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Observation of the Dynamic Colloidal Interaction Forces between Casein-Coated Latex Particles Brent S. Murray,* Eric Dickinson, Joseph M. McCarney, Phillip V. Nelson, and Martin Whittle Food Colloids Group, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, U.K. Received February 24, 1998 We describe a refined version of the particle scattering apparatus due to van de Ven (Langmuir 1994, 10, 3046) for measuring the dynamic interactions between colloidal particles in a flow field. Polystyrene latex particles (diameter 5.1 µm) were studied, stabilized by β-casein and Rs1-casein, at pH 7 and ionic strengths of 0.1 and 0.01 mol dm-3. The β-casein system exhibited stronger interparticle repulsion and less tendency for particle flocculation at the lower ionic strength. At ionic strength ) 0.1 mol dm-3 the Rs1-casein system exhibited stronger interparticle repulsion than the β-casein system. This behavior is consistent with other measurements on the properties of casein-coated particles.
Introduction The stability behavior (flocculation, coagulation, creaming, coalescence, etc.) and flow properties of all colloidal systems ultimately depend on the set of interaction forces acting amongst the particles. This includes colloidal forces acting directly between pairs of particles, such as electrostatic repulsion, van der Waals attraction, and steric repulsive forces due to adsorbed polymers and so forth, and the hydrodynamic forces acting indirectly between particles as a result of the flow conditions. In real systems it is important that the hydrodynamic forces are taken into account, not only because these may sometimes tend to dominate1 but also because the rate of approach of the particle surfaces and the intervening flow may affect the magnitude of the other forces. For example, for charged systems, it takes a finite time for counterion, co-ion, and surface charge distributions to come to equilibrium at a new interparticle separation,2 and depending on the rate of approach of the surfaces, this time may be too long for equilibrium to be established. Measured DLVO interactions for surfaces may fit neither constant surface charge nor constant surface potential models but lie somewhere in between.3 Furthermore, where polymers are adsorbed to surfaces, there may be effects of shear and compression on the structure of the adsorbed layers which may modify considerably the steric repulsive force due to such layers. Thus pronounced hysteresis of measured interaction forces has been observed for adsorbed polymers4 and proteins.5 The particle scattering apparatus (PSA) was introduced by van de Ven et al.6 as a means of measuring dynamic interaction forces between colloidal particles under the flow conditions relevant to real colloidal processing. This makes the PSA different from most other surface force * Author for correspondence. Telephone: 44 (0)113 2332962. Fax: 44 (0)113 2332982. E-mail:
[email protected]. (1) van de Ven, T. G. M. Colloidal Hydrodynamics; Academic Press: London, 1989. (2) Hsu, J. P.; Kuo,Y. C.; Tseng, S. J. J. Colloid Interface Sci. 1997, 195, 388. (3) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (4) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1990, 86, 1363. (5) Chowdhury, P. B.; Luckham, P. F. Colloids Surf. B, 1995, 4, 327. (6) van de Ven, T. G. M.; Warszynski, P.; Wu, X.; Dabros, T. Langmuir 1994, 10, 3046.
measuring techniques which have been developed for measuring the equilibrium interaction forces, for example, the surface force apparatus7 (SFA), atomic force microscopy (AFM),3 osmotic pressure,8 optical tweezers,9 total internal reflection spectroscopy,10 capillary techniques,11,12 and so forth. Whilst some of these are quite accurate and (in the case of the SFA) well-established, they commonly suffer from a number of disadvantages, such as being limited to specific materials (highly purified solids of exact geometry), being extremely sensitive to surface contamination, and being slow or difficult to use. In addition, only a few of these methods11,12 are suitable for measurements involving fluid particles. In contrast, the PSA is applicable to a wide variety of particle types, and it allows measurement of the effects of shear rate on the particle interactions. Recent publications13-15 by the originators of the method have also highlighted the higher sensitivity of the technique. For example, both long-range electrostatic forces and very short-range steric interaction forces (due to polymer chains on particle surfaces) have been measured.14,15 Up to now there have been no PSA studies on biopolymer-stabilized systems. Very many commercial formulations rely on the use of biopolymers for the stabilization of dispersions of solids and fluidssnot least food systems, where the principal stabilizing agents are proteins. The most widely used protein emulsifier is sodium caseinate, which contains β-casein and Rs1-casein as its major component proteins. Recent theoretical treatments and neutron reflectivity experiments16,17 on the caseins have (7) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1992. (8) Rohrsetzer, S.; Kova´cs, P.; Nagy, M. Colloid Polym. Sci. 1986, 264, 812. (9) Grier, D. G. Curr. Opin. Colloid Interface Sci. 1997, 2, 264. (10) Flicker, S. G.; Bike, S. G. Langmuir 1993, 9, 257. (11) Fisher, L. R.; Mitchell, E. E. In Food Colloids; Bee, R. D., Richmond, P., Mingins, J., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1989; p 123. Fisher, L. R.; Mitchell, E. E.; Parker, N. S. J. Colloid Interface Sci. 1989, 128, 35. (12) Aveyard, R.; Binks, B. P.; Cho,W. G.; Fisher, L. R.; Fletcher, P. D. I.; Klinkhammer, F. Langmuir 1996, 12, 6561. (13) van de Ven, T. G. M. Langmuir 1996, 12, 5254. (14) Wu, X.; van de Ven, T. G. M. Langmuir 1996, 12, 3859. (15) Wu, X.; van de Ven, T. G. M. J. Colloid Interface Sci. 1996, 183, 388. (16) Dickinson, E. J. Dairy Sci. 1997, 80, 2607. (17) Dickinson, E.; Pinfield, V. J.; Horne, D. S.; Leermakers, F. A. M. J. Chem. Soc., Faraday Trans. 1997, 93, 1785.
S0743-7463(98)00216-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/30/1998
Letters
Langmuir, Vol. 14, No. 13, 1998 3467
Figure 1. Schematic diagram of the PSA experiment. The paths A, B, and C illustrate repulsive, zero, and attractive interparticle potentials, respectively.
reached the stage that the essential features of the equilibrium adsorbed conformation of these proteins and the interactions of their adsorbed layers can be sensibly modeled. As yet, however, there have been few direct experimental tests of the predicted behavior. Much weak structuring (e.g., loose aggregation of proteins) is believed to occur at interfaces,18 and so methods for determining interaction forces must be properly sensitive to the effects of the hydrodynamics on the structure and dynamics of the stabilizing film. This letter describes the use of the PSA method to study a model latex system stabilized by the pure proteins β-casein and Rs1-casein. Materials A monodisperse polystyrene latex of particle diameter 5.1 µm was supplied by Bangs Laboratories Inc. (Fishers, USA). While the latex was nominally surfactant-free, the suspension was repeatedly diluted with water, centrifuged, and redispersed in water to remove any surface-active material. The Rs1-casein and β-casein were prepared at the Hannah Research Institute (Ayr, Scotland) from fresh skim milk by acid precipitation, washing, reprecipitation, and dissolution in urea, followed by ion-exchange chromatography and dialysis. Purity with respect to other protein contaminants was assessed by fast protein liquid chromatography. All other reagents were of AnalR grade. Deuterium oxide, imidazole buffer, potassium chloride, nitric acid, and hydrochloric acid were from Sigma Chemical Co. (Poole, U.K.). All water used was from a Milli-Q water purification system with a surface tension of 72.0 mN m-1 at 25 °C.
Methods The principle of the PSA method is illustrated in Figure 1. A flow cell is formed from two parallel plates, with a particle immobilized on the upper plate (e.g., by strong, irreversible adhesion). Freely mobile, neutrally buoyant particles are then introduced into the gap between the plates, and a mobile particle is brought to impinge upon the fixed particle by a laminar flow field induced by movement of the lower plate at constant velocity. In the absence of any colloidal interaction forces, the path of the mobile particle about the fixed particle will be symmetrical (path A) and the z coordinate before and after the “collision” will be the same. However, if there is a colloidal repulsive interaction between the particles, then the mobile particle will move away at a distance from the upper plate that is greater than that before the collision (path B). Conversely, a net attractive colloidal force will lead to path C. At distances far from the fixed particle the free particle moves parallel to the plates, with a velocity which is determined by its distance from the fixed upper plate, as described by Goren and O’Neill.19 On this basis, once the particle velocity is measured, the z-coordinate can be calculated. The PSA cell used in this study was designed to have a number of refinements over the originally described setup,6 in order to make the apparatus more robust and easier to use. The lower plate consisted of a circular piece of optically polished, clear-cut glass, 0.7 cm in thickness and 10 cm in diameter. Analysis with (18) Dickinson, E., Murray, B. S.; Stainsby, G. In Advances in Food Emulsions and Foams; Dickinson, E.; Stainsby, G., Eds.; Elsevier Applied Science: London, 1988; p 123. (19) Goren, S. L.; O’Neill, M. E. Chem. Eng. Sci. 1971, 26, 325.
a dial indicator showed that its surface was smooth and flat to within 2 µm over its entire diameter. This lower plate was fixed in a stainless steel support which formed the well containing the suspension of interest. The upper plate was one of a set of highquality microscope slides (of dimensions 76 × 26 × 1 mm) from Agar Scientific. Each slide was selected on the basis that its thickness did not vary by more than 2 µm at randomly selected points along the length. Slides were also examined using an atomic force microscope, and the surface roughness was observed to be negligible (i.e.,
