Langmuir 1992, 8, 1188-1194
1188
A Reevaluation of Temperature-Dependent Bilayer Interaction Forces As Determined by Surface Forces and Atomic Force Microscopy Measurements Y.-H. Tsao, S. X. Yang, and D. F. Evans' D e p a r t m e n t of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received November 13, 1990. I n Final Form: December 16, 1991 Direct force measurements on dihexadecyldimethylammonium bilayers in the presence of sodium acetate a t 25,40,50,and 60 "C and KBr a t 40 and 50 "C were carried out using a modified surface forces apparatus (SFA)with temperature control. The data were analyzed using the DLVO theory combined with hydration force terms. With increasing temperature, surface potential increases and counterion binding decreases. Under comparable ionic strength conditions, the degree of counterion binding is larger for bromide than for acetate. T h e change in surfactant aggregation with temperature is discussed in terms of the SFA data.
Introduction M u c h of our direct knowledge on interaction forces between amphiphilic aggregates comes from surface forces apparatus (SFA) or osmotic stress (OS) measurements. One simple system that is readily characterized by these techniques employs t h e dialkyldimethylammonium surfactants. T h e y self-assemble to form bilayers o n t h e mica surfaces employed in SFA measurements a n d a r e readily characterized by t h e X-ray methods used in OS studies. In t h e original SFA measurements,' t h e force curves for dihexadecyldimethylammonium acetate (DHDAA) bilayers with added sodium acetate were fitted t o the classical DLVO theory. T h e calculated surface potentials corresponded t o those for completely ionized surfaces. Recent OS measurements2 showed t h e presence of a strong shortranged repulsive hydration force and considerably lower surface potentials. In addition, t h e X-ray data indicated t h e presence of interdigitated bilayers. In order t o obtain more information on this simple model system, we have carried o u t SFA measurements o n DHDAA bilayers as a function of temperature a n d bilayer preparation method. W e have found that from different deposition methods, either normal or interdigitated bilayers can b e assembled o n t o mica surfaces. Atomic force microscopy images permit the interdigitated and normal bilayers t o b e directly visualized.
Experimental Section Dihexadecyldimethylammonium acetate (DHDAA) was prepared by passing dihexadecyldimethylammonium bromide (DHDABr, Sogo Pharmaceutical Co., Ltd.) dissolved in methanol through an anion-exchange column (Fisher REXYN 201) in the acetate form. The precipitated salt was purified by recrystallization from ether.3 Dieicosyldimethylammonium bromide (DEDABr) was a gift from Professor Robert Moss and was used as received. Analytical grade sodium acetate (Fisher Scientific) and potassium bromide (MCB Reagents) were used without further purification. Water was passed through a Millipore purification system, then just prior to injection into the SFA, it was processed with a Water Prodigy polishing unit (Labconco Corp.). (1) Pashley,R. M.;McGuiggan, P. M.; Ninham,B. W.;Brady,J.;Evans,
D.F. J . Phys. Chem. 1986, 90, 1637.
(2) Parsegian, V. A.; Rand, R. P.; Fuller, N.L. J . Phys. Chem. 1991, 95, 4777. (3) Brady, J.: Evans. D. F.: Ninham, B. W.; Kachar, B. J . A m . Chem. Soc. 1984, 106, 4279.
0743-7463/92/2408- 1188$03.00/0
The surface forces apparatus (SFA)4+ used to measure interaction forces as a function of temperature consists of two chambers (see Figure 1). The first chamber contains the double cantilever spring, the helical spring, and the upper and lower control rods. It is permanently attached via nylon bolts to a yoke which holds the stepping motors used to move the control rods. Thermal isolation of this control unit is achieved by inserting sapphire disks between the control rods and the stepping motors and by inserting a 0.5 mm thick sheet of Teflon between the yoke and the control unit. The second chamber consists of the solvent bath which contains the upper lens mount and the double-leaf spring onto which the mica-covered lenses are attached. The volume of the bath is 160 mL. While a much smaller volume could be used with the doubleleaf spring, our design also accommodates the variable-leaf spring used in other applications of the SFA. After the mica-covered glass lenses have been place in their mountings, the sample bath is bolted to the separation control unit and the double-leaf spring is attached to the cantilever spring. During experiments, the front of the control unit is covered with a fiber plastic plate to prevent contamination from the surroundings. A constant temperature air bath is permanently attached to the yoke. The rectangular bath is constructed from 3/pj in. linen filled phenolic sheets and contains a quartz disk in its bottom surface through which light from the light source passes. The annular spaces between the bath and the microscope objective and the control rods are closed by plastic bellows. The temperature is controlled by a Tronac, Inc., thermal regulator and a set of rigid heating tapes attached to the inner surfaces of the bath. A gentle air stream is circulated through the air bath during the experiments. The temperature in the solvent bath was monitored during experiments by a thermocouple contained in a stainless steel shell. The thermocouple tip was inserted through the left port and positioned within 5 mm of the glass lenses. Thermal equilibrium can be achieved within 2 h with temperature fluctuations less than 0.1 "C. The two different procedures used for assembling the DHDAA bilayers onto mica are illustrated in Figure 2. In the first procedure, the DHDAA bilayers are self-assembled onto bare mica surfaces. Warm water is injected into the SFA to reduce heating time, and when thermal equilibrium is achieved, the bare mica contact distance is determined and set equal to zero. An aliquot of a DHDAA solution giving a final concentration of 2 X 10 M is injected into the SFA and left for 12 h in order to permit the bilayers to form. ( 4 ) Tabor, D.; Winterton,
R. H. S. Proc. R. SOC.London,A 1969,312,
435.
( 5 ) Israelachvili, J. N.; Adams, C. E. J . Chem. Soc., Faraday Trans. 1 1978, 7 4 , 9 i 5 . ( 6 ) Evans, D. F.; Evans, J. B.; Sen, R.: Warr, G. G. J . Phys. Chem. 1988, 92. i84.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 4, 1992 1189
Bilayer Interaction Forces
iont5
10 n+3
1
T = 40'C
j
10 n+l
A
1
U
Light-
10
t
?herighthard Side is insulated
Figure 1. Modified surface forces apparatus. The left-hand side contains inlet/outlet ports.
[DHDAA] = 2 ~ 1 0 M .~
10 n+4 Monolayer contacl
Noma bilayer ADn = 40
A
Figure 2. Schematic drawing of formation of interdigitated and normal DHDAA bilayers. In a second series of experiments, mica surfaces containing hydrophobic monolayers of either DHDA or DEDA are employed. The hydrophobic monolayers are prepared by dipping the mica glued onto the glass hemispheres into a 2 x lo-' M cyclohexane solution containing either DHDAA or DEDABr. The surfaces are rinsed in cyclohexane to remove excess surfactant and then dried. After addition of water to the SFA, attainment of thermal equilibrium, and determination of the zero contact distance for the hydrophobic monolayers, DHDAA is injected into the SFA so that a bilayer forms on each of the hydrophobic mica surfaces. Subsequently, the force curves are determined as a function of added salt by injecting aliquots of electrolyte into the SFA. Atomic force microscopy (AFM) images of the hydrophobic DHDA monolayer and of the DHDAA bilayers were obtained using a Digital Instruments Nanoscope 11. In the AFM experiments, freshly cleaved mica was glued onto a stainless steel disk. To obtain hydrophobic monolayers, the mica was dipped into a 2 X lO-'M cyclohexane solution containing the surfactant before mounting the mica-stainless steel assembly in the AFM flow cell. The DHDAA bilayers were prepared in two ways which parallel those employed in the SFA experiments. A 2 X lo-' M DHDAA solution was injected into the AFM flow cell containing either bare mica or the hydrophobized mica and allowed to equilibrate for 12 h. In some experiments KBr was subsequently added in order to determine the effect of salt on the bilayers.
Results The interaction forces between dihexadecyldimethylammonium acetate bilayers as a function of added salt at
10 n+3
10
"
10 "1
10 "2
0
200
400
600
800
300
D (A) Figure 4. Forces measured between adsorbed DHDAA bilayers in 2 X lo-' M DHDAA solution as a function of added NaOAc a t 50 O C ,
25,40,50, and 60 O C were determined using the surface forces apparatus. The force curves for NaOAc covering the concentrationrange0 to 1 X 10-2M are shown in Figures 3-5, and corresponding data for KBr covering the concentration range of 0 to 2 X M are shown in Figures 6-9.
Tsao et al.
1190 Langmuir, Vol. 8,No. 4, 1992 10 n+5
T=60T
1°"+'1
10 "+2 ;
F/R 10 n+l
10
I
(vN/m)
10"'-
-I
j
10
10 n-1
j 4
[NaOAc] = l.Ox10-2M
10 n-2
0
200
400
600
800
D (A) Figure 5. Forces measured between DHDAA bilayers with addition of NaOAc at 60 "C. 10 n + 5
10 "-2
1000
0
400 600 D (A)
800
1000
Figure 7. Forces measured between DHDAA bilayers with addition of KBr at 50 "C.
I
T = 40°C
200
10 n+5 [DHDAA] = 2 ~ 1 0 .M~ 10 n+4
10 n+3
10
"+*
F/R
(Wm) 10 "+l KBr] = 6x10-4 M
10
10 w'
i [KBr] = 2x10-3 M
10 n-2
0
200
400
600
800
1000
0
D (A) Figure 6. Forces measured between DHDAA bilayers with addition of KBr at 40 "C.
200
400
600
800
1000
D (A) Figure 8. Forces measured between DHDAiDHDAA normal bilayers with addition of KBr at 25 "C. The contact distance between DHDA hydrophobic monolayers was set to be D = 0.
The data were analyzed using an equation of the form
FI277R = (E(attractive) + E(e1ectrostatic) + E(hydration)J (1) where F is the measured force between the two cylindrical surfaces employed in the SFA measurements and R is the mean radius of curvature of the mica surfaces. According to the Derjaguin appr~ximation,~ FI2xR equals the interaction energy per unit area between two infinite flat surfaces. E(attractive) was evaluated using the nonre(7) Derjaguin, B. V. Kolloid-2. 1934, 69,155.
tarded London dispersion equation with a Hamaker constant of J. E(e1ectrostatic) was calculated using a numerical solution to the Poisson-Boltzmann equation8
where D, is the separation between bilayers. (8) Verwey, D.J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids;Elsevier: Amsterdam, 1948.
Langmoir, Vol. 8, No. 4, 1992 1191
Bilayer Znteraction Forces
I"
I]
1 0n+4
Table I. Summary of Surface Potentials, Surface Charge Densities, and Debye Lengths as a Function of Added NaOAc and Temperature As Determined from SFA Measurements
Normal DEDADHDAA Bilayers T=2S°C [DHDAA] = 2x104 M
______
[NaOAc], M
+",mV
I(-1,
A
u", cm-*
C, M
T = 25 "C,[DHDAA] = 2 X lo4 M 0 3.0 x 10-4 1.1 x 10-3 1.0 x 10-2
1 on+3 3
10"+2
.
0 3.0 x 10-4 1.1 x 10-3 1.0x 10-2
F/R
W/m) lo"*'
I
10"
I
0 3.0 X 1V 1.1 x 10-3 1.0 x 10-2 0 3.0 x 10-4 1.1 x 10-3 1.0 x 10-2 0
200
400
600
800
1000
D (A) Figure 9. Forces measured between DEDA/DHDAA normal bilayers with addition of KBr at 25 "C. The contact distance between DEDA monolayers was set to be D = 0.
The hydration force9 was evaluated using an equation of the form FIR = 2rSmP0 D, exp(-D,l2.6) dD,
(2)
The value of PO equal to dyn/cm2 and the decay length of 2.6 8, were obtained from osmotic stress measurements on DHDAA at 252 and 40 "C.l0 Since the hydration force is identical for frozen chains (25 "C)and melted chains (40 "C),we assumed that the hydration force is independent of temperature and electrolyte concentration. The surface potentials, surface charge densities, and Debye lengths as a function of added electrolyte and temperature determined using eq 1 are tabulated in Tables 1-111. Also included are values for the previously published datal at 25 "C which have been reevaluated so as to include the hydration force contribution to the interaction energy. In these calculations the distance corresponding to the thickness of two interdigitated bilayers in contact was set equal to 55 A at 25 O C and 57 A at 40 "C.This corresponds to bilayer thicknesses of 27.5 and 28.5 A in agreement with those determined from X-ray diffraction data. For the normal bilayers, the contact distance was set equal to 40 A.
Discussion
DHDAA Bilayers on Mica Surfaces. In the paragraphs below, we present evidence that DHDAA bilayers which self-assemble onto bare mica are interdigitated whereas those formed by deposition of DHDAA onto the hydrophobic DHDA monolayers result in normal bilayers (see Figure 2). The evidence for this conclusion comes from measuring changes in the SFA contact distance and analysis of AFM images. (9) Parsegian, V. A.; Fuller, N.; Rand, R. P. R o c . Natl. Acad. Sci. U.S.A. 1979,76,2750. (10) Tsao, Y.-H.; Evans, D. F.; Parsegian, V. A.; Rand, R. P. To be submitted for publication.
220 290 0.044 207 185 0.054 185 95 0.069 185 30 0.217 T = 40 "C, [DHDAA] = 2 X lo-' M 280 285 0.110 260 180 0.120 240 100 0.149 210 35 0.237 T = 50 "C,[DHDAA] = 2 X lo4 M 300 280 0.135 280 190 0.139 250 100 0.153 220 36 0.250 T = 60 O C , [DHDAA] = 2 X 1W M 315 285 0.144 295 190 0.153 260 100 0.157 228 36 0.250
1.1 X 2.7 X 1.0 x 1.0x
lo-' lo-' 10-3 10-2
1.1 x 10-4 2.8 X lo-' 0.9 X 0.7x 10-2
1.1 X lo4 2.5 X lo4 0.9 X 0.7 X 1.1 X 2.4 X 0.9 X 0.7X
1oJ lo4
lW3 10-2
Table 11. Summary of Surface Potentials, Surface Charge Densities, and Debye Lengths as a Function of Added KBr and Temwrature As Determined from SFA Measurements [KBrl,M +", mV K - ~ ,A u", cm-2 +"N.OA~, mV T = 25 "C, [DHDAA] = 2 X lo-' M 0 220 290 0.044 1.0 x 10-5 208 310 0.027 6.0 X lo4 140 130 0.021 195 2.0 x 10-3 110 85 0.018 190 T = 40 "C, [DHDAA] = 2 X lo4 M 0 280 285 0.110 1.0 x 10-5 225 280 0.040 6.0 x 10-4 180 150 0.033 250 2.0 x 10-3 130 70 0.028 230 T = 50 OC, [DHDAA] = 2 X lo-' M 0 300 280 0.135 55 2.0 x 10-5 290 290 0.109 46 6.0 X loJ 240 140 0.092 262 2.0 x 10-3 190 70 0.075 242 Table 111. Summary of Surface Potentials, Surface Charge Densities, and Debye Lengths of Normal Bilayers as a Function of Addeq KBr at 25 "C As Determined from SFA Measurements [KBrl, M $", mV K - ~ ,A uo, cm-* C. M DHDA/DHDAA 0.072 9.0 x 10-5 0 250 320 9.6 x 10-5 1 x 10-5 240 310 0.061 6 X lo-' 175 120 0.045 6.4 X lo-' 2 x 10-3 130 75 0.030 1.6 x 10-3 DEDA/DHDAA 9.6 x 10-5 240 310 0.061 0 9.6 x 10-5 1 x 10-5 230 310 0.051 6 X lo-' 180 130 0.045 5.5 x 10-4 2.2x 10-3 130 65 0.034 2 x 10-3
When the bilayers self-assemble onto bare mica, the increase in contact distance in the SFA experiments is 55 A. This corresponds to the thickness of two bilayers; i.e. the thickness of each bilayer is 27.5 A. This value is in reasonable agreement with the value of 25 A determined from X-ray measurements on DHDAA bilayers at 25 "C.2 When the bilayers are formed in sequential steps, the increase in contact distance upon going from the hydrophobic monolayer to the bilayer is 40 A. We can estimate the increase in distance accompanying the formation of a DHDAA bilayer by calculating the length of two fully extended hexadecane hydrocarbon chains, 2L = 211.5 +
Tsao et al.
1192 Langmuir, Vol. 8, No. 4, 1992
Figure 10. (a, top left) AFM images of a DHDA monolayer on mica in water a t 25 "C. Individual methyl groups of the h y d " chains can be identified. (b, top right) AFM image of a DHDAA interdigitated bilayer showing a set of well-defined peaks with an average area of 34 i 4 A2,assumed to be equal to area of individual methyl groups of the head groups. (c, middle left) Cross-section profile of AFM image of a DHDAA interdigitated bilayer. The two methyl groups associated with a DHDA molecule are separated by 5.5 A. (d, middle right) AFM image of DHDAA normal bilayer giving an area per head oup of 40 i 2 A2. (e, bottom) AFM image of a DHDAA normal bilayer in 2 X M KBr solution showing a headgroup area of 38 f 2 Addition of KBr produces AFM images which are more ordered.
f2.
1.265(16)] = 40 A. Thus the predicted contact distance associated with the formation of two normal bilayers is =80 A,which is considerably larger than the value obtained with the self-assembled bilayer system. In Figure 10we compare AFM images of the hydrophobic DHDA monolayer and the DHDAA interdigitated and normal bilayers. With the monolayer (Figure loa), the resolution of images permits the individual methyl groups of the hydrocarbon chains to be identified. The area per
methyl group is 26 A2,which gives an area per surfactant molecule of 52 f 2 A2 (three samples, 60 images analyzed), which is almost e ual to the area per negative charge on mica which is 4812. With the interdigitated DHDAA bilayer, the AFM images (Figure lob) show a set of well-defined peaks with an average area of 34 f 4 A2 (three samples, 19 images analyzed). In interpretingthe bilayer images, we assumed that the methyl groups of the head group were imaged.
Bilayer Interaction Forces
Table IV. Estimates of the Volume of a Diheradecyldimethylammonium (DHDA) Molecule in the Monolayer and Bilayers from AFM and SFA Measurements area per DHDA molecule, thickness, volume, A2 (AFM) A @FA) A3 monolayer 52f2 20 f 2 O 1040 f 150 68 f 8 27.5 f 3 935 f 2Wb interdigitated bilayer 40f2 40 f 4 800 f 120b normal bilayer 0 As estimated from SFA measurements on normal bilayers, we assume the uncertainties in the SFA measurements to be *lo%. * Since the bilayer contains two surfactant molecules per measured area, the volume per DHDA is half the value obtained by multiplying area x thickness.
The justification for this assumption is shown in Figure 1Oc where cross-section profiles, i.e. change in measured height with distance, are shown. The two methyl groups associated with a DDHA molecule are separated by 5.3 while the average spacing between molecules is 7 A. Thus, the area per DHDA molecule is 68 f 8 A2. These results are consistent with the head group area of 67-76 A2 for interdigitated bilayers as determined by X-ray measurements. The variation in the X-ray values reflects the differences in the models used to estimate the head group areas. Addition of KBr (6 X M) to the bilayer results in only a small decrease in the head group area (62 f 6 A2, two samples, 16images analyzed) and no other discernible change in the AFM image. With the normal DHDAA bilayers, the AFM images (Figure 10d) give an area per head group of 40 f 2 A2 (two samples, 20 images analyzed). Addition of KBr (2 X M)decreasesthe head group area to 38 f 2 A2(twosamples, 21 images analyzed) (Figure 10e). Adding KBr also produces AFM images which are more ordered, an effect we attribute to preferential binding of bromide counterions and a correspondent increase in the Krafft temperature of the bilayer. We also noted a significant difference in the mechanical properties of the normal and interdigitated bilayers as detected in the AFM measurements (Figure 10b,d). Varying the voltage on the AFM piezo-crystal changes the force between the AFM tip and the sample. With the interdigitated bilayers, we could vary the force from -78 to 57 nN and obtain reproducible images upon repeated scanning. In these measurements zero force corresponds to zero net force between the AFM tip and the sample. With the normal bilayers, we could only obtain images over a narrow force range, +14 to +16 nN, and after one or two scans the images began to blur and fragment as a result of the interaction between the sample and the tip. However, addition of KBr to the normal bilayers increased their mechanical stability, and we could reproducibly scan the sample again and again over a wide range of forces (-33 to 90 nN). These observations are consistent with our expectation that the interdigitated bilayer should be robust and the normal bilayer somewhat more fragile. We can combine the AFM area measurements and the SFA distance measurements to obtain an estimate of the volume of a DHDA molecule in the monolayer and bilayers shown in Figure 10. These values are summarized in Table IV. Given the large uncertainties in AFM areas and SFA thicknesses, the volume estimates are in reasonable agreement. In conclusion, the SFA contact distances and AFM images show that either normal or interdigitated bilayers can be formed on mica depending on the method of preparation. Analysis of the Force Curves. In the previous paper on SFA DHDAA force measurements,' the data with added
Langmuir, Vol. 8, No. 4, 1992 1193
NaOAc were fit to the DLVO theory assuming that the surfaces were completelyionized; i.e. that they had a charge density of 0.27 C/m2. While the calculated DLVO curves showed good agreement with the measured curves, the assumption of a completely ionized surface was not consistent with results obtained from the study of micellar systems with acetate as the counterion. For example, micellar counterion binding obtained from the variation of cmc for the alkyltrimethylammonium acetates with added NaOAc gives values of 0.50." It is difficult to reconcile these two observations particularly since one would expect flat bilayers to have a greater degree of counterion binding than highly curved micelles. Recent osmotic stress (OS) measurements on DHDAA provide an explanation for this discrepancy.2 In the OS measurements, the force curves show a pronounced break at 18A. At shorter distances, a strong repulsive hydration force with an exponential decay of 3.2 8,is detected in the OS measurements, but no such break is detected in the SFA energy curves. A t longer distances the OS and SFA data give similar results. The reason for this difference can be understood by considering the difference in the geometries of the two techniques. In the OS experiments, forces are measured between bilayers which are in a parallel configuration. In SFA experiments, forces are measured between bilayers that are in a crossed cylindrical configuration, a geometry topographically equivalent to a sphere and a plane. Consequently, the interaction detected between the SFAs oppositely curved surfaces is an average over varying separation distances. According to Derjaguin, the force Fbetween a sphere of radius R and a surface can be equated to the energy E between two planes of the same material at the same separation D, as given in eq 1. Thus, in order to directly compare OS force curves with SFA energy curves, one must integrate the OS data and multiply it by 217. When this is done, the pronounced break in the force curves at 18A emerges as a much weaker break at 8 8,in the energy curves. In the SFA experiments, the forces between the oppositely curved surfaces are the integral that weighs interactions at distances greater than the minimum separation. Specifically, the hydration forces are hidden within the residual contribution of the longer range electrostatic interactions. In addition, with the acetate system, the repulsive force is so large that at distances less than 8 A it is very difficult to distinguish hydration forces because of the possibility of deforming the mica when the surfaces are pushed together. When the hydration force is included in the analysis of the SFA DHDAA data, the resulting estimates of counterion binding the the acetate bilayer system are consistent with those determined for micellar systems. For this reason we have included a hydration force term in eq 1 used to analyze the SFA data. Comparison of Force Curves for Normal and Interdigitated DHDAA Bilayers. In Figures 8 and 9 are displayed the force curves as a function of added KBr for the interdigitated bilayers and two normal DHDAA bilayers in which different underlying hydrophobic layers, DHDA and DEDA, are employed. The AFM images of the DHDA monolayer under water indicated that the hydrocarbon chains are in a melted state while those for DEDA monolayersare crystalline.12 We investigated both systems in order to determine whether the bilayer properties were affected by the state of the underlying hydrophobic monolayer. (11) Brady, J.;Evans, D. F.; Warr, G.G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986,90, 1853. (12)Tsao, Y.-H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Langmuir 1991, 7,3154.
1194 Langmuir, Vol. 8, No. 4, 1992 Table I11 lists surface potentials, charge densities, and Debye lengths obtained by fitting the force curves to eq 1. In these calculations, contact distances of 55 8, for the interdigitated layer and 40 8, for the normal bilayer were used. These values correspond to the contact distances measured in the absence of KBr and at the highest concentrations of salt where the lower surface potentials permit the contact distance to be measured with some assurance. Table 111shows that the parameters obtained from eq 1for the normal bilayer systems agree within experimental error. This suggests that the bilayer interactions as measured by the SFA are not affected by the nature of the underlying hydrophobic monolayer. While the surface potentials for the normal bilayers are slightly higher than those for the interdigitated system, the uncertainties in setting the contact distance makes it difficult to compare small differences between the two systems. Inspection of Figure 6 reveals another differene at higher KBr concentrations. With the normal bilayers, the force curves increase continuously as the surfaces move together. As the interdigitated bilayers are pushed together, they show a distinct jump from 80 to 55 8, a t 40 "C and 45 to 33 8, at 25 "C. This discontinuity in the force curve is reproducibly observed when the surfaces are repeatedly pulled apart and brought together again. This observation suggests a reproducible transformation of the adsorbed interdigitated bilayer under high force, but we can offer no detailed explanation. Comparison of Bilayer Force Curves as a Function of Temperature. At temperatures above 25 "C, it is difficult to obtain an unambiguous value for the bilayer contact distance from the SFA measurements. With increasing temperature, the bilayer surfaces become more ionized and the surface potential increases dramatically. Under these conditions, the forces exerted on the mica surfaces as they are pushed into contact can exceed 5 bar. Forces greater than this can deform and damage the mica. Consequently, we used the thickness of 57 8, for bilayers obtained from X-ray data in the melted state.I0 The Debye lengths obtained from the SFA force curves in the presence of NaOAc and KBr are in good agreement with those calculated from the expression
at all concentrations except a t [NaOAc] = 1 X M. A t this higher ionic strength, the Debye length calculated from eq 3 is 30 A, which is slightly smaller than the values (Table I) obtained from fitting the data. For a given electrolyte concentration, the Debye lengths are almost invariant because the changes in dielectric constant and temperature in eq 3 almost compensate each other. The Debye length obtained in the 2 X M DHDAA solutions in the absence of added electrolyte corresponds to a concentration of 1.1x M, a result consistent with the surfactant being in an aggregated state. The NaOAc datashow the followingfeatures. At a given temperature, addition of NaOAc results in a decrease in the surface potential and in a small increase in the charge density. With increasing temperature, the potential increases significantly and the charge density shows a corresponding increase. The data with added KBr are more difficult to interpret because of the presence of a counterion mixture of acetate from DHDAA and bromide (see Table 11). We focus on the highest bromide concentration of 2.0 X M where the ratio of Br- to OAc- is 10 to 1. With increasing tem-
Tsao et al. perature the surface potential and charge density all increase. However, these values are all significantly lower than the corresponding acetate values which were obtained from interpolation of the data in Table I and are given in column 6 ( I , ~ " N ~ O A ~of) Table 11. A t 25 and 40 "C the bilayer force curves with added bromide (6 X and 2 X M) both display a discontinuity as the surfaces are pushed together. However, at 50 "C, the force curves are continuous. The chain transition temperature for DHDABr is 44 "C, suggesting that the discontinuity in the force curves may be linked to a transition from solid to liquidlike bilayers. We now consider the implications of the SFA force measurements on the aggregation behavior of alkylammonium surfactants. The changes in the bilayer surface potential with counterion and increasing temperature are in accordance with the aggregation behavior of surfactants. Changing the counterion from bromide to acetate results in a decrease in the micellar aggregation number for single-chain surfactants3J3 and the transformation of bilayers to vesicles and/or micelles for dialkyldimethylammonium surfactants.13J4 The higher values of surface potential measured for acetate as compared to bromide in the SFA experiment confirm inferences drawn from less direct experiments. With increasing temperature, the aggregation numbers for spherical micelles formed from alkyltrimethylammonium bromides and acetates decrease.15 Dialkyldimethylammonium halides, which form bilayers at room temperature, transform to vesicles and eventually to micelles with increasing temperature. The increase in surface potentials with increasing temperature again confirms the inferences drawn from indirect experiments. Conclusions 1. When adsorbed onto mica, dihexadecyldimethylammonium acetate forms two different types of bilayers depending on the deposition method. Self-assembly onto bare mica leads to interdigitated bilayers similar to those observed in solution while deposition onto a hydrophobic monolayer results in normal bilayers. 2. With highly ionized dihexadecyldimethylammonium bilayers, short-ranged interactions such as hydration forces are difficult to detect. This is because the crossed cylindrical geometry employed in the SFA gives interaction forces which are an average over varying separations. This in turn can lead to surface potentials which are misleadingly high. 3. With increasing temperature, the surface potential of DHDAA bilayers increases in a manner which accounts for the transformation of charged bilayers to micelles and the decrease in aggregation number for ionic micelles. Acknowledgment. This research was supported by NIH Grant 2R01-GM-34341-06. We thank Professor R. Moss for the generous gift of dieicosyldimethylammonium bromide. We are grateful to V. A. Parsegian and R. P. Rand for valuable discussions. Registry No. DHDAA, 71326-37-9; DHDABr, 70755-47-4; DEDABr, 31500-63-7; NaOAc, 127-09-3; KBr, 7758-02-3. (13) Miller, D. D.; Magid, L. J.; Evans, D. F. J. Phys. Chem. 1990,93, 1895. (14) Miller. D. D.: Evans. D. F. J . Phvs. Chem. 1989. 93. 323. (15) Chen, V , Evans, D F , Warr, G G , Prendergast, F G J Phys Chem 1988, 92, 768
