientation. High hydrophilicity of these compounds may be a cause for such an orientation. On the other hand, the adsorption behavior of 2-methoxyethanol was different from that of 1-propanol [34] and l-butanol [2,5], which showed a change in the orientation with coverage. The interaction parameter in the Frumkin isotherm for
177 2 - m e t h o x y e t h a n o l v a r i e d f r o m a s m a l l n e g a t i v e v a l u e t o a p o s i t i v e v a l u e , i.e. - 0 . 0 6 t o 0.25, w h i l e t h a t f o r 1 - p r o p a n o l a n d 1 - b u t a n o l w a s l a r g e a n d p o s i t i v e i.e. 1.11 a n d 1.28, r e s p e c t i v e l y . T h e s e l a r g e p o s i t i v e v a l u e s i n d i c a t e t h a t a t t r a c t i v e i n t e r a c t i o n a c t s among adsorbed molecules through an intermolecular hydrophobic bond. Such a difference in the interaction parameters between 2-methoxyethanol and aliphatic alcohols suggests that introduction of a hydrophilic oxygen atom into a hydrocarbon chain results in weakening of the intermolecular hydrophobic bond. REFERENCES 1 0 . Ikeda, K. Kog~ and H. Tamura, J. Electroanal. Chem., 95 (1979) 177. 2 B.B. Damaskin, A.A. Survila and L.E. Rybalka, Elektrokhimiya, 3 (1967) 146. 3 C.P. Smyth, Dielectric Behavior and Structure, McGraw-Hill, New York, 1955. 4 S. Trasatti, J. Electroanal. Chem., 28 (1970) 257. 5 E. Dutkiewicz, J.D. Garnish and R. Parsons, J. Electroanal. Chem., 16 (1968) 505. 6 P. Buckley and M. Brochu, Can. J. Chem., 50 (1972) 1149. 7 M.V. Ramanamurti and B.V. Apparao, Bull. Chem. Soc. Jpn, 51 (1978) 456. 8 D.J. Schiffrin, J. Electroanal. Chem., 23 (1969) 168. 9 0 . Ikeda, Y. Matsuda, H. Yoneyama and H. Tamura, Electrochim. Acta, 21 (1976) 519. 10 A. De Battisti and S. Trasatti, J. Colloid Interface Sci., 63 (1978) 61. 11 M.A.V. Devanathan and P. Peries, Trans. Faraday Soc., 50 (1954) 1236. 12 A. De Battisti and S. Trasatti, J. Electroanal. Chem., 54 (1974) 1. 13 D.M. Mohilner and H. Nakadomari, J. Phys. Chem., 77 (1973) 1594. 14 D.M. Mohilner, L.W. Browman, S.J. Freeland and H. Nakadomari, J. Electrochem. Soc., 120 (1973) 1658. 15 A.N. Frumkin, B.B. Damaskin and A.A. Survila, J. Electroanal. Chem., 16 (1968) 493. 16 P. Delahay, Double Layer and Electrode Kinetics, Interscience, New York, 1965, p. 23. 17 D.C. Grahame, Chem. Rev., 41 (1947) 441. 18 B.E. Conway, H.P. Dhar and S. Gottesfeld, J. Colloid Interface Sci., 43 (1973) 303. 19 R. Parsons, Rev. Pure Appl. Chem., 18 (1968) 91. 20 R. Parsons, Trans. Faraday Soc., 55 (1959) 999. 21 A. De Battisti, V. Faggino and S. Trasatti, J. Electroanal. Chem., 73 (1976) 327. 22 F. Pulidori, G. Borghesani, R. Pedriali, A. De Battisti and S. Trasatti, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 79. 23 R. Amadelli, A. Daghetti, L. Vergano, A. De Battisti and S. Trasatti, J. Electroanal. Chem., 100 (1979) 379. 24 D.M. Mohilner, H. Nakadomari and P.R. Mohilner, J. Phys. Chem., 81 (1977) 244. 25 R. Parsons, R. Peat and R.M. Reeves, J. Electroanal. Chem., 62 (1975) 151. 26 O. Ikeda, H. Jimbo and H. Tamura, J. Electroanal. Chem., 137 (1982) 127. 27 S.S. Kurtz, A.R. Tompson and D.L. Kamin, J. Chem. Eng. Data, 10 (1965) 335. 28 B.B. Damaskin and A.N. Frumkin, Electrochim. Acta, 19 (1974) 173. 29- R. Parsons, J. Electroanal. Chem., 59 (1975) 229. 30 J. O'M. Bockris and M.A. Habib, Electrochim. Acta, 22 (1977) 41. 31 R. Parsons, Proc. R. Soc. (London), Ser. A, 261 (1961) 79. 32 K. MOiler, J. Res. Inst. Catal., Hokkaido Univ., 14 (1966) 224. 33 W.H. Byers, J. Chem. Phys., 7 (1939) 175. 34 A.N. Frumkin, B.B. Damaskin and A.A. Survila, Elektrokhimiya, 1 (1965) 738.
163
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ADSORPTION OF 2-METHOXYETHANOL AT THE MERCURY/WATER INTERFACE
O. IKEDA, K. K O G O and H. T A M U R A
Department of Applied Chemtstry, Faculty of Engineermg~ Osaka University, 2-1 Yamadaoka, Sulta, Osaka 565 (Japan) (Received 18th September 1979; in final form 30th December 1982)
ABSTRACT Adsorption of 2-methoxyethanol at the mercury/water interface was studied by means of electrocapillary curves. The isotherm was found to be congruent with respect to rational inner layer potential drop AM~/,. The adsorption of 2-methoxyethanol could be described by the Frumkin isotherm with 17s = 6.3 × 10-l0 mol cm -2 throughout the potential, but the interaction parameter was varied with the potential. Standard free energy of adsorption showed quadratic dependence on the potential, the m a x i m u m being observed at a = - 5 . 4 /tC cm -2 and AEM~= --0.22 V. However, the slope of this dependence changed at the m a x i m u m of adsorption, and the cause of this was discussed on the basis of a change in the surface water structure. It was concluded from the small positive effective normal dipole m o m e n t that 2-methoxyethanol takes a horizontal orientation with the positive end of the dipole toward the electrode surface.
INTRODUCTION
It has been shown that 2-(methyltlfio)ethanol [1] takes a vertical orientation with the methylthio (CH3S-) group on the electrode, and it was suggested that a somewhat larger inner layer capacity of 2-(methylthio)ethanol, compared to that of aliphatic alcohols [2], was due to a large electronic polarization of the sulfur atom. In order to make this point clearer, the adsorption behavior of 2-methoxyethanol was studied because the sulfur atom of 2-(methylthio)ethanol is replaced by an oxygen atom with a lower electronic polarizability than that of sulfur atom [3]. Further, 2-methoxyethanol is a derivative of ethylene glycol and its molecular structure resembles that of 1-butanol. The adsorption behavior of ethylene glycol [4] is quite different from that of 1-butanol [2,5]; however, it was of interest to know whether the adsorption behavior of 2-methoxyethanol resembles that of ethylene glycol or 1-butanol. Buckley and Brochu [6] determined the minimum energy molecular conformation and the dipole moment components parallel to three different rotation axes with respect to 2-methoxyethanol. Comparison of these data with the results obtained from the analysis of the double layer structure enabled us to determine a precise orentation of 2-methoxyethanol on the electrode surface. 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
164 Ramanamurti and Apparao [7] have already studied the adsorption of 2methoxyethanol in aqueous 0.1 M KC1 by means of differential capacity measurements. Some of their results were similar to ours, but the interpretation of the adsorption behavior was different. EXPERIMENTAL Commercial reagent grade 2-methoxyethanol was used and was purified by distillation. Sodium fluoride was of high purity reagent grade (Merck, ultrapure), and was used without further purification. Water and mercury were triply distilled before use. Nitrogen was deoxygenated by passing through copper mesh heated to 400°C. Electrocapillary measurement was carried out at 25 +__ 0.05°C by the m a x i m u m bubble pressure method [8,9]. In order to avoid the change in the caracteristics of the glass capillary [10], the measurement was carried out without intervals between runs. Further, the reading was corrected by measuring the standard solution (0.5 M Na2SO 4 [11]) after each 2 runs. Fourteen different concentrations of 2methoxyethanol were prepared to cover between 0.01 M and 2.0 M. A 1 M KC1 calomel electrode (MCE) and a platinum mesh were used as the reference and the counter electrode, respectively. The interfacial tension was measured at 50-mV intervals, except for the positive and negative extremes where 25-mV intervals were chosen. The error of the measurement was + 0.1 m N m - ~ near the electrocapillary maximum, and _+0.2 m N m - l at the positive and negative extremes. The medium effects on the activity of the supporting electrolyte [12,13], and on the activity coefficient of the adsorbate [14] were not taken into consideration. Most of the parameters essential to the analysis of the adsorption were obtained by analyzing the electrocapillary curves with a computer program. RESULTS AND DISCUSSION Electrocapillary curves
Figure 1 shows the electrocapillary curves for 0.4 M N a F containing 2methoxyethanol in various concentrations. The drop in the interfacial tension 7 was most remarkable at potentials around - 0 . 7 V vs. MCE. The potential of the electrocapillary maximum, Eec m (which is summarized in Table 1 with the interfacial tension at the maximum, Yecm) was shifted toward positive potentials with an increase in the concentration of 2-methoxyethanol. This indicates that the adsorbate on the electrode shows a positive effective normal dipole moment (i.e. a positive end of the dipole toward the electrode) or no effective normal dipole moment. Surface excess
Figure 2 shows the relation between the surface excess F and the electrode potential E at different concentrations of 2-methoxyethanol. The m a x i m u m of the
165
420
l
400
Z
380
l
l
0
l
-0.5 E / V
vs
-i.0 MCE
Fig. 1. Electrocapillary curves for 0.4 M NaF solution containing 2-methoxyethanol in various concentrations at 25°C. Concentration: (1) 0 (base solution, 0.4 M NaF), (2) 0.05, (3) 0.1, (4) 0.2, (5) 0.3, (6) 0.5, (7) 0.7, (8) 1.0, (9) 1.4, and (10) 2.0 M.
surface excess was observed at E = - 0 . 7 0 to - 0 . 7 5 V vs. MCE. On the other hand, the charge and the potential of maximum free energy of adsorption can be determined from the point of intersection in the potential-charge ( E - o ) curves [ 15]. In the following analysis, however, the rational inner layer potential drop A~q) was used in place of the electrode potential, because the discussion is focused on the properties of the inner layer. The value of A~q, was calculated from the next relation, A~q) = E - E z - A2~, where E and E z are the measured potential and the potential of zero charge for the electrolyte solution without a specific adsorption of ions ( - 0 . 4 7 2 V vs. MCE [16]), respectively, and A2q) is the potential drop across the diffuse double layer, which was calculated by using the Gouy-Chapman theory [17]. Figure 3 shows the A2Mq)-o curves for several concentrations of 2-methoxyethanol.
166 TABLE 1 Data of Eec m and "Yecm Co~g/mol 1- i
Eecrn/V vs. MCE
Yecm/mN m - 1
0(0.4 M NaF) 0.01 0.02 0.03 0.05 0.07
-
427.3 427.2 427.0 426.7 426.3 425.9 425.1 424.5 423.1 421.4 418.7 416.4 413.7 410.4 406.8
0.473 0.472 0.471 0.470 0.467 0.463 - 0.456 -0.451 - 0.447 -0.429 -0.411 -0.389 - 0.367 -0.345 -0.322
0.1
0.14 0.2 0.3 0.5 0.7 1.0 1.4 2.0
The curve for the base solution intersects those for the other solutions at one point, and the charge and the rational inner layer potential drop of the maximum free e n e r g y o f a d s o r p t i o n , Omax a n d A~,hma x w e r e f o u n d t o b e - 5 . 4 + 0.1 # C c m - 2 a n d - 0 . 2 2 0 ___0.005 V, r e s p e c t i v e l y .
C'4 I L)
1
,-q
2 3
O
4
O
x
/ 7 8 9
.~ i 0
~
~
-0.5
~
2i ~
10 11
-i.0
E / V vs M C E
Fig. 2. Variation of surface excess of 2-methoxyethanol at various concentrations with electrode potential. Concentration: (1) 2.0, (2) 1.4, (3) 1.0, (4) 0.7, (5) 0.5, (6) 0.3, (7) 0.2, (8) 0.14, (9) 0.1, (10) 0.07, (11) 0.05, and (12) 0.03 M.
167
1
-i0
2 ,,,o/
4
< I
3 5
C;max
-5
/ ///// 615
4
3
2
0.2
2#max
1 I
I~
0
-0.2
A~4> /
I -0.4
v
Fig. 3. Correlations between surface charge density and rational inner layer potential drop for several concentrations of 2-methoxyethanol. Concentration: 0 (base solution, 0.4 M NaF), (2) 0.14, (3) 0.3, (4) 0.5, (5) 1.0, and (6) 2.0 M.
The adsorption of organic c o m p o u n d s having no net dipole m o m e n t or showing no effective normal dipole m o m e n t gives Omax at o = --2 to - - 4 /~C cm -2, for example, - 2 / ~ C c m - 2 for pyrazine [18] and - 3 . 5 / ~ C c m - 2 for ethylene glycol [4]. Both the more negative value of Omax c o m p a r e d to values for the above two c o m p o u n d s and the positive shift of E e c m suggest that the positive end of the dipole of 2-methoxyethanol is directed toward the electrode surface. Surface excess at saturation
The value of the surface excess at saturation F s is necessary for the determination of the type of isotherm, and for the elucidation of the orientation at the interface. The estimation of this value was done by extrapolating the plot of F - i against Corg1 at constant m ~ to Corg - i = 0, where Corg is the concentration of 2-methoxyethanol in mol 1-1. Such a plot is shown in Fig. 4 and the estimated F s was about 6.2 to 6.3 x 10 -1°
168
o.71 ,~!
0.6
o u
0.5
o ! 0
M
/
/
o
0.4
I
0.3
0.2
0.1 t 0
I
I
!
!
I
1
2
3
4
5
Corg
-1
/
M-1
Fig. 4. Estimation of F s at different AM2~. T h e value of AM~ is indicated by each line.
mol cm -2 irrespective of A ~ . Then the mean value of 6.3 × 10-l0 mol cm -2 was employed as the FS. The same Fs was also estimated by the similar plot at constant charge density, though the plot showed some curvature.
Selection of electrical variables There are several types of analyses to select whether the free energy of adsorption is a function of charge density or of potential as the electrical variables [ 19]. One of them is based on the linearity of the plot for the change in a at constant AM~ due to adsorption (Fig. 5) or that for the change in A2M~ at constant a due to adsorption (Fig. 6). The linearity of the plot in Fig. 5 is better than that in Fig. 6 with respect to the adsorption of 2-methoxyethanol. This indicates that the free energy of adsorption of 2-methoxyethanol is a function of AM2~. A relation between the slope of the straight lines in Fig. 5, ( 0 o / 0 F ) Ar~ and AM2~
169
-10
-0.3
-5
o-o-o-o-~--~ --~
.
v
~
-0.2
I 0 U
~
0
o2
*
D
10
I
I
I
2
4
6
F x l O I0 / m o l
c m -2
Fig. 5. Change in surface charge density due to adsorption of 2-methoxyethanol at constant A ~ . The value of AM2~is indicated by each line.
is shown in Fig. 7. The slope in Fig. 7 can be correlated to quadratic dependence of the free energy of adsorption upon A2Me?[20]. but showed an inflection at the point of Ar~@r,ax. This was quite similar to the case of succinonitrile [21].
Adsorption &otherm In order to determine the adsorption isotherm of 2-methoxyethanol, several isotherms were examined, and it was found that the adsorption of 2-methoxyethanol could be described by the Frumkin isotherm
fix = [0/(1 - 8)] e x p ( - 2 a 8 )
(1)
170
AM4p=-O.
-5
.
o
.
"
_5 ~
o
o
.
o
V
"-- - 0 , 4 ( - 0 . 0 6 ) o---..o---o__-0.5(-0.06)
" o
3
(-0.06)
o
W
"
"
-0.2(-0.06)
o
:~
- 0 . 1 (0) 0 (0.i)
A (D
I
_5 "~ n
o v
•
.
.
u
,-4
0.2(0.25)
-4
0.3(0.25)
-3
0
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Fig. 6. Change m rational inner layer potential drop due to adsorption of 2-methoxyethanol at constant charge density. Charge density is indicated by each line.
r~ I
-2
L)
CD
I
o X
co
0
I
I
0.3
0
I
-0.3
~/v Fig. 7. Relationbetween(0o/OF) and A~@.
I
-0.6
171
where fl is the adsorption coefficient related to the standard free energy of adsorption AG° by fl = exp(-AG°/RT), x is the molar fraction of the adsorbate, 8 is the coverage, and a is the interaction parameter. Figure 8 shows the Frumkin isotherm plot using Fs = 6.3 × 10-10 mol cm -2 which has already been estimated. The value in the parentheses represents a at each Aft+. In the region of AM~ < m2MdPmax , the value of a was very close to zero. In fact, the adsorption of 2-methoxyethanol in this region could also be described by the Langmuir isotherm (i.e. a = 0), as already reported by Ramanamurti and Apparao [7]. The value of Fs evaluated from the fit to the Langmuir isotherm was 6.2 × 10-10 mol cm-2, which was almost the same as that used for the Frumkin isotherm. In the M . . . . however, the adsorption of 2-methoxyethanol could not be region of Ar~¢~> A2¢ described by the Langmuir isotherm, because the Langmuir isotherm plot ( x / F vs
-0.6
~=-8 pC cm-2 _
~
-7
-0,4
-0.2
"
>
~
~
~
-5
-4
0.2
2
4
0.4
I
I
i
2
4
6
F xlOI0 / mol cm-2 Fig. 8. Frumkin isotherm plot at different AM~. The value of AMq and the estimated interaction parameter a are indicated by each line and in the parentheses, respectively.
172 x) showed a deviation f r o m linearity at points for lower x, and the F s evaluated from the plot at higher x increased with an increase in A ~ , that was in conflict with the result of a constant F s deduced from Fig. 4. O n the contrary, the fit of the adsorption in the region of AM2q~> AM~bmax to the F r u m k i n isotherm results in an increase of a. Recent interpretation of positive a in the F r u m k i n isotherm due to Pulidori et al. [22] is as follows: A - S interactions are weaker than 1 / 2 ( A - A + S - S ) interactions, where A and S denote the adsorbate and the solvent, respectively. Furthermore, Amadelli et al. [23] proposed a parallelism between the interpretation of Mohilner et al. [24] that a positive value of a in the F r u m k i n isotherm corresponds to a positive deviation from the Raoult's law or the association of adsorbates in the surface solution, and the interpretation of Pulidori et al. cited above. These interpretations lead to a speculation that 2-methoxyethanol in AMEq,> A~q'ma × adsorbs through the formation of clusters or making w a t e r - w a t e r interactions somewhat stronger than those in A~,p < A ~ m a x.
Free energy of adsorption The free energy of adsorption expressed as In fl can be obtained from the extrapolation of the straight lines in Fig. 8 to 0 = 0. Figure 9 shows the plot of In fl against (A~q, - Aff~max) 2. Quadratic dependence of In fl on A~q, seems to be obeyed for the adsorption of 2-methoxyethanol, but the dependence is different in the two regions, namely A r ~ > AM~max and A r ~ < AMdPmax.
"''O." "'O..
(a)
O~
".. (b)
,-'4
3
"00
I
I
0.i
0.2 M
M
(A2~-A2~ma x)
2
/
0.3 V2
Fig. 9. Dependence of In fl on ( A 2M M ~ -- A2t~max ) 2. Dotted lines, (a) and (b) represent the slopes estimated from the two linear relations in Fig. 7 and eqn. (3). The points near the lines (a) and (b) correspond to the experimental values obtained for the region of AMe/,< AM~maxand A2~b M > A2d? M . . . . respectively.
173
The slope of the straight lines in Fig. 5 is related to the change in In fl due to A~q~
[20] ( a o / o r ) AM,~ = _ g T ( a In
B/OAMq,)
(2)
When In fl is a quadratic function of (AMq~-- AM~max), the following equation can be derived from eq. (2) 8(~o/~F)/8(AM~b -- AM~bmax)= -- 2 R T [8 In fl/8(AMd? -- AMqSmax)2].
(3)
One can estimate the value of the left hand side of eqn. (3) from the slope of the linear line in Fig. 8, and then elucidate the value of [3 In fl/8(A~2q~ - m M 2 ~ m a x ) 2 ] . The dotted lines in Fig. 9 correspond to the slopes obtained in the above way. The experimental points obtained from the fit to the Frumkin isotherm are well on the two dotted lines. This suggests that the experimental results are reliable and the analysis is self-consistent.
Orientation model of 2-methoxyethanol Figure 10 shows the top (a) and the side (b) views with respect to the scale drawing of a 2-methoxyethanol adsorbed on the electrode. In drawing these pictures, Fs of 6.3 x 10 - l ° mol cm -2, an estimated orientation of the positive end of the dipole toward the electrode, and the minimum energy conformation of 2methoxyethanol determined by Buckley and Brochu [6] were taken into consideration. The area of a rectangle enclosing the molecule in Fig. 10a is 0.624 nm 2 or Fs = 6.3 x 10-10 mol cm -2. The height of the molecule from the electrode surface was 0.44 nm. The dipole moment of 2-methoxyethanol (CH3OCH2CH2OH) determined from the measurements of the microwave spectrum [6} was 7.87 + 0.10 × 10 -30 C m. This is the maximum value obtained by adding vectorially the dipole moment components parallel to three rotation axes. Thus, the components parallel to the C H 3 - O axis ga, C H 2 - O (ether) axis/z b, and C H 2 - O (hydroxyl) axis/~c are
(a)
0.6 n m (b)
0.4
0.2
//////I/11/I////////11///11/ Electrode
Fig. 10. Scale drawing of a 2-methoxyethanol molecule adsorbed on the electrode: (a) top view, and (b) side view. The rectangle enclosing the molecule in (a) corresponds to the occupied area.
174
6.77, 3.84, and 0.8 X 10 -3o C m, respectively. When 2-methoxyethanol takes a vertical orientation where the methyl group is on the electrode, its effective normal dipole moment is expected to be /~a = 6.77 × 10 -3o C m. On the other hand, the dipole moment for the orientation where the hydroxyl group is directed toward the solution side and the hydrophobic groups are on the electrode is presumed to be about 3.3 × 10 -3° C m, because the angle of the C H 2 - O (ether) axis to the electrode surface is about 60 ° .
Evaluation of the dipole moment of an adsorbed 2-methoxyethanol Parsons et al. [25] have shown in the adsorption of urea that the dipole moment of the adsorbate at the interface can be evaluated from an analysis of the rational inner layer potential drop at constant charge density. We applied the analysis to the adsorption of tetramethylthiourea [26], where the difference in AzMq,was evaluated between the surface layer saturated with water and the adsorbate. In such a case, the analysis seems to be independent of the congruency of the isotherm. In analogy with tetramethylthiourea, the dipole moment of 2-methoxyethanol was obtained as a function of charge density, assuming the horizontal orientation shown in Fig. 10. The result is summarized in Table 2 with other inner layer parameters, where K l, e, and P are the integral inner layer capacity, the permittivity of the inner layer, and the effective normal dipole moment, and subscripts 0 and 1 mean the surface layer saturated with water and 2-methoxyethanol, respectively. The evaluated dipole moment PI is in the range of 4.7 to 3.0 × 10 -3o C m. These values agreed approximately with that estimated for the horizontal orientation in Fig. 10, 3.3 × 10 -3° C m. Figure 11 shows the variation of integral inner layer capacity K i due to adsorption of 2-methoxyethanol, which was used to obtain K~. The line for o = - 5 ~C cm -2 near the m a x i m u m adsorption was linear, but the lines for other charge densities were curves and those for o < - 5 #C cm -2 showed a negative deviation while those for o > - 5 /~C c m - 2 showed a positive deviation. Similar trends were also observed for the same plot of K i at constant A~q~ against F. If one assumes a constant orientation throughout coverage, the curve for o = 2 # C cm -2 means that
TABLE 2 Various inner layer parameters at saturation with water and 2-methoxyethanol o//~C cm -2
2
1
0
-l
-2
-3
-4
-5
-6
-7
-8
K~/#F cm -2 K6('°n) a / # F cm -2 K~/FF cm -2
29.7 24.1 16.0 7.0 4.8
29.3 23.7 16.0 7.0 4.8
28.8 23.4 16.0 7.0 4.7
28.2 23.0 15.8 7.0 4.6
27.6 22.7 15.5 6.8 4.7
26.9 22.2 15.0 6.6 4.4
26.2 21.8 14.3 6.3 3.9
25.5 21.3 12.6 5.5 3.4
24.7 20.8 11.1 4.9 2.9
23.8 20.2 9.5 4.2 3.1
23.2 19.7 8.8 3.9 3.2
elxl011/F m - ~ Plxl03°/C m
a Here K6 ('°") represents the integral inner layer capacity due to free charges, which is free from the contribution due to water dipoles.
175
o
o ~ ~ O
-"~-...~.,, 0=2 HC cm-2
30
¢q I O
20
10 ~L
0
I
I
1
2
4
6
r xl010 / mol cm-2 Fig. 11. Variation of K' at constant a due to adsorption of 2-methoxyethanol. the permittivity of the surface layer at lower coverage is larger than that for original pure water layer. Such a phenomenon seems to resemble that observed with respect to the refractive index in the mixing of water and 2-methoxyethanol [27].
Interpretation of adsorption behavior The self-consistent results so far obtained indicate that 2-methoxyethanol adsorbs horizontally and its orientation does not vary with the electric field. However, it is difficult to explain on the basis of these findings the presence of two different quadratic dependences of In/3 on A2Mq),or the inflection a t AM~max with the line in Fig. 7, which Battisti et al. encountered already in the case of succinonitrile [21]. The cause of the inflection seems to be related to two characteristic changes occurring around the maximum adsorption, namely (i) the positive deviation of K ' in the region of o > - 4 / ~ C cm -2, and (ii) the increase of the interaction parameter in the region of AMq,> AMq)max. The first seems to be related to the property of surface water structures. Especially, the values of K i for small coverage of 2-methoxyethanol at o = 2/zC cm -2 are larger than K~. From the definition of the integral inner layer capacity, this seems to suggest that preferential orientation of water molecules at this charge density is lowered by the presence of 2-methoxyethanol, that is to say, E e c m moves toward more positive potentials than those expected from only the orientation of 2-methoxyethanol owing to lowering in the orientation of water showing negative effective normal dipole moment.
176
Recent water adsorption models at the mercury/aqueous solution interface assume a water structure consisting of free molecules with [28,29] and without [30] chemisorption with the electrode, and cluster molecules. The cluster is in equilibrium with free water molecules, and the amount in the surface layer depends on the electric field. Moreover, the clusters are considered to have smaller [28,29] or not [30] effective normal dipole moment, compared to that of free water molecules. In a mixed solution of 2-methoxyethanol and water, Kurtz et al. [27] suggested that 2-methoxyethanol forms no hydrogen bonds with water. The above information also leads to the speculation, which is similar to that previously described, that adsorption of 2-methoxyethanol at positive electric field occurs through clusters of 2-methoxyethanol and accelerates the formation of water clusters with the consumption of free water molecules. This speculation may explain the second change in the interaction parameter in the Frumkin isotherm, if we follow the previously cited interpretation of a due to Pulidori et al. [22].
Comparison with other related compounds 2-Methoxyethanol takes a horizontal orientation, in which the hydrophobic groups contact to the electrode surface and the hydrophilic groups are directed toward the solution side. This orientation was different from that of 2(methylthio)ethanol, in which the methyl group and the sulfur atom contact to the electrode surface at the same time. Such a difference in the manner of contact seems to result from the fact that sulfur is a more polarizable and more hydrophobic atom than oxygen. Further, the permittivity of the inner layer saturated with 2methoxyethanol was 3.9 to 7.0 X 10- ii F m - 1 in the range of o = - 8 to 2 #C cm -2, and it was smaller than those of 2-(methylthio)ethanol which showed 7.1 to 8.8 × 10 -11 F m -~ in the same range of o. An exact theory of the permittivity of the double layer has not yet been given, because it contains problems somewhat different from those usually encountered in the dielectric theory, for example, a variable electric field and very thin layer of less than a nm. Therefore, the permittivity of the double layer with organic adsorbates has been explained in a qualitative way [21,31,32]. However, the somewhat lower inner layer permittivity with 2-methoxyethanol is thought to be due to the lower electronic polarizability of the oxygen than of the sulfur atom [3]. The adsorption behavior of ethylene glycol [4] has been studied by Trasatti. This compound showed a horizontal orientation with no effective normal dipole moment. The dipole moments obtained from permittivity measurements in solutions for both 2-methoxyethanol and ethylene glycol [33] are nearly the same, and are 6.8 and 7.7 x 10 -3o C m, respectively. Irrespective of these large dipole moments, both compounds showed a horizontal orientation. High hydrophilicity of these compounds may be a cause for such an orientation. On the other hand, the adsorption behavior of 2-methoxyethanol was different from that of 1-propanol [34] and l-butanol [2,5], which showed a change in the orientation with coverage. The interaction parameter in the Frumkin isotherm for
177 2 - m e t h o x y e t h a n o l v a r i e d f r o m a s m a l l n e g a t i v e v a l u e t o a p o s i t i v e v a l u e , i.e. - 0 . 0 6 t o 0.25, w h i l e t h a t f o r 1 - p r o p a n o l a n d 1 - b u t a n o l w a s l a r g e a n d p o s i t i v e i.e. 1.11 a n d 1.28, r e s p e c t i v e l y . T h e s e l a r g e p o s i t i v e v a l u e s i n d i c a t e t h a t a t t r a c t i v e i n t e r a c t i o n a c t s among adsorbed molecules through an intermolecular hydrophobic bond. Such a difference in the interaction parameters between 2-methoxyethanol and aliphatic alcohols suggests that introduction of a hydrophilic oxygen atom into a hydrocarbon chain results in weakening of the intermolecular hydrophobic bond. REFERENCES 1 0 . Ikeda, K. Kog~ and H. Tamura, J. Electroanal. Chem., 95 (1979) 177. 2 B.B. Damaskin, A.A. Survila and L.E. Rybalka, Elektrokhimiya, 3 (1967) 146. 3 C.P. Smyth, Dielectric Behavior and Structure, McGraw-Hill, New York, 1955. 4 S. Trasatti, J. Electroanal. Chem., 28 (1970) 257. 5 E. Dutkiewicz, J.D. Garnish and R. Parsons, J. Electroanal. Chem., 16 (1968) 505. 6 P. Buckley and M. Brochu, Can. J. Chem., 50 (1972) 1149. 7 M.V. Ramanamurti and B.V. Apparao, Bull. Chem. Soc. Jpn, 51 (1978) 456. 8 D.J. Schiffrin, J. Electroanal. Chem., 23 (1969) 168. 9 0 . Ikeda, Y. Matsuda, H. Yoneyama and H. Tamura, Electrochim. Acta, 21 (1976) 519. 10 A. De Battisti and S. Trasatti, J. Colloid Interface Sci., 63 (1978) 61. 11 M.A.V. Devanathan and P. Peries, Trans. Faraday Soc., 50 (1954) 1236. 12 A. De Battisti and S. Trasatti, J. Electroanal. Chem., 54 (1974) 1. 13 D.M. Mohilner and H. Nakadomari, J. Phys. Chem., 77 (1973) 1594. 14 D.M. Mohilner, L.W. Browman, S.J. Freeland and H. Nakadomari, J. Electrochem. Soc., 120 (1973) 1658. 15 A.N. Frumkin, B.B. Damaskin and A.A. Survila, J. Electroanal. Chem., 16 (1968) 493. 16 P. Delahay, Double Layer and Electrode Kinetics, Interscience, New York, 1965, p. 23. 17 D.C. Grahame, Chem. Rev., 41 (1947) 441. 18 B.E. Conway, H.P. Dhar and S. Gottesfeld, J. Colloid Interface Sci., 43 (1973) 303. 19 R. Parsons, Rev. Pure Appl. Chem., 18 (1968) 91. 20 R. Parsons, Trans. Faraday Soc., 55 (1959) 999. 21 A. De Battisti, V. Faggino and S. Trasatti, J. Electroanal. Chem., 73 (1976) 327. 22 F. Pulidori, G. Borghesani, R. Pedriali, A. De Battisti and S. Trasatti, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 79. 23 R. Amadelli, A. Daghetti, L. Vergano, A. De Battisti and S. Trasatti, J. Electroanal. Chem., 100 (1979) 379. 24 D.M. Mohilner, H. Nakadomari and P.R. Mohilner, J. Phys. Chem., 81 (1977) 244. 25 R. Parsons, R. Peat and R.M. Reeves, J. Electroanal. Chem., 62 (1975) 151. 26 O. Ikeda, H. Jimbo and H. Tamura, J. Electroanal. Chem., 137 (1982) 127. 27 S.S. Kurtz, A.R. Tompson and D.L. Kamin, J. Chem. Eng. Data, 10 (1965) 335. 28 B.B. Damaskin and A.N. Frumkin, Electrochim. Acta, 19 (1974) 173. 29- R. Parsons, J. Electroanal. Chem., 59 (1975) 229. 30 J. O'M. Bockris and M.A. Habib, Electrochim. Acta, 22 (1977) 41. 31 R. Parsons, Proc. R. Soc. (London), Ser. A, 261 (1961) 79. 32 K. MOiler, J. Res. Inst. Catal., Hokkaido Univ., 14 (1966) 224. 33 W.H. Byers, J. Chem. Phys., 7 (1939) 175. 34 A.N. Frumkin, B.B. Damaskin and A.A. Survila, Elektrokhimiya, 1 (1965) 738.