An experimental investigation of the stability of ethylcellulose latex: Correlation between zeta potential and sedimentation

ArticleinEuropean Journal of Pharmaceutical Sciences 26(2):170-5 · November 2005with35 Reads
DOI: 10.1016/j.ejps.2005.05.008 · Source: PubMed
Abstract
This paper aims at explaining the experimental observations of the stability and redispersibility of an aqueous ethylcellulose latex through the electrokinetic characterization of the particles. The surface charge and the electrical double layer thickness play an essential role in the stability of the system, hence the need for a full characterization of the polymeric particles. The effect of both pH and ionic strength of the dispersion medium were investigated. It was found that at acid pH values the latex displays "delayed" or "hindered" sedimentation: in such conditions, the electrophoretic mobility and zeta potential are rather low, indicating a small electrokinetic charge on the particles. At alkaline pH, when the dissociation of ionizable surface groups must be complete, the zeta potential is high and negative. The electrostatic repulsion between polymer particles is responsible for the low sedimentation volume and poor redispersibility of the latex. The effect of NaCl and CaCl(2) concentration on both the zeta potential and stability of the latexes was also investigated: it was found that CaCl(2) has the greatest influence, yielding flocculated, easily re-dispersible systems when its concentration in the dispersion medium is high enough. There qualitative observations were ascertained by means of calculations of the potential energy of interaction between particles. In the case of NaCl solutions, a high and relatively wide potential energy barrier was predicted, that may prevent the particle aggregation. Above 5mM NaCl a shallow minimum in the potential energy curves must lead to the formation of aggregates. Similar results were found with CaCl(2) solutions, although in this case the secondary minima are deeper and appear at lower concentrations.
    • "The increase in negative charge density increases the concentration of hydroxyl ions in a solution that tends to favor an increase in protons and a basic pH. On the other hand, a decrease in zeta potential, in module, as well as a more acidic pH can be explained by the neutralization of the negative regions as a result of the adsorption of hydrogen ions (Gallardo et al., 2005; Arias et al., 2008). The polydispersity index showed that all the microemulsions had narrow size distribution (Figure 6). "
    [Show abstract] [Hide abstract] ABSTRACT: Descrevemos o desenvolvimento de um produto farmacêutico contendo ativo vegetal baseado em uma planta medicinal brasileira. Foi avaliada a habilidade de formação de uma microemulsão à base do óleo de Pterodon emarginatus (sucupira) e seu potential anti-inflamatório. Uma formulação foi desenvolvida utilizando o óleo de P. emarginatus, mistura de óleo de mamona etoxilado (Ultramona(r) R-540)/propilenoglicol (2:1) (tensoativo/cotensoativo) e água destilada, na proporção de 10:15:75, respectivamente. A microemulsão selecionada foi submetida ao teste preliminar de estabilidade e foi analisada quanto ao diâmetro médio das gotículas, pH, potential zeta e índice de polidispersão, no 1º, 7º, 15º e 30º dias após o preparo e foram estocadas em diferentes temperaturas (5±2 °C, 25±2 °C e 40±2 °C). Avaliaram-se a atividade anti-inflamatória in vivo do óleo de sucupira e da formulação, usando o modelo experimental do edema de orelha induzido pelo óleo de cróton. No teste preliminar de estabilidade observou-se que a formulação estocada a 5 °C e a 25 °C mantiveram suas características originais durante 30 dias. O potencial anti-inflamatório de ambos, óleo de sucupira e formulação foi estatisticamente significativo (p <0.001), quando comparado ao controle, porém a microemulsão foi mais eficiente (p <0.05) que o óleo aplicado diretamente nas orelhas dos animais.
    Full-text · Article · Mar 2015
    • "The steric repulsion should be a function of surface adsorption density, . To better understand this system, zeta potential analysis was employed to characterize the surface charge density [37], hence the electrostatic repulsion, while GPC measurements of serum phase were conducted to study the surface adsorption of PVOH hence the steric repulsion. It should be noted that both analysis were conducted at a relatively dilute region of the formulation, meaning large separations between latex particles. "
    [Show abstract] [Hide abstract] ABSTRACT: To understand the impact of surface charge density on the redispersibility of spray dried powders, the redispersibility of styrene-butadiene (SB) powders prepared from latexes with various particle sizes and acid contents was investigated. The redispersibility of these powders showed a step function from fully redispersible to non-redispersible. The phase boundary between these two regimes is determined by the product of the latex particle size and acid content, which is strongly related to the surface charge density of the latex particle. Results indicate that to achieve fully redispersible polymer powder, a proper balance of latex particle size and surface charge is needed. For the class of SB latexes studied in this work, with a certain acid level, it is possible to predict the minimum latex particle size that enables redispersibility of its spray dried powder. This phenomenological observation is unique and it may help us to reveal the fundamentals which govern the redispersibility. To maintain the redispersibility of a spray dried powder, a colloidal stabilizer that adsorbs onto the surfaces of latex particles is required. When latexes are brought to close proximity, strong van der Waals forces may deplete the surface stabilizers, causing irreversible latex agglomeration at spray drying temperatures. A qualitative DLVO analysis was employed to study the impact of various colloidal interactions in this system, including van der Waals, electrostatic, and steric interactions. Zeta potential measurements of the mixture of SB latex and polyvinyl alcohol (PVOH, used as a colloidal stabilizer) suggest that the surface charge density plays a significant role on redispersibility, and strong electrostatic repulsive interactions are needed to overcome the attractive van der Waals interaction. Size exclusion chromatography of serum phase analysis indicated that PVOH adsorption onto latex particles did not change significantly at different acid levels, suggesting low impact of steric repulsion on redispersibility. Conclusions from this work provide additional means to control the performance of redispersible materials.
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    • "The loading of ACZ molecules inside the liposomes did not altered the zeta potential (−10.3 ± 3.9 SD mV). The low value of zeta potential associated with the ACZ loaded neutral liposomes is indicative of insufficient stability (Gallardo et al., 2005; Wang et al., 2011). Thus acetate gradient positive liposomes were screened for preparation of stable ACZ loaded liposomes (Table 2). "
    [Show abstract] [Hide abstract] ABSTRACT: In the present work comparative evaluation of acetate and pH gradient techniques for effective drug loading in liposomes has been investigated. The acetazolamide (ACZ) loaded liposomes prepared by two methods were analyzed by vesicle size analysis, zeta potential, percent encapsulation efficiency, in vitro drug release studies and intraocular pressure lowering activity. ICH guidelines were followed for determining stability of the prepared liposomes. The superiority of acetate gradient method for active loading of acetazolamide has been established. The prepared acetate gradient positive liposomes showed extended hypotensive effect when compared to other liposomal formulations. Thus ACZ loaded liposomes prepared by acetate gradient technique may serve as promising ocular delivery system in the treatment of glaucoma. The current work emphasizes the fact that the techniques used for active drug loading into liposomes strongly influence the pharmaceutical performance of the final formulation.
    Full-text · Article · Nov 2013
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