Langmuir 1990, 6, 132-136
132
of 450-500 A before reaching a stable size distribution. Aging appears to occur as a power law of time where the vesicle radius increases as and vesicle number density decays as t-0.6. Size distributions measured with QLS are in excellent agreement with those predicted by the Lifshitz-Slyozov and Wagner model of phase separation in colloidal systems. Vesicle stability is enhanced in NaCl solution due to a lowering of surfactant solubility.
Acknowledgment. This research was supported by the National Science Foundation (PYIA 8351179), the Shell Development Co., the 3M Co., and Johnson and Johnson. We acknowledge very helpful discussions with Professor Jacob Israelachvili. Registry No. SHBS, 67267-95-2.
Dilute Phase Behavior of Cetyl Alcohol, Sodium Lauryl Sulfate, and Water Richard J. Goetz and Mohamed S. El-Aasser* Department of Chemical Engineering, Emulsion Polymers Institute, Center for Polymer Science and Engineering, Lehigh University, 111 Research Drive Bldg. A , Bethlehem, Pennsylvania 18015 Received February 17, 1989. I n Final Form: June 12, 1989 A ternary phase diagram of cetyl alcohol (CA), sodium lauryl sulfate (SLS), and water is constructed at room temperature in the dilute corner with a composition range of 96-100 wt % water, and 4-0 wt % of the surfactant and alcohol. The phase behavior is examined though differential scanning calorimetry and associated with the polymorphism of CA. Three different phases are observed: (i) a gel phase, (ii) a coagel phase, and (iii) a micellar solution of SLS and crystals of CA. With aging, certain regions of the gel transform into the coagel phase. This transformation and the existence of the micelle + /3 crystal region are related to the collapse of the bilayer structure of the gel.
Introduction The phenomenon of the aggregation processes in aqueous surfactant solutions is important in understanding biomembrane dynamics, the properties of drug delivery vehicles, and the phase behavior of microemulsion systems.',2 Model systems typically consist of aqueous surfactant and alcohol mixtures, which aggregate in water to form a variety of liquid crystalline structures. The four common liquid crystalline structures are the lamellar, normal hexagonal, reversed hexagonal, and cubic phases. The physical nature of these phases and the location in the ternary phase diagram are strongly related to the chemical nature of the amphiphile, i.e., the balance between the hydrophilic and lipophilic moieties. These phases have been intensively studied over the past 30 years; however, except for the cubic phase, there is little controversy regarding their molecular arrangement.394 Because of their location in the ternary phase diagram, most of these studies have mainly focused on concentrated systems, using short-chain alcohols. The present study is aimed a t determining the phase behavior of dilute aqueous surfactant systems using a long-chain alcohol, cetyl alcohol (CA), which is a solid at room temperature. Recently, Benton and Miller have reported lyotropic liquid crystalline phases with 90% water in surfactant~
~~
~~
~
~~~~~
(1) Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adu. Polym. Sci. 1985, 64, 1.
(2) Bellocq, A,; Roux, D. Microemulsion Structure and Dynamics; Friberg, S., Botherol, P., Eds.; CRC: Boca Raton, FL 1987; pp 33-78. (3) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63, 59. (4) Ekwall, P. Advances in Liquid Crystals; Brown, G. H., Ed.; Academic: New York, 1975; Vol. 1. pp 1-142.
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alcohol-brine systems using various alcohol chain lengths up to decan01.~ These solutions are ideal for enhanced oil recovery due to the low viscosity and o/w interfacial tension, enhancing the ability to displace oiL6 Aqueous surfactant-alcohol systems with alcohol carbon chain lengths greater than 12 are used for the preparation of topical pharmaceutical and cosmetic creams. Cetyl and stearyl alcohols are commonly used since they impart a high consistency to the p r e p a r a t i ~ n .As ~ the concentration of the alcohol and surfactant in the aqueous solution increases, the system progressively changes from a fluid to being solidlike. Barry's738extensive rheological studies showed that the consistency is due to the formation of a viscoelastic network comprised of the mixed surfactants located in the aqueous phase. The surfactant network, observed through various microscopic techniques, changes in structure with increasing alcohol concentration from vesicular in form to a system consisting of lamellar liquid crystals nucleating from alcohol particle^.^,'^ However, the existence of this network depends on the polymorphism of the alcohol." Long-chain alcohols exist in two crystal forms stable over different temperature (5) Benton, W. J.; Miller, C. A. J . Phys. Chem. 1983,87, 4981. ( 6 ) Miller, C. A.; Ghosh, 0.; Benton, W. J. Colloids Surf. 1986, 19, 197. (7) Barry, B. W. Adu. Colloid Interface Sci. 1975,5, 37. (8) Barry, B. W.; Shotton, E. J. Pharm. Pharmacol. 1967,19, 1105, 1215. (9) Patel, H. K.; Rowe, R. C.; McMahon, J.; Stewart, R. F. Int. J . Pharm. 1985,37,899; 1985, 37, 564; 1985,25, 13. (10) Rowe, R. C.; McMahon, J. Colloids Surf. 1987, 27, 367. (11) Fukushima, S.; Takahashi, M. J . Colloid Interface Sci. 1976, 57, 201; 1977, 59, 159.
0 1990 American Chemical Society
P h a s e Behavior of Surfactant-Alcohol- Water
ranges." At room temperature, the CA molecules are arranged in orthorhombic crystals. This arrangement is referred to as the form. At 42 "C, the crystalline form transforms into a hexagonally packed bilayer structure, in which the hydrocarbon chains are aligned perpendicular to the plane of the hydroxyl group. This is known as the a form. The melting point of CA is 52 "C. Stable creams and emulsions are found when the hydrocarbon chains are arranged in the a form, which was confirmed by differential scanning calorimetry (DSC) and X-ray ~ c a t t e r i n g . l l ~ l ~Ex - 'cept ~ for the crystalline state of the hydrocarbon chains, the a form is structurally similar to the lamellar phase and can swell with water.16 With mixtures of CA and stearyl alcohol (SA), the P-a transition can be supercooled below room temperature, by adjusting the molar ratio of CA/SA.ll," Unstable pharmaceutical and cosmetic preparations are formed if CA and SA are used alone instead of their mixtures. Recently, a liquid crystalline phase has been reported in aqueous solutions of CA and sodium lauryl sulfate (SLS) containing 99% water." Similar to the preparation of pharmaceutical creams and emulsions, the CA and SLS must first be heated in water above the melting point of the alcohol. At room temperature, the solution exhibits birefringence when viewed between cross-polarized filters. After sonication, the intensity of the birefringence increases and is believed to enhance the rate of liquid crystal formation; since, liquid crystalline phases at high water content can take weeks to deve10p.l~ This mixed surfactant system is used in the formation and stabilization of miniemulsions, which are used in the preparation of pharmaceutical coatings and in emulsion polymerization Miniemulsions form spontaneously at the o/w interface if the oil phase has a relatively high water solubility.22 Conductivity and electrophoretic mobility measurements suggest a mechanism of emulsification by diffusion, involving the rupture of the lamellar phase upon swelling of the hydrophobic leaflets by the oil phase.23 Similar swelling mechanisms have also been reported, involving the formation and breakup of an intermediate liquid crystalline phase at the o/w i n t e r f a ~ e . ' ~No attempt will be made to review mechanisms of spontaneous emulsification in this report; however, an excellent review was recently written, discussing the occurrence of liquid crystals during emul~ification.'~ (12) Junginger, H.; Fuhrer, C.; Ziegenmeyer, J.; Friberg, S. J. Soc. Cosm. Chem. 1979,30,9. (13) de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1987, 265, 448. (14) de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1987, 265, 167. (15) de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1984, 262, 56. (16) Lawrence, A. C. S.; Al-Mamur, M. A,; McDonald, M. D. J . Chem. Soc., Faraday Trans. 1 1968,2789. (17) de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1986, 264, 691. (18) Lack, C. D.; El-Aasser, M. S.; Vanderhoff, J. W.; Fowkes, F. M.; Silebi, C. Langmuir 1987, 3, 1155. (19) Mandell, L.; Fontell, K.; Ekwall, P. Ordered Fluids and Liquid Crystals; Advances in Chemistry 63, American Chemical Society: Washington, D.C., 1967; pp 89-124. (20) Vanderhoff, J. W.; El-Aasser, M. S.; Ugelstad, J. U.S. Patent 4 177 177. 1979. - ( 2 l j Ugelstad, J.; El-Aasser, M. S.; Vanderhoff, J. W. J . Polym. Sci., Polym. Lett. Ed. 1973, 11, 503. (22) Brouwer, W.; El-Aasser, M. S.; Vanderhoff, J. W. Colloids Surf. 1986, 21, 69. (23) El-Aasser, M. S.; Lack, C. D.; Vanderhoff, J. W.; Fowkes, F. M. Colloids Surf. 1984, 12, 19. , (24) Benton, W. J.; Miller, C. A. J . Dispers. Sci. Technol. 1982, 3,
Langmuir, Vol. 6, No. I , 1990 133
In order to clarify the mechanism of spontaneous emulsification, further information on the phase behavior of this system is required. Although some studies have outlined the aqueous phase behavior of SA-CA and surfactant systems,26 this report represents the first study of the ternary phase behavior of CA-SLS-water at very dilute compositions. The ternary phase diagram of this system is constructed in the dilute corner at water concentrations between 96 and 100 wt % . The relation between the phase behavior and the polymorphism of CA is examined with DSC and compared to the concentrated systems used in pharmaceutical and cosmetics creams and emulsions. Finally, the emulsification mechanism is briefly discussed.
Methods and Materials SLS (Henkel-Texapon 100) was purified by recrystallization from absolute ethanol, dried under vacuum, Soxhlet extracted with diethyl ether for 48 h, and dried under vacuum. CA (99% pure Aldrich Chemical Co.) was used as received. Double-distilled deionized water was used. Solid mixtures of CA and SLS were prepared by mixing the components a t 70 "C. Once cooled to room temperature, the melts were ground to a fine powder and reheated. This process was repeated several times to ensure homogeneity. The aqueous mixed emulsifier solutions were prepared by mixing the components a t 70 "C for 2 h. The solutions were then cooled to room temperature with further mixing and then sonified with a Heat Systems Ultrasonic Model W 350 sonifier cell disruptor. Approximately 200 mL of the aqueous solutions was sonified a t a time until the solutions reached constant opacity. Thermograms were obtained by using a Mettler differential scanning calorimeter (Model TA-3000). Samples were hermetically sealed in aluminum pans. Water evaporation was checked by taring the pans before and after the DSC measurement. The aqueous solutions were heated a t 10 OC/min from 15 to 80 "C. The solid melts were heated from 15 to 115 "C a t a heating rate 2 OC/min. The slower heating rate was used to increase the separation of the a and 0 transitions. The calorimeter used in this study could not accurately determine the heat of the @-a or a-liquid transition of compositions in the phase diagram due to the extremely low solids content. However, the transition temperatures were reproducible.
Results and Discussion General Description of the Phase Diagram. Approximately 50 samples were used to prepare a partial ternary phase diagram in the composition range 96-100 wt 70 water and 4-0 w t 9'0 SLS and CA (Figure 1). Two different phase regions were observed: (i) a birefringent phase, which became gellike at high CA concentrations, and (ii) a clear micellar phase in equilibrium with crystalline CA. Initially, the birefringent region was located at molar ratios of CA/SLS greater than 1 with a minimum surfactant concentration of 0.125 wt % (5 mM). Below 0.125 wt % SLS, the solution appeared slightly opaque with CA crystals aggregating at the top of the solution. The amount of aggregation increased with alcohol concentration. After the solution aged for 3 months, shifts in the phase region decreased the birefringent area as CA crystals sedimented from the solution. This formed a third phase region, consisting of a mixture of the birefringent phase and white CA crystals. For SLS concentrations between 0.125 and 1 wt % (5-20 mM), the birefringent phase exists at molar ratios of CA/SLS greater than 1. However, for higher concentrations of SLS, greater than 1 wt YO,the birefringent phase exists if the molar ratio of CA/SLS is greater than 2. However, the appearance of the compositions containing less than 0.125 wt
I.
(25) Miller, C. A. Colloids Surf. 1988, 29, 89.
(26) Ekwall, P.; Aminoff, C. Acta Chem. Scand. 1956, 10, 237.
Goetz and El-Aasser
134 Langmuir, Vol. 6, No. 1, 1990 CA
1
20
30
40
50
Temperature
60
70
60
(C)
Figure 3. DSC thermogram of a solid mixture of 24 wt % SLS and 76 wt % CA.
Figure 1. Partial ternary phase diagram of CA-SLS-water in the dilute corner, after aging for 4 months. The dotted line
represents the original phase boundary between the birefringent and micelle + CA crystals regions immediately after the solutionshad been prepared. The black region outlines the compositions used to form miniemulsions. Concentrations shown are in weight percent. 20
30
c
40
50
Temperature
60
70
80
(C)
Figure 4. DSC thermogram of a composition in the micelle + CA crystal region of 2 wt 70 SLS, 0.5 wt % CA, and 97.5 w t 70
20
30
40
50
Temperature
60
70
(C)
Figure 2. DSC thermogram of pure CA used in this study.
70 SLS remained relatively the same. Due to the low concentration of SLS, this region was not studied further. The black region in Figure l outlines the compositions used to form and stabilize miniemulsions for emulsion polymerization reactions. The concentration of SLS is near the critical micelle concentration. Thus, the structure of a liquid crystalline phase present in this region would be highly susceptible to the effects of contaminants or impurities, which can be solubilized, affecting the aggregation of the aqueous mixed emulsifier structure. DSC. DSC was used to determine the crystal form of the hydrocarbon chains in the different phase regions of the phase diagram. However, the individual effects of SLS and water on the polymorphic transitions of CA were first examined. The CA used in this study has a @-CY transition at 42 "C and a melting point at 53 "C (Figure 2). The addition of water increased the temperature range in which the CY phase exists by lowering the @-CY transition. This is due to the swelling of the CY form with water through hydrogen b ~ n d i n g . ' ~The thermograms from the mixtures of solid CA and SLS showed remarkable behavior. There was little or no change in the P-CY phase transition by varying the composition of the melt. However, ( 2 7 ) Lawrence, A. S. C.; AI-Mamun, M. A. Trans. Faraday SOC.
1967,63, 2788.
30
40
50
Temperature
60
70
0
(C)
Figure 5. DSC thermogram of a composition in the birefringent region of 0.28 wt % SLS, 0.5 w t % CA, and 99.2 wt %
water.
the melting point increased by almost 20 "C (Figure 3). Thus, SLS only forms mixed crystals with CA in the CY form. The thermogram (Figure 4) from a sample in the twophase region of the phase diagram, Figure 1, showed two endotherms: (i) a P-a transition at 42 "C and (ii) the melting point at 52 "C. Although, the peak from the cyliquid transition is not distinct, it is the cause of the broad shoulder centered at 52 "C. Thus, this phase region consists of micellar SLS and P crystals of CA. The cy transition temperature of 52 "C indicated that mixed crystals of SLS and CA were not present. After the solid was separated, a mass balance, through gravimetry, accounted for all the alcohol. Thermograms from Samples in the birefringent region of the phase diagram show only one endotherm a t 70 "C (Figure 5), indicating the hydrocarbon chains in the birefringent phase are in the CY crystal form at room temperature. Thus, similar to
Phase Behavior of Surfactant-Alcohol-Water
Langmuir, Vol. 6, No. 1, 1990 135
CA
A
9,
Lk
Water
,,
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~~
~
~~
SLS
Figure 6. Complete phase diagram of CA-SLS-water in the dilute corner, after aging for 4 months. Concentrations shown are in weight percent.
pharmaceutical preparations, it is the polymorphism of the alcohol which determines the existence of the mixed surfactant phase of SLS and CA. More importantly, it is the combined effect of SLS and water which supercools the a form t o room temperature in absence of SA. The DSC thermograms showed that the hydrocarbon chains of the birefringent region are in the solid state. Thus, the structure in the birefringent region is a gel phase. In the gel phase, the surfactant bilayers are separated by alternating water layers similar to a lamellar liquid crystalline phase; however, the hydrocarbon chains are in a solidified a crystal structure with hexagonal subcell packing.z8,29 Vincent et a1.2' originally defined the gel phase, examining single-component systems consisting of potassium and rubidium soaps. The gel phase can also be formed by cooling a liquid crystalline mesophase below the temperature a t which the hydrocarbon chains crystallize. However, for any surfactant and lipid system, the gel phase does not form a t room temperature if the carbon chain length is less than 14.3' Attempts to determine the bilayer dimensions through X-ray scattering failed due to the high water content and large interlayer distance?' The complete phase diagram in the dilute corner is shown in Figure 6. The physical properties of the gel phase up to 75 wt % water have been reported for 1-acylglycerides by Larsson.30 The gel appeared birefringent and exhibited brilliant colors when illuminated by white light. These effects were due to the bilayer structure with large interlayer distances which were on the order of 4-7 pm. Larsson3* also studied the gel phases formed from nalkylamines with carbon chain lengths between 14 and 18. Two gel phases formed with similar molecular packing hut with different water layer spacing which varied between 14 and 1500 A. The increased swelling in these gel phases was due to the enhanced hydration of the surfactant a t high water content. Most gel phases are metastable and eventually transform into a coagel consisting of a mixture of the gel phase (28) Vincent, J. M.:Skoulious, A. Acta Crystallogr. 1966.20, 432. (ZS) L-ti. V. Biomembmnos: Physical Facts ond Function; C h a p man. D.. Ed.; Academic Press: London. 1966; pp 71-123. (30) Larsson, K.; Krog, N. Chem. Phys. Lipids 1973, I O , 177. (31) Fontell. K.,personal communication, 1985. (32) Larsson. K.; AI-Mamun, M. A. Chem. Phys. Lipids 1974. 12. 176.
and precipitated solid surfactant or alcohol. The DSC thermograms confirmed the presence of the B and a form of CA in compositions of the gel-phase region which formed white crystals upon aging and a t surfactant concentrations below 0.1 wt % SLS. However, this gel-coagel transformation does not occur uniformly throughout the gel. The gel is prepared by cooling the heated mixed surfactant solution below the melting point of the alcohol. Thus, it is unlikely that there exists an even distribution of SLS in the bilayers. The magnitude of the electrostatic repulsion between gel bilayers, which originates from the SLS, governs the bilayer spacing. Therefore, the gel phase can be considered to be lamellar in form with various degrees of order in different parts of the structure; this gives rise to the nonuniformity of the gel-coagel transition. However, this may not be the general description for singlecomponent gels. The gel-coagel transition upon aging is a common problem found in pharmaceutical and cosmetic formulations containing long-chain fatty alcohols, resulting in a decrease in the con~istency.'~Rowe et a1.9 studied the problem a t low-temperature storage and compared the crystallization to the fusion of phospholipid membranes. However, they never examined the important effects of the polymorphism of the alcohol in their studies. A more likely explanation for the transition focuses on the hydration of the surfactant in the gel bi1a~ers.l~ Once hydrated, the surfactant diffuses from the gel, decreasing the bilayer surface charge density. This reduces the electrostatic repulsion between adjacent bilayers. The expulsion of water upon the collapse of the bilayers results in the crystallization of CA to the /3 form. Recall that it is the important combination of SLS and water that supercools the a form of CA. The micellar + B crystal-phase region occurs in the region when the molar ratio of SLS/CA is greater than one. The inability to supercool the a form in this region is likely due to the limited amount of surfactant that can he solubilized or incorporated in the a form of CA. The excess surfactant then acts as an electrolyte, decreasing the repulsion between adjacent bilayers and reducing the swelling with water. Again, similar to the destabilization upon aging, the expulsion of water from the gel results in a structure transformation of CA from the a to the /3 form. However, this transformation occurs immediately, since the destabilization with aging depends on the slow diffusion of the surfactant from the solidified hydrocarbon bilayers. The water layer thickness can also he reduced by introducing an electrolyte to a composition in the gel region. Addition of 0.04 wt % NaCl to a monoglyceride gel containing 70 wt % water reduced the water layer thickness by 88%?3 The introduction of successive amounts of an electrolyte to the dilute CA-SLS gel destabilizes the gel into the coagel or the 0 crystal + micelle phase. The addition of 0.5 wt % NaCI transformed a gel, containing 0.42 wt % SLS and 0.72 wt % CA, to the coagel phase. However, 2 wt % NaCl destabilized the gel to the micelle + 0 crystal region. A complete electrolyte stability study was not conducted on this system. However, analogous to colloidal dispersions, increasing the valence of the cation should decrease the concentration required for phase transformation. The gel phase should not be confused with "gel-like" systems reported in the literature. For example, the hydrocarbon gels prepared by Hoffman's group can contain ~~~
(33) Larrson, K.; Krog. N. J.; Riisom, T. H. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1985; Vol. 2, pp 327-365.
136
Langmuir 1990,6, 136-142
up to 99.5% hydrocarbon, the remainder being surfactant and water.34 All the components are in the liquid state a t room temperature, and there is no evidence of crystallized hydrocarbon chains. The trick to prepare these systems is to slowly titrate the hydrocarbon into a foamy surfactant solution. Little information is known on the long-term stability of these systems; however, they do lose their consistency when sheared. Solans et al.35have recently prepared a gel containing 99% water using only a nonionic surfactant and a hydrocarbon. Although they give no physical properties of the surfactant, the gel forms only if the solution is first heated to 50 OC and then cooled to room temperature. A remarkable characteristic of the dilute CA-SLS gel is its ability to spontaneously form emulsions with submicron-sized emulsion droplets (miniemulsions). This research represents the first report of spontaneous emulsification involving a gel phase. However, the solid nature of the hydrocarbon chains of the gel prevents swelling and solubilization by an oil phase. A preliminary model, describing the initial stages of the emulsification process, must involve a change of the hydrocarbon chains of the gel from the solid to the liquid state. Moreover, if the bilayer structure is retained, the correct nomenclature for this system is a lamellar liquid crystalline phase. Due to the low SLS and CA concentration, the liquid crystalline phase formed is likely to be unstable, leading to emulsion droplet formation. However, further information is needed to clarify this preliminary mechanism. (34) Hoffman, H.; Ebert, G. Angew. Chem., Int. Ed. Engl. 1988,27, 902. (35) Solans, C.; Dominguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S. E. Colloid Polym. Sci. 1988, 266, 570.
Summary A ternary phase diagram of CA-SLS-water was constructed in the dilute region. The structure of the phases was determined through DSC and related to the polymorphism of the alcohol. A birefringent gel phase was found in regions with molar ratios of CA/SLS at least greater than one. The structure of the gel is similar to the lamellar phase; however, the hydrocarbon chains are arranged in a solidified a crystal structure with hexagonal subcell packing. The a form of CA is supercooled to room temperature through a unique interaction between SLS and water with CA. In certain regions of the phase diagram, the a form of CA crystallizes into the 0form, forming the coagel phase. This transformation is due to the decreased electrostatic repulsion between bilayers upon the desorption of SLS. The free SLS then acts to screen the charge on the bilayer, further reducing the interlayer distance. The expulsion of water from the bilayer removes a component necessary for the supercooling of the a crystal form of CA. Assuming that only a maximum amount of SLS can be incorporated in the a form, this mechanism also explains why the gel region is located in regions where the molar ratio of CA/SLS is greater than one. The structure of the gel and the emulsification mechanism are being examined through the measurement of the self-diffusion coefficients by Fourier transform spin-echo NMR. Acknowledgment. Support from t h e ColgatePalmolive Co. and discussions with Dr. Fred Fowkes from the Dept. of Chemistry and Drs. Krister Fontell and Ali Khan from the University of Lund, Sweden, are gratefully appreciated. Registry No. SDS, 151-21-3; CA, 36653-82-4.
NMR and Neutron-Scattering Studies on Poly(ethy1ene oxide) Terminally Attached at the Polystyrene/Water Interface Terence Cosgrove* and Keith Ryan School of Chemistry, Uniuersity of Bristol, Cantock's Close, Bristol, BS8 1 TS, U.K. Receiued February 6, 1989 Small-angle neutron scattering (SANS) and nuclear magnetic resonance spectroscopy (NMR) have been used to investigate the structure of terminally attached polymers at the polystyrene latex/water interface. In particular, volume fraction profiles obtained by SANS and by the Scheutjens-Fleer meanfield theory of polymer adsorption were used to calculate spin-spin relaxation functions for the adsorbed layer with different adsorbed polymer coverages and chain lengths. These are compared to the experimentally determined relaxation functions obtained by using NMR. There is a good qualitative agreement with the three approaches. The NMR data provide a new insight into the mobility and structure of the adsorbed layer.
Introduction Colloidal dispersions are now used extensively in a wide variety of industries, ranging from paint production to
* Author
to whom all correspondence should be addressed.
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plant protection. In many cases, polymers are added to a dispersion to modify its stability or rheological behavior. Of particular interest are those dispersions which are stabilized by a layer of adsorbed polymer.' If the stabilizing polymer is a homopolymer, then the level of 1990 American Chemical Society
