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Polyethylene Glycol Adsorption on Silica: From Bulk Phase Behavior to Surface Phase Diagram Nawal Derkaoui,† Sylve`re Said,‡ Yves Grohens,‡ Rene´ Olier,† and Mireille Privat*,† UMR 6521, Chimie, Electrochimie Mole´ culaires et Chimie Analytique, De´ partement de Chimie, UniVersite´ de Bretagne Occidentale, 6 AVenue Le Gorgeu, CS 93837, 29238 Brest Cedex 3, France, and Laboratoire Polyme` re, Proprie´ te´ s aux Interfaces et Composites, UniVersite´ de Bretagne Sud. Rue Saint Maude´ , BP 92116, 56321 Lorient Cedex, France ReceiVed January 23, 2007. In Final Form: March 14, 2007 A characterization of the bulk-phase diagram from literature data and new NMR and DSC measurements provided us with valuable elements that are helpful for gaining, from aqueous solution, better insight into the surface behavior of polyethylene glycol on Aerosil 200. Adsorption isotherms built further to measurements by a depletion method showed a strong and temperature-dependent variation of the isotherm shape in agreement with the variations of interactions already evidenced in the bulk. In temperature-concentration areas, where water is behaving as a helixpromoting solvent, the finding of positive PEG adsorptions and stairlike isotherms agrees with observations reported in the literature. We identified some of the vertical parts as corresponding to the formation of monolayers of helixshaped PEG molecules. In poor-solvent zones, adsorptions were null or negative, and the isotherms exhibited oscillations suggesting very different surface behavior. Our data analysis evidenced the presence of a much greater amount of water than in the previous surface states; however, the similar analysis of PEG behavior remains relevant. Indeed, the occurrence of first-order transitions in the surface layer implies some water reorganization, permitting the PEG molecules to move closer to the surface and become helix-shaped to rearrange in a monolayer. The surface phase diagram confirmed this analysis in a very satisfying way.
1. Introduction In many industrial fields, polyethylene glycol (PEG)-derived molecules are known to be involved as chemical agents, for example, co-surfactants or plasticizers in manufacturing processes. After mixing with other surfactants, they are routinely used as additives in industrial electrolytes to enhance the properties of deposited metal films. In particular, the importance of copper deposition in the fabrication of printed circuits has boosted the development of additive chemistry for this metal. Because of its current use in replacing aluminum in alloy metallurgy when making ultra-large-scale interconnects on silicon, a better understanding of additive mechanisms at the atomistic level is paramount for process control and microstructural engineering.1 Because PEGs are both hydrophilic and hydrophobic, their dual nature leads to numerous and various interactions with very different components, which are the basis of intermolecular associations aimed at either stabilizing structures in soft materials or strengthening fragile ones.2 Because of their great ability to adsorb on surfaces and to be grafted, they are frequently employed in modified surfaces employed for the manufacturing of electrochemical devices as well as for applications in relation with chromatographic or membrane selectivity.3-6 The use of PEGs is also expanding in medical fields; for example, interest among researchers for the development of micro- and nanoscale † ‡
Universite´ de Bretagne Occidentale. Universite´ de Bretagne Sud. Rue Saint Maude´.
(1) Walker, M. L.; Richter, L. J.; Josell, D.; Moffat, T. P. J. Electrochem. Soc. 2006, 153, C235. (2) Rosen, H. N. Wood Fiber Sci. 1975, 7, 249. (3) Suh, K. Y.; Khademhosseini, A.; Eng, G.; Langer, R. Langmuir 2004, 20, 6080. (4) Vo¨ro¨s, J.; De Paul, S. M.; Textor, M.; Abel, A. P.; Kauffmann, E.; Ehrat, M. Bio World 2003, 2, 16. (5) Nagasaki, Y.; Ishii, T.; Uchida, K.; Otsuka, H.; Kataoka, K. Eur. Cells Mater. 2003, 6 suppl. 1, 23. (6) Berzkin, V. G.; Korolev, A. A.; Malyokova, I. V. J. Microcolumn Sep. 1997, 8, 43.
structures3 for both extravascular/intravascular implants and analytical systems has been elicited by the availability of microscopic mechanical elements (micro-electro-mechanical systems (MEMS)) and the need for miniaturized electronic and microelectronic devices. However, protein adsorption onto implanted biomaterials has been acknowledged to be an initiation factor in the formation of systemic embolisms that are liable to result in strokes or heart attacks. Surfaces with tethered hydrophilic polymers such as PEGs are of interest as biomaterials because they reduce protein and cell interactions at the tissue-material interface. PEG-modified nanoparticles are employed in the manufacturing of new devices for the molecular recognition of proteins or antibodies4,5 or in chromatographic columns for hydrocarbon analysis.6 All together, these considerations explain why so many structural studies have been undertaken regarding PEGs in mixtures with several kinds of solvents and at surfaces. Investigations by IR7,9,10 and NMR7,8,11,12 as well as ultrasonic and photocorrelation spectroscopy experiments7 have evidenced structural modifications such as changes in conformation with respect to an ethylene bond between gauche and trans arrangements or modifications of intramolecular or intermolecular interactions (such as hydration) with their resulting changes in macroscopic properties. The factors leading to these evolutions are polymer concentration9-12 and temperature.7,8 Molecular dynamics simulations13 and spectroscopy data have provided (7) Faraone, A.; Magazu`, S.; Maisano, G.; Migliardo, P.; Tettamanti, E.; Villari, V. J. Chem. Phys. 1999, 110, 1801. (8) Branca, C.; Magazu`, S.; Maisano, G.; Migliardo, P.; Tettamanti, E. Physica B 1999, 270, 350. (9) Begum, R.; Yonemitsu, T.; Matsuura, H. J. Mol. Struct. 1998, 447, 11. (10) Rozenberg, M.; Loewenschuss, A.; Marcus, Y. Spectochim. Acta, Part A 1998, 54, 1819. (11) Liu, K. J. Polym. Solution 1968, 1, 213. (12) Hoffmann, M. M.; Bennett, M. E.; Fox, J. D.; Wyman, D. P. J. Colloid Interface Sci. 2005, 287, 712. (13) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459.
10.1021/la070199u CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007
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Figure 2. Scheme of possible conformation of PEG 400 aggregates.
Figure 1. General bulk phase diagram of mixtures of PEG 400 and water (according to refs 77 and 1616).
images of intramolecular conformations together with the resulting intermolecular arrangements. The most striking among these arrangements is the existence, under certain conditions of concentrations and/or temperature, of a helical conformation of the polymeric chain. Adsorption properties and the resistance of resulting films to protein adsorption are commonly explained by the induction, by these helical structures, of a dominant steric repulsion force.14,15 This steric force contains not only an elastic force but also an osmostic one because of the hydration of the PEG chain constantly implicated in the properties of PEG solutions or surface layers. We previously undertook a complementary extended study16 of the physicochemical properties of PEG 400 in water solutions versus temperature and concentration in order to gain more insight into the configuration of this particular polymeric chain and its interaction with water. Figure 1 summarizes these data together with those by other authors as synthetized in ref 7. It seemed necessary and relevant to explain the polymer behavior at the silica surface in contact with aqueous solutions. Indeed, under certain conditions vertical jumps or kinks17,18 have been observed on adsorption isotherms of polymers on solid particles such as silica beads in suspension in aqueous solutions. Although they had usually been identified as the signature of surface demixing, little has been done to determine their origin despite the publication of other surface studies carried out either in porous media19 or in Langmuir films.20 As highlighted in the present study of PEG 400 adsorption onto nonporous silica from surface phase diagrams, considering several behaviors together with their variations with variable zones, it seemed worthwhile to analyze them thoroughly in relation to those in the bulk. Let us first consider the bulk phase diagram presented in Figure 1: at very low temperatures, solid-liquid equilibria appear around a 60-40 eutectic point. Moreover, in liquid mixtures, aggregation takes place near room temperature but disappears at higher or lower temperatures; this process is at the origin of the bulk phase diagram shown in the 10-45 °C range, which was evidenced by (14) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. 1997, 101, 9797. (15) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426. (16) Derkaoui, N.; Said, S.; Grohens, Y.; Olier, R.; Privat, M. J. Colloid Surface Sci. 2007, 305, 330. (17) Trens, P.; Denoyel, R. Langmuir 1993, 9, 519. (18) Bjelopavlic, M.; Zaman A. A.; Moudgil, B. M. Kona 2000, 18, 60. (19) Scho¨nhoff, M.; Larsson, A.; Welzel, P. B.; Kuckling, D. J. Phys. Chem. B 2002, 106, 7800. (20) Islam, N. M.; Kato, T. Langmuir 2004, 20, 6297.
tensiometry and confirmed by a 13C NMR study on solutions. Our different experiments, in particular, the surface tension and NMR studies, confirmed the existence of viscous solutions and gels, whose temperature limits were also determined. As a whole, this bulk phase diagram is very similar to those presented by surfactant solutions (e.g., soaps). It is worth noting that at first we were puzzled by this surprising result. Then, we explained it essentially from the particular structure evidenced through a study of POE by molecular dynamics, undertaken by Tasaki13 and carried out to confirm the experimental data by NMR and Raman spectroscopy. The addition of an OH group at each end of a helical form similar to those studied by Tasaki leads to an amphiphilic structure that may be able to explain the formation of aggregates and gel in the areas where helices exist (Figure 2). The aggregates of Figure 2 could be considered to be a type of nonspherical micelle whose concentration could lead to hexagonal structures, lamellar phases, and gels. At this stage of our explanation, being aware that helices exist only in the goodsolvent area because of the hydrogen bonds between the water molecules trapped within the helix and the oxygen bridges of PEG is paramount. Then, the helical chain gains hydrophobic character resulting from the location of oxygen bridges within the helix with ethylene groups soaring outside.13-15 Out of the good-solvent zone, the shape of the chains is likely to be between the helix and coil. Whereas a persistent argument consists in contesting either the helical structure or its spontaneous appearance (some authors make impurities responsible of this conformation), strong experimental arguments are given in favor of such a conformation in IR studies,21 AFM spectroscopy,22 and NMR spectroscopy as early as in ref 23. Quite recently, Raman and Rayleigh scattering24 experiments provided evidence of helical structures in the crystalline state. These data, which were very well confirmed by molecular dynamic models using quite reasonable interaction hypotheses, allow us to discuss other experimental data by seriously taking into account this particular conformation. Our previous results,16 particularly the ones concerning a 13C NMR study of the different carbons along the chain during what appears to be an aggregation process taking place at some temperature and concentration, are hard to explain without this hypothesis, whereas they are quite compatible with all of the experiments suggesting to do so. Previous studies carried out on similar systems have highlighted how the bulk or surface properties of the system under study can be strongly affected by the structure in solution or on the surface. Most of the practical applications described in the literature have been found from modified or associated PEGs.15,17-20,25-26 Oxide (21) Begum, R.; Matsuura, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3839. (22) Oesterhelt, F.; Rief, M.; Gaub, H. E. New J. Phys. 1999, 6, 1. (23) Connor, T. M.; McLauchlan, K. A. J. Phys. Chem. 1965, 69, 1888. (24) Kozielski, M.; Mu¨hle, M.; Blaszczak, Z.; Szybowicz, M. Cryst. Res. Technol. 2005, 40, 466. (25) Raghavan, S. R.; Walls, H. J.; Khan S. A. Langmuir 2000, 16, 7920. (26) Zaman A. A. Colloid Polym. Sci. 2000, 278, 1187.
Polyethylene Glycol Adsorption on Silica
suspensions covered with PEGs are often used to adjust the liquid viscosity;25,26 the hydrogen bond plays a crucial role in the solution properties. Here, the PEG under study was single pure PEG 400 adsorbed onto nonporous silica, and surface phase diagrams were determined by adsorption measurements though this method is rather time-consuming and tedious. This kind of study is, indeed, a seminal way to identify, under precise conditions, the different surface phases and to relate them to solution behavior. The adsorption isotherms presented here were measured at precise positions on the bulk phase diagram. Three out of the five locations were at temperatures where aggregates had been previously evidenced (i.e., where helical forms were known to exist in the bulk); the last two locations were, respectively, above and below this zone. We explored a wide range of concentrations but avoided the gel area. All of these changes in concentration and temperature were aimed at gaining better insight into the relationships between bulk and surface behavior to eventually find some connection between the different aspects of surface properties. It is worth recalling that such observations usually have been the result of independent and fragmented investigations. To have clear insight into the implicated factors, we eliminated the role of the miscellaneous surface state by using a very homogeneous, dense, nonporous silica. Concerning the differences noticed in the diverse zones of concentration and temperature under study, our observations went far beyond our expectations: at low temperature (15 °C), adsorption isotherms exhibited negative adsorptions and oscillatory shapes. In the zone of temperatures where aggregation is known to occur (20-35 °C), isotherms were stair-shaped. At higher temperatures, they again became oscillatory. These variations will be further explained by conformational changes in the polymeric chain already in the bulk, leading to different interactions with the silica surface. The surface phase diagram obtained in the aggregation zone was similar to the bulk one at low temperature. 2. Experimental Section 2.1. Chemicals. PEG400 was supplied by Aldrich with Mn, Mw ) 400, so the polydispersity was supposed to be 1. In fact, the polydispersity in the stock sent at the time we conducted our experiments was measured at our request by Aldrich and was found to be 1.09 (method not given); our own verification by size exclusion chromatography (SEC) gave 1.1. Because the quality of such a sample was the same as the quality offered by suppliers specializing in polymer materials, we adopted at this time Aldrich as a supplier. The care taken in choosing the polymer sample is due to the drastic effects of mixtures of different homologues in batches whose polydispersity is too high. This complicates the interpretation of the data because even the characterization of the material is difficult and physical parameters can vary considerably with the supplier and the investigated batch. Water was purified on a milliQ device from Millipore. Silica was Degussa Aerosil 200 prepared by pyrogenation. Its specific area measured by BET adsorption isotherms for nitrogen was 196 m2 g-1. According to the supplier, silica particles were almost spherical, had a mean diameter of about 15 nm, and carried 3 OH groups/nm2. To standardize these conditions and avoid any variation with time induced by water uptake, silica was dried at 195 °C for 1 week. There were 1017 particles in 1 g of Aerosil. Most of the experiments were carried out on 0.4 g silica samples mixed with about 20 cm3 of solution, or 0.2 g for 10 cm3. A rough calculation showed that the shortest distance between two particles in solution varied between 64.0 and 40.4 nm. The silica porosity measured by BET adsorption with different gases was very low; the mean radius of pores was found to be 66.9 Å (mesoporosity), and microporosity was almost missing. These characteristics made capillary condensation in the pores quite unlikely. All of the tests carried out with other
Langmuir, Vol. 23, No. 12, 2007 6633
Figure 3. IR spectrum used to analyze the PEG 400 supernatant obtained at the end of the depletion method for isotherm determination. types of silica sometimes presenting very different adsorption values highlighted the lack of interference during centrifugation between adsorption (molecules fixed on the surface) and the trapping of molecules between the packed silica particles. 2.2. Adsorption Isotherm Measurement. We measured adsorption isotherms with the classical depletion method. Samples of silica powder (0.4 g) were weighed into centrifuge tubes before the addition of 18 g of an aqueous solution of PEG 400 and smooth agitation during the time required for equilibrium. Previous determinations of this duration by adsorption kinetics drove us to let adsorption develop for 40 h; this time was indeed considered to be sufficient to reach equilibrium. During agitation, the samples were kept in a thermoregulated oven at a given temperature; this temperature was known to (0.2 K. Prior to the adsorption determination, silica powder was removed from the suspensions by centrifugation at 2000g in a Jouan CT422 centrifuge until obtaining a clear supernatant, which was then titrated. Titration was carried out from the IR absorption spectrum (Figure 3) using the Beer-Lambert absorption law. We focused on distinct marker bands at about 1350 cm-1 assigned to the CH2 wagging mode of the O-CH2-CH2-O segment.9,10 Experiments were made with a CaF2 cell whose thickness was 50 µm. For each determination of supernatant concentration, the IR peak area was measured and compared to the calibration line previously determined and corresponding to Beer-Lambert behavior. Whenever a solution being tested was too concentrated, we proceeded to an exact dilution before using the calibration line. It is worth noting that the calibration line was obtained from linear regression with excellent fitting (peak area versus PEG molar concentration y ) 16.753x + 0.0367 with R2 ) 0.9999). In addition, repeatability measured in the linear area provided 0.26% as the coefficient of variation. Adsorption at the solid-liquid interface was calculated as the relative adsorption according to Gibbs Γ2,1 )
n0(x20 - x2) ms(1 - x2)
(1)
where subscript 2 denotes the solute (PEG400), subscript 1 represents the solvent (water), n0 is the total number of moles in the sample, x20 is the initial mole fraction of PEG 400, x2 is the equilibrium mole fraction of the supernatant after adsorption, m is the mass of silica, and s is the specific area of silica. By using (i) eq 1, (ii) the linear regression showing the relationship between the peak areas and the supernatant molar concentrations, and (iii) the relation between molar concentration and mole fraction, we determined the error bars on adsorption values; they showed no significant variation along an isotherm or even along a vertical part because of the occurrence of numerical compensations. Moreover, one should note that the mean relative error is less than 1%. Therefore, error bars cannot be shown in the Figures because they are about
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Figure 4. Adsorption isotherms of PEG 400 on Aerosil 200 from aqueous solutions: (a) at 25 and 30 °C and (b) at 35 °C. Table 1
Figure 5. Adsorption isotherms of PEG 400 on Aerosil 200 from aqueous solutions at 15 and 40 °C. as large as the symbols used to represent the experimental values. Denying the existence of vertical parts, which are quite visible in Figures 4-6, on the basis of experimental errors is impossible. Accidental errors are excluded by the repeatability of such special isotherm shapes. In addition, most of the points not lying along the lines used as visual guides look like an extension of plateaus, which is currently considered to be the signature of metastable states and is expected in the case of surface demixing or bulk demixing.
3. Results and Discussion 3.1. Adsorption Isotherms. 3.1.1. Stairlike Isotherms. In the following paragraph, the interpretation of the presented results will rely on the now well-enough-supported helical conformation of PEGs and PEOs under various concentration and temperature conditions. We will particularly use our previous results concerning the very special behavior of PEG 400 in the bulk,16 whose understanding, as we explained in the Introduction, essentially relies on this particular conformation. It can be considered that only a body of facts leads to take this conformation into account, but there are now enough data, experimental and theoretical, to constitute a true element of explanation. Figure 4 presents the stairlike adsorption isotherms obtained at 25 and 30 °C (Figure 4a) and 35 °C (Figure 4b) (i.e., in a region where aggregation has been proved in the bulk phase16). The vertical parts (1-4 at 25 °C, 1′-4′ at 30 °C, 1′′-4′′ at 35 °C) on isotherms can be understood as corresponding to equilibria between different surface phases. These surface phases are detected by plateaus, which are often extended by metastable points known to be another very precious indication of firstorder phase transitions. In the next paragraph, we will use vertical parts as tie lines; their variations in length as a function of
T/°C
25
30
35
step 1 2 3 4
0.2-0.5 0.8-2.2 2.5-3.8 3.8-5.5
0.2-0.5 0.9-2.6 2.8-5.0 5.0-6.8 (point of inflection)
0.2-0.5 0.8-1.1 1.5-1.7 1.7-2.8
temperature can be recorded and used to build a surface phase diagram. Let us now simply analyze the adsorption values obtained at both ends of the vertical parts; they roughly correspond to the ones read on each plateau. In contrast to the analysis in terms of surface phase changes, reaching a plateau on an isotherm has long been interpreted as the saturation of the surface,27,28 whereas the succession of several plateaus has been considered to be indicative of the deposition of successive layers. However, it is worth recalling that the term “layering” is applied only to isotherms where successive plateaus are separated by first-order phase changes (i.e., vertical jumps29). According to Figure 1, 25, 30, and 35 °C are the temperatures where PEG molecules are in their helical form leading to the formation of aggregates. These aggregates are, of course, in equilibrium with free particles.30 If we assume that because of their amphiphilic properties helices are very likely adsorbed either one by one or as aggregates and are fixed at a right angle to the surface, then the adsorption value at saturation can be evaluated by simply taking the inverse of the surface area, A, of the helix section. According to literature data,13 A is approximately equal to 75 Å2, which corresponds to ΓPEG ) 2.2 × 10-6 mol m-2. Table 1 gives the relative adsorptions corresponding to the two ends of the vertical parts of isotherms at different temperatures. The values in bold correspond to the second plateau and can be considered to be a saturated monolayer of helices at a right angle to the surface at 25 and 30 °C according to the previous rough model. Similarly, those written in bold italics at the same temperatures are equivalent to two layers. In between, the existence of another plateau reveals that something else happens. At 35 °C, the shapes of the isotherms are alike, and the existence of vertical sections corresponding to surface phase changes is confirmed by the existence of many metastable states. However, the values observed on the plateaus are different. Several causes can be considered. (27) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361. (28) Adamson, A. W. Physical Chemistry of Surfaces, 2nd ed.; Wiley, New York, 1976. (29) Ball, P. C.; Evans, R. J. Chem. Phys. 1988, 89, 4412. (30) Polverari, M.; van de Ven, T. J. Phys. Chem. 1996, 100, 13687.
Polyethylene Glycol Adsorption on Silica
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Figure 6. Analysis of surface phase changes at the interface between Aerosil 200 and aqueous solutions of PEG 400: (a) analyzed isotherm (15 °C); (b) schematic representation of the possible surface states; (c) similar representation of mechanisms proposed for the adsorption along the stairlike isotherms. Water is not represented but plays a very important role.
First, the measured values are relative ones31 with respect to water
ΓPEG,W ) ΓPEG -
( )
xPEG Γ xW W
(2)
where ΓPEG and ΓW are the Gibbs excesses of PEG and water, respectively, and xPEG and xW are the mole fractions in the bulk. To explain the lowest values of adsorption found at 35 °C (Figure 4a) compared to those at 25 and 30 °C (Figure 4b), let us now assume that ΓW can no longer be neglected, conversely to the previous assumption. Then, it is worth wondering why vertical parts are still observed, though they appear at bulk concentrations that are lower than those at the other two temperatures It is worth recalling that, according to Figure 1, the isotherm at 35 °C is close to a (T - c) zone of the diagram where water is a bad solvent. This means that the molecules are likely shaped more as coils than as helices and indicates the start of changes in interactions between them and water. It is a must for the conformation adopted by the polymer to be geometrically compatible with the water structure.13 Because of silica surface hydrophilicity, water adsorption may be favored by the reduction of compatibility between PEG and water in solution. However, because helical molecules also partially behave hydrophilically, the existence in the surface layer of more water than PEG (with respect to solution) does not imply that helical molecules are missing in the surface region. They can sometimes be very abundant. Let us now suppose that the vertical part denoted 2 (Figure 4b) still corresponds to the formation of a single monolayer. According to our previous evaluation, ΓPEG ) 2.2 × 10-6 mol (31) Andrieux, D.; Acharid, A.; Fritsch, M. C.; Garcia, J. M.; Martin Martin, M.; Me´ar, A. M.; Sadiki, M.; Huruguen, J. P.; Olier, R.; Privat, M. Langmuir 2004, 20, 11012.
m-2. In addition, Figure 4b indicates that ΓPEG,W ) 1.2 × 10-6 mol m-2 for xPEG ) 0.01, and then one can get
ΓW )
( )
xW (Γ - ΓPEG,W) ) xPEG PEG
(0.1/0.99) (2.2 10-6 - 1.2 10-6) ) 1.0 × 10-4 mol m-2 To calculate the value of ΓPEG corresponding to the fourth plateau, let us report in eq 2 this value together with the experimental values of ΓPEG,W, xPEG, and xW read at the start of this plateau. This rough approximation cannot be considered meaningless. It gives ΓPEG ) ΓPEG,W + (xPEG/xW)ΓW ) 2.8 × 10-6 + (0.016/ 0.984)(1.0 × 10-4) ) 4.4 × 10-6 mol m-2; despite the approximations of the treatment, this value is very close to the one obtained for a double monolayer. These estimations allow us to summarize as follows the evolution of adsorption with temperature toward a zone of poor solvent. Water adsorption is easier, but the behavior of PEG molecules and that of molecules in amphiphilic form are alike: the molecules constitute successive monolayers of helices perpendicular to the surface via first-order transitions. The layering is kept. It is worth noting that, in Figure 4a, moving toward another poor-solvent zone by decreasing the temperature, for instance, down to 20 °C, leads to the observation of a decrease of ΓPEG,W in agreement with the previous observations. Unfortunately, the precision of adsorption data was insufficient to draw a precise stairlike isotherm. 3.1.2. Oscillatory Isotherms. Figure 5 displays the isotherms obtained at 15 and 40 °C. Their shapes are radically different from the previous ones. Their main features are (i) null or negative adsorption values, (ii) several vertical parts leading to the most negative values and to the occurrence of well-like shapes, and (iii) the resulting oscillatory form. Similar oscillatory shapes have already been described in the literature for the adsorption of benzene onto graphon32,28 or dimethylpyridine onto Aerosil 200.33
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Figure 7. Surface phase diagram: (a) T - ΓPEG,W; (b) nondistorted shape, as it appears in the bulk for a system with two eutectics and a compound; (c) the same diagram in the T - µ2 representation, with µ2 being the chemical potential of the solute. As at equilibrium, this chemical potential is the same in both phases, and the double lines in part c tied by tie lines are reduced to a single one.
According to eq 2, a null adsorption means that the ratio of excesses of both components and the ratio of the molar fractions in the bulk are alike:
ΓPEG xPEG ) ΓW xW
(3)
The trend described in the previous paragraph about stairlike isotherms is more marked at temperatures close to the limit of the aggregation area; it likely results from either an increase in the amount of water present on the surface or from a decrease in PEG adsorption made far more difficult. Relation 4 expresses that the “wells” correspond to negative adsorptions:
ΓPEG xPEG < ΓW xW
(4)
The wells thus correspond to a massive entry or withdrawal of water in the surface region. As previously, the vertical parts are the reflection of first-order surface phase changes. Despite differences in the shape of isotherms at the temperature under examination, the physical behaviors of the system in the case of stairlike isotherms and of oscillatory isotherms are likely similar: the entry and withdrawal of water along these segments are ruled by the formation of ordered monolayers, likely under the same molecular form (i.e., helices). In very dilute solutions (i.e., to the left of the first “well” (Figure 5)), most of molecules in the bulk are likely “flock” (or coil)-shaped because at 15 °C Figure 1 shows that one stays in a poor solvent; it is also true at 40 °C. Because of their reduced affinity for water, a massive motion of these molecular forms toward the very hydrophilic silica surface is unlikely. However, according to eq 3, PEG molecules are surface-adsorbed. The structures of surface water and the bulk are different. The first vertical part suggests the occurrence of a change in the surface layer at the origin of a massive entry of water. The first surface transition on the silica-hold surface layer has often been attributed to the penetration of a solute into the water layer attached to silica surface sites by hydrogen bonds;31,35 as shown in the next paragraph, this assumption was confirmed by the analysis of the (32) Ash, S. G.; Bown, R.; Everett, D. H. J. Chem. Thermodyn. 1973, 5, 239. (33) Hamraoui, A.; Privat, M. J. Chem. Phys. 1997, 107, 6336. (34) Alexandridis, P.; Holmqvist, P.; Lindman, B. J. Colloid Interface Sci. 1997, 194, 166.
Figure 8. Surface phase diagram in the t - x bulk representation; xbulk represents the bulk concentration at which surface demixing occurs. The bold solid line represents the bulk aggregation line; thin lines indicate surface demixings; dashed lines recall to which series of demixings the upper points belong. Label LS shows the areas where the surface layer can be considered to be a liquid. Label SS shows the areas corresponding to solid states, with SS′ referring to a monolayer of helices and SS′′ referring to a double layer of helices; SS observed on every interface between silica and the aqueous solution likely corresponds to a dense water layer linked to the silica surface by hydrogen bonds.
surface phase diagram of the system under study. Let us consider Figure 6a and assume that the first vertical part, denoted 1, results from such a penetration accompanied by water intake. Indeed, in this case, water behaves on the surface as a good solvent; such behavior usually leads to chain swelling through a reinforcement of bonds with water.34 Any elevation of bulk concentration seems to lead to a new equilibrium. According to the vertical part, 2, this equilibrium corresponds to water withdrawal and suggests the formation of helices, likely at aright angle to the surface, by molecules placed in a good surface solvent and attached to silica by a hydroxyl terminal group. Because of the amphiphilicity of helices, some water remains stuck on silica or trapped inside the helices; the amount of water between the hydrophobic sides of the helices covered up with ethylene groups is likely very limited. Another portion of the water molecules stays around the terminal (35) Maze´as, I.; Pellerin, P.; Sellami, H.; Olier, R.; Privat, M. Langmuir 1999, 15, 2879.
Polyethylene Glycol Adsorption on Silica
Figure 9. Complete phase diagram of PEG 400 in water and at the interface between aqueous solutions and Aerosil 200. The bold line corresponds to the surface phase diagram (Figure 8) in a representation similar to that of Figure 7c.
O-H groups and forms secondary layers against silica and toward solution. Then, the second vertical part of the isotherm may correspond to the formation of a mixed layer containing helices in a monolayer, water, and more and more flocks (or coils). Upon the helix layer likely recovered by water molecules, the situation is similar to the one imagined at the bare surface of silica before part 1. The same two-step process takes place at the surface after the arrival of a sufficient number of flocks (or coils). Then the vertical part denoted 3 likely corresponds to the penetration of PEG molecules in the water layer held by the first helix layer, whereas part 4 indicates the formation of a second helix layer. Figure 6b schematically illustrates the proposed mechanism, whereas Figure 6c, for comparison, depicts the mechanism proposed in the previous paragraph for the adsorption along the stairlike isotherms. 3.2. Surface Phase Diagrams. Let us analyze surface phase diagrams to infer some of the previous assumptions. Figure 7 shows a (T - ΓPEG,W) diagram, which is the equivalent of a (T - c) diagram in the bulk. As previously indicated, this diagram was built by using the vertical parts of Figure 4 as tie lines for the new Figure. The numbers labeling the vertical parts of Figure 4 were reported on the corresponding tie lines of Figure 7a. It is obvious that the relative adsorption is not really a surface concentration, particularly in the case under study where the solvent term in eq 2 is unlikely to be negligible with respect to excess PEG. This explains why (i) we used only the value corresponding to positive values of ΓPEG,W (isotherms at 25, 30, and 35 °C) and (ii) the diagram is very distorted with respect to any known shape in the bulk. The difference obviously comes not only from the difference in composition variables between the bulk and surface phase diagrams but also from a physical and temperature-dependent phenomenon (i.e., the change in water adsorption, particularly in the region under study). However, it is relevant to accept a similarity with Figure 7b (i.e., the juxtaposition of two eutectic-like diagrams; there is no clear evidence of the existence of a compound on the surface, and the Figure concerned with the surface can correspond to the successive formation of two layers through, for each of them, a two-step process). The part corresponding to area 1 of Figure 7b, called a solid solution in the bulk, can be identified with a water layer structured by hydrogen bonds and linked to the surface by the same type of interaction. In this part of the surface diagram, the entry of PEG molecules is very limited. The first phase transition corresponds to the breaking of hydrogen bonds between water
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and silica; it allows the entry of PEG molecules into the layer and leads to a kind of liquid surface phase. Then, a second phase transition corresponding to the solidification of the layer occurs; it corresponds to the structuring of a monolayer of helix-shaped PEG molecules described above. Another way to draw a surface phase diagram is to use the bulk frame of Figure 1. Each surface dot is obtained by using, at a given temperature, the bulk concentration at which a vertical part occurs on an isotherm. Figure 8 exhibits such a diagram, which is the equivalent of Figure 7c with lines corresponding to two-phase coexistence, whereas in Figure 7 each phase is represented by its own line and is linked to the other phase in equilibrium by the proper tie line. This synthetic representation is not as easy to read as the previous one. The previously identified nature of phases was transferred to a new drawing (Figure 8), where it is also worth copying the points corresponding to the surface phase transitions observed at 15 and 40 °C because they were missing on the previous graph. Surprisingly, the 15 °C points were found to match the other ones well and thus confirmed the possible existence of surface eutectics. Conversely and in agreement with bulk observations, 40 °C points seem to follow another behavior: interactions are changing above 35 °C.
4. Conclusions Through a careful characterization of the bulk phase diagram from literature data and new measurements, this study provided additional insights into the surface behavior of a PEG oligomer on the silica surface. Adsorption isotherms issued from measurements by a depletion method showed strong temperaturedependent changes in shape, in agreement with the variations of interactions already evidenced in the bulk. In temperature areas where water behaves as a helix-promoting solvent, the finding of stairlike isotherms confirms the observation reported in ref 17. Moreover, some of the vertical parts were identified with the formation of monolayers of helix-shaped molecules of PEG. The oscillatory shape of isotherms in poor solvent area indicates, at first sight, very different surface behavior. Indeed, the analysis of data showed the presence of water in a greatly increased quantity with respect to the previous surface states. However, a similar analysis of the PEG behavior showed the occurrence, in the surface layer, of first-order transitions alternately implying water reorganization permitting PEG molecules to gain access to the surface, accompanied by the formation of a monolayer of helix-shaped molecules of PEG. A very satisfactory confirmation of this analysis was obtained from the surface phase diagrams. They evidenced a continuation between the surface phase changes observed at 15 °C and those between 25 and 35 °C. However, the behavior observed at 40 °C seems to belong to another sort of process. Altogether these results highlight the importance of helix shape in the surface properties of PEG 400. Monolayers or multimonolayers of molecules of such shape seem to be the rule at the silica surface. However, depending on the temperature and concentration, these layers are more or less embedded in adjacent water molecules. Therefore, the aggregation properties of silica beads or their rheological effect upon the liquids within which they are located will also depend on temperature and concentration. Finally, this kind of analysis is probably not specific to PEG 400 because recent data show that most of PEGs and PEOs show helical conformations36 and form clusters.24 LA070199U (36) Ho, D. L.; Hammouda, B.; Kline, S. R. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 135.
