Langmuir I994,10, 2985-2992
2985
Obvious and Nonobvious Influences of Surfactants on the Formation of Nanosized Particles Elva C. O’Sullivanf and Anthony J. I. Ward* Center for Advanced Materials Processing, Box 5814, Clarkson University, Potsdam, New York 13699-5814
Thomas Budd Department of Biology, St. Lawrence University, Canton, New York 1361 7 Received March 8, 1994. In Final Form: June 27, 1994@ Precipitation of P-FeOOH in the presence of aggregated surfactant domains shows expected influences on particle size and distribution. The crystal structure ofthe product is similartothat found in homogeneous aqueous solution. Surfactant stabilization of the nanoparticles in dispersion is observed over several months in lamellar liquid crystal media. A n important and unexpected observation is seen upon incubation ofthe precipitated material in the presence of excess surfactant. Particle size reduces and the size distribution becomes more narrow as the time of contact with the medium is increased.
Introduction Ferric oxyhydroxides have been studied extensively1 since they have a wide variety of applications including uses as pigments, catalysts, coatings, and flocculants and are important in recording media. Furthermore, the product of iron and steel corrosionis rust which, depending on the conditions, consists of colloidal aggregates of iron (hydr)oxides. Despite the attention paid to these compounds in the literature, little is understood about the mechanism of ferric oxyhydroxide precipitation especially in the ultrafine size range (0.1-50 nm). In fact, no methods have been reported for the production of these materials with the intention to sustain their dimensions in this nanometer size range. Synthetic strategies based on precipitation at high dilution or in the form of an aerosol are difficult to scale up and are realistic only for small-scale batch production of nanoparticles. Other methods involving chemical modification of the particle surface (“capping”)or encapsulation are successful in producing narrow size-distribution particles in the nanosize range; however, such procedures must also lead to modifications in the physicochemical properties of the materials. Finally, the use of surfactant aggregates has been reviewed2j3with most studies being made using microemulsion, micellar, or vesicular systems of a rather limited range of surfactants. The general view that has been adopted in this approach is the presence of ordered domains provided by the surfactant aggregates, leading to restrictions in particle growth and aiding the formation of a narrow size distribution. However, we have shown in some preliminary work4 that the presence of ordered domains, per se, is not a prerequisite for nanoparticle formation in media-containing surfactants. In general, particle formation involves complex formation, nucleation processes, particle growth, and dissolution. The presence of surfactant in the medium could be + Present address: American Cyanamid Co., Agricultural Ftesearch Division, P. 0. Box 400, Princeton, NJ 08543-0420. Abstract published in Advance ACS Abstracts, September 1,
expected to influence each of these proposed steps, a factor which has been much overlooked to date. A n example is provided by reactions (e.g.,formation of oxides, silica, etc.) where hydrolysis results from consumption of one of the reactant components, i.e., the medium. The consumption of the aqueous component in a surfactant system (e.g., microemulsion) can alter the range of stability of the phase which, in turn, can affect the reaction condition^.^ Complex formation between surfactant ions and multivalent ions can also be important since the solubility products for ionic surfactants are usually very low. This manifests itself in the observation of much higher Krafft temperatures for the multivalent ion surfactant pair; e.g., SDS (289 K) and CaDS (323 K) and precipitation of a surfactant complex could occur, depending upon the solubility product or the ability of the system to solubilize the complex. Confinement ofreactants to a medium that has definable aqueous and nonaqueous domains would be expected to affect the rates of both nucleation and particle growth. Preliminary measurements6,’have shown that nucleation processes are usually enhanced in the presence of surfactant, particularly in cases where one of the reactants is a component forming microdomains. In the classical model (La Mer), a rapid burst of nucleation s) is a primary requirement for the formation of fine particles, as is the restriction of processes such as Ostwald ripening, which leads to growth of the nuclei. In the presence of a structured surfactant medium this restriction in the growth processmay arise through the presence of adsorbed surfactant at the solidfluid interface andor through a kinetic restriction imposed by the presence of structured microdomains confining the reaction. The presence of surfactant in excess of that required to simply coat the surfaces of the particles also gives rise to the possibility of dissolution processes involving complexation between the reactant ions and the surfactant. This possibility is regularly overlooked when reactions are considered in these microdomains. Whether these associated surfactant media are merely passive hosts with no functions other than compartmentalization and geometrical control re-
@
1994. (1)Matijevic, E.;Scheener,P. J . Colloid Interface Sci. 1978,63,509.
(2) Fendler,J. H. Membrane Mimetic Chemistry;John Wiley & Sons: New York, 1982. (3) Ward, A. J. I.;Friberg, S. E. Muter. Res. SOC.Bull. 1989,xN, 41. (4) Donegan, S. B. W.D. Thesis, National Univ. of Ireland, 1992.
~~
~
(5) Osseo-Asare,K; Arriagada, F. J. Colloids Su$. 1990, 50, 321. (6) O’Sullivan, E.C.; Patel, R. C.; Ward, A. J. I. J . Colloid Interface Sci. 1991, 146, 582. (7) Ward, A. J. I.; Patel, R. C.; Donegan, S. B.; O’Sullivan,E. C.; Yang, Y. J . Colloid Interfuce Sci., in press.
o 1994 American Chemical Society
O'Sullivan et al.
2986 Langmuir, Vol. 10, No. 9, 1994
mains questionable. Some previous work26,27demonstrates that the interactions of the reactants with the reaction media is quite plausible, a n d models have been proposed for the solubilization ofNi,ll Co,ll a n d Fell1 ions into the inner water cores of wlo microemulsions. The relative roles of these various factors have been investigated here using the hydrolysis reaction of iron(111) chloride. This reaction was chosen because the amount of work previously done in homogeneous solution will allow some useful comparisons to be made. In this system, t h e effect of the anion on t h e nature of the iron oxide precipitated in homogeneous solution has a significant role in t h e hydrolysis process.' Acidic solutions of FeCl3 usually yield, on aging, j3-iron(III) oxide hydroxide-found naturally as the mineral akaganeite. The production of P-iron(II1) oxide hydroxide has been studied in detail,*-1° with some of the structural features being t h e subject of debate.'l Since the presence of C1- (or F-) is regarded as necessary for t h e crystal structure ofj3-iron(111) oxide hydroxide t o be maintained the general molecular formation is expressed as P-FeOOH or sometimes P-FeOOHnClz. Synthesis of monodisperse p-FeOOH using the long-range order found in associated surfactant media to influence particle forms the basis of the study presented here.
Hexane
- - - - -.. H O II
0.%25M FeCI, in 0.01 M HCI
I
Figure 1. Partial ternary phase diagram indicating the effect of ferric chloride on the microemulsion phase boundary. The two compositions that are referred to in later text are (i) composition A, 5% FeC13/48% hexane/47% C1z(E0)4 and (ii) composition B, 15% FeCls/42% hexane/43% C1z(E0)4 ([FeCl3I = (0.025 M in 0.01 M HC1). (Note: The presence of inverse micelles is not likely at these compositions since the surfactant/ water ratio is too high.)
Experimental Section Surfactant Systems. Commercial nonionic surfactants (Neodols from Shell Ltd.) were used as received. Combinations of Neodols 25-3 and 25-7 were mixed in proportions to give an average oxyethylenenumber of four moles per mole of surfactant. This surfactant will be referred to as C12E04 in the subsequent text. Control experiments using pure C12E04 (Nikko Chemical Co., Japan) showed essentially the same behavior as that for the mixed surfactant analogue. Doubly distilled deionized water and n-hexane were used for preparing the microemulsion and electrolyte solutions. Determination of Partial Phase Diagrams. The phase boundaries were determined for the following microemulsion regions: (1)ClzE04/n-hexane/0.025M ferric chloride in 0.01 M HCl; (2) ClzEOdn-hexanelwater. These diagrams (Figure 1)were produced by titration of the aqueous component into surfactantln-hexane mixtures. The resulting samples were mixed vigorously using a vortex mixer. The aqueous component was either water or a solution containing one of the ions. Samples with compositions on either side of the phase boundaries implied by the titration were then prepared by weight and stored at 298 K. Visual inspections were subsequently made to determine the nature of the system at these compositions. The boundaries obtained have a certainty of ca. 5%. Synthetic Methods. Ferric chloride (Fisher Scientific Co.) solution (0.025 M) was prepared in aqueous HCl(O.01 MI (Baker Chemical Co.) using doubly distilled water. With a 60:40 (w/w %)ratioofthe surfactant, ferric solutions were mixed on a vortex mixer and centrifuged to remove air bubbles. This composition lies in the lamellar liquid crystalline region at temperatures below ca. 341 K on the phase diagram (Figure 2). Batches of approximately 2.5 g were made up for each synthesis. Many syntheses were carried out by preparing different samples that were heated at 333 Kor at 353 Kin a water bath for varying time intervals. For the samples heated to 333 K it is important to note that the lamellar liquid crystalline phase was maintained while the samples heated to the higher temperature underwent a transition to the two-phase region; i.e., it contained an isotropic phase during the heating step. The samples were then washed (8)Raver, I.; Gourque, A.; Gabelica, Z.; Nagy, J. B. Proc. 81st Internatwnal Congress on Catalysis; West Berlin, 1984,Vol. IV,p 281. (9) Nagy,J. B.; Deroune, E. J.; Gourque,A.; Lifimpadio, N.;Raver, I.; Versaillie, J. P. Proceedings of the Sixth International Symposium on Surfactants in Solution-Modern Aspects; New Delhi, 1986. (10)B o b , J. Z. Anorg. Allg. Chem. 1925,149, 203. (11)Mackay, A. L. Miner Mag. 1960,32,545. Mackay, A. L. Ibid. 1962, 33, 270. Mackay, A. L.J . Phys. SOC.Jpn. 1962, 17, 317.
0
G ~ W I
H20 COMPOSITION (Wt. Pormnt)
Figure 2. Temperature variation ofthe lamellar liquid crystal phase compositions for the ClzEOdwater system. copiously with alcohol by continuous sonication and centrifugation (sec-butanol was used to break the phase, and 95% ethanol was used for subsequent washings). After the centrifugation step, most of the supernatant was removed and replaced with clean ethyl alcohol and re-sonicated. Centrifugation was carried out at 3200 rpm in 25 mL glass vials with foil-lined screw caps (Kimble). The washing procedure was repeated on average five times. When the surfactant had been removed, the samples were filtered (Poretics Corp., 0.05 pm pore size) under suction in preparation for X-ray analysis. Washing with alcohol proved far superior t o the use of double-distilled deionized water since the aqueous system tends to form a viscous liquid crystal phase component over a wide composition range. For the samples synthesized in the microemulsion region of the ClzEOdn-hexanel0.025M ferric chloride phase diagram, the same procedure was followed with regard to the heating times and heating strategy; Le., when the samples had been heated to either 333 or 353 Kfor the chosen time period, they were washed again in a similar manner. sec-Butanol was used to break the phase, but this time smaller aliquots were taken and microfuged at 12 000 rpm in a Fisher microfuge. Hard plastic 2 mL vials were used, and the samples were washed and sonicated as previously described. X-ray Dieaction. The precipitates obtained from the hydrolysis reactions were characterized by X-ray powder diffraction using a Siemens D501 diffractometer. Particles collected on filter paper were mounted on a glass slide by slightly damping
Obvious and Nonobvious Influences of Surfactants
Langmuir, Vol. 10, No. 9,1994 2987
the underside of the filter paper with ethanol. The X-ray source was copper with a Ka line of radiation of 0.01542 nm, and diffracting angles were observed from 5 to 65". The X-ray diffraction pattern, the angles of reflection, and the intensity of reflectionswere obtained from a computer analysis of the detector output. Transmission Electron Microscopy. Transmission electron microscopy was also used to determine particle size and morphology. Both a JEOL 120 and a Phillips 201 instrument, each having resolving power of 0.5nm, were used. The samples were prepared by directly dipping copper mesh grids (Fullam Inc.) previously coated with a poly(viny1formal) (Formvar) film into the appropriate dispersions. The grids were coated by taking half a petri dish and filling it with distilled water to the point where it was just about to overflow. A drop of Formvar solution (5%poly(viny1formal) in ethylene dichloride) was gently placed onto the surface of the water, and a few seconds later film formation occurred. Then the copper mesh grids were carefully laid onto this film, and when sufficient grids had been coated, a piece of filter paper was placed on top until it was fully wetted and then removed, bringing the coated grids as well. The photographs were taken using both a 35 mm camera and a plate film. Infrared Spectroscopy. Infrared spectroscopywas used for initial sample characterization. The samples were prepared by diluting the previously washed and dried ferric precipitates with anhydrous KBr and then pressing them into a thin transparent pellet. Spectra (4000-400cm-') were obtained using a Fourier transform infrared spectrometer (Mattson 2000). lSC Nuclear MagneticResonance (NMR) Spectroscopy. 13C NMR spectra were obtained using a spectrometer (IBM NR 250) operating in Fourier transform mode at a resonance frequency of 62.89MHz. Typically 1000transients were averaged using a 16Kdata block and spectral width of 100KHz. Relaxation delays of 3 s were employed with 6 ,us RF pulses (30"). Spin-lattice relaxation times, 2'1, were obtained using the standard inversion recovery sequence and were found to lie in the range of 0.1-10 s. A nonlinear regression procedure was used to fit the time evolution of the magnetization for each identifiable resonance. The data could be fitted to a single relaxation time parameter to better than 98%. The sample temperature was held constant to better than 10.2K over the studied temperature range (294-333 K) using a temperature controller (Briiker, VT 100). The components of the samples to be investigated were weighed directly into standard Pyrex NMR rubes (Wilmad, 10 mm o.d.), mixed by vortexing, and centrifuged to remove air bubbles. This procedure was repeated until the sample appeared homogeneous under polarized light (usually 3-4 times).
Results & Discussion The initial steps of hydrolysis are described by the following reactions:
+
[Fe(H,0)5(OH)]2+= [Fe(H20),(0H),1+ H+ (2)
+
2[Ff3(H,o)6l3+= [Fe(H,0)4(OH),Fe(H,0),34+ 2H'
(3)
identified. In low pH solutions the existence of the dihydroxo-bridged iron(II1) dimer bisb-hydroxo)octaaquodiiron has been postulated to be the predominant species on the basis of electrometric,15thermochemical,16 magnetic," and spectrali7J8studies. Even a t pH's in the range 2-3, the extent of the hydrolysis is very great, and in order to have solutions containing mainly Fe(II1)in the form of the purple hexaaquo ion, the pH must be around zero. The general equation for the hydrolysis reaction may be presented as
-
pFeOH2+
+ + F)H+
[Fe,0,(OH)~+rl(~p-29-')+(29
(4) The marked effect of pH on the final product of the hydrolysis has been reported.14 At low pH it appears that the formation of crystalline precipitates, e.g. a-FeOOH, P-FeOOH, predominates, while at high pH, the studies of Spiro et al.i9v20have established that hydrolyzed iron(II1) nitrate solutions contain large polycations. Subsequently, the rates of precipitation and polymerization reactions are proportional to some function of [H+l. Phase Behavior. Hydrolysis reactions of ferric chloride were investigated in both the lamellar liquid crystalline and microemulsion phases. The phase boundaries for the microemulsion regions containing the ferric ions were found to differ from the control case where no iron was present (Figure 1).This is an important experimental observation in terms of defining the phase being worked with and the compositions at which the phase is maintained. It is important to note that the introduction of appropriate metal ions could affect the structure and chemical composition of the phase and is a variable that often remains ignored. This, coupled with the fact that the phase boundaries for the nonionic surfactant are also temperature sensitive, means that these variables must be considered in this study if the effect of phase behavior on different reactions is to be understood. The presence of the ferric ions was found to decrease the maximum water uptake of the microemulsion phase from 36% to 26% (0.025M FeCls) through complexation ofthe ferric ions with the surfactant head group (see NMR data later), which causes dehydration and yields an effectively more ionic surfactant. The effect of the iron on the phase boundary was found to be more pronounced for the microemulsion region of the phase diagram than for the liquid crystal region. This is because the ratio of surfactant to iron is greater for the liquid crystal phase; hence complexation effects would be relatively smaller. The composition and morphology of the particles prepared in these associated surfactant systems are extremely sensitive to various parameters such as pH, concentration of the ferric ions, and the nature of the anion present. These observations are consistent with the findings from Matijevic: "Dispersions consisting of particles uniform in size and shape resulted-as a rule-over a narrow range of reactant concentrations and of pH." Under analogous conditions of ferric ion and acid concentration, the morphology of the products obtained in our study was similar to the homogeneous case.
(a pox0 complex)
K = 10-2.91 These equilibria have been studied exten~ively,~~-'~ and the monomeric species FeOH2+and Fe(OH)2+have been (12)Holm, N.G.Orzgzns Life 1985,15,131. (13)Galbraith, S.T.;Baird, T.; Fryer, J. R. Acta Crystallogr. 1979, 35,197.
(14)Sapieszko,R. S.; Patel, R. C.; Matijevic,E. J.Phys. Chem. 1977, 81,1061. (15)Strahm, U.;Patel, R. C.; Matijevic, E. J.Phys. Chem. 1979,83, 1689. (16)Knight, R.J.; Sylvia, R. N. J.Inorg. Natl. Chem. 1973,36,591. (17) Hedstrom, B. 0. A. Ark. Kemi 1935,6, 1. (18)Milbum, R.M. J.Am. Chem. SOC.1955,79,537. (19)Milbum, R.M.; Vosburgh, W. C. J.Am. Chem. SOC.1955,77, 1352. (20)Mulay, L.N.;Selwood, P. W. J.Am. Chem. SOC.1955,77,2693.
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2988 Langmuir, Vol. 10, No. 9,1994
d-spacing(A) 7.420 5.212 3.348 2.534 2.249 1.948 1.645
I [
Intensity Very strong Medium strong Very strong Strong Weak Medium Medium
I
I
420 41 1 521
310
1
110 I
I
-
a. (-
IC.*"*
211
1
200
i
= 100 nm) 18
'
31
44
57
70
DIFFRACTION ANGLE
d-spacing(A)
Intensity Medium Medium Medium Medium Weak
3.30 2.54 2.34
I
1.95 1.64
I I
420
1
Weak Weak
41 1 521
b. (-= 100 nm) Figure 3. (a)Particles formed on heating the lamellar phase to 353 K for 5 min. Phase not retained. (b)Particles formed on heating the lamellar phase to 333 K for 5 min. Phase maintained.
Hydrolysis of Femc Chloride in the Lamellar Liquid Crystalline Phase. The precipitation of hydrated copper sulfate from the hexagonal liquid crystalline phase of the w a t e r h e e n 80 binary system yields a completely different product than that precipitated from aqueous solution.2 However, the concentration required to initiate precipitation was decreased from the 2.5 M required in homogeneous aqueous solution to 0.025 M in the associated surfactant structure. On the contrary, the hydrolysis of ferric chloride in the lamellar liquid crystalline phase in this study yielded the same product as obtained in homogeneous aqueous solution, and the concentrations required to initiate precipitation were comparable. The usual method preparing a-FeOOH in homogeneous solution is the hydrolysis of a FeCl3 (>0.025M in 0.01 M HC1) solution at 373 K for 24 h;l however, this material can be prepared in 5 min by heating the lamellar phase comprised of a 3:2 ratio of C12E04: FeC13 (0.025M in 0.01 M HC1) solution (Figure 2). X-ray diffraction showed the product to be that of P-FeOOH, but the morphology and size could be varied by altering the dimensions of the media. In the experimentwhere the lamellar liquid crystal phase was heated to a temperature (353 K) where it was no longer stable (i.e., a phase transition to a two-phase microemulsionkquid crystal occurred), cigar-shaped crystals were obtained (Figure 3) which were approximately 100 nm in length and 4-6 nm wide. These particles were smaller in size than the particles obtainedon precipitation from aqueous solutions1which were -1 pm in dimension under similar conditions, i.e., 2-3 orders of magnitude
20
35
50
65
80
DIFFRACTION ANGLE Figure 4. X-ray diffractionof particlesformed in the lamellar phase [Fe3+0.025MI: (top) 353 K for 5 min; (bottom)333 K for 5 min.
larger. In contrast, if conditions were chosen such that the lamellar liquid crystalline phase was maintained throughout the heating period (318-333 K), the particles formed were much shorter in length but broader in diameter and irregular in shape, which is consistent with their growth being restricted by the dimensions of the water layers (Figure 3). The X-ray diffraction patterns for these particles were consistent with the observed decrease in particle size (Figure 4). Note: The same X-ray diffraction pattern was found at 333 and 318 K,i.e., when the lamellarphase was not broken. TheX-ray diffractionpatterns exhibit broader peaks (corresponding to smaller size particles) for the dispersion where the phase had been maintained. An explanation for this is that nucleation and particle growth occur in the aqueouslayer (whichcontains FeC13)between the surfactant bilayers. If the lamellar liquid crystalline phase is not maintained during the heating process, rapid nucleation occurs, but particle growth is not controlled by the domain boundaries, resulting in the formation of a smaller number of larger particles. In contrast, if the lamellar liquid crystalline phase is maintained, rapid nucleation still occurs, but this time the nuclei are immobilized as soon as they are formed by the presence of the boundaries of the microdomains. Consequently,
Obvious and Nonobvious Influences of Surfactants
a. (-=
1 ctm)
b. (-= 1 ctm) Figure 5. Particles formed a t high ferric chloride (0.1 M) in the lamellar phase: (a)particles formed on heating for 5 min to 353 K (b) particles formed on heating for 5 min to 333 K.
the particles can only grow by the diffision of ions. The final result is a large number of particles whose sizes are determined by the dimensions of the water layer. This synthesiswas also carried out using a higher concentration of ferric chloride (0.1 M) to see if there was any noticeable effect on the length and the width of the rods. The results showed that there was a bimodal distribution of sizes, and the larger rods reached approximately 2 pm in size when the lamellar phase was not maintained (Figure 5). If the samples are heated for longer periods of time (4 h as opposed to 5 min) a t 333 or 353 K, redissolution of the larger particles occurs to give smaller spherical primary particles (Figure 6). This implies that the presence of surfactant must greatly influence the basic chemistry,as indicated in the Introduction. Particles grow in homogeneous aqueous solution with increased heating times, and no observation has been made to suggest that the particles actually become smaller in size, while the same crystal structure is maintained. The X-ray difiaction patterns also support this hypothesis (Figures 4 and 7) since increased time of heating produced broader and more amorphous diffractionpatterns indicativeof smaller particle size. Another interesting phenomenon occurs when no heat is applied and the samples are allowed to age in the liquid crystal medium at room temperature. TEM examination of samples after 36 h showed the shorter acicular disks typical of preparations at 333 K (Figure 8). On long-term aging (7months),the samples showed long (35 nm) needles with 2-3 nm widths in equilibrium with tiny particles
Langmuir, Vol. 10, No. 9, 1994 2989
a. ( - = 9 n m )
b. ( - = 2-4 nm)
Figure 6. Particles formed upon prolonged heating in the lamellar phase for 4 h. (a) Particles formed on heating the lamellar phase to 353 K. Phase not retained. (b) Particles formed on heating the lamellar phase to 333 K. Phase maintained.
(Figure 8) with some flocculation and a small amount of redissolution (Figure 8). Experiments were performed at 318 K, and the dimension of the aqueous microdomain was varied by changing the surfactant:water ratio from 1:lto 4:l (w/w %) in order to see if this affected the particle formation. The results are summarizedin Table 1and show that decreasing the dimensions of the aqueous layer in the lamellar liquid crystalline phase (i.e., increasing the surfactantiwater ratio) leads to (a) a smaller median particle size, (r),and (b) decreasing amounts of flocculated material. The results show an approximately linear increase in (r) with an increase in mole ratio (nwater/nsUdaht), emphasizing the fact that the more viscous surfactant phases can play a role in defining the morphology of the product. The decrease in percent flocculated material is also consistent with the increased surfactant:ferricratio (i.e., the amount of surfactant present is decreased). Hydrolysis of Ferric Chloride in a Nonionic Microemulsion. The hydrolysis of ferric chloride was carried out in the microemulsion region of the Cl2EOJ hexane/water system. Two different .compositions are mentioned in this study for comparison, containing 5 and 15%of the aqueous component, respectively (Figure 1).It should be noted that only the 5% composition remained single-phased when heated, and at these compositions, the presence of inverse micelles is not likely since the ratio of surfactantiwater is too high. The same redisso-
O'Sullivan et al.
2990 Langmuir, Vol. 10,No. 9,1994
I
I
c
c
a. (
- = 40 nm)
i I
Flat irregular discs. a. (
- = 50 nm)
d
b. (35 nm needles and 5-6 nm spheres)
Figure 7. X-ray diffraction of particles formed by heating the lamellar phase for 4 h. (a) Particles formed on heating the lamellar phase to 353 K. (b) Particles formed on heating the lamellar phase to 333 K.
lution phenomena observed for the lamellar case were observed in these systems with increasing heating time (Figure 9). Particles formed were larger at the higher water contents, probably because of the increased iron content of the sample. It appears that inverse micelles are not a necessity for the formation of nanosized particles, reinforcing the fact that in the microemulsion region the effect of the surfactant is far more important than the effect of phase structure. NMR Investigation of Ion-Surfactant Complexation. Observations of (1)the presence of ions alters the phase boundaries and (2) the redissolution of particles upon heating for extended periods of time in the presence of surfactant imply that there is a strong interaction between the ions and the surfactant molecules. This is not surprising since it is well known that the physical properties of nonionic surfactants (e.g., the cloud temto the presence of ions. The p e r a t ~ r e ~ lare -~~ sensitive ) nature of the interactions is basically ion-dipole involving either the molecules solvatingthe surfactant head groups or a direct complexation between the ion and the surfactant. Displacement of molecules of hydration and their replacement by solvated ions lead to an entity with more ionic character than the original nonionic surfactant (21)Spiro, T. G.;Allerton, S. E.; Renner, J.;Terzis, R. J.Am. Chem. Soc. 1966,88,2721.
(22)Brady, G.W.;Kurkjian, C. R.;Lyden, E. F. X.; Robin, M. B.; Saltman, P.;Spiro, T. G.; Terzis, A. Biochemistry 1968,7,2185. (23)Ward, A. J. I. J . Pharm. Pharmacol. 1982,34,612.
b . ( - = 2-4 nm) Figure8. Effects of long-term aging of particlesin the presence of the lamellar phase. (a)Aging for 36 h at room temperature. (b) Aging for 7 months a t room temperature.
Table 1. Effect of Varying the Water Layer Dimensions water/surfactant ratio nwater/naurfactant (r)(nm) % flocculated 1:l 25 5 45 3:2 16.67 2-4 20 41 6.25 1-2
