Stability and electric parameters of AgJ-sols in the presence of a surface active substance

Abstract

AgJ-sols are widely used as a model system for the investigation of the double layer behaviour [1]. It is known relatively much about the structure of the Stern layer, the point of zero charge is got very precisely by different methods of investigation. In previous investigations it was found a special interrelation between stability and electric parameters [2,3]. Therefore it was interesting to study the influence of the adsorption of a surface active substance, in the present case DDAO (dimethyldodecylamine oxide) [4], on the electric parameters and the stability of the AgJsols. The electric parameters in the present study are the permanent dipole moment and the electrical polarizability. The last one is connected with the dynamical behaviour of the double layer. The electro-optical methods of investigation (electric light scattering, electric birefringence, electric dichroism) and the electro-orientational methods (conductivity, permitivity) give the possibility to characterize the electric state of anisometric particles by their electric moments (permanent and induced) [51. Theoretical considerationsThe orientation of anisometric particles in an electric field is due to the interaction of their electric moments with the applied electric field. The basic assumption of the theory is that the particles are considered as ellipsoids of rotation. The permanent dipole moment is assumed to be directed along the symmetry axis of the particles and the electric polarizability has two components, one along the symmetry and one along the transverse axis. The orientation of the particles lead to a change of the optical behaviour of the system, thus the scattered light changes and the system shows birefringence [6].For low electric field strengths with a frequency c0 the birefringence is given by the equation [7]:where Ano is the low frequency limit of dnst; 71, 72 are the polarizabilities along the symmetry and transverse axis, respectively, # is the permanent dipole moment along the symmetry axis, and D is the rotation diffusion coefficient.The electric birefringence in a dc-field is given by the equation [7]:where 2V is the number of the particles per unit volume, n the mean optical index of the medium, (gl--g2)opt the optical anisotropy factor, k is the Boltzmann constant, T the absolute temperature and /~= the dc-field W 977 1122Colloid and Polymer Science, VoL 257 • No. 10 (1979) strength. Equation [2] shows that the slope of the curve dn0 vers. E 2 at E= -+0 is proportional to the sum of the electric moments. The investigation of the electric birefringence at suitable frequencies gives the possibility to separate the action of the induced dipole moment from that of the permanent one. If the frequency of the applied electric field is high enough the action of the permanent moment disappears (provided that the nature of the permanent moment is connected with the volume of the particle). In this case the orientation of the particles is only due to the induced moment:Obviously the investigations of the electric biref...