J . Phys. Chem. 1990, 94, 5290-5292 A E ? KJimole
15 1-
1501
135) 0
'1
2
'
3
I
I ' I
4
5
1
I
I
6 7 8 9 At-A1 DISTANCE
I 10
(81
Figure 5. Framework stability as a function of AI-AI distance related to the location of Ni2+ions in Ni-Aluminosilicate mordenite. Decreasing AE means increasing Ni2+localization preference. The line is drawn as guide for the eye.
vations. Firstly, the small differences in the relative lattice energies for Ni2+located at the four different extra-framework sites indicate hardly any preferences on structural grounds. Secondly, the relative lattice energies of the Nialumina mordenites are observed to depend on the existence of specific A I U ( S i - O ) r A I sequences in combination with a Ni2+ ion in close proximity (compare relative lattice energies in Tables 111 and IV). In particular, sequences with N = 1 turned out to be very stable in comparison to sequences with N > 1. Not only the value of N but also the specific location of the AI-0-Si-0-AI sequences in the mordenite structure proves to
be important. In this case ( N = I), our calculations clearly show that Ni2+ is preferentially located in the 8-ring side pocket, followed by location in the 8-ring secondary pore system. Location of Ni2+in the 12-ring main channel proves to be less favorable. These findings are illustrated in Figure 4. It should be noted that the choice of particular AI-0-(SiO),-AI sequences studied in this paper is quite arbitrary. However, apart from N values for the various zeolite framework sequences, the AI-AI distances in the sequences also play an important role, allowing us to regard the sequences chosen as general representatives of sequences anywhere in the lattice. In particular in the case of Ni2+ located in the mordenite 8-ring secondary channel this AI-AI distance effect is clear. The fairly high stability of Ni2+ ions coordinated to AI-O-(Si-O)rAI sequences (N = 2, 3) in the mordenite 8-ring channel with respect to coordination to similar sequences in the 12-ring main pore system or the 8-ring side pocket is obviously (partly) related to AI-AI distances, as displayed in Figure 5 . At this point, it is worth noting that two AI framework atoms in close proximity to each other will result in an AI-AI repulsion due to the introduction of negative charge on the framework. However, this repulsion seems to be negligible compared to the interaction of these two AI atoms with a Ni2+ion. Figure 5 clearly visualizes this feature. In spite of the qualitative character of the calculations described in this paper, we feel that our study strongly supports the conclusions made earlier regarding the exchange properties of Ni2+ in siliceous zeolites.' The necessary occurrence of AI-0-(SiO)N-Al (with N I3) sequences in the zeolite lattice to anchor Ni2+explains the limited degree of Ni2+ ion exchange in siliceous zeolites: for a random distribution of aluminum (which we feel is for siliceous zeolites justified7) throughout the framework, the statistical probability of finding the necessary sequences is very small and thus exchange capacity is limited. Acknowledgment. We thank Dr. M. Leslie (SERC Laboratories, Daresbury, UK) for assistance with a number of computational problems experienced during this work. Registry No. Ni, 7440-02-0.
Closed and Open Structure Aggregates in Microemulsions and Mechanism of Percolative Conduction Amarnath Maitra,* Charily Mathew, and Manoj Varshney Department of Chemistry, Delhi University, Delhi I 10 007, India (Received: April I O , 1989; In Final Form: January 17, 1990)
The electrical conductivity and water self-diffusion coefficients of the oil-continuous microemulsions of Aerosol OT at the onset of the percolation process as well as in the bicontinuous system have been measured. Low self-diffusion coefficients of water indicate that the droplets maintain their "closed" shell structure. It has been concluded from the results that the percolative conduction arises due to the hopping of the charge carriers across the surfactant monolayer of the droplets, and the aggregates open up only at the onset of formation of bicontinuous microemulsions.
Introduction The electrical conductivity of the disordered systems is often interpreted by using the concept of percolation transition which signifies the existence of the conducting species in a conductorinsulator composite material.' Measurements of the conductivity ( I ) Percolation Structure and Process. Dcutscher, G.; Zallen, R.;Adler, J., Eds. In Annals of the Israel Physical Sociefy; Israel Physical Society: Jerusalem, Israel, 1983; Vol. 5 .
of oil-continuous microemulsions have indicated the existence of a percolation threshold which is dependent not only on the volume fraction but also on other factors like temperature, rigidity, and thickness of the surfactant monolayer of the droplets, etc., and by changing these parameters the percolation threshold may appear even at significantly small volume fraction of the conducting Some studies of these systems have at(2) Cazabat, A. M.; Langevin, D.; Meunier, J.; Ponchelin, A. J. Phys. Left. 1982, 43, L89.
0022-3654/90/2094-5290%02.50/0 0 1990 American Chemical Society
Aggregates in Microemulsions
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5291 HZO /O.lM AOT/lSOOCTANE
IO'C
-r
J
t
I
I
1
I - .
d
b
tt
d
t ,671
I
I6'O
20
30
,
1
40
l
