2380
Langmuir 1991, 7, 238G2384
Adsorption Studies of n-Hexadecyltributylphosphonium Bromide at the Mercury/Solution Interface through Differential Capacity Measurements Milliana K. Kaisheva' and Georgi Saraivanov University of Sofia, Faculty of Chemistry, No. 1 "A. Ivanov" Avenue, Sofia 1126, Bulgaria
Anastos Anastopoulos Laboratory of Physical Chemistry, Department of Chemistry 333-2, University of Thessaloniki, 54006 Thessaloniki, Greece Received January 2,1991. I n Final Form: April 29, 1991 The adsorption of n-hexadecyltributylphosphonium bromide (CTBPB) at the stationary mercury electrode/aqueous solution interface has been investigated by measuring the differential capacity of the electricaldouble layer at this interface. NaF (0.1moledm-3) was used as a supporting electrolyte. CTBPB concentrations varied from 10-7to 5 X 1Oa mol-dm-3. The analytical model of differential capacity that adequately described the experimental sample proved to be that based on the isotherm of Flory-Huggins. The estimates of the adsorption parameters of CTBPB in the adequate analytical model of capacity were found. The dependence of the outer Helmholtz plane potential on electrode potential E was estimated.
Introduction n-Hexadecyltributylphosphoniumbromide (CTBPB) is a cationic surfactant (SAA) recently applied in phasetransfer catalysis because of its markedly amphiphilic properties. However, its interfacial behavior is not yet well examined. Data are available from surface tension measurements of aqueous CTBPB solutions,' which show that the critical micelle concentration (cmc) of the surfactant is 1.6 X lo4 m~lndm-~.Results suggest the occurrence of structural changes in the adsorption layer of the surfactant. A number of authors2* have studied the adsorption of n-alkyltrimethyl- and n-alkyltrimethylammoniumcations on a mercury electrode. The systems examined have exhibited unusual behavior, explained by a two-dimensional condensation and by a structural rearrangement of the adsorption layer. The influence of adsorbed propargyltriphenylphosphonium,' tetraphenylarsonium, and methyltriphenylarsonium cations8 on ion-transfer and electron-transfer reactions has been ascribed to the same effects. It is of interest to study the adsorption of CTBPB on mercury by measuring the differential capacity of the electrical double layer and, by means of this precise method, find evidence concerning phenomena analogous to those observed in the above mentioned systems. In an earlier work amethod for the statistical estimation of adsorption parameters on the basis of an experimental sample of differential capacity data and application of nonlinear regression analysis was de~eloped.~ The method enables us to distinguish between several analytical models (1) Shen Hanxi; Zang Erle Fenxi Huaxue 1985, 13, 736. (2) ma, K. Nippon Kagaku Zapshi 1960,81,875.
(3) Damaskin,B.B.;Nikolaeva-Fedorovich,N.V.Zh.Fiz. Khim. 1961,
.15. 1279. --I
(4) Hayter, J. B.; Hunter, R. J. J. Electroanal. Chem. 1972, 37, 71.
(5) Hayter, J. B.; Humphreys, M. W.; Hunter, R. J.; Parsons, R. J. Ekctroanal. Chem. 1974, 66, 160. (6) Kaisheva,M. K.; Kaishev,V. K. Ann.Sofia Unio.Fac. Chem. 1975/ 1976, 70, 71. (7) Anastopoulos,A.; Christodoulou, A.; Moumtzis, I. Can.J. Chem. 1988,66, 1053. (8) Anaetopoulos, A.; Christodoulou, A.; Poulios, I. J. Electroanal. Chem. 1989,262, 235. (9) Kaisheva, M.; Kaishev, V. Langmuir 1985,1, 760.
possibly describing the differential capacity data and find the adequate one. Bearing in mind the size of the cetyltributylphosphonium (CTBP) cation, the analytical model that might be expected to adequately describe CTBPB adsorption on mercury is that derivedl0 on the basis of an adsorption isotherm of the Flory-Huggins type. The purpose of the present work is to investigate the adsorption of CTBPB a t the stationary mercury electrode/ aqueous solution interface by (i) measuring the differential capacity of the electrical double layer at this interface, (ii) finding a model adequately describing the experimental sample and estimating the adsorption parameters of CTBPB in the adequate analytical model of capacity, and (iii) determining the dependence of the potential ,!V of the outer Helmholtz plane of the electrical double layer on electrode potential E. The course of this determination involves assumptions and calculations that further elucidate the overall profile of the CTBPB interfacial behavior.
Experimental Section Pure NaF', produced by Wako Pure Chemical IndustriesLtd., was additionallypurified by recrystallizationand heating to 600 O C . Mercury, chemically purified, was twice distilled in vacuum. Three-times distilled water was used for experiments. CTBPB was obtained from Fluka AG. The experimental setupfor differentialcapacity measurements was based on the impedance bridge method and does not differ from that earlier described." The working electrodewas sealed in glass amalgamated platinum wire on which a stationary mercury drop of - 5 mma could be suspended in the course of the experiment. A three-electrodecell was used for experiments, working electrode potential cp being measured vs a saturated calomel electrode (SCE). Oxygen was removed from the solution by bubbling with hydrogen for 1 h prior to the experiment. Experiments were carried out at 20 0.5 "C and started at an initial electrode potential of -0.3 V SCE, which then changed in positive and negative directions. Measurements were carried out as well
*
~
~
~~
~
~~~~~
~~
(IO) Kaisheva, M.; Mataumoto, M.; Kita, Y.;Takenaka, T. Langmuir
1988, 4, 762. (11) Kaisheva, M.; Kaishev, V.; Mataumoto, M. J.Electroanal. Chem. 1984, 171, 111.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 10,1991 2381
n-Hexadecyltributylphosphonium Bromide Adsorption
4 E
6-0 10
I
60
30
t/min
Figure 2. Differential capacity for electrode potential -0.3 V (SCE) as a function of time with pure supporting electrolyte solution, 0.1 molodm-9 NaF (a) and with the addition of CTBPB in concentrations of 10-7 (b), 5 X lo-' (c), and 1od (d) molodm-8. I
0.0
-0.5
-1.0
I
I
'f/v(sce)
Figure 1. Differential capacity dependence on electrode potential measured vs saturated calomel electrode in the case of pure supportin electrolyte solution, 0.1 moledm-3 NaF(l), and with CTBPB ecfded in concentrationsof 5 X lo-' (2), 10" (3), (4), and 5 X lod ( 5 ) mol-dm".
starting from -1.7 V SCE with a positive scan. The ac frequency was 1010 Hz. An evaluation was made of the frequency dependence of the impedance for CTBPB solutions at -0.6 V (SCE). The straight vertical line, obtained for the dependence of the imaginary vs real component of the impedance, shows that adsorption of CTBPB is not complicated by processes connected with charge transfer.
Results and Discussion In the present investigation the differential capacity of the electrical double layer at the stationary mercury electrode was measured as a function of electrode potential for 0.1 molodm-3 aqueous solution of NaF (supporting electrolyte) and with the addition of CTBPB in concentrations (c) of lO-7,3 X lO-7,5 X 10-7, 104, 10-6, and 5 X 10-6 moledm-3. Some of the results from differential capacity measurements are illustrated in Figure 1. As seen from the figure, CTBPB is adsorbed at the electrode/ solution interface in the range of potentials more negative than 0 V (SCE), and two regions of adsorption in respect to concentration can be distinguished. On the one hand, for surfactant concentrations lower than 10-6 mol-dm-3, the capacity values in the interval of capacity minimum in Figure 1are at equilibrium and do not depend on the direction of the potential scan. They are obtained by waiting for the necessary length of time, as illustrated in Figure 2, to reach a constant capacity value. Each curve in Figure 2 is measured on a renewed electrode surface and is reproducible. As seen from Figure 2, differential capacity diminishes with time and for 5 X lW7 m01*dm-~CTBPB equilibrium is already reached mol~dm-~, within 20min. For an SAA concentration of the equilibrium capacity value is obtained 2 h after creating a fresh electrode surface. The cathodic maxima of curves 2 and 3 (Figure 1)show hysteresis, depending on the direction of the potential scan. The maxima illustrated in the figure are measured by changing the electrode potential from -0.3 V (SCE) to more negative potentials by steps of 0.05 V. If the initial potential is -1.8 V (SCE) and then changesto less negative values, the cathodic maxima are lower and wider than those shown in the figure.
I
10
1
I
5
10
I
15
*
20 t/min
Figure 3. Differential capacity dependence on time with a 1Od moledm-* solution of CTBPB and electrode potential cp -0.6 V (SCE).
On the other hand, for surfactant concentrations of m ~ l - d mand - ~ higher, hysteresis is observed in the whole interval of potentials from 0.1 to -1.8 V (SCE). The capacity values increase with time (Figure 3). Ae seen from Figure 1, curves 4 and 5, at negative potentiale differential capacity is much lower than and does not coincide with that for the supporting electrolyte solution. This result indicates that CTBP cations are not desorbed from the electrode surface. We suppose that the broad and low maxima observed in Figure 1,curves 4 and 5, are connected with a reorientation of the surface-activecations. At electrode potentials near to the potential of zero charge (pzc), corresponding to the minimum of differential capacity,the surfactant cations are adsorbed with the more hydrophobic hexadecyl radical oriented to the electrode surface and the hydrophilic phosphonium group to the aqueous solution. With an increase of the negative electrode charge, the energetically more favorable orientation becomes that with the positive "head" of CTBPB nearer to the metal surface, thus causing a reorientation of the surfactant. We suppose as well that a two-dimensional phase transition from a gaseous to a liquid-expanded state of the adsorbed layer of CTBPB takes place when its concentration increases to 10-5 m ~ l - d m -and ~ higher. During this process adsorbate and water molecules coexist in the adsorption layer. The separation of the interfacial layer into two new phases should be accompanied by an increase in differential capacity. A support of this view can be found in Figure 3 and in the comparison of curves 4 and 5 with curves 2 and 3 in Figure 1. We view the liquid-expanded state as a state having properties similar to those of the liquid phase. The hydrophobic "tails" interacting with each other are in a state of chaotic movements-rotating and vibrating as in the liquid hydrocarbon phase. Thermal motions are made by the
Kaisheva et al.
2382 Langmuir, Vol. 7, No. 10, 1991 hydrophilic "head" groups as well and water molecules are distributed between them. Such a transition from a gaseous to a liquid-expanded state has been observed as we1P in the case of dodecylammonium chloride adsorption at the waterlair interface. Investigations have shown12 that at a bulk concentration of the colloid surfactant equal to 0.1 cmc a first-order surface-phase transition occurs, which results in the formation of a condensed monolayer of the liquid-expanded type. In the case of CTBPB adsorption at the mercury/aqueous saline solution interface investigated in the present work, the same surfacephase transition occurs at a lower surfactant concentration-0.06 cmc, which is obviQusly connected with the higher energy of the interface and the presence of an electrolyte in the solution. In many investigations it has been shown that the adsorption of organic substances a t interfaces in the case of a noncondensed state of the adsorption layer is well described by an isotherm of the Flory-Huggins type:13-15 BC
e exp(-2na0)
n(1-e)" This isotherm can be obtained by a statistical analysis using the mean field approximation of the simultaneous adsorption of water and SAA molecule^.^^ Here one molecule of water occupies one elementary cell and that of SAA, n such cells at the interface. Equation 1 turns into the isotherm of Frumkin if n = 1. B is the equilibrium constant of adsorption, 8 the surface coverage of the electrode by SAA molecules, and a an effective constant, revealing the interaction between adsorbed SAA molecules. In order to estimate the adsorption parameters of CTBPB we have applied in this work the method developed earlier.gJ0 For this purpose we have created a sample $1 consisting of some of the experimental differential capacity values illustrated in Figure 1for different c and cp. We have restricted the interval of CTBPB concentrations and have included in SI only data for c lower than 10-5mol.dm-3, since in this case the adsorption layer is in a noncondensed state and the isotherm of FloryHuggins can be applied. The potentials in the sample ranged from -1.2 to 0.05 V (SCE), and the nonequilibrium capacity values of the maxima were not taken into consideration. Thus the experimental sample s1contained 41 capacity measurements. Further in this work electrode potentials E in the rational potential scale are applied, Le., vs the potential of zero charge in pure supporting electrolyte solution, which is -0.43 V (SCE). The mean square of the experimental error SE2was calculated as a result of repeating the experiment for 15 different values of electrode potential and surfactant concentration c and was found to be 0.836. The analytical model of differential capacity based on the isotherm of Flory-Huggins was derived earlier.lO For the influence of the electric field on the adsorption of SAA the following equation was obtained
- 0.5C'E2 + C'EEN Rmm in a way analogous to the one described earlier.16 Here BO (12) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. (13) Flory, P.J. J. Chem. Phys. 1941, 10, 51. (14) Huggins, M. L. J. Phys. Chem. 1942, 46, 151. (15) Nikitas, P.J . Chem. Soc., Faraday Trans. 1 1984, BO, 3315. (16) Hansen, R. S.; Minturn, R. E.; Hickeon, D. A. J. Phys. Chem. 1966,60, 1185.
= [~o.a;on(O.5)~-~ exp(na)]-' refers to B at E = 0, COis the differential capacity in the supporting electrolyte solution without a surfactant, C' is the double-layer capacity when the surface is entirely covered by SAA molecules (0 = l), rmis the maximum quantity of adsorbed substance per unit area of the electrode, ENis the shift of pzc caused by the oriented adsorption of CTBPB when 0 = 1,and c0.5;o is the bulk surfactant concentration corresponding to halfcoverage at E = 0. The dependence of differential capacity on t9 and E was obtainedlO by applying the physical idea about the existence of two parallel condensors at the interface
c = c,(i - e) + c'e +
where e is a normally distributed random variable, corresponding to the experimental error. Thus the analytical model of differential capacity consisted of eqs 1-3 and contained six unknown parama,and n. Using eqs 1 and 2 for eters: CO.S;O, C', EN,rm, a given set of arbitrarily chosen initial values of the unknown parameters, one can calculate the dependence of 6 on E and c. The insertion of 8 into eq 3 allows the description of the differential capacity as a function of c and E. By application of the nonlinear regression analysis and the method proposed earlier? it was shown in this work that the model based on the isotherm of Flory-Huggins adequately describes $1. This conclusionwas reached when the mean square error SM2computed for the model, using the predicted values of capacity and the experimental results, was compared to SE2by applying the statistical F criterion:17
F = S,,2/S: (4) For the applied model it was found that F = 1.109/0.836 = 1.33. The tabulated value of F0.01 for significance level 0.01 and degrees of freedom 35 and 15 is 3.12." The adequacy tosl of the model based on the isotherm of FloryHuggins is proved by the fact that F is smaller than F0.01. The estimates of the adsorption parameters obtained in this work are given in Table 1. As seen from the table, the concentration c0.5;0, for which the electrode surface is half-covered when E = 0, is rather low, which shows the very high surface activity of CTBPB. The latter is illustrated by the value obtained for the standard Gibbs energy of adsorption AGO' when E = 0 and the surface coverage, respectively a,tend to zero. In this case
Boo = [co,;on(0.5)n-1 1-1 (5) and AGoO = -RT In (55.5B0°) = -88 kJ-mol-'. In these equations R is the gas constant and Tis the absolute temperature. The standard Gibbs energy of adsorption AGO for E = 0, given by the equation AGO = -RT In (55.5BO)
(6)
is also high-63 kJ.mol-'. The value obtained for C' is typical for the adsorption of aliphatic alcohols, which confirms the idea that at E = 0 CTBPB is adsorbed with its hydrocarbon chain oriented toward the electrode surface. The shift of the potential (17) Draper, N. R.;Smith,H. AppliedRegression Analysis;John Wiley and Sons: New York, 1981 (translated in Russian: Moscow, 1987;p 272).
Langmuir, Vol. 7, No. 10,1991 2383
n-HexadecyltributylphosphoniumBromide Adsorption
f4 i iz
/
I +
,5
0
-45
-2
E/V
Figure 4. Electrode charge density as a function of potential E for a 1o-B mobdm-3 solution of CTBPB.
-4
?/,~~.cm-t
Figure 5. Dependence of the charge density of specifically adsorbed CTBP cations on electrode charge density for a 1o-B mol-dm-3 solution of CTBPB.
Table I. Estimates of Parameters Characterizing the Adsorption of CTBPB at the Mercury Electrode/Solution
Interface and Their Standard Deviations _..
1 A0.5
3.4 A0.6
0.47 AO.10
1.8 *0.2
1.0
10
k0.3
f4
of zero charge EN for the positively charged CTBP ions is positive, as should be expected. The maximum adsorbed quantity rmcorresponds to an area per adsorbed CTBP ion S = 90 A2, which shows that the radius of the tributylphosphonium head group in the adsorption monolayer is 0.54 nm, Le., the latter is rather densely packed. This result agrees with the calculations of Robinson and Stokes,18who on the basis of geometrical considerations obtained for the radius of the unhydrated tetrabutylammonium cation, supposing that the ion is spherical, the value 0.49 nm. In accordance with the dimensions of the adsorbed particle n is rather high and the value obtained for a indicates that attraction between hydrocarbon chains predominates. As a result of the quantitative investigations of CTBPB adsorption on mercury described above, it became possible to calculate the potential of the outer Helmholtz plane \k, of the electrical double layer for different electrode potentials E. To that purpose the equation of the GouyChapman theory \k, =
(mT/F)arcah (-qd/2AdG)
I
45
0
-45
Elv
Figure 6. Sum of electrode charge densityand the charge density of specifically adsorbed CTBP ions as a function of E for a 1o-B mol~dm-~ solution of CTBPB.
'I 3-
(7)
was used. Here F is the Faraday constant, -qd = q + qi is the charge density of the diffuse double layer, q is the electrode charge density, qi = -I'F is the charge density of specifically adsorbed CTBPB cations, r = Orm,c,1 is the concentration of the supporting electrolyte, and A = (2tt&")1/2. Since the bulk concentration of CTBP ions was several orders of magnitude lower than that of the ions of the supporting electrolyte solution, only ce1 was taken into consideration in eq 7. to is the permittivity of vacuum and t is the relative permittivity of the medium. On the basis of the adequate model of differential capacity and the statistical estimation of adsorption -~ parameters, B was calculated for lo* m ~ l - d m CTBPB solution. Thus, r = el', and correspondingly qi = -ITm were found. q was obtained by numerical integration of differential capacity results for 10-6 moledm-3 CTBPB solution. Integration started from the potential of zero charge of the electrode E = 0.47 V, which corresponds to (18) Robinson, R.; Stokes,R. Electrolyte Solutions; Butterworths: London, 1955.
Figure 7. Dependence of the outer Helmholtz plane potential on electrode potential E for 104 mol-dm-3 CTBPB solution. cp = 0.04 V (SCE). The value for the pzc in the presence of surfactant was taken to be approximately equal to EN from Table I. Results are illustrated in Figure 4 and are of the order of several microcoulombs per centimeter squared negative charge density for potentials from E = 0.5 to -0.5 V. The value of pzc in these calculations does not strongly influence the results for q. So, for example, if pzc were 0.25 V, q would vary from 2 to -5 pC.cm-2 in the same interval of potentials.
2384 Langmuir, Vol. 7, No. 10, 1991
As seen in Figure 5, where the dependence of qi on q is shown, and in Figure 6, in which the sum of qi and q is illustrated as a function of E, a superequivalentadsorption of CTBP cations is observed because of the high surface activity of the latter. The charge density of the inner Helmholtz plane qi is of the order of 16 pC.cm-2 and is much greater in absolute value than the charge density of the mercury surface q for both negative and positive polarizations. Applying eq 7,9, as a function of E was calculated, as illustrated in Figure 7. When these calculations were performed, the idea4 was taken into consideration that only 80% of the charge (Si + q ) is neutralized by ions in the diffuse part of the double layer and 20% of this charge
Kaisheva et al.
is neutralized by counterions (in the present case F) entering into the inner part of the double layer. Even if this assumptionwere not accepted,the resulta for q,,would only change with about 10 mV in comparison with those illustrated in Figure 7. As seen in Figure 7, for both negative and positive electrode potentials E, \k, is always positive and is almost independent of E. This result could be expected, bearing in mind the very high surface activity of the positively charged CTBP ion and ita superequivalent adsorption at the studied interface. Registry No. CTBPB, 14937-45-2; Hg,7439-97-6.
