878
Langmuir 1991, 7, 878-884
Micellization of Ultralong Chain Surfactantst D. J. Hodge,* R. G. Laughlin,g R. H. Ottewill,*J and A. R. Renniell School of Chemistry, University of Bristol, Bristol BS8 ITS, U.K., The Procter and Gamble Company, Miami Valley Laboratory, Cincinnati, Ohio 45239, and Institut Laue-Langevin, Grenoble, France Received June 21, 1990. In Final Form: September 28, 1990 The zwitterionic surfactant 6-(eicosyldimethylammonio)hexanoate ( C A H ) is highly soluble in water and has a low critical micelle concentration at ambient temperatures. Micellar solutions of this surfactant were examined by small angle neutron scattering using, the fully hydrogenated sample, material with a deuterated eicosylchain and material with deuterated N-methyl groups. From these studies it was possible to show that the micelles were spherical with the head groups forming a "shell" on the outside. The vector connecting the positively charged nitrogen with the negatively charged carboxylate group appeared to lie nearly parallel to the surface of the micelle. The pronounced effect on the scattering curves caused by the addition of electrolyte appears to be consistent with the idea that anions bind to the surface of these micelles more easily than do cations, or negative adsorption of cations occurs. In this work small angle neutron scattering studieshave Introduction been carried out on fully hydrogenated 6-(eicosyldimeAlthough zwitterionic surfactants are well-known,lS2 thylammonio) hexanoate, on 6-(perdeuteroeicosyldimethinvestigations of the structure of their micellar solutions ylammonio)hexanoate, and on 6-(eicosyl-ds-dimethylamare comparatively rare. The synthesis by McGrady and monio)hexanoate. This paper provides a preliminary Laughlinsreadily provides ammoniohexanoate surfactants report on the micellar structure of these surfactants in in pure form. These compounds, even with a CZO lipophilic water and the effect of electrolytes on the solution group, are readily soluble in water at ambient temperastructure. tures. Because such compounds are soluble at unusually long chain lengths, they have been termed 'ultralong chain" Experimental Section surfactants. Considerable interest centers on determining for solutions of this type of material the shape of the miMaterials. Doubly distilled water from a borosilicate glass celle. It is also of interest to examine changes in the miapparatus was used in all experiments involvingH20. Deuterium celle that occur as salt concentration and pH are varied. oxide (DzO) was obtained from Aldrich as 99.8 atom % deuterFor an examination of micellar systems, small angle ated material. neutron scattering offers many advantage^.^ Firstly, the 6-(Eicosyldimethylammonio)hexanoate,C d H . The hywavelength of the neutron beam, 12 A in the present work, drogenated6-(eicosyldimethylammonio)hexanoatewas prepared via base hydrolysis of the ethylammoniohexanoate bromide as is comparable to the size of a micelle; hence, the spectra described earliereg obtained over the Q range available contain considerable information about particle size, shape, and m o r p h o l ~ g y . ~ ~ ~ 6 4 (Perdeuteroeicosyl)dimethylammonio)hexanoate, C A H - Q . The 64(perdeuteroeicosyl)dimethylammonio)hexSecondly,the coherent scattering length density difference anoate was prepared starting with eicosanoic-dwacid (Merck). between hydrogen atoms and deuterium atoms makes it This compound was converted to the methyl ester, and the ester possible, by deuteration of selective parts of the molecule, reacted with excess dimethylamine in acetonitrile at 125 O C in to probe the spatial correlations between these regions in a sealed ampule for 18h to form the NJV-dimethylamide, This the micelle. Thirdly, the differences between the coherent amide was reduced, using LiAlDd in refluxing THF, to the eiscattering length densities of HzO and DzO make it possible cosyldimethylamine. The product was isolated by strongly to obtain solvent media which provide a wide range of acidifying the THF solution with concentratedhydrochloricacid; contrasts. As shown earlier, the variation of scattering the eicosyldimethylammoniumchloride precipitated on cooling. intensity with scattering vector as a function of contrast After the precipitatewas triturated with water at 0 OC, the product was dried and used for the next step. provides a sensitive method of examining particles with a core-shell m~rphology.~ The alkyldimethylammoniumchloride was dissolved in chloroform and the amine liberated by shaking this solution with 50% potassium carbonate until chloride free. The solvent was * To whom correspondence should be addressed. removed and the residue reacted with propyl 6-bromohexanoate t Invited Lecture given by R. H. Ottewill at Symposium on Zwitin refluxing dry acetonitrile for 72 h. The propyl ester was terionic Surfactants, ACS Meeting, Boston, MA, April 22-27,1990, hydrolyzed in 2-propanol with potassium hydroxide, the pret University of Bristol. cipitated potassium bromide filtered,and the residual potassium I The Procter and Gamble Company. bromide removed by using Rexyn 300 (Fischer)mixed bed ion 11 Institut Laue-Langevin. exchangeresin in 1:12-propanol-water.3 The initialconductance (1)Rosen, M.J. Surfactants and Interfacial Phenomena; Wiley: New was 2500 pfl cm-l and the final value was 4.2 pfl cm-1. The initial York, 1978. pH value was 12.8 and the final value 9.0. (2) Ottewill, R. H. Surfactants;Tadros, Th. F., Ed.;Academic Press: After the resin was filtered off, the water and 2-propanol were London, 1984. removed by azeotropic distillation using acetonitrile.g TLC (3) McGrady, J.; Laughlin, R. G.Synthesis 1984, 5426. ( 4 ) Hayter, J. B. Ber. Bunsen-Ces. Phys. Chem. 1981,86,887. indicated the presence of a nonpolar impurity, so the product (5) Hayter, J. B. Physics of Amphiphiles: Micelles, Vesicles and Miwas triturated with hexane, filtered, and dried; no impurities croemubions; Degiorgio, V., Corti, M.,Ede.; North Holland Amsterwere then visible by using TLC. The yield was 0.977 g (33.3% dam. 1985. overall). From the elemental analysis for carbon it waa inferred (6) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid that the sample contained about 6.4% water. The infrared Polym. Sci. 1987,265, 619,628. (7) Markovib, I.; Ottewill,R. H. Colloid Polym. Sci., 1986,264, 65. spectrum was consistent with the expected structure. Mass -----I
0743-7463191/2407-0878$02.50/0
0 1991 American Chemical Society
Ultralong Chain Surfactants spectrum (fast atom bombardment (FAB)) data indicated that the average deuterium content was DM instead of the DI1 anticipated. g(Eicoeyldimethyl-dga"onio)he.anoate, CpAH-& The preparation was similar to that described above except that deuterated dimethylamine(generatedby the action of 50% potassium hydroxide on dimethyl-de-ammonium chloride) was utilized to form the NJV-dimethyl-d6-eicosanamide.After several unsuccessfulattempts to reducethis amideusing LiAlD,, it wasreduced smoothly by using LiA1)4. The LiAlDd was found to have oxidized and/ or hydrolyzedon standing;this led to the formation of various byproducts. The final product showed no detectable impurities, usingTLC, and was found to be free of halide using a silver acetate/acetic acid reagent. Silver nitrate/nitric acid cannot be used for this test as it forms a precipitate of the nitric acid salt of the surfactant. Infrared and FAB mass spectral data were consistent with the expected structure. The exact level of deuteration could not be quantitatively determined, owingto fragmentation at the N-CHa bond. No halide anions were found during negative ion FAB mass spectrometry. The product was estimated to contain 2.5 % water based on elemental analysis. Critical Micelle Concentration. The critical micelle concentration (cmc)of C A H in water was estimated from a surface tension against log [concentration] plot to be (2.4f 1.0) X mol dm-8 at 25 "C. Density Meaeuremente. The partial molar density of CmAH was obtained with an Anton Paar DMA 60 digital density meter. The apparatus was calibrated by using air and water. The value obtained was 0.933 g cmJ at 25 OC. The density of the deuterated analogue CdH-dN, which was only available in small quantities, was estimated by scaling the density of the C A H material by the ratio of the molecular weights of the deuterated and undeuterated materials. This gave a value of 1.019 g cm+. Similarly, the material with deuterated N-methylgroups was estimated to have a density of 0.946 g cm-*. The densities of other materials were taken from the literature. The molecular weights, formulas, and densities are summarized in Table I. Small Angle Neutron Scattering (SANS). All the experiments were carried out at the Institut Laue Langevin (ILL), Grenoble, France, using the neutron diffractometer D17.8 The solutions were examined in optical-quality quartz cells with a path length of 1 mm. The cells were contained in a thermostated metal block at 25 OC. Measurements were carried out by using sample detector distances of either 1.40 or 3.40 m and a neutron wavelength of 12 A. The full width at half-height of the distribution of wavelengths was 10%. The scattering vector, for isotropic elastic scattering, can be defined by
x
Q = 4u sin (0/2) with 8 the scattering angle. Measurements were made over a Q range from 0.008 to 0.16 A-1. Standard ILL computer programs were used to process the basic data to give the intensity of scattering Z(Q) as a function of Q,normalized to watereg The appropriate background was subtracted from the scattered intensity of each of the samples, and corrections were made for attenuation of the beam due to absorption by the sample. The scattered intensities were converted to absolute units, dZ/dn using the scattering crosssection of water for the appropriate temperature and wavelength.
Theory Homogeneous Spherical Particles. For a monodisperse system of spherical, noninteracting particles, the scattered intensity in arbitrary units, Z(Q), as a function (8)Guide to Neutron Research Facilities at the ILL; Institut Laue Langevin: Grenoble, 1988. (9) Ghosh, R. E. A Computing Guide for Small Angle Scattering Experiments; Inrtitut Laue Langevin: Grenoble, 1989.
Langmuir, Vol. 7, No. 5, 1991 879
La Figure 1. Schematic illustration of cross section of core-shell structure: R,, core radius; 8, shell thickness; pm, p,, and p,, the coherent scattering lengths of medium, shell, and core, respectively. of the scattering vector Q can be written in the formlOJ1
with pp and pm being the coherent scattering length densities of the particles and the medium, respectively, Np the number of particles per unit volume, and Vp the volume of the particles. For a sphere of radius R the particle shape factor, P(Q),is given by
In order to convert arbitrary values of intensityto absolute units, dZ/dQ, the following equation was used (3) with NHD the number of water molecules per cm3 a t 25 "C,UH& the scattering cross-section of water, and T, and T, respectively the transmission of water and the sample. Concentric Sphere Particles. For the zwitterionic surfactant used, with a large head group and where independent deuteration of the eicosyl chain and the methyl groups on the quarternary nitrogen was possible, the simplest model to examine was that of a concentric sphere. This meant that the particle was composed of a core of hydrocarbon chains, radius R, and coherent scattering length density po and an outer shell of the head groups with a thickness, 6, and a coherent scattering length density, pe. This gives the total radius of the micelle, RT, a value of R, + 6. These parameters are illustrated in Figure 1. The basic scattering equation for this situation can be put in the form7
1(Q)= Np[b,
- Pm)(Az -AI)
+ (P, - pm)A1I2
(4)
with A, = 3V,[(sin QR, - QR, cos Q R , ) / Q 3 R ~ l and
A, = 3VT[(sin QRT - QRT cos QRT)/Q3R$] such that
+
= 41r(R, 6)3/3 and V, = 41rR:/3 Polydispersity. Polydispersity of the core particles was taken into account by using a zeroth-order log normal distribution12namely VT
(10) Guinier,A.;Fournet,G. Small AngleScattenngofX-Rays;Wiley: New York, 1956. (11) Jacrot, B. Rep. h o g . Phys. 1976,39,911. (12)Markovib, I.; Ottewill, R. H.;Underwood, S. M.;Tadroe, Th. F. Langmuir 1986,2, 625.
880 Langmuir, Vol. 7, No. 5,1991
p(RJ =
Hodge et al.
exp[-(ln R, - In Rm)2/2~,2] ( ~ T ) " ~ Uexp(u,2/2) ,,R~
(5)
wherep(R,) gives the proportion of core particles of radius R,, with Rmthe mean modal core radius and uoaparameter describing the width and skewness of the distribution.13 Particle Interactions. In dilute dispersions the number concentration is small and, hence, only a few particles interact on a time scale of experiments as a consequence of Brownian collisions. As the number concentration of scattering units, Np,increases, however, the particles are in constant interaction and consequently some degree of ordering can occur that is dependent on the number concentration and the strength of the interactions; for micellar systems the latter are likely to be repulsive. The spatial correlations produced as a consequence of the interactionslead to interparticle interferenceeffeds, which can be expressed as a structure factor, S(Q) given by
S(Q) 1 +
?Am[&)
- 11sin Qr dr
where g ( r ) is the pair correlation function and r is the center-to-center interparticle separation. For interacting particles eq 4 must therefore be written in the form
1(Q) = AN,[(p,
- pm)(A2 - A,) + (P, - p,)A1I2S(Q)
(7)
The form of S(Q) depends on the type of interactions between the particles. For the micellar systems investigated these are (i) repulsive hard sphere interactions14 and (ii) repulsive electrostatic interactions between the particles as a consequence of the formation of an electrical charge on the micelle surface.14 A straightforward solution for hard sphere interaction has been given by Ashcroft and Lekner,l6based on the Percus-Yevick approach,le in the form
with
where a,8, and y are given by a = (1+ 24eff)2/(1- 4,fJ4
P = - W e f f ( l +0*54eH)~/(1-4eff)'
-
Y = o * ~ e f f ( 24eff)2/(1 l+ +eff14
with 4eff an effective volume fraction given by 4eff 4~Re:Np/3 Reff is defined as an effective radius such that when two particles are separated by a distance 2Reff,the potential energy of repulsion becomes infinite. (13) Espemcheid, W. F.; Kerker, M.;Matijevib, E. J. Phys. Chem. 1964,68,3093. (14) Ottewill, R.H. ScientificMethodsfor the Study ofPolymer Colloids and their Applicatiom; Candau, F., Ottewill, R. H., Eds.;Kluwer: Dordrecht, 1990. (15) Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966,145,83. (16) Percue, J. K.; Yevick, G.J. Phys. Reo. 1968, 110, 1.
Table I. Formulae, Molecular Weightr, Denritier, and Coherent Scattering Length Denritier ( p ) formula
mol w t
density/ g cm-3
18.02 20.02 439.8 479.8 445.8 281.5 321.2 158.2 164.2 114.0 50.0
1.00 1.10 0.933 1.019 0.946 0.78 0.89 0.91 0.945 0.924 0.85
p/1010
cm-2
-0.56 6.40 -0.077 5.26 0.72 -0.34 6.60 0.497 2.66 0.368 6.42
For repulsive electrostatic interactions, a pair potential of the form given by Verwey and Overbeek" can be used, namely
V ( r )= 4m0tp214,2exp(2KR) exp(-h.r)/r (10) where e, is the relative permittivity of the medium, €0the permittivity of free space, the surface potential of the particles, and K the Debye-Huckel reciprocal double-layer thickness of the bulk electrolyte,which for a 1:lelectrolyte is given by = 2 ~ N ~ ~ e ~ 1 0 ~ / ~ ~ t , k T(11) where e is the fundamental charge on the electron, NAV is Avogadro'snumber, and c is the electrolyteconcentration in mol dm-3. Using this potential function, Hayter and Penfoldla and Hayter and Hansenl9have obtained a form for S(Q) which is applicable to an ensemble of charged interactingparticles such as micelles4 or polymer laticesm where multibody rather than pairwise interactions occur. The form of the equations required is discussed in detail by Hayter and 0, and high Penfold.18 For very low potentials, ~ 1 , the rescaled mean electrolyte concentration, K spherical approximation (RMSA)of Hayter and Hansenls reduces to the hard sphere interaction result. In the present work the scattering equations for particles with core-shell morphology were combined with the equations for the structure factor and used in a fitting routine to compare with the experimental data. K~
- -
Rasults Information on Materials. The formulas, molecular weights, densities,and coherent scattering length densities of the materials used are listed in Table I. In addition, the values for the scattering length densities of segments of the molecules are included in the table, particularly for the cases where deuterated materials were used. C d H in DzO. Figure 2 shows the experimental results obtained on an 0.03 mol dm-3 solution of the fully hydrogenated zwitterionic surfactant, C A H in D2O. These measurements were obtained at sample-detector distances of 1.4 and 3.4 m; as can be seen, good concordance was obtained with the data from the two experiments. The use of the RMSA model clearly indicated that the micelles under these conditions were uncharged. The fit shown in Figure 2 was obtained by using the hard sphere approximation to model the interactions. With the single sphere model, eq 1, a good fit was obtained by using pp = (17)Verwey, E. J. W.; Overbeek, J. Th. G.Theoryof StabilityofLyophobic Colloids; Elsevier: Amsterdam, 1948. (18) Hayter, J. B.; Penfold, J. Mol. PhyS. 1981,42, 109. (19) Hamen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (20) Ottewill, R. H. Langmuir 1989,5, 4.
Langmuir, Vol. 7, No. 5, 1991 881
Ultralong Chain Surfactants 12
[
Table 11. C A E , Variation of Parameters with Concentration concn/ concn/ volume mol dm-8 10-8 g cm-8 fraction Z(0) RJA a/A &/A
I
0.005 0.010 0.030 0.10 0.20
2.20 4.40 13.20 43.98 87.96
0.0024 0.0047 0.014 0.047 0.094
1.84 4.05 10.84 24.17 31.13
25 25 25 24 24
4.5 5.4 5.3 6.4
7.0
29.5 30.4 30.3 30.4 31.0
av
24.6
5.7
30.3
Q/P Figure 2. Z(Q) against Q for a solution of C A H in D2O at a concentration of 0.03 mol dma. Data were obtained at two sampledetector distances: 0,3.40m; Q1.40 m. Solid line (-) is fitted curve obtained by using hard sphere model. 1
,
I
---+a-
0
0.04
\ 0.08
0.12
0.16
QiK' 0.01
0.04
0.08
0.06
Figure 3. I ( Q )against Q for solutions of C d H in DaO at various Conccntrntion. s/g em-' concentrations (mol dm-8): 0, 0.2; 0, 0.1; 13, 0.03;A, 0.01; . , Figure 4. c / (dz/dQ) against concentration (gcm-s)for solutions 0.006. Continuous line is fitted curve obtained by using hard of C A H in D2O. sphere-core shell theory and values given in Table 11.
-0.077 X 1Olo cm-2, when RT was taken as 29.2 A. Using the core-shell model, eq 4,with p, = -0.34X 1Olo cm-2,that of the hydrocarbon tail, and ps = 0.497 X 1Olo cmm2,that of the head group, gave R, = 24 A and 6 = 6.2 A. This indicated that the total radius of the micelle was 29 f 1 A. It was not anticipated that a significant distinction between the two models would be possible under these conditions, since with pm = 6.40 X 1O'O cmm2for DzO, pm > pa and also pm > p,, conditions were not particularly sensitive to the core shell structure. The zeroth-order log normal distribution was used to introduce a small degree of polydispersity for the core radius of the micelles. The curves were not particularly sensitive to a variation of between 5% and lo%, but deviated beyond this. This implied that the size distribution of the micelles was rather narrow. It should be noted that the radius used to calculate the hard sphere interaction in eq 8 is the effective hard sphere radius and hence this would be slightly larger than the true radiue of the anhydrous micelle. ConcentrationDependence. C d H in DzO. Figure 3 records the results obtained on solutions of the fully hydrogenated material, C A H , in DzO at concentrations of O.OO6,0.01,0.03,0.10, and 0.2mol dm-3. The fits to the curves obtained indicated that substantial hard-sphere interactions occur between the micelles, particularly at the higher concentrations of surfactant. The parameters relevant to the fits are shown in Table 11. Volume fractions were calculated by using a density for the surfactant of 0.933 g ~ m and - ~values of I ( 0 ) were obtained from the
fitted curves; the latter were in good agreement with those obtained by using Guinier plots of In I(Q) against Qz.21 Molecular Weight of the Micelles. The mass of one micelle, %, is given by V,d, where d is the density of the surfactant. Hence, the molecular weight M = ~ J V A V , where NAV is Avogadro's number. Moreover, if the concentration of the surfactant, c, is expressed in g ~ m - ~ , we have, c = m a p ,giving in absolute scattering units for a homogeneous sphere
,
-
-
-
-
-
As Q 0, then P(Q) 1.0 and S(Q) S(0); the latter quantity is related directly to the osmotic compressibility. Thus
As c 0, the effect of the interactions becomes less significant and S(0) 1, giving
Figure 4 shows a plot of c/[dZ/dn] against c, which allowed a value of [ d I ; / d Q ] ~ , wto be obtained by (21)Hodge, D. J.; Ottewill, R. H. To be submitted for publication.
Hodge et al.
882 Langmuir, Vol. 7,No. 5, 1991 0.6
chloride as an added electrolyte. The change in the form I of the curve compared with those in the absence of
.q Bi, h
f
I
0.3
B I
0 0
0.04
0.08
0.12
0.16
QI8"
Figure 5. Z(Q) against Q for a solution of C d H - d M (0.05 mol dm-3) in DzO. Continuous line is fit to hard sphere core-shell theory with R, = 25 A and S = 5 A.
extrapolation and hence the micellar weight to be calculated. The value found was 47 750. The positive slope of the curve indicates the presence of strong repulsive forces between the micellar units. From the molecular weight of the anhydrous micelle and the density of the surfactant, 0.933 g cm-3, the radius of the spherical unit is calculated to be 28 A. C2oAH-d40i n D2O. The results shown in Figure 5 were obtained by using a sample of 6-((perdeuteroeicosy1)dimethy1a"onio)hexanoate (CZ&H&)) in D2O at a concentration of 0.05mol dm-3. Under these conditions the deuterated chain with a coherent scattering length density 6.60 X 1010 cm-2 is close to that of the medium, DzO, with pn! = 6.40X 1Olocm-2. Hence, the scattering is principally due to the shell of head groups, with ps = 0.497 X 1O'O cm-2. The enhancement of the peak a t ca. Q = 0.125 A-1 gives a clear indication of the presence of an outer shell, and andysis of the data using the core-shell model gave R, = 25 A and 6 = 5 A. Studies with CmAH-de. Experiments were carried out with the material, 6-(eicosyldimethyl-d6-ammonio)hexanoate in H20-D20 mixtures containing 10% ,15%,20 % , and 25% D20. This corresponded to coherent scattering length densities of the medium, pm,just below and just above the contrast match point of the compound. As shown in earlier work7 this approach provides a very sensitive means of examining particles with a core-shell geometry, since very significantchanges occur in the shape of 1(Q)against Q as the scattering length density of the medium is changed. In this region of pmvalues, moreover, the scattering observed was primarily that of the deuterated methyl groups. The curves of 1(Q)against Q obtained by using this approach are shown in Figure 6. Although the scattered intensity is weak, there is a considerable change in the form of the curves as the contrast changes. The curves obtained are of the form expected for a core-shell particle. The curves calculated to fit the data, based on the coreshell model described earlier, are shown as continuous lines. The parameters giving the best fit to the experimental data are listed in Table 111. These values indicate that the deuterated methyl groups are in a rather compact shell with a thickness of 3.3 f 0.4 A. This value suggests that there is little fluctuation with time of the charged nitrogen centers and that the layer is rather compact. As the total head group shell has a thickness of about 5 A, this suggests that the -COO- group is slightly further toward the external periphery of the micelle than the -+N(CD&- group. Effect of Electrolyte. Figure 7 shows a curve of Z(Q) against Q for a solution of C 2 d H at a concentration of 0.15 mol dm" in DzO containing 0.1 mol dm-3 sodium
electrolyte (Figures 2 and 3) is dramatic, suggesting that in the presence of electrolyte considerable repulsive interaction occurs between the micelles. These data were fitted using the RMSA model to calculate S(Q) with a floating value of the surface potential, $*. With R, = 25 A,b = 5 A,volume fraction = 0.071,and the scattering length densities in Table I, the best fit, shown in Figure 7, was obtained with $s = 32 mV. Since tsenters the equation as $,2, it was not possible to decide the sign of the potential from the fit. However, more detailed studies of the effect of saltzlshowed more pronounced effects with sodium bromide than with sodium chloride. This suggests that the anions are affecting the surface environment more than the cations. Discussion In this work micellar solutions of C2&H have been investigated by small angle neutron scattering in DzO as a means of obtaining maximum contrast between the micelle and the scattering medium. The fits of the data to amodel indicate that in D20 the micelle is uncharged, and if due allowance is made for the volume fraction dependence based on hard sphere interactions, excellent fits are obtained to a spherical unit containing a hydrocarbon core of 25 f 0.5A and a shell of 5 f A,thus giving a total radius of 30 f 1.5 A. The micelles appear to be rather monodisperse. This could signify that rather small fluctuations in shape occur in these zwitterionic micelles, possibly as a consequence of the electrostatic interactions occurring between the head groups. The molecular weight of the micellar units was found by extrapolation of the data to zero concentration and zero scattering vector. The value obtained was 47 750. The uncertainty in this value is of the order of f 5 % .Taking the mean value and the molecular weight of the monomer units as 440 (see Table I) indicates that a micelle on average contains about 109 monomer units. From the overall radius of 30 A, it follows that the area per head group in the micelle is about 100 A2. This is slightly larger than the value of 90A2 suggested by Brode22for the head group from adsorption studies on quartz. The radius of the micelle core was consistently found to lie between 24 and 25 A. T a n f ~ r dgives, ~ ~ for the maximum length of a hydrocarbon chain with n carbon atoms, the equation 1,
= 1.5 f 1.265n
which for a CZO chain gives 26.8A. However, this is a fully extended chain and hence a maximum value. For the present compound the -CHp group next to the nitrogen could be considered as part of the head group. It seems reasonable to conclude therefore that the measured value of 25 A is in reasonable agreement with expectations but that there could be some compression of the chains in the micelle core. The fully extended chain conformation becomes progressively less probable as the chain is lengthened. All the scattering measurements are consistent with the zwitterionic head groups forming a shell on the outside of the micelle with a thickness of ca. 5 A. The measurements obtained by using the material with deuterated methyl groups on the nitrogen, as a function of contrast, suggested (22) Brode, P.F.Langmuir 1988, 4, 176. (23) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1973.
Ultralong Chain Surfactants
Langmuir, Vol. 7, No. 5, 1991 883
0.12
0.10
0.06
(1.05
b
t
L
I
.-
. y1 . I
E
a
?
< . -
(1.10
a. t o
a
0.20
Q18-l
L
1
0.20
Q/8-’
m
a.2a
C
M
0.10
1
0.10
t
I
t
a
0.10
0.20
a
I 0.10
Q18-l
0.20
QI8-l
Figure 6. Z(Q) against Q for solutions of CAH-ds at a concentration of 0.038 mol dm4 in H20-D20 mixtures to illustrate variation with contrast: (a) 10% DzO; (b) 15% D20; (c) 20% D20; (d) 25% DzO. Continuous lines are fits to core-shell theory (see Table 111). Table 111. Contrast Variation Experiments with C*HII+N(CD~)~(CH~)~COOmedium pm/lOIO cm-2 atA RJA 0.136 2.90 25 10% Dz0 0.480 2.92 25 15% DzO 0.830 3.40 25 20% D20 25% D20 1.18 3.93 25
that the -+N(CD& group formed a shell with a thickness of 3.29 f 0.42 A. This indicates that the-+N(CD&-groups are arranged so that all the nitrogen atoms are at about the same distance from the center of the micelle. The larger overall thickness of the shell (about 5 A) obtained from measurements of the other compounds, suggests that the -C
40-
*O
group is slightly further from the center of the micelle than is the -+N(CH& group. However, it must be concluded that the +N+ and the -COO- charges must be closeto the surface of the micelle and that the vectorsjoining the charges must be almost parallel to the spherical surface of the micelle. From the area per head group, the distance between the two charges would appear to be of the order of 10 A. A major question which arises is the conformation of the five - C H r groups joining the positive nitrogen and the COO- group. These results imply that the +N+ and -0-are close to the surface, with the -(CH&- chain configured toward the center of the micelle. As pointed out by La~ghlin,~‘the distance between the charge centers is determined by the conformational structure of the bridgingmethylene groups and, except for the unique case of a monomethylene zwitterionic group, e-CHz-8, the bridge will almost certainly exist in a variety of conformations. Thus the actual conformational structure of the (24)Laughlin, R. G.Adu. Liq. Cryet. 1978, 3,W.
32
I
wR-’
Figure 7. Z(Q) against Q for a solution of C A H in DzO at a concentration of 0.1 mol dm-8 in the presence of 0.1 mol dm-8 sodium chloride: 0,experimental points; -, fitted curve with = 32 mV.
bridging group will be determined by the complex interplay between electrostatic forces and conformational energies, as influenced by the local environment. While specifying n in the -(CHZ)~-of the head group will define the structure of the zwitterionic group, it does not specify the distance between the charged groups nor even require that the distance be single valued. Further information on the conformation of the methylene groups could be obtained if material deuterated in the bridginggroup were available. The experiments on C A H in DzO without added salt as a function of concentration (volume fraction) indicate that hard-sphere interactions exist between the micelles. Since the micelles appeared to be uncharged and the interactions are repulsive, steric interactions arising as a consequence of the hydration and conformation of the head groups must exist. The available phase, basicity, and chromatographic data are consistent with the idea that the hydrophilicity of zwitterionic groups is influenced primarily by the nature of the anionic group. Thus, the strong hydrophilicity of ammoniocarboxylates not only contributes to its high solubility but also produces micelles with strongly hydrated head groups. A small dipolar component, acting in a direction perpendicular to the
884 Langmuir, Vol. 7, No. 5, 1991
surface, may also contribute to the repulsive interactions between the micelles. The pronounced effect on the scattering curves caused by the addition of electrolyte appears to be consistent with the idea that anions bind to the surface of these micelles more easily than do cations, or negative adsorption of cations occurs. The surface potential of A32 mV, produced on the micelle by the addition of 0.1 mol dm-3 sodium chloride, corresponds to a surface charge density of A2.55 pC cm-2. If all the potential drop in the electrical double layer were due to a layer of adsorbed chloride ions, this would correspond to approximately 18 chloride ions per micelle. This would imply that only about 17 % of the
Hodge et al. head groups are associated with a chloride ion or that the degree of dissociation is of the order of 0.17. There is also a possibility that a small degree of hydrolysis can occur in the presence of electrolytes to form +N-C02H and this certainly occurs a t acid pH values.21 Hydrolysis could be eliminated by using ammoniosulfonate or ammoniosulfate analogues,but these are not sufficiently soluble for these investigations.
Acknowledgment. Our thanks are due to the Procter and Gamble Company, Cincinnati, OH, for a studentship supporting D.J.H. Our thanks are also due to SERC and ILL for support and for the use of neutron facilities.
