23096
J. Phys. Chem. B 2005, 109, 23096-23102
Surface Behavior of the 1-Bromobutane with Isomeric Butanol Mixtures Beatriz Giner, Ignacio Gasco´ n, He´ ctor Artigas, Fe´ lix M. Royo, and Carlos Lafuente* Departamento de Quı´mica Orga´ nica-Quı´mica Fı´sica, Facultad de Ciencias, UniVersidad de Zaragoza, 50009 Zaragoza, Spain ReceiVed: June 22, 2005; In Final Form: September 29, 2005
Surface tensions of the 1-bromobutane with isomeric butanol mixtures were measured in the temperature range 283.15 K (or 298.15 K for 2-methyl-2-propanol) to 313.15 K with a drop volume tensiometer, and the corresponding surface tension deviations were calculated. Using this information together with bulk thermodynamic properties a thermodynamic study on surface formation was presented. This study includes the calculation of excess surface compositions and excess properties of surface formation.
Introduction The thermophysical study of liquids and liquid mixtures is significant for many purposes. There are multiple experimental methods of investigating intermolecular interactions and forces in liquids, but surface tension measurements can be differentiated from them due to their simplicity and precision. Surface tension can be considered the result of several phenomena that can take place not only in the surface of a liquid but also in the bulk. Understanding of structure and cohesion forces of the pure compounds and their mixtures can be obtained from surface tension data as well as information about specific molecular interactions because surface thermodynamics properties, such as entropy or enthalpy of surface formation, depend on the variation in molecular forces and that of the density of packing or molecular size.1 In this paper the formation of the 1-bromobutane + butanol mixture/air surface has been studied measuring the surface tension as a function of composition and temperature. We have investigated the mixtures formed by 1-bromobutane with each one of the butanol isomers (1-butanol, 2-butanol, 2-methyl-1propanol and 2-methyl-2-propanol) between 283.15 K (or 298.15 K for 2-methyl-2-propanol) and 313.15 K. Several magnitudes, such as surface tension deviations, excess surface molar fraction of butanol, excess entropy, enthalpy, and Gibbs function of surface formation, have been derived from experimental results. A review of literature shows that there is some experimental information on bulk thermodynamic properties of the mixtures studied here, such as excess volume, excess enthalpy, and isothermal and isobaric vapor-liquid equilibrium. These studies reveal that the mixtures containing primary isomers, that is, 1-butanol and 2-methyl-1-propanol, behave in a similar way, while behavior of mixtures formed by 2-butanol or 2-methyl2-propanol differ quite a lot, the rupture of hydrogen bonds between alcohol molecules being the factor that mainly governs the bulk behavior. Since the migration of alcohol molecules from the bulk to the surface depends on their interactions in the bulk, here, both surface and bulk properties have to be taken into account in order to understand the surface behavior of this kind of mixture. * To whom correspondence should be addressed. Phone: +34-946761202. Fax: +34-976-762295. E-mail:
[email protected].
TABLE 1: Experimental and Literature Values of Densities, G, and Refractive Indices, nD, of Pure Compounds at 298.15 K F/g‚cm-3
nD
compound
exptl.
lit.
exptl.
lit.
1-bromobutane 1-butanol 2-butanol 2-methyl-2-propanol 2-methyl-2-propanol
1.26840 0.80575 0.80241 0.79784 0.78110
1.2687 0.80575 0.80241 0.7978 0.7812
1.437850 1.397394 1.395264 1.393862 1.385074
1.4378 1.39741 1.39530 1.39389 1.3852
Experimental Section The liquids used were: 1-butanol (>99.8%), 2-methyl-1propanol, 2-methyl-2-propanol (>99.5%), 1-bromobutane, and 2-butanol (>99%) provided by Aldrich. The purities of these compounds were checked by measuring several thermophysical properties, such as density and refractive index, and the comparison between experimental and literature values2 at 298.15 K of densities and refractive indices is given in Table 1. The densities were measured by means of an Anton Paar DMA-58 vibrating tube densiometer, and refractive index measurements were made with an Abbemat-HP Dr. Kernchen refractometer. No further purification was considered necessary although the isomeric butanols were dried with an activated molecular sieve, type 0.3 nm, from Merck. The surface tensions, σ, of the pure liquids and their mixtures were determined using a drop volume tensiometer Lauda TVT2.3 Measurements were carried out in a range of temperatures from 283.15 to 313.15 K with an interval of 5 K between each temperature. This tensiometer measures the volume of a drop detaching from a capillary of known diameter. The temperature was kept constant within ( 0.01 K by means of an external Lauda E-200 thermostat. Densities needed to calculate surface tensions from volume drop determinations were measured using an Anton Paar DMA-58 vibrating tube densiometer. Details of the experimental procedure can be found in a previous paper.4 The accuracy of the surface tension measurement is better than 0.5% of the final value of surface tension and the corresponding reproducibility is better than 0.03 mN‚m-1. Surface tensions of the pure compounds at work temperatures are shown in Table 2. The mixtures were prepared by weight using a Mettler H20T balance. The maximum estimated error in the mole fraction is ( 1 × 10-4.
10.1021/jp053381h CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2005
Thermodynamic Study of Surface Formation
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23097
TABLE 2: Surface Tensions, σ, of the Pure Compounds σ/mN‚m-1 T/K compound
283.15
288.15
293.15
298.15
303.15
308.15
313.15
1-bromobutane 1-butanol 2-butanol 2-methyl-1-propanol 2-methyl-2-propanol
27.56 25.55 24.37 23.65
27.24 25.12 23.95 23.33
26.70 24.78 23.56 22.92
26.15 24.25 23.08 22.47 20.30
25.80 23.95 22.75 22.17 19.85
25.30 23.55 22.39 21.83 19.41
24.75 23.15 21.97 21.45 18.93
Results and Discussion Surface tension deviation, ∆σ, has been calculated from our measurements according to the following equation:
∆σ ) σ - x1σ1 - x2σ2
(1)
positive values only appear at high concentration of alcohol and when temperature is low. If the mixture contains 2-methyl-2propanol, surface tension deviations are negative in the whole range of composition and temperature, values being the most negative of all the mixtures. Finally, for all the mixtures, if
where σ is the surface tension of the mixture and xi and σi are the mole fraction and surface tension of component i respectively. The values of surface tension deviations are graphically represented in Figures 1 to 4. The surface tension deviations were correlated with temperature and composition by means of the following equation:
∆σ/mN m-1 ) x1(1 - x1)
r
p
Aij(2x1 - 1)i(T - T0)j ∑ ∑ i)0 j)0
(2)
where x1 is the mole fraction of 1-bromobutane, T is the temperature, T0 is a reference temperature, T0 ) 283.15 K for mixtures containing 1-butanol, 2-butanol, or 2-methyl-1-propanol, and T0 ) 298.15 K for 2-methyl-2-propanol, and Aij are adjustable parameters determined by the method of least-squares. The values of these parameters are given in Table 3 together with the corresponding standard deviations. Surface tension deviations of mixtures containing 1-butanol or 2-methyl-1-propanol show a similar sigmoidal shape, values being positive when composition is poor in bromobutane and if the temperature is not very high. In the case of mixtures formed by 1-bromobutane and 2-butanol, surface tension deviations present also a sigmoidal shape, but in this case,
Figure 2. Surface tension deviations, ∆σ, of 1-bromobutane (1) with 2-butanol (2): (b) exptl. data; (s) eq 2.
Figure 1. Surface tension deviations, ∆σ, of 1-bromobutane (1) with 1-butanol (2): (b) exptl. data; (s) eq 2.
Figure 3. Surface tension deviations, ∆σ, of 1-bromobutane (1) with 2-methyl-1-propanol (2): (b) exptl. data; (s) eq 2.
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Figure 4. Surface tension deviations, ∆σ, of 1-bromobutane (1) with 2-methyl-2-propanol (2): (b) exptl. data; (s) eq 2.
Figure 5. Excess surface mole fraction, xσ2 - x2, of 1-butanol.
TABLE 3: Parameters of Eq 2, Aij, and Standard Deviations, s (∆σ)
system 1-bromobutane + 1-butanol 2-butanol
2-methyl-1-propanol
2-methyl-2-propanol
A00 A10 A20 A30
A01 A11 A21 A31
A02 A12 A22 A32
0.3286 -0.7775 0.0600 -0.2065 -0.6445 -0.7993 1.2714 0.0353 0.1303 -1.2309 1.0031 -0.6450 -2.6612 -0.9849 -0.6115 -1.2392
-0.0442 0.0104 0.0271 0.0450 -0.0209 0.0003 -0.0829 -0.0521 -0.0673 0.0445 -0.0737 0.0788 -0.0228 -0.0037 -0.2370 -0.2156
-0.0002 -0.0012 -0.0010 -0.0012 -0.0002 0.0004 0.0002 -0.0002 0.0008 -0.0011 -0.0008 -0.0038 0.0007 -0.0005 0.0085 0.0085
s(∆σ) 0.01 0.01
0.01
0.01
temperature increases, surface tension deviations become more negative. The surface of a mixture is enriched usually in the component of lower surface tension (Gibbs adsorption). Quantitative information about this enrichment is given by the surface mole fractions, xσi . The composition of the surface mixture, that is, the surface mole fraction of component i, is defined by
xσi ) Γσi /
∑
σ iΓi
(3)
where Γσi is the surface excess concentration of component i, which are the real populations in the surface phase. This concentration is related with Γji, the relative surface excess concentration of component i with respect to component j, by means of the classical relationship
Γji ) Γσi - (xi/xj)Γσj ) - (∂σ/∂µi)T
(4)
Finally, the magnitudes Γσi and xσi can be evaluated taking into
Figure 6. Excess surface mole fraction, xσ2 - x2, of 2-butanol.
account that in the surface phase, composed by a monomolecular layer, the surface excess concentrations are related through the equation
∑iAσi Γσi ) 1
(5)
where Aσi is the partial molar area of component i, that can be calculated5 from this molar volume, V°i, as [NA(V°i)2]1/3. To calculate all these magnitudes it is necessary to know the variation of the chemical potential, µ, with composition. This information can be obtained from isothermal vapor-liquid equilibrium data.6-7 A representation of xσ2 - x2, that is, excess surface concentration of butanol, is shown in Figures 5 to 8. In all the cases, there is an amount of alcohol that migrates to the surface by a mechanism of adsorption. The maximum of xσ2 - x2 is placed
Thermodynamic Study of Surface Formation
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23099 According to the thermodynamic formulation of Aratono et al.5,9-10, ∆Ym consists of the differences in the partial molar thermodynamic quantities of the components between the interface, yσi , and the bulk, yi:
∆Ym ) xσ1 (yσ1 - y1) + xσ2 (yσ2 - y2)
(6)
These thermodynamic quantities of surface formation per mole of mixture can be rewritten as
∆Ym ) ∆Y°m + Yσ, M + (-YM)
(7)
σ σ, ∆Y°m ) xσ1 (yσ,° 1 - y° 1) + x2 (y2 ° - y° 2)
(8)
σ,° σ σ Yσ,M ) xσ1 (yσ1 - yσ,° 1 ) + x2 (y2 - y2 )
(9)
where
YM ) xσ1 (y1 - y°1) + xσ2 (y2 - y°2) Figure 7. Excess surface mole fraction, xσ2 - x2, of 2-methyl-1propanol.
since ∆Ym and ∆Y°m are evaluated from the experimental results and YM from the bulk properties, the thermodynamic quantity of mixing in the surface, Yσ,M, can be obtained. Finally, the excess thermodynamic magnitudes of surface formation per mole are defined by
∆YEm ) ∆Ym - ∆Yideal ) Yσ,E - YE m
Figure 8. Excess surface mole fraction, xσ2 - x2, of 2-methyl-2propanol.
around x1 ) 0.7 if the mixture contains 1-butanol or around x1 ) 0.8 for the rest of the mixtures. When temperature increases, the adsorption of alcohol into the surface decreases. It is remarkable that the sequence followed by xσ2 - x2 values: 1-butanol < 2-methyl-1-propanol < 2-butanol < 2-methyl-2propanol goes along with the difference of surface tensions between the pure compounds. It is also noticeable that for all the mixtures there is an inflection point before to the composition corresponding to the maximum of excess concentration in the surface. From the surface tension measurements vs temperature, the entropy, ∆S, enthalpy, ∆H, and Gibbs function, ∆G, of surface formation per unit surface area can be evaluated,8 and dividing these magnitudes by the total surface excess concentration, Γσ, the thermodynamic quantities of surface formation per mole of the mixture, ∆Ym, can be determined.
(10)
(11)
where Yσ,E is the surface excess property per mole (Yσ,E ) Yσ,M - Yσ,M,ideal) and YE is the corresponding bulk excess property per mole (YE ) YM - YM,ideal). The excess entropies and enthalpies of surface formation are positive at all temperatures over the whole composition range for all the mixtures, except for the following ones: mixture containing 1-butanol at 283.15 K at high mole fractions of 1-bromobutane, mixture with 2-methyl-1-propanol at 313.15 K at low mole fractions of bromoalkane, and mixture with 2-methyl-2-propanol at 313.15 K in the concentrated bromoalkane region. The excess Gibbs function of surface formation is represented in Figures 9 to 12. They show similar trends for all the mixtures showing lowest ∆GEm values around x1 ) 0.7 for 1-bromobutane with 1-butanol or around x1 ) 0.8 for the rest of the mixtures. In the region richer in alcohol, some mixtures, especially at low temperatures, show slightly positive values: when the mole fraction of bromoalkane increases, the excess Gibbs function of surface formation becomes negative. The minimum ∆GEm values follow the sequence in absolute value: 1-butanol < 2-methyl-1-propanol < 2-butanol < 2-methyl-2propanol, although at high temperatures the minimum values for the mixtures containing 2-butanol or 2-methyl-1-propanol are very similar. The temperature behavior of excess Gibbs function of surface formation is quite complex. At low and high mole fractions of bromoalkane the lower values are attained at high temperatures, although in these composition regions the values are similar at all the temperatures. On the other hand, in the middle composition range the lowest ∆GEm values correspond to the lower temperatures. The bulk and surface properties are intimately related; sometimes they are reported together with the aim of attain a better knowledge of the processes that occur in the mixture.11-13 So, let us revise the information obtained from several bulk properties, which values are the result of energetic and structural
23100 J. Phys. Chem. B, Vol. 109, No. 48, 2005
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Figure 9. Excess Gibbs function of surface formation of 1-bromobutane (1) with 1-butanol (2).
Figure 11. Excess Gibbs function of surface formation of 1-bromobutane (1) with 2-methyl-1-propanol (2).
Figure 10. Excess Gibbs function of surface formation of 1-bromobutane (1) with 2-butanol (2).
Figure 12. Excess Gibbs function of surface formation of 1-bromobutane (1) with 2-methyl-2-propanol (2).
effects. Excess Gibbs functions6,7 has been measured from 278.15 to 323.15 K; when temperature increases excess Gibbs function decreases, GE values for all the mixtures are positive, being higher for mixtures containing 1-butanol and 2-methyl1-propanol, which behavior is quite similar. Excess enthalpies14 that are directly related with energetic effects also have been measured at 298.15 and 313.15 K: HE values are positive and follow the sequence 1-butanol < 2-methyl-1-propanol < 2-butanol < 2-methyl-2-propanol. It is remarkable that the maximum of excess enthalpy values are shifted to rich mole fractions of 1-bromobutane. Excess volumes can provide information not only about energetic effects but also packing phenomena and structural effects. For our mixtures, excess volume15 is positive and the volumetric behaviors of mixtures containing 1-butanol and 2-methyl-1-propanol are again quite similar. The sequence followed by the values VE is the same as that in the case of
excess enthalpy. In light of the results, we can conclude that mixtures containing primary butanol isomers show similar excess bulk properties, while the corresponding properties of mixtures formed by 2-butanol or 2-methyl-2-propanol differ quite a lot. The different behavior observed between the isomeric butanols arises mainly from the differences in the strength and extent of their association in the pure state. The isomeric butanols present lower surface tension values than 1-bromobutane and are more surface active. Therefore the isomeric butanols are expected to be displaced to the surface while 1-bromobutane will tend to stay in the bulk, so surface tension deviation values should be negative (Gibbs adsorption). The bigger difference between surface tension of pure components, the more negative surface tension deviations should be. During the mixing process, new effects, such as changes in the structure of components, repulsion or attraction between unlike
Thermodynamic Study of Surface Formation molecules,16 can affect the surface behavior. These effects can cause, for instance, the alcohols to remain in the bulk, leading to positive values of surface tension deviations. Furthermore, the temperature dependences of all these processes are different, so the resulting process, which is a superposition of all the possible effects, may be very complex when the temperature changes. It is evident that it is not easy to determine what happens in the surface of a mixture, but fortunately, the thermodynamics properties of surface formation obtained from experimental surface tension data can supply useful information to help us understand the surface behavior. For the system 1-bromobutane with 1-butanol, at low temperatures and low or intermediate mole fractions of 1-bromobutane, excess enthalpies are slightly negative; that is, the 1-bromobutane does not break to many hydrogen bonds and the formation of specific interactions between 1-bromobutane and 1-butanol prevails. This fact means that surface tension of the mixture increases due to an enhanced interaction in the bulk, and therefore the surface tension deviation is slightly positive and the excess surface concentration of 1-butanol is small. When concentration of 1-bromobutane increases, the associated structure of 1-butanol is progressively broken, so there is a large amount of free 1-butanol that can migrate to the surface: the excess surface concentration of 1-butanol increases and the surface tension deviation decreases. The maximum excess surface concentration of 1-butanol is reached around x1 ) 0.7, coinciding with the minimum of surface tension deviation and the maximum of excess enthalpy. When temperature increases, the possibility of forming specific interactions decreases. The effect that dominates is the rupture of hydrogen bonds between the butanol molecules, and the surface tension deviation becomes negative, with respect to excess surface concentration of 1-butanol. It decreases because the difference between the surface tension of the pure components also decreases with temperature; that is, the Gibb’s adsorption diminishes. Similar considerations can be made to explain the behavior of the mixtures containing 2-butanol and 2-methyl-1-propanol. Although at low temperatures the mixing process is endothermic, excess enthalpies are not too high due to the existence of specific interactions between the unlike molecules, leading to positive surface tension deviations at low mole fractions of 1-bromobutane. When temperature increases, the possibility of establishing specific interactions decreases, and again the effect that dominates is the breaking of hydrogen bonds between the alcohol molecules, so the surface tension values become negative. It is remarkable that excess surface concentrations of 2-butanol and 2-methyl-1-propanol are higher than in the case of 1-butanol, due to the larger difference between surface tensions of the pure compounds that reveal the tendency to be adsorbed by the surface of the mixture. The behavior of the system 1-bromobutane with 2-methyl2-propanol is quite different; it may be due to several factors such as the structure of the tertiary alcohol, nearly globular, and the lower surface tension of the pure chemical, which denotes weaker interactions in the bulk than in the other isomers. In this case surface tension deviation is negative and much larger in absolute value than for the rest of the systems. Calorimetric behavior shows that excess enthalpy is positive and large and increases with temperature, so more and more hydrogen bonds are broken and there will be a large amount of 2-methyl-2propanol free to migrate to the surface. Excess surface concentration of 2-methyl-1-propanol decreases a little with temperature, because the difference between the surface tension of the pure component also slightly decreases with temperature.
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23101 As we have mentioned above, excess entropy and enthalpy of surface formation for all the mixtures are positive at practically the whole temperature and composition ranges. That is, during the surface formation, more energy is required than for the formation of an ideal surface, and the corresponding entropy is also greater. As the positive values of these properties indicated, the surface formation is driven by the entropy over the whole composition range. The maximum values of these properties are located surrounding a composition that corresponds with the region of greatest excess concentration of alcohol in the surface. In this sense, the amount of free alcohol in the surface seems to be the factor that dominates the surface behavior. We can notice a resemblance between Figures 5-8 and 9-12; maximum peaks in Figures 5-8 are located at the same compositions as minimum peaks in Figures 9-12. Changes in the curvature of the excess concentration figures are reflected in the excess Gibbs function figures; the temperature behavior of excess concentrations and excess Gibbs functions is also parallel, although with opposite sign. Furthermore, it can be pointed out that relevant features of the surface behavior occur in a composition range that coincides with the region in which the associated structure of the alcohol is definitively broken due to the great amount of bromobutane in the mixture. Conclusions Surface tensions of the 1-bromobutane with isomeric butanol mixtures have been measured in the temperature range from 283.15 K (or 298.15 K for 2-methyl-2-propanol) to 313.15 K. A comprehensive thermodynamic study of surface formation has been presented using both surface tension experimental and bulk thermodynamic property data. This study shows how the surface of the mixture is enriched in butanol to a great extent, with maximum surface composition of butanol around 0.7, and how the breaking of the associated structure of the butanols is the factor that mainly dominates the surface behavior. Acknowledgment. We are grateful for financial assistance from D.G.A. Universidad de Zaragoza (INFR 423-06) and Ministerio de Educacio´n y Cultura and Fondos FEDER (BQU 2003-01765). B.G. thanks the Ministerio de Educacio´n y Cultura for a predoctoral grant. References and Notes (1) Glinski, J.; Chavepeyer, G.; Platten, J.-K. J. Chem. Phys. 1995, 102, 2113. (2) Riddick, J. A.; Bunger, W. B.; Sanako, T. K. Organic Solvents. Physical Properties and Methods of Purification in Techniques of Chemistry, 4th ed.; Bernasconi, C. F., Ed.; Wiley-Interscience: New York, 1986. (3) Miller, R.; Hofmann, A.; Hartmann, R.; Schano, K.-H.; Halbig, A. AdV. Mater. 1992, 4, 370. (4) Giner, B.; Cea, P.; Lo´pez, M. C.; Royo, F. M.; Lafuente, C. J. Colloid Interface Sci. 2004, 275, 284. (5) Motomura, K.; Iyota, H.; Ikeda, N.; Aratono, M. J. Colloid Interface Sci. 1994, 126, 26. (6) Garriga, R.; Martı´nez, S.; Pe´rez, P.; Gracia, M. J. Chem. Eng. Data 2002, 47, 322. (7) Martı´nez, S.; Garriga, R.; Pe´rez P.; Gracia, M. J. Chem. Eng. Data 2003, 48, 294. (8) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997. (9) Aratono, M.; Toyomasu, T.; Villeneuve, M.; Uchizono, Y.; Takiue, T.; Motomura, K.; Ikeda, N. J. Colloid Interface Sci. 1997, 191, 146. (10) Motomura, K.; Aratono, M. Langmuir 1987, 3, 304. (11) Renuncio, J. A.; Coto, B.; Caban˜as, A.; Menduin˜a, C.; Rubio, R. G.; Pando, C. Fluid Phase Equilibr. 1996, 126, 177. (12) Smith, S.; Wiseman, P.; Bordreau, L.; Marangoni, G.; Palepu, R. J. Solution Chem. 1994, 23, 207.
23102 J. Phys. Chem. B, Vol. 109, No. 48, 2005 (13) Antipova, A. S.; Semenova, M. G.; Belyakova, L. E. Colloid Surface B 1999, 12, 261. (14) Artigas, H.; Lafuente, C.; Cea, P.; Lo´pez, M. C.; Royo, F. M. Z. Phys. Chem. 2001, 215, 933.
Giner et al. (15) Artigas, H.; Lafuente, C.; Rodrı´guez, V.; Royo, F. M.; Urieta, J. S. J. Chem. Thermodynam. 1994, 26, 151. (16) Glinski, J.; Chavepeyer, G.; Platten, J.-K. J. Chem. Phys. 1996, 104, 8816.
