12250
Langmuir 2005, 21, 12250-12256
Adsorption of Non-Ionic Surfactants on Hydrophobic Silica Particles and the Stability of the Corresponding Aqueous Dispersions Phillip J. Dale,† Johan Kijlstra,‡ and Brian Vincent*,† School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS United Kingdom, and Bayer Technology Services, D51368 Leverkusen, Germany Received August 5, 2005. In Final Form: October 1, 2005 The temperature stability of aqueous dispersions of hydrophobic monodisperse silica particles stabilized with nonionic surfactants has been investigated. Adsorption isotherms in conjunction with surface tension measurements showed that the surfactant formed a monolayer on the surface of the particles, where the adsorbed amount depended on the molecular weight of the ethylene oxide headgroup. The temperature stability of these dispersions has been measured by a standard turbidimetric technique and visual observations in terms of their critical flocculation temperature (CFT). Parameters controlling the CFT of the individual dispersions stabilized with a monolayer of surfactant include the thickness of the steric layer, the particle size, and the volume fraction of the particles. Calculations show that the van der Waals attraction between the particles with adsorbed polymer layers increases as the temperature of the dispersion increases, and this largely accounts for the observed CFT behavior.
1. Introduction This paper is concerned with the temperature stability of aqueous dispersions of hydrophobic particles, sterically stabilized with various adsorbed nonionic surfactants, all having an ethoxylate chain as the hydrophilic moiety. Such dispersions are used in a wide variety of industries, including the agrochemical, coatings, healthcare, food, and paint industries. Upon heating above some critical temperature (the so-called “critical flocculation temperature”, CFT) such dispersions are often found to slowly flocculate. Napper1,2 showed, for dispersions of poly(vinyl acetate) particles, with terminally attached poly(ethylene oxide) (PEO) chains, in a variety of electrolyte solutions, that a strong correlation exists between the CFT and the corresponding theta (θ) temperature of the PEO chains, at the electrolyte concentration concerned. Napper also showed that the CFT was more-or-less independent of the PEO molecular weight. By way of contrast, Luckham et al.3 found, for carbon black dispersions, stabilized by various adsorbed nonionic surfactants, in aqueous 0.1 M Na2SO4 solution, that the greater the molecular weight of the PEO group, the higher the CFT. Thompson and Pryde4 found that the CFT of aqueous dispersions of polystyrene latex particles, stabilized with adsorbed C12E8 (where C12 is an alkyl chain with 12 repeat units, and E8 is an ethoxylate chain with 8 repeat units) decreased with increasing particle size. Cowell and Vincent5 reported that the CFT of an aqueous dispersion of polystyrene latex particles, grafted with low molecular weight PEO chains, decreased with increasing particle volume fraction (φ). * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Bristol. ‡ Bayer Technology Services. (1) Napper, D. J. Colloid Interface Sci. 1970, 32, 106-114. (2) Napper, D. J. Colloid Interface Sci. 1970, 33, 384-392. (3) Luckham, P. F..; Bailey, A.; Miano, F.; Tadros, Th. F. Am. Chem. Soc. Symp. Ser. 1995, 615, 167-182. (4) Thompson, L.; Pryde., J. Chem. Soc, Faraday Trans. I 1981, 77, 2405-2410. (5) Cowell, C.; Vincent, B. J. Colloid Interface Sci. 1982, 87, 518526.
Similarly, Bevan and Scales6 found that the CFT of dispersions of polystyrene latex particles, with adsorbed PEO-poly(propylene oxide)-PEO (Pluronic F108), is φ dependent. These authors were able to demonstrate, using the total internal reflectance microscopy method7 to determine the depth of the free energy minimum (Gmin) in the pair-potential between the particles, that the magnitude of Gmin increased with increasing temperature (and hence the poorer solvent quality of aqueous solutions for the stabilizing PEO chains). Cowell and Vincent5 suggested that a strong analogy exists between the weak, reversible flocculation behavior of sterically stabilized dispersions and the condensation of a vapor for molecular systems. Plots of CFT versus φ are the direct analogues of vapor/solid (or liquid) temperature-molecule number density (or pressure) phase boundaries. Above the CFT, the dispersion separates into two coexisting colloidal phases: a low φ, dispersed phase (cf. the vapor), and a higher φ, reversibly flocculated phase (cf. a condensed molecular phase). This close correspondence arises because the form of the interparticle pair-potential is very similar to that for pair interactions between molecules in a vapor (i.e., the Lennard-Jones potential). The only major difference is that the interparticle pair potential is much more strongly temperature-dependent than the pair potential for molecules. For particles with adsorbed nonionic surfactants, because the solvency of the PEO chains in water decreases with increasing temperature, this means that CFT decreases with increasing φ, whereas for vapors, the temperature at which condensation usually occurs increases with increasing molecule density (or pressure). In this paper, we extend this previous work to describe the weak, reversible flocculation behavior of aqueous dispersions of hydrophobised silica particles, carrying adsorbed nonionic surfactants, all containing ethoxylate moieties. In particular, the CFT of these dispersions has been studied as a function of the following variables: the (6) Bevan, M.; Scales, P. Langmuir 2002, 18, 1474-1484. (7) Prieve, D.; Frej, N. Langmuir 1990, 6, 396-403.
10.1021/la052141q CCC: $30.25 © 2005 American Chemical Society Published on Web 11/15/2005
Stability of the Corresponding Aqueous Dispersions
Langmuir, Vol. 21, No. 26, 2005 12251
Table 1. Details of the Silica Particles Useda
of nitrogen gas on the particles (Quantachrome Autosorb-1), using the BET and t-plot methods. The density of the bare particles was measured using a DMA60 (Anton Paar) density meter. To investigate whether the grafting process had been successful, Fourier transform infrared (FTIR) spectra (Spectrum One FTIR Spectrometer, Perkin-Elmer) and X-ray photoelectron spectroscopy (XPS) spectra (ESCALAB 220 iXL, Thermo-VG Scientific) of the bare and 1-octadecanol grafted particles were compared. The grafting density of the 1-octadecanol layer on the silica surface was calculated from thermogravimetric analysis (TGA) measurements of the bare and grafted particles, using a STA 409EP instrument (Netzsch). To this end, the particles were initially dried to a powder from their respective dispersions in a vacuum oven at 40 °C overnight. Powdered samples were then placed into the TGA instrument and equilibrated at 120 °C for 3 h to remove surface water and then heated to 350 °C for a further 3 h. By subtracting the mass loss of the bare particles from that for the grafted particles over this temperature range, the mass corresponding to the grafted layer and the area per grafted molecule could be calculated. 2.3. Surfactant Adsorption Isotherms. Adsorption isotherms were determined for C12E24 and all of the Soprophor surfactants on the 1-octadecanol grafted silica particles by two methods. The first method involved simply mixing the dry, grafted particles with aqueous solutions of the appropriate surfactant. The second method involved dissolving the surfactant in tetrahydrofuran (THF), before mixing it with grafted particle dispersions in THF and then drying the resultant mixture in a vacuum oven overnight at 40 °C. Water was subsequently added to the dried surfactant/particle mixture. These mixtures were then left tumbling for one week and after this centrifuged at 3000 rpm to leave a clear supernatant phase. The C12E24 supernatant concentration was determined by the phosphomolybdic acid method,11 whereas the Soprophor surfactant concentration was determined using the corresponding UV-Vis spectra between wavelengths of 196 and 210 nm. 2.4. Critical Flocculation Temperatures. For the CFT measurements, all of the dispersions were prepared by the second method outlined in the adsorption isotherm section above. The equilibrium free surfactant concentration for all of the dispersions was adjusted to be at the onset of the plateau region of the corresponding adsorption isotherm. The dispersions are coded by their constituent particles and the surfactant used (see also Table 1). For example, GG1008Ts29 refers to the 1-octadecanol grafted Geltech silica particles, dispersed with Ts29. The dispersions were briefly sonicated to break up any particle aggregates. PCS was used to check that the dispersions were free from aggregates. The CFT of the aqueous dispersions of the 1-octadecanol grafted particles, with the various adsorbed surfactants present, was determined by two different methods. These are outlined below, dependent on the particle volume fraction range being studied. Dilute Dispersions (φ ) 9 × 10-5 to 3.6 × 10-3). The turbidity (τ)/wavelength (λ) spectra of the dispersions, containing 0.050.5 mol kg-1 MgSO4 were recorded as a function of increasing temperature. The method of Long et al.12 was then used to determine the CFT. At the CFT, a sharp break is observed in a plot of n () d log τ/d log λ) versus temperature. The wavelength range used was between 600 and 800 nm. Concentrated Dispersions (φ ) 0.02-0.2). Sealed, 1 mm path length cuvettes (Helma) containing the dispersions were equilibrated in a controlled temperature bath for 2 h and then visually inspected. Dispersions containing 0.3 mol kg-1 MgSO4 were compared with samples containing no added electrolyte, at the same particle volume fraction. Observations were made for the appearance of a flocc phase, in the form of a sedimented layer, which would indicate that the particles had (weakly) flocculated. If no sediment was observed, the temperature of the bath was increased by 2 °C and the samples were reequilibrated for a further 2 h. The dispersions were then observed again and the
diameter samplea
PCS/nm
TEM/nm
density/(kg m-3)
SB196 SG196 SB554 SG554 GB1008 GG1008
196 ( 2 195 ( 4 554 ( 4 550 ( 4 1008 ( 8 1080 ( 8
179 ( 9 192 ( 10 515 ( 15 521 ( 15 1106 ( 22 1133 ( 33
2200 1876 2022
a S ) Sto ¨ ber, G ) Geltech; B ) bare, G ) grafted with 1-octadecanol.
particle diameter and volume fraction, the structure of the surfactant, and the background electrolyte concentration. In a previous paper,8 we reported on how the thickness of one of the nonionic surfactants used in the present study, adsorbed on the same particles, varied as a function of temperature, as determined using small-angle neutron scattering. As will be demonstrated in this paper, this thickness variation is a key factor in controlling the stability of the dispersions as the temperature is increased. 2. Experimental Section 2.1. Materials. Two types of silica particles were used in this work: (i) commercial particles (ex Geltech) and (ii) those prepared using the Sto¨ber synthesis.9 Sto¨ber silica particles are formed by the aqueous hydrolysis and condensation of tetraethoxysilane (98% Aldrich) in the presence of ethanol (Fisher) and aqueous ammonia (33% Fisher). Both the Sto¨ber and Geltech silica particles were chemically grafted with 1-octadecanol (95% Aldrich) using the method of van Helden et al.10 Details of the various particles used are given in Table 1. Four nonionic surfactants were used in this work, an alkyl ethoxylate C12E24 (ex Dr. R. K. Thomas, University of Oxford) and three nonionic surfactants Ts16, Ts29, and Ts54, from the Soprophor series (ex Rhodia). These latter three each contained a tristyrylphenol (Ts) tail group and (nominally) 16, 29, or 54 ethylene oxide (EO) units, respectively. NMR analysis showed that in fact each Soprophor surfactant contained a mixture of biand tristyrylphenol tail groups. Matrix-assisted laser-desorption/ ionization mass spectrometry showed that Ts16, Ts29, and Ts54 had actual number averages of 18, 27, and 48 EO units, respectively. Gel permeation chromatography measurements showed that all the surfactants had Mw/Mn values of 1.12 or less. The surface area per surfactant molecule at the air/water interface was determined for the various surfactants used from the surface tension-concentration plots that were obtained using a K100 Processor Tensiometer instrument (ex Kruss). Water, with a resistivity of ∆Ghs in magnitude, then weak, reversible flocculation occurs, leading to colloidal phase separation, as demonstrated in the video experiments carried out in this work. The above rational may be used to account for the results shown in Figure 4. At a given value of φp, as the temperature is increased, so Gmin increases, until ∆Gi reaches the value of ∆Ghs for that value of φp. This defines the CFT “phase boundary” (i.e., ∆Gfloc ) 0, eq 1). With increasing φp, a lower value of ∆Gi and, hence, of Gmin is required, in order to reach the condition ∆Gfloc ) 0. Hence, the CFT decreases with increasing φp. Note that both ∆Ghs () -T∆Shs) and ∆Gi are temperature dependent, but since the absolute changes in T (K) over the range considered in this work are very small, it is the change in ∆Gi and, hence, of Gmin, with temperature, which must be considered to play the dominant role. To understand how Gmin varies with temperature, it is necessary to return to a consideration of the temperature (17) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (18) Vincent, B.; Luckham, P. F.; Waite, F. A. J. Colloid Interface Sci. 1980, 73, 508-521. (19) Feigin, R.; Dodd, J.; Napper, D. H. Colloid Polym. Sci. 1981, 259, 1027-1030. (20) Osmond, D. W. J.; Vincent, B.; Waite, F. A. J. Colloid Interface Sci. 1973, 42, 262-269. (21) Boucher, E.; Hines, P. J. Polym. Sci. 1976, 14, 2241-2251. (22) Israelachvili, J. Intermolecular and Surface Force, 2nd ed.; Academic Press: London, 1991. (23) Vincent, B. J. Colloid Interface Sci. 1973, 42, 270-285.
Stability of the Corresponding Aqueous Dispersions
Langmuir, Vol. 21, No. 26, 2005 12255 Table 4. Parameters Used in Calculating Gvdw for SG196 with Adsorbed C12E24, at 25 and 72 °Ca Hamaker constants 1020/J component particle inner layer outer layer medium
geometrical parameters /nm
25 °C 72 °C Ap Ai Ao Am
6.5 5.2 4.2 3.7
6.5 5.2 4.8 3.5
volume fractions
25 °C 72 °C R δi δo
93 2.3 7.3
93 2.3 4.3
25 °C 72 °C φi φo
1.00 0.18
1.00 0.30
a Hamaker constants were taken from ref 22, and δ and φ values from ref 8.
Figure 5. Schematic diagram of how Gi varies with separation (h) for two sterically stabilized particles, in a good solvent.
Figure 6. Symbols and geometry used in the calculation of Gvdw between two particles each with two sheaths.
dependence of the interparticle pair potential. As stated earlier, the value of Gmin depends on Gvdw, Gsteric, and Gelectro, each as a function of h. In this study, Gelectro can be neglected as electrophoresis measurements showed that the grafted particles stabilized by nonionic surfactants had zero mobility in 0.05 mol kg-1 MgSO4 solution, and all of the flocculation experiments were performed at higher electrolyte concentrations. Both Gvdw and Gsteric vary with temperature. These variations are considered in turn below. Under conditions where Gsteric is positive (repulsive), Figure 5 illustrates, schematically, the form of the interparticle pair potential. The only contribution to Gmin in this case comes from Gvdw.Osmond et al.20 derived equations for Gvdw between two spheres, each surrounded by a shell (or shells). It is proposed that the hydrophobised silica particles, with adsorbed nonionic surfactant, may be modeled using a double shell model, similar to that used to interpret the SANS experiments, and depicted in Figure 6. It is assumed that the inner shell consists of 1-octadecanol and the hydrophobic tails of the nonionic surfactant and has the same thickness (δi) at both temperatures. The outer shell is assumed to consist of PEO and water. The thickness of the outer sheath (δo) and the mean volume fraction of PEO (φo) in the outer sheath depend on the temperature. The values for δi and δo, and for φi and φo, determined using small angle neutron scattering, as reported in the previous paper,8 are listed in Table 4. The equation for calculating Gvdw between double-shell particles, namely the “Vincent-Vold” equation,20 is given in Appendix A. Figure 7 shows calculated plots of Gvdw(h) for the SG196 particles with adsorbed C12E24, at 25 and
Figure 7. Gvdw as a function of h, calculated using the VincentVold equation, for SG196C12E24 particles at 25 °C (s) and 72 °C (- - -). Gvdw(h) calculated for the bare SB196 particles at 25 °C (...) for comparison.
72 °C, based on the data listed in Table 4. Also plotted for comparison is Gvdw(h) for the core SB196 particles at 25 °C without the adsorbed shells (δ ) δi + δo ) 0). It can be seen that addition of the double shell significantly decreases the attraction between the particles, compared to the bare particles at 25 °C. It may also be seen that the particles with adsorbed C12E24 have a greater attraction at 72 °C than at 25 °C. As it is difficult to know the value of h to take to represent particle “contact”, the exact values of Gmin cannot be determined. However, one might expect h to be < 1 nm. This would give Gmin values of a reasonable order of magnitude and show that, indeed, Gmin is greater at 72 °C than at 25 °C, as mentioned above in relation to the discussion of Figure 4. In Figure 8, similar Gvdw(h) plots, at 25 °C, are shown for the SG196 and SG1008 particles, stabilized with either Ts29 or Ts54. In these calculations, it was assumed that φo ) 0.2 and that δo scales linearly with PEO chain length (using the value for E12 determined from the SANS experiments). It can be seen that for the SG196 particles there is little effect of PEO chain length on the Gvdw(h) plots, and hence Gmin, but a greater effect of PEO chain length is observed for the larger GG1008 particles. Moreover, Gmin would seem to be significantly greater for the GG1008 particles compared to the SG196 particles. These calculated results explain very well the trends for the various experimental CFT-MgSO4 concentration plots shown in Figure 3. The fact that the these CFT-MgSO4 concentration plots span the θ temperature-MgSO4 concentration plots for PEO (also shown in Figure 3) is interesting but probably of no direct significance. One might suppose that, under worse than θ conditions, the
12256
Langmuir, Vol. 21, No. 26, 2005
Dale et al.
-12Gvdw ) (Ao1/2 - Am1/2)2Ho + (Ai1/2 - Ao1/2)2Hi + (Ap1/2 - Ai1/2)2Hp + 2(Ao1/2 - Am1/2)(Ai1/2 - Ao1/2)Hio + 2(Ai1/2 - Ao1/2)(Ap1/2 - Ai1/2)Hip + 2(Ao1/2 - Am1/2)(Ap1/2 - Ai1/2)Hop (2) where Ap, Ai, Ao, and Am are the appropriate Hamaker constants for the particle, inner sheath, outer sheath, and the medium, respectively (see Table 4). Ho, Hi, Hp, Hio, Hip, and Hop are the geometrical functions defined below
H(x,y) )
y y + 2 + x + xy + x x + xy + x + y x2 + xy + x (3) 2ln 2 x + xy + x + y 2
x ) ∆/R1, y ) R2/R1 Figure 8. Gvdw between two particles stabilized with either Ts29 (s) or Ts54 (- - -), calculated using the Vincent-Vold equation, where the particle is either SG196 or GG1008, respectively.
Gsteric term would be negative. However, it seems highly improbable that the concept of a “θ temperature” per se applies to such short PEO chains as those present in the surfactants used in this work. This contrasts directly with the earlier findings of Napper et al.,1,2 referred to in the Introduction, who showed a strong correlation between CFT values and the corresponding θ temperature of the stabilizing polymer chains. However, in most of Napper’s work, these chains were of significantly higher MW than those used here. Moreover, as demonstrated in Table 3, the strong agreement between the As/w and Aa/w values for the various surfactants used in this work indicates that the adsorbed surfactants do pack very tightly at the solid/ solution interface. This implies that the PEO chains on two approaching particles would not interpenetrate to any significant extent, especially during the relatively short time scale of a typical Brownian collision, even under worse than θ conditions. Hence, one may conclude that Gsteric makes no real contribution to Gmin per se. The value of Gmin is primarily determined by Gvdw and the changes that occur to Gvdw on changing the PEO chain length and temperature, as described earlier. 5. Conclusions The critical flocculation temperature (CFT) of dispersions stabilized with nonionic surfactants has been found to depend on particle size, steric layer type and molecular weight, the nature of the solvent, and also the volume fraction of the dispersion. Small angle neutron scattering data suggested that the combined layer thickness of 1-octadecanol C12E24 adsorbed on SG196 decreases on raising the temperature from 25 to 72 °C. It has been shown that changes in the long range van der Waals attraction between approaching particles, with increasing temperature, largely accounts for the observed CFT behavior and that osmotic (mixing) effects associated with interpenetration of the adsorbed surfactant layers play little, if any, role. Appendix A: Calculation of Gvdw The Vincent-Vold equation for the total van der Waals attraction between two particles each with a double polymer sheath is given by eq 2.
where for
Ho:∆ ) h, R1 ) R2 + δ Hi:∆ ) h + 2δo, R1 ) R2 + δi Hp:∆ ) h + 2δ, R1 ) R2 ) R Hio:∆ ) h + δo, R1 ) R + δi, R2 ) R + δ Hip:∆ ) h + δo + δ, R1 ) R, R2 ) R + δi Hop:∆ ) h + δ, R1 ) R, R2 ) R + δ The parameters used to make the calculations are given in eq 4. On raising the temperature from 25 to 72 °C, four parameters change; these are Ao, Am, δo, and φo. The Hamaker constant for a pure material may be expressed by eq 4
A ) π2CF2
(4)
where C is the coefficient in the corresponding molecular pair potential and F is the number density of molecules per unit volume. Assuming C does not change with temperature, eq 4 can be used to calculate the Hamaker constant of water at 72 °C. It is assumed that the presence of MgSO4 has little or no effect on the Hamaker constant of the medium. It is assumed that the Hamaker constants of the silica particles and the alkane sheath do not change with temperature. The Hamaker constant of the outer sheath Ao may be calculated from the Hamaker constants of water (Am) and PEO (APEO) using the following equation derived by Vincent23
Ao ) [φoAPEO1/2 + (1 - φ0)Am1/2]2
(5)
where φo is the volume fraction of PEO in the outer sheath. Acknowledgment. We thank Dr. R. K. Thomas (Oxford University) and Rhodia for supplying the surfactants and Dr. S. Holding (RAPRA) for GPC measurements. P.D. acknowledges Bayer Technology Services, Bayer Crop Science, for (partial) financial and technical support, and the EPSRC for funding support for his Ph.D. studies. LA052141Q
