1662
Langmuir 2005, 21, 1662-1662
Effect of Varying the Oil Phase on the Behavior of pH-Responsive Latex-Based Emulsifiers: Demulsification versus Transitional Phase Inversion E. S. Read, S. Fujii, J. I. Amalvy, D. P. Randall, and S. P. Armes* Langmuir 2004, 20, 7422-7429. This paper is broadly correct in most aspects. However, based on our more recent experiments we believe that some of the text and two of the figures are not correct. It is true that the PDMA-PMMA-stabilized polystyrene latex acts as an effective particulate emulsifier for a range of oils. For example, in the case of n-dodecane, stable n-dodecane-in-water emulsions are obtained on homogenization at pH 8-9 but no emulsion is obtained on homogenization if the pH of the aqueous phase is first lowered to pH 2-3. On the other hand, using methyl myristate as the oil can lead to either a methyl myristatein-water emulsion or a water-in-methyl myristate emulsion, depending on the initial solution pH and the oil volume fraction. In these experiments it is clear that the degree of protonation of the steric stabilizer on the latex surface has a decisive influence on its emulsifier performance. However, it is not true that these emulsions, once formed at a particular pH, can be easily broken or inverted simply by changing the solution pH in situ. Instead, the emulsions appear to be relatively stable to pH variation, which is contrary to the impression given in our paper. Thus the phrases “stimulus-responsive”, “pH-responsive”, and “pHinduced”, which are used throughout the paper, are not appropriate for this class of particulate emulsifier. Bearing this in mind, it follows that the equilibrium arrows that connect the two physical states represented in Figures 1 and 2 are misleading and should be omitted; the corrected figures and figure captions are reproduced below. We have
confidence in the validity of the results reported in all the tables and other figures. However, the interested reader should re-examine our paper (and also two earlier papers: J. I. Amalvy, et al. Chem. Commun. 2003, 1826, and J. I. Amalvy, et al. Langmuir 2004, 20, 4345) in the light of the above discussion. It is emphasized that, for this latex-based particulate emulsifier, varying the solution pH prior to homogenization of the oil-water mixtures can have a significant affect on (i) whether a stable emulsion is formed and (ii) whether such emulsions, if formed, are of the oil-in-water or water-in-oil type. However, once homogenization has occurred, variation of the solution pH does not have a significant affect on the physical state of the system. In summary, the above system is pH-dependent, but not pH-responsive. I believe that this paper still has considerable scientific merit and novelty but perhaps not quite the same impact as originally intended. I only became aware of the above problem when a member of my research group recently discovered a new class of particulate emulsifiers that are genuinely “stimulus-responsive”, i.e., that allow emulsions to be coalesced rapidly simply by in situ adjustment of the solution pH. The synthesis and behavior of these new particulate emulsifiers will be reported in Advanced Materials. This unfortunate error was due to inadequate communication with my coauthors. As the corresponding author, I take full responsibility for this problem and I apologize unreservedly for unintentionally misleading the scientific community. LA047603Z 10.1021/la047603z Published on Web 01/15/2005
Figure 1. Schematic representation of the effect of solution pH on attempted emulsification for the n-dodecane/water system. Stable n-dodecane-in-water emulsions are obtained at pH 8, but no emulsion is formed at pH 3.
Figure 2. Schematic representation of pH-dependent transitional phase inversion. Oil-in-water emulsions are obtained on homogenization at pH 3, whereas water-in-oil emulsions are formed at pH 8. This behavior can be observed at certain volume fractions when using either methyl myristate or cineole as the oil phase (see Table 6 and main text for further details). 10.1021/la047603z CCC: $30.25 © 2005 American Chemical Society Published on Web 01/15/2005
