Langmuir 1996,11, 4019-4024
4019
Interaction Forces between Silica and Polypropylene Surfaces in Aqueous Solution L. Meagher and R. M. Pashley" Department of Chemistry, The Australian National University, Canberra, ACT 0200 Australia Received March 1, 1995. I n Final Form: June 30, 1995@ We have used an AFM technique t o study the interaction forces between silica glass and polypropylene in aqueous NaCl solutions in order to investigate the existence and properties of any solvation forces which might be generated specifically by the combination of a hydrophilic and a hydrophobic surface. The forces observed were found to be well-describedby DLVO theory and indicate that the interaction between these substrates does not generate any significant solvation effects. The low, pH dependent negative charge detected on the polypropylene surface was most likely due to the adsorption of bicarbonate ions from solution onto the low density of carbonyl groups formed by oxidation of the polymer surface.
Introduction The interaction between hydrophilic and hydrophobic surfaces occurs in a wide range of processes, such as in heterocoagulation, wetting, membrane fouling, and flotation. Both hydrophilic and hydrophobic surfaces typically display quite opposite non-DLVO behavior; thus, the interaction of a hydrophobic and a hydrophilic surface might be expected to display features of either one or the other or neither. Recent work in this area has produced conflicting results. Several groups have carried out surface force measurements using SFA,1,2the MASSIF,3and AFM.4 However, there is no consensus as to whether the forces should be strongly attractive, like the interaction between hydrophobic surfaces, or be consistent with DLVO theory. For example, the interaction between a negatively charged mica surface and a positively charged, hydrophobic mica surface coated with a Langmuir-Blodgett film was more strongly attractive than predicted by DLVO the0ry.l Similarly, the interaction measured between an air bubble or a silanated silica surface and a hydrophilic silica particle was also much more attractive than DLVO theory would predicts4 In both cases the authors suggested that the additional attractive force was related to a long range attractive force, the hydrophobic interaction, observed between two similar hydrophobic surfaces. By comparison, interaction forces between a hydrophilic silica surface and a silanated, hydrophobic silica surface were found to be in agreement with DLVO t h e ~ r y . ~ Despite this lack of consensus, there is much indirect evidence that the interaction between a hydrophilic and a hydrophobic surface in aqueous solution should be in reasonable agreement with DLVO theory. Support of this comes from the technologically important process of froth flotation. In this process, minerals made hydrophobic by the selective adsorption of collector (surfactant)molecules are separated from gangue material by attachment to air bubbles, as they rise through a mixed suspension. If the interaction force between a n air bubble (which has a hydrophobic surface) and a hydrophilic mineral was strongly attractive, then hydrophilic minerals would always float, i.e. there would be no need for the use of Abstract published in Advance A C S Abstracts, September 15, 1995. (1)Claesson, P. M.; Herder, P. C.; Blom, C. E.; Ninham, B. W. J. Colloid Interface Sci. 1987,118, 68-79. (2)Tsao, Y.-H.; Evans, D. F. Langmuir 1993,9, 779-85. (3)Parker, J. L.;Claesson, P. M. Langmuir 1994,I O , 635-9. (4)Ducker, W.A,; Xu, 2.; Israelachvili, J. N. Langmuir 1994,10, 3279-89. @
surfactant collectors. Related evidence also comes from the observed thickness and stability of aqueous wetting films on silica. The thickness of such wetting films is in reasonable agreement with the predictions of the DLVO theory ofcolloid ~ t a b i l i t y . ~By - ~comparison, wettingfilms on hydrophobic silica form films which rupture a t relatively large film thicknesses (50- 100 nm),6,8indicative of the presence of a strong hydrophobic attraction. In this study,we have directly measured the interactions in an asymmetric system for which the symmetric interaction between each of the surfaces is fairly wellunderstood. Furthermore, the type of hydrophobicsurface used in these measurements is a partially crystalline solid polymer, which is not subject to any of the instabilities of adsorbed surfactant layers, such as surfactant turnover, transfer, or des0rption.l These must be considered as real possibilities for substrates rendered hydrophobic by LB deposition or adsorption from solution.
Materials and Methods The interaction forces between Polysciences (about 6-7 pm diameter) glass spheres and flat polypropylene surfaces, immersed in aqueous NaCl solutions, were measured using a Nanoscope I1 and I11 scanning force microscope (Digital Instruments Inc.). The method used was that developed by Ducker et aL9J0Measurements were carried out in aqueous NaCl solutions at various concentrations and pH values at about 25 "C. The Polysciences spheres were cleaned using a water plasma prior to each experiment. To avoid particulate contamination all equipment was handled in a filtered air laminar flow cabinet whenever possible. The AF'M fluid cell and all its fittings were washed thoroughly in distilled ethanol followed by clean water. The technique used here produced glass AF'M probes which gave forces similar to those reported by Ducker et aL9Jo and demonstrate that the silica surfaces were negatively charged and hydrophilic. Polypropylene flat surfaces were produced by melting beads of the polymer held between two freshly cleaved sheets of mica under a slight pressure. To reduce surfaceoxidationthis process was carried out in a nitrogen gas glove bag. The mica which adhered to both sides was left in place, and the disks were stored (5) Read, A. D.; Kitchener, J. A. J. Colloid Interface Sci. 1969,30, 391-98. (6) Blake, T. D.: Kitchener, J. A. J. Chemical SOC.Faraday Trans. 1 1972,68,1435. (7)Aronson, M.P.;Princen, H. M. Colloid Polym. Sci. 1978,156, 140-149. (8)Yoon, R.-H.; Yordan, J. L. J. Colloid Interface Sci. 1991,146, 565-72. (9)Ducker, W.A.;Senden, T. J.;Pashley, R. M. Nature 1991,353, 2239-41. (10)Ducker, W.A,; Senden, T. J.;Pashley, R. M. Langmuir 1992,8, 1831-36.
0743-746319512411-4019$09.00/0 0 1995 American Chemical Society
Meagher and Pashley
4020 Langmuir, Vol. 11, No. 10, 1995
in nitrogen until required (usually 2-3 days). AFM imaging of the polypropylenesurface (in 0.01M NaCl solution), after lifting off the mica substrate, showed that the surfaces were relatively smooth, with a root mean square roughness of 1.3 nm with occasional 7-8 nm peaks over a 180 x 180nm region. Infrared and X-ray diffraction analysis of these samples indicated a high degree of crystallinity, with the chains packed predominantly in an isotactic configuration. It is likely that the natural tendency of the polymer to crystallize limits the degree to which a smooth polypropylene surface can be produced. The water used in these experiments was purified by passing through a Krystal Hear water purification system which consisted of a coarse filter, activated carbon, and reverse osmosis membrane, followed by distillation. Final purification was performed using a Milli Q Plus Ultrapure Water System housed in a laminar flow cabinet. The NaCl used to prepare solutions was of AR grade and was used without further purification. The nitrogen gas used was oxygen-free,high-purity nitrogen (CIG). X-rayphotoelectronspectra ( X P S )of polypropylenefilms were obtained by using a VG ESCALAB Mk I1 (base pressure
