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 ''mo del'' systems to study coagulation, rheology, contact angles, and surf ace forces. But the silica-water interface is highly changeable and re active. It has stubbornly refused to conform to theoretical 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 o f fused quartz swell under water to form layers of silica gel. We repo rt here on how this well-known effect shows up in surface force measur ements. Peculiar effects occur already at normal pH. Over a period of time after the surfaces are immersed in water, identical interaction p atterns occur on approach and on separation. The double-layer repulsio n extends from large distances down to the contact. Interaction hyster esis develops later. Adhesion and other specific interactions, particu larly at short range, develop with time. The evolution that extends fo r hours and days is variable in its manifestations from experiment to experiment. Precise conditions of solidification from the melt, and as pects of the thermal history of the glass transition during preparatio n of vitreous silica samples, can be factors in this variability. Surf ace degradation by formation of silica gel layers on contact with wate r can be enhanced by cold plasma treatment and by UV radiation. Pull-o ff forces increase with an increase of contact time. They also show a memory of conditions of previous contacts. Electrolytes enhance the ad hesion. Complicated polycondensation equilibria, influenced by nonspec ific and specific ion effects, pH, nonionic solutes, and temperature d istinguish the chemistry of silicic acid. All are involved in the inte raction. These curious, history-dependent, surface forces were first r eported half a century ago. They were attributed by Malkina and Derjag uin 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 surf ace gelation. These surface force observations have distinct parallels in thixotropy and other peculiarities of ''anomalous'' coagulation an d rheological behavior of concentrated and diluted dispersions of coll oid silica in water. The effect of ''cold fusion'' between macroscopic surfaces of pure silica in pure water is here studied at room tempera ture with a new interfacial gauge force measuring technique. This spon taneous 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 t hat led to theoretical notions of polywater and non-DLVO forces.