8820
J . Phys. Chem. 1993, 97, 8820-8834
Phase Behavior and Microstructure of Polyoxyethylene Trisiloxane Surfactants in Aqueous Solution M.He, R. M. Hill,+ Z. Lin, L. E. Scriven, and H. T. Davis’ Department of Chemical Engineering and Materials Science, University of Minnesota. 421 Washington Ave. S. E.. Minneapolis. Minnesota 55455, and NSF Centerfor Interfacial Engineering, 100 Union SI. S. E., University ofMinnesota, Minneapolis, Minnesota 55455 Received: March 24. 1993
Small-angle X-ray and neutron scattering, wide-angle X-ray scattering, cryc-transmission electron microscopy, andvidec-enhancedoptical microscopy areusedin thisworktodeterminethephasediagramsandmicrostructures of several trisiloxane polyoxyethylene surfactants (M(D’E.)M) in water. Similar to the phase behavior of hydrocarbon polyoxyethylene surfactants (C,E.), the phase behavior of the siloxane surfactants depends strongly on the size of the polyoxyethylene (E.) head group. As n rises from 5 to 8, 12, 16. and 18, the hydrophilicity increases and the surfactant microstructures tend toward higher curvature structures. For example, in the comparable rangeof temperatureand concentration in water, M(D’El8)M forms hexagonally packed cylindrical micelles and M(D’E8)M forms lamellar bilayers. Typical colloidal phases of hydrocarbon surfactants reported-the isotropic water-rich micellar phase (LI). the isotropic surfactant-rich inverse micellar phase (L2). as well as the normal lamellar and hexagonal liquid crystal phases (L. and Hl)-are all found in the trisiloxane surfactant/water systems. Trisiloxane surfactants with shorter Enchains, such as M(D’Es)M and M(D’E8)M. in water form the isotropic sponge-like bicontinuous phase (L,) similar to that found in some C,E./water binary systems. Surfactants with longer E. chains such as M(D’E12)M do not form the L3 phase. Instead, the waterrich isotropic micellar phase (LI) dominates at low surfactant concentration. The phase behavior and microstructures reported in this paper also shed light on the ‘superspreading” behavior’.* of M(D’E5)M and M(D’E8)M water dispersions.
IOtWdUCtiOO
Siloxanesurfactants find useincosmetic formulations,wetting facilitation, textile manufacture, and agriculture adjuvants.l-3 However, althoughwetting and surface activity have been studied, the phase behavior and microstructure of siloxane surfactants in aqueous condition were unknown until recently. Gradzielski et al.‘studied the aqueous aggregation behavior of several nonionic and ionicsiloxanesurfactants.They found micellar and lyotropic liquid crystal phases in siloxane surfactant/water systems similar to the phases found in hydrocarbon surfactant colloidal systems, although some of these siloxane surfactants have irregular branched structures. We recently reported the lyotropic phase behavior of a group of ABA- and rake-type nonionic siloxane surfactants.’ The structures can be diagrammed as:
system in which theaqueoussolution phase behaviorcan bestudicd as function of hydrophilic polyoxyethylene E. head groups. The only difference between trisiloxane surfactants M(D‘E.)M and alkyl polyoxyethylene surfactants C,E. is their hydrophobic moiety. As indicated in ref I,the packing area of the trisiloxane surfactant M(D‘EI5OMe)M or L17, obtained from the surfacc tension measurements at the surfactant solution-air interface, is about 70 A*. which is considerably larger than the 50 A2 of the CI,E7. Whereas CI2E)is likea chain with twoends. the trisiloxane surfactants studied here are more like a double-edged axe with a trisiloxane hydrophobic moiety as the head of the axe. This difference in the proportions of the hydrophobe compared with those of the hydrophile is an important feature of the trisiloxane surfactants. Whereas the CI2El consists of a long narrow hydrophobe attached to a fatter hydrophile
hydrophobic C12H23moiety E7 hydrophilic head group En hydrophilic head group
hydrophobic siloxane moiety
We found evidence that in the ABA- and rake-type of nonionic siloxanesurfactants.’ thesiliconechain is flexibleenough toadjust its conformation to accommodate to the interfacial area requirements of different sized E. head groups. Taking this chain flexibility into consideration, we proposed a simple model based on the molecular packing constraint6 to account for the phase behavior of these polymeric siloxane surfactants. The nonionic trisiloxane surfactant (Me3SiO)Bi(Me)(CH2)r (OCH2CH2).0H(n= 5,8,12,16,18)providesanovelsurfactant
To whom the mrcspondenee should be addrared. ?Central Research and Devclopmsnt. Dow-Corning Corp., 2200 W. Salzburg. Midland. MI 486864994.
the trisiloxane surfactants studied here consist of a much fatter and shorter hydrophobe. For example, the M(D‘E$)M molecule can be diagrammed as
a
€5
hydrophilic head group
hydrophobic uisilixane moiety M@B)M Becausethe C.,,E./water binary phase behavior has becn well studied,’-t’ comparisonof the phase behavior of M(D’E.)M with that of C,E. surfactants should lead to better understanding of
0022-3654/93/2097-8820$04.00/0 0 1993 American Chemical Society
Phase Behavior of POE Trisiloxane Surfactants the role the unusually shaped trisiloxane hydrophobic moiety plays in the surfactant microstructures. In what follows,we report phase diagrams and microstructures of aqueous solutions of siloxane surfactants. The experimental techniques employed were small- and wide-angleX-ray scattering, small-angle neutron scattering, video-enhanced optical microscopy, and cryo-transmission electron microscopy. In addition to looking for the more common lyotropic liquid crystal phases, we also searched in the phase diagrams for regions of the "anomalous" phase (L3), Le., the isotropic sponge-like bicontinuous phase.12-15 The L3 phase has been found previously in nonionic hydrocarbon surfactant/water binary sy~tems.~-lO Some of the surfactants studied in this paper are known in the literature as "super spreaders" because they speed the spread of water drops on hydrophobic surfaces such as paraffin wax film, which hydrocarbon surfactant solutions fail to wet.1.2J6 Previous studies indicated that the unique wetting problem relates to the T-shapedhydrophobeof the "superspreader" molecular structure.' The presence of turbidity in trisiloxane surfactant dispersions seems to be necessary for the "superspreading".z We attempted to correlate the superspreading property of the trisiloxane surfactant dispersions to their colloidal phase behavior and surfactant microstructure in water by determining what, if anything, is unique about the phases which are present in these solutions.
Experimental Techniques SAXS, WAXS, and SANS. Small-angle X-ray scattering (SAXS) experiments were performed on a modified Kratky camera from Anton Paar KG, Graz, Austria, equipped with an extended flight tube and a movable beam stop.'' The X-ray generator was a rotating anode ('ROTAFLEX" Model RU200B, Rigaku Corp., Japan) operating at 10 Kw, with a copper target. The K, wavelength of 1.54 A was selected by means of Nichol filters. The energy window on a Model MBRAUN OED100-M 10-cm linear position-sensitive detector (Innovative Technology, Inc., Newburyport, MA) was set to accept only the scattering photons with energy close to 1.54 A. The Kratky linear collimation produced a 15 X 0.13 mm2 X-ray area on the sample sealed in a 1.5-mm-i.d.glass capillary (Charles Super Co., Natick, MA). The sample-to-detector distance was set at 68.2 cm. The detectable wave vector q range was from 0.02 to 0.3 A-l, where q = 4i~/X)sin(8/2) and 8 is the scattering angle. The scattering data accumulated over 30-240 min were corrected for background scattering by subtracting the scattering intensity of water and the empty capillary. Then the slit-smeared SAXS intensities were converted numerically to pinhole, or unsmeared, intensities by means of Vonk's method.'* Crystal structures with unit-cell dimensions smaller than 20 A were determined by wide-angle X-ray scattering (WAXS) with a Model D-500 Siemens diffractometer (Siemens Corp., Iselin, NJ). Small-angle neutron scattering (SANS) experiments were performed on the 30-m camera of the National Institute of Standards and Technology (NIST) at Gaithersburg, Maryland. The samples were contained in disk-like quartz sample cells. The area illuminated by the neutron beam was 1.1 cm2, and the path length was 1 mm. The sample-to-detector distance was set at either 14.34 or 4.25 m. The detectable range of wave vector q was from 0.006 to 0.15 A-1. The scattering intensities were recorded on a two-dimensional position-sensitive detector. The scattering intensities were recorded on a two-dimensionalpositionsensitive detector. The scattering data accumulated over 2 0 4 0 min were corrected for background scattering by subtracting the scattering intensity of water and the empty capillary. Then, the scattering data were calibrated and converted to absolute intensity by using a polystyrene standard provided by NIST as reference. Cryo-TEM. Cryo-transmission electron microscopy (CryoTEM) experiments were carried out as previously described.19
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8821 Samples were prepared in the controlled environment vitrification system (CEVS). Before the sample was introduced into the CEVS, the environmental chamber was brought to steady state at the desired temperature with saturation of water. A 3-pL drop of sample was placed on a carbon-coated holey polymer support film mounted on a standard 200-mesh TEM grid.20 Blotting the liquid with filter paper created a thin film of the sample that spanned the holes. Then, the grid was plunged into liquid ethane and transferred under liquid nitrogen into the cryotransfer stage (Gatan 626, Gatan Inc., PA), which was inserted into the TEM (JEOL 1010, Japan Electron Optical Laboratory, Japan). The specimen was imaged at 100 KV and an underfocus of 4 pm. TEM images were recorded on SO-163 film (Eastman Kodak Co., Rochester, NY) and were developed with full-strength D-19 developer (Eastman Kodak Co.) for 12 min. VEM. The video-enhanced optical microscope (VEM) used to image some of the systems consisted of a Nikon Optiphot-Pol microscope (Nikon, Inc., Japan) fitted with rectified differential interference contrast (DIC) optics. The VEM also had phase contrast and polarization microscopy capabilities. The microscope was connected to a Dage Model NCG8 black and white television camera (Dage-MTI, Inc., Michigan City, IN) equipped with a newvicon imaging tube via a Nikon 0.9-2.25X zoom lens (Nikon, Inc., Japan). Improved image contrast was obtained through video and digital image processing by a Psicom 327 system (Perceptive Systems, Houston, TX), which allows a resolution as low as 0.025 pm. Phase Identification. The surfactant/water mixtures were prepared with distilled, deionized water. Samples of known composition were contained in 7-mL 1-cm4.d. sealed glass tubes with 0.1 mL volumetric tick-marks, which were held in constant temperature (fO.l "C) water baths. Submerged sample tubes were gently hand-shaked for about 1min and allowed to equilibrate for a half hour or more. Then the samples were inspected under transmitted visible light for turbidity and between two crossed polarizing films (Bausch & Lomb, Rochester, NY) for evidence of birefringence.21 The viscosity change of the samples was roughly estimated by the drainage time upon inverting the glass tubes. By repeating the observations at different temperatures 2 OC apart, and then at 0.2 OC apart near cloud points, one could locate the phase boundaries according to the following criteria? (1) A sharp interface that totally reflected light below some angle of incidence (and in many cases showed meniscus curvature near the tube wall) tells that two phases were definitely present; the onset or disappearance of turbidity may indicate a two-phase boundary. (2) Birefringence observed under polarized light suggests that liquid crystal phases may present. (3) A dramatic shift of the location of a sharp interface between two wellequilibrated phases (effectively discontinuous volume change) establishes the crossing of a three-phase line. (4) On the basis of the so-called rule of alternation,22when as temperature is raised or lowered a sample transforms from two phases (e.g., A and B) to another two phases (e.g., A and C) over a narrow temperature range (0.5-2 "C) in which a property associatedwith B disappears and/or one associated with C appears, three phases (Le., A, B, and C) maybe present in between. Such properties in our study were birefringence, viscosity, or SAXS scattering pattern. In general, this approach was called the isoplethal method because the compositions of the inspected samples were fixed.23 With the isplethal method, the temperature at onset of turbidity or birefringence was recorded by crossing a phase boundary from below and above the phase transition temperature. The hysteresis difference on the temperature at onset turbidity or birefringence determined from the two processes was smaller than 0.3 OC, and the average temperature was taken to be the phase boundary temperature.
He et al.
8822 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 TABLE I: Names and Molecular Formulas of the Trisiloxane Polyoxyethyleae Surfactants name molecular formula source (Me3SiO)2Si(Me)(CH2)s-(OCH2CH2)5-OH Dow-Corning M(D'E5)M (Me&0)2Si(Me)-(CH2)3-(OCH2CH2)rOH Dow-Corning M(D'E8)M (Me,Si0)2Si( M~)-(CH~)~-(OCH~CH~)&AC Dow-Corning M(D'E8OAc)M (Me3Si0)2Si(Me)-(CH2)3-(OCH&H2)7,s-OMe Union Carbide M(D'E7.50Me) Ma (MeaSiO)2Si(Me)-(CH2)3-(OCH2CH2) 1 d H Dow-Corning M(D'EidM (M~~S~O)~S~(M~)-(CH~)~-(OCH~CHZ)I~-OM~Goldschmidt M(D'ElsOMe)Mb M(D'Eis)M (M~~S~O)~S~(M~)-(CH~)~-(OCH~CH~)~E-OH Dow-Corning a
The name used in refs 1 and 3 is L 77. b The name used in ref 4 is PR28. PHASE DIAGRAM of M(D'E5)M I WATER
PHASE DIAGRAM of M(D'E5)M / WATER
b
L
1
3
L3
9
0,
ot, 0
I
,
I
20
I
I
I
I
40
I
I
60
,
I
I
I
I
0
80
M(D'E5)M % (W/W)
M(D'E5)M % (W/W) Figure 1. (a) Binary phase diagram of M(D'Es)M/water. (b) The temperatures and concentrations at which the samples were studied by SAXS, SANS, VEM, cryo-TEM, and visualization. SAXS and SANS, cryo-TEM, and VEM techniques add important microstructural information to the visually identified phase pattern. Materials
In the nomenclature adopted by Gradzielskiet a1.,4 the nonionic siloxanesurfactants were coded M(D'E,)M, in which M (CH3)3SiO-, D' =-(CH3)Si-, and the polyoxyethylenegroup E, = -CH2CH2CH2(0CH2CH2),0H, where n is the number of oxyethylene segments: see Table I. The surfactants prepared at Dow-Corning were made by hydrosilylation of 1,l ,I ,3,5,5,5-heptamethyltrisiloxanewith the correspondingallyl polyoxyethylene derivative using chloroplatinic acid catalyst.24 We presumed the other two materials were prepared in a similar way. Because of potential side reactions during hydrosilylation, the polyoxyethylene chain is frequently capped with OMe or some other protective group. We have recently shown that the end-capper strongly influences the phase For the surfactants prepared at Dow-Corning, the trisiloxane hydrophobe was distilled to 95 wt % or above monodispersity,as determined by a gas chromatographic method. The hydrophobe portion of the other two surfactants appeared to be of similar purity. All of the surfactants except M(D'E5)M contained the usual polydisperse oxyethylene chain length
distribution. M(D'E5)M was prepared from the purified C H ~ = C H C H ~ ( O C H ~ C H ~ ) Swith O H 95% (w/w) or above monodispersity. So the purity of M(D'E5)M was greater than 95 wt % monodisperse in both the trisiloxane hydrophobe and the E3 hydrophile. In theother surfactants, the polyoxyethylenechain length is polydisperse with standard deviation between 5% and 15%; the n of E, represents an average value. The M(D'E160Me)M was provided by the Chemische Fabriken, Th. Goldschmidt AG (Essen, Germany), and the binary phase behavior of the surfactant in water has been reported! M(D'E7.50Me)M (L,,) was a gift from the specialty chemical division, Union Carbide Corp. (Danbury, CT). The "superspreading" behavior of the M(D'E,.sOMe)M/water solution has been previously The nonionic trisiloxane surfactants mixing in distilled water (pH = 6.8) contained in sealed test tubes may decompose by a significant amount after aging for more than a few months. For example, after 3 months, the lamellar liquid crystal phases presented in M(D'E5)Mlwater and M(D'E~)M/water samples at midranges of surfactant concentrations disappeared, and an oily insoluble isotropic colorless layer was found on top of the water. According to the Si-29 NMR and mass spectroscopy studyt2.54 the oily layer that resulted from the aging should be (Me)3SiOSi(Me)3or Si(Me)30H. The decomposition rate in
Phase Behavior of POE Trisiloxane Surfactants
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8823
TABLE II: L3 Phase Boundary (Lower and Upper Temperatures at Onset of Turbidity) of M(IYE5)M in Water temp ( O C )
temp (OC)
wncn (%w/w)
lower points
upper points
mncn (%w/w)
lower points
upper points
20.06 21.76 25.20 27.59 31.21
19.5 20.0 20.5 23.5 24.5
19.8 21.5 25.5 26.5 29.0
35.17 39.88 44.58 49.10 56.93
31.5 34.0 36.5 38.5 44.0
33.5 38.5 42.5 47.5 48.5
distilled water (pH = 6.8), on the basis our observation of the aged aqueous samples, is always qualitatively proportional to the inverse of surfactant concentration. Under the condition pH > 9 or pH < 5, the trisiloxane surfactant in water can totally decompose within a few hours.
Results Phase Diagram and Microstructures of M(IY&)M/Water. Using the isoplethal method described in the Experimental Techniques section, we determined the phase diagram of M(D'Es)M/water as shown in Figure la. The samples whose microstructureswere studied and whose appearanceswerevisually examined by using the isoplethal method are indicated in Figure lb. A water-rich isotropic phase was found at M(D'E5)M concentrations (less than 0.01 wt %) over the temperature range we studied. Using the notation adopted in C,E,/water phase work,10 we denote the isotropic water-rich phase with extremely low surfactant concentration by the letter W. Between 20 and 55 OC, we found an optically isotropic phase with low viscosity located in a narrow temperatureconcentration domain between the W phase and the lamellar liquid crystal phase L, at higher concentration. At 25 OC, 4 h after sample preparation, a sample of 10 wt % M(D'E5)M in water was separated by gravity into two layers with a clear meniscus: this isotropic phase was at the top with the W phase at the bottom. We applied conventional desk-top centrifuge (2300 rpm, 102-3 g) over 10 min to this sample contained in a 7-mL 1.5-mm4.d. volumetric centrifuge glass tube. Right after the centrifuge was used, we found the clear colorless isotropic phase (top layer) had turned slightly translucent. Hand tilting induced flow birefringence and increased its volume fraction by about 10% when it coexisted with W.27 By analogy with the known phase behavior of C,E, systems? we conclude that this phase is the 'anomalous", or L3, phase. However, in contrast to some reports about it, the L3 phase at equilibrium showed neither flow birefringence nor strong light scattering. Using the isoplethal method described in the ExperimentalTechniquessection, we measured the clouding temperature on heating and cooling and the clearing temperatures as well. The clouding and clearing took place within 0.5 OC. The L3 phase boundaries are presented in Table 11. A lamellar liquid crystal phase was found at concentrations above 50-80 wt % up to 51.5 OC. Samples of this phase had no diffraction peaks in WAXS: we concluded that the trisiloxane chain of M(D'E5)M molecules in the lamellar bilayer is above the melting transition (Lainstead of LB). The samplesalso showed typical La focal conic texture in VEM.28 Between crossed polars, the samples showed bright birefringence with yellow color, by which the La phase can be easily distinguished from the isotropic liquid phases at lower and higher concentrations. The phase boundary of the Laphase was determined by observing the onset and disappearance of the birefringence which took place within 0.5 OC: see Table 111. On the higher concentration side of the La phase, a surfactant-rich, isotropic, low-viscosity liquid phase (L2) was found. Phase boundary data (temperatures of onset/ disappearance of cloudinesswithin 0.5 "C) were determined: see Table IV.
TABLE III: Phase Boundary (Melting Points) of the M(IYE5)M La Phase wncn (% w/w) temp ("C) mncn (% w/w) temp ( O C ) 49.10 49.80 56.93 58.32 62.50 65.45 66.32
18.5 20.0 40.5 40.5 52.0 32.5 31.5
67.40 69.12 70.46 7 1.02 76.60 80.00
29.5 27.5 27.0 26.5 20.5 11.0
TABLE Iv: Phase Boundary (Lower and Upper Temperatures at Onset and Disappearance of Cloudiness) of the M(IYE4M G Phase temp ("C) temp ("C) mncn (%w/w)
lower points
upper points
mncn (%w/w)
lower points
upper points
65.45 66.32 67.47 70.46
53.5 38.0 36.5 35.5
58.5 59.5 61.5
71.02 76.60 80.00 90.00
33.0 20.5 11.0 75 >75 >75
TABLE V Putative TIuee-Phase Coexistence Temperature (AT3) Ranges of the M(IYE,)M/Water Binary System coexistence phases W-L3-(L1?)
L3-(L1?)-La
AT3 ("C)
coexistence phascs
19.5-20.0 24.0-26.0
W-L3-L2 L3-Lm-L2
AT3 ("C) 52.0-55.0 51.5-52.0
Below the lower clouding temperature of the L3 phase, we found a concentration region between the water-rich isotropic phase W and the lamellar liquid crystal phase La in which the samples were cloudy as first mixed. But at 20 "C within 2 h after mixing of samples of less than 25 wt %, a translucent layer developed beneath a blurred interface with W above. In the range 25-40 wt %, the samples became translucent (less cloudy) but no layer separated even under centrifugation at 300 g for 30 min. Between 40 and 45 wt %, a translucent layer developed above a blurred interface with L, beneath. In all three cases, the translucent material exhibited blue birefringence under crossed polars. Because in the first and third cases the interface remained blurred and phase separation was evidently incomplete, we could not determine precisely the boundaries of the region around 25 and 40 wt % in which we maintain that an isotropic phase (LI) exists. Additional evidence is presented in this section. The temperature range (AT3) for three-phase coexistence was not precisely located because the temperature-cmmposition boundary of the translucent region (LI)was uncertain and the concentrations and temperatures tested were 5 wt % and 0.2 OC intervals, respectively. At fixed compositions, we observed that the progression from two to three to two phases took place within an interval of up to 3 OC. When three phases were not evident, the progression from two phases to cloudy material to two phases likewise took place within a 3 OC interval. The three-phase coexistence temperature region was estimated according to the rule of alternation:22see Table V. To detail how we located the temperature range of the threephase coexistencetemperatures, examples of our observations of samples at temperatures just in, right below, and right above the three-phase coexistence temperature range (AT3) are given in the Appendix. SANS, SAXS, cryo-TEM, and VEM techniques were used to characterize further the microstructures of the phases that had been identified by visual examination under unpolarized and polarized light. At 30 wt % and 28 OC, the L3 phase of M(D'E5)M in D20 was examined by SANS. As shown in Figure 2, there is one broad intensity maximum at a characteristic Bragg length of 157.2 A. This indicates that the microstructure of the L3 phase possesses local but not long-range order. The Bragg spacing in the L3
8824 The Journal of Physical Chemistry. Vol. 97. No.34. 1993
He et al.
.47.9 A
149.4 A
E
. . .
.
'
:
I i.
-
F
:.
0
0
0.05
0.1
0.15
0.2
9 ('4-9 Figure 2. SANS scattering pattern of M(D'E,)M L3 phase: 30 WI '% M(D'Es)M in DzO at 28 OC.
0'
0.1
0.2
0.3
0.4
0.5
9 ('4-9 (a) SAXSpatternof M(D'&)M Iphaac: 58 wt '% M(D'&)M in water at 20 O C . (b) SANS pattern of M(IYE5)M L.phase: 58 wt W M(V'b)M in DzO at 25 ' C . -4.
4
pA -,-.*
.. .'... .1: .
ri ,-*
q(A") Figure 6. (a) SANS pattern of the M(D'E5)M two phase mixture (L3 the translucent region): 30 wt 5% M(D'E5)M in D2O at 25 "C. (b) SAXS pattern of the M(D'E5)M translucent region: 38 wt W M(D'E5)M in water at 25 "C.
+
assuming that in the Lz phase the inverse micelles order in a distorted face-centeredcubic packing,55we calculated the average diameter of the individual inverse micelles to be 24 A. The translucent region (L,?)and L3 lie between the W and L. phases. The SANS and SAXSresults presented below provide moreevidence to qualify the translucent region as a unique phase. At 30 wt % M(D'E5)M in D20 and 25 OC,the L3 phase coexists with the translucent region. At this condition, as shown in Figure 6a, SANS detected two characteristic lengths: 154.8 and 87.0 A. The larger one, 154.8 A, is close to the characteristic length
i
0
0.1
0.2
0.3
0.4
q ('4-3 Figure 8. SAXS pattern of 50% M(D'E8)M in water (La)at 20 O C . of 157.2 A of the single L3 phase detected at 30 wt % and 28 OC (Figure 3). Therefore, the smaller characteristic length, 87.0 A,
represents the translucent region. This conclusion was further confirmed by SAXS. As shown in Figure 6b, at 38 wt % M(D'E5)M in water at 25 OC, in which only the translucent
8826 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993
He et al.
Figure9. AI ZO'C.Xwl% M(D'&)M in water observed under optical microscope indifferentialinterferenamntrastmode(D1C) withmagnification
scale of 1 an = 18.9 p: (a) X = IO. (b) X = 20, (c) X = 25. region presents, SAXS data show that the characteristic length of the translucent region is 96.3 A. This is not far from the 87.0 Ain30wt%solutiondetectedby SANS. Therefore,thescattering
evidence suggests that, besides the L3 phape. another distinct phase (proposed as L,) between the Wand L. phass appears in the translucent region.
Phase Behavior of POE Trisiloxane Surfactants
The Journal of Physical Chemistry. Vol. 97, No.34. 1993 8821
i:
surfactant-rich micellar phase (LI), the M(D’E5)Mlwater samples we studied were persistently slightly turbid as already described. So there may be dilute suspensions of lamellar structures, possibly vesicles, of the L . phase in clear micellar LI solution. Indeed, the slightly turbid samples are also slightly birefringent, asreportedabove. The hcuristicconceptofa packing constraint paramete+J.s.33.34 will let us further rationalize the analogy between the phase behavior of M(D’Er)M and C12E4in the Discussion section. Phase Diagram and Miemstnehocof M(W)M/Water. The phase diagram of M(D’Es)M/water on the surfactant-lean side of M(D’m)M/water is uncertain, because the samples over the range of concentrations from less than 0.1 to 40 wt % between the dilute water-rich solution W and the lamellar liquid crystal phase L. were persistently cloudy up to 60 OC, even with
c E
-
8-
3t3 z
i
z si w
%m 4
’.
0.2
molehl& in t i e lamellar biiayers were above the chain m e k g temperature T,. Between crossed polars, the liquid crystal samplesprsented a focalconictexture. From theevidenceabove, the lamellar liquid crystal phase is L . instead of LB(gel). Just below40 wt%,samplesofM(W)Min waterwerecloudy and birefringent. Confirmed by VEM, the L. phase was still present but dispersed in other isotopic diluted phases (to be discussed below) to form multilayer vesicles which were stable for 2-3 weeks and did not phase separate upon centrifugation at IOZ--lgfor I h. Therefore, the phase boundaryon thesurfactantlean side of the L . phase could not be determined precisely. As the temperature of samples of 5-15 wt % M(D’Es)M in water was raised from 20 OC by 5 OC intervals, little change was seen until therange4M0°C. In thatrange,thesilvery-hlueschlieren flow birefringence of milky-white suspension gave way to the brighter, streaky-yellowstatic birefringence. This transition was accompanied by a notable rise in viscosity. As the temperature of the samples of 3lWO wt % M(D’Es)M in water was raised from 20°C, theirfocalconicL.texture,observedbetweencrossed polars, persisted up to 80 “C and disappeared at higher temperature. Thus, weconclude that the L.region must extend toward the surfactant-lean side at higher temperature. Between 60 and EO OC in a narrow concentration range, an isotropic single phase with low viscosity was found between W and L. phases. These features resemble these of the L, phase found in M(D’Es)M/water and of the L, phase known in &Es/ water.’JI so it is reasonable to maintain that this M(D’Es)M
i
-
1
E y r e 10. (a) SANS patterns of 20 wt 7% M(D’E8)M in D20solvent. (b) Crya-TEM image of 5 w t 7% M(D‘E8)M dispersion in water. One
of the short warm-like elongated micelles is indicated by the arrow.
narrow one-phase region is the isotropicsponge-likebicontinuous phasel,. Theanalogybetween thephasebehaviorofM(D’Es)M and C12Erwill be further rationalized in the Discussion section. At high surfactant concentrations (above 85 wt % at 20 “C), a surfactant-rich isotropic phase Lz was found. As shown in Figure7,with temperatureincreasingthebphaseexpandstoward the surfactant-lean side of the phase diagram. The cloudy samples in the surfactant-lean region between L . and W below60°Cwereexamined byopticalmicroscope.SANS. SAXS. and cryo-TEM. The cloudy samples did not phase separate even after 14 days or centrifuging at 3M) g for 30 min. At 20 “C, a solution of 0.55 wt % M(D’b)M in water, observed between crossed polars, was turbid but not birefringent. Viewed under an optical microscope in the differential interference contrast mode (DIC), 0.55 wt % M(D’Es)M in water showed only small particles 0.5 pm (SO00 A) or less in size. The milkywhite suspension of 10 wt % M(D’Es)M in water showed strong flow birefringence with silvery-blue schlieren texture. Figure 9a is a DIC micrograph of IO wt % M(D’Es)M in water; the small particles present in the 0.55 wt % sample are still visible but are in an aggregated form. The system is plainly two-phase and is more or less continuous clusters of particles containing water-
He et al.
8828 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993
a PHASE DIAGRAM of M(D’E, 2>M / WATER
b
LOW CONSOLUTE BOUNDARY for M(D’E, 2)M ISOTROPIC PHASE
ISOTROPIC PHASE
0’
20
40
60
80
100
0
5
10
15
20
25
30
35
M(D’EI2)M % (W/W) M(D’EI2)M % (W/W) Figure 11. Binary phase diagram of M(D’E12)Mlwater. The temperatures and concentrations at which the samples were studied by SAXS, SANS, VEM, TEM, and visualization are indicated. (b) The lower consolute boundary of the M(D’E12)M isotropic phase. filed voids. The milky-white suspension did not change visibly over 2 weeks. At 20 wt % M(D’&)M in water, the microstructure was noticeably different. As shown in Figure 9b, a number of droplets of Lalarger than the small particles in Figure 9a were visible; they distinguished themselves by focal conic birefringence pattern typical of a multilamellar vesicle. Smaller particles, somewhat larger than those in (a), were also present, along with stretched-out tubular or fibrous structures that may have derived from them. At 24 wt %, the multilamellar vesicle droplets were larger and the small particles fewer, until beyond 25 wt % only multilamellar vesicle droplets could be seen: Figure 9c. Thus, this optical microscopy study revealed that at 20 O C there are two types of dispersed materials below 24 wt %--small isotropic particles and multilamellar vesicle droplets. The small particles dominated below 10 wt %, coexisted with multilamellar vesicle droplets between 10% and 24%, and could not be seen above 24 wt %, where the system was a dispersion of the droplets until it became one-phase Laaround 50 wt 96 at 20 OC. As shown in Figure loa, the SANS of a 20 wt % M(D’E8)M sample at 20 O C has a broad intensity maximum with the characteristic length at 199 A, evidently the interbilayer distance in the multilamellar vesicles seen in Figure 9b. In Figure lob, a cryo-TEM image indicates that 5 wt % M(D’E8)M in water formed short worm-like elongated micelles with chain length at about 3-5 times the diameter of the spherical micelles shown below. From all this experimental evidence about dispersed states of the surfactant in the cloudy samples, it is possible to infer likely equilibrium phase behaviors in the composition region between W and La. The small particles could be of an isotropic, microstructuredphaseintherangeofabout1&15 wt %surfactant of many short worm-like elongated micelles in a disordered array l i e balls of cooked spaghetti. Or they could beunilamellarvesicles
dispersed from the lamellar liquid crystal phase La,whereas the short worm-like elongated micelles are from the water-rich isotropic phase W. This alternative is favored by some.U$6 Judgment is difficult because the E. chains are distributed in length, making the system multicomponent. The samples of composition between W and Ladid not sediment detectablyunder the conditions that were available. The alternatives hinge on the nature of the small particles. If they are unilamellar vesicles dispersed from the lamellar liquid crystal phase La,then the entire composition range from W, with its elongated micelles, to Lacan be interpreted as a two-phase region. On the other hand, if the small particles are a dispersion of adistinct, liquid, isotropic phase of a range of intermediate compositions L1,then the range between W and Lamust consist of three parts. The first, from W to the surfactant-lean side of L1,should contain two-phase dispersions of LIsmall particles in elongated micelle-containing W, which is consistent with the observations, or closer to L1 boundary, droplets of W in L1,which were not detected. The second part should consist of homogeneous L1,it at equilibrium. The third part, from the surfactant-rich side of L1 to La,should contain liposomes, multilamellar vesicles, and/or uni-lamellar vesicles of Lain L1, which is consistent with the observations, or closer to the Laboundary, inclusions of LIin La,which were not detected. Because we did not find static birefringent objects, Le., liposomes or multilamellar vesicles below 5 wt % M(D’&)M, or elongated micelles above 30 wt %, we conclude that the weight of evidence favors the existence of the L1phase at equilibrium between about 10 and 15 wt %. Two other similar surfactants, M(D’E7.50Me)M and M(D’&OAc)M, in water also form cloudy samples at the composition range between W and La phases, similar to that presented by
Phase Behavior of POE Trisiloxane Surfactants
-a
The Journal of Physical Chemistry, Vol. 97. No. 34, 1993 8829
.. K:,
a 8
.
b
2
". .
.-.I
, , , , , , . , , I , , , , , , , , , , , , , , ,, , , . 00
0
1103 a'. (.
2 10''
3 103
~ .' 4
Figure 1L (a)CrwTEMima&cof5% M(D'E1r)Minwater. (b)SANS Guiniw pln of 0.5 wt % M(D'El2)M in DzO.
M(LY%)M in water above. The surfactant-water binary phase diagramsofthae twosurfactantsaredocumented inaseaparate report?' Pbrw Diagram and Microstnctws of M(IYElz)M/Water. The M(D'E12)M/water phase diagram is shown in Figure I l a with temperatures and concentrations at which samples studied by different techniques are indicated. In the vicinity of rwm temperature, M(D'EI2)Min waterformeda singleisotropicliquid phase at all the concentrations. The microstructure of this isotropicphaseon the surfactant-lean side was investigated using cryo-TEM and SANS. The microstructural evolution from the surfactant-lean side to the surfactant-rich side in this isotropic phase will be reported elsewhere?* In Figure 12a, the cryoTEM image of 5 wt W M(D'E1z)M in water at 20 OC shows spherical micelles. In Figure 12h,theGuinierplot'6oftheSANS data for 0.5 wt W M(D'EIz)M in DzO indicated that the radius of gyration of the micelles is 40.1 A. As shown in Figure I Ih, the lower consolute boundary (clouding temperature us composition) for this isotropic phase is above 40 OC. A hexagonal liquid crystal phase H I was found below 7 OC around the midrange of the concentrations. As shown in Figure 13a, SAXSof 50 wt W M(D'EIz)M solutionat 4 "Cdemonstrates
~~
Figme 13. (a) S A X S pattern of 50% M(D'E,z)M in water at 4 OC. (b) TcxtursofSO%M(D'E~,)Minwaterobservcdby VEMunderdiffcrcntial
interference contrast mode at 4
OC.
twodiffractionpcakswith wavevectorqintheratio I:&',which is characteristic for two-dimensional hexagonal packing. In Figure 13b, the VEM image presented the "fan-like" textures whicharealsoasignatureofthe HI phase.= Below 15OCaround 75 wt W M(D'E12)M, an L . phase locates on the surfactant-rich side of the H .. I ohase. which was confirmed by SAXS and VEM experiment results. Phase Diamns and M i c r c s ~ c t u r eof a M(D'E.AIMe)M . .- , and M(D'E1dMSm Water. Gradzielski et al. reported the phase behavior of M(D'E160Me)M in wate+ (the surfactant was denoted as PR28). In their binary phase diagram, a hexagonal liquid crystal phase HI, surrounded by isotropic liquid phases, spans much broader temperature and composition ranges than does the HI phase found in the M(D'EI2)M/water system above. No lamellar liquid crystal phase L . phase was observed. Visually detecting the onset/disappearance of cloudiness and birefrigenceoftheaqueoussamplesby varying their temperature. we determined the phase diagram of the M(D'Els)M/water system as shown in Figure 14, with samples studied by different techniques indicated.
8830 The Journal ofPhysica1 Chemistry. Vol. 97. No. 34,1993
He et al.
PHASE DIAGRAM of M(D'E, &M I WATER
l-----T
-i=
P (A-9 Flgm 16 SAXSand WAXS pttem of M(M?!I*)M (solid) at 20 O C .
00 M(D'EIdM a0 (wlw) F¶m 14. Binary phasc diagram of M(D'Et~)M/watcr.The t e m p atura and concentrationsat which the samples were studied by SAXS. SANS, VEM. TEM.and visualization are indicated.
TABLE VI: Spradhg AbiUty of 1 wt 46 Surfactant Solation on PanNm at 20 OC vs CornspOaaing F'hases surfactant
MWEW M(D'b)M
M(D'&OAc)M M(D'E,.,OMe)M
.
.
., i,..)i.. ..,
;.:+
klgnre IS. Cryo-TEM image of 5% M(D'E1a)M in water at 25 'C.
In this phase diagram, a hexagonal liquid crystal phase H I locates in the composition range between 50 and 70 wt W below 56 "C. An isotropic liquid phase LI presents on the surfactantlean side of the HI phase. In Figure 15. the cryo-TEM image indicatesthat 5 wt W M(D'Els)M in water (LI) formed spherical micelles at 20 'C. Below 25 'C, a solid crystal phase presents in the composition rangeabove90wtWM(D'Ela)M. UsingSAXSand WAXS,the microstructure of a solid anhydrous M(D'Ela)M sample was studied. The SAXS and WAXS scattering data are plotted in one graph in different wave vector 9 regions. Shown in Figure 16,SAXSofthesolidsampleat2O0Cprsentsthefirst-andthe second-order scattering peaks with wave vectors qt and q2in the ratio 12, which indicates lamellar ordering with an interlayer distance of 89.8 A. At higher 9 range p0.6 A-1). the WAXS diffraction peaks at a d-spacing of 3-5 A, due presumably to the long-range ordering of trisiloxane chain or head group of the
superspeading phases yo yo
yo yes
M(IYEdM ss2 (ref I )
not
M(D'ElsOMe)M
not not
M(D'EdM a
not
pmriblc m i a a V u c t u m
L, + W surfactant bilayer dbpnion L, + W* aggregate micellar dispersion' L,
+ W* amrcgstc micellar dispersion*
L,+ W* aggregate micellar dispersion' L, micellar solution L, micellar solution LI micellar solution LI
micellar solution
Somexa favor 1.+ W veaicle or bilayer diapcnion
surfactant molecules, demonstrate thesample'scrystallinity.The SAXS and WAXS data indicate that M(D'E,,)M itself formed an ordered lamellar phase in the absence of water or other solvents-i.e.,a thermotropicliquidcrystalat room temperature. In this solid lamellar phase, M(D'Els)M molecules organized themselves into lamellar bilayers with alternating domains of E. headgroupand trisiloxanemoieties. M(D'Ela)Mmoleculeswere frozen and packed with long-rangeorder in the lamellar bilayers. Wetting Ability of the Dilute M(D'E,)M Solutionson Panffii Wax F h and Related Pbsse Behavior. Some M(D'E.)M surfactants are well-known for their special ability to facilitate water wetting (sometimes called supersprcading) on certain hydrophobic solid surfaces such as paraffin wax. The spreading facilitation is not directly related to the lowering of the aqueous phase surface tension, since there are fluorocarbon surfactants and some other M(D'E.)M surfactants which cause a similar lowering but do not give rise to superspreading.1 It appears that the superspreading is enhanced when the surfactant is dispersed in water as small droplets of a surfactant-rich second phase.lJ6 Inourstudy, it was found that the wettingability ofthe M(D'EJM solutions on paraffin wax surfaces is closely related to the corrsponding aqueous phase behavior. In Table VI, the phases characterized for I wt%solutionsarelistedwith thecomsponding superspreadingabilities. In this tahle,superspreading means that a droplet of 1 wt 5% surfactant/water mixture when placed on a paraffin wax film (Parafilm, American Can Co.) spreads within a few seconds under saturated humidity to a thin layer whose
The Journal of Physical Chemistry. Vol. 97. No. 34, 1993 8331
Phase Behavior of POE Trisiloxane Surfactants
El
Qzzzzza Hydrophilic
I
I
5
I
12
Hydrophobic I
18
Polyoxyethylene (EO) Number 17. Schematicillustration of M(D‘F!JM microstructurein water as a function of the size of the EO head group.
apparent contactangleis toosmall tobejudged byvisua1ization.M In contrast, ‘nonsuperspreading” refers to just the opposite situation. Surface tensions measured at air-liquid interface for all the 1 wt S surfactantlwater binary systems range from 20 to 22 mNlM.16 For the.M(D’En)M surfactants with n from 5 to 8, the cloudy suspensions of the surfactant-rich isotropic phase in water superspreaded on paraffin wax film. In contrast, for the M(D’E.)M surfactantswithnfrom 12 to 18, theisotropicmicellar solutions formed smile d r o p on the paraffin wax film with large apparent contact angle (>90°). We found that the sessile drops of nonspreaders on paraffin wax film reached a steady configuration within 1 sand changed little upon observation for IO min under saturated humidity. Disetgsloll
The M(D’EJM aqueous phase behavior depends strongly on E. chain length. A longer E . chain length drives the surfactant microstructures to higher curvature toward the hydrophobicend of the molecule. For n = 5 and 8, M(D’E5)M and M(D’&)M in water form L3 and Laphase which dominate in middle and low surfactant concentration regions. Because the surfactant bilayer patchesare theelementary componentsof themicrostructuresin these phases, we conclude that the hydrophilic E. chain and the lipophilic trisiloxane moiety are balanced and that the mean curvatures of the monolayers making the bilayers tend to prefer valuesnearzero. Forn = 12,thephasediagramofM(D‘El~)M/ water is significantly different from those above. The L3 phase is not found in the M(D’Elz)M/water system. The L. phase retreats toward lower temperature and the surfactant-rich side. Isotropic micellar phase LIis found in a broad temperaturb composition region. A hexagonal liquidcrystal phaseHl emerges at midrange of concentrations from low temperature. So the LI and HIphases are favored when the E. chain length is raised
from n = 5 . 8 to n = 12. The microstructure of these phases, spherical micelles (LI) and cylindrical rods (HI), are comp0.d of monolayer surfactant sheets curved toward their hydrophobic side. In the case of n = 18. M(D’El8)M in water spans its H I phase over much broader ranges in composition and temperature than does the H I phase of M(D’EI>)M. The L . phase is not found. Instead, a solid lamellar phase is found at the surfactantrich corner of the phase diagram. Figure 17 illustrates the phase and micmstructuraltrends we have just summarized. The microstructura of the M(M3JM surfactants with a longer E. chain favor higher curvature toward the hydrophobicsideofthesurfactant monolayer sheets. Becausc a larger E. headgroupinduces stronger hydrationand headgroup repulsion, we deduce that the M(D’E.)M head group area rises when the E. chain length is longer. Consistent with the geometrical constraints of surfactant packing.6.3353‘ increasing the headgrouparea with the hydrophobicmoietyunvaryingdrives the surfactant microstructure toward a higher curvature to the hydrophobic side before flipping to inverted structures. which agrees with our experimental observations. A similar trend has been found for the binary systems of CIIE. (n = 4,6,8)/water’ and CloE. ( n = 4, 5)/water.l Theconcentrationand temperature effects on the aqueous phase behavior of M(D’E.)M are parallel towhat arereported fortheC,E./water~ystems~~~J~and willnot be further explored in this work. In terms of aqueous phase behavior. it is not easy to relate the hydrophobic trisiloxane moiety of M(D’E.)M to the single alkyl chain in C,E.surfactantsbecause themoleculargeometry ofthe two classes of surfactants is quite different. It has becn reported that the critical micelle concentration value of trisiloxane surfactants is comparable to that of undecane or dodecane surfactants with thesame hydrophilicgroups.’ Butthesignificant differences between the binary phasediagrams of CIzEsand that ofM(D’Es)Mmakeit apparentthat treatingM(D’Es)MasCI& is not accurate in terms of phase behavior. The L3 phase of M(D’Es)M has the lower eutectic temperature around 20 O C , whichis much lower than thoseof CloEs,CIzEs.CIZ& and CI&, which range over W O OC. Rather, the lower eutectic tem-
peratureoftheL3phaseofM(D’Es)MissimilartothatofCIzE>. One way to heuristically rationalize microstructural trends is to introduce the surfactant packing constraint parameter S = VIAL (V = hydrophobic volume, A = head group area, L = hydrophobic chain le~~gth).”.’~,~‘ According to this qualitative rationalization, there is a tendency to form spherical micelles, long circular cylindrical micelles, and infinite bilayer disk-like micelles at low surfactant concentration when S = I/>, 213. and 1, respectively. As the meanings of V,A, and L are somewhat uncertain, it appears to use safer to argue that as S increases from a small to a larger value the sequence of favored microstructures will be spherical micelles, cylindrical micelles, disklike micelles. or lamellar structures. Because the hydrophobic chain length Lcannot extend beyond the ‘all-tram” hydrophobic chain length and Vis roughly proportional to L,’ for each type of micelle there is a lower limit of the head group area A- for which S will have a given value. The value of A- also depends on the type of hydrophobic moiety. For single alkyl chain surfactants such as CIzE., Ad = 70 A 2 for S = 47 A 2 for S= and 23 A2 for S = 1.13 For the trisiloxane surfactants reported in this paper, the apparent volume of the trisiloxane moiety is V = 465 A.37 and the “all-trans” hydrophobic chain length is 4 = 9.2 A?s So the minimum head group area for the each type of micelle can be calculated, as shown in Table VII. From Table VII, we scc that the A- for M(D’E.)M is about twice that forC,zEaEoreach t y p o f micelle favored hy the packing constraint approach. So we can account for that, at similar temperature at low surfactant concentration, M(D‘E.)M and CIzE.. will form similar micellar solutions at certain n and n’ values where n > n’.
He et al.
8832 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 TABLE Vn. Apia for M(IYE,)M and C&, S
A n d M D’En)M) (k)
113 A- = 3V/& = 153 112 A& = 2V/& = 102 1 Amin= VI&= 51
A
‘ (C12Ed) 3 2 )
10 41 23
shape sphere favored long circular rod favored infinite bilayer disk favored
TABLE VIE Correlation of Similar Aqueous Binary Phase Magrams of M(IYE,)M and ClzE, M(D’En)M M(D’E5)M M (D’Esl M
CizEd
At higher surfactant concentration, it is suggested that intermicellar interaction leads to a disorder (micellar solution)order (micelle-packed liquid crystal) transition usually without change in micellar shape; the types of phase transition between liquid crystal phases depend mainly on the packing constraint parameter S.33 This simple model implies that S is actually the major factor in determiningthe microstructure and phase behavior of a nonionic surfactant in water, both in low- and high-surfactant concentration. Although the packing constraint approach does not count for micellemicelle fusion, the surface curvature at high surfactant concentration, and the lack of accurate relationships between S and temperature or concentration (which require a rather complicated theoretical approach39), we believe this approach does predict similar aqueous phase behavior between M(D’E,)M and when the S parameter for the two types of surfactants is close, which requires n > n’. From comparison of the phase diagrams of ClzE,#/water systems7-11with the phase diagrams of M(D’E,)M/water systems we studied, the binary phase diagrams with striking similarities are summarized in Table VIII. To further understand the phase diagram correlation shown in Table VIII, we found it helpful to compare the estimated values of the E,, head group area with the minimum effectivehead group area Aminfor each type of micelle. In Table IX, values of the estimated E, headgrouparea a7,*,4*,42at25 “C and low surfactant concentration are listed. By the packing constraint approach, the predicted shapes of micelles for the surfactants listed in Table VI11 are also summarized in Table IX. As predicted by the packing constraint approach in Table IX, the M(D’E,)M and C,E,mrfactants with similar aqueous phase behavior (see Table VIII) have the same micellar shape. This coincidenceindicates that the packing constraint concept provides a consistent rationalization of the major trends observed in binary nonionic surfactant/water phase diagrams. However, considering the simple model that the packing constraint parameter is based on, it is unrealistic to count on the packing constraint to provide all the details of the phase diagrams. For example, the packing constraint approach predicts that M(D’E1s)M and M(D’E12)M in water form rod-like micelles,
but thecryo-TEM image (Figures 12 and 15) found only spherical micelles. To explain this discrepancy, we speculate that temperature and surfactant concentration have quite a sensitive effect on the micellar shapes, which the packing constraint approach ignores or is insensitive to. Cubic phases reported for some C12E,/water systems (VI, V2, and I in refs 7 and 8) are not found in the phase diagrams of the M(D’E,)M surfactants we studied. It is worthwhile to mention that we have not found the small-angle scattering data for the cubic liquid crystal phases reported in the ClzE,/water systems, except for the cubic phase (11) of a commercial compound lauryl decaethylene glycol ether (C12Elo) in ~ a t e r . 4 ~The 3 ~ microstructures of the reported V1 and V2 cubic phases in C12E2/water systems were studied in our lab.45 In this study, the S A X S results for the samples of cubic phases revealed only one broad correlation maxima instead of the characteristic diffraction pattern indicating cubic symmetry. Because the exact phase boundary of a narrow cubic phase domain in those nonionic surfactant/water systems is usually difficult to locate and there is usually only a small energy barrier among the cubic phases in different space groups,46 it is not easy to capture a pure single cubic phase. We believe that the thecubic phases in C12E,/water systemsneed systematic study to prove further their existence and characterize their microstructures. The unique flexibi1itp7of the trisiloxane moiety may also play an important role in determining the aqueous phase behavior of the nonionic siloxane surfactants. Theoretical approaches to the phase behavior developed for nonionic systems such as binary C,E,/water might be useful to better understand the siloxane surfactant/water systems studied here.4G50 Themicrostructureofthecloudy samplesformedbyM(D‘E5)M and M(D’E8)M in water in the composition region between W and La remains to be understood. Although we suggest that the surfactant-rich isotropic phase L1, at equilibrium, locates in this region, there is still the possibility that other distinct phases exist between the W and La phases. The reports on other colloidal systems also indicate that novel phases between the water-rich isotropic phase and lamellar liquid crystal phases quite possibly exist, such as the nematic phases,sl the L3’,lZ and the La+,s2and the birefringent microemulsion.53 The LS phase and the translucent regions found in some M(D’E,)M/water systems may play some special role in the “superspreading”phenomena. Given that the surface tensions of M(D’E8)M and M(D’E12)M solutions beyond the cmc are 21.0 and 21.3 mN/M, and the dramatic difference in the spreading behaviors on paraffin wax film between the two solutions, it is relevant that the quite different phase patterns of the two surfactant-water binary systemsplay the key role in understanding the wetting problem. The result shown in Table VI indicates that the surfactant bilayer dispersions of L3, L1, or La phase aid water wetting on paraffin wax film, which agrees well with the model proposed by Zhu.2 The relation between the “superspreading” and aqueous phase behavior is under further investigation.25.24
TABLE Ix: Estimated Value of E,, Head Group Area ( a ) at 25 O C and the Micellar Shape Predicted by the Surfactant Packing Constraint Amroach n=2,3 n=4
30 -
21.0
a < 5 1 A2
inverse structure aC51
A2
inverse structure n=5,6 n=l,8
n = 12 n = 16,18
41 55 85 >lo0
38.1 ( n = 6) 48.0 ( n = 8)
-
a I 5 1 A2
bilayer or inverse structure 51 5 a c 102 A2
bilayer
51IOA2
sphere a>?OAZ sphere
Phase Behavior of POE Trisiloxane Surfactants
Summary
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8833
References and Notes
Polyoxyethylene trisiloxane surfactants constitute a new class of nonionic surfactants whose binary phase behavior is largely unexplored. We found that M(D’E,)M and C,,,E.. surfactants exhibit similar phase behaviors with respect to the size of E,, head group, concentration, and temperature. For M(D’E,)M and C,,,E,t surfactants with similar packing constraint S, the main features of the phase diagrams are similar. A bilayer dispersion of M(D’E5)M sponge-likebicontinuousphase L3 appears to greatly enhance the wetting ability of a water droplet on certain lowenergy surfaces such as paraffin wax film.
Acknowledgment. This project was supported by the National Science Foundation through the Center for Interfacial Engineering (CIE) at University of Minnesota. Dow-CorningCrop.%financial sponsorshipand technical supports are greatly appreciated. We appreciate Dr. F. Bates for many useful discussions and help regarding the SANS experiments. We thank CIE staff B. Trend and L. Sauer for their technical supports. M.H. thanks Dr. H. Hoffman for helping him obtain M(D’E16)M through Th. Goldschmidt AG. M.H. is also grateful to undergraduate researchers Mr. K. Sheikh and Mr. L. Yung for their valuable lab assistance. Thanks are due Dr. G. J. T. Tiddy and Dr. R. Strey for helpful discussions to understand the phase behaviors reported in this paper. Appendix Observation of some M(D’E5)Mlwater samples just in, just below, and just above the range of three phase coexistence. coexistence phases
visual observations
coexistence phases W-L3-(LI?) at AT3 three layers were found: the top layers are isotropic and clear (W, L3) and the bottom layer IS turbid showing slightly blue birefringence (LI?). W-(Ll?) AT3 two clear and isotropic layers (W,L3) separated b a sharp meniscus were found: composition: 38.0 wt % cloudy suspension were found L3-(LI?)-La at AT3 with strong yellow birefringence domains in sample. slightly turbid showin sli htly (LI?) AT3 two layers were found: the to layer is isotropic and clear TL,) and the bottom layer is slightly turbid showing strong yellow birefringence (La). composition: 18.3 wt %, 27.5 wt I W-L3-L2 at AT3 only cloud suspension showing no birefYingence was found. W-L3 AT3 two clear and isotropic layers separated by a sharp meniscus were found, but as shown in Figure 1(a), the volume fraction of the water layer increased discontinuously compared to that of W-L3 samples right below AT,. composition: 58.5 wt % L,-L,L* at AT3 cloud sus nsion was found with yeiow Gefringence domains in the sample. two layers were found: L3-La AT3 cloudy suspension with no birefringence was found.
(4)
(1) Ananthapadmanabhan, K. P.; Goodard, E. D.; Chandar, P. Colloids
Surf.1990, 44, 281. (2) Zhu, X, Surfactant Fluid Microstructure and Surfactant Aided Spreading; Ph.D. Thesis, University of Minnesota, 1992. (3) Gruning, B; Koerner, G. Tensidesurfactants, Deterg. 1989,26,312. Schmidt, G. Tensidesurfactants,Deterg. 1990,27,324. Schaefer, D. Tenside Surfactants, Deterg. 1990, 27, 154. Gould, C. Specialty Chemicals 1991, 354. Smid-Korbar, J.; Kristl J.; Stare, M. Int. J. Cosmet. Sci. 1990.12, 135. (4) Gradzielski, M.; Hoffman, H.; Robisch, P.; Ulbricht, W.; Gruning, B. Tenside Surfactants Deterg. 1990, 27, 366. (5) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Lyotropic Liquid Crystal Phase Behavior of Polymeric Siloxane Surfactants, submitted to Langmuir. (6) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Trans. II 1976, 72, 1525. (7) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; MacDonald, M. P. J. Chem. SOC.,Faraday Trans. I1983, 79, 975. (8) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. 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Surfactant Microstructures, Ph.D. Thesis, University of Minnesota, 1982. Foster, M. D. Small Angle X-ray Investigation of Pore Structure Change in Gas-Solid Reactions, Ph.D. Thesis, University of Minnesota, 1986. (18) Vonk, C. G. J . Appl. Cryst. 1971, 4, 340. (19) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87. (20) Vinson, P. K. Proc. 45th AnnualMeetingof the Electron Microscopy Society ofAmerica; Bailey, G. W., Ed.; San Francisco Press: San Francisco, 1987, p 644. (21) Bennett, K. E. Microemulsion phase behavior and Rheology, Ph.D. Thesis, University of Minnesota, 1985. (22) Purdon, F. F.;Slater, V. W. AqueousSolution andthephasediagram; Edward Arnold: London, 1946. (23) Laughlin, R. G. Aqueous Phase Science of CationicSurfactant Salts. In Cationfc Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series, Vol. 37; Marcel Dekker: New York 1991. (24) Contacting one of the authors, Hill, R. 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(33) Bleasdale, T. A.; Tiddy, G. J. T. Surfactant Liquid Crystals. In The Structure, Dynamics and Equilibrium Properties of Colloid Systems; Bloor, D. M., Wyn-Jones, E. Eds.;Kluwer Academic Publishers, 1990. (34) Ninham, B. W.; Evans, D. F. Faraday Discuss. Chem.Soc. 1986,81, 1. (35) He, M.; Hill, R. M.; Doumaux, H. A.; Scriven, L. E.; Davis, H.T. paper In preparation. (36) We only provide a phenomenological description of the “superspreading” because the complicated wetting processis both thermodynamically and dynamically controlled.lJ (37) The density of liquid M(D’E5)M is 1.04 at 1 atm and 22 OC and the MW is 500; the densities for the hydrophobic moiety and the EO head group are very close in aqueous solution (confirmed by SANS contrast matching experiments). The densities of liquid M(D’E,)M and M(D’E12)M are also very close to 1 at room temperature. (38) The “all-trans” chain length of - CH2)3SiOSi(CH3)3is 9.2 A. The bondlengths: Si-0 ( 1 . 5 0 A ) , C - C ( l . 5 4 a ) , S i ~ ( l . 8 6 , ) (data from CRC Handbook ofchemistry and Physics, 67th ed., CRC Press: Boca Raton, FL,
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1986-1987) or C-C-C (2.51 A), Si-C (1.86 A), Si-oSi (2.97 A), Si-C-H (2.41 A); bond angle (109O) (data from ref 5). (39) Jonsson, B.; Wennerstrom, H. J . Phys. Chem. 1987, 91, 338. (40) Private communication with Dr. G. J. T. Tiddy, Unilever Research, Port Sunlight Laboratories, Wirral, Merseyside L62, 4XN, UK. (41) Puwada, S.;Blanckschtein, D. J . Phys. Chem. 1992, 96, 5579. (42) Puwada, S.;Blankschtein, D. J . Chem. Phys. 1990, 92, 3770. (43) Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (44) Fontell, K.; Mandell, L.; Ekwall, P. Acta Chem. Scand. 1968, 22, 3209. (45) Hessel, F.; He, M. unpublished work. (46) Strom, P.; Anderson, D. M. Lungmuir 1992, 8, 691. (47) Eaborn, C. Organosilicon Compounds; Butterworths Scientific Publications, 1960; p 456. (48) Anderson, D.; Wennerstrom, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243. (49) Blankschtein, D.; Thurston, G. M.; Benedek, G. B. J . Chem. Phys. 1986,85,7268.
He et al. (50) Kjellander, R. J. Chem. Soc., Furaday Trans. XI 1982, 78, 2025. (51) Rizzatti, M.R.;Gault, J. D. J. Colloid. InterfacrSci. 1986,ll0,258. (52) Jonstromer, M.; Strey, R.J. Phys. Chem. 1992,96, 5993. (53) Taupin, C.; Auvray, L.; Di Meglio, J.-M. Film Flexibility of Amphiphilic Layers and Structure of Middle Phase Microemulsions. In Micellar Solutions andMicroernulsions; Chen, S.-H., Rajapopalan, R., Eds.; Springer-Verlag, 1990. (54) He, M.; Lin, Z.; Davis,T.; Scriven, L. E.;Snow, S.A. Phase Behavior and Microstructure of Cationic Trisilovne Surfactants in Aqueous Solution. Unpublished results. (55) Chen, S.-H.; Sheu, E. Y.Interparticle Correlations in Concentrated Charged Colloidal Solutions-Theory and Experiments. In Micellar Solutions and Microemulsionr; Chen, S.-H., Rajapopalan, R., Eds.; Springer-Verlag, 1990. (56) Cotton, J. P. Introduction to Scattering Experiments. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, Th., Eds.; North-Holland, 1991.
