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Surface orientation and electrical properties of n-butyl alcohol isomers at the free surface of water solution
Colloids and Surfaces,
42 (1989)
Elsevier Science Publishers
39-48
B.V.. Amsterdam
39
-
Printed in The Netherlands
Surface Orientation and Electrical Properties of n-Butyl Alcohol Isomers at the Free Surface of Water Solution PATRYCJA
DYNAROWICZ
Department
of General Chemistry,
Faculty of Chemistry,
Jagellonian
University,
Krakdw
(Poland)
(Received 6 February
1989; accepted 25 May 1989)
ABSTRACT Electric surface potential (d V) and surface tension (;3) measurements of aqueous solutions of n-butyl alcohol, iso-, set-, tert-butyl alcohol, butandiol, trichloro-tert-butyl alcohol and pivalaldehyde were used for the determination of the monolayer structure parameters, e.g. effective dipole moments connected with the reorientation of water molecules, hydrophilic and hydrophobic groups, local dielectric permittivities in the vicinity of these groups and surface orientation angles of adsorbed molecules at the water/air interface.
INTRODUCTION
The behaviour of adsorbed molecules is dependent on their chemical structure as well as on the kind of interface. Together they determine the orientation of the adsorbed substance at the interface. The free surface of water solutions has been discussed in many papers in terms of the interfacial region [l-4]. However, the structure of adsorbed films at the water/air interface has not been thoroughly examined yet. Thus, the aim of this paper is to present data concerning the structure of adsorbed films (e.g. the orientation angles of adsorbed molecules, electrical and thermodynamical parameters of monolayers) prepared from n-butyl alcohol isomers. All calculations concerning these parameters are based on surface tension ( y ) and electric surface potential (AV) measurements. If a dissolved substance is adsorbed at the free surface of water it alters both surface tension and electric surface potential at the water/air interface. The change of surface potential (A V) arising from the formation of a monolayer at the interface is known, according to Lange nomenclature [ 11, as change in contact potential (or Volta potential), Ax. The adsorption process change in surface potential is related both to the displacement of water dipoles by mol-
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
40
ecules of the adsorbed compound and to the simultaneous introduction of its own dipole moment of functional groups [ 51. Hence, the electric potential drop at the water/air interface either decreases or increases, depending on the chemical character of the adsorbed molecule. In the case of adsorption of molecules with polar (hydrophilic ) groups and non-polar (hydrophobic ) groups, aliphatic or aromatic, decrease of the surface potential of water (negative from the air side) is observed. If the adsorbed molecule, however, possesses two or more electronegative groups, and if one of them is more hydrophilic than the others (e.g. halogenophenols [ 6,7], halogenoacids [ 8,9] or halogenoalcohols [ 91) an increase in this negative potential occurs. The electric surface potential, AV, equals Ax only in the case of adsorbed films built up from non-ionized molecules, but if the polar group of adsorbed substance is ionized, the electrostatic potential ( !&) must be taken into account [5,10]. The adsorbed monolayer at the free surface of water produces an electrical double layer. However, it is impossible to determine this structure experimentally. In that case the aqueous solution/air interface is usually depicted by the structure of a parallel plate condenser, according to the Helmholtz idea: AV= 4m&/~
(1)
where ,L is the vertical component of the dipole moment of adsorbed molecule, n, is the total number of molecules present per unit area of the film, and e is the dielectric permittivity in the vicinity of the film dipoles. The value for E is usually taken to be unity, this is approximately true only for the relative permittivity in gaseous media. The Helmholtz equation can be, according to Demchak and Fort [ 111, rewritten in the form: AV=4nn,(Pu,/cl +We2
+&/e3)
(2)
wherein ,&, ,ii2 , & are effective dipole moments of water dipoles (pl ), hydrophilic (& ) and hydrophobic (& ) groups of the monolayer molecules. cl, E, and t3 are local dielectric permittivities in the vicinity of these groups, respectively. For the description of the surface film structure, the data of surface potentials (AV) and number of adsorbed molecules (n,) are necessary [ Eqn (1) 1. Values of surface potentials were obtained from direct measurements and the number of adsorbed molecules was calculated from the Gibbs adsorption equation, using data of surface tension measurements. From the linear dependence AV versus nt, the effective dipole moments of adsorbed molecules were estimated. Knowing the effective dipole moments (,u), dipole moments of free molecules (,u) and an angle between the latter and the main axis of the adsorbed substance, the surface orientation angle was obtained. The particular components of the effective dipole moments and local dielectric permittivities were determined using data of effective dipole moments of compounds with the same hydrophilic groups and different hydrophobic parts, and vice versa.
41
This paper presents the results of investigations of adsorbed films formed by molecules of n-, iso-, set-, tert-butyl alcohol, butandiol, trichloro-tert-butyl alcohol and pivalaldehyde. EXPERIMENTAL
The following compounds were used as experimental materials; n-butyl alcohol, produced by POCh, Poland, iso-butyl alcohol, produced by Ciech, Poland, set-butyl alcohol, produced by BDH Chemicals, England. These compounds were purified by redistillation. Purity of these alcohols was tested by surface tension measurements. The following compounds were used as received: 1,4-butandiol (pure), produced by Fluka Chemie AG, Buchs, Switzerland, tert-butyl alcohol (p.a. ), produced by Loba Chemie ( Wien Fischamend), pivaialdehyde (trimethylacetaldehyde), pure, produced by Fluka, /?,P,P-trichloro-tert-butyl alcohol (chloretone) , pure, produced by POCh, Poland. All solutions were prepared in four times distilled water, boiled down to 2/3 of its primary volume immediately before being used in order to remove CO, and trace impurities of organic compounds which were distilled with the water vapor. The surface tension of solutions was measured by the drop weight method with 20 s drop lifetime, at 20’ C. The time of the drop formation was chosen empirically so as to attain the equilibrium value of the surface tension. The accuracy of the measurements was -t 0.1 mN m-‘. The dynamic jet method [9,12] was applied to measure changes of the electric potential on the surface of solutions of the compounds investigated. The changes of the potential were measured for the following system: Hg,Hg,Cl,/ 0.1 M KCl/investigated solution/air/O.1 M KCl/O.l M KCl/Hg,Cl,,Hg. For the determination of the surface potentials, all the investigated solutions contained a constant amount (0.1 M) of potassium chloride in order to reduce the streaming potential which may arise by the flowing jet method. All measurements were made at room temperature (20’ C ) and with accuracy 2 5 mV. RESULTS
AND DISCUSSION
The results of surface tension and electric surface potential measurements are presented in Figs 1 and 2. The composition of solutions of /?,/?,P-trichloro-tert-butyl alcohol and pivalaldehyde was dependent on the water solubility of these substances. This solubility in 100 g of water is rather small and is 0.5 g and 0.86 g (at 2O”C), respectively. When the concentration of any of the investigated compounds was increased, considerable changes in the electric surface potential as well as surface
c [mole/l]
Fig. 1. Dependence of surface tension (y) on the concentration of aqueous solutions of butandiol (l), teti-butanol (2), set-butanol (3), n-butanol (4), iso-butanol (5), pivalaldehyde (6) and trichloro-tert-butyl alcohol (7).
tension at the solution/air interface were observed. The greatest decrease in the water surface tension was caused by trichloro-tert-butyl alcohol, then by pivalaldehyde, n-butanol isomers, and least of all, butandiol (Fig. 1). Within the range of concentrations O-O.1 A4, the surface tension of secondary and tertiary butyl alcohols is almost identical, and the difference occurs only in more concentrated solutions. The increase of surface activity on going from tertiary to normal alcohol is observed, and the difference between tertiary to secondary is less than from normal to secondary alcohol. However, in the case of n-butyl and iso-butyl alcohol, no significant difference in the surface activity occurs over a wide range of concentration. When comparing the decrease of surface tension caused by tert-butyl alcohol and trichloro-tert-butyl alcohol,
43
AV [mV]
6
i’_
100
0
0.3
0.6
c [mole/l]
0.9
1.2
1.5
Fig. 2. Dependence of the changes in surface potential (AV) on concentration of aqueous solutions of pivalaldehyde (1 ), tert-butanol (2)) n-butanol (3 ), set-butanol (4), isobutanol (5) and butandiol (6).
it is evident that the introduction of three halogen atoms at the P-carbon atom gives a more hydrophobic character to the molecule, resulting in a stronger decrease of surface tension. The opposite effect occurs when a hydrophilic (hydroxyl) group is introduced to the n-butanol molecule, which causes lower adsorption of butandiol at the water/air interface. Figure 2 shows changes in electric surface potential depending on concentrations. Molecules of all the investigated compounds (except trichloro-tertbutyl alcohol) decrease the natural surface potential of water. The greatest decrease of the electric potential is observed for pivalaldehyde, the lowest for butandiol. The difference in the surface behaviour for n-, set-, and tert-butyl alcohols appears only in diluted solutions (Fig. 3 ) . However, the sequence of surface potential changes for these butanol isomers is inverted, compared to surface tension changes. From the four investigated butyl alcohols, tert-butanol molecules cause the greatest change in electric surface potential. It is
44
250
200
150
100
50
c
0
0.02
0.04
0.06
[mole/l] 0.08
Fig. 3. A V versus concentration for n-butyl alcohol isomers: tert-butanol butanol (3) and iso-butanol (4).
( 1), set-butanol
(2 ), n-
quite logical when we compare the area occupied on the surface by one adsorbed molecule (Table 1, column 2) of n-butanol and its branched-chain isomers. Tert-butanol molecules occupy the largest (from butyl alcohol isomers) area on the surface and thus remove the greatest number of oriented water dipoles from the interface, so the decrease in the surface potential of water reaches a maximum value. When an adsorbed molecule possesses electronegative atoms (e.g. trifluoroethanol [ 9 1, chloro-, dichloro-, trichloroacetic acid [ 81, tribromo-, trifluoroacetic acid [ 131, p-fluorobenzoic acid [ 141)) an increase of surface potential was reported. However, in the case of /?,/!I,/?-trichloro-tert-butyl alcohol mole-
45 TABLE 1 Surface parameters of n-butanol isomers monolayers Compound
Area occupied on the surface by one molecule (A*)
Effective Dipole Effective dipole moment dipole group moment of free moment of molecule, (D) adsorbed ha (D) molecule, P‘2 flab fi (D)
butandiol n-butanol iso-butanol set-butanol tert-butanol pivalaldehyde trichloro-tertbutanol
62.3 29.0 29.3 31.2 35.3 38.3 47.4
0.161 0.230 0.210 0.247 0.300 0.410 -
2.40 1.96 1.70 1.65 1.66 -
0.522 0.400 1 .oo
&o/e1
- 177 - 197 - 160 -107 - 108
cz
4.3
e3
1.4
Orientation angle (deg)
69.8 69.1 70.6 72.4
dG” (kcal mol-‘)
-2.08 -3.19 -3.21 - 3.06 -3.01 -3.62 -3.89
“Valuesof fi adopted from Befs [ 151 and [ 181. bValueof fis adopted from Ref. [ 161.
cules an interesting phenomenon occurs. Adsorbed molecules of this compound almost do not change the surface potential of water at all, the value of AV versus concentration is nearly constant (ca - 10 mV). Probably it is connected with the fact that two electropositive methyl groups, concentrated near to the interface and having a wide angle with respect to each other, cancel electronegative -CC& groups out, and vice versa, thus the surface potential of water is almost unchanged. On the basis of the Gibbs isotherm equation it is possible to calculate the relative (to solvent) surface excess P, (1 refers to the solvent, 2 to a solute), and from the latter, the number of molecules (n) adsorbed on 1 cm2 of the surface. Adding the number of molecules already present on the surface (connected with the concentration of a solute) and number of molecules calculated from ra, the total number of adsorbed molecules (q) was estimated. From n, one can determine both the area occupied on the surface by one adsorbed molecule (Table 1, column 2) and the surface effective dipole moment (p) of a molecule at the surface layer. The effective dipole moments were estimated from the linear dependences AV versus nt (Fig. 4). Calculated values of p for adsorbed molecules are compiled in Table 1, column 3. The increase in the effective dipole moments on going from normal to tertiary alcohol is to be expected because of the increase of polarizable matter from n-butanol to tertbutyl alcohol [ 151. Knowing the effective dipole moments of adsorbed molecules (p) and dipole moments of free molecules (p) as well as considering the moment angle, it is possible to determine surface orientation angles. In alcohols [ 151 it was shown
46
AY
[df]
250
2 200
150
,-
6 100
,-
50 1
-
k O
50
100
150
Fig. 4. Surface potential versus the total number of molecules adsorbed on 1 cm2 of surface: pivalaldehyde (1)) tert-butanol (2 ), see-butanol (3 ), n-butanol (4), iso-butanol (5) and butandiol (6).
that the dipole moment can be treated as acting at an angle of about 62’ to the direction of C-C bond. Use of this angle and the angle between p and ,u (estimated from values of effective dipole moments and dipole moments of free molecule) gives the value of the orientation angle of an adsorbed molecule to the free surface (Table 1, column 10). As can be seen adsorbed molecules of nbutyl alcohol isomers take an almost perpendicular orientation to the interface. This can be expected considering the presence of the hydrophobic chains which have a tendency to avoid contact with the water phase.
47
Let us consider expressions for the vertical component of the dipole moment for adsorbed molecules of tert-butyl alcohol and pivalaldehyde
$H20h1
+&CHOhZ
+fi@-(CHde
410
mD
(4)
Solving the above equations (assuming that j&rzo/cl for both compounds is the same) and substituting values of effective dipole moments of hydrophilic groups (calculated from bond moments [16] and angles between them [ 171: j&-OH=522 mD [8] and &_cno = 1000 mD ), the local dielectric permittivity in the vicinity of these groups was obtained ( c2= 4.3 ) . For the determination of the dielectric permittivity in the vicinity of the hydrophobic groups it is necessary to choose compounds which have the same hydrophilic and different hydrophobic groups. Since it is impossible to compare values of ji for tert-butanol and trichloro-tert-butanol, the average value of E,= 1.4 (calculated from pivalic acid, acetic acid and its halogen0 derivatives) was adopted from a previous paper [ 131. Last, introducing values of pz, ,&, cB, Q, (Table 1, columns 6-9) into particular Demchak and Fort equations for all the investigated compounds, the contribution in the effective dipole moment connected with the reorientation of was determined (Table 1, column 5). As can be water molecules (j&o/cl) seen, the contribution to the surface potential of water molecules removed from the free surface by adsorbed molecules is rather insiginificant. The adsorption process of the investigated compounds can be described (in the low concentration region) by the integral form of Henry’s isotherm: In a +K= In&, where a is the activity of the surface active substance, dy= y. - y, y. is the surface tension of the solvent, y is the surface tension of the surface active agent solution, and K= -AG”/RT. By replacing activities by concentrations, the standard free energy of adsorption, AG”, was calculated (Table 1, column 11). Similar values of AG” for n-butyl alcohol isomers are logical on the basis of their similar surface behaviour and orientation at the water/air interface.
REFERENCES 1 2 3 4 5 6 7
J.T. Davies and E.K. Rideal, Interfacial Phenomena, Academic Press, New York, 1963, pp. 154-288 and 57-59. H. Zonntag, Koloidy, PWN, Warszawa, 1982. P. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1986. J.F. Scamehorn, Phenomena in Mixed Surfactants Systems, Am. Chem. Sot., Washington, DC, 1986. J.T. Davies and E.K. Rideal, Can. J. Chem., 33 (1955) 954. M. Filek, M. Paluch and B. Waligbra, J. Colloid Interface Sci., 89 (1982) 166. M. Paluch and M. Filek, J. Colloid Interface Sci., 73 (1980) 282.
48 8 9 10 11 12 13
P. Dynarowicz M. Paluch and P. Dynarowicz, R.J. Demchak M. Paluch and P. Dynarowicz
14 15
M. Paluch and P. Dynarowicz, J. Colloid Interface Sci., 115 (1987) 307. C.P. Smith, Dielectric Behavior and Structure, McGraw-Hill, New York, 1955.
16 16
J-W. Smith, Electric Dipole Moments, Butterworths, London, 1955. A.D. Mitchell and L.C. Cross (Eds), Tables of Interatomic Distances and Configuration of Molecules and Ions, The Chemical Society, London, 1958. Landolt-Biirstein Physicalisch-Chemische Tabelen, 5 Auflage, 3 Erg., 1 Teil, pp. 148-150, and 6 Auflage, 1 Band, 3 Teil, pp. 417-418, Springer-Verlag, Berlin, 1951.
18
and M. Paluch, J. Colloid Interface Sci., 107 (1985) 75. P. Dynarowicz, J. Colloid Interface Sci., 98 (1984) 131. M. Paluch and B. Waligora, J. Colloid Interface Sci., 124 (1988) and T. Fort, J. Colloid Interface Sci., 46 (1974) 191. P. Dynarowicz, Colloid Polym. Sci., 266 (1988) 180. and M. Paluch, J. Colloid Interface Sci., 129 (1989) 379.
436.
42 (1989)
Elsevier Science Publishers
39-48
B.V.. Amsterdam
39
-
Printed in The Netherlands
Surface Orientation and Electrical Properties of n-Butyl Alcohol Isomers at the Free Surface of Water Solution PATRYCJA
DYNAROWICZ
Department
of General Chemistry,
Faculty of Chemistry,
Jagellonian
University,
Krakdw
(Poland)
(Received 6 February
1989; accepted 25 May 1989)
ABSTRACT Electric surface potential (d V) and surface tension (;3) measurements of aqueous solutions of n-butyl alcohol, iso-, set-, tert-butyl alcohol, butandiol, trichloro-tert-butyl alcohol and pivalaldehyde were used for the determination of the monolayer structure parameters, e.g. effective dipole moments connected with the reorientation of water molecules, hydrophilic and hydrophobic groups, local dielectric permittivities in the vicinity of these groups and surface orientation angles of adsorbed molecules at the water/air interface.
INTRODUCTION
The behaviour of adsorbed molecules is dependent on their chemical structure as well as on the kind of interface. Together they determine the orientation of the adsorbed substance at the interface. The free surface of water solutions has been discussed in many papers in terms of the interfacial region [l-4]. However, the structure of adsorbed films at the water/air interface has not been thoroughly examined yet. Thus, the aim of this paper is to present data concerning the structure of adsorbed films (e.g. the orientation angles of adsorbed molecules, electrical and thermodynamical parameters of monolayers) prepared from n-butyl alcohol isomers. All calculations concerning these parameters are based on surface tension ( y ) and electric surface potential (AV) measurements. If a dissolved substance is adsorbed at the free surface of water it alters both surface tension and electric surface potential at the water/air interface. The change of surface potential (A V) arising from the formation of a monolayer at the interface is known, according to Lange nomenclature [ 11, as change in contact potential (or Volta potential), Ax. The adsorption process change in surface potential is related both to the displacement of water dipoles by mol-
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
40
ecules of the adsorbed compound and to the simultaneous introduction of its own dipole moment of functional groups [ 51. Hence, the electric potential drop at the water/air interface either decreases or increases, depending on the chemical character of the adsorbed molecule. In the case of adsorption of molecules with polar (hydrophilic ) groups and non-polar (hydrophobic ) groups, aliphatic or aromatic, decrease of the surface potential of water (negative from the air side) is observed. If the adsorbed molecule, however, possesses two or more electronegative groups, and if one of them is more hydrophilic than the others (e.g. halogenophenols [ 6,7], halogenoacids [ 8,9] or halogenoalcohols [ 91) an increase in this negative potential occurs. The electric surface potential, AV, equals Ax only in the case of adsorbed films built up from non-ionized molecules, but if the polar group of adsorbed substance is ionized, the electrostatic potential ( !&) must be taken into account [5,10]. The adsorbed monolayer at the free surface of water produces an electrical double layer. However, it is impossible to determine this structure experimentally. In that case the aqueous solution/air interface is usually depicted by the structure of a parallel plate condenser, according to the Helmholtz idea: AV= 4m&/~
(1)
where ,L is the vertical component of the dipole moment of adsorbed molecule, n, is the total number of molecules present per unit area of the film, and e is the dielectric permittivity in the vicinity of the film dipoles. The value for E is usually taken to be unity, this is approximately true only for the relative permittivity in gaseous media. The Helmholtz equation can be, according to Demchak and Fort [ 111, rewritten in the form: AV=4nn,(Pu,/cl +We2
+&/e3)
(2)
wherein ,&, ,ii2 , & are effective dipole moments of water dipoles (pl ), hydrophilic (& ) and hydrophobic (& ) groups of the monolayer molecules. cl, E, and t3 are local dielectric permittivities in the vicinity of these groups, respectively. For the description of the surface film structure, the data of surface potentials (AV) and number of adsorbed molecules (n,) are necessary [ Eqn (1) 1. Values of surface potentials were obtained from direct measurements and the number of adsorbed molecules was calculated from the Gibbs adsorption equation, using data of surface tension measurements. From the linear dependence AV versus nt, the effective dipole moments of adsorbed molecules were estimated. Knowing the effective dipole moments (,u), dipole moments of free molecules (,u) and an angle between the latter and the main axis of the adsorbed substance, the surface orientation angle was obtained. The particular components of the effective dipole moments and local dielectric permittivities were determined using data of effective dipole moments of compounds with the same hydrophilic groups and different hydrophobic parts, and vice versa.
41
This paper presents the results of investigations of adsorbed films formed by molecules of n-, iso-, set-, tert-butyl alcohol, butandiol, trichloro-tert-butyl alcohol and pivalaldehyde. EXPERIMENTAL
The following compounds were used as experimental materials; n-butyl alcohol, produced by POCh, Poland, iso-butyl alcohol, produced by Ciech, Poland, set-butyl alcohol, produced by BDH Chemicals, England. These compounds were purified by redistillation. Purity of these alcohols was tested by surface tension measurements. The following compounds were used as received: 1,4-butandiol (pure), produced by Fluka Chemie AG, Buchs, Switzerland, tert-butyl alcohol (p.a. ), produced by Loba Chemie ( Wien Fischamend), pivaialdehyde (trimethylacetaldehyde), pure, produced by Fluka, /?,P,P-trichloro-tert-butyl alcohol (chloretone) , pure, produced by POCh, Poland. All solutions were prepared in four times distilled water, boiled down to 2/3 of its primary volume immediately before being used in order to remove CO, and trace impurities of organic compounds which were distilled with the water vapor. The surface tension of solutions was measured by the drop weight method with 20 s drop lifetime, at 20’ C. The time of the drop formation was chosen empirically so as to attain the equilibrium value of the surface tension. The accuracy of the measurements was -t 0.1 mN m-‘. The dynamic jet method [9,12] was applied to measure changes of the electric potential on the surface of solutions of the compounds investigated. The changes of the potential were measured for the following system: Hg,Hg,Cl,/ 0.1 M KCl/investigated solution/air/O.1 M KCl/O.l M KCl/Hg,Cl,,Hg. For the determination of the surface potentials, all the investigated solutions contained a constant amount (0.1 M) of potassium chloride in order to reduce the streaming potential which may arise by the flowing jet method. All measurements were made at room temperature (20’ C ) and with accuracy 2 5 mV. RESULTS
AND DISCUSSION
The results of surface tension and electric surface potential measurements are presented in Figs 1 and 2. The composition of solutions of /?,/?,P-trichloro-tert-butyl alcohol and pivalaldehyde was dependent on the water solubility of these substances. This solubility in 100 g of water is rather small and is 0.5 g and 0.86 g (at 2O”C), respectively. When the concentration of any of the investigated compounds was increased, considerable changes in the electric surface potential as well as surface
c [mole/l]
Fig. 1. Dependence of surface tension (y) on the concentration of aqueous solutions of butandiol (l), teti-butanol (2), set-butanol (3), n-butanol (4), iso-butanol (5), pivalaldehyde (6) and trichloro-tert-butyl alcohol (7).
tension at the solution/air interface were observed. The greatest decrease in the water surface tension was caused by trichloro-tert-butyl alcohol, then by pivalaldehyde, n-butanol isomers, and least of all, butandiol (Fig. 1). Within the range of concentrations O-O.1 A4, the surface tension of secondary and tertiary butyl alcohols is almost identical, and the difference occurs only in more concentrated solutions. The increase of surface activity on going from tertiary to normal alcohol is observed, and the difference between tertiary to secondary is less than from normal to secondary alcohol. However, in the case of n-butyl and iso-butyl alcohol, no significant difference in the surface activity occurs over a wide range of concentration. When comparing the decrease of surface tension caused by tert-butyl alcohol and trichloro-tert-butyl alcohol,
43
AV [mV]
6
i’_
100
0
0.3
0.6
c [mole/l]
0.9
1.2
1.5
Fig. 2. Dependence of the changes in surface potential (AV) on concentration of aqueous solutions of pivalaldehyde (1 ), tert-butanol (2)) n-butanol (3 ), set-butanol (4), isobutanol (5) and butandiol (6).
it is evident that the introduction of three halogen atoms at the P-carbon atom gives a more hydrophobic character to the molecule, resulting in a stronger decrease of surface tension. The opposite effect occurs when a hydrophilic (hydroxyl) group is introduced to the n-butanol molecule, which causes lower adsorption of butandiol at the water/air interface. Figure 2 shows changes in electric surface potential depending on concentrations. Molecules of all the investigated compounds (except trichloro-tertbutyl alcohol) decrease the natural surface potential of water. The greatest decrease of the electric potential is observed for pivalaldehyde, the lowest for butandiol. The difference in the surface behaviour for n-, set-, and tert-butyl alcohols appears only in diluted solutions (Fig. 3 ) . However, the sequence of surface potential changes for these butanol isomers is inverted, compared to surface tension changes. From the four investigated butyl alcohols, tert-butanol molecules cause the greatest change in electric surface potential. It is
44
250
200
150
100
50
c
0
0.02
0.04
0.06
[mole/l] 0.08
Fig. 3. A V versus concentration for n-butyl alcohol isomers: tert-butanol butanol (3) and iso-butanol (4).
( 1), set-butanol
(2 ), n-
quite logical when we compare the area occupied on the surface by one adsorbed molecule (Table 1, column 2) of n-butanol and its branched-chain isomers. Tert-butanol molecules occupy the largest (from butyl alcohol isomers) area on the surface and thus remove the greatest number of oriented water dipoles from the interface, so the decrease in the surface potential of water reaches a maximum value. When an adsorbed molecule possesses electronegative atoms (e.g. trifluoroethanol [ 9 1, chloro-, dichloro-, trichloroacetic acid [ 81, tribromo-, trifluoroacetic acid [ 131, p-fluorobenzoic acid [ 141)) an increase of surface potential was reported. However, in the case of /?,/!I,/?-trichloro-tert-butyl alcohol mole-
45 TABLE 1 Surface parameters of n-butanol isomers monolayers Compound
Area occupied on the surface by one molecule (A*)
Effective Dipole Effective dipole moment dipole group moment of free moment of molecule, (D) adsorbed ha (D) molecule, P‘2 flab fi (D)
butandiol n-butanol iso-butanol set-butanol tert-butanol pivalaldehyde trichloro-tertbutanol
62.3 29.0 29.3 31.2 35.3 38.3 47.4
0.161 0.230 0.210 0.247 0.300 0.410 -
2.40 1.96 1.70 1.65 1.66 -
0.522 0.400 1 .oo
&o/e1
- 177 - 197 - 160 -107 - 108
cz
4.3
e3
1.4
Orientation angle (deg)
69.8 69.1 70.6 72.4
dG” (kcal mol-‘)
-2.08 -3.19 -3.21 - 3.06 -3.01 -3.62 -3.89
“Valuesof fi adopted from Befs [ 151 and [ 181. bValueof fis adopted from Ref. [ 161.
cules an interesting phenomenon occurs. Adsorbed molecules of this compound almost do not change the surface potential of water at all, the value of AV versus concentration is nearly constant (ca - 10 mV). Probably it is connected with the fact that two electropositive methyl groups, concentrated near to the interface and having a wide angle with respect to each other, cancel electronegative -CC& groups out, and vice versa, thus the surface potential of water is almost unchanged. On the basis of the Gibbs isotherm equation it is possible to calculate the relative (to solvent) surface excess P, (1 refers to the solvent, 2 to a solute), and from the latter, the number of molecules (n) adsorbed on 1 cm2 of the surface. Adding the number of molecules already present on the surface (connected with the concentration of a solute) and number of molecules calculated from ra, the total number of adsorbed molecules (q) was estimated. From n, one can determine both the area occupied on the surface by one adsorbed molecule (Table 1, column 2) and the surface effective dipole moment (p) of a molecule at the surface layer. The effective dipole moments were estimated from the linear dependences AV versus nt (Fig. 4). Calculated values of p for adsorbed molecules are compiled in Table 1, column 3. The increase in the effective dipole moments on going from normal to tertiary alcohol is to be expected because of the increase of polarizable matter from n-butanol to tertbutyl alcohol [ 151. Knowing the effective dipole moments of adsorbed molecules (p) and dipole moments of free molecules (p) as well as considering the moment angle, it is possible to determine surface orientation angles. In alcohols [ 151 it was shown
46
AY
[df]
250
2 200
150
,-
6 100
,-
50 1
-
k O
50
100
150
Fig. 4. Surface potential versus the total number of molecules adsorbed on 1 cm2 of surface: pivalaldehyde (1)) tert-butanol (2 ), see-butanol (3 ), n-butanol (4), iso-butanol (5) and butandiol (6).
that the dipole moment can be treated as acting at an angle of about 62’ to the direction of C-C bond. Use of this angle and the angle between p and ,u (estimated from values of effective dipole moments and dipole moments of free molecule) gives the value of the orientation angle of an adsorbed molecule to the free surface (Table 1, column 10). As can be seen adsorbed molecules of nbutyl alcohol isomers take an almost perpendicular orientation to the interface. This can be expected considering the presence of the hydrophobic chains which have a tendency to avoid contact with the water phase.
47
Let us consider expressions for the vertical component of the dipole moment for adsorbed molecules of tert-butyl alcohol and pivalaldehyde
$H20h1
+&CHOhZ
+fi@-(CHde
410
mD
(4)
Solving the above equations (assuming that j&rzo/cl for both compounds is the same) and substituting values of effective dipole moments of hydrophilic groups (calculated from bond moments [16] and angles between them [ 171: j&-OH=522 mD [8] and &_cno = 1000 mD ), the local dielectric permittivity in the vicinity of these groups was obtained ( c2= 4.3 ) . For the determination of the dielectric permittivity in the vicinity of the hydrophobic groups it is necessary to choose compounds which have the same hydrophilic and different hydrophobic groups. Since it is impossible to compare values of ji for tert-butanol and trichloro-tert-butanol, the average value of E,= 1.4 (calculated from pivalic acid, acetic acid and its halogen0 derivatives) was adopted from a previous paper [ 131. Last, introducing values of pz, ,&, cB, Q, (Table 1, columns 6-9) into particular Demchak and Fort equations for all the investigated compounds, the contribution in the effective dipole moment connected with the reorientation of was determined (Table 1, column 5). As can be water molecules (j&o/cl) seen, the contribution to the surface potential of water molecules removed from the free surface by adsorbed molecules is rather insiginificant. The adsorption process of the investigated compounds can be described (in the low concentration region) by the integral form of Henry’s isotherm: In a +K= In&, where a is the activity of the surface active substance, dy= y. - y, y. is the surface tension of the solvent, y is the surface tension of the surface active agent solution, and K= -AG”/RT. By replacing activities by concentrations, the standard free energy of adsorption, AG”, was calculated (Table 1, column 11). Similar values of AG” for n-butyl alcohol isomers are logical on the basis of their similar surface behaviour and orientation at the water/air interface.
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