Chapter 18
Interactions between Siloxane Surfactants and Hydrocarbon Surfactants
Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch018
Randal M. Hill Dow Corning Corporation, Midland, MI 48640-0994
The surface active properties of mixtures of siloxane surfactants and hydrocarbon surfactants are described. Our results include combinations of surfactants which differ in both their hydrophobic and hydrophilic groups. Previous studies have only looked at combinations which differ in one or the other. We found mixing behavior varying from antagonistic (positive -nonideal)to synergistic (negative-nonideal). We attempted to model the mixed CMC's in the usual way, using the formalism of the regular solution approximation, but found that the regular solution approach fails to account for our results in three significant ways: (i) the magnitude of the interaction depends on the proportions of the two constituents, (ii) the symmetry of the predicted C M C curves is incorrect, and (in) the measured C M C values for the nonionic/nonionic mixtures are much higher than the model can account for. Antagonistic mixing of fluorocarbon/hydrocarbon surfactant mixtures has been attributed to phobicity of these two moieties. However, since low molecular weight silicones and hydrocarbon solvents are generally mis cible, we propose another explanation for our systems based on molecular size and shape arguments.
Siloxane surfactants have received increasing interest recently because of their unique surface active properties (1,2). They find applications in such diverse areas as polyurethane foam additives, textile manufacture, cosmetic formulations, agricultural adjuvants, and paint additives (2). Most aqueous applications of siloxane surfactants actually involve their use in combination with organic surfactants and polymers. For instance, they are incorporated into cosmetic formulations containing a variety of other surface active ingredients. They are used to enhance spreading and plant penetration in agro-chemical formulations which also contain organic surfactants. Since it is well known that different classes of surfactants can interact strongly (3,4), itbecomes vital to understand the behavior of mixtures of siloxane surfactants and organic surfactants. How do such mixtures influence
0097-6156/92/0501-0278S06.00/0 © 1992 American Chemical Society
In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
18. HILL
Siloxane Surfactants & Hydrocarbon Surfactants
279
Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch018
the surface activity and performance of the formulation? Can the performance of products incorporating siloxane surfactants be improved by judicious choice of the other surfactants in the formulation? Are the unique surface active properties of siloxane surfactants simply additive to complex formulations, or do they interact, as other surfactants do, to determine the end-result? These are the questions we set out to answer in this study. Classification of Surfactants. Surfactants can be usefully classified in terms of the types of hydrophobic and hydrophilic groups they contain. The most common hydrophobic groups are hydrocarbon (linear and branched alkyl and alkylphenyl groups) and fluorocarbon (mostly branched alkyl groups containing various amounts of C-F functionality). Surfactants in which the hydrophobic group consists of dimethyl siloxane moieties are called siloxane surfactants. Siloxane is preferred over silicone because of the extremely wide use of the latter term to describe polydimethylsiloxane oils. Many siloxane surfactants are oligimers or polymers containing a broad distribution of molecular species. One very important exception to this is the trisiloxane-based surfactants which are the primary focus of this paper. Surfactants can also be classified in terms of their hydrophilic groups as nonionic, anionic, or cationic surfactants. Zwitterionic, or catanionic surfactants contain both an anionic and a cationic group. Common nonionic groups include polyethylene oxide (PEO), glucose and sucrose, amine oxide and phosphine oxide. Anionic groups include sulfate, sulfonate, carboxylate and phosphate. Cationic groups are usually quaternary ammonium salts of various structures. Siloxane surfactants have been prepared containing most of these hydrophilic groups (5). We will use the term siloxane polyethylene oxide (SPEO) surfactants to refer to the class of nonionic siloxane surfactants which contain polyethylene oxide hydrophilic groups. Nonionic surfactants may also contain polypropylene oxide (PPO) groups as well as PEO groups. PPO is actually a hydrophobic group, and siloxane surfactants which contain both PEO and PPO are referred to as siloxanepolyalkylene oxide (SPAO) surfactants in order to distinguish them from SPEO surfactants. SPAO surfactants are widely used as polyurethane foam additives. Purpose of Surfactant Classification. These surfactant classifications are useful because they represent the key to understanding the properties of surfactant mixtures - the behavior of surfactant mixtures depends on these class differences. For instance, the behavior of mixtures of nonionic hydrocarbon surfactants and anionic hydrocarbon surfactants is determined by interactions between the nonionic and anionic hydrophilic groups (3). The behavior of mixtures of anionic fluorocarbon surfactants and anionic hydrocarbon surfactants is controlled by the phobic interactions between the hydrocarbon and fluorocarbon tail groups (6-9). Previous studies of surfactant mixtures have investigated combinations which differ only in either their hydrophobe class or their hydrophile class, but not both. Mixtures of siloxane nonionic surfactants with anionic and cationic hydrocarbon surfactants differ in both their hydrophobic and hydrophilic groups. Therefore, previous work gives us no sure guidance to predict the behavior or our systems. Surfactant Mixing Behavior. Mixtures of surfactants belonging to the same hydrophile class and hydrophobe class, such as a pair of homologous nonionic hydrocarbon surfactants, mix ideally (10,11); the properties of the mixture can be predictedfromthe properties of the individual components by treating the micelle as a pseudo-phase and using the ideal solution equations (3):
In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
280
MIXED SURFACTANT SYSTEMS CMC CMC Y CMC + Y CMC A
A
B
B
B
A
VA qM
Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch018
CMC
A
where C M is the total monomer concentration, YAand Y B are the monomer mole fractions, and X A and X B are the micellar mole fractions. These equations contain no empirical parameters. Lange and Beck (10) showed mixed CMC's for mixtures of three alkyl ethoxylate nonionic surfactants. The measured values were almost exactly given by the ideal solution equations. Their data also showed that as the mixed CMC decreased, the value of the surface tension at the CMC also decreased. Meguro, Ueno and Esumi (11) also show data which displays this trend. They state that ideal mixing behavior is observed for mixtures of alkyl ethoxylates of varying alkyl chain length and ethoxylate chain length, and for mixtures of alkyl sulfoxides. In all these examples the hydrophobic portions of the molecules are similar, and consist of hydrocarbon moieties, whether linear or branched alkyl groups, or alkyl phenyl groups. In contrast to this, mixtures of siloxane and hydrocarbon surfactants involves mixing siloxane and hydrocarbon hydrophobic groups together in the micelle. The size, shape, and chain flexibility of these two types of hydrophobic group are markedly different, and this will certainly affect how they mix in the micelle. A well-studied example of a mixed surfactant system containing différait types of hydrophobic groups is the combination of fluorocarbon and hydrocarbon surfactants. Such mixtures show strong positive deviations from ideal behavior (antagonistic mixing), and sometimes form immiscible hydrocarbon and fluorocarbon micelles rather than mixed micelles (6-9). This behavior is generally attributed to the mutual phobicity between fluorocarbon and hydrocarbon moieties. Non-ideal surfactant mixing effects have been successfully modelled by treating the micelle as a pseudo-phase and using the formalism of the regular solution approximation (RSA) (3,12-15): C , » X i Cjexp(p Xl) = Y C
M
C -X
M
1
12
2
2
