Foamability of aqueous solutions: Role of surfactant type and concentration

Highlights

• Foaming and surface properties for ionic and nonionic surfactants are studied.

• New approach for determination of the dynamic surface properties is described.

• Two master curves, one for ionic and one for nonionic surfactants, are revealed.

• The difference is due to the electrostatic repulsion, important for ionic surfactants.

• Much lower surface coverage is sufficient for foam generation from ionic surfactants.

Abstract

In this paper we study the main surface characteristics which control the foamability of solutions of various surfactants. Systematic series of experiments with anionic, cationic and nonionic surfactants with different head groups and chain lengths are performed in a wide concentration range, from 0.001 mM to 100 mM. The electrolyte (NaCl) concentration is also varied from 0 up to 100 mM. For all surfactants studied, three regions in the dependence of the foamability, V_A, on the logarithm of surfactant concentration, lgC_S, are observed. In Region 1, V_A is very low and depends weakly on C_S. In Region 2, V_A increases steeply with C_S. In Region 3, V_A reaches a plateau. To analyse these results, the dynamic and equilibrium surface tensions of the foamed solutions are measured. A key new element in our interpretation of the foaming data is that we use the surface tension measurements to determine the dependence of the main surface properties (surfactant adsorption, surface coverage and surface elasticity) on the surface age of the bubbles. In this way we interpret the results from the foaming tests by considering the properties of the dynamic adsorption layers, formed during foaming. The performed analysis reveals a large qualitative difference between the nonionic and ionic surfactants with respect to their foaming profiles. The data for the nonionic and ionic surfactants merge around two master curves when plotted as a function of the surface coverage, the surface mobility factor, or the Gibbs elasticity of the dynamic adsorption layers. This difference between the ionic and nonionic surfactants is explained with the important contribution of the electrostatic repulsion between the foam film surfaces for the ionic surfactants which stabilizes the dynamic foam films even at moderate surface coverage and at relatively high ionic strength (up to 100 mM). In contrast, the films formed from solutions of nonionic surfactants are stabilized via steric repulsion which becomes sufficiently high to prevent bubble coalescence only at rather high surface coverage (> 90%) which corresponds to related high Gibbs elasticity (> 150 mN/m) and low surface mobility of the dynamic adsorption layers. Mechanistic explanations of all observed trends are provided and some important similarities and differences with the process of emulsification are outlined.

Introduction

Surfactants are essential ingredients in laundry, household and personal care products, and in various technological processes. In many of these systems, the foamability of the surfactant solutions appears as desired or undesired phenomenon, depending on the specific application. Therefore, understanding the process and revealing the key physicochemical and hydrodynamic factors which control the foaming process is very important from both scientific and practical viewpoints.

To build a general and universal interpretation of the experimental results about the foamability of surfactant solutions, one should consider the processes of air entrapment and bubble coalescence which have opposite effects on foam volume – see Fig. 1. The foam volume increases when a newly entrapped air during mechanical agitation or gas incorporation (via bubbling or from chemical reaction) is unable to coalesce with the large air-water interface. On the opposite, the coalescence between entrapped air bubbles and this large interface removes the trapped air and keeps the foam volume low. On its turn, the bubble coalescence depends on the competition between the rate of surfactant adsorption on the bubble surfaces and the drainage time of the foam films, formed between the air bubbles and the large air-water interface. If the adsorption rate is faster, the coalescence may be suppressed, due to the repulsion between the bubble and the large gas-liquid interface which may arise only when the gas-liquid intefaces are covered with a sufficient amount of adsorbed molecules. In contrast, if the rate of adsorption is slower, the formed foam films rapidly thin to their critical thickness at which the attractive forces between the film surfaces dominate, the foam films break and the bubbles coalesce before the protective adsorption layer is formed.

The physicochemical analysis of the above concepts is complicated by the fact that the various surfactants may have different stabilizing efficiency at the same surface coverage. For example, one may expect a significant difference between the ionic and nonionic surfactants because the surface forces between the foam film surfaces (electrostatic, steric) are expected to play a crucial role in foam film stabilization. Further complication is that one should consider the surfactant adsorption, surface properties (such as surface coverage and Gibbs elasticity) and surface forces (disjoining pressure) of the dynamic adsorption layers formed during foaming, which usually are very far away from the equilibrium ones.

All these complications lead to the fact that there is no unifying and self-consistent theoretical approach to include the above elements and to describe the available results from the foaming tests. There are different theoretical models which capture the role of one or another factor for foam film rupture and bubble coalescence, but they are all developed for more idealized dynamics of film thinning, e.g. for films with constant diameter and fixed capillary pressure like those formed in a capillary cell or between a large air-water interface and a rising bubble, pushed by buoyancy [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. The relation between the results from such model studies and the results from actual foaming experiments has never been clarified convincingly, mainly due to the enormous complexity of the dynamic processes of foaming.

In the various studies of foaming [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]] several physicochemical parameters were proposed to explain the variations in the foamability of the surfactants solutions: surfactant concentration [11] and its relation to the critical micellar concentration (CMC); dynamic surface tension (DST) [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]; surface mobility expressed through the Marangoni effect [28], surface modulus [29] or surface elasticity [30] of the adsorption layers; stability of the single foam films [31,32] expressed through the disjoining pressure [33] and its components, such as steric repulsion and structural forces [[33], [34], [35]]. Generally speaking, each of these characteristics could be important and their interplay should be understood much better if we want to describe and control the complex process of foam formation.

Most often, it is assumed in the literature that the volume of the generated foam correlates with the rate of surfactant adsorption, which is determined by measuring the DST, and with the amount of adsorbed surfactant at the air-water interface. Many researchers showed in their studies that lower dynamic surface tension often corresponds to higher foaminess of the solutions [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Also, at concentrations above the CMC, the kinetics of de-micellization and release of surfactant monomers from the micelles was found to play a role [16,17]. Less foam was generated in foaming processes with intensive agitation for surfactants with very low critical micellization concentration (which reduces the equilibrium concentration of monomers in the micellar solutions) and stable micelles, such as those of the nonionic surfactants and with longer chain length.

The observed correlation between the volume of generated foam and DST could be attributed to the more efficient suppression of the bubble coalescence in the case of rapidly adsorbing surfactants. One of the main mechanisms for dynamic stabilization of the freshly formed foam films is the Marangoni effect which may lead to significantly reduced rate of film thinning [15,28]. The Marangoni effect and the related deceleration of film thinning depend strongly on the instantaneous quantity of surfactant adsorbed on the film surface in the moment of film formation. Marangoni effect is related to the surface Gibbs elasticity, E_G, which acts to restore the homogeneous distribution of surfactant along the film surface. In [32] the properties of single vertical foam films and the foamability of solutions containing different surfactants were compared. The results showed that only the stability of black films under dynamic conditions has some correlation with the foamability of the same surfactant solutions for different surfactant types.

In other studies it was shown that the changes in the values of the surface dilational modulus may exhibit similar trends to the foamability of the respective solutions for surfactants with different molecular structures [30]. This relation shows that the surface elasticity could be an important factor for the processes of foam formation and stabilization. Also, the bubble break-up is an intrinsic process of foam generation. In a previous article [29] we studied the factors controlling the kinetics of bubble break-up in sheared foams and found that high surface modulus of the surfactant solutions (above 100 mN/m) leads to the formation of much smaller bubbles due to a rapid breakup of the initial bigger bubbles. Furthermore, in a later study [36] we showed that the higher viscoelasticity of the foam containing smaller bubbles may reduce the volume of the formed foam, thus suppressing the solution foamability. Thus we see that the effect of surface elasticity needs further clarification.

Some authors reported an important relation between the foam and surface properties, on one side, and the surfactant molecular structure, on the other side. At concentrations below and above the CMC, the dynamic surface activity was shown to increase with the increase of the molecular mass of the surfactant molecules, while the foamability was found to decrease due to slower diffusion of the surfactant molecules [11]. The length of the hydrophobic tail is identified as a parameter controlling the rate of diffusion, adsorption and arrangement at the interface [18,[20], [21], [22]]. For a given alkyl chain length, increasing the hydrophilicity of the molecules leads to boost in foamability [34]. For foams produced from solutions of small amphiphilic single- and double-tail surfactants, the number of the hydrophobic tails and their length play a crucial role for the foaming while the head group was reported to be of secondary importance [35]. The authors suggested that the critical aggregation concentration could be used as a predictor for the ability of the small amphiphilic molecules to enhance foaming.

All these results indicate that one should analyse much deeper the properties of the dynamic adsorption layers, formed on the bubble surface during foaming, in order to explain the observed trends in the foaming experiments and to identify the key physicochemical factors controlling this process.

Based on the above brief literature overview, we defined the following major aims of the current study:

• To study systematically the role of the various physicochemical factors on the foamability of surfactant solutions using a series of seven surfactants which differ in their type (ionic and nonionic), chain length (12 and 16), head group structure and charge (non-ionic Brij and Tween, cationic and anionic) and concentration (up to 100 mM). The role of ionic strength was also studied by varying the concentration of a neutral electrolyte (NaCl) between 0 and 100 mM.

• To analyse the experimental data by considering the properties of the dynamic adsorption layers, taking into account their rapid change with the time of surface aging. On this basis, to reveal the key physicochemical characteristics of the adsorption layers which govern the initial rate of foam generation and the volume of accumulated foam for the various surfactants. The idea is to identify those “universal” parameter(s) which could explain the data for the various surfactant solutions studied.

To achieve the above aims, we combine several experimental methods to obtain complementary information about the surface and foaming properties of the various surfactant solutions – foam tests, dynamic and equilibrium surface tension measurements. Self-consistent interpretation of the results obtained by all these methods is proposed. Note that we use a foaming method with very intensive mechanical agitation (Bartsch shaking test) and that many of the studied solutions are of relatively low surfactant concentration (below and around the CMC) – as a result, the bubble coalescence plays a crucial role for the volume of the foams studied. From this viewpoint, the current paper extends and complements our previous study [36] in which only the range of high surfactant concentration was investigated and the bubble coalescence was completely suppressed.

The article is organized as follows: Section 2 describes the materials and methods used. The experimental results are described in Section 3. Their interpretation and discussion is presented in Section 4. The main conclusions are summarized in Section 5.