Langmuir 1990,6, 1461-1464
1461
Phase Transition in Water Adsorbed on a-Ferric Oxide Surfaces A. Tsugita,**tT. Takei,t M. Chikazawa,t and T. Kanazawat Cosmetics Laboratory, Kanebo, Ltd., Kotobuki-cho, Odawara, Kanagawa 250, Japan, and Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Fukasawa, Setagaya-ku, Tokyo 158, Japan Received July 6, 1989. In Final Form: February 21, 1990 Phase transition in water adsorbed on nonporous a-FezOs has been studied by examining the vapor pressure (&temperature (T)relations. The range of P measured was from about 10-2 to 4.6 Torr. Phase transition, which was recognized by the presence of a break point on a log P vs 1 / T plot (ClausiusClapeyron plot), took place for the first time at about -38 "C at coverages of 1.7Vm,where Vm represents the monolayer capacity of water vapor. Isosteric heats of adsorption below and above the phase transition temperature (Tt,) agreed well with the heat of sublimationof bulk ice and the heat of evaporationof bulk liquid water, respectively. The temperature Tt, rose rapidly from about -36 O C at 2Vm to about -20 "C at 3Vm. This great rise in Tt, suggests that the structure and properties of adsorbed water film approach rapidly those of bulk liquid water in the range of amounts adsorbed between 2Vm and 3Vm. Above 3Vm, Tt, rose gradually up to 0 "C with increasing adsorbed film thickness. It was shown that the state of the adsorbed water film above -38 "C changed from localized to liquid-like and further to ice-like with an increasing amount adsorbed. However, direct change occurred below -38 "C from a localized to an icelike state. The adsorption isotherms obtained at various temperatures superimposed well, if the amounts adsorbed were plotted against the relative pressure with reference to the saturation vapor pressure of supercooled liquid water. The reason was discussed from the viewpoint of the BET theory.
Introduction The properties of physically adsorbed water films on solid surfaces generally differ from those of bulk liquids. In the calorimetric study of Morrison' on solid-liquid phase transition of argon layers adsorbed on rutile-type titanium dioxide, no phase transition was recognized at the amount adsorbed corresponding to 1.6Vm, where V, represents a monolayer capacity of argon which is determined from a BET plot of the adsorption data. At 2.9Vm,however, a broad transition peak appeared in a heat capacity curve at about 10 K below the triple point (83.95 K)of argon. The phase transition peak became sharper with increasing film thickness, and the phase transition temperature (Ttr) approached the bulk melting point of argon. In the case of nitrogen adsorption on rutile surface? a phase transition was observed above 2.2Vm and at a temperature below the bulk melting point of nitrogen. Such depression of Tt, has been assumed to occur by the influence of solid surfaces on arrangement of adsorbed molecules. If the normal lattice configurations are disturbed by such influence, the freezing phenomenon should become difficult to take place. Studies of phase transition in water adsorbed on nonporous solids are few compared with those for porous substances such as silica gels3or porous glasses! Plooster and Gitlin5 studied the phase transition in water adsorbed on nonporous silicas by calorimetric measurements. The adsorbed water on a silica surface having a small portion of polar sites on ita surface (SW/SN = 0.17) gave a phase transition for the first time at -14 "C when the adsorbed film thickness reached 0.6 nm. Here, the values + Cosmetics Laboratory. t Tokyo Metropolitan University. (1) Morrison, J. A.; Drain, L. E. J. Chem. Phys. 1951, 19, 1063. (2) Morrison, J. A.; Drain, L. E.; Dugdale, J. S . Can. J. Chem. 1952,
30,890.
(3) Berezin, G. I.; Kiaelev, A. V.; Kozlov, A. A. J. Colloid Interface Sci.
1973,45,190. (4) Sidebottom, E. W.; Litvan, G. G. Trans. Faraday SOC.1971, 67,
2726. (5) Plooster, M. N.; Gitlin, S. N. J. Phys. Chem. 1971, 75, 3322.
0743-7463I90 / 2406-146M02.50/O
SW and SNare the specific surface areas determined by water and nitrogen adsorptions, respectively, and the ratio represents the water vapor affinity of a solid surface. The temperature Ttr approached gradually the melting point of bulk ice (0 "C)with increasing amount adsorbed. Plooster et al. also showed that for the silica surfaces with a ratio of 1.3 Tt, depresses to a greater degree at a similar film thickness. However, no study to investigate the phase transition of water film adsorbed on nonporous solids in terms of the measurement of vapor pressure has been carried out. Red iron oxide, a-Fe203, is an important pigment for cosmetics, paint, or ceramics industries. Pigment dispersion is the most significant process in these industries. For example, in preparation of make-up cosmetics in cake form, iron oxide particles are not easy to disperse in white powders.6 They usually exist as agglomerates, because they are highly adherent. It is well-known that water adsorbed on powders affecta the adhesion behavior among their particles. The authors showed that the adhesive force between glass beads and a glass plate increased markedly between 2Vm and 3Vm.I The reason for this phenomenon has been discussed in relation to the state of water adsorbed on solid surfaces. In order to obtain well-dispersed systems, we need to consider more deeply the problems of the adherence among pigment particles and the physical properties of water adsorbed on pigment surfaces. In this study, the phase transition in adsorbed films of water on a-Fe203 surfaces has been investigated to obtain further information concerning the physical properties of water adsorbed on pigments. Experimental Section Material. Ferric oxide (a-FezOs)was of special reagent grade from Nakarai Kagaku, Japan, and had been produced by calcination of ferrous sulfate at 700 "C. Its specific surface area from (6) Tsugita, A.; Fukushima, T.; Yoneya, T.; Nishijima, Y. Int J. Cosmet. Sci. 1985, 7, 15. ( 7 ) Chikazawa. M.; Kanazawa, T.; Yamaguchi,T. Powder Sci. Technol. Jpn. 1984, 54.
0 1990 American Chemical Society
Tsugita et al.
1462 Langmuir, Vol. 6, No. 9,1990 N P adsorption measurements (SN) was 2.44 m2/g. The impurities estimated by X-ray fluorescence analysis were as follows: Si02 0.05 % , C1 0.33 7%, SO3 0.01 % . X-ray diffraction analysis
did not indicate the presence of any components other than a-FezO3. X-ray photoelectron spectroscopy showed the presence of a trace of C1. The peak areas of C1 before and after etching the surface with argon gas for 10 min were almost equal to each other. This fact shows that C1 is not concentrated on the surface. Water Vapor Adsorption. Prior to the adsorption measurement, the sample a-Fe2Oa in a Pyrex glass vessel was degassed at 50 "C for 18 h under a reduced pressure of 10" Torr to remove physically adsorbed water. Chemically adsorbed water is known not to be eliminated from the surface of a-Fe203 under such conditions.s The adsorption isotherm of water on a-FezO3 was determined volumetrically at 0 O C by using the same adsorption apparatus as described previously: which was equipped with a mercury manometer and a reading magnifier having a resolution of mm. Water Content Measurement. The water content in a-Fe~O3was determined by the successive ignition-loss method proposed by Morimoto et a1.lO After evacuation of the a-Fe2Oa sample at 50 "C for 18 h in a reduced pressure of 10" Torr, the sample was treated at 600 "C for 4 h and then at 1000 "C for 4 h. The amount of water generated by the heat treatment at each temperature was determined volumetrically with a mercury manometer. Determination of Water Vapor Pressure (0-Temperature ( T ) Relations. The relations of vapor pressure, P (Torr), to temperature, T, for water adsorbed on a-FezO3 were determined between about -50 and 0 "C by using the same apparatus as used in adsorption isotherm measurement. The vapor pressure in this case was measured with a Pirani gauge attached directly to the sample vessel. The Pirani gauge was calibrated with reference to the equation expressing the vapor pressure of ice as a function of the temperature ranging from -100 "C to the melting point (0 OC).ll The precision of P measurement was IO-' Torr. A sufficient amount of a-Fez03was placed in the sample vessel so that the changes in amount adsorbed with temperature could be negligibly small. After the sample was degassed at 50 "C for 18 h under a reduced pressure of Torr, a certain amount of water vapor was introduced into the sample vessel at 0 "C. The amount adsorbed can be known from the vapor pressure in equilibrium by using the adsorption isotherm obtained above, since the adsorption isotherm was reproducible. The sample vessel was gradually cooled down to about -50 "C by using an ethanol cold bath equipped with a cold N2 gas generator. It was then allowed to stand at that temperature to reach equilibrium. Temperature was controlled with the precision of 0.05 K. After the equilibriumpressure (P)was measured, temperature was raised at intervals of a few degrees to 0 "C. In each step, the equilibriumpressure was determined. Within 20 min the system came to equilibrium. The range of P measured was from about to 4.6 Torr. The addition of water vapor and the measurements of P and Tin equilibrium were repeatedly made. Results and Discussion C h a r a c t e r i z a t i o n of S u r f a c e of a-Fe2O3. T h e adsorption isotherm of water vapor on a-Fe203 at 0 "C is shown in Figure 1. The monolayer capacity, Vm, of water vapor was estimated to be 0.174 mL/m2 (STP) using the BET method. Assuming a cross sectional area of 0.105 nm2 for a water molecule, the specific surface area S w for the water adsorption was calculated t o be 1 . 2 3 m2/g. (8) McCafferty,E.; Zettlemoyer,A. C. Discuss. Faraday SOC.1971,52, 239. ~ . .
(9) Chikazawa,M.; Yamamoto, F.; Saita, E.; Kaiho, M.; Kanazawa, T. Bull. Chem. SOC.Jpn. 1977,50, 337. (10) Morimoto,T.;Shiomi, K.; Tanaka, H. Bull. Chem. SOC.Jpn. 1964, 37, 392. (11) Jancso, G.;Pupezin, J.; Hook,W. A. V. J. Phys. Chem. 1970, 74, 2984.
- 5
- 4
- 3:
E
- 2 - 1
0
-
0 0
0.2
0.4
0.6
0.8
1.0
PIP,
Figure 1. Adsorption isotherm of water vapor on a-Fe203 at 0 Table I. Water Content (OHGroudnm') of a-FeSOs temp, "C OH/nm2 50 600 lo00
9.8 0 0
Therefore, SW/SN becomes 0.50. This means that the water molecules are in a localized state a t monolayer completion and cover only one-half of the total surface area. The number of water molecules adsorbed on the surface is calculated to be 4.8 molecules/nm2. Thereafter, the amounts of water adsorbed are given in terms of the monolayer capacity (Vm). Water content of a-FezO3is shown in Table I; 9.8 OH groups/nm2 were estimated to exist on the a-Fe~O3 surface a t 50 "C. This value is almost equal to the result of Morimot0 et a1.12 and is consistent with the calculated value of 9 OH groups/nm2 for the fully hydroxylated surface of the (001) plane of a-FezO3 crystal assuming that one OH group is formed on each surface metal ion.12 The results obtained indicate that one water molecule adsorbs on two OH groups on the surface pretreated at 50 "C. Clausius-Clapeyron Plots. The Clausius-Clapeyron plots of water adsorbed on a-Fe-203at various amounts adsorbed are shown in Figure 2. When the amounts adsorbed were more than 1.7Vm, each plot gave two straight lines with different slopes. The slopes at lower and higher temperature sides were found to be approximately equal to the slopes of log P vs 1 / T plots for bulk ice and bulk water, respectively. From the values of these slopes, isosteric heats of adsorption (Q) in the lower temperature side (QlJ and in the higher temperature side ( Q h i ) were calculated and are shown in Figure 3. The values of Qi0 and QK almost agreed with the heat of sublimation of bulk ice (12.26 kcal/mol) and the heat of vaporization of bulk liquid water (10.68 kcal/mol), respectively. These observations indicate that the adsorbed films of water in lower and higher temperature sides are in ice-like and liquid-like states, respectively. The phase transition temperature (Tt,) in the adsorbed water film a t the amount adsorbed above 1.7Vm was determined from the intersecting point of two straight lines of each plot. Above 2.4Vm,the plots below their T+.r were superimposed upon the plot for bulk ice (line a in Figure 2). This means that the free energy of the adsorbed water film below Ttrbecomes equal to that of bulk ice and that bulk ice exists in equilibrium with the ad(12) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 74, 243.
Phase Transition in Adsorbed Water
Langmuir, Vol. 6, No. 9, 1990 1463
0.5
-
-50 0
v/v,
Figure 4. Phase transition temperature, VI Vm.
I
I
I
I I
4.0 4.2 (1/Tl x lo3
3.8
4,4
Figure 2. Clausius-Clapeyronplots of water adsorbed on cr-FezOa at various amounta adsorbed (VlV,): 0,4.95; 0,2.35; A, 2.12; 0,1.67; 0,l.W . , 1.22; A,1.03; e,0.50. loc, localized,liq, liquidlike; ice, ice-like. The arrows indicate phase transition points from liquid-liketo ice-likestates with the increase in temperature. Line a: the plot for bulk ice. Dotted lines: (b) T = -25 O C ; (c) T = -45 "C. denotes the region where the state of the adsorbed water film changes from localized to liquid-like, from liquid-liketo ice-like,or from localized to ice-like,as the amount adsorbed increases along dotted lines b or c.
-
lot 0
1
2
3
4
5
6
7
V/Vm
Figure 3. Iswteric heat of adsorption, Q, plotted against V/ V,. 81: Q in a localized state. Q b and Qhi: Q above 1.7Vmin the lower
and higher temperature sides, respectively.
sorbed water. Below 1.5Vm, phase transition did not take place, since the plots were found to be linear over whole temperature range. This suggests that the adsorbed molecules are sufficiently isolated, and thus any phases in a liquid-like or an ice-like state cannot be formed below 1.5Vm. Phase Transition Temperature Change in Tt, with increasing amount adsorbed is shown in Figure 4. A phase transition appeared for the first time at about -38 "C, when the adsorbed amount reached 1.7Vm. The temperature TO rose rapidly from about -36 "C at 2Vm to about -20 "C at 3 Vmand thereafter increased gradually up to 0 "C as the amount adsorbed increased. Below 2Vm, the sample surface should not be covered completely with water molecules, since the s W / s N value is 0.50. The fact that the phase transition appears a t 1.7Vm, however, suggests that an "island-like" adsorption region is formed on the surface of a-FezO3. The maximum Ttr depression observed in the adsorbed water film on the a-Fe203 surface ( s W / s N = 0.50) (!$')a,
T,,,plotted against
in this study (-38 "C) was much greater than that for the silica surface with the above s W / s N ratio of 0.17 (ca. -15 0C).5 This difference indicates that if the water vapor affinity of a surface is greater, the influence of the surface on arrangement of the adsorbed water molecules becomes more significant, and thus the freezing of the adsorbed water becomes more difficult to take place. A great rise in Tt, between 2Vm and 3Vm suggests that the influence of surface force decreased markedly with the increase in film thickness, and the structure and properties of adsorbed water approach those of bulk liquid water. The adhesion force has also been reported to increase markedly between 2Vm and 3Vm. This was explained in terms of the change in adhesion mechanism from hydrogen bonds to liquid bridge formation and the change in surface tension of the liquid bridge.7 States of Adsorbed Water Film. The way the state of the adsorbed water f i i at a certain temperature changes is now explained by Figure 2. At -25 "C, as the amount adsorbed increases along the dotted line (b), the state of the adsorbed water film changes from localized to liquidlike at 1.5-1.7Vm and further to ice-like at 2.4Vm. At -40 "C, as the amount adsorbed increases along a dotted line (c), the state of the film turns from localized to ice-like states at 1.5-1.7Vm. That is to say, above approximately -38 "C, the state changes in the following way:
-
-
localized liquid-like ice-like Below approximately -38 "C, however, the direct change from a localized state to an ice-like state occurs without passing through a liquid-like state. Adsorption Isotherms as a Function of Relative Pressure Based on the Saturation Vapor Pressure of Bulk Ice (Pso). Individual plots in Figure 2 indicate P vs T relationships at various amount adsorbed. Therefore, from the intersecting points of, for example, dotted line b ( T = -25 "C) with individual plots, the relationship between amount adsorbed and relative pressure (PI Pso), i.e., the adsorption isotherm at -25 "C, is obtained, where Pso denotes the saturation vapor pressure of bulk ice. In this way, adsorption isotherms as a function of PIPS0 a t various temperatures were obtained and are shown in Figure 5. Below 1.5-1.7Vm, the adsorbed water film is in a localized state. Above the amounts adsorbed, which are indicated on each isotherm with "-", the adsorbed film is ice-like. It is well-known that the amount adsorbed on a nonporous solid increases asymptoticallywith relative pressure approaching unity, if the adsorbed film is in a liquid-like state. On the other hand, when the adsorbed film is in an ice-like state, it is obvious from Figure 5 that PfPso reaches unity at relatively small amount adsorbed. For
1464 Langmuir, Vol. 6, No. 9,1990
Tsugita et al. Table 11. Calculated (V/V,) Values at Various
1.0
Temperatures
0.8
0.2
0
-
0.2
0.4
0.6
0.8
1.0
p/pso
Figure 5. Adsorption isotherms as a function of P/Pmat various temperatures: 0 "C; A,-10 "C; 0,-25 "C; 0 ,-35 "C; O , -45 "C. PSOdenotes saturation vapor pressure of bulk ice. denotes
-
the region where the state of the adsorbed water film changes from liquid-like to ice-like or from localized to ice-like with the increase in the amount adsorbed. 1.0
O'C 0.8
0.2
01
0
I
I
I
I
0.2
0.4
0,6
0.8
lo 1,O
PIPLO
Figure. 6. Adsorption isotherms as a function of P/Pm at various temperatures: 0 "C; A,-10 "C; 0,-25 "C; 0,-35 "C; CI, -45 "C. Pm denotes saturation vapor pressure of supercooled bulk liquid
water.
example, a t -45 "C, the adsorbed film turns ice-like a t 1.7Vm, and PIPS0 reaches unity at 2.4Vm, where the bulk ice phase begins to form in equilibrium with the adsorbed water. Adsorption Isotherms as a Function of Relative Pressure Based on the Saturation Vapor Pressure of Supercooled Bulk Liquid Water (PLo). Adsorption isotherms of water vapor on a-Fe203as a function of PIPm are shown in Figure 6. Those isotherms were greatly different in shape from the isotherms as a function of PI& (Figure 5). The isotherms obtained at temperatures between -35 and 0 "C were fairly well superimposed upon each other below 2Vm. Similar superimposition of isotherms was reported for the adsorption isotherms of water on porous glass below 0 O c a 4 Here, t h e reason why t h e isotherms a t various temperatures can superimpose well if amounts adsorbed are plotted against P I P L O is discussed from the viewpoint of the BET theory. The value of c in the BET equation is expressed as c = exp((Q1- QL)/RT)
C
0 -10 -25 -35 -45
74 87 114 138 172
0.1 0.99 1.01 1.03 1.04 1.06
VfV,atn 0.2 1.19 1.20 1.21 1.22 1.22
= 0.3
0.4
1.39 1.39 1.40 1.41 1.41
1.63 1.64 1.65 1.65 1.65
consequently the values of c at -10, -25, -35, and -45 "C were calculated and shown in Table 11. If x and V denote P I P L O and the amount adsorbed, respectively, then the BET equation is expressed as
0 0
temp, "C
(1)
where Q1 and QL are the heat of adsorption for the first adsorption layer and the heat of condensation,respectively. If Q1 is independent of T, then the value of c is determined only by T. From the adsorption data of water on a-FezO3 (Figure l), c was estimated to be 74. Substituting this value into eq 1, 81- QL is calculated to be 2.33 kcal/mol, and
v/vm= cx/[(l - x ) ( l - x + c x ) ]
(2)
Therefore, by use of eq 2, the values (V/ Vm) at various T for x values ranging from 0.1 to 0.4 are calculated and shown in Table 11. The values (VlV,) at various T for respective x agreed well with each other, especially for higher x values. Since the isotherms at, e.g., 0 and -10 "C are approxis taken as the saturation imately superimposed if PLO vapor pressure, the amounts adsorbed at, e.g., P I P L O = 0.600 are approximately the same value, VI, a t 0 and -10 "C. On the other hand, if the amounts adsorbed are plotted against PIPSO, the isotherms at 0 and -10 "C do not coincide. For example, a t the same amount adsorbed V1, PIPSOat 0 and -10 "C is 0.600 and 0.661, respectively. As shown in Figure 6, however, the isotherm at -45 "C deviated slightly upward above 1.7Vm( x > 0.4) from those at temperatures above -35 "C. This deviation may be attributed to the adsorbed film formed above 1.7Vm being in an ice-like state at -45 "C, in contrast to the film above -35 "C being in a liquid-like state. As P I P L O increased, the isotherms obtained at -25, -35, and -45 "C (Figure 6) began to deviate upward from the isotherm a t 0 "C above particular points. The points correspond to the transition points from liquid-like or localized to ice-like states with increasing amounts adsorbed. The vertical rises which appear on the isotherms between -25 and -45 "C are due to formation of bulk ice in equilibrium with the adsorbed water. The vapor pressures at which the vertical rises were observed agreed well with the vapor pressures of bulk ice at respective temperatures.
Conclusion Phase transition in water adsorbed on a-Fez03 has been studied by examining the P-T relations (ClausiusClapeyron plots). The results are summarized as follows: (1)As the amount of water adsorbed increased, the phase transition appeared for the first time at 1.7Vm. (2) Isosteric heats of adsorption below and above Tt, agreed well with the heat of sublimation of bulk ice and the heat of evaporation of bulk liquid water, respectively. (3) The minimum Tt, observed a t 1.7Vm was -38 "C. T h e temperature Tt, rose rapidly from -36 "C at 2 V m to -20 "C at 3Vm and further rose gradually till 0 "C with the increase of the adsorbed film thickness. (4) The state of the adsorbed water film above -38 "C changed from localized to liquid-like and further to ice-like with the increase of the amount adsorbed, while direct change occurred below -38 "C from localized to ice-like. (5)The adsorption isotherms obtained at various temperatures superimposed well, if the amounts adsorbed were plotted against P/PLO.
