Interaction between surfaces of fused silica in water. Evidence of cold fusion and effects of cold plasma treatment

Silica colloids and silica glass surfaces have often been used as "model" systems to study coagulation, rheology, contact angles, and surface forces. But the silica-water interface is highly changeable and reactive. It has stubbornly refused to conform to theoritical models of an ideal hydrophilic substrate. In this study we show why this is and demonstrate some of the diverse properties of this surface. Surfaces of fused quartz swell under water to form layers of silica gel. We report here on how this well-known effect shows up in surface force measurements. Peculiar effects occur already at normal pH. Over a period of time after the surfaces are immersed in water, identical interaction patterns occur on approach and on separation. The double-layer repulsion extends from large distances down to the contact. Interaction hysteresis develops later. Adhesion and other specific interactions, particularly at short range, develop with time. The evolution that extends for hours and days is variable in its manifestations from experiment to experiment. Precise conditions of solidification from the melt, and aspects of the thermal history of the glass transition during preparation of vitreous silica samples, can be factors in this variability. Surface degradation by formation of silica gel layers on contact with water can be enhanced by cold plasma treatment and by UV radiation. Pull-off forces increase with an increase of contact time. They also show a memory of conditions of previous contact. Electrolytes enhance the adhesion. Complicated polycondensation equilibria, influenced by nonspecific and specific ion effects, pH, nonionic solutes, and temperature distinguish the chemistry of silicic acid. All are involved in the interaction. These curious, history-dependent, surface forces were first reported half a century ago. They were attributed by Malkina and Derjaguin to "water structure". The effects that led later to contentious and disputed notions of hydration forces can be manifest as an "extra" repulsion or an "extra" attraction. They are here related to surface gelation. These surface force observations have distinct parallels in thixotropy and other peculiarities of "anomalous" coagulation and rheological behavior of concentrated and diluted dispersions of colloid silica in water. The effect of "cold fusion" between macroscopic surfaces of pure silica in pure water is here studied at room temperature with a new interfacial gauge force measuring technique. This spontaneous welding due to the presence of water can be hindered by stray contact shear, which interferes with observation by colloid probe and surface force techniques. The peculiar properties of the silica-water interface are discussed in connection with earlier experimental work that led to theoretical notions of polywater and non-DLVO forces.