Langmuir 1985,1, 456-461
456
Mechanisms of Water Interaction with an MgO Surface Elson Longo,? Jose A. Varela,f Albert0 N. Senapeschi,? and 0. J. Whittemore*s Universidade Federal de Sdo Carlos, Departamen_to de Quimica, 13.560 Sdo Carlos, SP, Brazil, Universidade Estadual Paulista, Znstituto de Quimica de Araraquara, 14.800 Araraquara, SP, Brazil, University of Washington, FB-10, Seattle, Washington 98195 Received October 8, 1984. Zn Final Form: April 4, 1985 The CNDO/2 (complete neglect of differential overlap) method was used to calculate the energies for water interaction with an MgO surface and vacancy formation. The calculations are in agreement with experimental results and show that for low water vapor pressures water dissociates at the surface with formation of a monoprotonated vacancy and MgOH or a double vacancy of magnesium and oxygen and Mg(OH)2. High water vapor pressures favor the formation of diprotonated magnesium vacancies and Mg(OH), or Mg(OH)2-H20.
Introduction The sintering rate of MgO compacts is dependent on the water vapor partial pressure.'-12 Water increases the rate of sintering probably by improving the vacancy formation of a cation or an anion in the lattice. However, the interaction mechanism of water with the surface of the crystal is not well understood. Experimental results show that the sintering rate is proportional to the water vapor partial pressure raised to 0.3-0.5 for low pressures8J0 and to 1.0-1.5 for high pressure.10J2 The currently proposed mechanisms are based on experimental data and can explain to a certain extent the influence of water on sintering for low partial pressures.8J(t12 Theoretical studies lead to two mechanisms to explain the action of water at different pressures. These results indicate the physical adsorption of water on the surface where water interacts with the magnesium in the first stage, followed by vacancy formation.l"l6 This last effect seems to be stronger, leading to the interpretation that water increases the rate of sintering through its decomposition and formation of magnesium hydroxide. In this study attempts will be made to explain the different mechanisms proposed for the influence of water vapor on the rate of sintering, on the basis of the theoretical results obtained by means of proposed models of the MgO crystal surface. Model and Method The calculations were performed using a quantum-mechanics method called CNDOI2 (complete neglect of differential overlap) developed by Pople and his coworker~.'~It applies approximations of ZDO type (zero differential overlap) to Roothaan's equation.l7Js All the electrons of the valence of atoms constituting the molecule are considered. Those of the internal shells plus the nuclei form the nonpolarized molecular "core". Wave functions and energies are computed for molecules of any set of nuclear coordinates. By changing the atomic positions one systematically obtains an energy surface as a function of conformation or geometry. The potential energy surface is employed to indicate which conformations are stable. Charge distributions were derived from the squared wave function at a point in space after integration over defined volumes. Results obtained with quantum-mechanical methods such as the CND0/2 are in agreement with those of ex+ Universidade
Federal de S&o Carlos.
* Universidade Estudual Paulista. University of Washington.
0743-7463/85/2401-0456$01.50/0
perimental methods. In this paper as in previous studies13-16the CNDOj2 method has been employed without modifications in its original structure as proposed by Pople and co-worker~.'~ All models used were optimized by computing the most stable supermolecule energies. In order to calculate the energy of the vacancy formation, the magnesium ion was displaced 0.05 nm from its equilibrium position in an axis direction (x or y or z ) . This direction of displacement was found to be the most probable as was formerly 0b~erved.l~ Figure 1shows the models for the following conditions: (a) a diprotonated magnesium vacancy after the formation of Mg(OH)2,(b) a vacancy with hydrogens in the position in which they would react to form molecular hydrogen, and (c) a magnesium vacancy and H2 formation. The following models are described in Figure 2: (a, a') interaction of the magnesium ion with one water and formation of a vacancy pair (VMg and VO), (b, b') interaction of the magnesium ion with two waters and formation of a vacancy pair (V, and Vo), (c, c' or c") interaction of the magnesium ion with a water molecule followed by its dissociation and formation of a vacancy pair (VMgand Vo) or formation of a magnesium vacancy with a proton (VhlgH),(d, d') interaction of the magnesium ion with two waters followed by their dissociation and formation of a magnesium vacancy with two protons (VZHMg), (e, e') interaction of a magnesium ion with three waters, followed (1)Eubank, W. R.J. Am. Ceram. SOC.1951,34 (8),225. (2) Razouk, R. I.; Mikhail, R. Sh. J . Phys. Chem. 1955,59 (7),636. (3)Anderson, P. J.; Morgan, P. L. Trans. Faraday SOC.1959,55(12), 2203. (4)Kotera, Y.;Saito, T.; Terada, M. Bull. Chem. SOC.Jpn. 1963,36, 195. (5)Clown, J.; Deacon, R. F. Trans. Br. Ceram. SOC.1964,63 (2),91. (6)Anderson, P. J.; Herlock, R. E.; Oliver, J. F. Trans. Faraday SOC. 1965,61 (12),2754. (7)Johnson, H. B.;Johnson, 0. W.; Cutler, I. B. J. Am. Ceram. SOC. 1966,49 (7),390. (8) Eastman, P. F.; Cutler, I. B. J. Am. Ceram. SOC.1966,49(lo),526. (9)Aihara, K.; Chaklader, A. C. D. Acta Metall. 1975,23 (7), 855. (10)Hamano, K.; Aeano, K.; Akiyama, J.; Nakagawa, Z. Rep. Res. Lab. Eng. Mater., Tokyo Inst. Technol. 1979,4,59. (11)Wong, B.;Pask, J. A. J. Am. Ceram. SOC.1979,62 (3-4), 141. (12)Varela, J. A.; Whittemore, 0.J. Mater. Sci. Monogr. 1982,14,439. (13)Longo, E.;Varela, J. A.; Assis, L. R.; Santilli, C. V. Ceramica (Suo Paulo) 1983,29(No. 163),165. (14)Longo, E.;Varela, J. A.; Santilli, C. V.; Whittemore, 0. J. Adv. Ceram. 1984,10,592-600. (15)Santilli, C . V.; Assis, L. R.; Longo, E.; Varela, J. A.; Whittemore, 0.J. Ci8nc. Cult. (Sao Paulo) 1984, 36 (lo),1778. (16)h i s , L. R.;Santilli,C. V.; Longo, E.; Varela, J. A. Ceramica (Suo Paulo) 1984,30 (No. 177),243. (17)Pople, J. A.;Santry, D. P.; Segal, G. A. J. Chem. Phys. 1965,43, s129. (18)Pople, J. A.;Santry, D. P.; Segal, G. A. J. Chem. Phys. 1965,43, S136.
0 1985 American Chemical Society
Water Interaction with an MgO Surface
Langmuir, Vol. 1, No. 4, 1985 457 Recently Sata and Sasamotolg characterized the mechanism of interaction by two reactions which depend on the environment. These can be written in the following forms: MgO(s) + '/2H2O
A
Mg(OH)(g) + '/402(g)
(5)
and MgO(s) + H20
0
b
C
Figure 1. Schematic representation of a diprotonated Mg vacancy: (a) most stable position with 0 = 4 = 4 5 O , (b) two protons in the vacancy at the same direction,and (c) molecular hydrogen in Mg vacancy. by the dissociation of two waters and formation of a magnesium vacancy with two protons (V2Hyg). Figure 3 describes the model of a magnesium vacancy with two protons, followed by the transfer of the protons to one nearest oxygen, with the formation of a new water molecule. This water molecule leaves the surface by forming an oxygen vacancy (Vo).
Results and Discussion In former the interaction of water with the MgO surface was characterized and the dissociation of the water with magnesium vacancy formation was analyzed. The results showed that the reaction path that includes water dissociation is the most probable, resulting in magnesium vacancy formation with Mg(0H) or Mg(OHI2. According to Eastman and Cutler? water is chemically adsorbed at the MgO surface in the following way:
+
Mg2+ 02-+ H20 e Mg2++ 20H-
+ VMg
(1)
where
On the other hand, Hamano et al.1° proposed another mechanism which can be explained in the following way: The water molecule collides on the MgO surface. This collision originates a hydroxide group autside of the lattice and a magnesium vacancy on the surface. This v%cancy can be filled by a magnesium ion of the lattice. After a short time retention, the water molecule leaves the surface creating an oxygen vacancy in the lattice. The proposed mechanism can be described by Mg2+ 02-+ H2O Mg2++ 2(OH-) + V M ~ Mg2+ 02-+ H2O + V M+~ Vo (4)
+
-
A
+
-.+
Mg(OH),(g)
(6)
The chemisorption reactions of water proposed by Eastman and Cutler (eq l ) , by Hamano et al. (eq 41, and by Sata and Sasamoto (eq 5 and 6) do not characterize the existence of the hydrogen atom bonded to the vacant site and can explain only the dependence of the rate of sintering for low water vapor pressures. In all these reactions the water is dissociated and in some cases regenerated, remaining directly bonded to the magnesium which will leave the lattice. As proposed in former theoretical studies,14 the protonated magnesium vacancy formation would be the most probable path. The first indication of the formation of a hydrogen atom directly bonded to the oxygen of the lattice was postulated by Anderson et al.,p6 These authors analyzed the infrared spectrum of magnesium oxide prepared from Mg(OH), and found two distinct bands for hydroxyl vibrations: one sharp band at 3752 cm-' which characterizes the free hydroxyl groups adsorbed at Mg2- ions and a broad band at 3610 cm-' which shows the possibility of OH groups forming hydrogen bridges. In ofder to form hydrogen bridges, the proton donor group (a-H) must approach the acceptor group, which in this case is the lattice oxygen of MgO or free OH. Then the interaction could occur between the s orbital of the hydrogen and the u and T orbitals of the acceptor group. The formation of a hydrogen bridge changes the force constant of the involved groups and shifts the axial and angular deformation frequencies to low values resulting in lower and broader bands. Such effects explain the broadening and the lowband frequency at 3610 cm-'. Boudart et d 2 0 examined highly active samples of MgO by electron microscopy after heat treatment in vacuum at 500 OC and observed thin platelets of Mg(OH)2. Using electron paramagnetic resonance they also found by exchanging the proton with deuterium at the MgO surface that the impurity associated with the active s i b is a proton. They observed that these sites called VI centers are gradually eliminated in vacuum as the temperature is increased from 500 to 900 "C. These centers were reactivated when the MgO sample was exposed to water vapor at room temperature. Finally they suggested that the active site appears to require a proton and a (111)orientation. Our recent theoretical studies show that water molecules interact with the magnesium atom located at the corner.14J5 The interaction of one water molecule is followed by its dissociation releasing -70.2 kJ/mol. Figure 4 shows the most favorable path for the displacement of the proton during the water dissociation. The effect produced is transfer of the water proton to one (MgO), group, decreasing the net electric charge of the lattice oxygen. This will partially compensate the negative charge transfer to the (MgO), group by the magnesium vacancy formation with release of MgOH. The interaction of a second water molecule with the same magnesium ion requires 65.4 kJ/mol for its dissociation. This is probably due to the electrostatic repulsion of the protons. In this case, two (19)Sata, T.;Sasamoto, T. Adu. Ceram. 1984,10, 541-552. (20) Boudart, M.; Delbouille, A.; Derouane, E. G.; Indovina, V.; Walters, A. B. J. Am. Chem. SOC.1972,94, 6622.
458 Langmuir, Vol. 1, No. 4 , 1985
Long0 et al.
H
I
Mg-0-Mg '
H '
H
O h
d'
a'
a
b'
Mo-0, H
'H
d'
C'
C
Figure 2. Schematic representation of MgO crystal interacting with water on the corner: (a) water at the most stable position, (a') formation of a MgO vacancy and Mg0.H20, (b) two waters at the most stable position, (b') formation of a MgO vacancy and Mg0.2H20, (c) dissociation of a water molecule at its most stable position, (c') formation of a MgO vacancy and Mg(OH)2, (c") formation of a protonated Mg vacancy and Mg(OH), (d) dissociation of two water molecules at their most stable positions, (d') formation of diprotonated Mg vacancy and Mg(OH),, (e) two dissociated and one undissociated water molecule at their most stable positions, (e') formation of diprotonated vacancy and Mg(OH)pH,O. Table I. Net Charge over the Involved Atoms in Vacancy Formation
qMi 40
Mg(MgO), 117 -360
H' 0 H
(MgOhMgHzO -14 -349 218 -198 187
(MgO)3HMgOH -27 -209 210 -452 57
(MgO)3H*-MgOH+ 156 -211 203 -449 69
(MgOhH-*MgOHd 565 -258 e 133 -457 82
(MgO)3Mg 2H20 -142 -348 218 -196 181
(MgO),2HMg(OH)z -225 -209 219 -475 31
(Mg0)32H-. Mg(OH)zd -106 -218 215 -478 47
(Mg0)32H-. Mg(OH)Ze 280 -315 152 -461 66
atom that accepts the H of the water. 'Displaced hydrogen to the oxygen of the lattice. "Mg atom that forms the vacancy. dDisplacementof 0.5 from equilibrium. 'Displacement of 2.0 A from equilibrium.
qOH; qOH
40-H'
(MgO)3MgHsO 619 1250 110
(MgO)3HMgOH 830 506 361
Table 11. Bonding Order (MgO)3H* (Mi@),**MgOH (MgO)y**MgOH Mg 2H20 794 621 817 513 512 1251 360 142 107
(MgO)B2HMg(OH)z 910 430 488
(MgO)32H*** (Mg0)32H*** Mg(0H)Z Mg(OH)* 878 868 516 513 544 174
a Mg atom that forms the vacancy. * 0 atom that accepts the H of the water. Hydrogen bridge between the hydrogen atom bonded to the lattice and the hydroxyl bonded to the magnesium that forms the vacancy.
water protons will be transferred to the (MgO), group with the formation of a dimotonated Me vacancv and release of Mg(OH),. The hydrogen whicgstays bonded to the oxygen of the lattice forms a hydrogen bridge with the hvdroxvl bonded to the magnesium formine the vacancv (Tables I and 11). These results agree with the observed broadened band of 3610 cm-', which is characteristic of a hydrogen bridge, and with the sharD band of 3752 cm-i. which shows the existence of free iydroxyls bonded to' the magnesium.6 These reactions of monoprotonated and diprotonated vaY
Y
cancy formations can be written in the following way, by usine Kroeer notations.21 ( M ~ ~ ) , M+; H,O .+ (MgO), + Mg(0H). + V H ' ~ g ,Ut= 88.6 kJ/mol ( 7 ) (MgO),Mg + 2H20 (MgO), + Mg(OH)2 + VBHMg, AE, = 47.3 kJ/mol (8) -+
where AEt is the total energy required for the interaction (21) Wengeler, H.; Freud, F. Mater. Res. Bull. 1980, 15, 1747.
Water Interaction with an MgO Surface
Langmuir, Vol. 1, No. 4, 1985 459 Table 111. Total Energy, ET (kJ/mol), for the Supermolecules with Optimization of Supermolecule Geometry
H
a
supermolecule
-ET, kJ/mol
(MgO)& (MgO)a*.*Mg (Mg0)ZMgHzO (Mg0)yMgHzO (Mg0)BMg 2Hz0
154 249.6 153 989.0 206 504.6 206 266.1 258 751.9
supermolecule (MgO)B*.*Mg2HzO (Mg0)2MgMgOHzO (MgO)zMgMg0*2HzO (Mg0)SHMgOH (Mg0)32HMg(OH)*
-ET, kJ/mol 258571.1 206 296.9 258 562.4 206 574.8 258686.5
Table IV. Total Energy, ET (kJ/mol), for the Supermolecule with Optimization of Supermolecule Geometry and Water Dissociation supermolecule -ET,kJ/mol (MgO)JWzO (MgO)SH*..MgOH (MgO)zMg.Mg(OH)z (M~O)&~.~HZO (Mg0)32H..*Mg(OH)Z
b
C
Figure 3. (a) Protonated Mg vacancy; (b) angular movement of a proton to the vacancy center; (c) displacement of a proton to form a water molecule.
showed that these sites are more active than other higher coordinated sites. Identical results were experimentally obtained by 3ones et a1.22and theoretically by Colbourn and M a c k r ~ r j t . ~ ~ These types of vacancies were suggested by Wengeler and Freudz1using infrared spectroscopy on a MgO single crystal. The observed two IR band absorption groups between 3600 and 3300 cm-’ which suggested the existence of two diamagnetic defects in the MgO lattice [OH. V”,.OH]” = V2HMg corresponding to the 3550-cm-l band and [ o H * v ” ~ ~=]V’ H ‘ ~corresponding , to the 3300-cm-I band. This selection was made on the basis of the assumption that the broad background absorption observed between 3600 ,and 3300 cm-l is caused by partly delocalized interstitial protons, H+. This assumption is supported by the fact that the background intensity between 3600 and 3300 cm-l decreases with the sharp increase of the 3300cm-’ band intensity above 770 K, indicating that, above this temperature, interstitial protons are used to fill up the more localized defects, which give rise to the quadruplet at the 3300-cm-’ band. The [OH.V”M,OH-]X defect occurs at high energy at about 3550 cm-l, where the Coulomb interaction between the two protons is likely to stiffen the OH- bond. These results agree with our calculation when the second water molecule dissociates at the MgO surface which has an energy variation that unstabilizes the supermolecule, (Mg0)32HMg(OH)2(Table 111). However, for the case of a monoprotonated vacancy [OH.V’Mg]’the defect is partially compensated and there is no electrostatic interaction between the protons, resulting in the low-energy band around 3300 cm-l. The theoretical results show that reaction 8 occurs more easily than reaction 7. On the other hand, the diprotonated vacancy VZHM, can create molecular hydrogen (Table IV), resulting in the equation
-
(MgO), + VzHMg I
0.8
13
18
I
2:
d21d)
Figure 4. Possible paths for displacement of a proton in the direction of the nearest oxygen of the MgO crystal. The energy barriers are in kcal/mol. dl is the distance 0-H of the water and dz is the distance between the proton and the nearest oxygen of the MgO crystal. The dashed line is the best path.
of the water, its decomposition, and vacancy formation, obtained by the theoretical model, Figure 2. In this model we considered the interaction of the water molecule with the corners as in previous studies15 which
206 504.6 206 416.1 206 434.4 258571.1 258 647.0
(MgO), + H2
+ VxMg,
AE, =
129.6 kJ/mol (9)
This fact was observed by Wengeler and Freud.21 They showed an infrared band around 4150 cm-l due to the enhancement of the vibration, indicating the presence of molecular hydrogen in the magnesium vacancy. The possibility of molecular hydrogen formation was analyzed throughout the isoenergy surface (Figure 5). This surface shows that the two hydrogens remain in the plane formed (22) Jones, C. F.; Segall, R. L.; Smart,R. Sp., Turner, P. S. J.Mater. Sci. Lett. 1984, 3, 810. ( 2 3 ) Colbourn, E. A.; Mackrodt Solid State Ionics 1983, 8, 122.
460 Langmuir, Vol. 1, No. 4, 1985
Long0 et al. On the other hand, eq 8 and 10 resulting from the decomposition and formation of a water molecule, respectively, correspond to eq 11,which occurs more favorably with no water decomposition. However, there is another mechanism in which a water molecule decomposes forming a double vacancy of Mg and 0: (MgO)sMg + H2O (MgO)ZMg,+ Mg(OH)z + V 0 + V”Mg, AEt = 70.2 kJ/mol (13)
-
Equations 11 and 13 explain the mechanisms proposed in eq 4 by Hamano et al.l0 and in eq 6 by Sata and Sasamotolgwith the double-vacancy formation of V”, and Vo However, one should note that the mechanism of water dissociation proposed by eq 13 is the most probable. Then, there are two possibilities for vacancy formation at low water vapor pressures, corresponding to eq 13 and 7. The equilibrium constants have the following form for these equations:
K1 = [Mg(OH)’l[ F M g ] / p H z O
(14)
for eq 7, where [VH’Mg]a P H ~ oand ~/~, 0
Figure 5. Isoenergy curves (eV) of the possible paths for molecular hydrogen formation. The dashed line is the energy valley indicating the most probable path. Table V. Total Energy, ET (kJ/mol), for the Supermolecule with a Corner Vacancy with Two Protons and Formation of a Hz and HzO Molecules suDermolecule -ET, kJ/mol 8 & 154577.6 154 570.0 154485.3 154452.8
(MgO).$H (MgO)$H (MgO)p*H2
-60’ 900 450
90’ 900 450
associated (45O
450)
(MgO)~Mg.*.H~O 154 348.7
by the two MgO groups, corresponding to a crystal face (Figure la-c). The position of lower energy is the well at 0 = 90” with 4 ranging from -60” to -45”. The dashed line of Figure 5 shows the most favorable path for molecular hydrogen formation. This figure shows that one of the protons changes its position to locate at 45”, releasing 48.1 kJ/mol, whereas the other proton remains in the original position (90”). As a consequence, this proton rotates 45” to achieve the same position as the first proton (45”,45”). Another alternative is the transfer of one of the protons to the protonated oxygen to form a water molecule (Table V). When this water molecule is displaced out of the lattice an oxygen vacancy and a magnesium vacancy are formed, according to
-
(MgO), + V 2 H ~ g (MgOI2Mg + V O + V ” M ~ + H20, AE,= 205 kJ/mol (10) Equation 10 shows a possible path for the proposed mechanism by Hamano et a1.,I0 where the (MgO)2Mg system has a double vacancy (V”Mg and V O ) . However, the calculation shows the existence of a more favorable path for the double-vacancy formation. This path would be the simultaneous exit of Mg and 0 from the lattice without the dissociation of the water molecule (Table 111), originating in these equations: (MgO),Mg + HzO (MgO)sMg + MgO.HZO+ V”hlg + V o , AE, = 217.3 kJ/mol (11) -+
(MgO),Mg
+ 2Hz0
-+
V”Mg
(Mg0)zMg + Mg0.2H20 + AE, = 190.3 kJ/mol (12)
+ Vo,
K2 = [Mg(OH)2][V”M,.V o l / p ~ ~ o
(15)
for eq 13, where [V”,,] a PH2p1/’. For high water vapor partial pressures there is the possibility of hydrated magnesium hydroxide, Mg(0H)2.H20,originating in the equation (MgO),Mg + 3Hz0 (MgO)3+ Mg(OH)2.H20+ = 76.2 kJ/mol (16) V 2 H ~ gAEt ,
-
Equations 8 and 16 are similar but the first is more energetically favored. The equilibrium constants have the following form for these equations: K3 = [Mg(OH)ZI[VzHMgI/ p H z 0 2
(17)
for eq 8, where [V2HMg]a PH,o,and
K4
= [Mg(OH)z.HzOl [ V 2 H ~/ gh]Z o 3
(18)
for eq 16, where [V2HMg]0: PHzo3i2. Equations 17 and 18 are consistent with the experimental results on rate of sintering obtained by Eastman and Cutler* and Varela and Whittemore.12 An analysis of the possible reaction path for vacancy formation at the MgO surface surface with the help of water vapor can be characterized in the following way: (a) Because the reactions are heterogeneous, there would be diffusion of water vapor to the MgO surface. (b) Then water would be adsorbed on the MgO surface, as characterized by former studies.13 The theoretical results show that steps a and b should be fast and are not rate determining. Table I11 and eq 11and 12 show that water helps vacancy formation but ita influence is very small compared with the reactiona that account for water dissociation. (c) The calculation shows that the surface of MgO acts as a positive catalyst for water dissociation. The first water molecule spontaneously dissociates at the MgO surface. The second water molecule needs, however, a small activation energy for its dissociation (Table 111). In this way, the MgO surface acts as an alternative path of lower activation energy for single (eq 7, 8, and 16) or double (eq 13) vacancy formation. A water molecule cah help vacancy formation by two different paths, for single (I&) or double (Mg, 0)vacancy formation. However, when two or three water molecules interact with the MgO Burface, there is only one path for vacancy formation. This can be considered as the limiting step, the rate-determining step for vacancy formation.
Langmuir 1985,1,461-464
Conclusions The theoretical results contribute to a better understanding of the most probable mechanism through which water molecules interact with magnesium oxide surfaces as well as for magnesium and oxygen vacancy formation. In addition to this, it may be postulated that after such interaction water is decomposed on MgO crystal surfaces. The latter act as a positive catalyst diminishing the dissociation energy barrier. The hydroxyl group generated in earlier steps will form an ionic bond with the magnesium atom. On the other hand, the proton and the oxygen atom of the crystal will establish a covalent bond. These bonds are responsible for generating the new mono- or diprotonated vacancies. If the vacancy is diprotonated, molecular hydrogen may consequently be formed. For low water vapor partial pressures one would expect
46 1
the monoprotonated vacancy and MgOH formation or a double vacancy of magnesium and oxygen and Mg(OH)2 formation, in agreement with the experimental results. For high water vapor partial pressures one would expect competitive mechanisms having a predomination of diprotonated magnesium vacancies and Mg(OH), or Mg(0H)2-H20,also in agreement with experimental data.
Acknowledgment. We acknowledge the computer center of Univenidade Federal de SBo-Carlos,the Conselho Nacional de Desenvolvimento Cientifico (CNPq), Grant 300964j83, the FundaGBo de Amparo h Pesquisa do Estado de SBo Paul0 (FAPESP), Grants 82/1491-8,82/1373-5,and 83/1325-3, and the National Science Foundation, Grant DMR 8,111,111. Registry No. HzO, 7732-18-5;MgO, 1309-48-4.
Determination of the Viscosity of Valinomycin Monolayers as a Function of Surface Density and a Comment on Conformation+ B . M.Abraham* Materials Science and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439
J . B. Ketterson Department of Physics & Astronomy, Northwestern University, Evanston, Illinois 60201 Received December 4, 1984. In Final Form: April 1, 1985 The surface viscosity of valinomycin spread on aqueous substrates buffered to pH 6.7 f 0.1 with trisphosphate has been determined over the surface pressure range 7-30 dyn/cm (area 3-160 A2/molecule). Four series of measurements were made: buffer alone, buffer with 0.1 M NaCl, and buffer with 0.1 M and 1 M KC1. The surface viscosity remained essentially constant at 1.8 (mdyn s)/cm over the entire compressional range, regardless of the presence of Na+ or K+. Apparently, the polar groups do not form an intermolecular network by hydrogen bonding as the polar groups of lecithin or the fatty alcohols appear to do. The surface pressure-molecular area diagrams (II-A) in all cases, with and without Na+ or K+ ions in the water, are monotonic and nearly identical. None of the diagrams display a signature that can be attributed to either a phase change, a conformation change, or a unique molecular area. It is concluded that all monolayers studied in this research had essentially the same conformation.
Introduction In the attempt to develop an understanding of the molecular basis for two-dimensional viscoelasticity, displayed by monolayers of the aliphatic alcohols' and by lecithin? we selected valinomycin as an appropriate compound to study because it is replete with polar groups but lacks pendant hydrophobic tails. The investigation was conducted as a function of surface density in order to define the state of the monolayer; consequently, the surface pressurejmolecular area diagram ( E A ) was obtained along with the viscosity data. The information developed might resolve certain discordances between the ll-A diagrams published by Kemp and Wenner3 and by Ries and Swift.4 Valinomycin has been the subject of extensive investigation as it is one of a class of compounds, ionophores, that
* Present address: Department of Physics & Astronomy Northwestern University, Evanston, IL 60201. 'Work supported by the U S . Department of Energy at the Argonne National Laboratory.
facilitate transport of cations across biological membranes. Of the biologically important cations, Na+ and K+, valinomycin has been shown to be highly selective for K+;5 considerable effort has, therefore, been directed toward establishing the conformation in various environments as an aid to elucidating the mechanism of transport. Both uncomplexed and potassium-complexed crystalline valinomycin have been examined with x-ray^.^^^ Solutions in a variety of polar and nonpolar solvents have been examined with NMR8-l0and with both infrared and Raman (1)Abraham, B. M.; Miyano, K.; Su, S. Q.; Ketterson, J. B. Reu. Sci. Instrum. 1983,54,213-219. ( 2 ) Abrahma, B. M.; Miyano, K.; Ketterson, J. B. Ind. Eng. Chem. 1984,23,245-249. (3)Kemp, G.;Wenner, C. E. Biochim. Biophys. Acta 1972,282,l-7. (4)Ries, H. E.: Swift, H. J. Colloid Interface Sci. 1978,64,111-119. (5)Pressman, B. C. Annu. Reu. Biochem. 1976,45,501-530. (6)Neupert-Laves, K.; Dolber, M. Helu. Chim. Acta 1975,58,432-442. ( 7 ) Smith, G.D.; Dum, W. L.; Langs, D. A.; DeTitta, G. T.; Edmonds, J. W.; Rohrer, D. C., Weeks, C. M. J.Am. Chem. SOC.1975,97,7242-1247. (8)Haynes, D. H.; Kowalsky, A.; Pressman, B. C. J. Biol. Chem. 1969, 244, 502-505.
0743-7463/85/2401-0461$01.50/00 1985 American Chemical Society
