Influence of the Molecular Geometry of Nonionic Surfactants on

Apr 3, 1996 - ... were determined as a function of the total hydrocarbon number (n)m=const in the lipophilic part and oxyethylene group number (m)n=co...
16 downloads 3 Views 193KB Size
Download PDF
+

+

Langmuir 1996, 12, 1765-1770

1765

Influence of the Molecular Geometry of Nonionic Surfactants on Surface and Micellar Properties in Aqueous Solutions Krystyna Kratzat* and Heino Finkelmann Institut fu¨ r Makromolekulare Chemie, Universita¨ t Freiburg, Sonnenstr. 5, D-79104 Freiburg, Germany Received October 31, 1995. In Final Form: January 12, 1996X The influence of the molecular geometry of corresponding differently branched nonionic surfactants on surface and micellar properties in aqueous solution was investigated by surface tension measurements. The nonionic surfactants with a branched hydrophilic moiety are described as Y surfactants (CnG(Em/2M)2) and with a branched lipophilic moiety as V surfactants: symmetrical Vs (Cn/2)2GEmM and asymmetrical Va (Ck)(Cn-k)GEmM; where Cn and Ck denote an alkyl chain, G a triglyceryl unit, and EmM an oligo(oxyethylene) monomethyl ether with k ) 4, n ) 10, 12, 14, 16, and m ) 6, 8, 10. The critical micelle concentration (cmc), the standard free energy of micellization (∆Gm), the equilibrium surface tension (γcmc), and areas per molecule at cmc (Acmc) at the air-water interface were determined as a function of the total hydrocarbon number (n)m)const in the lipophilic part and oxyethylene group number (m)n)const, in the hydrophilic part of the surfactant. The results are discussed on the basis of structural factors for the corresponding Y, Vs, Va surfactants and compared with data of the conventional unbranched surfactants CnE8M (I surfactants). Within these homologous series the increase of (n)m leads to the lowering and an increase of (m)n to the increasing of the cmc’s and ∆Gm’s. For the corresponding surfactant series the negative contribution to the cmc and ∆Gm of each CH2 group promotes micellization and increases in the sequence I > Y > Vs. On the other hand the positive contribution of each EO group opposes the micellization and is larger for Y than for Vs surfactants. The γcmc and Acmc for Y surfactants are significantly larger than for V surfacants. In the gorup of the V surfactants the cohesive forces of the hydrocarbon chains significantly influence the γcmc’s.

1. Introduction A characteristic property of nonionic surfactants in water is the tendency to adsorption at the air-water interface and, above a critical concentration, the formation of micelles. The micellization and surface properties of (alkoxy)oligo(oxyethylenes) in water have been extensively studied.1 Only a few studies, however, exist dealing with the influence of the molecular geometry of the surfacant on surface and micellar properties.2-4 The influence of the surfactant structure on the micelle organization is well-known. The factors controlling micellar shape are the average molecular area at the micelle surface, the length, the volume, and a structure parameter of the hydrocarbon part of the surfactant.5-7 If these parameters are known the micellar shapes can be predicted. At higher concentration the interaction between micelles may lead to the formation of lyotropic liquid crystlaline (LC) phases. Theory suggests that the type of LC phases is determined by the shape of the micelles in the micellar solution in equilibrium with these phases. In previous papers7-11 we presented the influence of the X

Abstract published in Advance ACS Abstracts, March 1, 1996.

(1) Magid, L. J. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker Inc.: New York, 1987; p 677. (2) Elworthy, P. H.; Florence, A. T. Kolloid Z. Z. Polym. 1964, 195, 23. (3) Takahashi, H.; Kuwamura, T. Bull. Chem. Soc. Jpn. 1973, 46, 623. (4) Yeates, S. G.; Craven, J. R.; Mobbs, R. H.; Booth, C. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1865. (5) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. N. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (6) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (7) Kratzat, K.; Finkelmann, H. submitted to J. Colloid Interface Sci. 1995. (8) Kratzat, K.; Finkelmann, H. Liq. Cryst. 1993, 13, 691. (9) Kratzat, K.; Finkelmann, H. Colloid Polym. Sci. 1994, 272, 400. (10) Kratzat, K.; Schmidt, C.; Finkelmann, H. J. Colloid Interface Sci. 1994, 163, 190.

Figure 1. Molecular models of comparable (a) I surfactant C14E8M, (b) Y surfactant C14G(E4M)2, asymmetrical Va surfactants (c) (C4t)(C10)GE8M and (d) (C4)(C10)GE8M, (e) symmetrical Vs surfactant (C7)2GE8M in an all-trans conformation.

molecular geometry of corresponding nonionic Y8 and V surfactants,7,9-11 with branched hydrophilic or lipophilic moieties, respectively, on the formation of LC phases. The systematic variation of the molecular structure of these surfactants, presented in Figure 1, results in characteristic changes of the LC phase morphology. In this paper we present investigations on the influence of the moleclar geometry of corresponding Y and V surfactants (i) on the process of micellization, critical micelle concentration (cmc), standard free energy of micellization ∆Gm, and (ii) on the properties of the surface film at the cmc, the equilibrium surface tension γcmc, surface pressure πcmc, and the area per molecule Acmc. The results will be compared with literature data of corresponding homologous series of linear I surfactants CnE8M4 in order to answer the following questions: (i) How does the cmc, ∆Gm, γcmc, and Acmc in each homologous series of Y, Vs, Va, (11) Kratzat, K.; Stubenrauch, C.; Finkelmann, H. Colloid Polym. Sci. 1995, 327, 257.

+

+

1766

Langmuir, Vol. 12, No. 7, 1996

Kratzat and Finkelmann

Table 1. Surface and Micellar Properties of Nonionic Y, Vs, Va, and I Surfactants Obtained from Surface Tension Curvesa k

4t 4 4

n

m

CMC × 10-5 (mol/L)

γcmc (mN/m)

πcmc (mN/m)

Acmc (Å2)

A × m-1/2 (Å2)

∆Gm (kcal/mol)

71 80 86 87 67 98

25.1 28.3 30.4 30.8 27.4 31.0

-6.26 -7.36 -8.53 -9.34 -8.85 -8.22

10 12 14 16 14 14

8 8 8 8 6 10

120.0 18.0 2.4 0.6 1.4 4.1

35.7 36.0 36.2 36.5 33.6 37.0

Y: CnG(Em/2M)2 36.2 35.9 35.7 35.4 38.3 34.9

12 14 16 14 14

8 8 8 6 10

84.0 17.0 3.0 12.0 17.5

28.4 27.8 28.0 27.4 28.0

Vs: (Cn/2)2GEmM 43.5 43.6 43.9 44.5 41.9

73 72 75 75 77

25.8 26.1 26.5 30.6 24.3

-6.46 -7.39 -8.40 -7.60 -7.38

14 14 16

8 8 8

19.0 10.5 1.5

31.3 31.1 31.4

Va: CkCn-kGEmM 40.6 40.8 40.5

70 69 77

24.7 24.4 27.2

-7.33 -7.67 -8.81

6 8 10 12 14

8 8 8 8 8

9100.0 520.0 136.0 15.4 1.1

20.9

-3.744 -5.404 -6.184 -7.454 -8.99

I: CnEmM

34.6

37.3

96 99 103 95 59

a

cmc ( 10% or 15% mol/L ) critical micelle concentration; γcmc ( 0.4 mN/m ) surface tension at the cmc; πcmc ( 0.4 mN/m surface pressure at the cmc; Acmc ( 8% Å2 ) molecular area on adsorbed film at the cmc; ∆Gm ( 0.05-0.1 kcal/mol ) standard free energy of micellization.

and I surfactants depend on the total hydrocarbon number (n)m of the lipophilic part at constant m or on the oxyethylene (EO) group number (m)n at constant n in the hydrophilic part. (ii) How does the systematic variation of the molecular geometry influence cmc, ∆Gm, γcmc, and Acmc when the volume ratio (Vl/Vh) of the surfactants are kept constant. 2. Experimental Section 2.1. Materials. The homologous series of Y surfactants CnG(Em/2M)2,8 symmetrical Vs surfactants (Cn/2)2GEmM,9 and asymmetrical Va surfactants (Ck)(Cn-k)GE8M,7 with k ) 4, n ) 10, 12, 14, 16, and m ) 6, 8, 10, were prepared according to methods previously described. The monodisperse products were carefully purified by repeated flash chromatography with a diethyl ether/acetone mixture as eluent until the minimum in the surface tension curves disappeared. Their purity (98-99%) was confirmed by high-pressure liquid chromatography. Tetradecylocta(oxyethylene) monomethyl ether C14E8M (I surfactant) was prepared as follows: 0.1 mol of octa(oxyethylene) monomethyl ether was added to 0.1 mol of sodium hydride in 50 mL of dry diethyl ether under nitrogen atmosphere. After stirring for 5 h at room temperature, 0.1 mol of tetradecyl bromide was added, and the mixture was stirred at room temperature for 24 h. Sodium bromide was filtered off and the product purified by flash chromatography with a diethyl ether/acetone (5/1 v/v) mixture as eluent. C14E8M was obtained as a colorless, viscous oil. Melting point F: 17 °C. Anal. Calcd for C14E8M (C31H64O9): C, H, 64.1; H 11.1. Found C, 63.9; H, 11.4. 1H-NMR (80 MHz, CDCl3/TMS; ppm): 0.9 (t, CH3), 1.3 (m, CH2), 3.4 (s, OCH3), 3.6 (m, OCH2). 2.2. Surface Tension Measurements. Surface tension measurements of aqueous solutions were carried out at (20.0 ( 0.1) °C over a wide range of concentrations comprising two decades around the cmc with Digitaltensiometer K10 (Fa Kru¨ss) by using the Wilhelmy plate method. The usual precautions were taken to ensure clean glassware. Deionized water (Milipore system Milli-Q plus) was used for the preparation of solutions. The external accuracy of measurements, checked by replicate experiments and by frequent measurements of γ for pure water, was (0.4 mN/m. The values of γ were determined after a period of 20-60 min to ensure equilibration conditions. The time needed for equilibration surface tension at concentration below the cmc is considerably slower for Y (30-60 min) than for Vs or Va surfactants (20-30 min). For Y surfactants with alkyl chain length of n ) 16 at concentrations below the cmc, the surface

tension equilibrium is reached at such a slow rate that direct measurements are difficult. The error in the cmc values was calculated to about 10%, in the case of C16G(E4M)2 even higher 15%. This also holds for (C7)2GE6M due to the miscibility gap close to the cmc because the preparation of aqueous solutions of (C7)2GE6M at concentrations about 10-4 mol/L were difficult and therefore incorporated a higher experimental error.

3. Results and Discussion The data obtained for cmc, ∆Gm, γcmc, πcmc, and Acmc derived from the equilibrium surface tension vs log concentration isotherm of the corresponding Y, Vs, Va, and I surfactants are summarized in Table 1. These nonionic surfactants exhibit strong surface activity and low cmc values. In the following we will discus in more detail the results of the cmc’s and the features of the adsorbed films at the air-water interface (i) in dependence of (n)m and (m)n in each series of the Y, Va, Vs, and I surfactant and (ii) in dependence of the molecular geometry, due to the variations of the chain structure at equal total compositions of the Y, Vs, Va, and I surfactants. 3.1. Critical Micelle Concentration of the Homologous Y, Va, Vs, and I Surfactant Series. The surface tension as a function of concentration of corresponding homologous Y and Vs surfactant series are shown in Figure 2. The sharp bend in the γ-log c curves characterizes clearly the cmc. In each of the Y, Vs, Va, and I surfactant series an increase of the total hydrocarbon number (n)m in the lipophilic part leads to in a shift of the γ-log c curves, i.e. of the cmc’s to lower concentrations, typically shown in Figure 2a. On the other hand, an increase of the (EO) number (m)n results in a shift of the γ-log c curves and the cmc’s to higher concentrations (Figure 2b) due to the increase of the surfactant solubility. This dependence of cmc on (n)m or (m)n is typical for nonionic surfactants. The influence of the hydrocarbon chain length on the cmc within the homologous Y, Va, Vs, and I surfactant series is shown in Figure 3a, where plots of log cmc as a function of (n)m are depicted. Measurements can be

+

+

Surface and Micellar Properties of Nonionic Surfactants

Figure 2. Surface tension γ vs concentration c of Y surfactants CnG(Em/2M)2 and Vs surfactants (Cn/2)2GEmM at 20 °C in dependence of (a) n ) 10, 12, 14, 16 at constant m (m ) 8), and (b) m ) 6, 8, 10 at constant n (n ) 14). For clarity not all data points are shown.

evaluated according to the empirical equation of Klevens:12

Langmuir, Vol. 12, No. 7, 1996 1767

Figure 3. log cmc at 20 °C vs (a) (n)m)8 and (b) (m)n)14 for Y surfactants CnG(Em/2M)2, Vs surfactants (Cn/2)2GEmM, Va surfactants (C4)(Cn-4)GE8M, and I surfactants CnE8M at 26 °C from ref 7. The error bars except for I surfactants are within the symbols.

(m)n. This relationship is represented by the following equations:

log cmc ) A - Bn

Y series: log cmc ) 0.12m - 5.55 at 20 °C

6 e m e 10 (4)

where A and B are constants. The following equations with data for A and B were obtained:

Vs series: log cmc ) 0.04m - 4.14 at 20 °C

6 e m e 10 (5)

Y series: log cmc ) -0.39n + 0.93 at 20 °C

10 e n e 16 (1)

Vs series: log cmc ) -0.36n + 1.28 at 20 °C

12 e n e 16 (2)

I series: log cmc ) -0.45n + 1.50 at 26 °C

6 e n e 14 (3)4

The positive slope of the linear relations 4 and 5 shows the contribution of each EO group to the cmc. This contribution is larger for the Y surfactant series than for the corresponding Vs series for the sequence of EO units concerned. It can be concluded that the influence of an increasing the EO number in the hydrophilic moiety on the cmc is distinctly smaller than an increasing CH2 number in the lipophilic part. The different positions of the straight lines of the relations between log cmc and n or m show the influence of the different molecular architecture of the corresponding I, Y, Va, and Vs surfactants on cmc. 3.2. Standard Free Energy of the Micelle Formation. The standard free energy ∆Gm was calculated according to Molyneux et al.13 with the equation

The value of the negative slope of the linear relations 1-3 (with the error of 0.01) shows the influence of each methylene group (CH2) on the cmc. For the corresponding surfactant series this influence and therefore the tendency to form micelles increases with increasing n in the sequence I > Y > Vs. The influence of the hydrophilic moiety on the cmc in each series of Y and Vs surfactant is shown in Figure 3b, where the log cmc is ploted against the total (EO) number (12) Klevens, H. B. J. Phys. Colloid Chem. 1948, 52, 130.

∆Gm ) 2.303RT(log cmc - log ω)

(6)

where ω is the molar concentration of water at 20 °C. The (13) Molyneux, P.; Rhodes, C. T.; Swarbric, J. Trans. Faraday Soc. 1965, 61, 1043.

+

+

1768

Langmuir, Vol. 12, No. 7, 1996

Kratzat and Finkelmann

total free energy change in the micellization includes contributions from methyl, methylene, and hydrophilic groups of the Y, Vs, Va, and I surfactant molecules. Since ∆Gm is related to log cmc, it is evident that ∆Gm decreases with increasing (n)m and increases with increasing (m)n for the Y, Vs, Va, and I surfactants. The contribution of the lipophilic moiety to ∆Gm can be obtained by combining eqs 1-3 with eq 6. The following equations are obtained,

Y series: ∆Gm ) -0.52n - 1.09 at 20 °C

12 e n e 16 (7)

Vs series: ∆Gm ) -0.49n - 0.63 at 20 °C

12 e n e 16 (8)

I series: ∆Gm ) -0.60n - 0.39 at 26 °C

6 e n e 14 (9)

Figure 4. Area per molecule Acmc at the air-water interface vs (m)n)14 for Y surfactants CnG(Em/2M)2 and Vs surfactants (Cn/2)2GEmM.

where the negative slope of the linear relations 7-9 shows the contribution of each CH2 group to ∆Gm for Y, Vs, and I surfactants, respectively. This contribution decreases in the sequence I > Y > Vs. From eqs 7-9 it is clearly seen that the contribution of the lipophilic moiety to ∆Gm promotes the micellization and increases with increasing hydrocarbon number. Thus, the lipophilic surfactant part is the main driving force for micellization. The contribution of the hydrophilic moiety to ∆Gm can be obtained by combining eqs 4 and 5 with eq 6. The following equations are obtained:

Y series: ∆Gm ) 0.16m - 9.78 at 20 °C

6 e m e 10 (10)

Vs series: ∆Gm ) 0.05m - 7.90 at 20 °C

6 e m e 10 (11)

The contribution of each EO group to ∆Gm is derived from the positive slope of the linear relations 10 and 11 for Y and Vs surfactants, respectively. This contribution to the ∆Gm is rather small for both surfactant types and strongly depends on the molecular structure. Thus, it is only about one-third for the Vs structure as compared to the Y one. The positive contribution of the hydrophilic moiety to ∆Gm opposes micellization. 3.3. Area per Molecule at the Air-Water Interface at the cmc. The Acmc values of Y, Vs, Va, and I surfactants were calculated using the Gibbs’ adsorption isotherm from the slopes in the γ-log c curves just below the sharp bend at their cmc. As seen from Table 1 the Acmc values are independent of (n)m within the experimental error. This result is in agreement with findings of Yeates et al.4 on the CnE8M series. On the other hand, the Acmc for Y surfactants C14G(Em/2M)2 increases with (m)n, but for Vs surfactants (C7)2GEmM the Acmc is almost independent of (m)n (Figure 4). The influence of the molecular geometry on the Acmc can be analyzed on closer inspection the dependence of Acmc/m1/2 vs (m)n. Van Voorst Vader14 gave theoretical explanation for the observed phenomenon that in I surfactants the Acmc is proportional to the square root of m, i.e. the Acmc/m1/2 is constant. We found for the branched Y and Vs surfactants that the Acmc/m1/2 is not constant (Figure 5). With increasing (m)n the Acmc/m1/2 relation increases or decreases for the Y and Vs series, respectively. This different behavior between the Y, Vs, and I surfactant series can be explained as follows: In soluble films of EO surfactants the EO chain penetrates into the aqueous (14) van Voorst Vader, F. Trans. Faraday Soc. 1960, 56, 1078.

Figure 5. Acmc/m-1/2 vs (m)n)14 for Y surfactants CnG(Em/2M)2 and Vs surfactants (Cn/2)2GEmM.

phase in form of a coil.15 For I surfactants the size of the coil and therefore Acmc increases with increasing m, but Acmc/m1/2 remains constant. For Y surfactants the presence of the two short EO chains linked to the glyceryl results in large Acmc values. The lengthening of the both EO chains leads to an increase of the area per EO chain and thus leads to an increase in both Acmc and Acmc/m1/2. The Vs surfactants which contain two hydrocarbon chains form monolayers with closely packed n-alkyl blocks. The cohesive forces between these chains prevent an increase of Acmc with m resulting in a decrease of Acmc/m1/2. The lengthening of the EO chain leads to a further penetration of the EO coil into the aqueous phase. This might serve as a reasonable explanation for the behavior of Acmc in dependence of m. 3.4. Features of the Adsorbed Surfactant Film at the Air-Water Interface. The adsorbed films of Y, Va, Vs, and I surfactants are effective in reducing the surface tension γ at the air-water interface. The γ values of highly diluted nonmicellar solutions decrease with increase of the surfactant concentration (c) until cmc is reached. The influence of the number of (n)m or (m)n on a specific γ value below cmc for the Y and Vs series can clearly be seen in Figure 2. A specific γ value is shifted to lower c with increasing (n)m and to higher c with increasing (m)n. When the cmc is reached, the surface tension (γcmc) does not change with an increase of c. As seen from Table 1 (15) Lange, H. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker Inc.: New York, 1967; p 443.

+

Surface and Micellar Properties of Nonionic Surfactants

+

Langmuir, Vol. 12, No. 7, 1996 1769

values γcmc and πcmc (π ) γ0 - γ; where γ0 ) surface tension of pure water) are almost independent of (n)m for all Y, Va, and Vs series. However, as shown in Figure 6 γcmc values as a function of (m)n show a different behavior for Y and Vs series. For Y surfactants γcmc increases with increasing (m)n, indicating that πcmc decreases. For Vs surfactants both γcmc and πcmc remain almost constant with increasing (m)n. Since γcmc is clearly related with Acmc15 these observations are in good agreement with the behavior of Acmc in dependence of (n)m and (m)n discussed before. 3.5. Influence of the Molecular Geometry on the Micellization and Surface Properties. It is interesting to compare the results of cmc, ∆Gm, γcmc, and Acmc (Table 1, Figure 7) with respect to the different molecular geometry of the corresponding compounds: Y, C14G(E4M)2; Vs, (C7)2GE8M; Va, (C4)(C10)GE8M; (C4t)(C10)GE8M (where C4t denotes a tert-butyl chain); and I, (C14E8M) (Figure 1). The branched Y and V surfactants have the same HLB and Vl/Vh ratio. Although for the I surfactant HLB slightly differs from the other systems due to the absence of the glyceryl branching unit, it will be included into these considerations. The single hydrocarbon chain surfactants (Y and I) with the same alkyl chain have similar cmc and ∆Gm values. The difference in the cmc’s (∆Gm’s) may be explained by the glyceryl unit, which is equivalent to about two EO groups.16 As mentioned before these results show the

small influence of the branched hydrophilic moiety on the micellization. It is remarkable that the cmc values of the corresponding Y and V surfactants, varying only in the molecular structure, differ about one decade. On the other hand the cmc’s of the V surfactants, which differ only in the hydrocarbon part structure, are very similar. The reason for this different behavior may be found in the differences of the structure of their lipophilic part. The double hydrocarbon chain V surfactants are more compact and present a smaller surface area to the solvent than the single hydrocarbon chain analogues. The hydrocarbon surface area to the solvent decreases in the sequence of (C14) > (C4)(C10) > (C4t)(C10) > (C7)2. Therefore, the loss of the iceberg structure of water surrounding the hydrocarbon part is smaller in the case of V than in the case of I or Y surfactants. This results in a smaller interfacial energy change per molecule on micellization. This observation can be compared with the well-known influence of the molecular structure on the free energy of hydration (∆G°). For example the ∆G° for n-butane is 8.7 kJ/mol, for 2-methylpropane 9.7 kJ/mol.17 According to these observations the I and Y surfactants are more surface active than their comparable V surfactants. The differences in the molecular structure of the Y, I, and V surfactants have influence on the surface concentration Γcmc and therefore on Acmc. The values of Γcmc decrease progressively from I > Va ∼ Vs > Y; consequently the area per molecule Acmc increases. The Acmc for the Y surfactant with the branched hydrophilic part is remarkably larger than for the corresponding I and V surfactants. Although the I and V surfactants have the same octa(oxyethylene) chain in the hydrophilic part, the Acmc’s differs distinctly. This indicates an additional contribution of the glyceryl unit and the second alkyl chain of the V surfactants to the Acmc. Finally we will consider the influence of the surfactant structure on the surface tension and pressure at constant temperature, which depend on the conditions of the surface film. These conditions depend on lateral forces from the polar head-group region (steric constrains between EO chains) and the apolar hydrocarbon chain region (cohesive forces). It is interesting to note that the γcmc (πcmc) values of comparable Y and V surfactants distinctly differ as shown in Figure 7. A possible reason for this behavior may be explained as follows. The different γcmc’s between Y and V surfactants are obviously due to distinct differences in the Acmc values. The one alkyl chain Y surfactant with its very large Acmc forms an expanded hydrocarbon film (Figure 8a) of small πcmc. The differences of γcmc’s within the V surfactants (Va, Vs) which have the same Acmc’s result from the different hydrocarbon film structure (Figure 8b,c). The two alkyl chain V surfactants from monolayers with close-packed hydrocarbon blocks. The hydrocarbon film of the Vs surfactant is closest packed due to the strong cohesive forces between the equal hydrocarbon chains. The differences in the hydrocarbon chain lengths of Va surfactants lead to smaller cohesive forces between alkyl chains and thereby to the formation of more expanded monolayer films. Therefore the πcmc increases in the sequence Y < Va < Vs. It is interesting to note that a correlation seems to exist between the macroscopic γcmc and the microscopic micellar mean curvature. In both cases the surfactant film at the air-water interface as well as in bulk water depend on the balance developing in the lateral forces from the head group and the hydrocarbon chain region. From the light

(16) Hall, C.; Tiddy, G. J. T.; Pfannemu¨ller, B. Liq. Cryst. 1991, 9, 527.

(17) Cabani, S.; Gianni, P.; Mollica, V.; Lepori, L. J. Solution Chem. 1981, 10, 563.

Figure 6. Surface tension γcmc at the cmc vs (m)n)14 for Y surfactants CnG(Em/2M)2 and Vs surfactants (Cn/2)2GEmM.

Figure 7. Surface tension γ vs concentration c of Y, C14G(E4M)2; Va, (C4t)(C10)GE8M, (C4)(C10)GE8M; Vs, (C7)2GE8M at 20 °C.

+

1770

+

Langmuir, Vol. 12, No. 7, 1996

Kratzat and Finkelmann

Similar observations between γcmc and the micellar shape can be made for linear I surfactants C12Em where m ) 4-12 by the consideration of the γcmc20,21 and the phase behavior.6,22 4. Conclusion

Figure 8. Schematic presentation of surface film of (a) Y surfactants; (b) Va surfactants; (c) Vs surfactants.

The results of cmc, ∆Gm, γcmc, and Acmc distinctly show the influence of the molecular geometry of corresponding Y, V, and I surfactants on the conditions of the surface and micellar properties. While the molecular geometry clearly determines the micellar formation in a very dilute region and the LC phase polymorphism at higher surfactant concentrations, the micellar shape in the isotropic phase regime between cmc and LC phases is still an open question. Although we know that the Y surfactants form small spherical micelles above cmc, the micellar geometry of the V surfactants with concentration and temperature still has to be analyzed. These measurements will clarify whether the systematic results for the cmc, γcmc, Acmc, and LC phase polymorphism also holds in isotropic solution with respect to the relation between the micellar shape and the molecular geometry. This study is in progress and will be reported elsewhere.

scattering results and freeze fracture electron microscopy18 of the isotropic micellar solutions as well as from the LC phase behavior of Y, Va, Vs surfactants7-9 it can be concluded that with decreasing γcmc values the micellar mean curvature decreases as well in the sequence Y > Va > Vs-surfactant. According to these observations the stability of the dilute micellar solution7-9 which correlates to the micellar shape19 decreases in the same sequence.

Acknowledgment. We are grateful to Dr. K. Lunkenheimer and Dr. B. Pfannemu¨ller for helpful discussions. Financial support from Fond der Chemischen Industrie and Deutsche Forschungsgemeinschaft (SFB60) is gratefully acknowledged.

(18) Kratzat, K.; Holtz, J.; Speth, V.; Finkelmann, H. Unpublished results. (19) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Progr. Colloid Polym. Sci. 1990, 82, 253.

(20) Lange, H. Koll. Z. Z. Polym. 1965, 201, 131. (21) Schick, M. J. J. Colloid Sci. 1962, 17, 801. (22) Stray, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86 (12), 2253.

LA9505428