1682
J. Phys. Chem. 1991, 95, 1682-1684
Thermodynamics of Adsorption and Mkeliization in Linear and Guerbet Sulfate and Ethoxy Sulfate Surfactants Ramesh Varadaraj,* Jan Bock, Paul Valint, Jr,+ and Stephen Zushma Exxon Research and Engineering Company, Corporate Research Science Laboratories, Clinton Township, Annandale, New Jersey 08801 (Received: January 9, 1990; In Final Form: September 11, 1990)
The thermodynamics of adsorption and micellization in linear and Guerbet sodium ethoxy sulfate and sulfate surfactants have been described by using standard thermodynamic free energy, entropy, and enthalpy parameters. Branching of the linear hydrophobe to give the Guerbet structure results in thermodynamic favorability for adsorption at the air-water interface compared to micellization in bulk. In Guerbet surfactants steric features disfavor micellization, and entropy is the major thermodynamic factor contributing to the free energy of adsorption and micellization.
Introduction Previous studies on fundamental interfacial properties of linear and Guerbet surfctants have revealed that Guerbet branching of the hydrophobe results in significant changes in surfactant properties and performance.' This paper deals with a thermodynamic approach to ascertain how Guerbet branching affects adsorption at the air-water interface and micellization in bulk. Thermodynamic properties of aqueous solutions of sodium salts of c16 Guerbet sulfate and monodisperse (EO), ethoxy sulfate are compared to their c16 linear counterparts (Chart I). This comparative study was expected to lead to an understanding of the effect of hydrophobe structure on the thermodynamics of aggregation at the air-water interface and in bulk solution. Experimental Section Synthesis and purification of the ethoxy sulfates and sulfates used in this study were described in a previous paper.I A Kruss 10 tensiometer was used for surface tension determinations in water and 0.1 N NaCl solutions. Rosen et al. have shown that standard thermodynamic parameters for adsorption at the air-aqueous solution interface and micellization in bulk can be obtained from plots of surface tension versus log concentration.24 By following the methodology described in these reports, standard free energy, entropy, and enthalpy of adsorption and micellization, AGOadrAGOmic,Moa,+ ASomiC, AHoad, and AHomiCwere determined for the linear and Guerbet ethoxy sulfates and sulfates in water and in 0.1 N NaCl solutions at 25 and 40 OC. To ascertain structural effects on adsorption and micellization, the "work of transfer" quantities for standard free energy, entropy, and enthalpy (AGO,, - AGOad), (AS",, - hSoad), and (AHomiC- AHoad) were calculated. Results and Discussion Tables I and I1 lists the thermodynamic parameters of adsorption and micellization for the linear and Guerbet ethoxy sulfates and sulfates in water and 0.1 N NaCl solution. The standard free energies of adsorption and micellization, AGOadand AGOmic,are more negative by about 6-8 kJ/mol for the linear ethoxy sulfate (C16LEsS)than those for the corresponding Guerbet compounds (C,6LGESSand CI6BGESS,Figure 1). This indicates that thermodynamic favorability for adsorption and micellization is lower for the Guerbet surfactants compared to the linear counterpart and reflects of the effect of hydrophobe structure on the thermodynamics of aggregation at interfaces and in bulk. As the hydrophobe bulk increases due to branching, more steric hindrance is offered for intermolecular interactions at the interface and for aggregation into micelles. It is observed that AGOadis more negative than AGOmicfor both linear and Guerbet surfactants, indicating that adsorption at the air-water interface is thermodynamically preferred to micellization in both these systems. Additionally, the observation that AGOad 'Current address: Bausch & Lomb,Rochester, NY.
CHART I
X
= SO,',
(EO)
SO
4 '
is less negative by about 11 kJ/mol in the Guerbet systems compared to 6 kJ/mol in the linear system indicates that branching of the linear hydrophobe more strongly favors adsorption at the air-water interface over micellization. In micelles the surfactants need to pack in a manner with a certain degree of curvature. Because of the curvature in micellar packing, intermolecular interactions would be significantly higher in micellar aggregation than in aggregation at the flat air-water interface. Since Guerbet branched hydrophobes are bulkier and offer greater steric hindrance, micellar packing is relatively less favored than adsorption at the air-water interface., The effect of EO groups on the standard free energies of adsorption and micellization in Guerbet systems is evident through a comparison of C16LGhydrophobe with five, three, and zero EO groups and CI6BG hydrophobe with five and zero EO groups (Figure 1). AGOad and AGOmicbecome more negative with an increase in the number of EO groups, indicating that adsorption at the air-water interface and micellization in bulk becomes more favorable with increasing ethoxylation. This effect is similar to that observed by Rosen et al. for linear ethoxy ~ u l f a t e s . ~ The standard entropies of adsorption and micellization s o a d and Gomic are all positive for the linear and Guerbet systems, indicating increased randomness in the system upon adsorption and micellization. The slight decrease in entropy between linear C,,LE,S and the Guerbet compounds reflects differences in desolvation of the oxyethylene units with change in hydrophobe ( I ) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.;Thomas, R.; Brons, N. J . Phys. Chem., previous three articles in this issue. (2) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J . Phys. Chem. 1982, 86, 541. (3) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J . Phys. Chem. 1986, 90,2413. (4) Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982,5,
159.
(5) Safran, S . A.; Turkevich, L. A.; Pincus, P. A. J . Phys. Leu. 1984,45, 19.
0022-3654/91/2095-1682$02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1683
Guerbet Sulfate and Ethoxy Sulfate Surfactants
TABLE I: Standard Thermodynamic Parameters of Adsorption and Micellization for CI6Linear and Guerbet Ethoxy Water temp, "C LESS LGESS BGESS LGE$ free energy, kJ/mol -48.1 -40.9 -36.5 -38.4 25 AGOmic -56.0 -47.5 -41.0 -43.0 40 AG",i, -54.3 -5 1.2 -45.3 -47.2 25 AGoad -61.7 -56.1 -49.3 -5 1.3 40 AGoad 8.8 8.8 6.2 10.3 25 AGomic- AGoad 5.7 8.6 8.3 8.3 40 AG",i - AGoad entropy, kJ/(mol K) 0.52 0.44 0.30 0.3 1 25 somic 0.27 0.49 0.33 0.26 25 soad 9.60 3 1.58 11.73 11.37 25 T(s"mic - m o l d ) enthalpy, kJ/mol 52.7 1 107.50 88.97 53.45 AH",i, 25 91.72 47.14 33.26 32.18 25 AHoad 15.78 20.19 20.53 25 41.83 AHomic- AHoad mASmIc
TABLE 11: Standard Thermodynamic Parameters of Adsorption and Micellization for CI6 Linear and Guerbet Ethoxy Sulfates in 0.1 N NaCl Solution
LGS
BGS
-27.3 -30.3 -40.8 -42.5 13.5 12.2
-19.7 -22.2 -3 2 -33.3 12.3 11.1
0.20 0.1 1 28.00
0.16 0.09 21.98
33.52 -8.02 41.54
29. IO -5.18 34.28
=lasad
Standard Entrow kJ/mol/K
0.5
free energy, kJ/mol AGO,* AG",i, AGoad
AG'mic L\Gomic- AGoad AGomic- AGoad entropy. kJ/(mol K) somi,
soad
T ( s o m i c- ASoad) enthalpy, kJ/mol AH",i, AHoad
AHomic- AHoad ~
25 40 25 40 25 40
-51.9 -61.4 -56.5 -64.1 4.6 2.7
-48.4 -53.8 -54.5 -59.1 6.1 5.3
-47.9 -50.9 -54.9 -56.1 7.O 5.2
-28.5 -3 1.7 -37.6 -40.7 9.1 9.0
25 25 25
0.63 0.5 39.04
0.36 0.31 15.65
0.20 0.08 36.46
0.21 0.2 1 1.10
25 25 25
136.13 92.50 43.63
59.68 37.88 21.80
12.41 -3 1.06 43.47
35.13 24.98 10.15
0.4
0.3 0.2 0. I 0 CleLE,S
C,&GE,S
m A H mlc
C,QG€$
C1&GE,S
Cq,LGsS
C+,LGS
C,@S
I A H a d
Standard enthalpy kJ/mol
120
C,,L%S
C,&G%S
C,#GE,S
Figure 2. Standard entropies of adsorptiion and micellization for linear and Guerbet sulfates and ethoxy sulfates at 25 "C in water.
A mlcG D A G ad
-(Standard free enemy kJ/mol)
Cl,LE,S
Sulfates and Sulfates in
C1,LGS
C1(pGS
Figure 1. Standard free energies of adsorption and micellization for linear and Guerbet sulfates and ethoxy sulfates at 25 "C in water.
C1OLQ$S
Ci@GBS
CIsLGgS
GeLGS
GgQS
Figure 3. Standard enthalpies of adsorption and micellization for linear and Guerbet sulfates and ethoxy sulfates at 25 "C in water.
accordance with previous findings:+ w o a d is less positive than AHomicas more hydrogen bonds are broken in the process of micellization compared to adsorption at the air-water interface. structure.6*' The expected decrease in s o a d and somic with Similar reasoning holds for the observed decrease in AHoad and a decrease in the number of oxyethylene units occurs in the AHomicwith decrease in the degree of ethoxylation. Guerbet systems (Figure 2). Such effects have been reported To obtain further insight into the structural effects on the for linear ethoxy sulfates also.3 thermodynamics of adsorption and micellization in linear and s o a d values are observed to be slightly lower than somic for Guerbet surfactants, the "work of transfer" (AGOmk- AGOad)was both linear and Guerbet systems. This is indicative of reduced calculated from the standard thermodynamic parameter values. freedom of motion for the hydrophobes packed at the air-liquid The work of transfer function, which measures the ease of adinterface than in the micelle. sorption to form a monolayer at zero surface pressure relative to The standard enthalpies of adsorption and micellization w a d the ease of micellization, shows an increase upon branching of and AHomicare higher for the linear ethoxy sulfate (Figure 3). the linear chain to a Y-branched Guerbet structure. As pointed This indicates that a greater number of bonds between poly(oxout by Rosen et al.? positive values for this work of transfer stem yethylene) chain oxygen and water are broken during adsorption from two sources: (a) a greater positive entropy change upon and micellization in linear than in Guerbet surfactants. Also, in adsorption than upon micellization i.e., high T(Somic- S o a d ) values, and (2) a smaller positive enthalpy change upon adsorption (6) Crook, E. H.; Trebbi, G. F.; Fordyce, D. B. J . fhys. Chem. 1964.68, than upon micellization, i.e., small (AHomic- AHoad). Tables I 3592. and 11 list the T(hSomic - s o a d ) and (AHomic- AHoad) values (7) Schick, M. J. J . fhys. Chem. 1963.67, 1796. ~~
~~~~
~~
~
~~~
1684
J . Phys. Chem. 1991,95, 1684-1689
for the linear and Guerbet compounds. It is observed that the greater positive entropy contribution governs the magnitude of the work of transfer in the ethoxy sulfates. This positive entropy contribution is greater for the Guerbet surfactants than the linear ones. The structural features of the Guerbet hydrophobe that can cause steric disfavorability to micellization may be the cause for the large entropy contribution to the positive value of the work of transfer. The thermodynamics of aggregation at the airaqueous solution interface and in bulk were investigated in 0.1 N NaCl solution for the CI6ethoxy sulfates. Results are shown in Table 11. Trends in free energies of adsorption and micellization observed in water (and discussed above) were similar to those in 0.1 N NaCl solution also. Guerbet branching of the hydrophobe results in a decreased thermodynamic favorability for micellization and increased fa-
vorability for adsorption at the air-water interface.
Conclusion Branching the linear hydrophobe to give the Y-branched Guerbet structure results in an increase in thermodynamic favorability for adsorption at the air-water interface compared to micellization in bulk. In Guerbet surfactants steric features tending to disfavor micellization result in entropy being the major thermodynamic factor. Because of the relative thermodynamic favorability for adsorption at the air-water interface, Guerbet surfactants are very effective in reducing the air-water surface tension. Guerbet surfactants would offer unique advantages in surfactant applications such as foaming which depend on surfactant adsorption at the air-water interface and the effectiveness of surface tension reduction.
Uptake of N,05 and HNO, by Aqueous Sulfuric Acid Droplets J. M. Van Doren,t L. R. Watson: P. Davidovits, Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 021 67
D. R. Worsnop,* M. S. Zahniser, and C. E. Kolb Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821 (Received: March 20, 1990; In Final Form: August 24, 1990)
Uptake coefficients for gaseous NzOs and HN03 into liquid 0.33 HzS04mole fraction (73 wt %) droplets at 283 K have been measured. HN03 in the gas phase was directly observed as a product of Nz05uptake. The experimental method employs a monodisperse train of droplets (-200-pm diameter) in a low-pressure flow reactor. Droplet-trace gas interaction times are 1-2 ms. Uptake coefficients are 0.058 f 0.006 for NzOSand 0.1 1 f 0.01 for HN03. These values are 40% larger and smaller than for NzOSand HN03 uptake, respectively, into pure water. The product branching ratio for gaseous evolution of H N 0 3 from N 2 0 5heterogeneous reaction ranged from 0.25 to 0.37 for gas/liquid contact times of (1.1-1.6) X s. Both the reduction in HN03 uptake coefficient (relative to that on water) and the HN03 gaseous evolution from N I O Suptake can be explained by limited H N 0 3 solubility in aqueous sulfuric acid. A value of 4 ( f 2 ) X IO3 M atm-l at 283 K IS derived for the Henry’s law constant based on a time-dependent model of this limitation. Nitric acid production from the heterogeneous reaction of N205with aqueous sulfuric acid may provide an important mechanism for removal of NO, from the stratosphere.
-
Introduction The heterogeneous interactions of gas-phase species with aerosols play an important role in atmospheric chemistry. In the polar stratosphere heterogeneous reactions have been pivotally involved in the ozone destruction process.’-‘ Heterogeneous reactions provide both a mechanism for the removal of gaseous species and a means for chemical transformation of these species. An important process in Antarctic ozone depletion is the removal of nitrogen oxides from the gas phase (denitrification), which inhibits the formation of chlorine nitrate leading to large concentrations of ozone reactive CIO. In this context the heterogeneous interactions of nitric acid ( H N 0 3 ) and dinitrogen pentoxide (N205)with polar stratospheric cloud particles have been postulated as important steps in the catalytic destruction of ozone.1*2*4 An outstanding question is whether the heterogeneous reactions implicated in the polar stratosphere occur in the nonpolar stratosphere as well. Aerosols in the mid-latitude stratosphere, 1625-km altitude region, are principally composed of aqueous sulfuric acid in the range 60-80 wt 9% (0.2-0.4 mole fraction) sulfuric acid as governed by the water vapor mixing r a t i ~ . ~The . ~ size of the aerosols is in the range 0.1-1 pm, and the surface area is estimated to be in the range 10-9-10-8 cm2/cm3.’ The ambient temperature in this region is between 21 5 and 220 K where the acid aerosol ‘Current address: Air Force Geophysics Laboratory, Hanscom AFB, MA 01731-5000.
0022-3654/91/2095- 1684$02.50/0
is expected to be a supercooled liquids6 There have been several attempts to assess the impact of heterogeneous processes on global ozone reductions outside the polar regions.’~* Hofmann and Solomon modeled the observed ozone depletions following the eruption of El Chichon and concluded that heterogeneous reactions may have been responsible for part of the anomalously low ozone levels observed a t midlatitudes in early 1983.’ To properly assess the importance of heterogeneous processes, reliable experimental kinetic data must be obtained for these reactions as a function of temperature and aerosol composition. Particularly important in their model is the conversion of N 2 0 5to HNO,
N205+ H20(aq)
-
2HN03(aq or g)
(R1)
(1) Solomon, S.Reu. Geophys. 1988, 26, 131-148. (2) Wofsy, S. C.; Molina, M. H.; Salawitch, R. J.; Fox, L. E.; McElroy, M. B. J. Geophys. Res. 1988, 2442-2450. (3) Salawitch, R.J.; Wofsy, S.C.; McElroy, M. B. Geophys. Res. Lett. 1988, IS, 871. (4) KO,M. K. W.; Rodriguez, J. M.; Sze, N. D.; Profitt, M. H.; Starr, W.
L.; Krueger, A.; Browell, E. V.;McCormick, M. P. J. Geophys. Res. 1989, 94, 16, 683. (5) Rosen, J. M. J . Appl. Meteorology 1971, 10, 1044-1046. (6) Steele, H. M.; Hamill, P.; McCormick, M. P.; Swissler, T. J. J . Amos. Sci. 1983, 40, 2055-2067. (7) Hofmann, D. J.; Solomon, S . J. Geophys. Res. 1989,94,5029-5041. (8) Rodriguez, J. M.; KO,K.W.; Sze, N. D. Geophys. Res. Lett. 1988, I S , 257-260.
0 1991 American Chemical Society
