Stabilization of Latex Dispersions Using a Graft Copolymer of Inulin

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Articles Stabilization of Latex Dispersions Using a Graft Copolymer of Inulin Based Surfactants Jordi Esquena,*,† Francisco J. Domı´nguez,† Conxita Solans,† Bart Levecke,‡ Karl Booten,‡ and Tharwat F. Tadros§ Departament de Tecnologia de Tensioactius, Institut d’Investigacions Quı´miques i Ambientals de Barcelona (IIQAB), CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain and ORAFTI, Aandorenstraat 1, B-3300 Tienen, Belgium Received June 20, 2003. In Final Form: September 10, 2003 Polystyrene (PS) latex dispersions were prepared by surfactant-free emulsion polymerization. Poly(methyl methacrylate) (PMMA) latex dispersions were prepared using the same procedure except by adding INUTEC SP1 as emulsifier. The two latex dispersions were characterized by photon correlation spectroscopy (PCS) to obtain the average particle size and the polydispersity index. The stability of these latex dispersions was measured by determination of the critical coagulation concentration (CCC) of three electrolytes, namely, NaCl, CaCl2, and Al2(SO4)3. The CCC was 0.375 mol‚dm-3 for NaCl, 0.007 mol‚dm-3 for CaCl2, and 0.0004 mol‚dm-3 for Al2(SO4)3. A polymeric surfactant, namely, a graft copolymer of polyfructose on which alkyl groups were grafted to the backbone, were added to latex dispersions and their stability was investigated. On addition of this polymeric surfactant, the stability of the latex dispersions was significantly increased and the CCC became very high above a critical polymer concentration. For the PS latex, the CCC of CaCl2 was higher than 4.3 mol‚dm-3 when the polymeric surfactant concentration was 0.25 wt %. The results could be rationalized in terms of the enhanced steric repulsion resulting from the adsorption of the graft copolymer. It was assumed that the molecule produces large “loops” of polyfructose between the adsorbed alkyl groups, forming a hydrated layer thickness of approximately 4 nm. In addition, these polyfructose chains were still hydrated even in the presence of high electrolyte concentrations.

Introduction The stabilization of solid-liquid dispersions is of considerable importance for many technological applications, particularly in the fields of paints, coatings, adhesives, printing inks, paper, textiles, and so forth. Appropriate steric stabilization can be achieved by adsorbing neutral polymers on particle surface, as described in the literature.1-5 Polymeric surfactants are commonly used in formulations that require good stability under conditions of high electrolyte concentrations and high temperature. For this purpose, block (A-B and A-B-A) and graft (BAn) surfactants, where B is the anchor chain and A is the stabilizing chain, are commonly used.6-9 The * To whom correspondence should be addressed. E-mail: [email protected]; phone: +34934006159; fax: +34932045904. † Institut d’Investigacions Quı´miques i Ambientals de Barcelona. ‡ ORAFTI. § 89 Nash Grove Lane, Wokingham, Berkshire, RG40 4HE, United Kingdom. (1) Napper, D. H. Polymeric Stabilization of Dispersions; Academic Press: New York, 1983. (2) Tadros, Th. F. Polym. J. 1991, 23, 683. (3) Einarson, M. B.; Berg J. C. J. Colloid Interface Sci. 1993, 155, 165. (4) Ortega-Vinuesa, J. L.; Martı´n-Rodrı´guez, A.; Hidalgo-A Ä lvarez, R. J. Colloid Interface Sci. 1996, 184, 259. (5) Romero-Cano, M. S.; Martı´n-Rodrı´guez, A.; Chauveteau, G.; de las Nieves F. J. J. Colloid Interface Sci. 1998, 198, 273. (6) Zhulina, E. B.; Borisov, O. V. Makromol. Chem., Macromol. Symp. 1991, 44, 275. (7) Bijsterbosch, H. D.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1999, 210, 37. (8) Tadros, Th. F. In Novel Surfactants; Holmberg K., Ed.; Marcel Dekker: New York, 2003. (9) Tadros, Th. F. et al. Adv. Colloid Interface Sci. 2003, 104, 191.

B chains are highly insoluble in the medium and strongly adsorbed on the surface with several “anchor” points. The A chains are highly soluble in the medium, where they are strongly solvated by the medium under conditions of high electrolyte or high temperature. The above conditions are satisfied using a graft copolymer consisting of polyfructose chains in which alkyl groups are grafted. The latter provide the anchor points, whereas the polyfructose chains will produce large loops between the alkyl chains, thus providing strong steric repulsion. These polyfructose chains remain strongly hydrated in high electrolyte concentrations (up to 5 mol‚dm-3 NaCl and 1.5 mol‚dm-3 MgSO4).10 The present work describes the use of the above polymeric surfactant for the stabilization of PS and PMMA latex dispersions, which are prepared by emulsion polymerization techniques. The stability of latex particles in the presence and absence of polymeric surfactant was investigated by measuring the critical coagulation concentration (CCC) in the presence of three different electrolytes, namely, NaCl, CaCl2, and Al2(SO4)3. The effect of increasing the polymer concentration on CCC was systematically investigated. Materials and Methods Materials. The main surfactant used throughout this study was INUTEC SP1, supplied by ORAFTI Non-Food (Tienen, Belgium). This is essentially a copolymer made of a polyfructose backbone in which some alkyl groups are grafted.11,12 Its average (10) Tadros, Th. F.; Vandamme, A.; Booten, K.; Levecke, B.; Stevens, C. V. Colloids Surf., A submitted for publication.

10.1021/la035092v CCC: $25.00 © 2003 American Chemical Society Published on Web 11/06/2003

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molecular weight is 4525 g‚mol-1 and the cmc is between 5 × 10-7 and 6 × 10-6 mol‚dm-3. The purity of such surfactant was higher than 99% and it makes a clear solution at concentrations lower than 0.1 wt %, above which a turbid solution appears, which could be due to some chain aggregation. For comparison, many surfactants were used: modified Maltodextrin DE 28, Glucidex DE 28, and Glucidex DE 47, which are maltodextrin backbones on which alkyl chains were grafted (surfactants supplied by ORAFTI Non-Food). Surfonic N-800, which is a nonyl phenol ethoxylate with 80 ethylene oxide units, was supplied by Huntsman (Belgium). Other surfactants used in this work include Brij 30 and sodium dodecyl sulfate (SDS), both supplied by SigmaAldrich and Synperonic PE L64 supplied by UNIQEMA (ICI). These surfactants were used as received. The styrene and methyl methacrylate used to prepare the latex dispersions were supplied by Sigma-Aldrich and both were purified in an alumina column so as to remove the hydroquinone, a polymerization inhibitor. As initiator, K2S2O8 from Fluka with a purity higher than 99% was used. The electrolytes used were NaCl, CaCl2‚2H2O (purity >99%), and Al2(SO4)3‚16 H2O (purity >98%), all supplied by Sigma-Aldrich. Water was deionized and Millipore filtered by a Milli-Q system. Emulsion Polymerization. It was performed in a 500-mL four-neck glass reactor equipped with a mechanical stirrer, a reflux-cooler, a thermometer, and a nitrogen flushing. A thermostated oil bath was employed to control the reaction temperature. The polymerization was performed according to the method described in the literature:13 deionized water was first poured in the glass reactor, then air was purged with nitrogen to remove oxygen, and finally the monomer and the initiator were poured. The reaction conditions were 70 °C, approximately 1 atm of pressure, and a stirring speed of 200 rpm for 24 h of polymerization time. PMMA particles were prepared using the same procedure but using INUTEC SP1 as emulsifier. Particle Size Determination. The mean particle size and size distribution were determined using photon correlation spectroscopy (PCS), also known as dynamic light scattering (DLS). A Malvern 4700 photon correlation spectrometer (Malvern Instruments, Malvern, United Kingdom) was used for this purpose. An argon laser (λ ) 488 nm) with variable intensity was used to cover the wide size range involved. Measurements were always carried out at a scattering angle of 90° and at a constant temperature of 25 °C. The scattering vector q, as defined in ref 14 is calculated from the equation:

q)

θ sin (4πn λ ) (2)

(1)

where n is the refractive index of the medium (n ) 1.332 for water at 25 °C), λ is the wavelength, and θ is the scattering angle (90°). The particle size is measured from the autocorrelation function of the intensity fluctuation of scattered light. From this, one could obtain the diffusion coefficient, D, and the latter is used to calculate the particle hydrodynamic radius, Rh using the Stokes-Einstein equation:

D)

kT 6πηRh

(2)

where k is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the continuous phase. The value of η was 8.9‚10-4 Pa‚s for water at 25 °C. The PCS data were analyzed by (a) the standard program obtaining the Z average value, (b) the CONTIN method,15,16 and (11) Stevens, C. V.; Meriggi, A.; Peristeropoulou, M.; Christov, P. P.; Booten, K.; Levecke, B.; Vandamme, A.; Pittevils, N.; Tadros, Th. F. Biomacromolecules 2001, 2, 1256. (12) Stevens, C. V.; Meriggi, A.; Booten, K. Biomacromolecules 2001, 2, 1. (13) Kotera, A.; Furusawa, K.; Takeda, Y. Kolloid Z. Z. Polym. 1970, 227, 677. (14) Cotton, J. P. In Neutron, X-ray and Light Scattering: Introduction to an Investigative Tool for Colloidal and Polymeric Systems; Lindner, P., Zemb, Th., Eds; North-Holland: Amsterdam, 1991. (15) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (16) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229.

(c) a constrained regularization calculation algorithm known as REPES,17-19 incorporated in the analysis package GENDIST (interactive program for the analysis of homodyne PCS data). Critical Coagulation Concentration (CCC) Determination. In this study, the experimental CCC values of the latex dispersions, defined as the minimum concentration of electrolyte required to induce the coagulation of a stable colloidal suspension, were determined as follows: a volume of a latex dispersion was added to an equal volume of electrolyte solution or water. Once the sample was prepared, the test tube was kept in a thermostated water bath at 25 °C for 12 h. After that period of time, part of the sample was introduced in a Quartz cell of 10 × 10 × 45 mm so as to measure the optical density in a wavelength range of 400-700 nm,20-23 whereas the other part was used for optical microscopy observations. Log-linear plots of transmission versus wavelengths were linear and the slopes n were determined. The CCC value was obtained from a plot of n versus electrolyte concentration, since a sharp decrease of n was observed at the CCC. Optical Microscopy. Samples were observed with a Reichert Polyvar 2 optical microscope, supplied by Leica, equipped with polarizers and interference contrast prism. Images were acquired with a digital camera and an IM 500 software system.

Results and Discussion Effect of Polymer on Polystyrene Surfactant-Free Particles. Polystyrene particles with a 10 wt % monomer and 0.06 wt % K2S2O8 were prepared using the surfactantfree polymerization process described above.13 The mean diameter of these particles was 210 nm with a polydispersity index of 0.0272. The mean particle size obtained was smaller than the size described in the literature,13 probably because of different stirring and purity conditions. In a preliminary study, it was observed that the presence of 1 mol‚dm-3 NaCl induced coagulation of the polystyrene particles. However, by adding a 0.25 wt % of the polymeric surfactant at the same electrolyte concentration, PS particles remain stable. This well-known stabilization is due to steric interactions produced by polymer adsorption on particle surface.1-5 As expected, this behavior was dependent on the order of addition, since the polymeric surfactant could not stabilize particles already aggregated by a previous addition of electrolyte. Therefore, all stability studies were performed adding the surfactant before the electrolyte. The critical coagulation concentrations, CCC, were measured by spectrophotometry. As an illustration, the transmittance, τ, of samples containing PS latex, the polymeric surfactant, and different Al2(SO4)3 concentrations were measured within the wavelength range of 400-700 nm (Figure 1). The transmittance was plotted as a function of λ in double logarithmic and in log τ versus λ plots. Potential and exponential equations, described respectively as τ ) aλm and τ ) benλ, were fitted to the experimental data, being a, b, m, and n the fitting parameters. The best correlation factors, r2, where obtained for the exponential dependence. Figure 1 shows an example of the linear dependence observed in a log τ versus λ plot. The correlation factors r2 corresponding to the fits (17) Jakes, J. Czech. J. Phys. B. 1988, 38, 1305. (18) Nikolai, T.; Brown, W.; Johnsen, R. M.; Stepa´nek, P. Macromolecules 1990, 23, 1165. (19) Johnsen, R. M. In Light Scattering in Biochemistry; Harding, S. E., Sttelle, D. B., Bloomfield, V. A., Eds.; The Royal Society of Chemistry: Cambridge, United Kingdom, 1992. (20) Heller, W.; Bhatnagar, H.; Nakagaki, M. J. Chem. Phys. 1962, 36, 1163. (21) Long J.; Osmond, W.; Vincent, B. J. Colloid Interface Sci. 1973, 42, 545. (22) Snowden, M. J.; Clegg, S. M.; Williams, P. A.; Robb, I. D. J. Chem. Soc., Faraday Trans. 1991, 87, 2201. (23) Sharma, A.; Tan, S. N.; Walz, J. Y. J. Colloid Interface Sci. 1997, 190, 392.

Stabilization of Latex Dispersions

Figure 1. Variation of the transmittance, τ, as a function of the wavelength, λ, for two Al2(SO4)3 concentrations.

Figure 2. Representation of the slope value (n) of the straight line, obtained from the variation of the transmitance in front of the wavelength, versus Al2(SO4)3 concentration.

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Figure 3. Variation of the CCC values of surfactant-free PS particles as a function of the [INUTEC SP1]/[PS] concentration ratio of using CaCl2 and Al2(SO4)3 as electrolytes.

Figure 4. Variation of the hydrodynamic diameter as a function of the [INUTEC SP1]/[PS] concentration ratio of particles obtained by surfactant-free emulsion polymerization.

shown in the figure for 1.9‚10-3 and 6.3‚10-4 mol‚dm-3 aluminum sulfate concentrations are 0.984 and 0.989, respectively. The (n) parameter, which is the slope of the above representation, was plotted versus Al2(SO4)3 concentration, as shown in Figure 2. For Al2(SO4)3 concentrations lower than 6.3‚10-4 mol‚dm-3, the slope n remained constant, whereas for samples containing the Al2(SO4)3 concentration higher than 1.9‚10-3 mol‚dm-3, the slope value was lower than the samples mentioned before. The samples were also observed under the optical microscope. Flocculation was observed at Al2(SO4)3 concentrations higher than 1.9‚10-3 mol‚dm-3, whereas samples containing Al2(SO4)3 concentrations lower than 6.3‚10-4 mol‚dm-3 were stable. As a consequence, both methods provided the same CCC value and therefore optical microscopy was preferred because of the simplicity of its methodology. The stability of PS particles was measured as a function of ([INUTEC SP1]/[PS]) ratio for a 1:1 (NaCl), a 2:1 (CaCl2), and a 3:2 (Al2(SO4)3 ) electrolytes. For NaCl, the dispersions remained stable up to the maximum concentration investigated, namely, 5.2 mol‚dm-3. The results for the other two electrolytes are shown in Figure 3. In absence of the polymeric surfactant, the CCC value for NaCl was 0.375 ( 0.025 mol‚dm-3, that for CaCl2 was 0.0075 ( 0.0015 mol‚dm-3, and the CCC value for Al2(SO4)3 was 0.0004 ( 0.0002 mol‚dm-3. The ratios of these CCC values for 1:1, 2:1, and 3:2 electrolytes are (938:18.8:1), which is in accordance with the results obtained for the same type of electrolytes on the basis of the perturbation solution of the Poisson-Boltzmann equation (920:18.5:1) described by Hsu et al.24,25 The results obtained are also in accordance with the Derjaguin approximation (810:10.8:1), with the exception of the ratio between NaCl and CaCl2.26-29

From the results obtained with CaCl2 as electrolyte, it can be concluded that there is no influence of the [INUTEC SP1]/[PS] weight ratio up to 0.001. At approximately the polymeric surfactant/PS weight ratio of 0.0015, there was a sharp increase in the stability of PS particles, reaching to a CCC value of 1.54 ( 0.14 mol‚dm-3. At [INUTEC SP1]/[PS] weight ratio of 0.0017, the CCC value was 1.88 ( 0.13 mol‚dm-3. Using Al2(SO4)3 as a electrolyte, there was a progressive increase in the stability of PS particles with no sharp increase in stability at the critical polymer concentration. As expected, the stability of PS particles using Al2(SO4)3 was lower than that using CaCl2 in the whole range of [INUTEC SP1]/[PS] weight ratio studied. Hydrodynamic Diameter of Polymer-Coated PS Latex Dispersions. Samples containing PS particles were prepared and different amounts of the polymeric surfactant were added. The experiments were performed without electrolyte in the solution and with a NaCl concentration of 0.025 mol‚dm-3. Figure 4 shows that the hydrodynamic diameter of PS particles in the absence of the polymeric surfactant and without electrolyte was approximately 211 nm, whereas with a NaCl concentration of 0.025 mol‚dm-3, the average hydrodynamic diameter was approximately 208 nm. In the presence of electrolyte, a constant hydrodynamic diameter was reached at a very low [INUTEC SP1]/[PS] weight ratio. From the results obtained, the hydrodynamic particle diameter increases by approximately 7 ( 2 nm in the absence of electrolyte and 8 ( 2 nm in the presence of electrolyte, which are nearly equal within experimental error. The layer thickness of adsorbed INUTEC SP1 on PS particles could be estimated as half the increase in the

(24) Hsu, J. P.; Kuo, Y. C. J. Colloid Interface Sci. 1995, 171, 254. (25) Hsu, J. P.; Kuo, Y. C. J. Colloid Interface Sci. 1997, 185, 530. (26) Derjaguin, B. V.; Landau, L. Acta Physicochim. 1941, 14, 633. (27) Verwey, E. J.; Overbeek, J. Th. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.

(28) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: London, 1989; Vol. 1. (29) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain, where Physics, Chemistry, Biology and Technology meet; VCH Publishers: New York, 1994.

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Table 1. CCC Values of Surfactant-Free PS Particles (5 wt %) Using CaCl2 as Electrolyte and for 0.25 wt % of Different Surfactants Tested surfactant

CCCCaCl2/M

without surfactant INUTEC SP1 Surfonic N-800a Brij 30 Synperonic PE L64 sodium dodecyl sulfate modified maltodextrin DE 28 Glucidex DE 28 Glucidex DE 47

0.0075 ( 0.0015 >4.37 3.14 ( 0.28 0.25 ( 0.05 0.088 ( 0.013 0.035 ( 0.001 0.025 ( 0.005 0.046 ( 0.015 0.046 ( 0.015

a Considering that the active substance is 30 wt %, the real surfactant concentration is 0.075 wt %.

hydrodynamic particle diameter. The resulting layer thickness, 4 ( 1 nm, is approximately the same in the presence and absence of NaCl. Comparison of the Stability of Surfactant-Free PS Particles on Addition of Different Surfactants. The stability of surfactant-free PS particles was studied on addition of different surfactants and using CaCl2 as electrolyte. The concentration of PS particles in the dispersion was 5 wt % and the surfactant concentration added was 0.25 wt %. The results obtained are shown in Table 1. As mentioned above, the stability of PS particles without adding any surfactant to the dispersion was low (0.0075 ( 0.0025 mol‚dm-3) and it increased considerably when adding a 0.25 wt % of INUTEC SP1 up to a CCC value higher than 4.37 M. When adding the Surfonic N-800 at an active surfactant concentration of 0.075 wt %, the stability was lower. The other surfactants tested showed low stability to PS particles, even Maltodextrin DE 28, Glucidex DE 28, and Glucidex DE 47, which are molecules with similar chemistry. PMMA Particles Using INUTEC SP1 as Emulsifier. In this study, PMMA particles were prepared by surfactant-free emulsion polymerization with the same initiator, monomer concentration, and reaction conditions as surfactant-free PS particles described above. However, flocculated particles were obtained. PMMA particles were then prepared by adding INUTEC SP1 as emulsifier keeping MMA and K2S2O8 concentrations at a constant value of 5 wt % and 0.0125 wt %, respectively, for INUTEC SP1 concentrations ranging from 0.005 to 0.5 wt %. Stable PMMA dispersions were only obtained for INUTEC SP1 concentrations of 0.005 and 0.007 wt %. The diameter of these particles were 129.1 and 148.9 nm, respectively, and they were obtained after 24 h of reaction. The stability of PMMA particles containing 0.007 wt % in INUTEC SP1 was studied using CaCl2 and a CCC value of 0.010 ( 0.005 mol‚dm-3 was obtained. However, their stabilities were significantly increased on addition of more INUTEC SP1 after the polymerization up to final concentration of 0.25 wt % polymeric surfactant. The CCC value obtained using CaCl2 was higher than 3.48 mol‚dm-3. PMMA particles were also prepared varying the monomer concentration from 5 to 40 wt % and keeping constant the surfactant concentration (0.007 wt %). The MMA contents, the diameter, and the polydispersity indexes of the resulting particles are shown in Table 2. Figure 5 shows that the stability of PMMA particles depends on particle size and the bigger the particles the less stable they are, which is in accordance with the literature on particles with an adsorbed layer of nonionic surfactants.30,31

Table 2. Diameters and Polydispersity Indexes Obtained in PMMA Particles Synthesized for Different Monomer Concentrations at [INUTEC SP1] ) 0.007 wt % and [K2S2O8] ) 0.0125 wt % [MMA]/wt %

diameter/nm

polydispersity index

5 10 15 20 30b 40b

148.9 189.3 484.3 (512.8a) 445.5 (450.5 a)

0.023 0.038 0.045 0.172

a Samples were repeated and similar results were obtained. Samples were aggregated and particle size could not be determined.

b

Figure 5. Variation of CCC values measured with CaCl2 and diameter of PMMA particles as a function of MMA concentration in the final dispersion. MMA concentration is half the initial because of dilution after CaCl2 addition.

Figure 6. Variation of PS and PMMA particle diameter, obtained by emulsion polymerization using INUTEC SP1 as emulsifier, as a function of the [INUTEC SP1]/[Polymer] concentration ratio.

Comparison of PS and PMMA Particles Obtained Using INUTEC SP1 as Emulsifier. PS and PMMA particle size was studied as a function of INUTEC SP1 concentration keeping constant the monomer and initiator concentration at 10 and 0.0125 wt %, respectively. The results obtained are shown in Figure 6. It can be observed that PS and PMMA particle size decreases when increasing [INUTEC SP1]/[Polymer] concentration ratio. Very similar sizes were obtained for both PS and PMMA particles, indicating that size is mainly determined by the surfactant concentration. PS and PMMA particles containing 0.001 wt % in INUTEC SP1 were used so as to study their stability on addition of the polymeric surfactant. The electrolyte used for these experiments was again CaCl2. Figure 7 shows that for both PS and PMMA particles, the lowest values of CCC measured with CaCl2 correspond (30) Otewill, R. H.; Walker, T. J. Chem. Soc., Faraday Trans. 1 1974, 70, 917. (31) Hsu, J. P.; Liu, B. T. J. Colloid Interface Sci. 1998, 198, 186.

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Conclusions

Figure 7. Variation of CCC values of PS and PMMA particles, obtained by emulsion polymerization using INUTEC SP1 as emulsifier, as a function of the [INUTEC SP1]/[Polymer] concentration ratio.

to the samples without surfactant: 0.02 ( 0.01 and 0.0075 ( 0.0025 mol‚dm-3, respectively. A sharp increase in stability was found above a [INUTEC SP1]/[Polymer] concentration ratio of 0.024, independently of the monomer. As a result, it can be said that INUTEC SP1 could stabilize PS and PMMA particles with a relatively low [INUTEC SP1]/[Polymer] concentration ratio of 0.03.

INUTEC SP1, which is a new class of graft copolymer surfactant, can stabilize surfactant-free PS particles. The CCC measurements, obtained by optical microscopy observations, showed that the stability of PS particles can be achieved on addition of INUTEC SP1 at [INUTEC SP1]/[PS] ratios higher than 0.002. Such stabilization effect could be explained in terms of steric repulsion due to the formation of surfactant loops on the particle surface, which would happen at low surfactant concentrations. The adsoption of this surfactant, detected by an increase of the particle hydrodynamic diameter, rapidly increased at small surfactant concentrations. The stability effect of INUTEC SP1 has not been observed among other kinds of surfactants studied, namely, ionic, alcohol ethoxylated, and other polymeric surfactants. Acknowledgment. The authors gratefully acknowledge financial support from ORAFTI Non-Food and Generalitat de Catalunya DURSI (grant 2001 SGR-00357). The authors also want to acknowledge Nu´ria Azemar for her assistance with the PCS measurements. LA035092V