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Chemical structure and thermodynamics of amphiphile solutions 2. Effective length of alkyl chain in oligooxyalkylenated alcohols
239
Colloids and Surfaces, 56 (1991) 239-249 Elsevier Science Publishers B.V., Amsterdam
Chemical structure and thermodynamics of amphiphile solutions 2. Effective length of alkyl chain in oligooxyalkylenated alcohols*>** Adam Sokolowski Institute
of Organic and Polymer
Technology,
Technical
University
of Wroctaw, 50-370
Wroctaw (Poland)
(Received 5 June 1990; accepted 15 October 1990)
Abstract Surface tension measurements were performed for individual monoethers of oligooxyethylene glycols and individual oligooxypropylene analogs in aqueous solution at 293.15 K. The experimental dependence of surface pressure x on concentration z (assuming ideality of the solution) was well described by a new empirical adsorption equation which enables us to interpret quantitatively the relationship between chemical structure and surface activity for a homologous series of amphiphiles. On the basis of this equation a new concept of effective length of alkyl chain, neff, was developed. The results are discussed in terms of the dual hydrophilic/hydrophobic nature of the oxyethylene, -OCH,CH,-, and oxypropylene, -OCH,CH (CH,)-, groups.
INTRODUCTION
In our previous work, we have investigated the surface activity of individual alkyl monoethers of oligooxyethylene glycols (1, C,H2,+ 1( 0CH2CHz),0H, n=4-8; z= l-5) [ 21, and individual oligooxypropylene analogues (2, CH3 G&L+ 1 (OCH, C’H ),OH, n= l-4; z= l-7 [ 31 at the aqueous solution-air interface. A quantitative correlation has been found between the standard free energy of adsorption, AGZ, and the structure of the studied monoethers (1,2) and that of the aliphatic alcohols (3, C,H2,+ 10H, n= 4-8) [ 2,3] as well as by using the appropriate multiple regression equation. It has been found that within the range of surface tension decrease studied, where n ranges from 2 to *For Part 1, see Ref. [ 11. **Presented at the conference on Application of New Trends in Surfactant and Colloid Chemistry, Torino, Italy, 5-9 June 1989.
240
25 mN m-l, not only the methylene group, -CH2-, of the hydrocarbon chain and the oxypropylene group, -OCH&H ( CHB)-, but also the oxyethylene group, -OCH2CH2-, can show hydrophobic character. However, the hydrophobic character of the oxyethylene and oxypropylene groups is not constant. For the groups of compound investigated it decreases with increasing length of hydrocarbon chain and with the increase in n. The hydrophobic character of the methylene groups of monoethers is also not constant. It is known that in many homologous series of surfactants a correlation exists between the number of carbon atoms in the alkyl group of a monomeric amphiphile and the c.m.c.; it is represented by the equation log c.m.c. =A +B n
(1)
where n is the number of -CH,- groups in the alkyl chain, and A and B are experimental constants for a particular series of surfactants [4-71. The concept of the effective length of an alkyl chain, neff, was developed to generalize the influence of structural modifications on the surface phenomena and colloidal properties of surfactants [8-lo]. The value neff, formulated in terms of -CH2- groups, is the value of n for which the analogous straight-chain compound indicates the same surface activity as the structurally modified compound. At present the criteria for determining neff are as follows: c.m.c., polarity indexes etc. As an example, Eqn (1) for structurally modified surfactants takes the form log c.m.c. =A +B neff
(2)
The calculated values of neff for the investigated surfactants containing a definite length of the hydrocarbon chain, n, make it possible to determine the socalled hydrophobicity index (HI) [ 8,9] which is defined as
HI=n,ff n
(3)
Hitherto the concept of neff shows several limitations such as difficulties in finding the appropriate “standard” straight chain length surfactants, and the impossibility of determining neff for surface active agents which do not form micellar solutions. The aim of this work was to test the new concept of the effective length of the alkyl chain in non-ionic amphiphiles. It was expected that the new concept of neffwould make it possible to compare directly the hydrophilic/hydrophobic nature of the oxyethylene and oxypropylene groups in oxyalkylenated alcohols, diols, thiols, amines etc. The present discussion will be limited to only alkyl monoethers of oligooxyethylene glycols (1))alkyl monoethers of oligooxypropylene glycols (2 ) and aliphatic alcohols (3 ) as “standard” amphiphiles.
241 MATERIALS AND METHODS
Pure homogeneous n-butyl, n-amyl, n-hexyl, n-heptyl and n-octyl monoethers of oligooxyethylene glycols containing from one to five oxyethylene groups per molecule were obtained by fractional distillation of the polydisperse products of the addition of oxirane (ethylene oxide) to the corresponding aliphatic alcohols [ 251. Individual methyl, ethyl, n-propyl and n-butyl monoethers of oligooxypropylene glycols containing from one to seven oxypropylene groups per molecule were obtained by fractional distillation of the products of the addition of methyloxirane (propylene oxide ) to aliphatic alcohols [ 3,111. The GLC analysis of these compounds (a Giede 18.3.6 chromatograph; stationary phases, Apiezon APZL on Chromosorb Q and Carbowax 20M on Chromosorb G/AW DMCS) did not reveal any impurities (limit of detection, 0.1% ). The equilibrium surface tension of aqueous solutions of monoethers 1 and 2 was determined at 293.15 t 0.1 K [ 2,3]. Water was triply distilled from alkaline permanganate solution. The maximum bubble pressure method was accurate to 20.1 mN m-‘. The experimental data for the surface tension of aqueous alcohol (3) solutions at 298.15 K were taken from the paper of Posner et al. [ 121. RESULTS AND DISCUSSION
The experimental values of the equilibrium surface tension for aqueous solutions of monoethers 1 and 2 and aliphatic alcohols 3 were found to be well described by a simplified form of the equation for Temkin’s isotherm [ 13-151. The Temkin isotherm is valid for the coverage range 8=0.2-0.8 and describes well the adsorption process of non-ionic amphiphiles at the aqueous solutionair interface [ 16-211. This equation can be written in the following form convenient for further considerations: In x= -In B+n’j2
(g/RT)
(4)
where x is the molar fraction of a substance (in the bulk of the solution) at a given surface pressure n, B is the equilibrium constant of the adsorption process, and g is a constant. The correlation coefficients, R, which characterize the quality of the fit, are in the range 0.996-0.999 for all the compounds studied (l-3). In order to find a quantitative relationship between the adsorption of the compounds and their structure we propose to describe the process by a new empirical adsorption equation: In x=b,+b,n+b,z+b,n
~+~“~(b~+b,n+b,z+b,n
2)
(5)
242
In this equation which is based on Temkin’s isotherm (Eqn. (4) ] In x means A@, (in RTunits), n is the number of carbon atoms in the hydrocarbon moiety, z is the number of oxyalkylene units and b& are regression coefficients. The proposed equation not only reflects the correlation between surfactant concentration (x) and surface pressure (z to the power 0.5) but also the effect of the structural elements of the molecules on the adsorption process for the whole group of amphiphiles C,A,W, for example, &Hz,+ 1(OCH,CH,).OH, HO ),OH, CJ%n+~ XCH,CH, ( 0CH&H2).0H, C,Hz,+ I (OCI-WH(CHs) (CH2CH20)z,2(CH,CH(CH,)O),(CH,CH,0)z,,Hetc. where X=S, NH. This new adsorption equation makes it possible to recalculate Temkin’s constants for the compounds studied ( 1)- (3 ) In B= - (b,+b,n+b,z+b,nz) g= (b,+b,n+bgZ+b,nz)
Jlooo
(6)
RT
(7)
and if we increase the number of variables related to amphiphile structure it will also be useful for more structurally-modified amphiphiles, for example, and &Hz,+ I (OCH2CH(CHB)),(OCH&H2),0H CH[O(CH,CH,O).C,H,+,l,. CnH~n+i The number of products in Eqns (5)-( 7) can be reduced if all non-significant products bixi, biXi3Cj, biXi3CiXk etc. are rejected [ 2,191. The coefficients bi and their standard errors S (bi) for the compounds studied are presented in Table 1. By using t statistics, it has been shown that at a confidence level of cr= 0.05, the product b-gun “’ can be neglected in Eqns (5) and (7). In all cases a high coefficient R justifies the form of Eqn (5) and Table 1. The details of the analysis of regression used may be found in the papers of Bartkowiakowa [ 221 and Jennrich [ 231. For monoethers of oligooxyethylene glycols (i= 1))monoethers of oligooxypropylene glycols (i = 2 ) and alcohols (i = 3 ) the constants In B and g were calculated from the following equations: (In Bcalc)i= 1= 7.014 + 0.8418n+ 1.01062 - 0.04640nz
(8a)
(lnB,,,,)i=,=8.237+0.8487n+1.3839z-0.04134nz
(8b)
(In B,,i,)i=3=4.998+
(8~)
1.1300n
(gcalc)+i= (1.2716-0.07949n+0.11221z)7.7075~104
(9a)
(gca,c)i=2= (1.3522-0.05390n+0.124492)7.7075~104
(9b)
(gca,c)i=3= (0.9373-0.03334n).7.8390.104
(SC)
and are listed in Tables 2-4 along with the values determined directly from the Temkin isotherm [Eqn (4) 1. It is seen that a good agreement between calculated and determined constants of Temkin’s isotherm was obtained which indicates that the applied mathematical model has been properly selected.
243 TABLE 1 Coefficients of the new empirical adsorption equation, the regression equation [Eqn (5) 1; z range from 2 to 25 mN m-r Coefficient
bo S(b,)
b, S(b)
b, S(b,)
b, SCM b, S(h) bs St&,) bs S(b) b, S(b) N R
Monoethers ( 1)
Monoethers (2 )
Eqn (5a)
Eqn (5b)
-7.014 (0.14) -0.8418 (0.022) - 1.0106 (0.038) 0.04640 (0.0063) 1.2716 (0.035) - 0.07949 (0.0051) 0.11221 (0.0053)
- 8.237 (0.10) -0.8487 (0.033) - 1.3839 (0.019) 0.04134 (0.0062) 1.3522 (0.029) -0.05390 (0.0075) 0.12449 (0.0043) -
138 0.9984
132 0.9990
Alcohols (3 ) Eqn (5~)
- 4.998 (0.10) - 1.1300 (0.021)
0.9373
(0.036) - 0.03334 (0.0056)
49 0.9993
In the case of amphiphiles with a bipolar structure, CHB(CH2),_1W, according to Eqn (10) the standard free energy of adsorption AG” comprises two main components: AG”=nAG”[-CH2-]
+AG”[Xw]
(10)
where AG”[-CH,-] and AG” [X,] values are related to the energy of adsorption of the methylene group in the hydrocarbon chain and the remaining parts of molecules, respectively, according to the equation AG”[X,]=AG”[W]+(AG”[CH,-]-AG”[-CH,-1)
(II)
For aliphatic alcohols (i= 3 ) as “standard’ amphiphile we can write In x=b0+blrz,ff+7c”2 AG[CH,(CH,),_,OH]
(bq+b5neff) =n,ffAG”[-CH2-]i=,+AG”[Xw]i=3
(12) (13)
A combination of Eqn (12) with Eqn (13) for alcohols (3) yields AG”[-CH,-]i=,=b,+b,n”2
(14)
AG”[Xw]i=3=bo+b,n”’
(15)
where index 3 denotes alcohol 3.
244 TABLE 2 Constants in the equation for the Temkin adsorption
isotherm of monoethers
of oligooxyethylene
glycols (1)
No.
Structure
g-10-4 (N1” m3” mol-‘) Deb”
InB
Caleb
Det.”
Calc.’
8.00
8.22 9.08 9.94
11.01 12.07 12.65
11.21
8.93 9.66 10.82
10.81 11.67
13.55 14.57
13.68 14.51 12.00 12.78
11.73 6
7.87
7.60
12.00
7
8.67 9.08
8.47
13.05 13.50
8 9 10 11 12 13
10.07 11.19 7.53 8.25
14
8.58 9.57
15
10.61
16 17
6.44
9.33 10.20 11.06 6.99
13.01
12.80
7.85 8.72
13.94
13.53 14.26
9.58 10.45
14.21 14.87 15.92 13.54
8.97
15.63
22
6.03 6.23 6.64
5.76 6.63 7.49
23
9.05
8.36
14.45 14.84 15.38 16.49
18 19 20 21
13.56 14.34
14.27 15.18
6.38 7.24 8.11
6.94 7.75 9.21
12.03 12.86
14.18 14.94
15.12
14.99 15.73 13.59 14.28 14.96 15.65 14.39 15.03 15.67 16.31
“From Eqn (4). bFrom Eqn (9a). ‘From Eqn (8a).
A combination of Eqn (5) with Eqn (10) for monoethers 1 and 2 yields AG”[-CH,-]i=b,+ AG”[Xw]i=b,+b2z+
t bg~+b,n”~+
1 b7~n”~
4 b,n+b,n”2+bgzn1’2+
(16) f b,nn”2
(17)
where the i indices are 1 and 2. The new concept of effective length of alkyl chain in amphiphiles, neff, on the grounds of the proposed new empirical adsorption equation and taking aliphatic alcohols 3 as “standard” amphiphile, is as follows:
245 TABLE
3
Constants
in the equation for the Temkin
adsorption
isotherm
of monoethers
of oligooxypropy-
lene glycols (2 ) No.
Structure
(N1"m3” n
In B
g-10-4
z
mol-‘)
Det.
Calc.b
Det.
Calc.’
12.58 12.00
10.97 11.93
10.74 11.73
10.43 11.77
12.54
12.88
13.11
13.52 14.68
13.84 14.80
14.50 15.92
13.11 14.46
14.95 16.24
15.76 16.72
16.82 18.23
8
10.64
10.55
11.34
9 10
12.13
11.51
12.55
12.54
12.98
12.47
13.92
13.84
11 12
13.71 14.82
13.43
15.19
15.91
16.68 17.95
15.14 16.44
13 14
14.39 15.35
17.74
16.69
16.31
19.03
19.04
10.14 11.09
11.87
12.04 13.30
15.80 17.14 18.48 11.24
15
10.03
16 17
10.38 12.22
12.05
13.00 14.73
18
13.09 14.00
13.01 13.97
15.92 17.15
15.02 15.69
14.93 15.89
18.41 19.45
18.34 19.60 12.85
19 20 21
14.56 15.82 17.08
22
8.93
9.72
12.42
23 24
10.41 12.02
10.68 11.64
14.03
14.07
25
12.60
26 27
12.89 13.73 14.32
15.65 16.80
15.29 16.51 17.72
28
15.31
13.56 14.52 15.48
17.84 18.76 20.03
18.94 20.16
“From Eqn (4). bFrom Eqn (9b). ‘From Eqn (8b ) .
AG” [ -CH,- ] i neff=AGo[_CH,_]i=,‘n+
AG”[Xw]i-AG”[Xw]i,, AG” [-CH,-] i=x
(18)
The incremental values of AG” [ Xw ] i and AG” [ -CH2-] i were calculated on the basis of Eqns (14)-(17).
246 TABLE
Constants No.
4
in the equation for the Temkin adsorption Structure
isotherm
g-10-4 (N’12 m3’* mol-‘)
of alcohols (3) In B
n
1
4
2
5
3 4
6 7
5
8
Det.
Calc.b
6.39 6.04
6.30 6.04
5.80 5.74 5.17
5.78 5.52 5.26
Det.” 9.49 10.69 11.82
Calc.” 9.52 10.65
13.09
11.78 12.91
13.93
14.04
“From Eqn (4). bFrom Eqn (9c ) . ‘From Eqn (8~).
It was expected that the proposed neff concept would make it possible to compare quantitatively the hydrophilic-hydrophobic nature of oligooxyalkylenated derivatives. The values of both neff (see Eqn ( 18) ) and ( neff- n) for oxyethylenated alcohols (i = 1) and oxypropylenated alcohols (i = 2 ) at various surface pressures are shown in Table 5. All neff values for the &HZ,+ 1(A),OH monoethers studied increase with the decrease in surface pressure and with the increase in the oxyalkylene group number (A=OCH,CH,- or -OCH,CH(CH,)-) contained in the molecules. Thus, the effective alkyl chain length, neff, in the amphiphiles investigated (independently of the oxyethylenated or oxypropylenated derivatives) is higher than a nominal hydrocarbon chain length of the amphiphile molecules, taking aliphatic alcohols as a reference. Values of ( neff- n) (see Table 5 and Fig. 1) at 5 mN m -’ for C4HSOCH2CH20H and C,H,OCH,CH ( CH3) OH indicate that the incorporation of an oxyethylene (-OCH,CH,-) and an oxypropylene (OCH,CH (CH,)-) group between the alkyl chain and hydroxyl group of the nbutanol molecule has a similar effect to that induced by incorporation of additional 0.90 and 1.9 (-CH,-) groups respectively. Incorporation of the next oxyalkylene groups causes a further increase in neff (Table 5); thus the oxyethylene and oxypropylene groups of monoethers 1 and 2 show a hydrophobic nature at a surface pressure ranging from 5 to 25 mN m-‘. It can be seen from the results shown that (n,,- n) decreases with the length of hydrocarbon chain (for the corresponding size of the oligooxyalkylene group, (A),). As was expected, the values of neff for all oxypropylenated alcohols (2 ) are higher than those for oxyethylenated ones ( 1). The new measure of hydrophobicity, neff, proposed here, makes it possible to compare the oligooxyethylenated and oligooxypropylenated chains in a simple way by
247 TABLE 5 Effective length of alkyl chain, nefffor monoethers 1 and 2 Structure 71 (mN m-‘) n
2
1
1
1
3
1
5
2
1
2
3
2
5
3
1
3
3
3
5
4
1
4
3
4
5
5
1
5
3
5
5
6
1
6
3
6
5
Monoether ( i = 1)
Monoether ( i = 2 )
hf
fbff -
ndf
5 25 5 25 5 25
2.48 1.54 3.70 2.20 4.92 2.85
1.48 0.54 2.70 1.20 3.92 1.85
3.60 2.32 5.39 3.46 7.19 4.60
2.60 1.32 4.39 2.46 6.19 3.60
1.1 0.8 1.7 1.3 2.3 1.7
5 25 5 25 5 25
3.29 2.46 4.47 3.08 5.65 3.70
1.29 0.46 2.47 1.08 3.65 1.70
4.36 3.15 6.12 4.26 7.89 5.36
2.36 1.15 4.12 2.26 5.89 3.36
1.1 0.7 1.7 1.2 2.2 1.7
5 25 5 25 5 25
4.09 3.37 5.24 3.96 6.38 4.54
1.09 0.37 2.24 0.96 3.38 1.54
5.13 3.98 6.86 5.05 8.59 6.12
2.13 0.98 3.86 2.05 5.59 3.12
1.0 0.6 1.6 1.1 2.2 1.6
5 25 5 25 5 25
4.90 4.29 6.00 4.84 7.11 5.39
0.90 0.29 2.00 0.84 3.11 1.39
5.90 4.80 7.60 5.85 9.29 6.89
1.90 0.80 3.60 1.85 5.29 2.89
1.0 0.5 1.6 1.0 2.2 1.5
5 25 5 25 5 25
5.71 5.21 6.77 5.72 7.83 6.23
0.71 0.21 1.77 0.72 2.83 1.23
6.67 5.63 8.33 6.64 9.99 7.65
1.67 0.63 3.33 1.64 4.99 2.65
1.0 0.4 1.6 0.9 2.2 1.4
5 25 5 25 5 25
6.51 6.12 7.54 6.60 8.52 7.07
0.51 0.12 1.54 0.60 2.52 1.07
7.44 6.46 9.06 7.43 LO.69 8.41
1.44 0.46 3.06 1.43 4.69 2.41
0.9 0.3 1.5 0.8 2.2 1.3
n
(n,ff)i=z-
k-n
(%)i,,
248
nelf -
Fig. 1. Effective length of alkyl chain, neff, for alkyl monoethers of ethylene glycol (i = 1, z = 1) and alkyl monoethers of propylene glycol (i= 2, z = 1) at 5 mN m-l. A: 1, C,H,,+ 10CH,CH,OH; 3, 3, C,H,,+,OH. &Hz,+, OH. B: 2, C,H,,+, OCH,CH(CH,)OH;
evaluating the following expression: (~r)i,~(n,ff)i,l. The value of (n eff ) C,H~,+IOCHZCH(CH~)OH - (%ff) CnHzn+ 10CHzCHzOH ) independently of the size of the substituent &Hz,+ 1, equals approximately 0.5-1.0 unit of (-CH,-) (see Table 5, column 8). The above hydrophobicity difference is understandable because in the monoether of propylene glycol there is a secondary hydroxyl group. More interesting effects have been observed after calculation of [ (neff)i,a(n,ff)i=l] for di-, tri-, tetra- and penta-oxyalkylenated monoethers. Changing the oligooxyethylenated chain to an oligooxypropylenated one causes the molecule hydrophobicity to increase, expressed only as insertion of l-2 methylene groups into the hydrocarbon chain. It is a very small difference considering that every next -OCH,CH ( CH3 ) - group in the oligooxypropylenated chain introduces an additional methyl group to the monoether molecule. It can be concluded that the adsorption on a free surface of oligooxyalkylene glycol monoethers having short hydrocarbon chains proceeds in such a way that both oxyethylene and oxypropylene groups can show hydrophobic character. This effect decreases with the hydrocarbon chain increase. When the hydrocarbon chain is long enough one may expect that this effect may disappear; however, even in the case of dodecyl ethers of monooxyethylene- and dioxyethylene glycol the dual hydrophilic/hydrophobic nature of isolated -CH,CH,O- groups is demonstrated by showing that they give a negative con-
249
tribution to the standard free energy of adsorption from both water and heptane to the heptane-water interface [ 241. The method proposed for the calculation of neffmakes it possible to provide a direct comparison of the hydrophilic/hydrophobic character of various surfactant fragments in the adsorption process.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Part 1: A. Sokolowski, J. Phys. Chem., 93 (1989) 8223. A. Sokoiowski and B. Burczyk, J. Colloid Interface Sci., 94 (1983) 369. A. Sokoiowski and J. Chlebicki, Tenside Deterg., 19 (1982) 282. K. Shinoda, T. Nakagawa, B. Tamamushi and T. Jsemura, Colloidal Surfactants, Some Physicochemical Properties, Academic Press, New York, 1963. S. Kucharski, A. Sokotowski and B. Burczyk, Rocz. Chem., 47 (1973) 2045. M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1978, pp. 94,164. M. Nakagaki and T. Handa, in M.J. Rosen (Ed.), Structure/Performance Relationships in Surfactants, American Chemical Society, Washington, 1984, p. 73. I.J. Lin, J.P. Friend and Y. Zimmels, J. Colloid Interface Sci., 45 (1973) 378. I.J. Lin, B.M. Moudgil and P. Somasundaren, Colloid Polym. Sci., 252 (1974) 407. I.J. Lin and L. Marszali, Tenside Deterg., 14 (1977) 131. J. Chlebicki, Rocz. Chem., 48 (1974) 1241. A.M. Posner, J.R. Anderson and A.E. Alexander, J. Colloid Sci., 7 (1952) 623. P. Parsons, Trudy IV Soveschanija po Electrokhimii, Acad. Sci. USSR Ed., Moscow, 1959, p. 42. P. Delahay, Double Layer and Electrode Kinetics, Wiley, New York, 1965, p. 81. A. Couper, in Th.F. Tadros (Ed.), Surfactants, Academic Press, London, 1984, p. 19. J. Lucassen and D. Giles, J. Chem. Sot., Faraday Trans. 1,71 (1975) 217. A. SokoIowski and B. Burczyk, Proc. XII Jornadas de1 Comite Espailol de la Detergencia, Barcelona, 1981, pp. 491-507. A. SokoIowski, B. Burczyk and J. Beger, Colloids Surfaces, 36 (1989) 373. A. Sokolowski, B. Burczyk and J. Beger, Colloids Surfaces, 44 (1990) 89. H. Lange and P. Jeschke, in M.J. Schick (Ed.), Nonionic Surfactants-Physical Chemistry, Marcel Dekker, New York, 1987, p. 1. J. van Hunsel and P. Joos, Colloids Surfaces, 24 (1987) 139. A. Bartkowiakowa, Zastosowanie Matematyki (Appl. Math.), 16 (1978) 293. R.J. Jennrich, Stepwise regression, in K. Enslein, A. Ralaston and H.S. Wilf (Eds.), Statistical Methods for Digital Computers, Wiley, New York, 1977, p. 58. R. Aveyard, N. Carr and H. Slezok, Can. J. Chem., 63 (1985) 2742.
Colloids and Surfaces, 56 (1991) 239-249 Elsevier Science Publishers B.V., Amsterdam
Chemical structure and thermodynamics of amphiphile solutions 2. Effective length of alkyl chain in oligooxyalkylenated alcohols*>** Adam Sokolowski Institute
of Organic and Polymer
Technology,
Technical
University
of Wroctaw, 50-370
Wroctaw (Poland)
(Received 5 June 1990; accepted 15 October 1990)
Abstract Surface tension measurements were performed for individual monoethers of oligooxyethylene glycols and individual oligooxypropylene analogs in aqueous solution at 293.15 K. The experimental dependence of surface pressure x on concentration z (assuming ideality of the solution) was well described by a new empirical adsorption equation which enables us to interpret quantitatively the relationship between chemical structure and surface activity for a homologous series of amphiphiles. On the basis of this equation a new concept of effective length of alkyl chain, neff, was developed. The results are discussed in terms of the dual hydrophilic/hydrophobic nature of the oxyethylene, -OCH,CH,-, and oxypropylene, -OCH,CH (CH,)-, groups.
INTRODUCTION
In our previous work, we have investigated the surface activity of individual alkyl monoethers of oligooxyethylene glycols (1, C,H2,+ 1( 0CH2CHz),0H, n=4-8; z= l-5) [ 21, and individual oligooxypropylene analogues (2, CH3 G&L+ 1 (OCH, C’H ),OH, n= l-4; z= l-7 [ 31 at the aqueous solution-air interface. A quantitative correlation has been found between the standard free energy of adsorption, AGZ, and the structure of the studied monoethers (1,2) and that of the aliphatic alcohols (3, C,H2,+ 10H, n= 4-8) [ 2,3] as well as by using the appropriate multiple regression equation. It has been found that within the range of surface tension decrease studied, where n ranges from 2 to *For Part 1, see Ref. [ 11. **Presented at the conference on Application of New Trends in Surfactant and Colloid Chemistry, Torino, Italy, 5-9 June 1989.
240
25 mN m-l, not only the methylene group, -CH2-, of the hydrocarbon chain and the oxypropylene group, -OCH&H ( CHB)-, but also the oxyethylene group, -OCH2CH2-, can show hydrophobic character. However, the hydrophobic character of the oxyethylene and oxypropylene groups is not constant. For the groups of compound investigated it decreases with increasing length of hydrocarbon chain and with the increase in n. The hydrophobic character of the methylene groups of monoethers is also not constant. It is known that in many homologous series of surfactants a correlation exists between the number of carbon atoms in the alkyl group of a monomeric amphiphile and the c.m.c.; it is represented by the equation log c.m.c. =A +B n
(1)
where n is the number of -CH,- groups in the alkyl chain, and A and B are experimental constants for a particular series of surfactants [4-71. The concept of the effective length of an alkyl chain, neff, was developed to generalize the influence of structural modifications on the surface phenomena and colloidal properties of surfactants [8-lo]. The value neff, formulated in terms of -CH2- groups, is the value of n for which the analogous straight-chain compound indicates the same surface activity as the structurally modified compound. At present the criteria for determining neff are as follows: c.m.c., polarity indexes etc. As an example, Eqn (1) for structurally modified surfactants takes the form log c.m.c. =A +B neff
(2)
The calculated values of neff for the investigated surfactants containing a definite length of the hydrocarbon chain, n, make it possible to determine the socalled hydrophobicity index (HI) [ 8,9] which is defined as
HI=n,ff n
(3)
Hitherto the concept of neff shows several limitations such as difficulties in finding the appropriate “standard” straight chain length surfactants, and the impossibility of determining neff for surface active agents which do not form micellar solutions. The aim of this work was to test the new concept of the effective length of the alkyl chain in non-ionic amphiphiles. It was expected that the new concept of neffwould make it possible to compare directly the hydrophilic/hydrophobic nature of the oxyethylene and oxypropylene groups in oxyalkylenated alcohols, diols, thiols, amines etc. The present discussion will be limited to only alkyl monoethers of oligooxyethylene glycols (1))alkyl monoethers of oligooxypropylene glycols (2 ) and aliphatic alcohols (3 ) as “standard” amphiphiles.
241 MATERIALS AND METHODS
Pure homogeneous n-butyl, n-amyl, n-hexyl, n-heptyl and n-octyl monoethers of oligooxyethylene glycols containing from one to five oxyethylene groups per molecule were obtained by fractional distillation of the polydisperse products of the addition of oxirane (ethylene oxide) to the corresponding aliphatic alcohols [ 251. Individual methyl, ethyl, n-propyl and n-butyl monoethers of oligooxypropylene glycols containing from one to seven oxypropylene groups per molecule were obtained by fractional distillation of the products of the addition of methyloxirane (propylene oxide ) to aliphatic alcohols [ 3,111. The GLC analysis of these compounds (a Giede 18.3.6 chromatograph; stationary phases, Apiezon APZL on Chromosorb Q and Carbowax 20M on Chromosorb G/AW DMCS) did not reveal any impurities (limit of detection, 0.1% ). The equilibrium surface tension of aqueous solutions of monoethers 1 and 2 was determined at 293.15 t 0.1 K [ 2,3]. Water was triply distilled from alkaline permanganate solution. The maximum bubble pressure method was accurate to 20.1 mN m-‘. The experimental data for the surface tension of aqueous alcohol (3) solutions at 298.15 K were taken from the paper of Posner et al. [ 121. RESULTS AND DISCUSSION
The experimental values of the equilibrium surface tension for aqueous solutions of monoethers 1 and 2 and aliphatic alcohols 3 were found to be well described by a simplified form of the equation for Temkin’s isotherm [ 13-151. The Temkin isotherm is valid for the coverage range 8=0.2-0.8 and describes well the adsorption process of non-ionic amphiphiles at the aqueous solutionair interface [ 16-211. This equation can be written in the following form convenient for further considerations: In x= -In B+n’j2
(g/RT)
(4)
where x is the molar fraction of a substance (in the bulk of the solution) at a given surface pressure n, B is the equilibrium constant of the adsorption process, and g is a constant. The correlation coefficients, R, which characterize the quality of the fit, are in the range 0.996-0.999 for all the compounds studied (l-3). In order to find a quantitative relationship between the adsorption of the compounds and their structure we propose to describe the process by a new empirical adsorption equation: In x=b,+b,n+b,z+b,n
~+~“~(b~+b,n+b,z+b,n
2)
(5)
242
In this equation which is based on Temkin’s isotherm (Eqn. (4) ] In x means A@, (in RTunits), n is the number of carbon atoms in the hydrocarbon moiety, z is the number of oxyalkylene units and b& are regression coefficients. The proposed equation not only reflects the correlation between surfactant concentration (x) and surface pressure (z to the power 0.5) but also the effect of the structural elements of the molecules on the adsorption process for the whole group of amphiphiles C,A,W, for example, &Hz,+ 1(OCH,CH,).OH, HO ),OH, CJ%n+~ XCH,CH, ( 0CH&H2).0H, C,Hz,+ I (OCI-WH(CHs) (CH2CH20)z,2(CH,CH(CH,)O),(CH,CH,0)z,,Hetc. where X=S, NH. This new adsorption equation makes it possible to recalculate Temkin’s constants for the compounds studied ( 1)- (3 ) In B= - (b,+b,n+b,z+b,nz) g= (b,+b,n+bgZ+b,nz)
Jlooo
(6)
RT
(7)
and if we increase the number of variables related to amphiphile structure it will also be useful for more structurally-modified amphiphiles, for example, and &Hz,+ I (OCH2CH(CHB)),(OCH&H2),0H CH[O(CH,CH,O).C,H,+,l,. CnH~n+i The number of products in Eqns (5)-( 7) can be reduced if all non-significant products bixi, biXi3Cj, biXi3CiXk etc. are rejected [ 2,191. The coefficients bi and their standard errors S (bi) for the compounds studied are presented in Table 1. By using t statistics, it has been shown that at a confidence level of cr= 0.05, the product b-gun “’ can be neglected in Eqns (5) and (7). In all cases a high coefficient R justifies the form of Eqn (5) and Table 1. The details of the analysis of regression used may be found in the papers of Bartkowiakowa [ 221 and Jennrich [ 231. For monoethers of oligooxyethylene glycols (i= 1))monoethers of oligooxypropylene glycols (i = 2 ) and alcohols (i = 3 ) the constants In B and g were calculated from the following equations: (In Bcalc)i= 1= 7.014 + 0.8418n+ 1.01062 - 0.04640nz
(8a)
(lnB,,,,)i=,=8.237+0.8487n+1.3839z-0.04134nz
(8b)
(In B,,i,)i=3=4.998+
(8~)
1.1300n
(gcalc)+i= (1.2716-0.07949n+0.11221z)7.7075~104
(9a)
(gca,c)i=2= (1.3522-0.05390n+0.124492)7.7075~104
(9b)
(gca,c)i=3= (0.9373-0.03334n).7.8390.104
(SC)
and are listed in Tables 2-4 along with the values determined directly from the Temkin isotherm [Eqn (4) 1. It is seen that a good agreement between calculated and determined constants of Temkin’s isotherm was obtained which indicates that the applied mathematical model has been properly selected.
243 TABLE 1 Coefficients of the new empirical adsorption equation, the regression equation [Eqn (5) 1; z range from 2 to 25 mN m-r Coefficient
bo S(b,)
b, S(b)
b, S(b,)
b, SCM b, S(h) bs St&,) bs S(b) b, S(b) N R
Monoethers ( 1)
Monoethers (2 )
Eqn (5a)
Eqn (5b)
-7.014 (0.14) -0.8418 (0.022) - 1.0106 (0.038) 0.04640 (0.0063) 1.2716 (0.035) - 0.07949 (0.0051) 0.11221 (0.0053)
- 8.237 (0.10) -0.8487 (0.033) - 1.3839 (0.019) 0.04134 (0.0062) 1.3522 (0.029) -0.05390 (0.0075) 0.12449 (0.0043) -
138 0.9984
132 0.9990
Alcohols (3 ) Eqn (5~)
- 4.998 (0.10) - 1.1300 (0.021)
0.9373
(0.036) - 0.03334 (0.0056)
49 0.9993
In the case of amphiphiles with a bipolar structure, CHB(CH2),_1W, according to Eqn (10) the standard free energy of adsorption AG” comprises two main components: AG”=nAG”[-CH2-]
+AG”[Xw]
(10)
where AG”[-CH,-] and AG” [X,] values are related to the energy of adsorption of the methylene group in the hydrocarbon chain and the remaining parts of molecules, respectively, according to the equation AG”[X,]=AG”[W]+(AG”[CH,-]-AG”[-CH,-1)
(II)
For aliphatic alcohols (i= 3 ) as “standard’ amphiphile we can write In x=b0+blrz,ff+7c”2 AG[CH,(CH,),_,OH]
(bq+b5neff) =n,ffAG”[-CH2-]i=,+AG”[Xw]i=3
(12) (13)
A combination of Eqn (12) with Eqn (13) for alcohols (3) yields AG”[-CH,-]i=,=b,+b,n”2
(14)
AG”[Xw]i=3=bo+b,n”’
(15)
where index 3 denotes alcohol 3.
244 TABLE 2 Constants in the equation for the Temkin adsorption
isotherm of monoethers
of oligooxyethylene
glycols (1)
No.
Structure
g-10-4 (N1” m3” mol-‘) Deb”
InB
Caleb
Det.”
Calc.’
8.00
8.22 9.08 9.94
11.01 12.07 12.65
11.21
8.93 9.66 10.82
10.81 11.67
13.55 14.57
13.68 14.51 12.00 12.78
11.73 6
7.87
7.60
12.00
7
8.67 9.08
8.47
13.05 13.50
8 9 10 11 12 13
10.07 11.19 7.53 8.25
14
8.58 9.57
15
10.61
16 17
6.44
9.33 10.20 11.06 6.99
13.01
12.80
7.85 8.72
13.94
13.53 14.26
9.58 10.45
14.21 14.87 15.92 13.54
8.97
15.63
22
6.03 6.23 6.64
5.76 6.63 7.49
23
9.05
8.36
14.45 14.84 15.38 16.49
18 19 20 21
13.56 14.34
14.27 15.18
6.38 7.24 8.11
6.94 7.75 9.21
12.03 12.86
14.18 14.94
15.12
14.99 15.73 13.59 14.28 14.96 15.65 14.39 15.03 15.67 16.31
“From Eqn (4). bFrom Eqn (9a). ‘From Eqn (8a).
A combination of Eqn (5) with Eqn (10) for monoethers 1 and 2 yields AG”[-CH,-]i=b,+ AG”[Xw]i=b,+b2z+
t bg~+b,n”~+
1 b7~n”~
4 b,n+b,n”2+bgzn1’2+
(16) f b,nn”2
(17)
where the i indices are 1 and 2. The new concept of effective length of alkyl chain in amphiphiles, neff, on the grounds of the proposed new empirical adsorption equation and taking aliphatic alcohols 3 as “standard” amphiphile, is as follows:
245 TABLE
3
Constants
in the equation for the Temkin
adsorption
isotherm
of monoethers
of oligooxypropy-
lene glycols (2 ) No.
Structure
(N1"m3” n
In B
g-10-4
z
mol-‘)
Det.
Calc.b
Det.
Calc.’
12.58 12.00
10.97 11.93
10.74 11.73
10.43 11.77
12.54
12.88
13.11
13.52 14.68
13.84 14.80
14.50 15.92
13.11 14.46
14.95 16.24
15.76 16.72
16.82 18.23
8
10.64
10.55
11.34
9 10
12.13
11.51
12.55
12.54
12.98
12.47
13.92
13.84
11 12
13.71 14.82
13.43
15.19
15.91
16.68 17.95
15.14 16.44
13 14
14.39 15.35
17.74
16.69
16.31
19.03
19.04
10.14 11.09
11.87
12.04 13.30
15.80 17.14 18.48 11.24
15
10.03
16 17
10.38 12.22
12.05
13.00 14.73
18
13.09 14.00
13.01 13.97
15.92 17.15
15.02 15.69
14.93 15.89
18.41 19.45
18.34 19.60 12.85
19 20 21
14.56 15.82 17.08
22
8.93
9.72
12.42
23 24
10.41 12.02
10.68 11.64
14.03
14.07
25
12.60
26 27
12.89 13.73 14.32
15.65 16.80
15.29 16.51 17.72
28
15.31
13.56 14.52 15.48
17.84 18.76 20.03
18.94 20.16
“From Eqn (4). bFrom Eqn (9b). ‘From Eqn (8b ) .
AG” [ -CH,- ] i neff=AGo[_CH,_]i=,‘n+
AG”[Xw]i-AG”[Xw]i,, AG” [-CH,-] i=x
(18)
The incremental values of AG” [ Xw ] i and AG” [ -CH2-] i were calculated on the basis of Eqns (14)-(17).
246 TABLE
Constants No.
4
in the equation for the Temkin adsorption Structure
isotherm
g-10-4 (N’12 m3’* mol-‘)
of alcohols (3) In B
n
1
4
2
5
3 4
6 7
5
8
Det.
Calc.b
6.39 6.04
6.30 6.04
5.80 5.74 5.17
5.78 5.52 5.26
Det.” 9.49 10.69 11.82
Calc.” 9.52 10.65
13.09
11.78 12.91
13.93
14.04
“From Eqn (4). bFrom Eqn (9c ) . ‘From Eqn (8~).
It was expected that the proposed neff concept would make it possible to compare quantitatively the hydrophilic-hydrophobic nature of oligooxyalkylenated derivatives. The values of both neff (see Eqn ( 18) ) and ( neff- n) for oxyethylenated alcohols (i = 1) and oxypropylenated alcohols (i = 2 ) at various surface pressures are shown in Table 5. All neff values for the &HZ,+ 1(A),OH monoethers studied increase with the decrease in surface pressure and with the increase in the oxyalkylene group number (A=OCH,CH,- or -OCH,CH(CH,)-) contained in the molecules. Thus, the effective alkyl chain length, neff, in the amphiphiles investigated (independently of the oxyethylenated or oxypropylenated derivatives) is higher than a nominal hydrocarbon chain length of the amphiphile molecules, taking aliphatic alcohols as a reference. Values of ( neff- n) (see Table 5 and Fig. 1) at 5 mN m -’ for C4HSOCH2CH20H and C,H,OCH,CH ( CH3) OH indicate that the incorporation of an oxyethylene (-OCH,CH,-) and an oxypropylene (OCH,CH (CH,)-) group between the alkyl chain and hydroxyl group of the nbutanol molecule has a similar effect to that induced by incorporation of additional 0.90 and 1.9 (-CH,-) groups respectively. Incorporation of the next oxyalkylene groups causes a further increase in neff (Table 5); thus the oxyethylene and oxypropylene groups of monoethers 1 and 2 show a hydrophobic nature at a surface pressure ranging from 5 to 25 mN m-‘. It can be seen from the results shown that (n,,- n) decreases with the length of hydrocarbon chain (for the corresponding size of the oligooxyalkylene group, (A),). As was expected, the values of neff for all oxypropylenated alcohols (2 ) are higher than those for oxyethylenated ones ( 1). The new measure of hydrophobicity, neff, proposed here, makes it possible to compare the oligooxyethylenated and oligooxypropylenated chains in a simple way by
247 TABLE 5 Effective length of alkyl chain, nefffor monoethers 1 and 2 Structure 71 (mN m-‘) n
2
1
1
1
3
1
5
2
1
2
3
2
5
3
1
3
3
3
5
4
1
4
3
4
5
5
1
5
3
5
5
6
1
6
3
6
5
Monoether ( i = 1)
Monoether ( i = 2 )
hf
fbff -
ndf
5 25 5 25 5 25
2.48 1.54 3.70 2.20 4.92 2.85
1.48 0.54 2.70 1.20 3.92 1.85
3.60 2.32 5.39 3.46 7.19 4.60
2.60 1.32 4.39 2.46 6.19 3.60
1.1 0.8 1.7 1.3 2.3 1.7
5 25 5 25 5 25
3.29 2.46 4.47 3.08 5.65 3.70
1.29 0.46 2.47 1.08 3.65 1.70
4.36 3.15 6.12 4.26 7.89 5.36
2.36 1.15 4.12 2.26 5.89 3.36
1.1 0.7 1.7 1.2 2.2 1.7
5 25 5 25 5 25
4.09 3.37 5.24 3.96 6.38 4.54
1.09 0.37 2.24 0.96 3.38 1.54
5.13 3.98 6.86 5.05 8.59 6.12
2.13 0.98 3.86 2.05 5.59 3.12
1.0 0.6 1.6 1.1 2.2 1.6
5 25 5 25 5 25
4.90 4.29 6.00 4.84 7.11 5.39
0.90 0.29 2.00 0.84 3.11 1.39
5.90 4.80 7.60 5.85 9.29 6.89
1.90 0.80 3.60 1.85 5.29 2.89
1.0 0.5 1.6 1.0 2.2 1.5
5 25 5 25 5 25
5.71 5.21 6.77 5.72 7.83 6.23
0.71 0.21 1.77 0.72 2.83 1.23
6.67 5.63 8.33 6.64 9.99 7.65
1.67 0.63 3.33 1.64 4.99 2.65
1.0 0.4 1.6 0.9 2.2 1.4
5 25 5 25 5 25
6.51 6.12 7.54 6.60 8.52 7.07
0.51 0.12 1.54 0.60 2.52 1.07
7.44 6.46 9.06 7.43 LO.69 8.41
1.44 0.46 3.06 1.43 4.69 2.41
0.9 0.3 1.5 0.8 2.2 1.3
n
(n,ff)i=z-
k-n
(%)i,,
248
nelf -
Fig. 1. Effective length of alkyl chain, neff, for alkyl monoethers of ethylene glycol (i = 1, z = 1) and alkyl monoethers of propylene glycol (i= 2, z = 1) at 5 mN m-l. A: 1, C,H,,+ 10CH,CH,OH; 3, 3, C,H,,+,OH. &Hz,+, OH. B: 2, C,H,,+, OCH,CH(CH,)OH;
evaluating the following expression: (~r)i,~(n,ff)i,l. The value of (n eff ) C,H~,+IOCHZCH(CH~)OH - (%ff) CnHzn+ 10CHzCHzOH ) independently of the size of the substituent &Hz,+ 1, equals approximately 0.5-1.0 unit of (-CH,-) (see Table 5, column 8). The above hydrophobicity difference is understandable because in the monoether of propylene glycol there is a secondary hydroxyl group. More interesting effects have been observed after calculation of [ (neff)i,a(n,ff)i=l] for di-, tri-, tetra- and penta-oxyalkylenated monoethers. Changing the oligooxyethylenated chain to an oligooxypropylenated one causes the molecule hydrophobicity to increase, expressed only as insertion of l-2 methylene groups into the hydrocarbon chain. It is a very small difference considering that every next -OCH,CH ( CH3 ) - group in the oligooxypropylenated chain introduces an additional methyl group to the monoether molecule. It can be concluded that the adsorption on a free surface of oligooxyalkylene glycol monoethers having short hydrocarbon chains proceeds in such a way that both oxyethylene and oxypropylene groups can show hydrophobic character. This effect decreases with the hydrocarbon chain increase. When the hydrocarbon chain is long enough one may expect that this effect may disappear; however, even in the case of dodecyl ethers of monooxyethylene- and dioxyethylene glycol the dual hydrophilic/hydrophobic nature of isolated -CH,CH,O- groups is demonstrated by showing that they give a negative con-
249
tribution to the standard free energy of adsorption from both water and heptane to the heptane-water interface [ 241. The method proposed for the calculation of neffmakes it possible to provide a direct comparison of the hydrophilic/hydrophobic character of various surfactant fragments in the adsorption process.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Part 1: A. Sokolowski, J. Phys. Chem., 93 (1989) 8223. A. Sokoiowski and B. Burczyk, J. Colloid Interface Sci., 94 (1983) 369. A. Sokoiowski and J. Chlebicki, Tenside Deterg., 19 (1982) 282. K. Shinoda, T. Nakagawa, B. Tamamushi and T. Jsemura, Colloidal Surfactants, Some Physicochemical Properties, Academic Press, New York, 1963. S. Kucharski, A. Sokotowski and B. Burczyk, Rocz. Chem., 47 (1973) 2045. M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1978, pp. 94,164. M. Nakagaki and T. Handa, in M.J. Rosen (Ed.), Structure/Performance Relationships in Surfactants, American Chemical Society, Washington, 1984, p. 73. I.J. Lin, J.P. Friend and Y. Zimmels, J. Colloid Interface Sci., 45 (1973) 378. I.J. Lin, B.M. Moudgil and P. Somasundaren, Colloid Polym. Sci., 252 (1974) 407. I.J. Lin and L. Marszali, Tenside Deterg., 14 (1977) 131. J. Chlebicki, Rocz. Chem., 48 (1974) 1241. A.M. Posner, J.R. Anderson and A.E. Alexander, J. Colloid Sci., 7 (1952) 623. P. Parsons, Trudy IV Soveschanija po Electrokhimii, Acad. Sci. USSR Ed., Moscow, 1959, p. 42. P. Delahay, Double Layer and Electrode Kinetics, Wiley, New York, 1965, p. 81. A. Couper, in Th.F. Tadros (Ed.), Surfactants, Academic Press, London, 1984, p. 19. J. Lucassen and D. Giles, J. Chem. Sot., Faraday Trans. 1,71 (1975) 217. A. SokoIowski and B. Burczyk, Proc. XII Jornadas de1 Comite Espailol de la Detergencia, Barcelona, 1981, pp. 491-507. A. SokoIowski, B. Burczyk and J. Beger, Colloids Surfaces, 36 (1989) 373. A. Sokolowski, B. Burczyk and J. Beger, Colloids Surfaces, 44 (1990) 89. H. Lange and P. Jeschke, in M.J. Schick (Ed.), Nonionic Surfactants-Physical Chemistry, Marcel Dekker, New York, 1987, p. 1. J. van Hunsel and P. Joos, Colloids Surfaces, 24 (1987) 139. A. Bartkowiakowa, Zastosowanie Matematyki (Appl. Math.), 16 (1978) 293. R.J. Jennrich, Stepwise regression, in K. Enslein, A. Ralaston and H.S. Wilf (Eds.), Statistical Methods for Digital Computers, Wiley, New York, 1977, p. 58. R. Aveyard, N. Carr and H. Slezok, Can. J. Chem., 63 (1985) 2742.