Interaction between Nonionic Polymer Hydroxypropyl Methyl

Article pubs.acs.org/IECR

Interaction between Nonionic Polymer Hydroxypropyl Methyl Cellulose (HPMC) and Cationic Gemini/Conventional Surfactants Najam Sardar,† Mohammad Kamil,*,† and Kabir-ud-Din‡ †

Department of Petroleum Studies, Aligarh Muslim University, Aligarh-202002, India Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India

‡

S Supporting Information *

ABSTRACT: The purpose of this study was to investigate the interaction between a nonionic polymer, (hydroxypropyl)methyl cellulose (HPMC), and cationic gemini surfactants, bis(hexadecyldimethylammonium)hexane dibromide (16-6-16), bis(hexadecyldimethylammonium)pentane dibromide (16-5-16), and their corresponding monomeric counterpart cetyltrimethylammonium bromide (CTAB), by using electrical conductometry, fluorescence, and viscometry methods. It was found that the gemini surfactants interact strongly with HPMC as compared to conventional surfactant CTAB. The free energies of aggregation, ΔGagg, micellization, ΔGmic, and transfer, ΔGt, associated with the binding interaction between surfactant and polymer, have also been evaluated. The negative values of ΔGt confirm the feasibility of interaction between the surfactant and polymer. The aggregation number (Nagg) obtained from steady state fluorescence measurement with CTAB was found to be more than with the geminis. A significant viscosity increment was observed in the case of gemini surfactants as compared to CTAB. The rapid increase of the viscosity with surfactant concentration was, therefore, attributed to the considerable cross-links among micelles and polymers (transient network).

1. INTRODUCTION Polymer−surfactant systems are the subject of significant research.1−3Aqueous solutions containing water-soluble polymers and surfactants are generally useful as these systems display varied and bewildering pattern of properties, due to many variations in molecular structures available to the formulator. Areas where polymers and surfactants are frequently present include pharmaceutical formulations, personal care products, food products, household and industrial detergents, paints and coatings, oil drilling, and enhanced oil recovery fluids, along with fundamental interest in intermolecular interactions and hydrophobic aggregation phenomena.4−9 The polymer−surfactant interaction leads to the formation of polymer−surfactant complex. The well accepted morphology of the complex is the necklace model established by Shirahama and Cabane.10,11When surfactant is added to an aqueous polymer solution, no interaction is detected up to a specific concentration denoted as critical aggregation concentration (cac). At [surfactant] ≥ cac, the surfactant starts to adsorb to the polymer chains in a cooperative manner in the form of small micelles (clusters). The adsorbed clusters increase in size with both polymer and surfactant concentration up to a certain limit. The number of cluster binding “sites” on the polymer increases strongly as the adsorption begins and continues until the entire polymer gets saturated. When more surfactant is added, normal micelles begin to form; this concentration is called critical micelle concentration (cmc). The interaction becomes stronger as the amount of the surfactant is increased, reflecting the greater drive to substitute surfactant−polymer for surfactant−water interactions. The properties of the solution will be altered as a result of such changes. For certain polymers, at higher polymer concentrations, the clustering adsorption tends to become intermo© 2011 American Chemical Society

lecular in nature; i.e., one cluster is shared by two or more polymer molecules creating a three-dimensional network (transient network). On the other hand, for certain polymers, when surfactant concentration reaches cmc, the polymer molecule may then solubilize its hydrophobic “sites” also in normal “free” micelles. This competition will favor the normal micelles as their relative number increases with increasing total surfactant concentration, and the networking tendency of the polymer solution will eventually be lost. In the past few years, a novel class of surfactants called gemini (dimeric), consisting of two hydrophobic chains and two hydrophilic head-groups united by a short (rigid or flexible) spacer, has emerged.12−15 These gemini surfactants impart better surface properties as compared to their singlehead, single-tail counterparts. The geminis have much smaller cmc values, much greater efficiency in reducing surface tension than expected, better wetting properties, and other unusual behaviors. Despite many studies on interactions between neutral polymer and anionic surfactant, the understanding of the nature of the neutral polymer-cationic surfactant interaction is still incomplete. Therefore, the purpose of this study was to investigate the interaction between a nonionic polymer, (hydroxypropyl)methyl cellulose (HPMC), and cationic gemini surfactants (16-6-16 and 16-5-16) and the corresponding simple monomeric counterpart (CTAB) and to see the effect of hydrophobicity and the molecular architecture in such Received: Revised: Accepted: Published: 1227

August 4, 2011 November 3, 2011 December 23, 2011 December 23, 2011 dx.doi.org/10.1021/ie2010725 | Ind. Eng.Chem. Res. 2012, 51, 1227−1235

Industrial & Engineering Chemistry Research

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was determined by the intersection of first and second linear parts and the cmc in this case was the intersection point of the second and third linear parts. 2.3. Fluorescence Measurements. The micellar aggregation numbers (Nagg) of polymer and surfactant solutions were determined using steady-state fluorescence measurements. Pyrene and cetylpyridinium chloride were used as probe and quencher, respectively. Fluorescence measurements were taken on a Hitachi F-2500 fluorescence spectrophotometer with excitation and emission slit width of 2.5 nm and scan speed 60 nm·min−1. Excitation was done at 335 nm, and emission was recorded in the range 350−450 nm. A 3 μM pyrene concentration was made in different solvent mixtures (polymer + surfactant). A surfactant concentration (4.0 mmol·dm−3 for CTAB and 2.0 mmol·dm−3 for 16-6-16 and 16−5−15) above the cmc was used in all the measurements. All spectra had one to five vibronic peaks (Figure S1) (Supporting Information). The first and the third vibronic peaks appeared at 373 and 384 nm, respectively. [Quencher] was varied slightly to ensure the Poisson distribution18 for the quencher. 2.4. Viscosity Measurements. The viscosities were measured using an Ubbelhode suspended level capillary viscometer. The viscometer was always suspended vertically in a thermostat with a temperature stability of ±0.1 K in the investigated region. The requisite amount of surfactant was added in HPMC solution. These solutions were used as stock solutions to see the effect of surfactant concentration. A nearly saturated solution was prepared for a typical surfactant, and further lower concentrations were made by dilution from the above stock. Viscosities of such solutions under Newtonian flow conditions were obtained as described elsewhere.19The viscosity runs were carried out at HPMC concentrations of 1.0, 0.5, and 0.1 wt % with different surfactant concentrations at 298.15 K. The relative viscosity was calculated from the equation

interactions by using conductometry, fluorescence, and viscometry techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. (Hydroxypropyl)methyl cellulose, HPMC (mol. wt ∼10,000, hydroxylpropyl content ∼9%, Fluka, Switzerland), and cetyltrimethylammonium bromide, CTAB (≥99.0%, Merck, Germany), were used as received. Gemini surfactants α,ω-bis(hexadecylammonium)alkane dibromides (16-6-16 and 16-5-16) were prepared and purified as described elsewhere,16 which gave 1H NMR spectra and C,H,N analyses data consistent with their assigned structures.17 For synthesis, the following materials were used without further purification: 1,5-dibromopentane (Himedia, India), 1,6- dibromohexane (≥98%, Fluka, Switzerland), N,N-dimethylhexadecylamine (≥95%, Fluka, Switzerland), and dry ethanol (99.9%, Changshu Yangyuan, China). The molecular structures of CTAB, 16-6-16, and 16-5-16 along with HPMC are shown in Figure 1.

ηr = t /t0

(1)

where t and t0 are the flow time of solution and water, respectively. Density corrections were not made since these were found negligible.20

Figure 1. Structures of CTAB, gemini surfactants 16-s-16 (s = 5, 6) and HPMC.

3. RESULTS AND DISCUSSION 3.1. Conductivity Results. The conductance was measured for CTAB and two gemini surfactant (16-6-16 and 16-5-16) solutions in the presence and absence of different weight percent of HPMC at 298.15 K, 303.15 K, 308.15, and 313.15 K, respectively, to calculate cac and cmc values. The specific conductivity (κ) profiles as a function of concentration of CTAB and two gemini surfactants (16-6-16 and 16-5-16) at 298.15 K are presented in Figure 2(a-c). The intersection point of the two straight lines represents the usual cmc of surfactants, and the corresponding values at four different temperatures for CTAB, 16-6-16, and 16-5-16 are presented in Table1, which are in agreement with the reported values in the literature.21 In Figure 3, the plot shows two breaks in the presence of HPMC in comparison to a single break observed in the absence of polymer. The two breaks in the presence of polymer are ascribed to the occurrence of two kinds of aggregation phenomena. The first break is called cac where the interaction of polymer chain with surfactant starts. The second break point is called cmc, where the polymer chain with surfactant and/or

2.2. Conductivity Measurements. In the present investigation, the conductivity measurements were performed on a digital conductivity meter (Systronics Conductivity-TDS Meter 308, range 0.1 μS to 100 mS, accuracy ±1% of F.S. ± 1 digit, India). The conductivity runs were carried out by adding progressively concentrated surfactant stock solution into the thermostatted solvent (demineralized double distilled water of specific conductivity 1 to 2 × 10−6 S·cm−1 or solvent containing different weight percent HPMC. The conductivity runs were carried out at different weight percent of HPMC (0.05 to 1.0 wt %) and at four different temperatures 298.15 K, 303.15 K, 308.15, and 313.15 K. The temperature was maintained in a thermostatted water bath with thermal stability of ±0.1 K. The critical micellar concentration of the pure surfactant used was obtained from the plots of specific conductivity (κ) as a function of the surfactant concentration. The cmc values were taken from the intersection of the two straight lines drawn before and after the intersection point in the κ versus surfactant concentration plots. In case of the polymer−surfactant mixtures the plots of κ versus [surfactant] showed two breaks, the cac 1228

dx.doi.org/10.1021/ie2010725 | Ind. Eng.Chem. Res. 2012, 51, 1227−1235


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The cac and cmc values for CTAB and gemini surfactants at 298.15 K, 303.15 K, 308.15, and 313.15 K in different weight percent HPMC (0.05 to 1.0 wt %) are recorded in Table 1. It can be seen that the cmc values in the presence of HPMC are higher than the values in the absence of the polymer molecule, which indicates aggregation with the surfactant prior to micellization. For the same HPMC concentration, the cac and cmc follow the trend as follows: 16-6-16 < 16-5-16 < CTAB. Thus, it can be concluded that gemini surfactants and the surfactant with longer spacer chain interact strongly as compared to surfactant with shorter spacer and the conventional counterpart.22 The oxygen atom in the OH group of HPMC helps in dispersing the positive charge of the headgroup of surfactants which results in an ion-dipole type of interaction between polymer and surfactants. Second, the van der Waals type of interaction occurs between the chain length of cationic surfactants and hydrophobic carbon backbone of the polymer. It is further noticed that both cac and cmc values increase with an increase in polymer concentration for all three surfactants. Whereas for 16-5-16 and 16-6-16 after 0.2 wt % the cmc value becomes constant but for CTAB it increases slightly as shown in Figure S2 (Supporting Information), which may be due to the availability of more and more reactive binding sites to the surfactant monomers23 or micelle-like aggregates (not the true micelles). Thus, a greater amount of the surfactant is required to bind to the polymer. After the total binding sites are occupied, the surfactant becomes free to form micelles. From Table 1, it is clear that, as the temperature increases, both cac and cmc values increase for all concentrations of polymers. In general, the effect of temperature on the cmc of surfactants in aqueous media is complex. On one hand, temperature increase causes decreased hydration of the hydrophilic group, which favors micellization, but, on the other hand, temperature increase also causes disruption of the structured water surrounding the hydrophobic group, which disfavors micellization. The relative magnitude of these two opposing effects, therefore, determines whether the cmc increases or decreases over a particular temperature range. In the present study, the cmc values increase with an increase in temperature indicating that the micellization is less favored in these three systems, i.e., HPMC/CTAB, HPMC/16-6-16, and HPMC/16-5-16 in water. The degree of micelle ionization (g) was calculated by taking the ratio between the slopes of the linear portions above and below the break point in the conductivity profiles. Hence, two values, i.e., g1 and g2, were obtained (Table T1) (Supporting Information). The larger value of g for the complex micelle is an indication of increased degree of ionic dissociation as a result of the interaction of the surfactants with the polymer.24 The free energy of aggregation, ΔGagg, and the free energy of micellization, ΔGmic, can be calculated using the following equations25 Figure 2. (a−c) Plots of specific conductivity (κ) versus [surfactant] at different temperature in water (a) for CTAB, (b) for 16-6-16, and (c) for 16-5-16.

micelle like aggregates gets saturated, followed by formation of normal micelles on adding a greater amount of surfactant. Figure 3(a), (b), and (c) shows the effect of increasing concentration of HPMC on specific conductivity profiles of CTAB, 16-6-16, and 16-5-16.

ΔGagg = RT (2 − g1)ln Xcac

(2)

ΔGmic = RT (2 − g2)ln Xcmc

(3)

In case of gemini surfactants, the above two equations become as follows26

ΔGagg = 2RT (1.5 − g1)ln Xcac 1229

(4)



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Table 1. cac and cmc Values for CTAB, 16-6-16, and 16-5-16 in Solutions Containing Different Weight Percentages HPMC (Determined from Conductivity Measurements) CTAB HPMC (wt %) 0.00

0.05

0.10

0.20

0.50

1.00

a

temperature (K) 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

cac (mmol·dm−3) 0.508 0.576 0.625 0.659 0.542 0.560 0.591 0.693 0.560 0.644 0.693 0.693 0.576 0.644 0.674 0.727 0.610 0.727 0.791 0.991

16-6-16

cmc (mmol·dm−3) 0.956 (0.990) 1.040 1.123 1.240 1.320 1.390 1.440 1.520 1.350 1.390 1.490 1.590 1.500 1.600 1.620 1.750 1.590 1.720 1.820 2.420 1.950 2.100 2.220 2.490

cac (mmol·dm−3)

a

0.0312 0.0322 0.0388 0.0395 0.0312 0.0329 0.0371 0.0395 0.0322 0.0329 0.0388 0.0412 0.0329 0.0405 0.0412 0.0437 0.0412 0.0420 0.0461 0.0478

16-5-16

cmc (mmol·dm−3) 0.0429 (0.041) 0.0478 0.0520 0.0578 0.1195 0.1236 0.1361 0.1402 0.1204 0.1312 0.1378 0.1470 0.1204 0.1419 0.1429 0.1478 0.1212 0.1178 0.1329 0.1495 0.1387 0.1503 0.1537 0.1610

a

cac (mmol·dm−3)

cmc (mmol·dm−3)

0.0495 0.0503 0.0554 0.0579 0.0495 0.0512 0.0579 0.0603 0.0503 0.0529 0.0579 0.0630 0.0520 0.0562 0.0630 0.0703 0.0562 0.0613 0.0630 0.0745

0.0321 (0.031)a 0.0363 0.0419 0.0453 0.1495 0.1544 0.1595 0.1686 0.1520 0.1569 0.1603 0.1744 0.1554 0.1578 0.1627 0.1752 0.1578 0.1595 0.1686 0.1720 0.1779 0.1869 0.1886 0.1911

Literature value (Kabir-ud-Din,* M. A. Rub, A. Z. Naqvi, J. Phys. Chem. B 2010, 114, 6354).

ΔGmic = 2RT (1.5 − g2)ln Xcmc

{[S] − cmc}. For CTAB, Nagg values of the system are larger than pure geminis as shown in Table 2. The aggregation number is the total number of species in the cluster (polymer and surfactant). We can see that the addition of even a small amount of HPMC decreases the Nagg. Also, the aggregation number, first decreases, then slightly increases as HPMC wt % increases indicating that the polymer bound cluster size increases, and finally becomes constant as [HPMC] further increases. The decrease in Nagg may be due to binding of surfactant micelles to HPMC chains (or the “adsorption” of the HPMC on the micelles) and the resulting increase of the micelle ionization degree. As a result, the micelles formed on polymer chains are smaller than in pure water. This behavior is fairly general as reported by earlier investigators.30−36 No change in Nagg when HPMC concentration is larger than 0.5 wt % indicates that the surfactant clusters in HPMC hydrophobic chains no longer separate into smaller clusters on further addition of HPMC.30,31 The above results can further be explained on the basis of quenching. The strength of hydrophobic environment can be evaluated by determining the first order quenching rate constant, the so-called Stern− Volmer binding constant (KSV), using the relation

(5)

The free energy of transfer, ΔGt, associated with the binding interaction between surfactant and polymer, is given by

ΔGt = ΔGagg − ΔGmic

(6)

The negative values of ΔG t (Table T1) (Supporting Information) confirm the feasibility of interaction between the surfactants and polymer. In case of geminis, the negative values correspond to stronger interactions. The value is largest in case of 16-6-16 and least in the case of CTAB, which confirms that the gemini surfactants show better properties as compared to their monomeric counterpart. 3.2. Micellar Aggregation Number in Polymer− Surfactant Systems. Steady-state fluorescence quenching method was used for estimating aggregation number (Nagg) of various amphiphiles in different mixed media as reported earlier by a number of researchers.26−28 By using the fluorescence quenching method at different weight percent of HPMC + surfactant mixtures, the aggregation number (Nagg) was calculated and is presented in Table 2. Nagg was deduced from the following equation29

ln(I0 / I ) = Nagg[Q ]/([ S] − cmc)

I0 / I = 1 + KSV [Q ] (8) K SV gives an idea about bimolecular quenching and unimolecular decay as it being the product of rate of the quenching process and lifetime of the probe in the absence of bimolecular quenching.32,33 The greater the solubility of the probe and quencher, the higher would be the KSV value. The observed high KSV values (Table 2) suggest an increase in quenching due to the presence of both pyrene and quencher in a strong hydrophobic environment.

(7)

where [Q] = quencher concentration, [S] = total surfactant concentration, [I0] = fluorescence intensity in the absence of quencher, and [I] = fluorescence intensity in the presence of quencher. The total surfactant concentration for CTAB + HPMC was 4.0 mmol·dm−3 and total concentration for gemini surfactant + HPMC was 2.0 mmol·dm−3. Equation 7 predicts a linear plot of ln(I/I0) versus [Q] with a slope equal to Nagg/ 1230



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suspended hard spheres. When the polymer solution contains surfactant micelles, one intuitively expects that the resistance to flow is dominated by the entanglement effect associated with polymer chains. As a consequence, the viscosity of the mixture is essentially the same as that without micelles if the interaction between macromolecule and micelle is absent.26 No significant changes in relative viscosity were observed in HPMC with all three surfactants up to the cmc of the surfactants, and the change in relative viscosity was observed only at higher concentration of surfactants >10 mmol·dm−3. Figure 5(a), (b), and (c) demonstrates the variation of the solution viscosities with surfactant concentration obtained for different fixed polymer concentrations. Figure 5(a) is for 1.0 wt % HPMC which shows that the behavior of cationic surfactant CTAB and cationic gemini surfactants 16-6-16 and 16-5-16 follow our aforementioned anticipation. In the case of CTAB, the leading contribution comes from the polymer solution. The extra contribution due to micelles is significant only at higher surfactant concentrations (>100 mmol·dm−3). Note that the viscosity of pure CTAB solution in 1.0 wt % HPMC at 50 mmol·dm−3 is only 5.441. This is due to the very weak interaction between macromolecule and micelles as was evident by conductivity data too. The HPMC/16-6-16/water and HPMC/16-5-16/water systems show significant viscosity increment due to the existence of micelles at much lower surfactant concentrations. This shows strong interaction between HPMC and the gemini surfactants. An effort was also made to develop an empirical correlation between [surfactant] and relative viscosity in the following form as given below

ηr = k1· exp(k 2·c)

(9)

values k1 and k2 are given in Table T2 (Supporting Information) . For all the systems the mean absolute deviation (MAD) was found to be less than 4.43%. The viscosities of the mixtures (50 mmol·dm−3 16-6-16 + HPMC + water) and (50 mmol·dm−3 16-5-16 + HPMC + water) were around 2.4 times and 3.5 times as large as of the pure polymer solution (ηr = 2.247). Hence, it was concluded that the gemini surfactants interact strongly with HPMC as compared to their conventional counterpart CTAB. Figure 5(b) and (c) are for 0.5 and 0.1 wt % HPMC, respectively, and displays again that the addition of cationic surfactant CTAB plays a minor role and the gemini surfactants enhance the viscosity of the polymer solution substantially. Also, 16-5-16 gives a viscosity about 1.5 times as large as that of pure polymer (ηr = 2.260) for 0.5 wt % HPMC and 2.25 times as large (ηr = 1.253) for 0.1 wt % HPMC. We can see that ηr increases with [surfactant] for all the surfactants, but the increase is significant with the gemini surfactants as compared to CTAB. Also, higher viscosities observed for 16-5-16 for the same [surfactant] shows the ability of gemini surfactants of shorter spacer to give rise to rod shaped micelles at fairly low surfactant concentration.37 Viscometric data were also converted to reduced specific viscosity (ηsp/c) where c is the polymer concentration and ηsp is defined by

Figure 3. (a−c) Plots of specific conductivity (κ) versus [surfactant] at 298.15 K in different wt % HPMC (a) for CTAB, (b) for 16-6-16, and (c) for 16-5-16.

3.3. Viscosity Results. Figure 4 shows the relative viscosity of polymer solution as a function of polymer concentration, which increases with the [polymer]. The large resistance to flow is generally attributed to the entanglement among polymer chains. In the dilute limit it can be simply explained by the Einstein relation, which gives the extra contribution due to the resistance to deformation by

ηsp = (η − ηo)/ηo

(10)

Here η and ηo are the viscosities of solution (HPMC/ surfactant/water) and solvent (surfactant/water), respectively, i.e., the system was treated as a quasibinary one.38 Extrapolating ηsp/c to zero polymer concentration gives the intrinsic viscosity 1231



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Table 2. Aggregation Number (Nagg) and KSV Values for CTAB, 16-6-16, and 16-5-16 in Solutions Containing Different Weight Percentages of HPMC CTAB HPMC (wt %)

Nagg

0.00 0.05 0.1 0.2 0.5 1.0

67 (70 ± 8) 48 49 41 56 56

16-6-16

KSV/104 (mmol·dm−3)−1 a

2.9533 2.2640 2.2903 1.9774 3.0248 2.1434

39 (28) 39 50 33 35 34

16-5-16

KSV/104 (mmol·dm−3)−1

Nagg b

5.3934 3.5874 3.2373 1.5458 1.5799 1.8387

KSV/104 (mmol·dm−3)−1

Nagg 41 (27) 30 41 40 36 36

c

2.4994 2.3087 2.7056 2.8714 2.3928 2.3535

a

Literature value (J. Phys. Chem. B 2009, 113, 786). bLiterature value (J. Colloid Interface Sci. 2008, 328, 429). cLiterature value (Azum, N. Studies on Surfactant-Additive Systems., Thesis, Department of Chemistry, Aligarh Muslim University, 2009).

The viscosity increment due to ionic surfactant is explained by the so-called “polyelectrolyte effect” based on the necklace picture. As multiple ionic micelles bind to a nonionic homopolymer polymer, parts of the backbone of the chain repel each other due to electrostatic repulsions with the result that the polymer coils become more elongated (rod-like) and the solution viscosity increases.40 For 0.1 wt % HPMC, the reduced viscosity decreases up to 15 mmol·dm−3 of [CTAB], then increases, again decreases, and then becomes highly sensitive to the amount of CTAB present. The decrease in reduced specific viscosity with an increase of CTAB concentration indicates that the aggregate formation leads to a more compact polymer structure albeit the electrostatically bound along the polymer chain. Figure 6(b) is for 16-6-16 gemini surfactant, it can be seen that for 1.0 and 0.5 wt % HPMC, the reduced specific viscosity was found to be fairly insensitive to surfactant concentration below 80 mmol·dm−3, after which the viscosity decreases significantly. For 0.1 wt %, the reduced specific viscosity is constant up to 50 mmol·dm−3 and then increases to reach a maximum at around 80 mmol·dm−3, thereafter decreasing sharply showing pronounced interaction between HPMC and 16-6-16 at this concentration. Reduced specific viscosity curves for 16-5-16 have been shown in Figure 6(c). The viscosity decreases at all HPMC concentrations but becomes constant after around 35 mmol·dm−3. Thus, the reduced specific viscosity becomes rather insensitive to changes in [16-5-16] at higher 16-5-16 concentrations, showing a strong interaction between HPMC and 16-5-16 at lower concentrations. As per the above discussion, a schematic illustration of the interaction between polymers and surfactants is presented in Figure 7.

Figure 4. Relative viscosities of different wt % HPMC solutions in water at 298.15 K.

[η], using the expression

ηsp / c = [η] + kH[ηsp]2 c + .......

(11)

where higher terms have been omitted and kH (Huggins constant) is a hydrodynamic measure of the intensity of the polymer/polymer interaction. When surfactants are added to different HPMC solutions, the intrinsic viscosity is altered as shown in Figure 6(a-c). Similar observations have been made by others also.29,30 For each set of measurements the polymer concentration has been kept constant while the total [surfactant] varied. From Figure 6(a), it is seen that for 1.0 and 0.5 wt % HPMC, the reduced specific viscosity increases up to CTAB concentration ∼15 mmol·dm−3. This change in rheological behavior coincides with the onset of binding of CTAB and HPMC fractions and due to the result of a coil expansion from the formation of charged CTAB aggregates along the polymer chain. As the mean charge density is low, the CTAB-polymer complex behaves as a weakly charged polyelectrolyte.39 Hence, weak interaction occurs between the polymer and surfactant molecules and after that viscosity decreases with [CTAB] due to an increase in bromide ion concentration which reduces the electrostatic repulsion between the aggregates. After attaining a minimum value, ηsp/ c gradually increases again up to 250 mmol·dm−3, showing weak interaction between HPMC and CTAB. After 250 mmol·dm−3 it increases significantly due to formation of multiple micelles.

4. SUMMARY The above results show that the water-soluble nonionic polymer HPMC interacts with cationic gemini surfactants, (16-6-16) and (16-5-16), and their corresponding monomeric counterpart (CTAB). These facts agree with the conclusions of other authors23,34 who found that the polymer’s hydrophilic− lipophilic balance determines its interaction with a given surfactant. Further, the interaction results in the formation of polymer−surfactant micelles. The aggregation of polymer− surfactant micelles takes place at a surfactant concentration higher than critical micelle concentration of micelle without polymer. The results also show that the gemini surfactants interact strongly as compared to the conventional one. With a fluorescence probe technique it was observed that the Nagg value for CTAB was more as compared to gemini surfactants. Nagg decreases on addition of polymer, then slightly increases, 1232



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Figure 6. (a−c) Plots of reduced specific viscosity versus [surfactant] at different wt % HPMC at 298.15 K (a) CTAB, (b) 16-6-16, and (c) 16-5-16. Figure 5. (a−c) Plots of relative viscosity versus [surfactants] at (a) 1.0 wt % HPMC, (b) 0.5 wt % HPMC, and (c) 0.1 wt % HPMC at 298.15 K.

surfactant interaction is also studied by examining the change in the transport properties. The results show the increase in viscosity with an increase in surfactant concentration. A significant viscosity increment was observed in the case of geminis surfactant. This rapid growth of the viscosity with surfactant concentration may be attributed to the formation of

indicating an increase in polymer bound cluster size, and then becomes constant after 0.5 wt % HPMC indicating that clusters in HPMC hydrophobic chains no longer separate into smaller clusters on further addition of HPMC.30,31 The polymer− 1233



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(5) Goddard, E. D.; Gluber, J. V. Principles of Polymer Science and Technology in Cosmetics and Personal Care; Marcel Dekker: New York, 1999. (6) Chandar, P.; Somasundaran, P.; Turro, N. J. Fluorescence Probe Investigation of Anionic Polymer-Cationic Surfactant Interactions. Macromolecules 1988, 21, 950. (7) Hansson, P.; Lindman, B. Surfactant-polymer Interactions. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (8) Goddard, E. D.; Ananthapadamanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, 1993. (9) Kwak, J. C. T. Polymer-Surfactant Systems, Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998. (10) Shirahama, K. The Binding Equilibrium of Sodium Dodecyl Sulfate to Poly(ethylene oxide) in 0.1 m Sodium Chloride Solution at 30 °C. Colloid Polym. Sci. 1974, 252, 978. (11) Cabane, B. Structure of some Polymer-Detergent Aggregates in Water. J. Phys. Chem. 1977, 81, 1639. (12) Menger, F. M.; Littau, C. A. Gemini-Surfactants: Synthesis and Properties. J. Am. Chem. Soc. 1991, 113, 1451. (13) Zana, R.; Talmon, Y. Dependence of Aggregate Morphology on Structure of Dimeric Surfactants. Nature 1993, 362, 228. (14) Menger, F. M.; Keiper, J. S. Gemini Surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906. (15) Zana, R. Dimeric and Oligomeric Surfactants. Behavior at Interfaces and in Aqueous Solution: a review. Adv. Colloid Interface Sci. 2002, 97, 203. (16) Ali, M. S.; Suhail, M.; Ghosh, G.; Kamil, M.; Kabir-ud-Din. Interactions Between Cationic Gemini/conventional Surfactants with Polyvinylpyrrolidone: Specific Conductivity and Dynamic Light Scattering. Colloids Surf., A 2009, 350, 51. (17) Kabir-ud-Din.; Fatma, W.; Khan, Z. A.; Dar, A. A. H NMR and Viscometric Studies on Cationic Gemini Surfactants in Presence of Aromatic Acids and Salts. J. Phys. Chem. B 2007, 111, 8860. (18) Kabir-ud-Din.; Siddiqui, U. S.; Kumar, S.; Dar, A. A. Micellization of Monomeric and Dimeric (Gemini) surfactants in polar nonaqueous-water-mixed solvents. Colloid Polym. Sci. 2006, 284, 807. (19) Kabir-ud-Din.; Kumar, S.; Aswal, V. K.; Goyal, P. S. Effect of the Addition of n-alkylamines on the Growth of Sodium decyl sulfate Micelles. J. Chem. Soc., Faraday Trans. 1996, 92, 2413. (20) Ozeki, O.; Ikeda, S. The Viscosity Behavior of Aqueous NaCl Solution of Dodecyldimethylammonium Chloride and the Flexibility of its Rod-Like Micelle. J. Colloid Interface Sci. 1980, 77, 219. (21) Zana, R.; Benrraou, M.; Rueff, R. Alkanediyl-.alpha.,.omega.bis(dimethylalkylammonium bromide) Surfactants. 1. Effect of the Spacer Chain Length on the Critical Micelle Concentration and Micelle Ionization Degree. Langmuir 1991, 7, 1072. (22) Zana, R. Dimeric (Gemini) Surfactants: Effect of the Spacer Group on the Association Behavior in Aqueous Solution. J. Colloid Interface Sci. 2002, 248, 203. (23) Romani, A. P.; Gehlen, M. H.; Rosangela, I. Surfactant-polymer Aggregates Formed by sodium Dodecyl Sulfate, Poly(N-vinyl20pyrrolidone), and Poly(ethylene glycol). Langmuir 2005, 21, 127. (24) R. Zana, R.; J. Lang, J.; P. Lianos, P. Flourescence Probe Studies of the Aggregation State of Sodium Dodecyl Sulfate in Aqueous Solutions of Polyoxyethyleneglycol and Poly N-vinylpyrrolidone. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1982, 23, 39. (25) Lu, J. R.; Marrocco, A.; Su, T. J. Adsorption of Dodecyl Sulfate Surfactants with Monovalent Metal Counterions at the Air-Water Interface Studied by Neutron Reflection and Surface Tension. J. Colloid Interface Sci. 1993, 158, 303. (26) Wang, S. C.; Wei, T. C.; Chen, W. B.; Tsao, H. K. Effects of Surfactant Micelles on Viscosity and Conductivity of Poly(ethylene glycol) Solutions. J. Chem. Eng. Phys. 2004, 120, 4980. (27) Rodriguez, A.; Graciani, M. M.; Fernandez, G.; Moya, M. L. Effects of Glycols on the Thermodynamic and Micellar Properties of TTAB in Water. J. Colloid Interface Sci. 2009, 338, 207. (28) Rodriguez, A.; Graciani, M. M.; Angulo, M.; Moya, M. L. Effects of Organic Solvent Addition on the Aggregation and Micellar Growth

Figure 7. Schematic representation of the conformation of HPMC and surfactant complexes.

cross-links among micelles and polymers, i.e., transient network as observed by earlier investigators.26

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ASSOCIATED CONTENT

S Supporting Information *

Representative fluorescence (emission) spectra of 3 × 10−6 M pyrene in aqueous micellar solution of 16-6-16 (2.0 mmol·dm−3) and HPMC at different quencher concentrations is presented in Figure S1. Variation of polymer−surfactant cmc with polymer concentration is shown in Figure S2. Degree of micelle ionization (g1 and g2) and free energies (ΔGagg, ΔGmic, and ΔGt) for CTAB, 16-6-16, and 16-5-16 in solutions containing different weight percentages of HPMC (determined from conductivity) are given in Table T1. The values k1 and k2 for empirical correlation between [surfactant] and relative viscosity are given in Table T2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0571-2501887. E-mail: [email protected].

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REFERENCES

(1) Glass, J. E. Polymers in Aqueous Media: Performance Through Association, Advances in Chemistry Series 223; American Chemical Society: WA, 1989. (2) Shalaby, S. W.; McCormic, C. L.; Butler, G. B. Water Soluble Polymers, Synthesis, Solution Properties and Applications; ACS Symposium Series 467, American Chemical Society: WA, 1991. (3) Ranade, V.; Hollinger, M. A. Drug Delivery Systems, 2nd ed.; CRC Press: Boca Raton, 2003. (4) Stahl, G. A.; Schulz, D. N. Water−Soluble Polymers for Petroleum Recovery; Plenum Press: New York, 1986. 1234



Article

of Cationic Dimeric Surfactant 12-3-12, 2Br-. Langmuir 2007, 23, 11496. (29) Carlsson, A.; Karlstrom, G.; Lindman, B. Synergistic SurfactantElectrolyte Effect in Polymer Solutions. Langmuir 1986, 2, 536. (30) Wang, G.; Olofsson., G. Ethyl Hydroxyethyl Cellulose and Ionic Surfactants in Dilute Solution. Calorimetric and Viscosity Study of the Interaction with Sodium Dodecyl Sulfate and Some Cationic Surfactant. J. Phys. Chem. 1995, 99, 5588. (31) Ruiz, C. C.; Bolivar, J. A. M.; Aguiar, J.; MacIssac, G.; Palepu, S. Thermodynamic and Structural Studies of Triton X-100 Micelles in Ethylene Glycol−Water Mixed Solvents. Langmuir 2001, 17, 6831. (32) Rohatgi-Mukherjee, K. K. Fundamentals of Photochemistry; Wiley Eastern: New Delhi, 1992. (33) Azum, N.; Naqvi, A. Z.; Akram, M.; Kabir-ud-Din. Studies of Mixed Micelle Formation Between Cationic Gemini and Cationic Conventional Surfactants. J. Colloid Interface Sci. 2008, 328, 429. (34) Turro, N. J.; Yekta, A. Luminescent Probes for Detergent Solutions. A Simple Procedure for Determination of the Mean Aggregation Number of Micelles [7]. J. Am. Chem. Soc. 1978, 100, 5951. (35) Zana, R.; Panagiotis, L.; Jacques, L. Fluorescence Probe Studies of the Interactions Between poly(oxyethylene) and Surfactant Micelles and Microemulsion Droplets in Aqueous Solutions. J. Phys. Chem. 1985, 89, 41. (36) Fang, L.; Ganzuo, X. Guiying, Hanqing, W. Formation and Aggregation Number of Sodium Dodecyl Sulphonate-polyvinylpyrrolidone Aggregates. Chinese Sci. Bull. 1997, 42, 1810. (37) Kabir-ud-Din.; Siddiqui, U. S.; Kumar, S. Viscometric Studies on Aqueous Gemini Micelles in the Presence of Additives. Colloids Surf., A 2007, 301, 209. (38) Nilsson, S. Interaction between Water-Soluble Cellulose Derivatives and Surfactants. 1. The HPMC/SDS/Water System. Macromolecules. 1995, 28, 7837. (39) Francois, J.; Dayantis, J.; Sabbadin, J. Hydrodynamical Behaviour of the Poly(ethylene oxide)-sodium dodecylsulphate Complex. Eur. Polymn. J. 1985, 21, 165. (40) Goddard, E. D. In Surfactants in Solution; Mittal, K. L.; Shah, D. O., Eds.; Plenum: New York, 1991.

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