Interactions between Bisphosphate. Geminis and Sodium Lauryl Ether

Transcription

1 Chapter 5 Interactions between Bisphosphate Geminis and Sodium Lauryl Ether Sulphate 110

2 5.1 Introduction The physiochemical and surface active properties of mixed surfactants are of more interest and useful than pure surfactants, for industrial applications. By virtue of differences in the tail and head groups of the surfactants, mixed surfactants may show composition dependent micellization, mutual interaction, solvation, micellar shape, etc. For the mixture of two surfactants undergoing micelle formation above a critical micelles concentration (CMC), the solution properties fall either between or outside the solution properties of the two-single surfactant solutions. This is also the case for the CMC of a binary surfactant solution. Clint [Clint, 1975] has given the relation between mole fraction and CMC of the i th component for ideal mixtures, and Rubingh [Rubingh, 1979] has made a comprehensive theoretical attempt to deal with non-ideal mixture on the basis of the regular solution theory (RST). In solution containing two or more surfactants, the tendency of aggregated structures to form is substantially different from that in solutions having only pure water [Tikariha et al., 11]. Such different tendency results in dramatic change in properties and behavior of mixed surfactants compared to that of a single surfactant. Practical formulations often requires the addition of surfactants to help in regulating the physical properties of the product or improve it s stability. The stability of the mixed micelles depends on two factors (i) coulombic interaction between ionic head groups and (ii) chain length of the surfactant tail groups. In many practical applications, the properties of surfactants are important and attractive [Rosen, 1989]. A mixed micellar solution is a representation of a mixed micelle, mixed monolayer at the air/water interface and mixed bilayer aggregate at the solid interface [Tikariha et al., 11]. In the present work mixed micellization of anionic bisphosphate gemini surfactants with sodium lauryl ether sulphate (SLES) was studied. Gemini surfactants were used as an additive. The purpose of the present study is to investigate the interactions between a mixed surfactant system (anionic monomeric surfactants with sulphate and anionic gemini surfactant system with phosphate head group). To our knowledge there hasn t been any report published on the mixed micellization of the surfactant system consisting 111

3 of a phosphate gemini and SLES. SLES is a very important surfactant in many surfactant based formulations, owing to it s very good foaming power. The present study is an attempt to find out the compatibility of phosphate gemini surfactants with SLES. This study has been carried out by surface tension measurements, dynamic surface tension analysis and foamability of the mixed surfactant systems (SLES + m 3 m geminis and SLES + m 5 m geminis). The effect of chain length of the gemini surfactant on the interaction parameter was studied. 5.2 Materials and Methods The as synthesized six bisphosphate gemini surfactants (m 3 m and m 5 m geminis), described in chapter 2, were used. Commercial sample of sodium lauryl ether sulfate (SLES) was obtained from M/s Galaxy Surfactants Pvt. Ltd., India. SLES comprised of % C 12 chain and % C 14 chain surfactant and the ethoxylation was 2 mol per mol carbon chain. Distilled water was used for preparing all the surfactant solutions. The equilibrium surface tension, dynamic surface tension and foamability measurements were carried out using the same procedures discussed in earlier chapters. Horizontal Impinging Jet, foaming apparatus was used for foamability studies. O O NaO S O O 2 R R = C 12 H 25 sodium lauryl ether sulphate Figure 5.1: Structure of SLES 112

4 5.3 Results and Discussion Critical Micelle Concentration (CMC) The CMC of mixed micellar systems of SLES and anionic phosphate gemini surfactants (m 3 m and m 5 m) in aqueous solutions was investigated, using surface tension measurements. The surface tension was measured using Wilhelmy plate method on Kruss K-11 tensiometer, at temperature 25 ± 1 0 C. The CMC value of SLES was found to be 0.99 mm, A min value found to be 61 A 2. The CMCs and interfacial properties of the mixture of SLES/geminis was reported in Table The surface tension plots were shown in figures The surface tension results were accurate within the range of ±0.2 mn/m. It was observed that with increasing mole fraction of gemini surfactants the CMC values decreases, this was observed for both m 3 m and m 5 m geminis. The A min values were changed drastically for the mixtures, more than that of individual surfactants, which indicates that the adsorption of the mixed surfactants at air/water interface is less than compared to that of the individual surfactants. Authors Rosen and Zhou [Rosen, 1982; Zhu et al., 1991] also observed the same expansion behavior which was attributed to the dissimilarity in the nature of interaction among hydrophobic groups and hydrophilic groups in the mixed adsorbed layer. In case of structurally similar hydrocarbon tails, hydrophobic interactions occur at small distances, whereas ion dipole interactions among anisotropic head groups are effective at relatively larger distances. In the case of SLES/m 3 m and SLES/m 5 m systems, larger A min values were found because of the repulsive interactions instead of the attractive forces between the hydrophobic as well as hydrophilic head groups of SLES and gemini surfactants Interactions between mixed anionic surfactants The commercial products are always comprised of a mixed surfactant system, because economically synthesis of each component is not viable option. A mixed surfactant system is often superior in performance to individual surfactants. There is a substantial 113

5 difference in the micellization tendency of mixtures of two or more surfactants as compared to a single surfactant. This results in a dramatic change in properties and behavior of mixed surfactants as compared to any single surfactant. Hence it is necessary to investigate the nature of interactions (synergistic/antagonistic) and the factors affecting the interactions [Suradkar and Bhagwat, 06]. A lower CMC of mixture than individual surfactants is considered as synergy. The synergistic interactions between the mixed surfactants is useful from the application point of view. The interaction between the surfactants can be determined using models for mixed micellization. These models are based on an equilibrium thermodynamic approach [Ogino and Abe, 1993]. The pseudo-phase separation model assumes that that the mixed micelle can be treated as a separate phase. The pseudo-phase separation approach is a very useful tool for the description of micelle formation [Hassan et al., 1995]. Clint [Clint, 1975] proposed an equation, for the CMC of the ideal mixture of two surfactants: 1 1 = x + (1 x 1 ) (5.1) C mix C 1 C 2 Where x 1 is the bulk solution mole fraction of surfactant 1 in the mixture; C 1, C 2 and C mix are the CMCs of the pure surfactant 1, 2 and mixed system, respectively. The ideal solution theory has been successful in explaining the properties of mixtures composed of surfactants with similar chemical structures, however deviations occur for mixtures containing chemically dissimilar surfactants. The non-ideal behavior of mixed surfactant systems was described by Rubingh [Rubingh, 1979], the model was based on Regular Solution Theory. The non-ideal form of equation 5.1. can be given as; 1 C mix = x 1 + (1 x 1 ) (5.2) C 1 f 1 C 2 f 2 ln( f 1 ) = β (1 x 1 ) 2 (5.3) ln( f 2 ) = β (x 1 ) 2 (5.4) 114

6 where x 1 and x 2 are the mole fractions of the surfactant 1 and surfactant 2, respectively, in the mixed micelle. β is the interaction parameter that is usually obtained by fitting the experimental data of mixture CMCs as a function of bulk mole fractions x 1 of surfactant. Assuming a constant value of interaction parameter β, across the whole range of mole fractions, it is possible to solve for x 1 and hence predict the mixed CMCs. The interaction parameter is a measure of the extent of net (pairwise) interaction between the surfactants within the micelles resulting in their deviation from the ideal behavior. In order to obtain valid interaction parameter β values that do not change significantly with change in the ratio of surfactant in the mixture, the following conditions must be met [Rosen, 04]; 1) The two surfactants must be molecularly homogeneous and free from surface active impurities. 2) Since the derivation of equation 5.2 and 5.4 are based upon the assumption that the mixed micelle or monolayer can be considered to contain only surfactants, these structures are considered to contain no free water, and all the present water can be considered to bound to the hydrophilic head groups, 3) Since equations 5.2 and 5.4 neglect counterion effects, all solutions containing ionic surfactants should have the same total ionic strength, with a swamping excess of any counterion. The surfactant forms an aggregate or remains as a free monomer in a solution. The total surfactant concentration is just incrementally larger than C mix, then the monomer composition coincides with the overall surfactant composition. This indicates that more number of free surfactant monomers are present in the solution rather than micelles. The number of micelles will be increased with an increase in total surfactant concentration. The mixture CMC, C mix, is fitted with eq 5.2, which is also known as a Margules oneconstant equation. Such a treatment gives a constant value of interaction parameter at all bulk solution mole fractions x 1 [Suradkar and Bhagwat, 06]. The value of interaction parameter is then substituted in eq 5.2 to compute the values of micellar mole fraction x 1 at each bulk solution mole fraction x 1. The plots of C mix 115

7 against Gemini bulk solution mole fraction x 1 are shown in Figures The conditions for synergism or negative synergism in a mixture containing two surfactants (in the absence of second liquid phase) have been shown mathematically [Rosen, 1989] to be the following: (1) For synergism, the interaction parameter must be negative and β > ln(c1/c2). (2) For negative synergism or antagonism, the interaction parameter β must be positive and β > ln(c1/c2) where C 1 and C 2 are the CMCs of individual surfactants. Interactions between the surfactants in binary mixtures are the result of mainly two contributions, one associated with interactions between hydrophobic moieties of the two surfactants in the micellar core and the other with electrostatic interactions between the head groups of both surfactants at the interface, besides the possibility of hydrogen bonding cannot be ruled out [Sheikh et al., 11] SLES/m-3-m gemini surfactants The one parameter Margules equation was fit to the experimental data, to obtain single β value for the entire mole fraction range of gemini surfactants. For the SLES/ system, negative deviation was observed from the ideal behavior, except at gemini mole fraction 0.6. At 0.6 mole fractions of gemini the C mix value increased, more than ideal C mix. The margules equation was fitted to the experimental C mix values and the single negative β value was obtained (-2.82) which means there are attractive interactions or synergistic interactions exists between the mixed surfactants. A negative interaction parameter means that the attractive interaction between two different surfactant monomers is stronger that the attractive interaction between the two individual surfactant monomers with themselves or that the repulsive interaction between two different surfactant monomers is weaker than the self repulsion of the two individual surfactant monomers. However positive β value was obtained for the SLES/ and SLES/ (0.13 and 0.69 respectively) which indicates there is negative synergism, i.e. antagonistic effect was observed. For SLES/ system positive deviation was observed in C mix, but at 0.8 mole fraction of gemini the C mix 116

8 value was found to be almost similar to ideal C mix which also suggests that micellization is favored by gemini surfactant at higher gemini surfactant concentration. Similarly the SLES/ system also exhibits negative synergism and at mole fractions 0.6 and 0.8, micellization was favored by gemini surfactant. A positive interaction parameter implies that the attractive interaction between the two different surfactant monomers is weaker than the attractive interactions between the individual surfactant monomers themselves or the self repulsion between two different surfactant monomers is stronger than the self repulsion between the individual surfactant monomer themselves SLES/m-5-m gemini surfactants A positive β value was obtained for these systems. The positive deviation from ideal behavior shows antagonistic interactions between mixed surfactant. The β value was found to be in the order of, > > (1.90 > 0.39 > 0. respectively). Overall in the case of both m 3 m and m 5 m gemini surfactants the β value increases with the increasing carbon chain length in the tail group of gemini surfactants, as shown in fig The positive deviations can be attributed to the unfavorable interactions or repulsive interactions between the sulphate head group of SLES and phosphate head groups of geminis, also similar kind of interactions are possible between the unequal chains of SLES/gemini surfactants Dynamic surface tension Dynamic surface tension measurements were carried out for the SLES (at CMC mm) and SLES (at CMC)/m 3 m geminis (0.1 and 0.5 mm) and m 5 m (0.1 and 0.5 mm) gemini surfactants, using Maximum bubble pressure method. The principle and procedure of maximum bubble pressure was described in earlier chapters. The dynamic surface activity parameters were listed in table 5.3. It was found that with increasing gemini surfactant concentration in the mixture of SLES/m 3 m and SLES/m 5 m, the rate of dynamic surface tension reduction decreases, as shown 117

9 in figures 5.16, 5.18, 5., 5., 5.22, 5.24, The reduced dynamic surface tension of the mixtures was studied, the plots of RDST versus log t are shown in figures 5.17, 5.19, 5.21, 5.23, 5.25, The t values and R 1/2, found to decrease for the SLES/ in the order of (0.1 mm) > (0.5 mm). Similar trend was observed for the SLES/ gemini surfactant, the t values found to decrease in the order of (0.1 mm) > (0.5 mm). However the trend was different for the SLES/ , the t values and R 1/2 values increased in the order (0.5 mm) > (0.1 mm). The effect of the increasing chain lengths of the geminis can be seen, as with the increasing chain length, the R 1/2 values decreases which suggests that the increased hydrophobicity, causes decrease in the adsorption of the molecules under dynamic condition. It was found that for SLES/m 5 m system, the SLES/ at 0.1 & 0.5 mm gemini concentration the surface activity was found to increase than SLES (at CMC) alone. The dynamic surface activity of at 0.1 mm concentration found to increase by times than that of SLES. The m 5 m gemini surfactants found to have good surface activity under dynamic conditions compared to the m 3 m geminis Foamability An apparatus for measurement of foamability of surfactant solution is recently developed in our laboratory. The setup generates foam by impacting a stream of liquid on to a flat horizontal surface of the polydispersed foam generated during the process, the setup separates the fine bubbles from coarse one. The rate of collection of fine foam volume gives a measurement of foamability of the test solution. The details of this method is described in earlier chapter. Experiments were carried out at an ambient temperature (2 ± 2 K). Foam generation of various gemini surfactant solutions and their monomeric surfactants were investigated by Horizontal Impinging Jet method. The foamability of SLES (at CMC) and SLES/gemini surfactants aqueous solutions was studied. The Foamability plots were shown in figures , and the foamability results was enlisted in table 5.4. Overall it was found that the foamability of 118

10 SLES in the presence of the gemini surfactants decreases with the increase in gemini surfactant concentration. This is due to the decreased surfactant availability for adsorption at the interface. Since the newly formed interface must be stabilized by the adsorption of surfactant to produce foam. The interface creation must be immediately followed by interface stabilization in order to avoid coalescence of the formed bubbles. The rate of the stabilization depends on the rate of interface stabilization. The reason can be correlated to the surface density of the monomers of mixed surfactants present at the interface. From table 5.1 and 5.2, it was found that the A min values of the mixtures of SLES/gemini, increased significantly, which means the area per molecule at the interface is larger means very less number of surfactant monomers are available to adsorb at the interface, this results in the lowering of foamability of SLES. Also the low foamability can be a attributed to the slow dynamics of SLES/gemini surfactant mixture. The chain length effect was not observed in the case of m 5 m gemini surfactants, however at 0.1 mm m 3 m geminis the foamability increases in the order of > > but less than that of SLES without any additives. 119

11 Table 5.1: m 3 m gemini bulk solution mole fraction x 1, Mixture CMC C mix, Micellar mole fraction x 1, and Interaction Parameter β and interfacial properties for SLES/m 3 m gemini surfactant system. C mix measured mm C mix ideal mm Γ max A min mol/cm 2 A 2 Gemini x 1 β

12 Table 5.2: m 5 m gemini bulk solution mole fraction x 1, Mixture CMC C mix, Micellar mole fraction x 1, and Interaction Parameter β and interfacial properties for SLES/m 5 m gemini surfactant system. C mix measured mm C mix ideal mm Γ max A min mol/cm 2 A 2 Gemini x 1 β

13 Table 5.3: Dynamic surface activity parameters of SLES and SLES/geminis Surfactant Conc. n t γ m R 1/2 (mm) (mn/s) SLES Table 5.4: Foamability of SLES and SLES/m 3 m and SLES/m 5 m geminis Surfactant system Conc. (mm) Foamability (ml/s) SLES at CMC, SLES/ SLES/ SLES/ SLES/ SLES/ SLES/

14 Surface tension (mn/m) Concentration (mm) Figure 5.2: Surface tension plot of SLES 123

15 Surface tension (mn/m) x 1 = x 1 = x 1 = x 1 = Concentration (mm) Figure 5.3: Surface tension plots of SLES with gemini surfactants 124

16 Surface tension (mn/m) x 1 = x 1 = x 1 = x 1 = Concentration (mm) Figure 5.4: Surface tension plots of SLES with gemini surfactants 125

17 70 x 1 = x 1 = 0.6 Surface tension (mn/m) x 1 = 0.4 x 1 = Concentration (mm) Figure 5.5: Surface tension plots of SLES with gemini surfactants 126

18 Surface tension (mn/m) x 1 = x 1 = x 1 = x 1 = Concentration (mm) Figure 5.6: Surface tension plots of SLES with gemini surfactants 127

19 Surface tension (mn/m) α = α = α = α = Concentration (mm) Figure 5.7: Surface tension plots of SLES with gemini surfactants 128

20 Surface tension (mn/m) x 1 = x 1 = x 1 = x 1 = Concentration (mm) Figure 5.8: Surface tension plots of SLES with gemini surfactants 129

21 3 2 m-3-m geminis m-5-m geminis 1 β Carbon chain length of gemini surfactants Figure 5.9: Plot of interaction parameter (β) between SLES and geminis versus chain length 1

22 1 0.8 C mix measured Margules equation fit C mix ideal β = 2.87 CMC (mm) Mole fraction of gemini Figure 5.10: Plot of C mix against mole fraction of gemini β = 0.13 C mix measured Margules equation fit C mix ideal CMC (mm) Mole fraction of gemini Figure 5.11: Plot of C mix against mole fraction of gemini

23 β = 0.69 C mix measured Margules equation fit C mix ideal CMC (mm) Mole fraction of gemini Figure 5.12: Plot of C mix against mole fraction of gemini C mix measured Margules equation fit C mix ideal 0.9 β = 0. CMC (mm) Mole fraction of gemini Figure 5.13: Plot of C mix against mole fraction of gemini

24 1 0.8 β = 0.39 C mix measured Margules equation fit C mix ideal CMC (mm) Mole fraction of gemini Figure 5.14: Plot of C mix against mole fraction of gemini C mix measured Margules equation fit C mix ideal β = 1.90 CMC (mm) Mole fraction of gemini Figure 5.15: Plot of C mix against mole fraction of gemini

25 SLES at cmc without additives SLES at cmc mm SLES at cmc mm Dynamic Surface tension (mn/m) Time (s) Figure 5.16: Dynamic surface tension plot of SLES / gemini SLES (at cmc) (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic surface tension (mn/m) RDST / sqrt t t Figure 5.17: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 134

26 55 SLES at cmc without additives SLES mm SLES mm Dynamic Surface tension (mn/m) Time (sec) Figure 5.18: Dynamic surface tension plot of SLES / gemini surfactant SLES SLES (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic surface tension (mn/m) RDST t -1/ t Figure 5.19: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 135

27 55 SLES at cmc, without additives SLES mm SLES mm Dynamic Surface tension (mn/m) Time (s) Figure 5.: Dynamic surface tension plot of SLES / gemini SLES SLES (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic Surface tension (mn/m) RDST t -1/ t Figure 5.21: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 136

28 55 SLES at cmc, without additives SLES mm SLES mm Dynamic surface tension (mn/m) Time (s) Figure 5.22: Dynamic surface tension plot of SLES / gemini SLES (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic surface tension (mn/m) RDST /sqrt t t Figure 5.23: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 137

29 55 SLES (0.1mM) SLES (0.5mM) Dynamic Surface tension (mn/m) t (sec) Figure 5.24: Dynamic surface tension plot of SLES / gemini surfactant SLES SLES (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic surface tension (mn/m) RDST t -1/ t Figure 5.25: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 138

30 55 SLES at cmc without additives SLES mm SLES mm Dynamic surface tension (mn/m) Time (s) Figure 5.26: Dynamic surface tension plot of SLES / gemini surfactant SLES SLES (0.1 mm) (0.5 mm) (0.1 mm) (0.5 mm) Dynamic surface tension (mn/m) RDST t -1/ t Figure 5.27: Plots of dynamic surface tension versus t 1/2 and RDST versus t of SLES / gemini 139

31 25 SLES mm SLES + 0.5mM SLES without additive Foam Volume (ml) Time (min) Figure 5.28: Foamability of SLES / gemini 25 SLES mm SLES mm SLES without additive Foam Volume (ml) Time (min) Figure 5.29: Foamability of SLES / gemini 1

32 25 SLES without additives SLES mm SLES mm Foam volume (ml) Time (min) Figure 5.: Foamability of SLES / gemini 25 SLES mm gemini SLES mm gemini Foam Volume (ml) Time (min) Figure 5.31: Foamability of SLES / gemini 141

33 25 SLES mm SLES mm Foam Volume (ml) Time (min) Figure 5.32: Foamability of SLES/ gemini 25 SLES mm SLES mm Foam Volume (ml) Time (min) Figure 5.33: Foamability of SLES/ gemini 142

34 Table 5.5: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) Table 5.6: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m)

35 Table 5.7: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) Table 5.8: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m)

36 Table 5.9: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) Table 5.10: Equilibrium surface tension data for SLES/ gemini mixture α = 0.2 α = 0.4 α = 0.6 α = 0.8 Conc. γ Conc. γ Conc. γ Conc. γ (mm) (mn/m) (mm) (mn/m) (mm) (mn/m) (mm) (mn/m)

37 Table 5.11: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant SLES (at CMC) (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m) t (sec) γ (mn/m)

38 Table 5.12: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m)

39 Table 5.13: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m)

40 Table 5.14: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m)

41 Table 5.15: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m)

42 Table 5.16: Dynamic surface tension data of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (sec) γ (mn/m) t (sec) γ (mn/m)

43 Table 5.17: Foamability data of SLES at CMC and SLES/ gemini surfactant SLES (at CMC) (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml) t (min) Foam (ml)

44 Table 5.18: Foamability of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml)

45 Table 5.19: Foamability of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml) Table 5.: Foamability of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml)

46 Table 5.21: Foamability of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml) Table 5.22: Foamability of SLES at CMC and SLES/ gemini surfactant (0.1 mm) (0.5 mm) t (min) Foam (ml) t (min) Foam (ml)