112
R. J. MEAKINS, M. G. STEVENS, AND R. J. HUNTER
A Comparison of Triethyl- and Trimethylammonium Headgroups in the Adsorption of n-Alkyl Quaternary Ammonium Ions at the Mercury Surface
by R. J. Meakins,Q M. G. Stevens,lb and R. J. Hunterlb Division of Applied Physics, National Standards Laboratory, CSIRO,Sydney, Australia
(Received July 3, 1068)
A series of n-alkyltriethylammoniumcompounds with n-alkyl chains between 4 and 16 carbon atoms has been prepared and submitted to electrocapillary and differential capacitance measurements at a dropping mercury electrode in 1 N potassium chloride. The results indicate that these are more strongly adsorbed than the corresponding compounds with trimethylammonium headgroups. The ratios of the differential capacitances for the two types of compound, calculated from the thicknesses of the adsorbed layers, are in good agreement with the experimentalvalues.
Introduction
Experimental Section
Previous papers described interfacial tension and differential capacitance measurements of a dropping mercury electrode with n-alliyltrimethylammonium ions in 1 N sulfuric acid2a and 1 N potassium chloride.2b From the concentration dependence of the interfacial tension it was calculated that at favorable potentials and concentrations the compounds with the longest alkyl chains give adsorbed layers in which the quaternary ammonium ions are almost close packed. The large displacement of the electrocapillary maximum toward anodic potentials in each case indicated that the quaternary ammonium ions are adsorbed with the positively charged headgroups toward the mercury surface. This is in contrast to the aliphatic dipolar compounds, which are probably adsorbed with the polar group in the ele~trolyte.~ The adsorbed layer caused a large decrease in the differential capacitance, considered to be due mainly to hindrance of the approach of hydrated cations from the solution toward the metal surface. Other authors, working with adsorbed layers of long-chain aliphatic alcohols, have assumed either complete penetration4 or complete exclusion5of hydrated ions. It seemed likely that the hindering effect of the adsorbed layer would be increased by increasing the size of the headgroup, and this should therefore lead to a further reduction in the differential capacitance. A series of n-alkyltriethylammonium compounds was therefore prepared and submitted to electrocapillary and differential capacitance measurements a t a dropping mercury electrode in 1 N potassium chloride. The present paper compares the results for these compounds with the corresponding compounds with trimethylammonium hcadgroups.
The apparatus was similar to that used in previous work,2&,8 except that the counterelectrode was a platinum gauze cylinder of about 70 cm2. An automatic phase-sensitive device7 was used to determine the time interval from the birth of the mercury drop to bridge balance and also the total drop lifetime from which the interfacial tension was calculated. All measurements were made at a frequency of 1.57 1tHz. For the preparation of the n-alkyl quaternary ammonium compounds the n-alkyl bromides of highest available commercial purity were first distilled through a short column packed with stainless steel gauze cylinders, and the purity was checked by vapor-phase chromatography and mass spectrometry. The method of preparation of the n-aIkyltrimethylammonium bromides was similar to that previously described. To obtain the 12-alkyltriethylammonium bromides, the appropriate n-alkyl bromide was mixed with cxcess triethylamine in nitrobenzene solvent and the mixture was heated under reflux for 1 hr. Dry ether 'was then added gradually, with shaking, to give a deposit of fine
The Journal of Physical Chemistry
(1) (a) Division of Applied Physics, National Standards Laboratory, CSIRO, Sydney, Australia; (b) Department of Physical Chemistry, University of Sydney, Sydney, Australia. (2) (a) R. J. Meakins, J. A p p l . Chem., 15, 416 (1965); (b) R. J. Meakins, ibid., 17, 157 (1967). (3) J. O'M. Bockris, M. A. V. Devanathan, and K. Muller, Proc. Roy. SOC.,A274, 55 (1963). (4) M. A. V. Devanathan, ibid., A267, 256 (1962). (5) A. N. Frumkin and B. B. Damaskin, "Modern Aspects of Electrochemistry," 6 . O'M. Bocltris and B. E. Conway, Ed., Butterworth and Co. Ltd., London, 1964, p 149. (6) R. J. Meakins, Australasian Corrosion Eng., 11, 5 (1967). (7) J. B. Hayter, J. Electroanal. Chem., in press. (8) R. J. Meakins, J. AppZ. Chem., 13, 339 (1963).
A COMPARISON OF TRIETHYLAND TRIMETHYLAMMONIUM HEADGROUPS
+/- *-.$../ 'Or
I
I
I
'-0.2 - 0 . 4
I
I
J
1
l
l
l
-0.6 -0.6 -1.0 -1.2 -1.4 APPLIED POTENT1AL. VOLTS, S.C.E.
l
~
-1.b
l
-1,f
~
l
~
Figure 1. Electrocapillary and differential capacitance curves for a dropping mercury electrode in 1 N KC1 containing n-octyl quaternary ammonium chlorides. Comparison of the trimethylammonium and triethylammonium headgroups.
crystals. These were filtered off under dry conditions, washed several times with dry ether, and then warmed repeatedly in a vacuum desiccator to remove the last traces of nitrobenzene. Alternatively, the crystals were extracted with dry ether for about 10 hr in a Soxhlet apparatus. The n-alkyl quaternary ammonium bromides were converted to the corresponding chlorides by shaking in alcohol with two separate quantities of freshly prepared silver chloride. After filtration and evaporation of the solvent, the products were recrystallized from dry acetone-ether. The chlorides were more difficult to crystallize than the bromides. The compounds with the shorter alkyl chains, particularly the chlorides, are deliquescent and were therefore stored in desiccators over silica gel or phosphorus pentoxide.
Results and Discussion Figure 1 shows a comparison of the behavior of the triethylammonium and trimethylammonium headgroups in the 1%-octylquaternary ammonium compounds. The chlorides were used for these measurements because the more strongly adsorbed bromide ion would tend to mask the anodic desorption peaks, particularly at the higher concentrations. I n measurements with the longer chain compounds, the anion is less significant, since smaller concentrations
)
*.'
/
'
I
-02
,
I
-04
,
I
I
20'C
I
I
I
1
1
'
"
'
-06 -06 -io I2 1 4 APPLlED POTENTIAL, VBLTS, S C E
'
-I6
-18
Figure 2. Comparison of electrocapillary properties of n-dodecyltriethylammoiiium and n-dodecyltrimethylammonium bromides in 1 N KCl.
are required to give saturated adsorbed monolayers. Some results for the n-dodecyltrimethylammonium and n-dodecyltriethylammonium bromides are shown in Figure 2. I n all of these results the compounds with the triethylammonium headgroup give the smaller values of the cathodic interfacial tension, indicating that they are more strongly adsorbed than the corresponding n-alkyltrimethylammonium compounds. This conchsion is supported by the wider separation between the anodic and cathodic desorption regions in the differential capacitance curves, indicating greater persistence a t the mercury surface against the competition of other ions and water molecules. The latter point was investigated further by studying the behavior of each of the quaternary ammonium compounds at several diff erent concentrations. The results for n-hexyltriethylammonium chloride are illustrated in Figure 3. With increasing concentratioo the anodic and cathodic desorption peaks are both d b placed so as to move further apart, but the displacemend of the cathodic peak is always larger. This is presumably because at cathodic potentials the quaternary ammonium ions are competing merely with electror statically attracted potassium ions, whereas on tEre anodic side they compete with the "specifically adsorbed" chloride ions. The electrocapillary maxima (ecm) for these sohations were obtained from the corresponding interfacial Volume 79, Number 1 January 1969
114
R. J. MEAKINS, 84. G. STEVEXS, AND R. J. RUNTER 25Oc
Table I: Desorption Polentials Relative to the Ecm for Various Concentrations of n-l-Iexyltriethylammor~iiim Chloride, C~HI~N(C,H,),C~, a t a Dropping illerciiry Electrode in 1 iV KCl
Concn, 116
V
Anodic
-0.51 -0.45 -0.41 ,-0.38 -0.34 -0.31 -0.32
*,.
,-. *
-0.38 -0.34 -0.32 -0 28 -0 24 -0.23
-1.36 -1.43 -1.47 -1.64 -1.60 -1.62
0
8 . 9 X lo-' 4.5 X 8.9 X 3.2 X LO-' 6.4 X 8 . 9 X 10+
L
-
APPLIED i POTENTIAL, : YOLTS,? S . C-114 .E
-116
Desorption potential -(see), V--
Ecm (sce),
Cathodic
Desorption potential (relative t o w m ) , -----V---Anodic Cathodic
...
...
3-0.07 t0.07
-0.91 -1.04 -1.09 -1.20 -1.20 -1.30
+0.06 +O.OG 4-0.07
4-0.09
-ia
Figure 3. Differential capacitance curves for various coliceatrations of n-hexyltriebhylammonium chlorfde in 1 N KCI,
/ -5
-4
-3
I _
-2
-1
LOG COWCEITRhTIOY (HOLARI
tension curves by computation from isotensiori potentials. This method was first suggested by GrahaiizeD but bas not been widely used because of the extensive calculations involved. It is accurate t o about 1 niV and involves only the assumption that equal drop times at two different applied potentials correspond to equal values of the interfacial tcnsion. The results obtained by this method are shown in Table together with the observed desorption poientials and also the desorption potentials relative to the olecirocapillary maxima. Similar tables were prepared for each of the coinpounds investigated, and the values were then used to obtaip the relationship between the cathodic desorption potential and the logarithm of the concentraiion. The rcsutts are shown in Figure 4. In each case the rclationship is liriear, which is in agreement with the results for many dipolar compounds.10 The gradients are approximately equal for alkyl chains up to eight carbon &turns, but there is a sharp increase for the Clz and CM compounds, with Cl0 intermediate. The increase in slope for the longer alkyl chains may be due to the preserioe of a bimolecular layer of adsorbed ions.2s11 If the results for corresponding n-alkyltriethylainirlpniuin arid ~~-alkyltrimethylammoniuin compounds are compared, it is seen that at any payticular conJj
The Journal: of Physical Chenvistry
Figure 4. Concentyation depeiideilcc of the cathodic desorption potelitids for n-all~ylt~im~thyl~mmoliium and n-alliyltriethylammoiiiurn chlorides a t a droppiiig merci:ry electrode in 1 ,V KC1.
centration the former persist at tlae mercury surface to higher negative potenti als. This supporis Lhe above conclusion that the ~~-al!~yltrietl~ylan~monium ions are the more strongly adsorbed. The main factors lcading to this stronger adsorption would be: (a) the larger hulk of nonpolar material in the head group which mould favor expulsion from the aqueous phasc; and (b) the strongcr van dcr Waals iorces of cohesion between the triethylammonium headgroups. Other factors favoring or opposing adsorption, e.g., the van der Waals attraction between the alkyl chains, the mutual repulsion hetween the positive charges of the headgroups, and the competition by other ions and water molecules for the mercury surface, would be similar for both types of compound at any particular surface concentration and potential. (9) D. C. Graharne, R.P. Larsen, and M. A. Poth, 1.A n w . C ~ C ? I Z . Soc., 76, 4819 (1954).
(10) W. Lorenr, and F. Mockel, Z.Eleklrochcm., 60, 507 (1956). (11) J. T. Davies and E. K. Rideill, "Interfncinl Plienon~e~l:~," Academic Press, New Yorlr, N. Y.. lSGl,p 437.
115
A COUPliRISON O F TRIETHYLAND TRIMETHYLAMMONIUM HEADGROUPS A further point from Figure 3 is that the heights of the desorption peaks for the n-hexyltriethylammonium ions increase with increasing concentration. This was also observed with the C, and C8 compounds. From the work of Frumkin and Damasliin6 it appears that the relationship between the peak height and the logarithm of thc concentration should be linear, but in the present work this is approached only in the compounds with the shorter alkyl chains at the highest measured concentrations. At lower concentrations, particularly with the longer chain compounds, the adsorption-desorption process is not able to keep in step with the applied measuring frequency, and hence the true value of the peak capacitance is not obtained. I n agreement with this explanation it was found that some measurements with the 71-octyl compound at a lower frequency gave higher desorption peaks. For further discussion of these results it is useful to determine the maximum surface concentrations of the various n-alkyl quaternary ammonium ions. These were calculated, as previously describedJZafrom the dependence of thc interfacial tension, y, on the bulk concentration, c, using the Gibbs adsorption equation. The appropriate experimental relationships between y and log c are shown in Figure 5, the applied potentials being chosen as those giving the maximum values of the differential capacitance of each compound. The chlorides were used in all cases except n-dodecyltrimethylammonium, which was added as the bromide. The surface concentrations calculated from these results are shown in Table 11, together with the derived values of the surface areas per adsorbed ion and the excess surface charges due to the adsorbed layers. The values for the n-alkyltrimethylammonium com-
Table I1 : lfaximiim Swface Concentrations of n-Alkyl Quaternary Ammonilim Ions with Trimethylammonium and Triethylammoninm Headgroups Max surface concn of the
No. of carhon atoms in n-alkyl chain
adsorbed ion, mol/cmz X 10-10
Trimethylammonium n-But yln-0 c t y1 n-Dodecyln-Tetradecyl-
4 8 12 14
1.70 2.29 2.93 5.63
98 73 57 30
16 22 28 54
Triethylammonium n-Biit yln-Hexyln-Oct,vln-Decyln-Dodecyln-Tet,radecyl-
4 6 8 10 12 14
2.46 2.29 2.39 2.46 3.08 4.22
68 73 70 68 54 39
24 22 23 24 30 41
n-.\lkyl qiiaternary ammonium ion
Surface area per adsorbed ion,
Excess surface charge, pC/cm*
4lOy
-4
-3
-8'
'
LOO CONCENTRATION IMOLIR)
Figure 5. Concentration dependence of the interfacial tension at a dyne in 1 N KC1 containing various n-alkyltrimethylammonium and n-alkyltriethylammollium chlorides. The applied potentials quoted are those giviiig the minimum differential capacitance.
pounds are similar to those previously obtained with the bromides in 1 N sulfuric acid.2a I n that work it was pointed out that the surface area requiredoby the trimethylammonium headgroup is about 40 A*, which is double the space required by the extended n-alkyl chain, From the surface areas shown in Table I1 it is evident that the adsorbed ions of the n-butyl-, n-oct yl-, and n-dodecyltrimethylammonium compounds are somewhat less than close packed.o The Cl,compound, on the other hand, has only 30 A2/adsorbed ion, suggesting the presence of at least a partial bimolecular layer. From scale moIecuIar models of the triethylammonium headgroup it is found that its surface projection can be approximately represented by an equilateral triangle with a side of about 12 A, giving a minimum possible surface area of 62 A t . The experimental values in Table 11 for the compounds with alkyl chains between four and ten carbon atoms are betmeen 6s and 73 A g , indicating that the headgroups are almost close packed. The lower values for the C12 and CU compounds suggest the presence of partial bimolecular layers. It is interesting now to compare the smallest values of the minimurn differential capacitance for compounds with the two types of headgroup. These results are given in Table 111, together with the corresponding concentrations and potentials. I n each case the minimum differential capacitance for the triethylammonium compound is smaller than for the trimethylammonium compound. This applies even where the surface concentrations are approximately equal, as, for example, with the CSand Clz compounds (see Table 11). I n considering a possible structure for the electrical double layer with adsorbed quaternary ammonium ions it seems unlikely that the hydrophobic alkyl chains would extend outward into the electrolyte. They would most likely be bent over and packed together to Volume 73,Number 1 January 1960
11G
R. J. MEAKINS, &/I.G.STEVENS, AND R. J. HUNTER
II*
%ble 111: Smallest Values of the Minimum Differential Capacitance for a Dropping Mercury Electrode in 1 N KC1 Containing n-Alkyltrimethylammonium and n-Alkyltriethylammonium Ions
n-Alkyl quaternary ammonium ion
Anion of compd used
Nil, i e . , 1 N KCI
Min differential capacitance,
Bulk concn giving min differential capacitance,
rF/cm
M
Applied potential giving min differential capacitance (see),
v
-1.10
16.2
solution Trimethylammonium n-Butyln-Octyln-Dodecyln-Te tradecyln-HexadocylTrie thylammoiiium n-Butyln-Hexyln-Oc tyln-Decyln-Dodecyln-Tetradecyln-Hexadecyl-
Chloride Chloride Bromide Chloride Bromide Bromide
10.6 8.6 7.7 7.8 7.9 7.4
Chloride Chloride Chloride Chloride Chloride Bromide Chloride Bromide
6.9 6.6 6.3 6.2 0.5 6.5 6.7 6.3
form a hydrocarbon layer above the headgroups, thus excluding water and also satisfying the van der Waals forces of attraction between the chains. The excess positive charge due to the quaternary ammonium ions would be balanced by chloride counterions, which, if dehydrated, could fit into spaces near the headgroups without having much effect on the thickness of the layer. The scale molecular models indicate that the thickness of the layer would be about 6.9 for t i e n-alkyltriinethylammonium ions and about 8.8 A for the it -nlliyltriethylnmmonium ions. A calculation from molar volumes and densities gives somewliat smaller tliicltnesses, but the ratio is approximately the same. At, these thicknesses the packing is almost perfect for an alkyl chain of ten carbon atoms, With shorter chains there would be some indentations and with longer chains there would be an excess of hydrocarbon, leading probably to a slight thickening of the layer. Figure G shows the proposed model of the electrical double-layer “capacitor,” with n-decyltriethylammoriium chloride as the “dielectric,” with the metal surface as one “plate,” and with the other “plate” consisting of the line through the centers of closest approach of the tirated potassium ions. .‘ic;surning that the relative permittivities are similar (01 both types of compound, the ratios of tho capacit n i i c r s should be equal to the inverse of the ratios of the total thicknesses; i.e., CTA~A/CTEA = 12.9/11.0 = 1.17, where C T U A and CTaA are the capacitances of the The Journal of Physical Chemistry
1 1 5 1
x x x x 1x
10-1 10-2 10-8 10-4 10-4 1.5 x 10-4
-1.00 -0.90 -0.80 -0.75 -0.75 -0.75
1 x 10-8 9 x 10-4 5 x 10-4
-1.05 -0.95 -0.85 -0.80 -0.75 -0.75 -0.75 -0.80
x x 5x 7x
8 5
6 X
10-5
10“ 10-6 10-6
LAYER At’ CLOSE-PACKED FDECYL CHAINS c
LAYER
b F TRIETHYLAMM.
3HEAD-FROUPS
Figure 6. Electrical double layer with specifically adsorbed n-decyltriethylammoiiit1m ions.
n-alkyltrirnethylammonium and the n-allryltriethylammonium compounds, respectively. Under the conditions where the model applies, the capacitance due to the adsorbed layer mill be approximately equal to the measured values of the minimum differential capacitance shown in Table 111. The
Table IV No. of carbon atoms in the n-alkyl chain
4 8 12 14 16
(CTMA)min/ (CTEA)rnin
1.54 1.37 1.18 1.16 1.17
117
PYROCHLOROPHYLL-SENSITIZED PHOTOREDUCTION OF NITROCOMPOUNDS ratios from these experimental values are given in Table IV. The larger values for the compounds of shorter chain length are probably due to imperfect packing in the ~~-alkyltrimethylammoniumlayer, leaving gaps which would be occupied mainly by water molecules, leading to an enhancement of the effective relative permittivity. With the longer chain compounds the agreement between the theoretical and experimental ratios is quite good.
Conclusions I n measurements with a dropping mercury electrode in 1 hr potassium chloride it is found that n-alkyltriethylammonium ions are adsorbed more strongly than the corresponding n-alkyltrimethylammonium ions. The ratios of the minimum values of the capacitnnc
