Langmuir 1994,10, 2213-2218
2213
Lamellar Aggregates in the LBPhase of a Nonionic Silicone Surfactant (L77-OH) David C. Steytler,*D. Lee Sargeant, and Brian H. Robinson School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Julian Eastoe School of Chemistry, Cantock's Close, University of Bristol, Bristol BS8 ITS, U.K.
Richard K. Heenan ISIS,Rutherford Appleton Laboratory, Chilton, Oxon OX11 OQX, U.K. Received January 27, 1994. In Final Form: April 26, 1994@ The ternary phase diagram for the silicone-based,nonionic surfactant L77-OH,water, and cyclohexane shows a narrow Lz region in the oil-rich corner. At 25 "Cthe maximum water solubilization, as determined by the water-to-surfactant molar ratio, W m m ,is 22.0. Since this is close to the estimated hydration number of the surfactant hydrophilic EO groups, reversed micellar aggregates,rather than water-in-oilmicroemulsion droplets, are believed to be present. Analysis of the SANS profiles from the dilute (4 < 0.05) Lz phase indicates that discrete lamellar aggregates are present at all w values. The SANS data are quantitatively fitted using a disk form factor P(Q)which determines the bilayer thickness ( t )and allows an estimation of the lateral dimension through the disk radius ( r ) , At low water content (w 5.0) the value oft suggests that the EO chains are in an interdigitated (or coiled) configuration. It was found that t increases with w until at w m m the EO chain is apparently fully extended. The radius also increases with w from 77 A (w = 3.4)to larger dimensions (> 100A)which cannot accurately be measured using the available Q-range. Further support for a reversed bilayer structure in the Lz phase is obtained from studies of binary L77OWwater mixtures. Polarizing microscopy shows the phase to be lamellar (La).A similar layer thickness in the binary and ternary phases at the same w is found by SANS.
-
1. Introduction Traditionally, water-in-oil (wlo) and oil-in-water (o/w) microemulsions have been prepared using hydrocarbon oils and hydrocarbon-based surfactants. The structural and dynamic features of these systems have been extensively studied by a variety of investigative techniques.' However, microemulsions formed from silicone-based oils and surfactants have received little attention to dak2t3 In this paper we present a small-angleneutron scattering (SANS) study of the LZphase composed of the siliconebased, nonionic surfactant L77-OH in cyclohexane(C6D12). The results indicate that for small volume fractions of the dispersed phase (4 < 0.05) large discrete lamellar aggregates are formed for all values of w (= [HzOy[L77OH]). Poly(dimethylsi1oxane) (PDMS), or silicone, oils are available in both linear and ring configurations covering a wide range of molecular weight. PDMS oils of high molecular weight spread like surfactants at the aidwater interface and lower the surface t e n s i ~ n . This ~ ? ~ behavior is believed to be caused by a difference in polarity between the methyl groups and the silicon-oxygen backbone such that the oxygen atoms are oriented into, and the methyl groups away from, the water surface. The dimethylsiloxane unit differs from that of hydrocarbons (CHz) in terms of both polarity and size. At the surfactantfoil/ water interface it may be expected that the large molecular Abstract published in Advance A C S Abstracts, June 1, 1994. (1)Lindman, B.; Ninham, B. In Progress in Microemulsions; Martellucci, s.,Chester, A. N., Eds.; Plenum Press: New York, 1989;p 85. (2) Katayama, H.; Tagawa, T.; Kuneida, H. Poster presented at ICSCS Symposium, Compiegne, France, July 7-13, 1991. (3)Messier, A,; Schorsch, G.; Rouviere, J.; Tenebre, L. Prog. Colloid Polym. Sci. 1989,79, 249. (4)Jarvis, N. L.J . Colloid Interface Sci. 1969,29,647. ( 5 ) Mann E. K.; Langevin, D. Langmuir 1991,7 , 1112. @
diameter of PDMS oils will restrict their penetration into the surfactant layer. Such factors affect fundamental properties a t the oil/water interface (interfacial tension, preferred curvature, rigidity) which determine the size and shape of aggregates in ternary (oil/water/surfactant) system^.^-^ Similar effects will also govern the oilsurfactant interactions in systems containing silicon-based surfactants. L77 is a member of a large group of nonionic, siliconebased surfactants marketed by Union Carbide under the trade name Silwet.lo The molecular structure (Figure l a ) is derived from the silicone oil octamethyltrisiloxane (Figure lb), a propyl group replacing one of the central methyl groups and bearing the hydrophilic poly(ethy1ene oxide) (PEO) chain. The common form of the surfactant has the PEO chain terminated by a methoxy group, making it less hydrophilic than the OH-terminated derivative as used in this study. To distinguish the two forms of the surfactant we will refer to the methoxy- and hydroxyterminated derivatives as L77-OCH3 and L77-OH, respectively. Davis et aZ.ll have recently reported an extensive structural study of a range of trisiloxane-based surfactants, including L77, in water. They observed increasing curvature of aggregates with increasing EO number of the hydrophilic chains of the surfactants in both the liquid crystal and micellar states. As far as we (6) Fowkes, F. M. J. Phys. Chem. 1062,66, 382. (7)Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I. Prug. Colloid Polym. Sci. 1990,81,36. (8)Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloom, D. M., Wyn-Jones, E., Eds.; Kluwer: Netherlands, 1990;p 557. (9)Aveyard, R.; Binks, B. P.; Mead, J. J. Chem. SOC.,Faraday Trans. 1 1986,82,1755. (10)Silwet Surfactants, available from Union Carbide, 39 Old Ridgeway Road, Danbury, CT. (11)He, M.; Hill, M.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993,97,8820.
0 1994 American Chemical Society
2214 Langmuir, VoZ. 10, No. 7, 1994
Steytler et al. L77-OH
a
/
/ / WatU
\ 26
\
..qcpc Dilute L 2
cyd-
Figure 2. Schematic ternary phase diagram indicating (arrows) the lacation of the h phase and binary L77-Owwater (dphaee) samples studied by SANS (LL77-OHI = 0.083 mol dm-9.
particularly topical concerns the formation of cylindrical reversed micelles and w/o microemulsions. Divalent metal sdtg of d n i c surfadants such as Aerosol OTlS-l7and &chained cationics such as DDAB’*have been shown to exhibit c)rlindrical strucwes over much of the LZphase. Naturally omwring soyban ledthin d s o forms cylindrical strwtura, a d large entangled “polymer-like” &versed zniceUar aggregate8 amprasent in the L2 phase, giviagrise’tohigh viacoaities.18The forination of extended bilayer Btractures (lamella micelles) is believed to be energetically unfavorable due to edge effects.14 Indeed evideuce for such structures either in water (L1)or nonpolar media (L) is scarce.2o
Figure 1. (a)Structure of the nonionic surfactants L77-OCHs (R = CHs) and L77-OH (R = H). (b) Octaxnethyltrisiloxane, frm which the L77 surfactants are derived.
2. Experimental Section
The phase behavior and strUctgl.al features of aggregates formed by the surfactant L77-OH have been examined as a are awam, there have k n o previous studies of the function of w both in the oil-rich corner (La)and on the b i n m surfhctantfwatmridedtheternaryphasediagram as represented structuree of 4phases h m e d by nonionfcsilicone schematicallyin Figure 2. Surfactants. An u n w d &,mcturdf-tum of the L77 srnfaatants a.1. Phnse Behavior. The extent of the single-phase LZ region can be represented on a pseudobinary phase diagram is the ~ ~ d i g u m t i oofnthe eilicone-based hydrophobe, showing the m a x i “ uptake of watm as a function of which is much less extended than in c o n v e n t h d hydrocarbon-badnonionicsurfactan~derivedfiramlitlear t e m p e ” at a fixed surfactant concoatration. To determine phase bound” on the diagram,-pies (5 mL)were prepared alcohols (CnEmS, Furthemawe, due ta the highdbneicy of Bontaining 5% L77-0H (0.083 mol dm-9 with varying amounts methyl p u p s , the methyldoxane p u p s oantribute ofwater, The phaee boundaries wegv determined by inspection app&b€eapMty, makingthe swf&”&Imably and approached by two different pathe. (i) iaothermally, by more hydrophobic lbhan ~~-dkaae-baaed~ swf8ctank, d increpsingw ,and (ii) by &anging temperature at fixed w. Due similar m o l d a t weight. The of L774CHs ,to to close refkactive-indexmatching between the dispersedphase wet solid surfaces and its aaeociatRd physical groperties and the oil, phase transitions were sometimesdifficultto observe. have been repcmbd previmly by AnantibpaaibrnabtLan The concentrationof h e (monomeric)surfactant in the oil phase etaZ.12 Inthiestudythe s r n f a c t a n t w w b u n d # o h a e e was d k A ” d by the procedure described by Fletcher et al.21 thesuface~~oll(y)d~eair/waterinterfaos~eOmNfor nodonic surfadants oftype CnE,. Polarizingm ? i c ~ ~on~ ~ p y thebiaargL7?-0I#ataraamples wascarriedoutuaingaVickeickerr, m-l and to be a highly e f f i h t wetting agwC for btrumentmptkalmimicroecopefittedwithatherm~~sample hydrophobiCdaeemeuchaa@ye&yhe. Thiebdmvior stage and m e s e d polariaera. can be underatood ia;termeof the kydmphbbe &m&ure, BB. M S . SANSmeaeurammtswereperfarmedonthetimewhichi~thatghtb~epcssentanfoptimtmtx&gu&h o W g h t IBQspectrometerwingthe ISISpulsed neutron source for efficientdilm propagation. M h y e t r packing: of the of the sER(= Rutherlord Appleton Laboratory, U.K. The surfactant at the .aihqter intarfa“ was .f@ ta be l i m i t e d b y t h e l a t e r a l ~ o n o f t h e s ~ h y c k o p h o b e magnitude of the momentum transfer vector Q ia given by
Ww
so&afitheareapepd~tisgjrSater(70&9tita&for nonionic S~W&&II~ OftyaeCnE, ofshilar Eo number. There is mtrch int.+r~~& in the factors wbich determine the formation of nonspherical s y z f e t aggregates in apolar media,and.v&w theories d a t i n g shape to intwfuial cnniaturem‘giditp and slnfwmlt geometry“ have been presdted. One area which is
c“r&
(12) A m m h p d a a d b q K P.; Goddad, E. D.;C b d a r , P. couoide suq. 1 m , 44,281. (13) Safran, S.h,Turkevich, L.A; Pincua, P.J. Phya. btt.1984, 45, L69.
(14) LrPelechvili,J. N.; Mitchel, D. J.; Ninham, B.W.J. Chem Soc., Faroday !&am.0 1@76,72,16a5. (15)Eaetoe, J.: Towey, T. F.: Robinson,B. H. Williams J.;Heenan, R.K Langmuir lS@3,97,1459. (16)Petit, C.; Lixon, P.; Wed, M. P. Larylmrru 1891, 7,2620. (17)Eaetoe, J.; Robhaon, B, H.; *meto, 0,; Towey, T, F.; Heenan, R. K; Leng, F.J. J. Chena..sOe.,Fercrdqy Tmm. lOe2,88,461. (18) Ekmtoe, J. Langmuar 1991,8,1603. (19)scartazppq * R.; Luisi, P.L. J. Phy.9. Chsm.1988,92,829. (20)Maser, N.A; Benedek, 0.B.j Carey, M.C. Biochemistry, 1980. 19, 601.
(21)Aveyard, R.; Binke, B. P.;Eletcher, P.D.I. Langnwir l@SS,5, 1210.
Lamellar Aggregates in the L2 Phase of L77-OH
Langmuir, Vol. 10, No. 7, 1994 2215
(1) where A is the incident wavelength (2.2-10.0 A), determined by time-of-flight, and 8 is the scattering angle. The intensity of neutrons was recorded on a position-sensitive 64 x 64 pixel 2-D detector at a fixed sample-to-detectorposition (4.43 m)providing an effective &-range from 0.005 to 0.20 A-1 in a single measurement. The raw data were corrected for transmission and incoherent background scattering and normalized to absolute scattering probabilities (cm-1) using standard procedures. Further details of technical and experimental aspects together with data reduction procedures are given elsewhere.22Sampleswere contained in stoppered, matched LO-" Hellma cells and thermostated at 20 "C.For Lz samples the alkane medium was cyclohexane-& (MSD Isotope 99.5 atom D)so as to provide a contrast in mean scattering length density of approximately 6 x 10'0 cm-z against the L77-0WHzo aggregates. For binary L77-OWwater mixtures (lamellar phase) DzO (MSD 99.9 atom % D) was used to provide the neutron scattering contrast.
3. SANS Theory For structures such as spheres, rods, disks, or ellipsoids ofvolume V, present at number density ne and ofcoherent scattering length density e,, dispersed in a medium of em, the normalized SANS intensityZ(Q)(cm-l) may be written
I(&) = n,(e, - em>2V~{S(Q)(lP(Q)12> + V(Q))12 - (IP(Q)I2>1(2) P(Q) is the single particle form factor describing the angular distribution of the scattering owing to the size and shape ofthe particle. Expressions forP(Q) of spheres, rods, disks, ellipsoids, etc. can be used to model SANS data.23 S(Q) is the structure factor which arises from spatial correlations between particles. To a first approximation, when the particle volume fraction 9 is low (-0.05) and in the absence of interactions, S(Q) 1.0 across the Q-range accessed by SANS. Under these conditions I(&) is a direct measure of P(Q), i.e.,
-
I(Q) = n,(Ae)2V:P(Q)2
0
10
20
30
40
50
T (OC)
Figure 3. Binary w-T phase diagram for 5%wfv L77-OH in cyclohexane showing the extent of the single-phase LZregion.
For t-1 > Q > r-l and r >> t the limiting Guinier law for disks should be valid.25
4. Results
4.1. Phase Behavior. L2 Phase. A plot of the maximum water solubilized us surfactant concentration was used to determine the concentration of free monomeric surfactant (cpc)a t 20 "C. The intercept on thex-axis gave cpc = 0.98%w/v(0.016 mol dm-3). Similar measurements a t 30 "C gave, within error, the same intercept, demonstrating that the cpc is not significantly temperaturedependent over this range of temperature. A temperature invariance of the cpc has been noted previously for the haze boundaries of conventional nonionic surfactants.z8 To calculate the true w value, which is a fundamental parameter characterizing the aggregates, the cpc must be taken into account, and so w was calculated according to
(3)
Since P(Q) = 1when Q = 0, the scale factor, A, between I(Q)andP(QY for any given model representing the SANS profile is n,(k)2Vp2. If the data are fitted in absolute units, the value of A is a self-consistency check on the model since both npV, (=@,the volume fraction) and A$ are known. The fitting program we have developed allows us to examine a wide range of models, physically unrealistic solutions can then be eliminated using, in part, the scale factor criteria.23 For a dilute system of monodisperse, rigid disks the form factor may be writtenz4
where H = t/2, t = disk thickness, r = disk radius, and ,8 = a variable of integration representing the rod orientation. (22)Heenan, R. IC;King, S. M.; Osbom, R.; Stanley, H. B. Rutherford Appleton Laboratory Report RAL-89-128,Rutherford Appleton Laboratory: Chilton, U.K.,1989. (23) Heenan, R. K. FISH Data Analysis Program. Rutherford Appleton Laboratory Report RAL-89-129,Rutherford Appleton Laboratory: Chilton, U.K., 1989. (24)Livsey, I.; J. Chem. SOC.,Furaduy Trans. 2 1987,83,1445.
with [L77-OHl the total added concentration of L77-OH. The pseudobinary w- - T diagram, which indicates the extent of the LZphase, is shown in Figure 3. Interestingly, the phase diagram does not show the characteristic narrow "funnel" extending to high w over a narrow temperature range as found for hydrocarbon-based, nonionic surfactants of type C,E,,21 If it is assumed that each EO group of the surfactant within the aggregates requires two to three water molecules of hydration,26then the surfactant would be completely hydrated when w = 16-24. Since the maximum solubilization is wmax 22, this suggests that hydrated reversed micelles are present rather than w/o microemulsion droplets. Strictly speaking, in a w/o microemulsion system the activity coefficient of the dispersed water approaches unity and is "bulk-like". This is unlikely to be the case even at w = 22 for L77-OH LZ phases. Qualitatively, the solution viscosity (7)was greater than that of the cyclohexanesolvent and increased with w . It should be noted that the relatively high
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(25)Cabane, B. In Surfactant Solutions-New Methods of Inuestigution;Zana,R., Ed.; Marcel Dekker: New York, 1987. (26)Christenson, H.; Friberg, S. J. Colloid Interface Sci. 1980,75, 276. (27)Schurtenberger, P.;Scartazzini, R.; Magid, L. J.; Leser; M. E.; Luisi, P.L. J. Phys. Chem. 1990,94, 3695. (28)Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. J. Chem. Technol. Bwtechnol. 1992,54, 231.
Steytler et al.
2216 Langmuir, Vol. 10, No. 7, 1994 lo’
1.0
P
3.0
2
2 =b
2.0
C
LO
0.0
0.00
0.01
02
0.02
(2)
Figure 5. Guinier plot for disks (log(I(Q)Q2)us Q2) for L77OwH2O/CsD12. [L77-OH] = 3.5%wlv (0.058 mol dm-3), w = ,001
.01
.1
1
Figure 4. SANS data in log (Z(Q)) us log(Q) form for L77OH/H~O/C~D~Z. [L77-OH] = 3.5%w/v, w values are as indicated. Also shown as solid lines are the fits to P(Q)disk. The I(@ values have been scaled and displaced for each data set in order to aid visualization. The scaling factors employed are 5” where n represents the ( n 1)th data set (w = 3.4, n = 0; w = 6.9, n = 1; ...).
+
Table 1. Morphology (from Microscopy) and Structural Parameters (from SANS) for Binary L77-OWDa0 Samples structure repeat distance ( d ) (A) 6.9 viscous liquid isotropic 44 13.8 “gel” lamellar, mosaic 49 20.7 “gel” lamellar, mosaic 54 w
physical state
viscosities observed at low volume fraction (4 < 0.05) are characteristic of anisotropic surfactant aggregate^.^^,^^ Binary L77-OH1Water. Observation ofthe higher water content L77-OWwater samples (w = 13.8and 20.7)under a polarizing microscope showed a mosaic pattern under crossed polars characteristic of a lamellar phase (La).The lowest water content sample (w = 6.9) exhibited no birefringence. The results are summarized in Table 1 along with layer thicknesses derived from SANS measurements. 4.2. SANS. Lz Phase. SANS results are shown in Figure 4 for samples in the LZphase for w = 3.4-17.1 at fxed L77-OH concentration. The data have been plotted on a log(I(Q))us log(&)plot for clarity of presentation. For all samples a t w > 6.9 and over the Q-range 0.01 < Q < 0.10 A-1 the data decay with an exponent of “-2, characteristic of bidimensional structures (lamellae). At Q =- 0.10 A-1 the exponent changes to --4.0, as is expected when scattering is sensitive to the interface (Porod scattering). Agradient in excess of -4 in the Porod regime, as found for the highest w system, is suggestive of a distribution of scattering length density at the oil/ surfactant interface caused by oil penetration into the surfactant layer.29 Measurements made as a function of volume fraction (4 = 0.025-0.075) demonstrated the absence of intermicellar correlations (i.e., S(Q) = 1) in this concentration regime. The I ( Q ) profiles of Figure 5 therefore represent the form factor P(Q). As described in section 3 a range of different functions forP(Q)were tested. A model for rigid monodisperse disks was the only form factor to realistically and quantitatively fit the entire &-range of the profiles. The fitted functions are shown in Figure 4. The disk thickness t and radius r were the only floated parameters, with the caveat that the scale factor A = f 1 0 % of the value calculated from the sample (29) Strey, R.; Winkle, J.; Magid, L. J.Phys. Chem. 1091,95,7502.
17.1.
Table 2. Fitted Parameters (Monodisperse Disk Model) C @ ~ Z Phase of the L ~ ~ - O ~ ~ O /Systema for the t/A 3.4 31.5 6.9 43.8 10.3 48.9 13.7 51.0 17.1 53.0 a T = 20 “C, [L77-OH] = 3.5%w/v (0.058 mol W
r/A 77.0 > 100 ’100 > 100
’100 dm-9
composition. The contrast employed (L77-OWHzO/ C6D12)means that the disk dimensions are measured at the oiysurfadant interface. Although the fitting procedure employed was sensitive to t , it was insensitive to r above a w value of approximately 7 where the disk radius becomes too large to be accurately defined within the limited Q-range of the instrument. A summary of the parameters obtained for the fits is given in Table 2. The data can also be analyzed using a Guinier limiting law. However, it should be noted that a full quantitative fit (as detailed above) is always preferable to a Guinier analysis. The plots for spheres exhibited positive deviation from linearity as Q2 0, and for cylinders no clear linear region was found. When displayed on a plot of log(I(Q)Q 2 ) us Q2 as in Figure 5, the data exhibit the features characteristic of small-angle scattering from disks. I t is noteworthy that the good linearity obtained by representing the data in Figure 5 by a Guinier disk model would not prevail for the corresponding sphere and rod models in which different quantities, namely, ln(I(Q))for spheres and ln(I(Q)Q)for rods, are plotted against Q2. The slope of the linear region in Figure 5 is proportional to t2, and the downturn at very low Q is caused by the finite size of the object. The gradient gives the disk thickness as 53 A, in excellent agreement with that obtained from fitting the form factor for disks (see Table 2). Binary L77-OHlWater. The SANS profiles for the higher water content binary L77-0H/D20 samples (w = 30.8,20.7) has a sharp Bragg-type peak at Qmax(data not shown). For Lalamellar phases the peak position can be used to obtain the repeat distance, d, via the relationship d = 2n/Qmax. The w = 6.9 sample, which is not La (Table 11, also shows strong structural correlations characterized by a peak a t higher Q. It is believed that liquid nonionic surfactants when weakly hydrated contain small partiallyhydrated reversed micelles.30 For the L77-OH system, which forms a lamellar phase at higher w ,such aggregates could conceivably be small bilayer “segments” of limited lateral extension. The peak in the SANS profile must
-
(30) G . J. T. Tiddy, private communication.
Lamellar Aggregates in the LZPhase of L77-OH
Langmuir, Vol.10,No. 7,1994 2217
then arise from intermicelle correlations and so again represent the closest contact distance between micelles, i.e., the bilayer thickness. 6. Discussion
Lamellar phases are reasonably common for ternary mixtures of poly(oxyethy1ene)surfactants, oil, and water. Interestingly they are often observed at the haze boundary of the LZregion near the phase inversion temperature, and it seems reasonable that lamellar aggregate structure may exist in the L2 phase close to this boundary. Both the phase behavior and SANS data for the L77OH surfactant show similarities to results previously reported by Bouzier and Ravey31a2of a SANS investigation of the structure of hydrated micelles formed from C,E, surfactants in oil media. In this earlier work it was concluded that the extent of solubilization of water (w”) in the L2 phase did not exceed the maximum hydration requirement of the hydrophilic EO groups. Their analysis is also consistent with the formation of small bilayer structures which become thicker and more extended as w increases. At low w the thickness of the bilayer micelles (“Hanks”) was found to be less than twice the fully extended conformation of the surfactant, suggesting that the EO chains of the surfactant are interdigitated (or coiled) in the interior of the micelle. As w increases, the bilayer thickness increases, until a t w,, the EO chains are fully hydrated and extended. It was concluded that the limit of stability was reached at the point when the surfactant becomes fully hydrated and that it is not possible to form a discrete water “droplet” within the micelles. The maximum aggregation number was found to be 1000, corresponding to a radius of approximately 75 A if the aggregate is considered to be a regular disk. Similar conclusionscan be drawn for the phase behavior of L77-OH in cyclohexanewhere the limit of solubilization w,, is close to the hydration number for the surfactant (estimated to be 18-24 water molecules). The PEO chain of the L77-OH surfactant would have a length of approximately 25 A in the fully extended, all-trans configuration. At low w the bilayers are observed to be of thickness significantly less than twice the fully extended surfactant configuration (approximately 50-60 A),suggesting again interdigitation (or coiling) of the EO groups in the bilayer under conditions of low hydration. With increasing w the bilayer also swells to a maximum value corresponding to a fully extended EO configuration which is completely hydrated. However, the lateral dimension of the L77-OH aggregates, as given by the radius r, also increases with w but to an extent much greater than that found for the C,E, surfactants. A further consistency check is to compare the repeat distance obtained for the lamellar liquid crystal phase (L77-OWwater)with the disk thickness measured in the Lz phase as a function of hydration (w).In the La phase (L77-0WD20) the diffraction peak gives the spacing between adjacent water layers as shown schematically in Figure 6. This corresponds exactly to the bilayer thickness in the lamellar phase and should therefore be in agreement with the disk thickness in the L2 phase for the same w value. The results obtained for thickness t in the L2 and La phases are compared in Figure 7. The close agreement obtained lends additional support to the formation of lamellar aggregates in the L2 phase. With this in mind it is important to state that we have no direct evidence that the aggregates are regular disks. Attractive inter(31) Ravey, J. C.; Bouzier, M.; Picot, C..J. Colloid Interface Sci. 1984,97,9. ( 3 2 ) Ravey, J. C.; Bouzier, M. J.ColloidInterface Sci. 1987,116,30.
1
........
Figure 6. Schematic representation of the lamellar phase showing the SANS contrast profile for the repeat distance (d) and disk thickness ( t )in the LZ phase. 60
,
20
10
0
30
W
Figure 7. Dependence on w of the disk thickness ( t )in the Lz phase (A),and repeat distance (d)for the lamellar Laphase (m).
micellar interactions, predominately at the edge of the lamellae (see below), will act to reduce the stability of the aggregates and can be minimized by formation of disks which present the minimum perimeter of any lamellae shape. However, owing to the dynamic nature ofmicelles, and the process of monomer exchange, a more realistic picture is probably one of irregularly-shaped lamellar structures. From simple geometrical arguments the relationship between the bilayer thickness, d , and w is obtained as
dlA
59.8w A,
-
(7)
The linear region in Figure 7 gives a gradient of 1.42, from which the area of the surfactant headgroup at the interface is obtained as 85 k,which is higher than that measured for the related L77-OCH3surfactant at the air/ water interface (A, = 70 A2).12 It is also informative to consider the formation ofbilayer aggregates in relation to the surfactant geometry represented by the headgroup area, A,, and the hydrophobe volume, V, and length, l . 1 4 On the basis of simple packing arguments, the limiting values of the packing parameter (p = V I M . ) for (reversed) bilayer formation are 1 < p < 2, with an optimum condition whenp = 1. This approach can only be strictly applied to the current system when the chains are not interdigitated. When this is the case, as shown schematically in Figure 6 for the “parent” La phase, the area A, is effectively doubled so that the optimum value for p will be higher.
2218 Langmuir, Vol. 10, No. 7, 1994 Formation of disk-shaped (and rod-shaped) micelles has previously been criticized on the grounds of unfavorable packing constraints or exposure of the surfactant to water (or oil) at the edge of the disk. We would argue that exposure of the EO groups to cyclohexanein an “uncapped” bilayer of L77-OH would not contribute a significant instability to the structure on the grounds that the surfactant has a relatively high cpc, i e . , the EO/oil interaction is not highly unfavorable. It is also informative to consider the relative area exposed by the edges of disks to that presented by the ends of rod-shaped micelles. For the same volume of surfactant forming either disk- or rod-shaped micelles the ratio of exposed areas (AdiBk/Ard) is given by the ratio of the larger dimensions (length of rod (111 radius of disk (r)). It is found that the exposed area for disks of radius, e.g., 300 A is equivalent to that of rods of the same length. Considering recent evidence
Steytler et al. for the formation of rod-shaped reversed micelles in this size range,15-17 we conclude that the formation of disks cannot be ruled out on the grounds of “edge effects” alone. The results presented here show that despite these edge effects lamellar structures in apolar media will form for an appropriate surfactant molecular geometry.
Acknowledgment. The authors would like to thank Professor Gordon Tiddy (Unilever Research, Port Sunlight Laboratory) for assistance with the polarizing microscopy as well as for helpful discussions concerning this work. Support from the NBRC of the SERC is also gratefully acknowledged for provision of beam time a t the ISIS facility in the U.K. and for a grant for deuterated chemicals. We also thank Ted Sadler and Grant Cameron of OSi Specialties (U.K.) Ltd. for provision of the L77-OH surfactant used in this work.
