Langmuir 1989, 5, 1123-1126 a on the surface, the boundary conditions are
dY y, dx
+
0 as x
1123
L j s y ods = 2 In s o
+ (1 + s2/4)'l2
+
I:
- -[(I
+
4irea ~ K k for T x = KR
dY _ --s=--
dx where R is the radius of the cylinder or sphere. Integrating eq A2 with respect to y, we obtain (f)=4sinh2(a)-
Y2m -dy dy x dx
where "In [ ( l + z)/2] dz = Dl[ In
The planar limit m = 0 is easily solved to yield dY dx
- = -2 sinh
x - KR = 1 In
2
22-
(i)
(A151 where D l ( x ) is the Debye function7
(-)(e) z + l z-1
zo+l
where z = cosh (y/2) and zo =cash (yOl21, yo = e\ko/kT, and \ko is the surface potential. We wish to solve eq A6 for large KR. We note that except for very small y (exponentially small in K R ) x = KR. Therefore, to first order in 1/&,using this approximation and eq A7, we have
To obtain the next order approximation, we use eq A8 and A9, which yield
(2)' .
=
.
8m 4(z2 - 1) + -(z - 1) KR
+ 8m(m -
(KR)~
ln
(F) (Ai01
Dl(x) =
1
" t dt
et - 1
For the inside of a cylinder or sphere we can still apply eq A2. However, the boundary conditions are modified. Assuming charge neutrality, we have dY y, - finite as x dx
-
0
For large KR, y is exponentially small at x = 0. Therefore, we will still obtain to the required accuracy eq A6. The only difference is that dyldx and therefore x - KR will now have the opposite sign. Consequently, the expressions for yo and gelwill be the same but with R replaced by -R. (7) Abramowitz, M.; Stegun, I. A. Handbook of Mathematical Functions; National Bureau of Standards, U S . Dept. of Commerce, 1970; p 998.
Applying this equation at the surface (x = K R )where z = zo and dyldx = -s yields a relation between s and zo or yo. It is straightforward to invert this equation to obtain (to leading orders in ~ I K R ) Z"
(T)] l+x
1
= (1 + s2/4)'/2 -
Preparation of Monolayers and Stacked Layers of 1-Octadecanethiol A. Itaya,' M. Van der Auweraer,* and F. C. De Schryver
( K R ) ~+(~~' / 4 ) ' / ~In
[
1
4m(m - 1) s(~R)'(l+ s2/4)'l2 In
[
1
m(m - 1)
+ (1 + s2/4)'/2 2
or
Department of Chemistry, K. U. Leuuen, Celestijnenlaan 200 F, B-3030 Heverlee, Belgium
1
(All)
I
(-412)
Yo =
+ (1 + s2/4)'/' 2
The electrostatic free energy per unit area is
Received August 12, 1988. I n Final Form: February 8, 1989
Introduction It has been reported that stable monolayers of long n-alkyl alcohols (R-OH) are formed on a water surface and can be deposited on substrates.',' Although monolayers of n-alkanethiols (R-SH) adsorbed on gold from dilute solution have recently been reported: there are few reports concerning stable monolayers of thiols on a water surface. Sobotka and Rosenberg reported that 1-octadecanethiol did not form stable monolayers on a water surface by itself but that a mixture of 1-octadecanethiol and stearic acid formed stable mixed monolayers. They furthermore suggested that the thiol was air-oxidized to the disulfide in mixed layers on a water ~ u r f a c e . ~ Livingstone and Swingley also reported that 1-octadecanethiol did not form a stable film on an aqueous 0.01 mol L-' solution of alu*Author to whom all correspondence should be addressed. +On leave from the Kyoto Institute of Technology.
where 0743-7463/89/2405-ll23$01.50/0
0 1989 American Chemical Society
1124 Langmuir, Vol. 5, No. 4 , 1989
1
Notes Table I. Relative Decrease per Minute of the Area of the Layers of 1-Octadecanethiol under Constant Surface Pressure surface pressure, relative area surface pressure, relative area mN/m decrease, % mN/m decrease, % 0.042O 16.9 11.1 0.261b 26.2 0.083O 20.6 O.40gb 34.1 0.201" 27.3 O.78gb
LO
E
-. z E I
a,
a Concentration of CdC12, 5 X lo4 mol L-*; pH 7.5;temp, 21 "C. "Concentration of BaCl,, 5 X mol L-I; pH 7.5;temp, 21 "C.
L
3 in
VI
: 20 a 01 U
m
. I L -
3
m
0
10
20 8
2
A r e a per m o l e c u l e ( A ) Figure 1. Surface pressure-area isotherms for 1-octadecanethiol (1and 2) and 1-octadecyldisulfife (3). 1 and 3, CdCl,; 2, BaCl,; mol L-l; pH 7.5; temp, 21 "C. concentration, 5 x
minum, lead, or zinc salts. However, when it was spread on an aqueous 0.01 mol L-' solution of potassium permanganate at pH between 2 and 14 and at a temperature between 0 and 25 "C, oxidation to 1-octadecanesulfonic acid occurred. They suggested that monolayers of the sulfonic acid are formed with an area per molecule of ca. 33 A 2 . 5 In this paper, we report on the formation of stable monolayers of 1-octadecanethiol on an aqueous solution of BaC12. Increasing the surface pressure or using an aqueous solution of CdClz as a subphase leads to the formation of stable stacked layers that can be deposited on substrates.
Experimental Section The synthesis of 1-octadecanethiolwas accomplished according to a procedure reported in the literature? 1-Octadecyldisulfide was obtained as a byproduct of the reaction. These products were purified by column chromatography on silica gel using a mixture of n-hexane and toluene as eluent. Surface pressure-area ( P A ) isotherms were measured by using a circular Langmuir trough (Mayer Feintechnik). The subphase was pure water from a Milli-Q (MilliporeIntertech) system to which BaCl, or CdClzwas added. The pH of the subphase was controlled by addition of an aqueous solution of NaOH or HC1. Chloroform (Fluka, p.a.) was used as a spreading solvent. Results and Discussion A typical r A isotherm of 1-octadecanethiol spread on an aqueous CdC12 solution (5 X mol L-') with pH 7.5 at 21 "C is shown in Figure 1. The Cd2+ion is known to give excellent structural stability to monolayers of long straight-chain fatty acids. The area per molecule obtained by extrapolating the rising part to T = 0 (limiting area) is 9 f 0.5 A2. This value is much less than the cross section perpendicular to the long axis of the n-alkyl chain obtained (1)Nutting, G. C.; Harkins, W. D. J . Am. Chem. SOC.1939,61,1180. ( 2 ) Shiozawa, T.;Fukuda, K. Nippon Kagaku Kaishi, 1987,2076. ( 3 ) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SoC. 1987,109,3559. (4) Sobotka, H.; Rosenberg, S. Monomolecular Layers; Sobotka, H., Ed.; Amer. Ass. Advan. Sci.: Washington, DC, 1954; p 175. (5)Livingston, H.K.;Swingley, C. S. J. Colloid Interface Sci. 1972, 38,643. (6) Vogel, A. I. A Text-book of Practical Organic Chemistry, 3rd ed.; Longmans, Green and Co. Ltd.: London, 1956;p 497.
2 00
250 300 Wavelength (nm)
350
Figure 2. Absorption spectra of films of 1-octadecanethiol(- - -) and 1-octadecyldisulfide (-). The films were deposited at 11.2 mol L-' CdC1, and pH 7.5. mN/m, 5 X for n-alkyl alcohols (ca. 21 A2) or long-chain carboxylic acids.' This indicates that the film formed is no monolayer. However, as is shown in Table I, the layers were quite stable under a surface pressure of ca. 17, 26, and 34 mN/m. When the conditions for the preparation of the monolayer were changed (pH 6.2 and 8.2, concentration of CdC12 5 X mol L-', temperature 33 and 40 "C), similar isotherms were obtained. The structure formed on the water surface could be deposited successfully on hydrophilic glass plates at 12 and 27 mN/m by an upward stroke ( deposition ratio respectively 0.9 f 0.1 and 1.0 h 0.1 ). The absorption spectrum of the 1-octadecanethiol film spread on CdC12and deposited at 11.2 mN/m has a maximum a t 265 nm where the optical density amounts of 0.046 f 0.003 (Figure 2). Taking in account the area per molecule determined from the T-A curve and assuming a random orientation of transition dipoles, this amounts to an extinction coefficient of 2.1 f 0.2 x IO4 L mol-' cm-'. This value can be compared with respectively 1.6 f 0.1 X L mol-' cm-l obtained for 10 L mol-' cm-' and 1.2 X respectively 1-octadecanethiol in isooctane and Cd(C104)2 in water a t 265 nm (Figure 3). This intense absorption band corresponds to the absorption band found in a cadmium cysteine complex' (Amu = 245 nm, emax = 1.0 X lo4
Langmuir, Vol. 5, No. 4, 1989 1125
Notes
\
I
2 00
i 25 0
,~
-i'
'\
I
'--I
3 00
350
Wavelength l n m )
Figure 3. Absorption spectra of 1-octadecyldisulfide (- - -) and 1-octadecanethiol(-1 in isooctane and of Cd(C104)2in water (-e).
L mol-' cm-') and a cadmium thioneid complex (Amm = 250 nm, em= = 1.4 X lo4 L mol-' cm-') and is probably due an S Cd charge-transfer t r a n ~ i t i o n . ~According to
-
Kagli! this transition will only be observed when the Cd2+ is coordinated with more than one sulfide group. The pressure area-curves and the stability of the layers formed on the water surface suggest the formation of stable stacked layers in equilibrium with the monolayer. The formation of stable stacked multilayers similar to those of 1-octadecanethiol was found for other systems such as metal-free phthalocyanine filmslo and squarylium dyes.'l Also, the equilibrium between a monolayer and a bulk phase12 could give rise to the observed a-A curves. The possibility that 1-octadecanethiol is air-oxidized to the disulfide and that stable layers of the disulfide are formed on the water surface can be excluded. As is shown in Figure 1, the isotherm of the disulfide is different from that of 1-octadecanethiol, and the limiting area is ca. 13 f 0.5 A2/molecule, which is different from twice the limiting area obtained for 1-octadecanethiol. Moreover, the absorption spectrum of stacked layers of 1-octadecyl disulfide, deposited a t 11.2 mN/m from a 5 X CdC1, solution, is characterized by a maximum at 220 nm. Assuming a random orientation of the transition dipoles in the monlayer, the extinction coefficient amounts to 2.4 f (7) Drum, D. E.; Vallee, B. L. Biochem. Biophys. Res. Commun. 1970, 41, 33. ( 8 ) Kagli, J. H. R.; La Vallee, B. L. J. Biol. Chem. 1961, 236, 2435. (9) Swenson, D.; Baenziger, N. C.; Coucouvanis,D. J . Am. Chem. SOC. 1978, 100, 1932 (10) Baker, S.; Petty, M. C.; Roberta, C. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (11) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 159, 337
(12) Harvey, N.; Rose, P.; Porter, N. A.; Huff, J. B.; Arnett, E. A. J. Am. Chem. SOC.1988, 110, 4395.
0.3 X lo4 L mol-' cm-l at 220 nm (Figure 2). The extinction coefficients of 1-octadecanethiol and 1-octadecyl disulfide in isooctane amount to respectively 163 f 10 and 255 L mol-' cm-l a t 230 nm (Figure 3). The data obtained for the film of 1-octadecanethiol deposited from the CdClz subphase are quite different from those obtained for 1octadecyl disulfide deposited from the same subphase. These results and the excellent stability of the stacked layer of 1-octadecanethiol on a water surface indicate that air oxidation of 1-octadecanethiolto the disulfide does not occur. As is shown in Figure 1, a a-A isotherm of l-octadecanethiol spread on an aqueous BaC12 solution (5 X lob4 mol L-l) with pH 7.5 at 21 OC is remarkably different from that of 1-octadecanethiol spread on an aqueous CdC12 solution. It shows a plateau at a surface pressure of ca. 15 mN/m. The limiting area in the low surface pressure region below 15 mN/m is ca. 17 f 1 A2. This value is somewhat smaller than the value obtained for n-alkyl alcohols and is consistent with the cross sectional area perpendicular to the long axis of the n-alkyl chain. Eventual differences could be due to the incorporation of some of the octadecanethiol molecules in a condensed phase when the monolayer was spread. This indicates that a stable monolayer of 1-octadecanethiol can be formed on a water surface at low surface pressures up to ca. 15 mN/m. The limiting area at high surface pressures (more than ca. 20 mN/m) is ca. 5 f 0.5 A2. Therefore, the plateau is considered to be caused by either the transition from a monolayer to stacked layers, the dissolution of the thiol into the subphase, or the formation of a bulk phase. Films were deposited successfully with a deposition ratio of 1.0 f 0.1 on hydrophilic glass plates a t an upward stroke at 10.3 mN/m. At 20.6 mN/m, deposition occurred with a deposition ratio of 0.5 f 0.1. No deposition of the film occurred on hydrophobic glass plates at a downward stroke a t 10 mN/m, but deposition occurred at 20 mN/m to a certain extent (deposition ratio 0.3 f 0.1). The optical density of 1-octadecanethiol deposited at 10 and 20 mN/m is below 0.005 at all wavelengths up to 250 nm. As the transmission of the reference already becomes wavelength dependent in this range, it would be dangerous to obtain an absorption spectrum or an extinction coefficient from these data. However, one can assume that the extinction coefficient will always be below respectively 5000 and 1500 L mor1 down to 250 nm. As the extinction coefficient of this film is about 1 order of magnitude smaller than that of the 1-octadecyl disulfide film, the absorption spectrum indicates that the sulfur atoms are not involved in a disulfide bond. The pressure-area curve indicates that a monolayer film is formed below 15 mN/m and that a t a higher surface pressure a fraction of thiol molecules is squeezed out from the monolayer to form a stable stacked layer. Although it is likely that the film deposited a t 10 mN/m is a monolayer, it is not possible to draw any conclusions on the nature of the film deposited at 20 mN/m. For mixed layers of a squarylium dye and eicosanoic acid, a similar behavior was found.13 Here, a mixed monolayer was formed a t low surface pressure, and the squarylium dye was squeezed out from the monolayer at high surface pressure to form a stable multilayer. The difference between the absorption spectra obtained for films of 1-octadecanethiol deposited from respectively a BaC1, and a CdClz solution indicate that Cd2+ions are incorporated in the film deposited from the CdClz solution. As neither the film deposited from the BaC12solution nor (13) Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Onda, K.; Manda, E. J. Am. Chem. SOC.1985, 107, 5270.
1126 Langmuir, Vol. 5, No. 4, 1989 Table 11. Stability Constants of Cadmium ComelexesuJ6 K,,O mol L-l K,,*mol L-' CHBCOO2.0 x 10' 1.0 x 10' SH4.0 X lo7 1.0 x 107
----I_____
" K , = [ML-]/([M2+][L-]).* K 2 = [ML,]/([ML+][L-]).
the solution of 1-octadecanethiol or of Cd(C104)2give any indication of an intense absorption band around 265 nm, this band must be due to the presence of a cadmium sulfur bond. When a concentrated CHC1, solution of the thiol (1.4 X mol L-') was applied to an aqueous subphase containing 5 X mol L-' CdC12 at pH 7.5, very thin crystalline-like plates were formed on the water surface. This did not occur when BaClz was used instead of CdCl,. This fact, as well as the difference in the P A isotherms, indicate that (C18H37S)2Cd has a much higher tendency to form microcrystalls or stacked layers than (ClaH37S)2Ba.This tendency to form microcrystalls forced us to use very diluted solutions for spreading and to start the compression a t a large area per molecule. This had as a consequence that t1.3 total area of the compressed film was rather low, leading to a large relative error on the area per molecule in the compressed film. 1-Octadecanol and stearic acid form stable monolayers on a water surface.lS2 On the other hand, 1-octadecanethiol does not form stable monolayers except under special conditions (aqueous BaClzsubphase and a surface pressure of 15 mN/m). Ionic radii of S-, 0-,Cd2+,and Ba2+are 2.19, 1.76, 0.97, and 1.35 A, respectively. In the case of the monolayers of long straight-chain fatty acids, the distance between two neighboring chains amounts to 4.8-4.9 A.l4*vb In multilayers of lead stereate,15 an orthorombic packing with in-plane lattice constants of 7.38 and 4.96 leads, for two molecules per unit cell, to a distance of 4.44 8, between the backbones of the alkyl chains. Starting from this orthorombic packing,16 one can, assuming a side-on (bidentate) coordination of the carboxylate groups17 and using a value of 2.24 Ala for the distance between the oxygen atoms in a carboxylate, find a value of 3.50 A for the closest distance between the oxygens atoms of the carboxylate atoms of two neighboring chains. For freely rotating chains, the distance between the oxygens of two neighboring chains would vary between 2.20 and 4.44 A for a distance of 4.44 A between the backbones. Assuming an end-on coordination (monodentate), this distance will amount to 4.44 A, the distance between the backbones of the alkyl chains. Therefore, a good coordination of the Cd2+ion by the oxygens of two neighboring ionized carboxylic groups is possible. On the other hand, if the thiol forms monolayers,the distance between neighboring sulfur atoms is ca. 4.54 A,I9 and a good coordination of Cd2+by two neighboring ClaH37S-groups is not possible. Although the formation of clusters of thioglycolate and cadmium ions, where the cadmium is in a tetrahedral coordination (Cd-S equals 2.51 Azo or 2.69 has been reported, the distance between neighboring sulfide ions equals only 3.07, 3.73, or 4.05 These data indicate that the distance (14)(a) Schlotter, N. F.; Porter, M. D.; Bright T. B.; Allara D. L. Chem. Phys. Lett. 1986,132,93.(b) Vogel, V.; Woll, C. J.Chem. Phys. 1986,84,5200. (15)Prakash, M.;Ketterson, J. B.; Dutta, P. Thin Solid Films 1985, 134,1. (16)Naselli, J. F.;Rabolt, J. F.; Swalen, J. D. J.Chem. Phys. 1985,82, 2136. (17)Kimura, F.;Umemura J.; Takenaka, T. Langmuir 1986,2,96. (18)Speakman, J. C. Struct. Bonding 1972,12,141. (19)Strong, L.;Whitesides, M. Langmuir 1988,4,546. (20)Strickler, P.Chem. Commun. 1969,655. Helu. Chim. Acta 1974,57, 513. (21)Burgli, H.-B.
Notes
between neighboring sulfur atoms in the monolayer is too large to form a network of mercaptide bridges on top of a layer of cadmium ions. Therefore, although the interaction between the Cd2+ion and C1aH37S-groups remains possible, this interaction will not be able to bridge the cadmium ions in the same way as the carboxylate groups bridge the cadmium ions in cadmium arachidate monolayers. On the other hand, the larger stability constants of CdSH' and Cd(SH), compared to Cd(CH,COO)+ or Cd(CH3COO)2(Table 11) suggest that the bond between Cd2+ and (ClaH3,S-), is stronger than that between Cd2+and a carboxylate group. The lower electronegativity of sulfur compared to oxygen will lead to an increased covalent character of the sulfur-cadmium bond.22 p v d r backdonations will increase the covalent character of this bond. As the lateral distance between two sulfur atoms in the films is too large to coordinate the Cd2+ions, saturation of the valences of the Cd2+ion can only occur with sulfur atoms from oppositely oriented monolayers in a head-tohead configuration. This could explain the tendency to form stable stacked layers in the presence of CdC12. These stacked layers could also explain why the absorption spectrum of the deposited film of (ClaH37S)2Cdshows features characteristic for a Cd2+ion coordinated by more than one sulfide groupa7 Also, the complex between cadmium and thioglycolate has been reported to form infinite layem2' In this latter system, a spiro structure avoids the steric interactions between alkyl chains of the thioglycol and allows a closer approach of the sulfur atoms than in the stacked layers discussed in this paper. In analogy to the fact that a carboxylate group in a monolayer has a smaller tendency to bind to a Ba2+ ion than to a Cd2+ one could suppose that the thiolate group also has a smaller tendency to bind to a25126Ba2+ion than to a Cd2+ ion. This is confirmed by the fact that although complexes between cadmium and thiolate ions have been observed2' no experimental evidence for the formation of complexes between the barium ion and thiolates exists. Therefore, on a barium chloride subphase, the tendency of the barium ion to saturate its valences with sulfur atoms from an oppositely oriented monolayer in a head-to-head configuration will be too small to overcome the increased hydrophobic interactions. Furthermore, the larger ionic radius of Ba2+compared to Cd2+will lead to a better coordination of the Ba2+by neighboring thiolate groups of the same monolayer. Therefore, a stable monolayer is supposed to be formed on aqueous BaC1, solution a t a surface pressure below 15 mN/m.
Acknowledgment. This work was partly supported by a Grant-in-Aid from the International Scientific Research Program. Joint Research from the Japanese Ministry of Education, Science and Culture (63044082) was organized by Prof. H. Masuhara of the Kyoto Institute of Technology. M.V.d.A. is a research associate of FKFO. We thank the Belgian Ministry of Scientific Programming, the FKFO, and the ERO for financial support to the laboratory. Registry No. CaC12,10108-64-2;BaCl,, 10361-37-2;l-octadecanethiol, 2885-00-9;1-octadecyl disulfide, 2500-88-1. (22)Livingstone, S.E. Q.Reu. Chem. SOC.1965,19,386. (23)Irving, R. J.; Frenelius, W. C. J. Phys. Chem. 1956,60,1427. (24)Kobayashi, K.; Takaoka K.; Ochiai, S. Thin Solid Films 1988,159, 267. (25)Kuehn, C. G.;Isied, S. S. Prog. Inorg. Chem. 1980,27, 153. (26)SillBn, L. G.;Martell, A. E. In Stability Constants of Metal-Ion Complexes; The Chemical Society: London, 1964.
