Effect of Polymer Concentration on the Rheology and Surface Activity of Cationic Polymer and Anionic Surfactant Mixtures

Abstract

The effects of polymer concentration on rheology, surface tension, and electrical conductivity of polymer–surfactant mixtures are investigated experimentally. The polymer studied is a cationic quaternary ammonium salt of hydroxyethyl cellulose, and the surfactant used is anionic sodium lauryl sulfate. The polymer concentration is varied from 1000 to 4000 ppm, and the surfactant concentration varied from 0 to 500 ppm. Polymer concentration affects the properties of the mixtures substantially. At a given surfactant concentration, the consistency of the polymer–surfactant mixture rises sharply with the increase in polymer concentration. The mixture also becomes more shear-thinning with the increase in polymer concentration. The surface tension decreases substantially, and the electrical conductivity increases with the increase in polymer concentration at a fixed surfactant concentration. At a given polymer concentration, the consistency index generally exhibits a maximum and the surface tension exhibits a minimum at some intermediate surfactant concentration. With the increase in polymer concentration, the maximum in the consistency index and the minimum in surface tension shift to higher surfactant concentrations. Although the exact mechanisms are not clear at present, a possible explanation for the observed initial changes in rheological and surface-active properties of polymer–surfactant mixtures with the addition of surfactant is charge neutralization and entanglement of polymer chains. At high surfactant concentrations, recharging and disentanglement of polymer chains probably take place.

1. Introduction

Controlling or manipulating the viscosity and rheology of polymer solutions by the addition of surfactants is very important in many practical applications dealing with enhanced oil recovery, drug delivery, hydraulic fracturing and drilling, formulation of foods, cosmetics, paints, and household products, etc. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. The surface activity of solutions characterized by surface tension is also equally important in practical applications [26,27]. Surface tension influences the wettability, adhesion, stability, and functionality of materials [27]. For example, a material with low surface tension (high wettability) spreads easily on a surface, whereas a material with high surface tension does not spread readily and adhere to the surface. Surface activity also plays a crucial role in enhanced oil recovery operations [26]. A high surface activity (low interfacial tension) facilitates the displacement of oil by injected water, resulting in high recovery rates of oil. The addition of surfactants to polymeric additives is also being explored to facilitate the flow of liquids in pipelines and other process equipment by reducing the frictional drag [22,23,24].

Surfactants are often added to enhance the performance of polymer-based hydraulic fracturing and drilling fluids. Surfactants can alter the viscosity of the fluid during pumping by causing disentanglements of polymer chains. The decrease in viscosity facilitates flow and reduces the consumption of pumping energy. The shear stability of the fluid is also enhanced by surfactants due to the formation of hydrated layers around polymer chains [21]. This prevents extensive stretching and breakup of the polymer chains under high shear stress. Surfactants can also help to control the rapid shear-thinning behavior of polymeric fluid. This is helpful in retaining fluid viscosity during high-shear operations, which is important for the transport of proppant to the fracture zone [21]. Surfactants also play a role in enhancing the suspension of solid particles and in the prevention of sedimentation. Homogeneous distribution of solid particles is helpful in the development of optimum filter cakes on the walls of the wellbore, resulting in improved sealing of formations [21].

The studies published on polymer–surfactant interactions are generally focused on polymer–surfactant aggregation at low surfactant concentrations [5]. The interaction between surfactants and polymers depends on several factors, such as the types of surfactants and polymers, composition or concentrations of species, and solution conditions such as the pH and temperature [5,28,29,30]. For example, the interaction between an electrically charged surfactant (negative, positive, or zwitterionic) and an electrically charged polymer is of the electrostatic type, whereas the interaction between a nonionic surfactant and nonionic polymer is of the hydrophobic type, involving hydrophobic portions of the surfactant and polymer [5,21]. The critical concentration where interaction between polymer and surfactant molecules begins is usually lower than the CMC (critical micelle concentration) of the pure surfactant solution [4].

Despite a considerable number of studies published on the understanding of polymer–surfactant interactions, the complex behavior of mixed surfactant and polymer additives in solutions is far from well understood.

Clearly, more research is needed to improve our understanding of the effects of interactions between surfactants and polymers on the properties of their solutions. The main objective of this work is to determine the effects of polymer concentration on anionic surfactant–cationic polymer interactions through measurements of steady rheological properties and surface activity of surfactant–polymer solutions.

2. Materials and Methods

2.1. Materials

The polymer used was a cationic quaternary ammonium salt of hydroxyethyl cellulose (referred to as CHEC), manufactured by Dow Chemical, New Milford, CT, USA, under the trade name of UCARE Extreme Polymer. The chemical structure of the polymer is shown in Figure 1. This polymer is a bio-derived and biodegradable polymer used extensively in the manufacture of conditioners, leave-on products, and shampoos.

The surfactant used was anionic sodium lauryl sulfate, supplied by Stepan Company, Northfield, IL, USA, as a dry white powder under the trade name of Stepwet DF-95. The chemical structure of the surfactant is CH₃(CH₂)₁₀CH₂OSO₃Na. It is used in many applications such as dentifrices, hand cleaners, powdered baths, liquid hand soaps, and shampoos.

Water used throughout was deionized with a very low electrical conductivity of about 2.2 µS/cm.

2.2. Preparation of Polymer Solutions and Surfactant–Polymer Mixtures

Polymer solutions were prepared in batches of about 1 kg at room temperature (22 ± 1 °C). The predetermined amounts of deionized water and polymer were mixed using a Gifford-Wood homogenizer (Federal Equipment Company, Cleveland, OH, USA) at an appropriate speed for nearly 1 h until complete dissolution of the polymer. A known amount of surfactant was added to the known amount of polymer solution and mixed thoroughly at room temperature using the homogenizer for about 1 h to prepare the surfactant–polymer solution. Care was taken to avoid entrapment of air during the mixing process.

2.3. Measurement of Steady Rheological Properties

Steady rheological properties of polymer solutions and surfactant–polymer mixtures were measured at room temperature (≈22 °C) using the co-axial cylinder type of viscometers, namely Fann and Haake viscometers. A Fann viscometer was more suitable for relatively low-viscosity solutions, whereas a Haake viscometer was used for solutions of high viscosities. The dimensions of the viscometers are listed in

Table 1. The dimensions of the Fann and Haake viscometers used in this work.

Viscometer	Inner Cylinder Radius, ${\mathit{R}}_{\mathit{i}}$	Outer Cylinder Radius, ${\mathit{R}}_{\mathit{o}}$	Length of Inner Cylinder	Gap Width
Fann 35A/SR-12	1.72 cm	1.84 cm	3.8 cm	0.12 cm
Haake Roto-visco RV 12 with MV I	2.00 cm	2.1 cm	6.0 cm	0.10 cm

. Note that the rotational speeds of the Fann viscometer varied from 0.9 to 600 rpm, whereas the rotational speeds in the Haake viscometer varied from 0.01 to 512 rpm. The devices were calibrated using viscosity standards of known viscosities.

2.4. Surface Tension Measurement

The surface tension of surfactant and surfactant–polymer solutions was measured using the pendant drop tensiometer at room temperature. The tensiometer was manufactured by Droplet Lab, Markham, ON, Canada. This tensiometer is based on the pendant droplet method of estimating the surface tension. A pendant droplet is generated and imaged at high resolution using a smartphone camera. The surface tension is estimated using specialized software by fitting the droplet profile with the Young–Laplace equation [31]. Measurements were performed twelve times for each fluid, and the average value was calculated.

2.5. Electrical Conductivity Measurement of Solutions

A Thermo Orion 3 Star conductivity meter (Thermo Fisher Scientific Inc., Waltham, MA, USA) was utilized to measure the electrical conductivity of solutions. The measurements were performed at room temperature.

3. Results and Discussion

3.1. Rheology and Surface Activity of Polymer Solutions Without Surfactant

Figure 2 shows the rheological behavior of polymer solutions without surfactant addition. In Figure 2a, shear stress is plotted as a function of shear rate on a log–log scale for polymer solutions at different concentrations. In Figure 2b, the same data are plotted as viscosity versus shear rate on a log–log scale. Clearly, polymer solutions at different concentrations are non-Newtonian shear-thinning. The shear stress versus shear rate data or viscosity versus shear rate data are described satisfactorily using the following power law model [32]:

$$\tau =K{\dot{\gamma }}^n$$

(1)

$$\eta =\tau /\dot{\gamma }=K{\dot{\gamma }}^{n-1}$$

(2)

where $\tau$ is shear stress; $\dot{\gamma }$ is shear rate; $K$ is the consistency index; $n$ is the flow behavior index; and $\eta$ is the viscosity. The flow behavior index $n$ of the shear-thinning polymer solutions is less than unity.

Table 2. Power-law parameters of CHEC polymer solutions at different concentrations.

Polymer Concentration (ppm)	Consistency Index, K (mPa·sn)	Flow Behavior Index, n	R2
1000	321.23	0.423	0.9833
2000	356.36	0.485	0.993
3000	368.7	0.538	0.9827
4000	642.22	0.505	0.9947

summarizes the power-law parameters of polymer solutions at different concentrations. To assess the goodness-of-fit of the power law model, the coefficient of determination R² values were also calculated and summarized in the table. The minimum R² value is 0.9827. This is still high enough to justify the usage of the power law model to describe the data.

Figure 3 shows the variations in electrical conductivity and surface tension of polymer solutions with polymer concentration. As expected, the electrical conductivity of polymer solutions increases with an increase in ionic polymer concentration. However, the surface tension is almost independent of polymer concentration. The surface tension of polymer solutions is only slightly less than that of water, indicating negligible surface activity of the polymer.

3.2. Rheology and Surface Activity of Polymer Solutions with Added Surfactant

Figure 4 shows the rheograms (viscosity versus shear rate plots) of polymer–surfactant mixtures at different polymer concentrations. For the sake of clarity, the data at all surfactant concentrations are not shown. All the mixtures are shear-thinning in that the viscosity decreases with the increase in shear rate. More importantly, the polymer–surfactant mixtures obey the power-law model (Equation (1) or (2)).

The power-law parameters $K$ and $n$ vary with both polymer and surfactant concentrations. The plots of power-law parameters as functions of surfactant concentration are shown in Figure 5 at different polymer concentrations. Note the following: (a) At a low polymer concentration of 1000 ppm, the consistency index initially increases above that of the polymer alone with the addition of the surfactant. It reaches a maximum value around surfactant concentration 100–200 ppm. Above 200 ppm surfactant concentration, the consistency index falls off. It becomes equal to or less than the consistency index of the polymer alone when the surfactant concentration exceeds about 350 ppm. The flow behavior index is above that of the polymer solution until a surfactant concentration of 200 ppm, although it falls below that of the polymer when the surfactant concentration exceeds 200 ppm. In other words, the polymer–surfactant mixtures become more shear-thinning at high surfactant concentrations above 200 ppm. (b) At a polymer concentration of 2000 ppm, the consistency index is much higher than that of the polymer solution alone over the entire surfactant concentration range, and it exhibits a maximum at the surfactant concentration of 200–300 ppm. The flow behavior index is much lower than that of the polymer solution over the entire surfactant concentration range, and it exhibits a minimum at the surfactant concentration of 200–300 ppm. Thus, the polymer–surfactant mixtures are much more shear-thinning than the polymer solution alone, and the degree of shear-thinning is maximum at the surfactant concentration of 200–300 ppm. (c) At a polymer concentration of 3000 ppm, the consistency index is again much higher than that of the polymer solution alone over the entire surfactant concentration range, and it exhibits a maximum at the surfactant concentration of 300 ppm. The flow behavior index is also much lower than that of the polymer solution over the entire surfactant concentration range, and it exhibits a minimum at a surfactant concentration of 300 ppm. (d) Finally, at a polymer concentration of 4000 ppm, the consistency index and the flow behavior index exhibit behaviors similar to a polymer concentration of 3000 ppm; however, the maximum in the consistency index and the minimum in the flow behavior index are shifted to a higher surfactant concentration of 400–500 ppm.

The exact mechanisms causing changes in the rheological properties and surface tension are not clear at present. One possible explanation for the initial rise in the consistency index and a corresponding decrease in the flow behavior index, that is, an increase in shear-thinning, is charge neutralization of polymer chains caused by oppositely charged surfactant, resulting in entanglement of polymer chains. At higher surfactant concentrations, it is likely that the polymer chains develop a negative charge (opposite to the original positive charge) due to excessive adsorption of negatively charged surfactant. Consequently, the polymer chains become disentangled at high surfactant concentrations due to electrostatic repulsion between the chains. This causes a decrease in consistency and shear-thinning of the solution. However, this mechanism of observed changes in the rheological properties and surface tension is speculative at best, and further research is needed to explore the exact mechanisms for changes in these properties.

Figure 6 compares the rheological properties (consistency index and flow behavior index) of polymer–surfactant mixtures at different polymer concentrations. At any given surfactant concentration, the consistency index increases substantially with the increase in polymer concentration. The flow behavior index generally decreases with the increase in polymer concentration; that is, the polymer–surfactant mixtures become more shear-thinning with the increase in polymer concentration. The increase in consistency and an increase in the degree of shear-thinning (decrease in the flow behavior index) with the increase in polymer concentration is due to an increase in polymer chain density, resulting in increased interactions and entanglements of polymer chains.

Figure 7 shows the conductivity and surface tension plots of polymer–surfactant mixtures at different polymer concentrations as functions of surfactant concentration. For comparison purposes, the surface tension plots of pure surfactant (without polymer) are also shown. At a low polymer concentration of 1000 ppm, the conductivity of cationic polymer solution remains nearly constant initially with the addition of anionic surfactant. When the surfactant concentration exceeds 200 ppm, the conductivity increases with the increase in surfactant concentration. One possible explanation for this behavior is competition between charge neutralization of polymer molecules by surfactant molecules and an increase in conductivity due to the availability of free ionic surfactant in the solution. At higher polymer concentrations, the conductivity decreases due to the charge neutralization of polymer molecules and the unavailability of free surfactant in the solution. Most of the surfactant at high polymer concentrations is likely absorbed by the polymer molecules.

The surface tension of surfactant–polymer mixtures is substantially lower than that of the surfactant solution without polymer. Furthermore, the surface tension versus surfactant concentration plots tend to exhibit a minimum at the intermediate surfactant concentrations, especially at low polymer concentrations of 1000 and 2000 ppm. At high polymer concentrations, the minimum in surface tension is either shifted to high surfactant concentrations or the surface tension levels off at high surfactant concentrations. The surface tension plots are consistent with the rheological data (see Figure 5) in that the consistency index exhibits a maximum where the surface tension is observed to be minimum. Interestingly, the surface tension of polymer–surfactant mixtures is generally much lower than that of the pure surfactant solution when compared at the same surfactant concentration. This indicates that the polymer–surfactant complexes are more surface active than surfactant molecules, although the exact mechanism is not known at present.

Figure 8 compares the surface tension and electrical conductivity of polymer–surfactant mixtures at different polymer concentrations. At any given surfactant concentration, the surface tension decreases substantially with the increase in polymer concentration. For low polymer concentrations of 1000 ppm and 2000 ppm, minima occur in the surface tension, although at high polymer concentrations of 3000 and 4000 ppm, no minima are observed. However, the surface tension is the lowest at the highest surfactant concentration of 500 ppm. The surface tension at all polymer concentrations is generally much lower than that of the surfactant solution alone at the same surfactant concentration.

The conductivity of the surfactant–polymer mixture increases with the increase in polymer concentration. At a low polymer concentration of 1000 ppm, the conductivity generally increases with the increase in surfactant concentration, whereas at higher polymer concentrations of 2000, 3000, and 4000 ppm, the conductivity generally decreases with the increase in surfactant concentration.

3.3. Reliability of Rheological and Surface Tension Measurements

The rheological data obtained for polymer solutions and polymer–surfactant mixtures were highly reproducible and reliable. As an example, Figure 9 shows comparisons of two sets of data obtained for the polymer solutions at the same concentration. The polymer solutions were prepared in a completely fresh way on two separate occasions and the viscometer data were collected. At any given polymer concentration, the rheological data obtained for the two samples overlap. Clearly, the measurements are highly reproducible.

To ensure that the data were reliable and free of any wall effects (slip effects), the data were collected for the same sample using both Fann and Haake viscometers with measuring heads of different dimensions (see Table 1). Figure 10 shows the comparison of data obtained by Fann and Haake viscometers for the same fluid, that is, 4000 ppm polymer with 50 ppm Stepwet. The data obtained from the two viscometers almost overlap with each other. This clearly indicates that the data are reliably free of any wall effects.

Measurements of surface tension were performed twelve times for each fluid, and the average value was calculated. The variability in the data was small. As an example, consider the polymer concentration of 2000 ppm.

Table 3. Mean surface tension and standard deviation of polymer–surfactant mixtures at a CHEC polymer concentration of 2000 ppm.

Surfactant Concentration (ppm)	Sample Size	Mean Surface Tension (mN/m)	Standard Deviation
0	12	64.09	0.9788
50	12	50.253	1.5325
100	12	38.323	1.5588
150	12	32.223	1.9077
200	12	29.446	2.0281
250	12	27.058	1.6438
300	12	28.084	2.1795
350	12	29.025	3.1398
400	12	37.057	1.7749
450	12	46.064	2.1008
500	12	49.3	0.9688

shows the mean surface tension and standard deviation. The standard deviation was calculated from the following formula:

$$s=\sqrt{\sum {\displaystyle \frac{{\left(x_i-\overline{x}\right)}^2}{N-1}}}$$

(3)

where $s$ is the standard deviation; $x_i$ is the individual measurement; $\overline{x}$ is the sample mean; and $N$ is the sample size. Clearly, the spread of data about the mean value is small.

Figure 11 shows the error bars on the surface tension data plotted as a function of surfactant concentration at a fixed polymer concentration of 2000 ppm. The spread or variability of data about the mean is generally small at all surfactant concentrations, indicating the reliability of the surface tension data.

4. Conclusions

The addition of anionic surfactant (sodium lauryl sulfate) to cationic polymer (quaternary ammonium salt of hydroxyethyl cellulose) solution alters the rheological and surface-active properties substantially. Both polymer and surfactant concentrations have a strong effect on the rheological and surface-active properties. The rheological behavior of all polymer–surfactant mixtures can be described adequately using the power-law model. At any given polymer concentration, the consistency index increases substantially and goes through a maximum value at some intermediate concentration of surfactant. With the increase in polymer concentration, the maximum in the consistency index is observed to shift to higher values of surfactant concentration. At a low polymer concentration of 1000 ppm, the maximum is observed at a surfactant concentration in the range of 100–200 ppm; at 2000 ppm polymer concentration, the maximum shifts to 200–300 ppm surfactant; at 3000 ppm polymer concentration, the maximum shifts to 300 ppm; and at 4000 ppm polymer concentration, the maximum shifts to 400–500 ppm surfactant. The degree of shear-thinning in polymer–surfactant mixtures, as reflected in the flow behavior index, also increases significantly with the increase in both polymer and surfactant concentrations. The surface tension behavior of polymer–surfactant mixtures is consistent with the rheological behavior. However, at a given polymer concentration, the surface tension drops substantially and goes through a minimum (instead of a maximum observed in consistency) at some intermediate concentration of surfactant. The minimum in surface tension generally shifts to higher surfactant concentration with the increase in polymer concentration. The exact mechanisms for the observed changes in rheological and surface-active properties with the increase in surfactant concentration are not clear at present. Further studies are needed to uncover the exact mechanisms causing changes in the properties. In future work, the microstructural changes in polymer–surfactant mixtures should be confirmed through some independent means. Dynamic light scattering (DLS) of polymer–surfactant mixtures might provide useful information about the formation of polymer–surfactant complexes. Dynamic rheological study and measurement of the intrinsic viscosity of polymer–surfactant mixtures would also provide useful information about the microstructure.