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Electrostatic Effects on Interfacial Film Formation in Emulsion Systems
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
179, 537–543 (1996)
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Electrostatic Effects on Interfacial Film Formation in Emulsion Systems MARTIN MALMSTEN,* ,1 ANNA-LENA LINDSTRO¨M,†
AND
TORBJO¨RN WA¨RNHEIM†
*Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden; and †Pharmacia Hospital Care, S-112 87 Stockholm, Sweden Received April 17, 1995; accepted October 31, 1995
The adsorption at silica from dilute emulsion systems was studied with in situ ellipsometry. In particular, the effects of electrostatic interactions on the adsorption rate and the adsorbed layer structure and formation were investigated by varying the emulsion droplet and surface charge, as well as the electrostatic screening, accomplished by varying pH and the excess electrolyte concentration. Electrostatic interactions were found to markedly affect the adsorption rate, but not the adsorbed layer structure or the mechanism for the adsorbed layer formation. For all cases investigated, the adsorbed layer thickness corresponds to emulsion droplets or multilamellar liposomes, and the adsorbed layer formation proceeds through attachment of emulsion droplets and/or multilamellar liposomes at the surface without extensive droplet spreading or liposome collapse. When the droplets and the surface are similarly charged, the adsorption is facilitated by increasing the electrostatic screening or by decreasing the emulsion droplet and surface charge, accomplished by increasing the excess electrolyte concentration and decreasing pH, respectively. When the droplets and the surface are oppositely charged, the adsorption rate is much higher than that observed when the droplets and the surface are similarly charged, although the adsorbed layer structure and the mechanism for the adsorbed layer formation are similar. Qualitatively, these effects may be understood by considering only electrostatic and van der Waals interactions. q 1996 Academic Press, Inc. Key Words: adsorption; DLVO; electrostatic; ellipsometry; emulsion; film.
INTRODUCTION
The deposition at macroscopic surfaces from colloidal systems is of interest in a variety of practical applications, including pharmaceuticals, foods, paints, and coatings. The interaction between the colloidal system and the surface is of importance, e.g., for the long-term colloidal stability of the system. Of particular interest to the present study are oil-in-water (o/w) emulsions used in pharmaceutical applications such as nutrition and delivery of lipophilic drugs (1– 3). For an emulsion to be applicable intravenously, the emulsion droplets must be small and of a narrow size distribution. Furthermore, the emulsions need to be stable, show1
To whom correspondence should be addressed.
ing little growth in the mean droplet size over extended periods of time. This means that high requirements are put on both the emusifier used and the material of the containers in which the emulsions are stored. Despite the need for an improved understanding of the interaction between colloidal systems and macroscopic surfaces, there have been very few previous studies on these processes, mainly due to lack of experimental methods allowing detailed information to be obtained regarding the adsorbed layer structure and the mechanism for the adsorbed layer formation. Recognizing the importance of the interaction between colloidal systems, particularly emulsions, and macroscopic surfaces, Malmsten et al. previously investigated the applicability of in situ ellipsometry for studies of the interfacial film formation in emulsion systems (4). In this previous study, the adsorption from a dilute emulsion system at (hydrophilic and negatively charged) silica and (hydrophobic) methylated silica was investigated. It was found that the adsorbed layer at both these surfaces consists of small or ‘‘flattened’’ emulsion droplets or multilamellar liposomes, and that the adsorbed layer formation proceeds through attachment of these colloids at the surface. Furthermore, the droplet ‘‘spreading’’ was more pronounced for methylated silica than for silica, as would be expected from contact angle considerations (5). In the present study, the effects of electrostatic interactions on the adsorption at silica were investigated by varying the droplet and surface charge (by varying pH) as well as the screening of the electrostatic interactions (by altering the excess electrolyte concentration). As in the previous investigation, in situ ellipsometry was used for these studies (4, 6). EXPERIMENTAL
Materials Emulsion preparation. The model o/w fat emulsion studied consisted of purified soybean oil dispersed in distilled water and stabilized by fractionated egg phosphatides. The major constituents of the phosphatides were phosphatidylcholine and phosphatidylethanolamine. Smaller amounts of negatively charged lipids were also present. The concentration of phospholipids and soybean oil was 1.2 and 20
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wt%, respectively. Sodium hydroxide was added to obtain pH 8 in the resulting system. The emulsion was prepared by using a conventional high-pressure homogenization process (2, 3), followed by heat sterilization. In order to obtain a small and uniform droplet size, the emulsion was passed several times through a high-pressure homogenizer. The emulsion was diluted (to 2 1 10 03 wt%) in 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2, 0.01 M phosphate buffer, pH 7.2, 0.01 M phosphate buffer, pH 4.9, or 0.01 M phosphate buffer, pH 2.5, in the adsorption experiments. Emulsion characterization. Since the dilution with buffer could affect the stability of the emulsions, this was investigated at different dilutions. For example, emulsion samples were studied microscopically with an Axioskop (Zeiss). Sampling and examination were performed at 0, 6, and 18 h after dilution. The mean droplet size ( z-average) and size distribution were determined using a Malvern 4700 photon correlation spectrometer (Malvern Instruments), equipped with an 8-multibit correlator and a 35 mW He/Ne laser. The light scattered by the oil droplets was detected at 907 scattering angle. Again, the measurements were performed at 0, 6, and 18 h after dilution at 257C. The determination of the z -potential of the emulsion droplets was performed using a Malvern Zetasizer 4, with a 4 mW He/Ne laser (Malvern Instruments). The instrument is provided with a software routine (duty cycling), allowing the use of an AZ104 cell for samples diluted in medium with high ionic strength. Switching off the applied voltage between each cycle eliminated the distortion by sample heating. Water was first purified by a Milli-RO 10PLUS pretreatment unit, including depth filtration, carbon adsorption, and decalcination preceding reverse osmosis. Subsequently, it was led through a Milli-Q PLUS185 unit, which treats the feed water with UV light (185 and 254 nm) before leading it into a Q-PAK unit consisting of an active carbon unit followed by a mixed bed ion exchanger, an Organex cartridge, and a final 0.22-mm Millipak 40 filter. Surfaces Silica surfaces were prepared from polished silicon slides (p-type, boron-doped, resistivity 7–13 Vcm; Okmetic, Finland). In short, these were oxidized thermally in pure and saturated oxygen, followed by annealing and cooling in argon flow, which resulted in an oxide layer thickness of about 40 nm. The slides were then cleaned as described previously (4). This procedure rendered the surfaces hydrophilic, with a contact angle (in air) of less than 107. The silica slides were then kept in ethanol until use. Just prior to use, the slides were rinsed twice with, in order, ethanol and water, followed by drying with filtered nitrogen (Aga, Sweden). Ellipsometry The ellipsometry measurements were all performed by means of null ellipsometry ( 6 ) . The instrument used was
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an automated Rudolph thin-film ellipsometer, Type 436, controlled by a personal computer. A xenon lamp, filtered ˚ , was used as the light source. A thorough deto 4015 A scription of the experimental setup has been given previously ( 7 ) . In the case of a layered substrate, e.g., oxidized silicon, a correct determination of the adsorbed layer thickness and mean refractive index requires determination of the silicon bulk complex refractive index ( N2 Å n2 0 ik2 ) as well as of the thickness ( d1 ) and the refractive index ( n 1 ) of the oxide layer ( 6 – 9 ) . This is achieved by measuring the ellipsometric parameters c and D in two different media, e.g., air and buffer. From the two sets of c and D, n2 , k2 , d1 , and n1 can be determined separately. All measurements were performed by so-called four-zone null ellipsometry in order to reduce effects of optical component imperfections ( 6 ) . After the optical analysis of the bare substrate surface, the emulsion was added to the cuvette to a final concentration of 2 1 10 03 wt%, and the values of c and D were recorded. ( The adsorption was monitored in one zone, since the four-zone procedure is rather time-consuming and since corrections for component imperfections already had been performed.) Stirring was performed by a magnetic stirrer at about 300 rpm. From c and D, the mean refractive index (n f ) and average thickness ( del ) of the adsorbed layer were calculated numerically according to an optical four-layer model (6–9). The mean refractive index and average thickness were finally used to calculate the adsorbed amount ( G ) according to Cuypers et al. (10), using a molar refractivity of 3.2 (10) and a specific volume of 1.0 (11). Due to the similarity of these parameters for soybean oil and lecithin, ellipsomety provides the total adsorbed amount. Measurements were performed at 207C. RESULTS AND DISCUSSION
Emulsion Stability The egg lecithin used in the present investigation contained a small fraction of ionized lipids, resulting in an emulsion droplet charge. The charge of the droplets (and of the silica surface) will depend on pH and the excess electrolyte concentration. The emulsion droplet size and z -potential at different pH and excess electrolyte concentration are given in Table 1 together with the silica surface potential. The low z -potential of the emulsion droplets ( zdroplet ) obtained for the emulsion diluted in buffer and electrolyte indicated a risk for instability of the systems, e.g., through flocculation or coalescence. However, the absence of aggregated and/or larger droplets was ensured for all samples by light microscopy. Furthermore, for all samples the mean droplet size and the droplet size distribution, obtained by PCS, gave no indications of significant flocculation or coalescence over a period of at least 18 h.
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TABLE 1 System Characteristics at Different pH and Excess Electrolyte Concentration Buffer
Diameter (nm)
zdroplet (mV)
zsurface (mV)a
0.15 M NaCl pH 7.2 pH 7.2 pH 4.9 pH 2.5
307 307 308 323
04 022 016 /7
060 060 030 07
Note. In all cases 0.01 M phosphate buffer was used. a Obtained from electroosmosis and surface force measurements (20, 21).
From the total interfacial area in the emulsion system and the surface area occupied per emulsifier molecule (12), it is clear that the system studied contained phospholipids in excess of that required to stabilize the emulsion. Consequently, it is possible for liposomal structures to form within the emulsion (13). Adsorbed Layer Structure and Formation The amount adsorbed ( G ) at silica from the diluted emulsion at different pH and excess electrolyte concentration is shown in Fig. 1a. At pH 7.2, where both the emulsion droplets and the surface are negatively charged ( zdroplet Å 04 and 022 mV for 0.01 M phosphate buffer in the presence and absence of 0.15 M NaCl, respectively; zsurface É 060 mV), the adsorption is enhanced by increasing the excess electrolyte concentration. Decreasing pH to 4.9, which reduces both the droplet and surface negative charge (Table 1), results in a slightly higher adsorption than that at pH 7.2. Decreasing pH further to 2.5, resulting in a reduction in the surface negative charge ( zsurface Å 07 mV) as well as in a charge reversal of the emulsion droplets ( zdroplet Å /7 mV), gives rise to a dramatically enhanced adsorption. A notable feature from Fig. 1a is that the adsorption proceeds up to 25,000 s seemingly without limit. At most, we have followed the adsorption for 10 h, and have seen no signs of adsorption saturation. As will be discussed below, the reason for the slow adsorption is not so much the low droplet concentration, but rather a quite low ‘‘sticking probability’’ for the droplets at the surface, simply inferred from the finding of quite different adsorption kinetics at a fixed droplet concentration, but different pH and excess electrolyte concentration (Fig. 1a), and also at different surfaces (4). Thus, changing the electrostatic interaction between the droplets and the surface by varying pH and the excess electrolyte concentration seems to modulate the droplet sticking probability (see below). In contrast to the adsorbed amount, the adsorbed layer thickness ( del ) reaches a limiting value very rapidly, and at adsorption times larger than about 1000 s, the adsorbed
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layer thickness is essentially constant ( Fig. 1b ) . The adsorbed layer thickness was found to be 90 { 15 nm for all systems investigated, and similar kinetics were observed for all pH and excess electrolyte concentrations. It is interesting to compare the adsorbed layer thickness with the dimension of the emulsion droplets and the phospholipid molecules. Clearly, the adsorbed layer thickness corresponds more closely to the droplet size ( 307, 307, 308, and 323 nm for 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2, 0.01 M phosphate buffer, pH 7.2, 0.01 M phosphate buffer, pH 4.9, and 0.01 M phosphate buffer, pH 2.5, respectively ) , than to mono- or bilayers of phospholipid molecules ( expected to result in an adsorbed layer thickness of about 2.5 and 5 nm, respectively ) ( 14 ) . This is the case particularly since the emulsion droplets are size polydisperse and since droplets as small as 50 nm are present in the emulsion. A difficulty in the interpretation of the optical thickness in terms of structural parameters is that resulting from the interfacial ‘‘film’’ being inhomogeneous, as seen clearly from the adsorbed amount and the adsorbed layer thickness taken in conjunction. For systems with a density distribution normal to the surface it has been shown that the ellipsometrically determined thickness is related to the first moment of the density distribution, i.e., an average thickness is obtained with ellipsometry (15). The root-mean-square thickness is obtained from the optical thickness by dividing by a factor of 1.47–1.75, depending on the density distribution normal to the surface (16). Furthermore, the size of the droplets is comparable to the wavelength, and therefore, the applicability of the effective medium theory may be discussed. Another problem in the interpretation of the ellipsometric results is that due to the lateral heterogeneity of the adsorbed layer. In principle, this can be investigated ellipsometrically, employing either multiple angle of incidence or spectroscopic ellipsometric techniques coupled to an assumed type of heterogeneity. Although certainly urgent, these studies are outside the scope of the present investigation. From the considerations above, it is clear that the interpretation of the adsorbed layer optical parameters in terms of structural properties should be made somewhat cautiously. Nevertheless, the main point is that it is clear from the ellipsometric results that the adsorbed layer consists of adsorbed emulsion droplets or multilamellar liposomes, and not by a phospholipid bilayer. Since the adsorbed layer thickness reaches a limiting value already after about 1000 s, whereas the adsorbed amount increases without limitation for at least 10 h, the adsorbed layer density must increase monotonously during the adsorption process. It is important to note, however, that also at rather high adsorbed amounts, the average concentration of the adsorbed layer is low, and the mean distance between the adsorbed droplets (d) large compared to the droplet radius (rh ; d/rh É 8–65).
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FIG. 1. (a) Adsorbed amount ( G ) and (b) adsorbed layer thickness ( del ) on silica as a function of time. The adsorption was performed from 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (diamonds), 0.01 M phosphate buffer, pH 7.2 (squares), 0.01 M phosphate buffer, pH 4.9 (triangles), and 0.01 M phosphate buffer, pH 2.5 (circles).
The good time resolution of ellipsometry, and the simultaneous recording of the adsorbed amount, the adsorbed layer thickness, and the mean adsorbed layer refractive index, allow the mechanisms for the adsorbed layer formation to be investigated in some detail with this technique. As can be seen in Figs. 2–5, the adsorbed layer thickness is essentially independent of the adsorbed amount, while the mean adsorbed layer refractive index increases essentially linearly with the adsorbed amount. Thus, the adsorbed amount increase is accomplished by increasing the average adsorbed layer concentration at a constant adsorbed layer thickness. Together with the finding of the adsorbed layer thickness corresponding to emulsion droplets or multilamellar liposomes, these findings mean that the adsorption occurs through attachment of these colloids at the surface in a largely intact shape, with only a limited droplet spreading or liposome collapse. Keeping the mechanism of adsorption in mind, it is interesting to once more consider the effects of pH and the excess electrolyte concentration on the adsorption at silica from the emulsion under investigation. As a first approximation we consider only electrostatic and van der Waals interactions to be of importance for the drople-droplet and droplet-surface (adsorption) fusion. The stability in such a colloidal system is described by the DLVO theory (17). In the present case, the composition of the emulsion, and in particular of the dispersed phase, as well as the droplet size were essentially constant, meaning that the van der Waals interactions were fixed. However, the droplet–surface electrostatic interactions were strongly affected by varying pH and the excess electrolyte concentration. At pH 7.2, both the surface and the emulsion droplets are negatively charged. More precisely, the z -potential of the surface and the droplets is about 060 and 022 mV, respectively. This generates a repulsive electrostatic droplet–surface interaction opposing adsorption, which is counteracted mainly by an attractive van der Waals interaction. At addi-
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tion of excess electrolyte, the electrostatic repulsion is screened, which means that the attractive component to the droplet–surface interaction will become more dominant. Within the framework of the DLVO theory, this means that the magnitude of the maximum in the droplet–surface interaction curve is reduced (Fig. 6). Since the probability for achieving close droplet–surface contact increases with a decreasing energy barrier, and since close contact is most probably necessary for adsorption to occur, increasing the excess electrolyte concentration is expected to result in an increased probability of close droplet–surface contact (adsorption), analogous to the increased probability of close droplet–droplet contact (flocculation) in the emulsion system. Except for an increased probability for close droplet–surface contact, the increase in the excess electrolyte concentration is not expected to markedly affect the adsorbed layer structure or the mechanisms for the adsorbed layer formation ( zdroplet ! zsurface ; no signs of flocculation even at high excess electrolyte concentrations). Considering this, the finding of an increased adsorption rate, but unchanged adsorbed layer structure and formation (Figs. 1–3), is expected. On decreasing pH to 4.9, both the droplet and the surface negative charge is reduced (Table 1), resulting in a reduction in the energy barrier for close droplet–surface contact (Fig. 6). From the considerations above we would therefore expect that the adsorption rate should be somewhat higher at pH 4.9 than at pH 7.2, although the adsorbed layer structure and formation are still expected to be similar to that at pH 7.2. As can be seen in Figs. 1, 3, and 4, this is indeed what is found experimentally. Quantitatively, the effects on the adsorption are rather small, and the adsorbed amount after an adsorption time of 25,000 is 0.15 and 0.35 mg/m 2 at pH 7.2 and pH 4.9, respectively. The reason for the small difference in adsorption rate at pH 7.2 and 4.9 is that the charge density of both the droplets and the surface is only moderately reduced on decreasing pH from 7.2 to 4.9 (Table 1). At pH 2.5, the negative charge of the silica surface is
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FIG. 2. (a) Adsorbed layer thickness ( del ) and (b) mean adsorbed layer refractive index (n f ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (4).
quite low ( zsurface É 07 mV), while the emulsion droplets are weakly positively charged ( zdroplet Å /7 mV). Consequently, there is an attractive electrostatic driving force for adsorption, apart from the van der Waals attraction, and the total droplet–surface interaction is therefore monotonously attractive (Fig. 6). We would therefore expect a much more pronounced adsorption at pH 2.5 than at pH 4.9 and pH 7.2, as also observed experimentally (Fig. 1). [The marginally higher total electrolyte concentration at pH 2.5 (0.013 M) than at pH 4.9 and pH 7.2 (both 0.01 M) is expected to marginally reduce the adsorption rate at the former pH.] From Figs. 1, 3, 4, and 5, we see that despite the higher adsorption rate at pH 2.5 the adsorbed layer thickness and the mechanism for the adsorbed layer formation are similar to that at pH 4.9 and 7.2. This result is not self-evident since we would expect the droplets to strive at being as close to the surface as possible from the droplet–surface interaction curve (Fig. 6). We would therefore expect a larger degree of droplet spreading and liposome collapse at pH 2.5 than for the higher pH values. The fact that this is not observed experimentally seems to indicate that the interfacial spread-
ing favored by van der Waals and electrostatic interactions at pH 2.5, and by van der Waals interactions alone at pH 4.9 and 7.2, is limited by the surface tension of the emulsion droplets and liposomes. Since the composition of these is identical at all the conditions investigated, identical limiting adsorbed layer thicknesses are expected for all cases. The interfacial deposition of colloidal particles has not been studied extensively previously, despite the fundamental and practical interest of these processes. Giesen et al. previously studied the adsorption of unilamellar liposomes at silicon slides with ellipsometry and found that the liposomes adsorbed at this surface, although collapsing and forming phospholipid bilayers at the interface (18, 19). With the assumption that the liposomes present in our system would behave similarly to the system studied by these authors, this previous finding indicates that the adsorbed layer in the present investigation is not likely to consist of unilamellar liposomes, since collapsed unilamellar liposomes are incompatible with the adsorbed layer thicknesses observed. From the same reasoning, and from the finding of an essentially identical adsorbed layer thickness at pH 2.5, 4.9, and 7.2,
FIG. 3. Adsorbed layer thickness ( del ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 7.2.
FIG. 4. Adsorbed layer thickness ( del ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 4.9.
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FIG. 5. (a) Adsorbed layer thickness ( del ) and (b) mean adsorbed layer refractive index (n f ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 2.5.
even multilamellar liposomes are less likely to form the adsorbed layer, although this is less conclusive and requires further studies. Considering the lack of studies reported in literature on the adsorption from colloidal systems at macroscopic surfaces, there is a need for further investigations in this area. In particular, these should involve model systems, such as monodisperse latices, liposome-free emulsions of a narrow droplet size distribution, and unilamellar liposome dispersions. SUMMARY
The adsorption from emulsion systems at silica, and the formation of interfacial films, was studied with in situ ellipsometry. Electrostatic interactions were found to be crucial for the adsorption rate in the emulsion system investigated, but not for the adsorbed layer structure or the mechanism
for the adsorbed layer formation. When the emulsion droplets and the surface are similarly charged, the adsorption rate is increased by an increased screening of the electrostatic interaction or by a reduction in the droplet and silica surface charge. The adsorbed layer structure and formation, on the other hand, were unaffected by these changes. When the droplets and the surface are oppositely charged, the adsorption rate is much higher than that observed when they are similarly charged, although the adsorbed layer structure and formation were similar. In all cases, the adsorbed layer is composed of emulsion droplets or multilamellar liposomes, and the formation of the adsorbed layer proceeds through attachment of largely intact droplets and/or liposomes, with little droplet spreading or liposome collapse. The dependence of the adsorption rate on the excess electrolyte concentration and pH may be qualitatively understood by considering only electrostatic and van der Waals interactions. ACKNOWLEDGMENTS Annika Dahlman is thanked for help with surface preparations. This work was financed by Pharmacia Hospital Care, Sweden, and the Foundation for Surface Chemistry, Sweden.
REFERENCES
FIG. 6. DLVO interaction force (normalized with the droplet radius) between an emulsion droplet and the silica surface in 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (diamonds), 0.01 M phosphate buffer, pH 7.2 (squares), 0.01 M phosphate buffer, pH 4.9 (triangles), and 0.01 M phosphate buffer, pH 2.5 (circles). Experimentally obtained values of zdroplet and zsurface (Table 1) were used in the calculations. The Hamaker constant was taken to be 1.0 1 10 020 J throughout.
1. Hanin, I., and Papeu, G., ‘‘Phospholipids—Biochemical, Pharmaceutical, and Analytical Considerations.’’ Plenum, New York, 1990. 2. Becher, P. (Ed.), ‘‘Encyclopedia of Emulsion Technology: Basic Theory,’’ Vol. 1. Dekker, New York, 1983. 3. Becher, P. (Ed.), ‘‘Encyclopedia of Emulsion Technology: Applications,’’ Vol. 2. Dekker, New York, 1983. 4. Malmsten, M., Lindstro¨m, A.-L., and Wa¨rnheim, T., J. Colloid Interface Sci. 173, 297 (1995). 5. Johansson, D., Bergensta˚hl, B., and Lundgren, E., J. Am. Oil Chem. Soc. 72, 921 (1995). 6. Azzam, R. M. A., and Bashara, N. M., ‘‘Ellipsometry and Polarized Light.’’ North-Holland, Amsterdam, 1989. 7. Landgren, M., and Jo¨nsson, B., J. Phys. Chem. 97, 1656 (1993). 8. Tiberg, F., and Landgren, M., Langmuir 9, 927 (1993).
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FILM FORMATION IN EMULSION SYSTEMS 9. Malmsten, M., J. Colloid Interface Sci. 166, 333 (1994). 10. Cuypers, P. A., Corsel, J. W., Jansen, M. P., Kop, J. M. M., Hermens, W. Th., and Hemker, H. C., J. Biol. Chem. 258, 2426 (1983). 11. Small, D. M. (Ed.), ‘‘Handbook of Lipid Research,’’ Vol. 4. Plenum, New York, 1986. 12. Bergensta˚hl, B., and Fontell, K., Progr. Colloid Polym. Sci. 68, 48 (1983). 13. Fe´re´zou, J., Lai, N.-T., Leray, C., Hajri, T., Frey, A., Cabaret, Y., Courtieu, J., Lutton, C., and Bach, A. C., Biochim. Biophys. Acta 1213, 149 (1994). 14. Marra, J., and Israelachvili, J., Biochemistry 24, 4608 (1985). 15. Charmet, J. C., and de Gennes, P. G., J. Opt. Soc. Am. 73, 1777 (1983).
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16. Stromberg, R. R., Tutas, D. J., and Passaglia, E., J. Phys. Chem. 69, 3955 (1965). 17. Israelachvili, J. N., ‘‘Intermolecular and Surface Forces.’’ Academic Press, London, 1992. 18. Giesen, P. L. A., Willems, G. M., Hemker, H. C., Stuart, M. C. A., and Hermens, W. Th., Biochim. Biophys. Acta 1147, 125 (1993). 19. Giesen, P. L. A., Willems, G. M., Hemker, H. C., and Hermens, W. Th., J. Biol. Chem. 266, 18720 (1991). 20. Van Alstine, J. M., Burns, N. L., Riggs, J. A., Holmberg, K., and Harris, J. M., Colloids Surf. A 77, 149 (1993). 21. Grabbe, A., and Horn, R. G., J. Colloid Interface Sci. 157, 375 ( 1993 ) .
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0247
Electrostatic Effects on Interfacial Film Formation in Emulsion Systems MARTIN MALMSTEN,* ,1 ANNA-LENA LINDSTRO¨M,†
AND
TORBJO¨RN WA¨RNHEIM†
*Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden; and †Pharmacia Hospital Care, S-112 87 Stockholm, Sweden Received April 17, 1995; accepted October 31, 1995
The adsorption at silica from dilute emulsion systems was studied with in situ ellipsometry. In particular, the effects of electrostatic interactions on the adsorption rate and the adsorbed layer structure and formation were investigated by varying the emulsion droplet and surface charge, as well as the electrostatic screening, accomplished by varying pH and the excess electrolyte concentration. Electrostatic interactions were found to markedly affect the adsorption rate, but not the adsorbed layer structure or the mechanism for the adsorbed layer formation. For all cases investigated, the adsorbed layer thickness corresponds to emulsion droplets or multilamellar liposomes, and the adsorbed layer formation proceeds through attachment of emulsion droplets and/or multilamellar liposomes at the surface without extensive droplet spreading or liposome collapse. When the droplets and the surface are similarly charged, the adsorption is facilitated by increasing the electrostatic screening or by decreasing the emulsion droplet and surface charge, accomplished by increasing the excess electrolyte concentration and decreasing pH, respectively. When the droplets and the surface are oppositely charged, the adsorption rate is much higher than that observed when the droplets and the surface are similarly charged, although the adsorbed layer structure and the mechanism for the adsorbed layer formation are similar. Qualitatively, these effects may be understood by considering only electrostatic and van der Waals interactions. q 1996 Academic Press, Inc. Key Words: adsorption; DLVO; electrostatic; ellipsometry; emulsion; film.
INTRODUCTION
The deposition at macroscopic surfaces from colloidal systems is of interest in a variety of practical applications, including pharmaceuticals, foods, paints, and coatings. The interaction between the colloidal system and the surface is of importance, e.g., for the long-term colloidal stability of the system. Of particular interest to the present study are oil-in-water (o/w) emulsions used in pharmaceutical applications such as nutrition and delivery of lipophilic drugs (1– 3). For an emulsion to be applicable intravenously, the emulsion droplets must be small and of a narrow size distribution. Furthermore, the emulsions need to be stable, show1
To whom correspondence should be addressed.
ing little growth in the mean droplet size over extended periods of time. This means that high requirements are put on both the emusifier used and the material of the containers in which the emulsions are stored. Despite the need for an improved understanding of the interaction between colloidal systems and macroscopic surfaces, there have been very few previous studies on these processes, mainly due to lack of experimental methods allowing detailed information to be obtained regarding the adsorbed layer structure and the mechanism for the adsorbed layer formation. Recognizing the importance of the interaction between colloidal systems, particularly emulsions, and macroscopic surfaces, Malmsten et al. previously investigated the applicability of in situ ellipsometry for studies of the interfacial film formation in emulsion systems (4). In this previous study, the adsorption from a dilute emulsion system at (hydrophilic and negatively charged) silica and (hydrophobic) methylated silica was investigated. It was found that the adsorbed layer at both these surfaces consists of small or ‘‘flattened’’ emulsion droplets or multilamellar liposomes, and that the adsorbed layer formation proceeds through attachment of these colloids at the surface. Furthermore, the droplet ‘‘spreading’’ was more pronounced for methylated silica than for silica, as would be expected from contact angle considerations (5). In the present study, the effects of electrostatic interactions on the adsorption at silica were investigated by varying the droplet and surface charge (by varying pH) as well as the screening of the electrostatic interactions (by altering the excess electrolyte concentration). As in the previous investigation, in situ ellipsometry was used for these studies (4, 6). EXPERIMENTAL
Materials Emulsion preparation. The model o/w fat emulsion studied consisted of purified soybean oil dispersed in distilled water and stabilized by fractionated egg phosphatides. The major constituents of the phosphatides were phosphatidylcholine and phosphatidylethanolamine. Smaller amounts of negatively charged lipids were also present. The concentration of phospholipids and soybean oil was 1.2 and 20
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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wt%, respectively. Sodium hydroxide was added to obtain pH 8 in the resulting system. The emulsion was prepared by using a conventional high-pressure homogenization process (2, 3), followed by heat sterilization. In order to obtain a small and uniform droplet size, the emulsion was passed several times through a high-pressure homogenizer. The emulsion was diluted (to 2 1 10 03 wt%) in 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2, 0.01 M phosphate buffer, pH 7.2, 0.01 M phosphate buffer, pH 4.9, or 0.01 M phosphate buffer, pH 2.5, in the adsorption experiments. Emulsion characterization. Since the dilution with buffer could affect the stability of the emulsions, this was investigated at different dilutions. For example, emulsion samples were studied microscopically with an Axioskop (Zeiss). Sampling and examination were performed at 0, 6, and 18 h after dilution. The mean droplet size ( z-average) and size distribution were determined using a Malvern 4700 photon correlation spectrometer (Malvern Instruments), equipped with an 8-multibit correlator and a 35 mW He/Ne laser. The light scattered by the oil droplets was detected at 907 scattering angle. Again, the measurements were performed at 0, 6, and 18 h after dilution at 257C. The determination of the z -potential of the emulsion droplets was performed using a Malvern Zetasizer 4, with a 4 mW He/Ne laser (Malvern Instruments). The instrument is provided with a software routine (duty cycling), allowing the use of an AZ104 cell for samples diluted in medium with high ionic strength. Switching off the applied voltage between each cycle eliminated the distortion by sample heating. Water was first purified by a Milli-RO 10PLUS pretreatment unit, including depth filtration, carbon adsorption, and decalcination preceding reverse osmosis. Subsequently, it was led through a Milli-Q PLUS185 unit, which treats the feed water with UV light (185 and 254 nm) before leading it into a Q-PAK unit consisting of an active carbon unit followed by a mixed bed ion exchanger, an Organex cartridge, and a final 0.22-mm Millipak 40 filter. Surfaces Silica surfaces were prepared from polished silicon slides (p-type, boron-doped, resistivity 7–13 Vcm; Okmetic, Finland). In short, these were oxidized thermally in pure and saturated oxygen, followed by annealing and cooling in argon flow, which resulted in an oxide layer thickness of about 40 nm. The slides were then cleaned as described previously (4). This procedure rendered the surfaces hydrophilic, with a contact angle (in air) of less than 107. The silica slides were then kept in ethanol until use. Just prior to use, the slides were rinsed twice with, in order, ethanol and water, followed by drying with filtered nitrogen (Aga, Sweden). Ellipsometry The ellipsometry measurements were all performed by means of null ellipsometry ( 6 ) . The instrument used was
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an automated Rudolph thin-film ellipsometer, Type 436, controlled by a personal computer. A xenon lamp, filtered ˚ , was used as the light source. A thorough deto 4015 A scription of the experimental setup has been given previously ( 7 ) . In the case of a layered substrate, e.g., oxidized silicon, a correct determination of the adsorbed layer thickness and mean refractive index requires determination of the silicon bulk complex refractive index ( N2 Å n2 0 ik2 ) as well as of the thickness ( d1 ) and the refractive index ( n 1 ) of the oxide layer ( 6 – 9 ) . This is achieved by measuring the ellipsometric parameters c and D in two different media, e.g., air and buffer. From the two sets of c and D, n2 , k2 , d1 , and n1 can be determined separately. All measurements were performed by so-called four-zone null ellipsometry in order to reduce effects of optical component imperfections ( 6 ) . After the optical analysis of the bare substrate surface, the emulsion was added to the cuvette to a final concentration of 2 1 10 03 wt%, and the values of c and D were recorded. ( The adsorption was monitored in one zone, since the four-zone procedure is rather time-consuming and since corrections for component imperfections already had been performed.) Stirring was performed by a magnetic stirrer at about 300 rpm. From c and D, the mean refractive index (n f ) and average thickness ( del ) of the adsorbed layer were calculated numerically according to an optical four-layer model (6–9). The mean refractive index and average thickness were finally used to calculate the adsorbed amount ( G ) according to Cuypers et al. (10), using a molar refractivity of 3.2 (10) and a specific volume of 1.0 (11). Due to the similarity of these parameters for soybean oil and lecithin, ellipsomety provides the total adsorbed amount. Measurements were performed at 207C. RESULTS AND DISCUSSION
Emulsion Stability The egg lecithin used in the present investigation contained a small fraction of ionized lipids, resulting in an emulsion droplet charge. The charge of the droplets (and of the silica surface) will depend on pH and the excess electrolyte concentration. The emulsion droplet size and z -potential at different pH and excess electrolyte concentration are given in Table 1 together with the silica surface potential. The low z -potential of the emulsion droplets ( zdroplet ) obtained for the emulsion diluted in buffer and electrolyte indicated a risk for instability of the systems, e.g., through flocculation or coalescence. However, the absence of aggregated and/or larger droplets was ensured for all samples by light microscopy. Furthermore, for all samples the mean droplet size and the droplet size distribution, obtained by PCS, gave no indications of significant flocculation or coalescence over a period of at least 18 h.
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TABLE 1 System Characteristics at Different pH and Excess Electrolyte Concentration Buffer
Diameter (nm)
zdroplet (mV)
zsurface (mV)a
0.15 M NaCl pH 7.2 pH 7.2 pH 4.9 pH 2.5
307 307 308 323
04 022 016 /7
060 060 030 07
Note. In all cases 0.01 M phosphate buffer was used. a Obtained from electroosmosis and surface force measurements (20, 21).
From the total interfacial area in the emulsion system and the surface area occupied per emulsifier molecule (12), it is clear that the system studied contained phospholipids in excess of that required to stabilize the emulsion. Consequently, it is possible for liposomal structures to form within the emulsion (13). Adsorbed Layer Structure and Formation The amount adsorbed ( G ) at silica from the diluted emulsion at different pH and excess electrolyte concentration is shown in Fig. 1a. At pH 7.2, where both the emulsion droplets and the surface are negatively charged ( zdroplet Å 04 and 022 mV for 0.01 M phosphate buffer in the presence and absence of 0.15 M NaCl, respectively; zsurface É 060 mV), the adsorption is enhanced by increasing the excess electrolyte concentration. Decreasing pH to 4.9, which reduces both the droplet and surface negative charge (Table 1), results in a slightly higher adsorption than that at pH 7.2. Decreasing pH further to 2.5, resulting in a reduction in the surface negative charge ( zsurface Å 07 mV) as well as in a charge reversal of the emulsion droplets ( zdroplet Å /7 mV), gives rise to a dramatically enhanced adsorption. A notable feature from Fig. 1a is that the adsorption proceeds up to 25,000 s seemingly without limit. At most, we have followed the adsorption for 10 h, and have seen no signs of adsorption saturation. As will be discussed below, the reason for the slow adsorption is not so much the low droplet concentration, but rather a quite low ‘‘sticking probability’’ for the droplets at the surface, simply inferred from the finding of quite different adsorption kinetics at a fixed droplet concentration, but different pH and excess electrolyte concentration (Fig. 1a), and also at different surfaces (4). Thus, changing the electrostatic interaction between the droplets and the surface by varying pH and the excess electrolyte concentration seems to modulate the droplet sticking probability (see below). In contrast to the adsorbed amount, the adsorbed layer thickness ( del ) reaches a limiting value very rapidly, and at adsorption times larger than about 1000 s, the adsorbed
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layer thickness is essentially constant ( Fig. 1b ) . The adsorbed layer thickness was found to be 90 { 15 nm for all systems investigated, and similar kinetics were observed for all pH and excess electrolyte concentrations. It is interesting to compare the adsorbed layer thickness with the dimension of the emulsion droplets and the phospholipid molecules. Clearly, the adsorbed layer thickness corresponds more closely to the droplet size ( 307, 307, 308, and 323 nm for 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2, 0.01 M phosphate buffer, pH 7.2, 0.01 M phosphate buffer, pH 4.9, and 0.01 M phosphate buffer, pH 2.5, respectively ) , than to mono- or bilayers of phospholipid molecules ( expected to result in an adsorbed layer thickness of about 2.5 and 5 nm, respectively ) ( 14 ) . This is the case particularly since the emulsion droplets are size polydisperse and since droplets as small as 50 nm are present in the emulsion. A difficulty in the interpretation of the optical thickness in terms of structural parameters is that resulting from the interfacial ‘‘film’’ being inhomogeneous, as seen clearly from the adsorbed amount and the adsorbed layer thickness taken in conjunction. For systems with a density distribution normal to the surface it has been shown that the ellipsometrically determined thickness is related to the first moment of the density distribution, i.e., an average thickness is obtained with ellipsometry (15). The root-mean-square thickness is obtained from the optical thickness by dividing by a factor of 1.47–1.75, depending on the density distribution normal to the surface (16). Furthermore, the size of the droplets is comparable to the wavelength, and therefore, the applicability of the effective medium theory may be discussed. Another problem in the interpretation of the ellipsometric results is that due to the lateral heterogeneity of the adsorbed layer. In principle, this can be investigated ellipsometrically, employing either multiple angle of incidence or spectroscopic ellipsometric techniques coupled to an assumed type of heterogeneity. Although certainly urgent, these studies are outside the scope of the present investigation. From the considerations above, it is clear that the interpretation of the adsorbed layer optical parameters in terms of structural properties should be made somewhat cautiously. Nevertheless, the main point is that it is clear from the ellipsometric results that the adsorbed layer consists of adsorbed emulsion droplets or multilamellar liposomes, and not by a phospholipid bilayer. Since the adsorbed layer thickness reaches a limiting value already after about 1000 s, whereas the adsorbed amount increases without limitation for at least 10 h, the adsorbed layer density must increase monotonously during the adsorption process. It is important to note, however, that also at rather high adsorbed amounts, the average concentration of the adsorbed layer is low, and the mean distance between the adsorbed droplets (d) large compared to the droplet radius (rh ; d/rh É 8–65).
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FIG. 1. (a) Adsorbed amount ( G ) and (b) adsorbed layer thickness ( del ) on silica as a function of time. The adsorption was performed from 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (diamonds), 0.01 M phosphate buffer, pH 7.2 (squares), 0.01 M phosphate buffer, pH 4.9 (triangles), and 0.01 M phosphate buffer, pH 2.5 (circles).
The good time resolution of ellipsometry, and the simultaneous recording of the adsorbed amount, the adsorbed layer thickness, and the mean adsorbed layer refractive index, allow the mechanisms for the adsorbed layer formation to be investigated in some detail with this technique. As can be seen in Figs. 2–5, the adsorbed layer thickness is essentially independent of the adsorbed amount, while the mean adsorbed layer refractive index increases essentially linearly with the adsorbed amount. Thus, the adsorbed amount increase is accomplished by increasing the average adsorbed layer concentration at a constant adsorbed layer thickness. Together with the finding of the adsorbed layer thickness corresponding to emulsion droplets or multilamellar liposomes, these findings mean that the adsorption occurs through attachment of these colloids at the surface in a largely intact shape, with only a limited droplet spreading or liposome collapse. Keeping the mechanism of adsorption in mind, it is interesting to once more consider the effects of pH and the excess electrolyte concentration on the adsorption at silica from the emulsion under investigation. As a first approximation we consider only electrostatic and van der Waals interactions to be of importance for the drople-droplet and droplet-surface (adsorption) fusion. The stability in such a colloidal system is described by the DLVO theory (17). In the present case, the composition of the emulsion, and in particular of the dispersed phase, as well as the droplet size were essentially constant, meaning that the van der Waals interactions were fixed. However, the droplet–surface electrostatic interactions were strongly affected by varying pH and the excess electrolyte concentration. At pH 7.2, both the surface and the emulsion droplets are negatively charged. More precisely, the z -potential of the surface and the droplets is about 060 and 022 mV, respectively. This generates a repulsive electrostatic droplet–surface interaction opposing adsorption, which is counteracted mainly by an attractive van der Waals interaction. At addi-
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tion of excess electrolyte, the electrostatic repulsion is screened, which means that the attractive component to the droplet–surface interaction will become more dominant. Within the framework of the DLVO theory, this means that the magnitude of the maximum in the droplet–surface interaction curve is reduced (Fig. 6). Since the probability for achieving close droplet–surface contact increases with a decreasing energy barrier, and since close contact is most probably necessary for adsorption to occur, increasing the excess electrolyte concentration is expected to result in an increased probability of close droplet–surface contact (adsorption), analogous to the increased probability of close droplet–droplet contact (flocculation) in the emulsion system. Except for an increased probability for close droplet–surface contact, the increase in the excess electrolyte concentration is not expected to markedly affect the adsorbed layer structure or the mechanisms for the adsorbed layer formation ( zdroplet ! zsurface ; no signs of flocculation even at high excess electrolyte concentrations). Considering this, the finding of an increased adsorption rate, but unchanged adsorbed layer structure and formation (Figs. 1–3), is expected. On decreasing pH to 4.9, both the droplet and the surface negative charge is reduced (Table 1), resulting in a reduction in the energy barrier for close droplet–surface contact (Fig. 6). From the considerations above we would therefore expect that the adsorption rate should be somewhat higher at pH 4.9 than at pH 7.2, although the adsorbed layer structure and formation are still expected to be similar to that at pH 7.2. As can be seen in Figs. 1, 3, and 4, this is indeed what is found experimentally. Quantitatively, the effects on the adsorption are rather small, and the adsorbed amount after an adsorption time of 25,000 is 0.15 and 0.35 mg/m 2 at pH 7.2 and pH 4.9, respectively. The reason for the small difference in adsorption rate at pH 7.2 and 4.9 is that the charge density of both the droplets and the surface is only moderately reduced on decreasing pH from 7.2 to 4.9 (Table 1). At pH 2.5, the negative charge of the silica surface is
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FIG. 2. (a) Adsorbed layer thickness ( del ) and (b) mean adsorbed layer refractive index (n f ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (4).
quite low ( zsurface É 07 mV), while the emulsion droplets are weakly positively charged ( zdroplet Å /7 mV). Consequently, there is an attractive electrostatic driving force for adsorption, apart from the van der Waals attraction, and the total droplet–surface interaction is therefore monotonously attractive (Fig. 6). We would therefore expect a much more pronounced adsorption at pH 2.5 than at pH 4.9 and pH 7.2, as also observed experimentally (Fig. 1). [The marginally higher total electrolyte concentration at pH 2.5 (0.013 M) than at pH 4.9 and pH 7.2 (both 0.01 M) is expected to marginally reduce the adsorption rate at the former pH.] From Figs. 1, 3, 4, and 5, we see that despite the higher adsorption rate at pH 2.5 the adsorbed layer thickness and the mechanism for the adsorbed layer formation are similar to that at pH 4.9 and 7.2. This result is not self-evident since we would expect the droplets to strive at being as close to the surface as possible from the droplet–surface interaction curve (Fig. 6). We would therefore expect a larger degree of droplet spreading and liposome collapse at pH 2.5 than for the higher pH values. The fact that this is not observed experimentally seems to indicate that the interfacial spread-
ing favored by van der Waals and electrostatic interactions at pH 2.5, and by van der Waals interactions alone at pH 4.9 and 7.2, is limited by the surface tension of the emulsion droplets and liposomes. Since the composition of these is identical at all the conditions investigated, identical limiting adsorbed layer thicknesses are expected for all cases. The interfacial deposition of colloidal particles has not been studied extensively previously, despite the fundamental and practical interest of these processes. Giesen et al. previously studied the adsorption of unilamellar liposomes at silicon slides with ellipsometry and found that the liposomes adsorbed at this surface, although collapsing and forming phospholipid bilayers at the interface (18, 19). With the assumption that the liposomes present in our system would behave similarly to the system studied by these authors, this previous finding indicates that the adsorbed layer in the present investigation is not likely to consist of unilamellar liposomes, since collapsed unilamellar liposomes are incompatible with the adsorbed layer thicknesses observed. From the same reasoning, and from the finding of an essentially identical adsorbed layer thickness at pH 2.5, 4.9, and 7.2,
FIG. 3. Adsorbed layer thickness ( del ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 7.2.
FIG. 4. Adsorbed layer thickness ( del ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 4.9.
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FIG. 5. (a) Adsorbed layer thickness ( del ) and (b) mean adsorbed layer refractive index (n f ) versus the adsorbed amount ( G ) at silica. The adsorption was performed from 0.01 M phosphate buffer, pH 2.5.
even multilamellar liposomes are less likely to form the adsorbed layer, although this is less conclusive and requires further studies. Considering the lack of studies reported in literature on the adsorption from colloidal systems at macroscopic surfaces, there is a need for further investigations in this area. In particular, these should involve model systems, such as monodisperse latices, liposome-free emulsions of a narrow droplet size distribution, and unilamellar liposome dispersions. SUMMARY
The adsorption from emulsion systems at silica, and the formation of interfacial films, was studied with in situ ellipsometry. Electrostatic interactions were found to be crucial for the adsorption rate in the emulsion system investigated, but not for the adsorbed layer structure or the mechanism
for the adsorbed layer formation. When the emulsion droplets and the surface are similarly charged, the adsorption rate is increased by an increased screening of the electrostatic interaction or by a reduction in the droplet and silica surface charge. The adsorbed layer structure and formation, on the other hand, were unaffected by these changes. When the droplets and the surface are oppositely charged, the adsorption rate is much higher than that observed when they are similarly charged, although the adsorbed layer structure and formation were similar. In all cases, the adsorbed layer is composed of emulsion droplets or multilamellar liposomes, and the formation of the adsorbed layer proceeds through attachment of largely intact droplets and/or liposomes, with little droplet spreading or liposome collapse. The dependence of the adsorption rate on the excess electrolyte concentration and pH may be qualitatively understood by considering only electrostatic and van der Waals interactions. ACKNOWLEDGMENTS Annika Dahlman is thanked for help with surface preparations. This work was financed by Pharmacia Hospital Care, Sweden, and the Foundation for Surface Chemistry, Sweden.
REFERENCES
FIG. 6. DLVO interaction force (normalized with the droplet radius) between an emulsion droplet and the silica surface in 0.01 M phosphate buffer, 0.15 M NaCl, pH 7.2 (diamonds), 0.01 M phosphate buffer, pH 7.2 (squares), 0.01 M phosphate buffer, pH 4.9 (triangles), and 0.01 M phosphate buffer, pH 2.5 (circles). Experimentally obtained values of zdroplet and zsurface (Table 1) were used in the calculations. The Hamaker constant was taken to be 1.0 1 10 020 J throughout.
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FILM FORMATION IN EMULSION SYSTEMS 9. Malmsten, M., J. Colloid Interface Sci. 166, 333 (1994). 10. Cuypers, P. A., Corsel, J. W., Jansen, M. P., Kop, J. M. M., Hermens, W. Th., and Hemker, H. C., J. Biol. Chem. 258, 2426 (1983). 11. Small, D. M. (Ed.), ‘‘Handbook of Lipid Research,’’ Vol. 4. Plenum, New York, 1986. 12. Bergensta˚hl, B., and Fontell, K., Progr. Colloid Polym. Sci. 68, 48 (1983). 13. Fe´re´zou, J., Lai, N.-T., Leray, C., Hajri, T., Frey, A., Cabaret, Y., Courtieu, J., Lutton, C., and Bach, A. C., Biochim. Biophys. Acta 1213, 149 (1994). 14. Marra, J., and Israelachvili, J., Biochemistry 24, 4608 (1985). 15. Charmet, J. C., and de Gennes, P. G., J. Opt. Soc. Am. 73, 1777 (1983).
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16. Stromberg, R. R., Tutas, D. J., and Passaglia, E., J. Phys. Chem. 69, 3955 (1965). 17. Israelachvili, J. N., ‘‘Intermolecular and Surface Forces.’’ Academic Press, London, 1992. 18. Giesen, P. L. A., Willems, G. M., Hemker, H. C., Stuart, M. C. A., and Hermens, W. Th., Biochim. Biophys. Acta 1147, 125 (1993). 19. Giesen, P. L. A., Willems, G. M., Hemker, H. C., and Hermens, W. Th., J. Biol. Chem. 266, 18720 (1991). 20. Van Alstine, J. M., Burns, N. L., Riggs, J. A., Holmberg, K., and Harris, J. M., Colloids Surf. A 77, 149 (1993). 21. Grabbe, A., and Horn, R. G., J. Colloid Interface Sci. 157, 375 ( 1993 ) .
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