radius of the order of 15 nm in 2-butoxyethanol (abbreviated In the temperature-composition diagram of the system 2-butoxy-C 4 E 1 )/water mixtures is studied in the vicinity of the ethanol (abbreviated C 4 E 1 )/water, SiO 2 [mass fraction of SiO 2 , lower critical point of that system. It is found that SiO 2 y(SiO 2 ) É 0.01; volume fraction f(SiO 2 ) É 10 03 ] there exists a particles dispersed in C 4 E 1 /water mixtures with a composiflocculation temperature-composition curve running below the tion x ú x c (x, mole fraction of C 4 E 1 ; index c, critical compobinodal curve for x ú x c (x, mole fraction of C 4 E 1 ; x c Å 0.0598).
sition) flocculate at a certain flocculation temperature T floc .
The concentration of the colloidal particles is low enough to con-T floc is lower than the temperature of phase separation T P of sider them an ''impurity.'' In the region of the phase diagram the mixture and is a function of the composition of the bound by the binodal and the flocculation curve the suspension of dispersion medium. For x õ x c the suspensions are stable the colloidal SiO 2 particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase sepa-up to T P . The system C 4 E 1 /water was chosen for this study ration and the flocculation temperature increases with increasing for two reasons: (1) information is available about the prefvalues of x 0 x c up to a maximum value of x (x max ). For x ú x max erential adsorption of the components of the mixtures on the suspensions are unstable at all temperatures studied. For porous glass with a hydrophilic and hydrophobic surface x õ x c the suspension of the SiO 2 particles is stable up to the near the binodal curve (10, 11); (2) results of the stability temperature of phase separation of the C 4 E 1 /water mixture. The of a dilute suspension of polystyrene spheres in C 4 E 1 /water flocculation curve is assumed to reflect the influence of local conmixtures have been reported (12). centration fluctuations with long range correlations on the stability of the SiO 2 suspensions. For x ú x c these concentration fluctuations II. EXPERIMENTAL AND RESULTS are water rich and interact with the hydrophilic surface of the colloidal SiO 2 particles by forming an adsorption layer. This layer The colloidal SiO 2 suspension used in this study is a comis assumed to modify the interparticle potential energy-distance mercial product (Levasil 100/30, Bayer AG, Leverkusen, curve and to trigger flocculation. For x õ x c the concentration fluctuations are C 4 E 1 rich and no adsorption layer is formed at Germany). The mass fraction of the solid material in the the hydrophilic surface of the colloidal particles. ᭧ 1997 Academic Press aqueous stock dispersion has a value of about y(SiO 2 ) É Key Words: suspension; colloidal silica; stability; binary liquid 0.30 (13). The size distribution of the particles determined mixture; binodal curve.
by electron microscopy is available from Dr. Schober (Bayer AG, Leverkusen, Germany). The distribution is broad [6 nm õ (2r) õ 150 nm] with a pronouced maximum