Langmuir 1988,4, 1247-1251 rings are parallel to the water surface and multilayers on solid substrates in which the rings appear to be parallel to the substrate surface. It has “head” and “tail” characteristics arising from the combination of its trihexylsiloxy and hydroxy groups and organic solubility because of its trihexylsiloxy group. The compound has the ability to be compressed into a monolayer that is robust enough to be formed into multilayers due, in part, to its thick, stiff ring stack. The multilayers, formed by the compound are almost certainly highly anisotropic. In addition, they are thinner than aliphatic multilayers with comparable numbers of layers because their monolayers are thinner than those of the aliphatic multilayers. For the same reason the thickness of the new multilayer can be adjusted more precisely than can the thickness of the aliphatic multilayers. The monolayer and multilayers are stable dielectrics. It appears that the conduction mechanism in the multilayers
1247
is a hopping-type mechanism. It further appears that the overall conduction mechanism in the devices is a hopping-type mechanism modified by effects due to the oxide layers on the electrodes. The monolayer and multilayers are members of a new class of dielectrics having the potential to yield microelectronic devices engineered to a high level of precision. Among the types of devices for which they appear suited are those in which semiconductivity and photoconductivity play a role.
Acknowledgment. We thank Professors J. B. Lando, J. A. Mann, and W. H. KO of Case Western Reserve University for help with the analysis of the data. The generous support of DARPA and the Office of Naval Research (Grant N00014-83K-0246) is gratefully acknowledged. Registry No. (HO)G~PCOS~PC(OS~(~-C~H~~)~), 115461-96-6.
Effect of Added Sodium and Lithium Chlorides on Intermicellar Interactions and Micellar Size of Aqueous Dodecyl Sulfate Aggregates As Determined by Small-Angle Neutron Scattering Stuart S. Berr*>tand Richard R. M. Jonesf Magnetic Resonance Imaging Facility, Box 190, Medical Center, University of Virginia, Charlottesville, Virginia 22908, and Industrial and Consumer Sector Research Laboratory, 3M Company, 3M Center-201-4N-01, St. Paul, Minnesota 55144 Received January 9, 1987. I n Final Form: June 9, 1988 The effect of added electrolyte on ionic micelle size and interparticle interactions has been studied by small-angle neutron scattering (SANS). The surfactants, lithium and sodium dodecyl sulfate (LDS and SDS), were kept at a fixed concentration (0.05 M), while their respective chloride concentrations (LiC1 and NaCl) were increased. As the concentration of added salt was raised, intermicellar Coulombic repulsions decreased to the point where attractive (possibly van der Waals) forces could dominate. Furthermore, the added electrolyte appeared to screen intramicellar head-grouprepulsions, allowing the micelles to grow in terms of an increased aggregation number, N . Sodium was found to be much more effective than lithium at screening charge. N for SDS micelles increased from 54 to 928, and the contact potential decreased from 16 to -221 kJ/mol when [NaCl] was increased from 0.0 to 0.6 M. N for LDS, on the other hand, went from 53 to 91, and the contact potential decreased from 14 to -2.5 kJ/mol as [LiCl] was increased from 0.0 to 0.8 M.
Introduction The interactions between charged micelles are dominated by Coulombic repulsive interactions when the ionic strength of the solution is low. These interactions prevent micelles from approaching closely enough to experience strong van der Waals attractions. The ionic strength of the solution can be increased through the addition of an electrolyte. The effect of doing so has been previously studied by small-angle neutron scattering (SANS) for large concentrations of lithium dodecyl sulfate (LDS) with added LiC1’ and by static and dynamic light scattering for t Magnetic Resonance Imaging Facility, University of Virginia.
This paper was drawn from work done at Wake Forest University, Winston-Salem,NC, and Oak Ridge Associated Universities, Oak Ridge National Laboratory, Oak Ridge, TN, as partial fulfillment of requirements for the Ph.D. degree. BITNET address is ssb4ba Virginia. *Industrialand Consumer Sector Research Laboratory, 3M Company.
0743-7463/88/2404-1247$01.50/0
sodium dodecyl sulfate (SDS) a t just above the cmc with added NaC1.2 In this work, an effective intermicellar potential is used to account for the small-angle neutron scattering that is due to the interparticle structure as described by the function S(Q). Once S(Q) has been taken into account, the intraparticle form factor, F(Q),is calculated and used to determine the micellar size. The basic micelle model utilized consists of an elliptical hydrocarbon core, surrounded by a shell of constant thickness that contains the sulfate head groups, associated counterions and water, and possibly some surfactant hydrocarbon.
Experimental Section Surfactants. The preparation of SDS-I,I-d2and LDS-I,I-d2 was carried out as previously described?~~ These surfactants were (1)Bendedouch, D.; Chen, S.-H. J. Phys. Chem. 1984,88, 648. (2) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711.
0 1988 American Chemical Society
1248 Langmuir, Vol. 4, No. 6, 1988
Berr and Jones
, 0 0 M LlCl
I
03 M
0 0 \i sac1
I
0 os \I
.
0 'I\I
0 1 hl
*\\
I
1 a
2'1
"3
-
1
0 1 I?
3 2.;
c
Q
0
0 21
C?1
3 24
Q
3 24
0 2 ?.i
:; 1 n-
0
Figure 1. SANS intensities versus Q for SDS as a function of added NaCl (0.0 M I[NaCl] I0.6 M). The large increase in zero-angle scattering may be due to either an increase in interparticle interactions (due to a decrease in Coulombic intermicellar repulsions allowing van der Waals attractions to predominate) and/or an increase in micelle size. prepared from dodecanol-l,l-dz, which was made by the reduction of dodecanoic acid with LiA1D4. Gas chromatography and mass spectroscopy of the alcohol showed no other homologues. Deuterium was incorporated into these surfactants so as to establish better contrast between the various regions of the micelle and the solvent. Solutions were prepared by dissolving appropriate masses of surfactant and salt into weighed DzO (Aldrich, 99.8% atom D). Molarities were calculated by using well-accepted molar volume^.^*^ The surfactant concentration was kept constant a t 0.05 M. Small-Angle Neutron-Scattering Measurements. Neutron-scattering measurements were performed on the 30-m (maximum source-to-detector distance) SANS instrument a t the High Flux Isotope Reactor of the National Center for Small-Angle Scattering Research (NCSASR) at Oak Ridge National Laboratory (ORNL). The sample-to-detector distance was chosen to be 2.0 m for all runs. The momentum transfer was 0.02 5 Q I0.24 A-' (Q = (4n/h) sin 0, where 20 is the scattering angle and h = 4.75 A is the neutron wavelength). The samples were contained in UV grade quartz cells with a 0.2-cm path length, sealed with tightfitting Teflon stoppers. The cell temperature was maintained at 25.0 0.1 "C by means of an external circulating bath and was monitored by a thermocouple inserted into the sample holder. Scattering intensities from the surfactant solutions were corrected for detector background and sensitivity, empty cell scattering, incoherent scattering, and sample transmission. Solvent intensity was subtracted from that of the sample. The resulting corrected intensities were converted to radial averages versus Q
*
Phys. Chem. 1986, 90,6492.
(4) Szajdzinska-Pietek,E.; Maldonado,R.; Kevin, L.; Jones, R. R. M.; Berr, S. S. J . Phvs. Chem. 1985.89, 1547. (5) Marcus, Y: J.Solution Chem. 1983, 12, 271. (6) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (7) Hayter, J. B.; Penfold, J. J. Chem. Soc., Faraday T r a m 1 1981, 77, 1851. (8) Hansen, J.-P.; Hayter, J. B. Mol. Phys. 1982, 46, 651.
0 6 \I
Q
0 24
Q
0 24
Figure 2. SANS intensities versus Q for LDS as a function of added LiCl (0.0 M 5 [LiCl] 5 0.8 M). Although the zero-angle scattering increases, the amount of increase is not as dramatic as that of SDS with respect to added NaCl. by using computer programs provided by the NCSASR. Absolute cross sections, da/dfl, were computed from calculations based on known scattering from pure HzO. The resulting intensities versus Q are shown as open symbols in Figures 1 and 2. The error bars reflect counting statistics. The solid line curves in these figures are calculated by using a nonlinear least-squares fitting routine as described in the following section.
Analysis of SANS Data The micelle model used to calculate F(Q) is described in detail e l ~ e w h e r e . ~ JThis ~ model consists of a dry hydrocarbon core surrounded by a Stern region that contains sulfate head groups, associated counterions and water, and possibly some surfactant hydrocarbon. The following definitions apply to this model: N is the aggregation number (number of surfactant monomers per micelle), 2 is the difference between Nand the number of counterions in the Stern region, p ZIN, o is the number of water molecules per monomer in the Stern layer, A, is the double axis of the core, A, is the double axis of the total micelle, B, is the single axis of the core, E,is the single axis of the total micelle, THIK is the thickness of the Stern layer, WET is the number of "wet" methylene units per mono(9) Berr, S. s.; Caponetti, E.; Johnson, J. s.,Jr.; Jones, R. R. M.; Magid, L. J. J . Phys. Chem. 1986,90, 5766. (10) Rutgers, A.J.; Hendrikx, Y. Trans. Faraday SOC.1962,58, 2184. (11) X = [ x ( [ ( I d d- I. ~)/ER]2/DF)]o~6 where the sum runs over all the data points. I d d and are the calculated and experimental data points, ER is the error associated with a particular data point due to counting statistics, and DF = Npb- Nw + 1 is the degrees of freedom with Npbthe number of data points and Nw the number of free-fitting parameters (in this case 4). If X 5 1, the calculated points lie, on the average, within the error of the data. (12) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (13) Hamaker, H. C. Physica 1937,4, 1058. (14) Van Megen, W.I.; Snook, I. Faraday Discuss. Chem. SOC.1978, 65,92. (15) Hayashi, S.; Ikeda, S.J. Phys. Chem. 1980,84, 744. (16) Berr, S.S . J. Phys. Chem. 1987, 91, 4760.
x,
Effect of Added Salts on Micelles
Langmuir, Vol. 4, No. 6, 1988 1249
Table I. Determination of Approximate Values of and WET' A, THIK WET 2 N
SDS-d2 LDS-d2
1.82 1.65
0.63 0.81
0.0 0.3
20.1 19.4
55.7 55.0
THIK j3 0.36 0.35
OWith AR set equal to unity for micellar solutions of 0.05 M surfactant without added salt at 25.0 OC.
mer that reside in the Stern region, and AR Bt/At is the axial ratio of the micelle (AR > 1 indicates a prolate ellipsoid, AR < l an oblate ellipsoid, and AR = l a sphere). All lengths are in nanometers, and volumes are in cubic nanometers. In earlier s t u d i e ~ , ~we , ~ assumed J~ that 2 was the effective charge on the micelle. This charge was used in calculations of both S(Q) and F(Q). However, when salt is added to ionic micellar systems, the repulsions between micelles decrease due to charge screening, and attractive forces, such as those due to van der Waals interactions, can predominate,' making it impossible to use Hayter and Penfold's routine to calculate S(Q)since their theory assumes the intermicellar interactions are strictly Coulombic. For this reason, 2 will only be used to calculate the scattering length density of the shell for the function F(Q). The intermicellar interaction potential, and hence S(Q), will be calculated from the free-fitting parameter GEK as explained below. It is desirable to minimize the number of free-fitting parameters, not from an aesthetic standpoint, but from a practical one; a large number of fitting parameters often leads to unstable solutions. To this end, THIK and WET were determined from salt-free solutions with AR set equal to unity. Hydration numbers were assigned as 13 for Na+ and 22 for Li+.9 From previous work3 it was found that w for SDS was 23. From these values, the hydration of the sulfate head group can be calculated for SO, to be 10. The results of using these values and allowing N , 2, THIK, and WET to float in a nonlinear least-squares fit to the experimental data are given in Table I. The value of 0for SDS is in agreement with the value found by Corti and Degiorgio ( p 0.4).2 In subsequent calculations of F(Q),the values given in Table I for THIK, WET, and 0will be used. N will be allowed to vary (which will result in the variation of AR), along with the parameters used to calculate S(Q),in order to extract information of interest from the experimental SANS data. It should be noted that the values of THIK and WET are only approximate. The purpose here is not to establish the fine details of micellar structure but to investigate the differences between Na+ and Li+ with regard to intermicellar interactions and micellar growth as a function of added chloride salt. To this end, approximate values for THIK and WET are sufficient. Calculation of S ( Q ). The repulsive potential U(x) between two identical impenetrable spherical micelles of effective diameter u separated by a center-to-center distance r is given by U(x)/kBT = y exp(-kx)/x x 1 1 -- m x
