Langmuir 1995,11, 1715-1719
1715
Effects of Urea on Dioctadecyldimethylammonium Monolayers Silvia M. B. Souza, Hernan Chaimovich, and Mario J. Politi” Departamento de Bioquimica and Laboratbrio Interdepartamental de Cinktica Rapida, Instituto de Quimica, Universidade de Sdo Paulo, Caixa Postal 26077, Sdo Paulo, SP, 05599-970 Brazil Received May 3, 1994. I n Final Form: January 31, 1995@ Surface pressurelarea compression isotherms of dioctadecyldimethylammonium salts ((21-, Br-, F-, Ac-, Nos-, and OH-) with variable amounts of urea in the subphase were obtained. The parameters calculated from of the compressioncurves (e.g. phase transitions, compressibility,minimum area per head group) are strongly dependent on the nature of the counterion, indicating ion specificity. The minimum NOS- > OH-, F-, CH~COZ-.Urea caused film head group area increased in the order Br- > C1expansion, loss of defined phase transitions, and increase in the calculated minimum head group area. The ion specificityobserved without urea is lost upon increasingthe concentration of the additive suggesting the absence of a Stern layer at the interface. The effect of urea is interpreted in terms of urea association to the interface rather than as a general effect on the properties of the solvent. These results support both theoretical calculations and experimental results, indicating direct interaction of urea on amphiphile aggregates in which urea replaces water molecules from the interface.
Introduction Urea, a well-known protein denaturant,ladincreases the critical micelle concentration (cmc) of ionic and nonionic surfactants.2 The effects of urea on macromolecular conformation and supramolecular aggregate’s structure have been extensively explored, but molecular details remain a matter of controversy.l Two extreme forms of rationalizing the outcome of urea addition on the properties of several systems have been proposed. The additive may affect properties either indirectly, by affecting the solvent structure, o r conversely, the effects of urea can be explained by assuming a direct contact of the additive with the dissolved macromolecule or supramolecular a g ~ e g a t e .The ~ ~ indirect ~ mechanism is most widely accepted, since many experimental results support that the addition of urea to water strongly affects the solvent ~ t r u c t u r e . l -Increasing ~ experimental evidence, as well a s statistical mechanics simulations, however, show that urea can directly interact with hydrophobic groups and aggregate’s interface^.^-' It is of relevance, therefore, to examine the relative importance of solvent
* Author for correspondence. FAX, (55)(11)815 5579;E-mail,
[email protected] Abstract published in Advance A C S Abstracts, April 1, 1995. (1)(a) Frank, F. Water-A Comprehensive Treatise; Plenum Press: New York 1963.(b) Ruppley, J. A. J. Phys. Chem. 1964,68, 2002.(c) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press, London; 1969.(d) von Hippel, P: H.; Wong, K-Y. Science 1964,145,577.(e) Thayer, M. M.; Haltiwanger, R. C.; Allured, V. S.; Gill, S. C.; Gill, S. J. Biophys. Chem. 1993,46,165.(0 Sijpkes, A.H.; van de Kleut, G . J.; Gill, S. J. Biophys. Chem. 1993,46, 171.(g) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945,13,507. (2)(a)Burning, W.; Holzer, A. J. Am. Chem. SOC.1961,83,4865. (b) Mukejee, P.;Ray, A.J.Phys.Chem. 1963,67,190.(c) Piercy, J.; Jones, M. N.; Ibbotson, G. J. Colloid Interface Sci. 1971,37,165. (3) (a)Franks, H. S.; Franks, F. J. Chem. Phys. 1968,48,4746. (b) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. L. J.Am. Chem. SOC.1964,86,508. (4)(a) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1963,238,4074. (b) Roseman, M.; Jencks, W. P. J . Am. Chem. SOC.1975,97,631. (5)(a) Baglioni, P.; Ferroni, E.; Kevan, L. J. Phys.Chem. 1990,94, 4296. (b) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys.Chem. 1990,94,8218(c)Nakanishi, K. Chem. SOC.Rev. 1993,22, 177.(d) Cristinziano, P.; Lejl, F.; Amodeo, P.;Barone, G.; Barone, V. J. Chem. SOC.Faraday Trans. 1 1989,85,621.(e) Wen, N.; Brooker, M. H. J. Phys. Chem. 1993,97,8608. (0 Hoccart, X.;Turrel, G. J. Chem. Phys. 1993,99,8498. (6)Almeida, F. C. L.; Chaimovich, H.; Schreier, S. Langmuir 1994, 10, 1786. @
0743-746319512411-1715$09.00/0
changes and urea binding since the effect of the additive may result, at a molecular level, from both direct and indirect mechanisms. The study of micellar properties has contributed to our understanding of self-aggregation and has also served as a n model to analyze the effect of additives on several phenomena that are dependent on hydrophobic interactions. Urea not only increases the cmc but also increases the extent of ion dissociation (p) from ionic micelle^.^^^ The urea concentration dependence of the cmc and ,!3 are markedly different and, a t constant urea concentration, ,!3 changes with detergent concentration.8 The nature of the counterion strongly affects the cmc increments produced by urea.g It is not evident how these results can be accommodated by a purely indirect, solvent dependent change, produced by the additive. The determination of ion binding and ion selectivity in micelles and other amphiphile aggregates are very sensitive to the method used to differentiate between “free” and “bound” ion.1° In particular, the determination of the effects of a n additive on ion binding must take into account the effects of the additive on the method employed to differentiate between free and bound ion. This difficulty has been recognized when using urea as a n additive and conductance to determine the extent of ion dissociation.8 Monolayers provide a convenient system to investigate the effects of additives on ion binding, since the measured properties are independent of the determination of free and bound ions. The monolayer compression isotherms, measured on a n aqueous electrolyte subphase with or without additives, yield information about lateral interactions between the surfactants.ll Therefore, it is expected that ion selectivity and additive effects on ion binding will change the isotherm parameters. The compression (7)Duffy, E. M.; Kowalczyk, P. J.; Jorgensen, W. L. J. Am. Chem. SOC.1993,115,9271. ( 8 ) (a) Causi, S.; De Lisi, R.; Milioto, S.; Tirone, N. J.Phys. Chem. 1991,95,5664. (b) Caponetti, E.; Causi, S.; De Lisi; Floriano, M. A. R; Milioto, S., Tirone, R. J. Phys. Chem. 1992,96,4950. (9)Schick, M. J . J. Phys. Chem. 1964,68, 3585. (10)Morgan, J. D. P. The Kinetics and Selectivity of Ion Flotation, PhD Thesis, University of Sydney, Section of Physical and Theoretical Chemistry, 1994. (11)Roberts, G. Langmuir-BlodgettFiZms;PlenumPress:NewYork, 1990.
0 1995 American Chemical Society
Souza et al.
1716 Langmuir, Vol. 11, No. 5, 1995 isotherms of monolayers of dioctadecyldimethylammonium (DODA)are sensitive to the ionic composition of the subphase and change markedly with temperature.12 The extrapolated minimum area per head group of DODA monolayers decrease with saltlZcand depends on the nature of the counterion.lZa The shape of the compression isotherms are temperature dependent and monolayer phases are lost as the temperature increases.lZb Here we have studied the effects of ionic composition of the subphase on the compression isotherms of monolayers of DODA. Our results confirm and extend previous findings regarding the specific effects of monoanions on the monolayer properties.lZa We have determined the effects of urea on the properties of monolayers of dioctadecyldimethylammonium (DODA)(monovalent)salts (X). Urea addition to the subphase produced a n expansion of the area occupied by the head groups. In addition we demonstrated that the ion selectivity observed in the absence of urea is progressively lost upon addition of the additive. The data strongly suggest urea intercalation between surfactant head groups prevents the non-Coulombic, ion-selective head group-ion interactions.
Materials And Methods Dioctadecyldimethylammonium Bromide (DODABr,Eastman Kodak) was triply recrystallized from hot acetone:ethanol85:15 (v:v). Hoffman degradation followed by gas chromatography of DODAOH prepared from DODABr, demonstrated the presence of only CIS alkyl chains.13 Urea (Merck) was successively recrystallized from hot ethanol until the conductance of a -10 M solution was lower than -10 mS cm-l.14 Salts were analytical grade or better and stock solutions were titrated by standard quantitative methods. Water was glass bidistilled, deionized, and ultrafiltrered through a Milli-Q system (Millipore, resistivity t 18.2 MQ). Compression curves were obtained in a KSV 5000 (Finland) automatic Langmuir-Blodgett apparatus. The instrument was mounted in a dust free acrylic box, provided with a gas (Nz) ultrafiltration unit (Engefiltro, Brazil) for purging the system after monolayer deposition. Typically the surface of the aqueous subphase was cleaned by aspiration until compression showed no surface contamination. Subsequently, 50 p L of a solution of DODABr (1mg/mL) in C H C l h e x a n e 1:4 (v:v) was spread on the top of the aqueous phase using a microsyringe. The deposited DODABr film was temperature-equilibrated for about 45 min before compression a t 4 mm min-l. Subphase and monolayer temperatures were kept constant a t 15 0.1 "C (unless specifically stated) by a water circulating bath. Surface pressure (n)against monomer surface area (A) was automatically recorded. Different salts were added to the subphase a t 1mM final concentration. Given the dimensions of our trough (30.0 x 50.5 x 0.5 cm) the excess salt in the subphase is ca. 1000-fold, an excess sufficient to displace the added bromide from DODABr even with the ions that weakly bind to the tetraalkylammonium head group.lO Reliability of the instrumental setup was confirmed by examining monolayer compression and expansion curves of stearic acid (Sigma Chemical Co.). The minimum area per monomer obtained in our setup was identical to the literature value (25.5 A2/monomer).liJ5
*
Results The usual definition of surface pressure (ll)is nIY0-Y where yo and y are the surface tensions in the absence and in the presence of the monolayer, respectively. II was obtained directly from the output of the KSV (12)(a) Marra, J. J. Phys. Chem. 1986, 90, 2145. (b) Bonosi, F.; Gabrielli, G. Colloids Surf 1991, 52, 277. (c) Claesson, P.; Ribeiro, A. M. C.; Kurihara, K. J. Phys. Chem. 1989,93,917.
Table 1. Compressibility Modulus Ranges of Different Phases in a Monolaserl6 monolayer state (phase) clean surface gas (G) expanded liquid (L1) intermediate liquid (I) condensed liquid (Lz) condensed solid (S)
C,-l 0-12 12-50 50-100 100-250 1000-2000
monolayer setup. The monomer surface area (A) was obtained from the added monomer concentration and the through area. As the monolayer is compressed the state of the film changes from an ideal gas to a condensed solid. A very convenient form of phase classification, proposed by Davies and Rideal,lG employs the bidimensional compressibility (C,) given by
CS = - l/A(cWdII), C, can be obtained experimentally from the lT vs A curves. Our setup can calculate C, directly by computing the differential from the pressurelarea data stored in a microcomputer. The reciprocal of C,, the compressibility modulus (C,-l), has the dimensions of force per unit length and is therefore a measure of the monolayer compressional elasticity. The classification of monolayer phases as a function of the compressibility modulus range is presented here in order to facilitate data presentation (Table 1).16 Another parameter that has been shown to be essential for the comparison of several effects on monolayer compression isotherms as well as for the extrapolation of properties of monolayers to other aggregated amphiphile systems is the limiting area per molecule (Ao), i.e., the area, extrapolated to zero pressure under maximum packing conditions.ll A, was calculated by extrapolation with a computer program from the stored data from the linear portion of the compresion isotherm, obtained before film colapse. The isothermal compression curves for DODA monolayers with different added salts in the subphase can be considered as DODAX curves (where X is the added monovalent ion), since the amount of added salt vastly exceeds that of the amphiphile (see Materials and Methods). The compression curves obtained with different monovalent ions (Figure 1) are very similar to those previously obtained by MarralZaand extend the data to include the nitrate ion. The shape and parameters of the compression curves are strikingly different as X changes from acetate through bromide ion (Figure 1). The isotherms for ions such as Br-, C1- and Nos-, which strongly bind to micelles and vesicles prepared with the same amphiphile,1°J7 show clear changes in slope, suggesting the coexistence of several phases during compression. The isotherms for ions that bind weakly to the same amphiphile, e.g. OH-, F-, and Ac- are simpler and more expanded (Figure 1).18 Calculation of the isotherm parameters (Table 2) quantifies the monolayer properties (13) . Ribaldo, E. J.; Bonilha, J. B. S.; Politi, M. J.;Chaimovich, H.; Quina, F. H.; Bunton, C. A.; Petty, R. L.; Sartori, R.; Romsted, L. S. J. Colloid Interface Sci. 1984, 97, 115. (14) Politi, M. J.; Chaimovich, H. J. Sol. Chem. 1989, 18, 1055. (15)Abraham, B. M.; Ketterson, J. B.; Miyano, K.; Kueny, A. J. Chem. Phys. 1981, 75,3137. (16) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963. (17)(a)Abuin, E. B.; Lissi, E.; Araujo, P. S.; Aleixo, R. M. V.; Chaimovich, H.; Bianchi, N.; Miola, L.; Quina,F. H. J. Colloid Interface Sci. 1983,96,293. (b) Cuccovia, I. M., Chaimovich, H., Lissi, E.; Abuin, E. Langmuir 1990,6, 1601. (c) Bunton, C. A,; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991,24, 357. (18) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986,90, 1637.
Effects of Urea on DODA Monolayers I 50
-
40
-
30
-
.
E
I
k
,
I
.
'
I
Langmuir, Vol. 11, No. 5, 1995 1717 1
I
-NaAc
......., , . NaBr .. . - ...NaCl
\
The addition of urea to the subphase produces dramatic changes in the compression curves obtained with all ions analyzed. As a n example we present the result of adding various urea concentrations to DODACl (Figure 2). Two distinct effects are apparent upon inspection of the effect of increasing urea on the shape of the compression curves. As the urea concentration increases, the monolayer becomes more expanded a t any given pressure (Figure 1). It is also evident that some of the phases seen without urea are progressively lost upon increasing the concentration of the additive (Figure 1). These results are presented in a quantitative form in Table 3. We decided to compare the effect of urea on DODA monolayers prepared with different monovalent anions using a single, fixed urea concentration: 2.0 M. Table 4 presents the data collected from compression curves, obtained as described above for DODACl, for the anions analyzed in this work. A direct comparison of the effect of urea on the limiting area per head group (A,) of DODA monolayers with different counterions shows that the ion selectivity observed without urea is essentially lost upon addition of 2.0 M urea to the subphase (Table 5). The values of A, for the different ions are very similar, and the relative
I
-
-. ._NaOH
_ _ _ _ NaN03
.
2 . .E20
-
10
-
a
0 40
80
60
100
120
140
A (A' ) Figure 1. DODA monolayer compression isotherms (pressure (P)versus area per monomer (A)) for different electrolytes in the subphase.
shown in Figure 1. The A, of the DODA monomer decreased progressively as the affinity of the counterion for the tetraalkyl ammonium group increases. Similar results can be calculated from data obtained previously. 12a
Table 2. DODA Monolayer Parameters (A, ACS-l, Phase, n, and A,) in the Presence of 1 mM NaX (X-= NOS-, Br-, C1-, OH-, F-, and Ac-) at T = 16 "C
XNos-
Br-
c1-
OH-
FAc-
phase G L1 I L2 G L1 I L2 G L1 I L2 L1 I
G L1 I L1 I
AC8-l (mN m-l) 0-12 12-50 50-100 100-177.8 0-12 12-50 50-100 100-138.3 0-12 12-50 50-100 100-190.3 12-50 50-54.0 0-12 12-50 50-53.9 12-50 50-63.7
A (A2 molec-1) > 125.8 125.8-52.7 52.7-49.7 49.7-46.2 > 100.0 100.0-50.2 50.2-45.6 45.6-42.2 Z125.8 125.8-55.1 55.1-50.9 50.9-47.3 >57.6 57.6-54.8 > 126.40 126.4-71.5 71.5-54.7 > 76.0 76.0-58.4
n (mNm-1) x0.45 0.45-22.6 22.6-28.0 r28.0 xo.9 0.9-22.8 22.8-30.1 >30.1 31.4 x29.4 >29.4 x0.68 0.68-17.9 >17.9 x16.1 '16.1
A,, (Azmolec-l)
59.4
56.6
58.3 86.3 87.8 87.7
Table 3. Effect of Urea on DODACl Monolayer Compression Parameters urea (M) 0
0.015 0.1
phase G L1 I L2 G L1 I G L1 I L2
0.3
G L1 I
0.5
L1 I L1 I L2 L1 I L1 I L1
1
2 3 5
AC,-l(mN m-l) 0-12 12-50 50-100 100-190.3 0-12 12-50 50-102.9 0-12 12-50 50-100 100-103.1 0-12 12-50 50-90.9 12-50 50-70.2 12-50 50-100 100-126.7 12-50 50-57.4 12-50 50-52.3 12-48.3
A
(A2 molec-1)
> 125.8
125.8-55.1 55.1-50.9 50.9-47.3 120.1 120.1-63.1 63.1-48.0 > 133.1 133.1-58.1 58.1-51.9 51.9-50.9 > 133.9 133.9-61.9 61.9-52.6 264.1 64.1-52.9 137.0-53.6 59.6-49.6 49.6-46.1 142.5-69.8 69.8-57.7 >88.6 88.6-66.1 >78.1
(mN m-l) x0.7 0.7-25.7 25.7-31.4 >31.4 x0.45 0.45-21.0 21.0-43.2 x0.45 0.45-27.6 27.6-36.4 36.4-38.7 x0.45 0.45-29.6 29.6-42.1 x27.4 27.4-40.9 128.7 28.7-35.3 >35.3 x26.24 226.2 (18.5 18.5-35.5 x26.9
A, (A2 molec-l)
58.3 64.9
69.5 75.3 82.2 83.3 98.4 106.5 118.1
1718 Langmuir, Vol. 11, No. 5, 1995
Souza et al.
Table 4. DODA Monolayer Parameters (A, ACS-l, Phase, ll and A,) in the Presence of 1 mM NaX and 2 M Urea (X-= NOS-, Br-, OH-, F-,and Ac-)
XNOSBr-
OHFAc-
phase
AC,-l(mN m-l)
A (Azmolec-l)
I'I (mN m-l)
L1 I L1 I L1 I L1 I L1 I
12-50 50-63.1 12-50 50-56.6 12-50 50-61.8 12-50 50-52.8 12-50 50-61.8
271.1 71.1-57.8 292.5 92.5-75.4 '91.0 91.0-72.5 >81.6 81.6-69.4 291.0 91.0-70.8
20.4 14.5 123.5 '23.5 < 14.5 214.5
A, (Azmolec-l) 88.34 114.3 103.1 110.8 104.1
Table 5. Effect of 2 M Urea on the Limiting Area per Head Group (A, in k )of DODA Monolayers with Different Counterions Obtained at Several Temperatures 10 "C
15 "C
20 "C no urea urea
urea
no urea
urea
F-
no urea 80.35
108.1
BrAcNO3OH-
53.32 93.27 56.59 73.76
111.7 102.8 84.63 103.4
89.85 58.26 56.60 87.67 58.59 86.33
110.8 98.43 114.3 104.1 88.34 103.1
c1-
50
1
'
1
'
1
-
1
.
Urea 0.I015M Urea 0.1M ....... Urea 0.3M Urea 1 .OM -..-. ..Urea 2.OM Urea 5.OM
____ -
40
-
84.45 62.61 54.55 86.65 60.54 86.66
t 20
10
0 I
40
.
I
60
.
I
80
,
I
100
,
,
120
,
I
140
83.63 70.35 56.34 86.91 65.44 83.69
I
160
A(A?
Figure 2. DODACl monolayer compression isotherms (pressure ( P ) versus area per monomer (A)) for differnt urea concentrations in the subphase.
difference decreases even more as the temperature is increased (Table 5).
Discussion Surface pressure-area (ll-A) measurements can give valuable information about how amphiphile pack together, which in turn relates to the intermolecular forces involved. In particular, with charged amphiphiles ll-A functions strongly depend on the type of counterion, indicating that Coulombic as well a s specific interactions leading to a pronounced ion specificity are mayor contributors to the properties of the monolayer. The acetate, fluoride, and hydroxide DODA monolayers were essentially in the liquid expanded type phase, characterizing a relatively large electrostatic repulsion between the head groups. The bromide, chloride, and nitrate monolayers distinctly show transitions between liquid and condensed phases. Differences in ion association in micelles produce aggregates that differ in the degrees of partial ion coverage.1°J7 In the case of micelles of hexadecyltrimethylammonium hydroxide, it is likely that, a t low detergent concentration, micelles are highly d i s s o ~ i a t e d . ~This ~ J ~has led to the interpretation that in micelles of CTAOH a mayor part of the counterions li,e in a plane that is distant from the head groups by 2-4 A.17c
112.2 105.4 115.7 105.5 95.55 104.0
30 "C no urea
urea
89.60 80.69 64.56 91.89 75.25 90.62
118.4 109.5 112.7 112.1 100.3 107.4
Limited counterion insertion in a Stern layer in the monolayer could explain the increase in head group repulsion leading to the higher A,, values than observed for fluoride, hydroxide, and acetate DODA monolayers. From the values of A, for the different ions a relative affinity of Br- =- C1- NOa- > Ac- OH- F- can be deduced. This lyotropic series is typical of ion binding to both micelles and vesicles prepared with the same amphiphile type.1° The data obtained here are in good agreement with others obtained under similar conditions in the same system.12 The effect of urea on the monolayer properties is remarkable and can be detected a t urea concentrations as low as 0.1 M. Two phenomena are evident upon inspection of the curves obtained in the presence of urea. Firstly, the distinct phase transitions obtained using chloride as a counterion are progressively lost as the urea concentration increases. Furthermore the limiting area per monomer increases with urea concentration. As stated in the introduction, the effect of urea on the properties of proteins and aggregates has been attributed to either direct urea interactions or to changes in the properties of the s ~ l v e n t . However, ~,~~ the intermolecular forces determining the monolayer properties are sensitive to urea in the interface a t concentrations ofurea well below those that produce a major change in water properties.21 In 2 M urea, the A, for DODACl becomes similar to that obtained with fluoride ion in the absence of urea, and the ll-A curve shows essentially only a liquid-expanded phase. The DODACl monolayer, consequently behaves as if the counterion, in the presence of urea, is a weakly bound and the non-Coulombic,specific term has been lost. This finding implies that, in a monolayer with urea, ions are weakly bound and that binding is mainly Coulombic. The simplest explanation for this phenomena is that urea binds to the interface and that water dislocation from the interface causes a sufficient spatial displacement of the bound ion as not to allow specific interaction with the
-
30
E E
111.2 103.5 116.2 105.4 92.19 103.4
25 "C no urea urea
- -
(19). Chaimovich,H.; Cuccovia, I. M.; Bunton, C. A,; Moffatt, J. R. J . Phys. Chem. 1983,87,3584. (20) Finer, E. G.: Franks, F.; Tait. M. J. J. Am. Chem. SOC.1972.94. 4424. (21)(a) Isemura, T.; Hamaguchi, K.; Kawasato, H. Bull. Chem. SOC. Jpn. 1955, 38, 185. (b) Lo Nostro, P.; Gabrielli, G. Langmuir 1993, 9, 3132.
Effects of Urea on DODA Monolayers head group. Using a simple adsorption isotherm, taking the bulk urea concentration and the data shown in Figure 2, we estimate that urea may bind to the monolayer 50 times more efficiently than water. Intercalation of urea in both surfactant head groups, water replacement at the interface, and even deeper penetration into the micelle have been proposed to explain the effects of urea in the photoyield of radicals in the photoionization of N-alkylphenothiazines in micelles of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.22 Urea intercalation at the interface of reverse micelles of AOT explains the effect of the additive on the transition from a separated particle regime to a highly conductive aggregated or bicontinous phase.23 The addition of higher urea concentrations, by changing the solubility of the apolar part of the amphiphile in the subphase, could in part explain the effects here observed by increasing the probability of chain contact with the water surface. It should be stressed, however, that the (22) Kang, Y. S.;McManus, H. J. D; Kevan, L. J. Phys. Chem. 1992,
96, 10049.
(23)Amaral, C. L. C.; Brino; 0.; Chaimovich, H.; Politi, M. J. Langmuir 1992,8, 2417.
Langmuir, Vol. 11, No. 5, 1995 1719 effect of urea on the cmc of detergents is not large and is dependent on the nature of the counterion. For example, the cmc of n-dodecyltrimethylammonium bromide is only 3-fold higher in 6.0 M urea when compared to pure water.2a Another important effect of urea on the properties of DODA monolayers is the additive ion selectivity. In micelles, ion selectivity is progressively lost as the distance from the interface is increased.24 It is attractive to postulate, therefore, that upon urea insertion at the monolayer the counterions, Coulombically attracted to the oppositely charged surface, fail to interact specifically. This, in turn produces a loss in the ion specificity presumablyby not permitting the formation of a true Stern layer at the surface.
Acknowledgment. We thank Prof. 0.N. Oliveira and his group from the Physics Institute of the University of SBo Paulo at Sao Carlos for their help with the monolayer setup. We acknowledge financial support from the Brazilian Granting Agencies FAPESP, CNPq, and FINEP. LA940366N (24) Zanette, D.; Chaimovich, H. J. Phys. Org. Chem. 1991,4,643.
